The Testing of Ceramics in Construction (Testing in Construction)

Testing in Construction Volume 2 This book is a comprehensive guide on the testing of the main ceramic elements used in the construction industry. Standard tests on fundamental products such as bricks, tiles and pipes are described and how these translate into practical procedures. This is a thorough treatment of European practice which finds acceptance worldwide. Chapters covering larger elements such as walls, full scale tests, as well as model and accelerated testing, are included. Tests concentrate mainly on new products or structures and techniques for dealing with the performance and condition of existing structures are included. Codes and Standards by which brick masonry is designed are described. The future role of test method development using IT is discussed with its associated potential benefits. Testing of Ceramics in Construction will be of considerable practical value to consulting engineers and designers, civil and structural engineers, manufacturers, contractors and testing houses on the international scene. It will also form a useful reference for architects, chartered surveyors and students in all these disciplines.

Geoff Edgell is Manager of the Building Technology Division at CERAM and has supervised testing programmes relating to many major construction projects. He is Chairman of the CEN TC125 Working Group on test methods for masonry units, mortars and ancillary components and he chairs the BSI committee responsible for masonry standards and test methods. Geoff is also Visiting Professor in the School of Civil Engineering at the University of Leeds.

G.Edgell

Whittles Publishing

Testing of Ceramics in Construction

An authoritative account of the test methods for ceramic products used in construction

Testing in Construction Volume 2

Testing of Ceramics in Construction

Edited by G.Edgell

The Testing of Ceramics in Construction

Testing in Construction Series Series editors David Doran, Consultant, London and Clive Cockerton, Associate, GBG Structural Services Ltd., Garston, UK

The growing importance of quality, durability and cost-effectiveness within construction means that testing is central to most sectors of the industry. Testing is necessary world-wide as part of attestation procedures and in Europe harmonisation and compliance have become watchwords. Each volume in this series stands independently but together will form an invaluable library of data and practical help for anyone involved with testing; whether relying upon the results of a test programme or working firsthand with the materials. This series provides a comprehensive guide, making it highly relevant for consulting engineers and designers, civil and structural engineers, manufacturers, contractors and testing houses on the international scene. It will also form a useful reference for architects, chartered surveyors and students in all these disciplines.

Volume 1 Volume 2

Principles and Practice of Testing in Construction The Testing of Ceramics in Construction

The Testing of Ceramics in Construction

edited by

Geoff Edgell Manager, Building Technology Division, CERAM, Stoke-on-Trent and Visiting Professor, School of Civil Engineering, University of Leeds

Whittles Publishing

Published by Whittles Publishing Limited, Dunbeath Mains Cottages, Dunbeath, Caithness, KW6 6EY, Scotland, UK www.whittlespublishing.com

© 2005 G. Edgell

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, recording or otherwise without prior permission of the publishers.

ISBN 1-870325-43-5

The publisher assumes no responsibility for any injury and/or damage to persons or property from the use or implementation of any methods, instructions, ideas or materials contained within this book. All operations should be undertaken in accordance with existing legislation and recognised trade practice. Whilst the information and advice in this book is believed to be true and accurate at the time of going to press, the author and publisher accept no legal responsibility or liability for errors or omissions that may be made.

Every effort has been made to contact the appropriate copyright owners for illustrations. We apologise for any oversights and ask that any such instances be referred to the publishers.

Printed and bound in Poland, EU

Contents (A contributor’s name indicates authorship of all subsequent sections up to the next name)

1

Introduction G. Edgell .................................................................................... 1

2

Materials testing ............................................................................................. 6 General G. Edgell ............................................................................................. 6 Standard tests on masonry units ....................................................................... 17 Compressive strength J. Lomax Water absorption Density Soluble salts Moisture expansion Frost resistance tests on clay masonry units F.Peake Standard tests on clay pavers ............................................................................ 43 Modulus of rupture C.Beardmore Slip and skid resistance Frost resistance tests on clay pavers F.Peake Standard tests on wall, floor and roofing tiles .................................................. 70 General W.Walters & G.Edgell The European testing systems for wall and floor tiles W.Walters Tile tests Test limitations The ISO testing system for tiles Slip resistance Inclined platform Other factors Clay roofing tiles G.Edgell Standard tests on vitrified clay pipes ............................................................... 83 General Geometry Crushing strength test Bending moment resistance test Fatigue strength Abrasion resistance Watertightness Chemical resistance Standard tests on hollow clay pot flooring ....................................................... 91 General Punching bending test Longitudinal compression test Dimensions –v–

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Moisture expansion Fracture energy Other tests ...................................................................................................... 95 Thermal expansion Tensile testing 3

Element testing ............................................................................................. 105 Compressive strength ..................................................................................... 105 General Specimen format Construction of test specimens and test procedure Flexural strength ............................................................................................. 110 General Specimen format Construction of specimens and test procedure Shear strength ................................................................................................. 114 General Specimen format Construction of test specimens and test procedure Application of results Brickwork containing sheet damp-proof course materials Bond strength testing R. de Vekey ................................................................... 119 Introduction Direct tension testing Flexural tension testing Bond wrench structural principles Bond wrench calibration procedure Variability of bond wrench measurements Test specimens and apparatus Testing of in situ masonry: method of use Accuracy Reporting bond wrench results for in situ tests

4

Prototype testing G.Edgell ........................................................................... 139

5

Full scale testing ........................................................................................... 150 Wall testing ..................................................................................................... 150 Compression Testing machines Construction of test specimen and test procedure Lateral load Construction of walls and test procedure Shear Tests on shear walls – vi –

C ONTENTS Reinforced and prestressed masonry testing .................................................. 164 Flexure and shear Racking shear Compression Prestressed brickwork Bed joint reinforcement Arch bridges C.Melbourne ............................................................................. 172 Explosions G.Edgell ....................................................................................... 181 Fire resistance of brickwork K.Fisher ............................................................ 185 Introduction History Development of fire testing methods Practical testing and performance of clay brickwork Legislation, application of test data, regulatory requirements Future developments Impact testing B.Hobbs .................................................................................. 198 Introduction Vehicle impact testing Development of laboratory test arrangement Teesside wall test arrangement Initial test programme Results Conclusions 6

Model testing G.Edgell ................................................................................. 215

7

Accelerated testing ....................................................................................... 219 Rain penetration testing: Laboratory and on site F.Peake .............................. 219 Background Laboratory testing Site testing Water testing pipelines and manholes G.Edgell ............................................. 224

8

Long-term testing ......................................................................................... 228 Creep testing J.Brooks .................................................................................... 228 Definition of terms Measurement of creep Measurement of strain Experimental procedures

9

Appraisal of existing materials .................................................................... 238 Structural testing G.Edgell ............................................................................. 238 Flat jack testing R. de Vekey ........................................................................... 247 Introduction – vii –

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Form Applications in masonry Why measure stress or elastic behaviour? The principle Determination of the effective area of flat jacks Calibration of flat jacks Procedure for measurement of stress in situ using flat jacks Procedure for measurement of stress-strain behaviour in situ using flat jacks Pull-out, penetration and impact hammer tests .............................................. 258 Introduction Individual test methods 10 Codes and standards G.Edgell ..................................................................... 265 11 Future developments G.Edgell ..................................................................... 270 Index .............................................................................................................. 273

– viii –

Authors Chris Beardmore, Technical Consultant, Stoke-on-Trent Dr Jeff Brooks, Visiting Research Fellow, School of Civil Engineering, University of Leeds Dr Ken Fisher, Technical Consultant, Whyteleafe, Surrey Professor Brian Hobbs, Director of the School of Science and Technology, University of Teesside Jeff Lomax, Technical Consultant, CERAM Building Technology, Stoke-on-Trent Professor Clive Melbourne, Professor of Structural Engineering, Research Institute for the Built and Human Environment, University of Salford Frank Peake, Technical Consultant, Stoke-on-Trent Dr Bob de Vekey, Associate Technical Director, Building Research Establishment, Garston, Watford Bill Walters, Tile Consultant, CERAM, Stoke-on-Trent

Acknowledgement Thanks are due to CERAM for permission to use photographs and extracts from their publications and archives and to BSI for permission to use in particular diagrams from Standard Test Methods.

– ix –

Preface Ceramics have been used in construction for thousands of years and organised testing on site was well known in the 19th century. The role of testing in the laboratory and on site developed significantly throughout the 20th century. This book comes at a period of historic significance in Europe as in an attempt to remove barriers to trade, test methods for construction products are being harmonised including those for most ceramic products. Whilst the market for ceramic products is being made more transparent through common specifications and test methods, the code of practice for the use of masonry is being developed on a Europe wide basis. Consequently the market for clay bricks and the specification for their design and use are both being changed. In order that design is not too prescriptive one measure being introduced, as a first option, is the evaluation of brickwork properties by agreed test methods. There has therefore been a significant development of test methods for clay bricks and brickwork as well as other masonry materials. The principal author, Professor Geoff Edgell, has been convenor of the Working Group of CEN (Committee for European Standardisation) Technical Committee 125, which has produced all of the test methods for masonry and masonry products. The contributing authors are all experts in their fields and many have represented the UK in the international drafting work. The book is therefore timely because in addition to explaining the principles behind the test, it provides the background to the situation currently pertaining and points to limitations on what could be agreed internationally. The book will prove a valuable reference to manufacturers, engineers, architects, builders, students and testing professionals. It goes beyond standardised testing and deals with bespoke testing at full scale and for specific hazards e.g. impact or structural forms such as reinforced and prestressed brickwork. The chapter on appraisal is especially useful as there are numerous current projects where modern development is linked to preservation e.g. The Channel Tunnel Rail Link at St. Pancras. Professionals involved in such projects can draw on the vast experience of testing to make informed judgements on structural and environmental performance over a reasonable remaining economic life. Testing ceramics is a mixture of science, engineering, experience and judgement and continues to develop. What is a definitive view today may not be so tomorrow but the European perspective makes the timing of this publication very relevant to the specification and use of ceramics in construction.

Clive Cockerton and David Doran

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1 Introduction

This volume This volume of the series Testing in Construction deals with ceramics. Ceramics in the form of bricks, blocks, tiles and pipes have been used in construction for thousands of years, as is illustrated by the many examples of buildings, aqueducts, hypocausts, etc. dating from Roman times and before. In times past much was learnt, by trial and error, about the behaviour of structures; there were many collapses, often causing loss of life. Health and safety at work was considered far less important than it is today. It is also true that architects and engineers were often the builders or contractors for their own projects. In many cases, the construction project would require bricks and those would be made at the site under the control of the same person. Consequently, there was less reliance in compliance with laid down standards to ensure suitability for purpose, however there was a great deal of skill in the sorting of bricks at the place of use. For example, clay brickwork structures would be built with the smaller, darker, better fired bricks in the more exposed parts and the softer less durable bricks were used in protected or internal locations. This is not to say that testing in construction was unknown however. Sir Marc Brunel, for example, was famous for testing to prove the structural performance of his designs, perhaps the most famous being the reinforced brickwork beams built in 1838, at Nine Elms (Beamish, 1862). A replica of the Nine Elms Beam was built for the Great Exhibition of 1851, with the newly discovered Portland cement being used instead of Roman cement. However the earlier test had confirmed the value of the reinforcement, as opposed to the cement type. Brunel was of course exceptional and perhaps the first organised approach to testing was by military men in both England and the USA. It was a British Colonel (Pasley, 1837) who was responsible for numerous tests on reinforced brickwork structures in the 1830s. At the turn of the 20th century, a French structural engineer, Paul Cottancin was acting as both engineer and contractor for a number of buildings using his own patented system for reinforcing brickwork and concrete (Edgell, 1985). At St Sidwells Wesleyan Church in Exeter there is a fine example of his work. The church has many notable features not least of which is the horseshoe shaped gallery which cantilevers some 4.2 m off the walls, see Figs 1.1 and 1.2. The building did not satisfy the local building bye-laws and as part of an exercise to prove the safety of the building, a loading test was carried out on the balcony. Fig. 1.3 is a photocopy of the test report. –1–

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Figure 1.1

Cross section of St Sidwells Wesleyan Church.

Figure 1.2

Gallery at St Sidwells during construction. –2–

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Figure 1.3

Copy of test report on load testing of Gallery.

Figure 1.4

One of the 282 tests carried out by Sir Alexander Brebner. –3–

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In the 1920s, in Bihar and Orissa in India, a significant amount of testing for both structural and fire performance was carried out by Sir Alexander Brebner (1923), an example of which is shown in Fig. 1.4. These early examples illustrate that for many years there have been practitioners who could identify a clear role for testing in construction and it continues to be true that, despite the availability of modern computer techniques, there are circumstances where the most satisfactory way of proving a design is by testing. Throughout the 20th century, testing as a separate expert discipline developed with the emphasis more towards supporting products which, in modern European parlance, are to be placed on the market. The role of the British Standards Institution (BSI) has been central in the development of standard test methods and it is a role that was vital to the reduction of waste in industry. The railway-age led to much greater mobility of not just people, but products and components, for these could be sourced from a much wider area than had hitherto been possible. This led to significant problems as components with ostensibly the same description and purpose could in fact be quite different and could not be used without some modification. The problem reached a head at the end of the 19th century, when in addition, professional architects and engineers were specifying all sorts of sizes and sections of mild steel and wrought iron girders and suppliers were continually having to change the manufacturing process at great cost. In 1901 Sir John Wolf Barry, a leading consulting engineer, arranged for the Institution of Civil Engineers (ICE) to appoint a committee to consider the standardisation of the sizes and shapes of iron and steel sections. The Institution of Mechanical Engineers (IMechE), the Institute of Naval Architects (RINA) and the Iron and Steel Institute (ISI) were all invited to nominate representatives and on 26 April 1901, the Engineering Standards Committee (ESC) was formed, the forerunner to BSI (Douglas Woodward, 1972). The work quickly developed and expanded with some great early success in the reduction of variety, steel sections in common use reduced from 175 to 113, spectacularly tramway rails from 75 to 5. The work of the committee developed tremendously during the two World Wars, where for example standardisation of aircraft parts was a major issue in World War I and perhaps more mundane, but of great importance, black-out material and stirrup pumps in World War II. The standardisation of test methods to support product specifications was of great importance throughout this period, so that purchasers could be satisfied that results from different test houses were comparable. As time has passed and Standards work has become more international, the importance of test methods has grown for another reason. Standardisation in the early years in the UK was about the reduction of variety, but in the international scene, it is about not inhibiting national diversity, while ensuring fitness for purpose. Test methods need to support performance-based standards and not restrict the use of traditional products in differing countries. –4–

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I NTRODUCTION This volume aims to cover ceramics in construction of all types. The early chapters deal with the products as units and aim to give the latest details and reasoning behind standard tests. In this regard, it is an interesting and exciting time in that new European Standards for products are being developed and, although there is an ebb and flow in this international work, in many cases the current thinking can be displayed. There are chapters on the testing of elements as structures, rainscreen and for resistance to fire and explosion. Sadly, during the writing of this book Dr H.W.H. ‘Timber’ West, who was to have contributed the section on explosions, died. Timber was an author of reports on work for which he was officer in charge examining the performance of brickwork apartment buildings following the worries generated by the collapse at Ronan Point in 1968. In his absence, a very brief summary of the work has been produced here and the inquisitive reader is referred to the original reports for details. This volume ends with a forward look on Codes and Standards and developments in testing. This has been most difficult to write with any certainty, as the views as to what can be achieved politically, technically and culturally on the international stage can change quickly. My thanks go to the contributory authors, all of whom are experts in their field. They do of course stand on the shoulders of giants such as Arnold Hendry and Timber West, but have all made significant contributions to the development of modern testing thinking. I hope this volume is a useful guide to where we are and how we got here. In Europe, Standards can take up to 10 years to develop and it is within that sort of timescale that new and younger people join the committees to further the work. I hope this helps them to develop what has been done, to not repeat mistakes and to advance the techniques, science and engineering of testing ceramics in construction.

References Beamish, R. 1862. Memories of the Life of Sir Marc Isambard Brunel. Longman, Green, Longman and Roberts, London. Brebner, A. 1923. Notes on Reinforced Brickwork, Technical Paper No. 38, Vols 1 and 2, Calcutta Public Works Department, Government of India. Edgell, G.J. 1985. ‘The remarkable structures of Paul Cottancin’, The Structural Engineer, 63A(7), pp.201–8. Pasley, R.E. 1837. Brick and Cement Beams, Civil Engineers and Architects Journal, Vol 1. Douglas Woodward, C. 1972. The Story of Standards, British Standards Institution, London.

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2 Materials testing

General The majority of tests covered in this section are described as Standard Tests and this implies that they have been adopted, usually by the BSI, as tests to support product specifications. Consequently, they represent procedures that have been developed to the point that a committee of experts in BSI representing manufacturers, specifiers, the government, academia and research interests, have agreed that they represent the state of the art for determining those properties needed to enable the product to be traded. Ideally, the test methods should be simple so that costs are minimised. They should also give unique answers, which puts a great responsibility on the drafters, as any ambiguity which can lead to differences in interpretation, can lead to costly disputes. The best way that areas open to interpretation are found is by physically trying to carry out test procedures and for many of our established British Standard Test Methods, at least some inter-laboratory comparisons have been carried out. In some cases and certainly ideally, statements can be made about the reproducibility and repeatability of test methods. These words have special meaning in relation to testing; reproducibility is the ability to reproduce results in one testing laboratory and means that the procedures are not so sensitive that an operative cannot ostensibly following the same test procedure reproduce his or her results. Repeatability is a measure of the ability of different laboratories to produce comparative results when testing the same product by the same procedure. A procedure (BSI, 1987) exists for determining reproducibility and repeatability, however in relation to clay building products, this has rarely been followed. One reason for this is cost, if one takes for example the compressive strength test for clay bricks, the range is so great that the procedure is needed to be carried out at a large number of strengths in order to validate it over the whole field of application and that, taken with the number of laboratories to be involved, leads to a very extensive programme. We are now seeing the introduction of a range of new European Tests for a wide range of properties and products and their development has not been easy. Some of them represent a compromise between different approaches adopted in different countries and some will be applied to some products for the first time. The drafters have gone to great lengths to ensure there are no problems, but the amount of application has been patchy and it would be a surprise if no problems of interpretation arose. If one takes a simple example where, in the case of the –6–

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compressive strength of a brick, the UK procedure to compensate for any irregularities in the bed faces of the brick to be crushed, is by putting a 4 mm thick piece of plywood above and below the brick. However, the European approach is to grind the surfaces of the brick flat with a surface-grinding machine. This raises questions about the comparison of results between the two methods, which the Specifications and Codes do not deal with and much work has been done in the laboratory to investigate procedures. However, when commercial test houses consider how they will offer services, the need to provide an efficient service is important and the procedure for grinding needs to move away from the investigatory procedure to a multiple grinding procedure. This may lead to further problems as the process may then be determined by the largest unit in the sample and hence it receives more surface treatment than others. It is this kind of consideration (and other commercially important factors such as grinding speed), that may affect the outcome. It is only through experience of application that the various nuances of the new test methods will be discovered over the years ahead. Before outlining the details of testing for the various properties of clay bricks, it is worth taking some time to understand the various types of brick, their uses and their history. The results of tests will then have more meaning and will enable the person perusing the results to spot mistakes, as these do occur even in the best-regulated laboratories. In the UK there is an enormous range of types of clay bricks. They may be described by the type of clay used, the manufacturing process, the geometry and the end use. They may also be classified and in the UK, this is traditionally by compressive strength and water absorption. In addition, each individual type will have a name in the manufacturer’s catalogue and these are chosen to reflect their durability, rustic nature or traditional geographic area of use. What is written above relates to bricks whose shape can be enclosed in a rectangular parallelepiped, although to use them in straight runs of walling may well involve the use of purpose made or cut bricks with a variety of special names in order to complete the bonding pattern. In addition to these, there is an extensive range of special shapes, which are used with great effect to generate the aesthetic appeal the architect is aiming to achieve. All special shapes have names and are included in a Standard Catalogue, BS4729 (BSI, 1990). There are also non-standard special bricks, which may be produced for special reasons initiated by the manufacturer as a novel new product, or to suit a particular design. Generally, it would be quite uncommon to find a brick design that has not been made. Special shaped bricks are often crucial to the appearance of a finished building and are intended to be so. However, they are often the most difficult for the brickmaker to make. Often, the regular shaped bricks for a project are ordered well ahead of time, historically, often from more than one manufacturer. However it is often much later that an order is received for the special shaped bricks, which are needed usually at the same time as the others. By their nature, special shaped bricks contain a different mass of clay than the regular shaped bricks –7–

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and consequently may need a different firing regime in order to appear the same colour. In addition because of the small quantities by comparison, they may be produced in a batch kiln instead of a continuous process kiln, perhaps using different fuel. Consequently, to produce ‘specials’ of matching colour requires great skill, especially as the site from which clay is being won may be subtly different from that a few weeks earlier. It is thus a great compliment to the brickmaker when fine buildings, which generate appeal by their careful use of special shapes, grace our towns and cities. It is also perhaps sad to reflect how little is taught in our schools of architecture about bricks and brickwork, as it is probable that architects in many of the most famous practices are unaware of the complexities of manufacturing and how such a small task of timely ordering the ‘specials’ in a large project, can be so important. Special shaped bricks are used, for example to provide a dog leg shape (angle) at the corner of a building, as shown in Fig. 2.1. There are specialist contractors who manufacture these shapes by cutting regular shaped bricks as required and sticking parts of them together to form the required shape. Although brick manufacturers would generally want to provide the specially made and fired product, they will often have a preferred cutting specialist.

Figure 2.1 Dog leg shape (external angle).

Figure 2.2

Acoustic brick.

One particular type of special shaped brick is the acoustic brick, which has been made in fairly limited quantities. This product has an unusual shape, as shown in Fig. 2.2. The key feature is the small void in the centre of the brick, which is connected to the outer face by a slot. The void forms what is known as a Helmholtz Resonator (Hodgkinson, 1976) and sound which enters it is absorbed in part and only part is reflected back. These units tend to be quite frequency specific, but at the right frequency can be almost perfect absorbers. Walls made from these units have been used in such places as auditoria and swimming baths, but rarely externally. Where it is intended to provide sound barriers externally which do absorb rather than simply reflect sound, for example alongside many continental roads, it is common to see perforated bricks laid on their side with an absorptive material such as mineral wool behind them. In the USA, it is more usual to see reflective rather than absorptive barriers and these are often protecting –8–

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relatively isolated buildings from the noise from an interstate road rather than the protection of a large number of buildings in a town or city, see Fig. 2.3.

Figure 2.3a barrier.

Reflective acoustic

Figure 2.3b barrier.

Isolated building behind

In relation to testing, it should be pointed out that because of their shape it may not be possible to carry out some tests that one would for standard format bricks. This is one reason why the Specification deals with dimensions only. Although they are made to be used with standard format bricks, because of their size and shape, their geometry in terms of perforations or voids may differ and consequently compressive strength, for example, may be affected. If this is important, the manufacturer’s advice should be sought and certain ad hoc tests may be needed, for example on brickwork details. BSEN 771-1 has been published and a revised version of BS 4729 will be published by BSI. One function of this revised standard will be to provide a common interpretation, for each special shape, of how the BSEN tests should be applied. In some cases the guidance will be that the performance requirement is not relevant to the particular special shaped brick. Regular shaped bricks in the UK are manufactured with a nominal or work size of 215 mm × 102.5 mm × 65 mm and are generally intended to be used with a 10 mm joint on both the bed and header faces, so that the void into which they are intended to fit, the co-ordinating size, is 225 mm × 112.5 mm × 75 mm. The dimensions at first sight are somewhat unusual and so, for example, the 102.5 mm is used as it relates to the imperial equivalent 41⁄8 inches, while maintaining the length:width ratio of the co-ordinating dimensions. Historically, bricks were made larger than today and Imperial sized bricks (85⁄8 in × 41⁄8 in × 25⁄8 in) are still produced, especially for use in extensions to existing buildings. Many manufacturers will make such bricks on request and some do so as a stock item. A standard does exist for modular bricks, i.e. those intended to co-ordinate with window sizes, etc. The standard size is 200 mm × 100 mm × 75 mm but these are not commonly specified. There are systems of wall panel construction that have the appearance of brickwork, but consist only of the stretcher face of a brick with a thickness of –9–

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25 mm or sometimes greater. The brick slips as they are known are sometimes glued to insulation, which may in turn be fixed to a concrete substrate; mortar may be used to point the face of the panel in order to produce the desired appearance. Brick slips are supplied by many manufacturers and are not specially fired but are manually cut from the face of production run bricks and are hence relatively expensive. Another way of producing the appearance of brickwork, is mathematical tiling, which is the vertical hanging of special tiles where the exposed face is rectangular, as shown in Fig. 2.4. This type of construction was first recorded in the 18th century and was a means of improving the appearance of wattle and daub construction at a lower cost than using brickwork. At first it was not pointed, but now accepted practice where the system is used is flush pointing. This method of cladding became especially popular in parts of Kent and Sussex; there are also examples in London, including some with a black glazed finish.

Figure 2.4a

Mathematical tiles.

Figure 2.4c

Figure 2.4b in progress.

Mathematical tiling

Mathematical tiling: completed wall. – 10 –

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Clay bricks can be related to the type of clay that they are made from and although this is of no great interest to the user or even test house, a manufacturer will know the degree of difficulty he will have in making bricks from a particular clay and also the likely characteristics of the finished product. There are quite a number of types of clay used in the UK: Etruria marl, Keuper marl and Coal measure shales are all quite extensively used, although the most well known is the Lower Oxford clay, which is used to make Fletton bricks. Flettons as they are known, have dominated the UK brick market because of the unique properties of the Lower Oxford clay. The name Fletton derives from the village in Huntingdon, where a works using this clay was established in 1879, by James McCallum Craig. Lower Oxford clay is special in three ways. First, when it is won, it is at the right moisture content (16–20%) to be moulded into bricks unlike most clays which need mixing with water which then needs removing by drying prior to firing. Second, the unfinished bricks are sufficiently strong to be stacked in the kiln straight away, without drying them first. The third important characteristic is that the clay contains carbonaceous material, which is more than sufficient to act as the fuel to fire the bricks. The history of the Fletton brick is a story in its own right and can be recommended (Hillier, 1981). As Fletton bricks were cheap to produce, they did at one time form over half the UK supply, and significant use was made of so-called Fletton commons as the inner leaf of the cavity wall in houses. The common brick is an expression less well used now than in the past and describes a product making no claim for its appearance. Although they are still made, it is not in the quantity they once were and this is largely associated with their replacement, as the material for the inner leaf of cavity walls, by concrete blocks. Fletton bricks are made for use in situations when appearance is important and a range of surface treatments are used to make them attractive, e.g. texturing, staining or sand facing. When bricks are made with an attractive appearance, they are usually described as facing bricks and most of the brick types in the UK are facing bricks. In terms of maintaining the definition, the distinction between facing and common is not the issue it once was and the current British Standard refers to it only when explaining a check for visual acceptability of brickwork using a reference panel in an appendix (BSI, 1985) or the more recent Publicly Available Specification (BSI, 2003). There is a stronger feeling over the use of the definition in mainland Europe and this may well relate to local traditions of using clay units as inner leaves and for infilling frames as well as in facing work. It is also easy to see how the definitions were an issue historically in the UK, as it is not unusual to see Victorian housing where the front is in fine facing brickwork, often with narrow mortar joints and the remainder in less attractive common brickwork. The majority of clay bricks made in the UK are made by the extrusion process, which is when a rectangular column of clay is forced through a die and subsequently cut into brick-shaped units. They may then be re-pressed depending on the moisture content of the clay and then dried, stacked into kiln cars and passed slowly down a tunnel kiln. Extruded bricks can be fired in batches in – 11 –

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intermittent kilns, but the typical modern works is one that uses a tunnel kiln working to maximum capacity to ensure economic production. Extruded bricks generally contain some perforations and the pattern of three round holes is very popular with manufacturers. Perforations are important from the manufacturing point of view, as they mean that less clay and less fuel is used in the drying or firing. In terms of brick properties, the effect of perforations is less critical than one might expect. Although brick strength can be affected, research has shown that for levels up to 25%, the relationship between brickwork strength and brick strength is not affected (West et al., 1968). Frost resistance is more related to clay type and the firing conditions than to geometric form and resistance to rain penetration is more related to standards of workmanship. The 25% limit is controversial in Europe where other values have traditionally been used, consequently no definition of perforated brick based upon a limit on the volume of the perforations could be agreed. The European approach is for the manufacturer to declare the geometry of the unit and for the consequences, for example, on structural design to be dealt with in design codes. Hand moulded bricks are still produced by skilled craftsmen, whereby a clot of clay is thrown by hand into a rectangular mould of wood or metal. The base of the mould usually has a raised feature in the centre, which helps to ensure the clay properly fills the corners of the mould. The base of the mould is known as the stock and hence the expression ‘stock bricks’. The depression in the bed face of the brick caused by the stock is known as a frog, reputedly because of a similar frog-shaped depression in the underside of a horse’s hoof. Perhaps the best known stock brick is the yellow London stock, which is commonly used in London. These bricks contain numerous black spots, which add to their attractive appearance. This is in fact carbonaceous material, not fully burnt out in the firing process and they derive from old domestic refuse, which is mixed with the clay prior to moulding. The refuse was taken from London, down the Thames, to the brickfields of Kent, as a return load on barges that brought either cement or bricks to the capital. The use of refuse in this way is an early example of the use of waste materials in brickmaking. More recently, pulverised fuel ash, glass cullet and sewage sludge have all been tried with less spectacular success. Machine pressed bricks often contain frogs, the most well known being the Fletton and in some cases frogs, usually of different sizes, would feature in both top and bottom bed faces forming the double frogged brick. In view of the labour intensive process to produce hand-moulded bricks these are usually made at relatively small works, but because of their attractive character they attract a price premium. In order to try to enter this sector of the market, brick makers have developed a machine-based process aiming to produce the appearance of handmade bricks. These bricks are very popular and can look especially attractive in internal work, which often incorporates deep raked mortar joints to give texture and good use of light and shade. The presence of a frog in a brick does have implications for its testing but research (West et al., 1972) has shown that the design rules for solid or perforated bricks do apply. – 12 –

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Tunnel kilns and intermittent kilns have been mentioned as means of firing bricks, but there are others. The oldest method is to set a large pile of perhaps a million bricks onto a bed of solid fuel and the fire burns for a few weeks before it dies out and the results inspected. This is known as clamp firing and is used, for example, on several stock brick types. Although there is great skill in clamp firing, the fire invariably burns at a greater temperature in some places than in others and there is a considerable amount of sorting by size and colour. Occasionally the fire burns too hot and causes bricks to fuse together and these darkened masses of fired clay can be seen incorporated into brickwork walls near to the works. Although clamp firing has a long tradition, bricks are made in the UK where the fire beneath the bricks is fuelled by gas, but this is not common. The next simplest approach to firing is when a clamp is surrounded by walls with no roof. This is a Scotch Kiln. The benefit of this arrangement is that the outer layers of bricks are permanently protected from cooling winds and hence there is better temperature distribution among the setting of the bricks. Efforts are often made to protect clamps by temporary means such as using corrugated iron, but these are less effective. Another type of continuous kiln is the Hoffmann Kiln, which is traditionally used for Fletton bricks. This kiln consists of a series of chambers arranged about a central chimney and the fire moves from chamber to chamber. Consequently one chamber would be filling up, those next to it drying bricks, further on bricks would be firing, cooling and being drawn. Works using this process could have several kilns and forests of brick built brick kiln chimneys existed to fire Flettons, for example, at Stewartby and Ridgmont in Bedfordshire. The classification by bricks in the British Standard is two-fold: compressive strength and water absorption are used to define types by name; frost resistance and soluble salt content are used to give a durability designation consisting of two letters. Engineering bricks are divided into two classes A and B. Class A engineering bricks have compressive strengths in excess of 70 N/mm2 and water absorption of less than 4.5%, the figures for class B bricks are 50 N/mm2 and 7%. The uses to which engineering bricks are put are not closely related to their classification, as many facing bricks could be sold on their properties as engineering bricks. As the name suggests, traditional uses are engineering structures such as the many retaining walls and bridges in railway and canal systems, which have been made from blue engineering bricks. Other less visible applications are in sewerage systems, where man entry sewers were often lined with engineering bricks. In fact, the Victorian sewerage system under Manchester contains many fine chambers in red engineering bricks, where considerable attention to detail led to a quite surprisingly extensive use of special shapes. The flexibility of brickwork makes engineering bricks a popular choice for manholes, where there may be numerous entry and exit pipes at various positions and angles, which can be easily accommodated. The blue colour of some bricks typical of the so-called Staffordshire Blues is produced when red burning clays are subjected to a period in the kiln of reduced – 13 –

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oxygen conditions. The blue colour may penetrate the whole brick, or may occur in some parts of the brick only where the surface has an attractive mix of blue and red colours; this brick is often described as blue brindle. The Standard also defines bricks suitable for use to construct a damp proof course. There are two classes: DPC1 and DPC2, which are defined by limiting the water absorption to 4.5% and 7%, respectively. In this case, there are no strength requirements, which may be a consideration in the structural design and dealt with separately. Essentially DPC1 bricks are suitable for use in buildings and DPC2 bricks for external works, for example engineering structures. Damp Proof Course bricks are defined in the same way in the European Specification, despite some countries appearing not to use any obvious method to restrict rising damp. In contrast, engineering bricks are not classified in the European Specification, as is the case for many traditional names, e.g. German Klinker brick. All bricks other than engineering bricks are required to have a strength exceeding 5 N/mm2, which is usually easily achieved using any of the various production methods. In practice, most bricks lie in the strength range of 10 N/mm2 to 70 N/mm2, although there are some engineering bricks where individual brick strengths have exceeded 200 N/mm2. The durability designation links the frost resistance classification of F, M and O, which relate to Frost Resistant, Moderately Frost Resistant and Not Frost Resistant, with the level of water soluble salts which are Low (L) or Normal (N). Consequently, designations are of the form FL, FN, MN, etc. and the Code of Practice (BSI, 2001) gives guidance on the minimum requirements for given types of exposure to the elements to minimise the risk of frost attack on the bricks or sulfate attack on the mortar. A further type of brick covered by a separate standard are the Acid Resisting Bricks. These products are somewhat different to standard building bricks, as their properties are such that the market for them is truly international. They are used with expensive specialist mortars for situations such as the linings of power station or industrial chimneys, where acid gases and sometimes liquid acid can be present. Industrial acid resisting storage facilities can be of extremely complex shapes and the joints are very narrow, which means the units need to be made to very tight tolerances. The computer-aided-design programmes now available enable the shapes to be modelled, the individual unit shapes defined and the moulds needed to be made allowing accurately for drying and firing shrinkages. At one time, units for such shapes would be physically modelled in wood to ensure everything fitted together prior to the mould manufacture. There are approximately 55 companies and 100 works in the UK manufacturing clay bricks and in the past, there have been many more. It was quite common for manufacturers to press names into the surfaces of the brick, especially in the bottom of frogs. The letters could be simple such as LBC or quite extensive for example STAFFORDSHIRE IRON and there are books, which relate these marks to the individual works (The British Clayworker, 1951). This marking and the ability to trace bricks back to what are now often defunct works can be – 14 –

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very useful in the appraisal of existing buildings. During the investigation of one of the buildings following the terrorist bomb in Manchester in 1995, some bricks were sampled from a number of locations and sent for testing. It was immediately suspected that there was more than one type of brick and their strengths seemed to confirm the fact, although with aged bricks the manufacture could lead to very variable products. Fortunately, the manufacturers marks could be used to trace the bricks to four different manufacturing sites and consequently the building could be considered as consisting of different brickwork structures in its different parts. With the advent of formal Quality Approval systems in the UK industry in the early 1980s, many manufacturers introduced marking on their products that enabled them to be traced back to the date of manufacture. Although this has more usually been used to deal with difficulties prior to walling them, it could potentially help in dealing with the appraisal of existing buildings. Clay bricks dominate the UK brick market, but there are also bricks made from calcium silicate and from concrete. Calcium silicate bricks now form a tiny percentage of the UK market, although historically they were more significant. Over the years, the industry suffered greatly from blacklisting by insurance companies; this was never justifiable and related to the fact that long ago some calcium silicate products contained asbestos. However, the economic facts led to the decline of the industry. Calcium silicate bricks (BSI, 1978) are made from either sand or flint and lime and are moulded into solid or frogged shape and then autoclaved at low pressure. A wide variety of colours can be made using pigments and a variety of surface finishes is available. The bricks are generally more regular in shape than clay bricks and are generally in the strength range 5 N/mm2 to 60 N/mm2. The main difference between clay and calcium silicate bricks is that the former expand with time to a greater or lesser degree due to the long-term take up of moisture from the atmosphere, and the latter shrink with time. Provision to accommodate movement is necessary in the design of buildings and the spacing of movement joints needs to be closer for calcium silicate brickwork than for clay. On the other hand, there is no record of frost failure in the UK, although it has been experienced in Germany. Calcium silicate units are used extensively in continental Europe and very large units, up to 600 mm high, are produced in Holland and Germany. These units are often used in the internal leaves of cavity walls with thin mortar or glued joints, the more common practice of using cranes on small building sites makes handling of such units less of a health and safety issue. Concrete bricks (BSI, 1981) have made inroads into the clay brick market and they are very popular in some regions. Most concrete bricks are solid or frogged, although there are examples of three-holed perforated bricks. The strength range is similar to that for calcium silicate bricks. Concrete bricks shrink with time, as do calcium silicate bricks and they are frost resistant. Where it is required, they can be made from sulfate-resisting cement and be durable in harsh surroundings. The weight of the bricks generally exceeds that of clay and calcium silicate and although this is a small benefit in relation to acoustic isolation, it does mean the laying of them is more demanding. – 15 –

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There is a market in the UK for bricks reclaimed from buildings when they are demolished. On the face of it, this is a fairly sensible way of preserving raw materials; the buildings can look attractive and the demolition contractor produces less waste for disposal. However, there is a difficulty in that such bricks may be towards the end of their working life. They may have been subjected to numerous cycles of freezing and thawing and although showing no obvious signs of damage, may start to fail after a relatively short life in a new building. This possibility is made more likely if the new building is in a more severe climate in terms of the frequency of freeze/thaw cycling, when the brickwork is saturated. Another relevant factor in this regard is that in older buildings, it was often the practice of the skilled bricklayer to use the lighter coloured, less well fired bricks for the internal work and the more well fired bricks for the outside. It is obvious when looking at some buildings made from reclaimed bricks that they can be mixed up on demolition, some even showing paint or plaster adhering to them and this may mean less well fired bricks which may be susceptible to frost damage have been used externally. Some manufacturers sell new bricks that give the appearance of reclaimed ones. Most of the text so far has referred to standard format bricks as they are known in the UK and although reference has been made to special shaped bricks, there have been over the years many types of unit that could be described other than as a brick. Large clay blocks, usually multi-perforated, are common in continental Europe and are used for monolithic wall construction in some parts of Central and Southern Europe, especially when rendered. Clay blocks with a high level (up to 70%) of horizontal perforations are made in France and used typically as infill to concrete framed structures. In the UK, clay blocks have been made for use in the inner leaf, but have not been produced since the 1970s. Probably the ultimate in clay blocks was the M–G plank (West, 1970) a storey height unit made with 50% perforations with an overall plan size of 300 mm × 100 mm, see Fig. 2.5. The units were designed to interlock with one another on their vertical edge, the joint being sealed with a neoprene gasket. The units were also made with smaller heights, which could be glued together to form storey high units but these ideas were never taken up commercially in the UK, although similar products have been produced commercially in France. On occasions there have been attempts to make clay units of greater width to form, for example, a 9 inch wall in the case of the V (for vertically perforated) brick or in the case of the Calculon, a 6.75 inch wide unit, which was intended to be used for load bearing inner walls of multi-storey construction. Although it has been demonstrated in the UK that units other than the standard format can be produced and indeed are in many countries of the world, they are not popular in the UK. Clearly there is scope for innovative design using other than the current standard unit and perhaps with greater mobility of design professionals and more influence from mainland Europe, the UK may see it realised.

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Figure 2.5a height unit.

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M–G Plank: storey

Figure 2.5b M–G Plank: smaller block showing cross section.

Standard tests on masonry units Specification Clay bricks made in the UK are currently manufactured to a British Standard specification, BS3921 (BSI, 1985) to which amendments were introduced in December 1995. However, it is intended that this standard will be superseded shortly by a European specification BSEN 771–1(European Committee for Standardisation, 2002) and an associated BSEN 772 series of test methods. The European specification relates to two groups of clay masonry units referred to as HD and LD units, which are characterised by their intended exposure to natural weathering and their density. HD (high-density) units include all masonry units that in use are exposed to the elements and those that in use are not but have a gross dry density greater than 1000 kg/m3. In contrast, LD (low-density) units relate to those units, which in use are protected from the elements and have a density of not more than 1000 kg/m3. The two groups essentially differentiate between the brick type units widely manufactured in Northern Europe and the more highly perforated block type units widely produced in Southern and Central Europe which when used externally are rendered. While as might be expected, both the BS and BSEN specifications refer to similar properties, there are some distinct differences in the respective specifications and methods of testing that need to be appreciated. Dimensions Some dimensional consistency of bricks is required to enable brickwork to be built with an acceptable appearance in terms of joint thickness and perpend alignment. – 17 –

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The standard UK work size is 215 mm long × 102.5 mm wide × 65 mm high, which after allowance for a nominal 10 mm joint thickness gives a co-ordinating size of 225 mm × 112.5 mm × 75mm. In setting limits for deviations from the work size, BS3921 relates to the overall measurement of a sample of 24 bricks, none of which shall have a dimension greater than the co-ordinating size. After removal of any superfluous material to the faces and edges, the bricks are laid in contact in a straight line on a flat horizontal surface, header to header, stretcher to stretcher and bed to bed in turn and the total length in each format measured. Alternatively, the sample may be considered as two sets of 12 bricks and the measurements combined. The overall simplicity of the test means it can be easily carried out on site if required. The BS limits on size are given in Table 2.1. Table 2.1

BS3921 – Limits on size Overall – 24 brick measurement

Work size (mm) Length (215) Width (102.5) Height (65)

Maximum (mm) 5235 2505 1605

Minimum (mm) 5085 2415 1515

Expressed another way, the limit on length is ± 75 mm on 24 times the 215 mm work size and for width and height ± 45 mm on 24 times the respective 102.5 mm and 65 mm work size. While these tolerances may appear to be lenient, they were derived from statistical analysis of works control data and when introduced were considered to provide sufficient consistency about the work size from batch to batch, to achieve an acceptable uniformity of brickwork, while at the same time accommodating characteristic variations in the product and so not imposing undue restrictions on the manufacturer. Having said this, where there are short runs of brickwork, for example in piers between window openings, there is less scope to accommodate localised dimensional variations by minor adjustment of the joint thickness during laying. In this situation, the provision of bricks to a tighter dimensional specification is required and can be achieved by selection on site or, by prior agreement with the manufacturer, by sorting at the factory. With modern manufacturing control, some manufacturers supply as standard some brick types to a tighter size tolerance than that required by the BS. In contrast to the BS, the BSEN specification gives requirements for individual brick dimensions. Measurements are carried out in accordance with BSEN 772–16 (BSI, 2000). The individual dimensions of a sample of ten bricks are measured to the nearest 0.2 mm or 0.5 mm (depending on the tolerance class) using a calliper. – 18 –

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For length, measurements are taken with the jaws of the calliper positioned along the centre of the header face running parallel to the bed face. Similarly for width, the calliper jaws are positioned along the centre of the stretcher face again running parallel to the bed face. For height, the specification requires the mean of two measurements are taken, one with the jaws of the calliper running across the centre of the brick parallel to the stretcher face and one with the jaws positioned across the centre of the brick but parallel to the header face. Significantly, it is required that for all measurements the jaws of the calliper overlap the brick and as such will need to have a reach of over 215 mm when measuring width and height. The jaws should also have a thickness of not less than 5 mm and not greater than 10 mm. The measurement arrangements are illustrated in Fig. 2.6, using a vertical height gauge fitted with extended jaws.

Length

Width

Height (1)

Height (2)

Figure 2.6

Dimension measurements to BSEN 772–16. – 19 –

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The BSEN specification requires that the maximum deviation from the declared work size shall be stated in terms of a tolerance category for the mean length, width and height of a sample of 10 bricks. For UK-sized bricks, the mean of the measured dimensions should be expressed to the nearest mm. There are two standard tolerance categories referred to as T1 and T2. Because of different work sizes in different countries, the limits for these categories expressed to the nearest mm are derived by a simple calculation: T1 = ± 0.40 √worksize (min. 3 mm) T2 = ± 0.25 √worksize (min. 2 mm) Table 2.2 gives the calculated limits for the mean length, width and height dimension for UK standard sized bricks. If desired, there is also the option for a manufacturer to declare his own tolerance class, Tm, the limits for which may be tighter than the T2 limits, or more lenient than the T1 limits. Table 2.2

Mean – Tolerances Tolerance category

Work size (mm)

T1 (mm)

Length (215) Width (102.5) Height (65)

±6 ±4 ±3

T2 (mm) ±4 ±3 ±2

Tm Manufacturer’s own limit

In addition to a tolerance category for mean dimensions, a manufacturer may also be required to declare a tolerance category in respect of the maximum variation in size within a test sample. Batches of bricks supplied to a tighter range specification would be of benefit where there are short runs of brickwork and hence less scope to adjust the joint thickness to account for variations in size, or for example, where tighter joints are specified. As with the tolerance categories for the mean dimensions T1 and T2, there are two standard tolerances categories for range, R1 and R2, the limits for which are again derived by a simple calculation based on the declared work size and expressed to the nearest mm. R1 = 0.6 √worksize mm R2 = 0.3 √worksize mm Again there is the option for a manufacturer to declare his own tighter or more lenient tolerance for size variation within a test sample, Rm. Table 2.3 shows the calculated limits for the range tolerances for UK sized bricks.

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Range – Tolerances Tolerance category

Work size (mm)

R1 (mm)

Length (215) Width (102.5) Height (65)

9 6 5

R2 (mm) 4 3 2

Rm Manufacturer’s own limit

The implications for UK brick manufactures of working to the BSEN specification have been considered through a government support funded initiative (DOE Partners in Technology, 1997). As part of this, dimensional measurements of a wide range of bricks including extruded, soft-mud moulded and handmade types have indicated that bricks complying with the BS dimensional tolerances, are likely to satisfy the tighter T2 BSEN category limits for mean dimensions. However, in terms of the range, it was concluded that the more lenient R1 category is more likely to be declared.

Compressive strength The compressive strength of a brick is a measure of its ability to resist crushing. Depending on the type of clay and method of manufacture the compressive strength characteristics of clay bricks vary significantly ranging from as low as 5 N/mm2 to more than 100 N/mm2. The compressive strength of a brick is important in structural design but is of no significance in non-load bearing situations such as traditional house construction. Furthermore, it is not an indicator of frost resistance. To determine the compressive strength, BS3921 requires that the bed face of a brick is loaded at a rate of 35 N/mm2/min to about half its expected strength with a gradual transition to a rate of 15 N/mm2/min thereafter to failure. To assist uniformity of loading, 4 mm thick plywood packings are located between the bed faces and the crushing machine platens. Prior to crushing, samples are soaked in water at ambient temperature for 24 hours or in boiling water for 5 hours. Testing in a saturated state gives a slightly lower strength than when testing dry. Where bricks have a single frog and in use are intended to be laid frog up, which is the usual practice, the frog of the soaked bricks is filled with mortar which is then trowelled flush with the surface. At the same time mortar cubes are cast and the brick crushed when the mortar cubes cured under water have attained a compressive strength in the range 28–42 N/mm2. With double frogged bricks both frogs are filled with mortar. Several options for the composition of the mortar are given in BS3921 but it is usual to use a rapid setting 1:3 high alumina cement:sand mix, which hardens sufficiently in about 3 hours to enable

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the frog filled bricks to be re-immersed in water at ambient temperature, typically attaining the required mortar strength in about 24 hours. BS3921 requires that the mean strength of a sample of 10 bricks shall not be less than that declared by the manufacturer with a minimum value of 5 N/mm2. There are also minimum compressive strength and associated maximum water absorption values relating to two classes of engineering bricks. For Class A engineering bricks the strength shall not be less than 70 N/mm2 while for Class B bricks the strength shall be not less than 50 N/mm2. Importantly, there are significant differences between the BS and BSEN 772–1 (BSI, 2000) method of test for compressive strength. The BSEN method requires that the bed faces of a brick are surface ground to a parallel tolerance, which dispenses with the need for a plywood surface packing to assist uniformity of loading. Bricks are also tested dry rather than wet. Conditioning to the required air-dry state is achieved by drying the bricks at 105ºC ± 5ºC for at least 24 hours. Importantly, in the BSEN test, most if not all frogged bricks manufactured in the UK will be tested without filling the frog(s) with mortar, with the strength then based on the load contact area rather than the gross area of the bed face. Frogs are only filled with mortar when the contact load area is less than 35% of the gross bed face area, which is unlikely to be found with bricks manufactured in the UK. If a frog is to be filled, a cement:sand mix is used which when tested in accordance with BSEN 1015–11 (BSI, 1999) will have a strength of 30 N/mm2 or at least that expected for the brick whichever is the lesser, at the time the brick is tested. In the unlikely event of the surface grinding reducing the height:width ratio to less than 0.4, i.e. to a height less than 40 mm for a UK standard sized brick, a composite specimen consisting of two surface ground bricks shall be tested. Except for initial type testing and in the case of dispute, the BSEN specification allows compressive strength testing by alternative methods providing an acceptable correlation with results from the BSEN method can be demonstrated. Like the BS, the BSEN specification requires that the mean compressive strength characteristic of a sample of ten bricks shall not be less than that declared by the manufacturer. In addition, a declaration shall also be made in terms of one of two categories, of the statistical probability of not achieving the declared strength. The probability of not achieving the required level can not be greater than 5% for category I units and will be greater than 5% for category II units. Where relevant to the use of a unit, a declaration must also be made as to whether individual brick strengths in a test sample will be greater than 80% of the declared mean strength. Such a declaration of consistence may be required for example, where masonry may be subjected to concentrated loads or for construction in seismic areas. Where necessary, a normalised compressive strength value relating to an equivalent air-dry 100 mm wide × 100 mm high unit shall also be declared. However, unlike the BS, the BSEN does not have a minimum strength requirement or classes of strength but in the case of the latter there is a provision to relate to a national classification system. Importantly, comparative testing of a wide range of brick types to the BS and BSEN method, through the Partners in Technology initiative, has shown that – 22 –

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almost without exception the BSEN method of test with surface grinding gives appreciably higher strength values than when testing to the BS method. While this difference is clearly illustrated in Fig. 2.7, it can be seen from the scatter in the results that there is no reliable single correlation that can be applied. In some instances the compressive strength was of the order of 100% higher with an average increase of about 50% for the generally higher strength extruded bricks and about 30% for the usually lower strength soft-mud and handmade units. Obviously, against this background it is important for manufacturers, specifiers and designers to appreciate the potential differences in compressive strength resulting from the two methods of test. In particular in retaining the identity of class A and B engineering bricks in the UK, it will be necessary to increase the respective minimum mean compressive strength values appreciably. These are given in the national annex to BSEN 771-1.

Figure 2.7 Compressive strength of clay bricks. Comparison of BS3921 and BSEN 771–1.

Water absorption All clay bricks are porous to varying degrees and will therefore absorb water in use. As with compressive strength, depending on the type of clay and method of manufacture, the water absorption property of bricks vary appreciably. Clays are tempered to a lower moisture content for forming by extrusion than they are for forming by moulding. As a consequence, bricks formed by extrusion and under vacuum are characteristically denser and hence on firing less porous than bricks formed by a moulding process. Under saturated conditions the increase in weight expressed as a percentage of the dry weight of a brick can range from less than 2% for a dense semi-vitreous extruded brick to more than 30% for a more open pored – 23 –

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hand made unit. However, it is important to appreciate that a high characteristic water absorption does not necessarily mean a brick is more susceptible to frost attack, i.e. a high water absorption brick can be just as frost resistant as a low water absorption brick. Having said that, the water absorption property of a particularly type of brick is sensitive to the degree of heatwork the brick has been subjected to on firing, the greater the heatwork the lower the characteristic water absorption and visa-versa. As a consequence, a water absorption test is often used by manufacturers for quality control purposes and in some instances has provided a means of establishing an upper limit in relation to frost resistance for a particular product type. It is also a misconception that the performance of brickwork in terms of its ability to resist rain penetration will be adversely affected when constructed in bricks having higher water absorption characteristics. Design details and quality of workmanship, in particular joint filling, are the critical factors in this case. The simplest test for water absorption involves immersing a dried brick in water at ambient temperature, a so-called ‘cold soak’. In most cases, a sufficiently constant level of saturation is reached after 24-hour immersion and this has generally been adopted as the standard soak time, although longer times may be used. Two other methods of test were specified in the previous version of BS3921 (BSI, 1974). One involved boiling bricks in water for 5 hours and the other, a quicker test, involved evacuating the bricks at a pressure of less than 20 mm of mercury followed by immersion in water for 10 min. Both the test options were introduced to give a truer indication of the water absorption property. The reason for this is that while the majority of pores in a brick are open, some of these are very fine and with a cold soak the water does not readily fill the finer pores. In contrast, temperature effects involved with immersion in boiling water causes air in the finer pores to expand and so promote its displacement with water and hence achieve a higher level of saturation. Similarly, the vacuum test will remove the air in the finer pores and so promote a higher level of saturation. Both the test methods were considered to give similar results although a comparative study has questioned this (Peake and Ford, 1981). The ratio between the soaked and boiled water absorption value for a particular brick is referred to as the saturation coefficient and is typically in the range 0.7–0.9. A lower saturation coefficient is thought to indicate a better frost resistance although the correlation is not very reliable. The current BS specification refers only to the boiling water test. The test method requires that ten bricks are oven dried to a constant mass and when cool, weighed before being immersed in water at ambient temperature in such a way that the water is free to circulate around them. The water is then brought to the boil in approximately 1h and after continuous boiling for 5 hours, left to cool naturally. Thereafter the bricks are removed, lightly shaken and the surface wiped with a damp cloth before being re-weighed. The percentage gain in weight of each brick is then determined and a mean value calculated for the set. BS3921 states that the water absorption property of all bricks shall be declared and for Class A engineering strength bricks not exceed 4.5% and for Class B engineering strength bricks not exceed 7%. Furthermore, for bricks to be – 24 –

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used in buildings as damp proof courses the water absorption should not exceed 4.5%, while for bricks providing a damp proof course in external work the limit is 7%. The introduction to the standard also points out that in structural design the water absorption property is used as a determinant in assessing the flexural strength of masonry. For this purpose BS5628:Part 1 (BSI, 1992) refers to three ranges of water absorption, less than 7%, between 7 and 12% and greater than 12%. The boiling water test is also referenced in the BSEN specification but only for determining the water absorption of bricks to be used as damp proof courses, test method BSEN 772-7 (BSI, 1998). The water absorption properties of bricks to be used for this purpose shall be declared and not exceeded by a test sample, although unlike the BS, no actual limits are given in the specification. For all other bricks that are to be used externally and their face exposed, the BSEN specification requires that the water absorption shall also be determined but based on a 24-hour soak in water at ambient temperature. The method is given in Annex C to the specification. In this case a manufacturer shall declare the water absorption within a consignment of bricks which the mean of a test sample of five bricks shall be less than, thereby setting an upper limit. The BSEN specification also requires that where relevant to the use of a brick a manufacturer shall declare the initial rate of water absorption, that is, the initial rate of suction which the mean of a test sample of at least six bricks shall be within. When laying bricks with a very low initial suction there is a tendency for the bricks to ‘float’ on the mortar bed, which restricts the number of courses that can be laid until the mortar begins to stiffen. In contrast, when laying bricks with a very high initial suction, water is rapidly absorbed from the mortar making the brickwork less flexible to minor adjustment before the mortar starts to stiffen. A very high rate of initial suction can reduce the strength of the mortar and weaken the bond between the mortar and the brick and hence reduce the flexural strength of the brickwork. To minimise this risk, ‘floating’ bricks should be stored dry and a stiffer mortar mix used, while a high initial rate of suction can be reduced by the practice of ‘docking’ bricks in water for up to 2 min immediately prior to walling. The BS specification does not require a declaration of the initial rate of suction but does give a test method. Both the BS and BSEN 772–11 (BSI, 2000) test methods involve supporting the bed face of a dry brick of known weight on two bearers such that the face is kept immersed in water to a depth of 3 mm ± 1 mm for the BS method and 5 mm ± 1 mm for the BSEN method, for 60 s. The brick is then re-weighed and the water absorbed by the bed face area expressed in kg/m2/min. As a general guide bricks should be ‘docked’ if the initial rate of water absorption (suction) is greater than 1.5kg/m2/min.

Density A brick density is often sought, particularly when considering the sound or thermal insulation performance of a masonry structure. Although there is no requirement for – 25 –

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this in BS3921, most UK brick manufacturers declare the gross density of their bricks. Gross density, or bulk density as it is sometimes referred to, is determined by dividing the dry mass of a brick by its volume (length × width × height) and is expressed in kg/m3. Due to differing material densities and varying levels of perforation or void size, the gross density of bricks made in the UK range from about 1500 kg/m3 to more than 2300 kg/m3. The BSEN specification requires that where relevant to a brick’s use and for acoustic requirements, a manufacturer shall declare the gross and net density. In addition, a tolerance category D1 or D2, which relates to the maximum deviation that the mean gross and net density of a sample of ten bricks from a consignment shall be from the declared value, shall be stated. D1 – 10% D2 – 5% As with dimensional tolerances a manufacturer will also have the option to declare his own density tolerance, Dm, which may be greater than the D1 limit or less than the D2 limit. The mean gross and net density is determined in accordance with BSEN 772–13 (BSI, 2000). The respective densities are calculated by dividing the dry mass of a unit by the corresponding volume. The gross volume is derived from the units overall length, width and height measured in accordance with BSEN 772–16, with a deduction of the volume of any perforations or voids etc. intended to be filled with mortar, while the net volume, is the gross volume less the volume of any perforations or voids not intended to be filled with mortar and is determined by hydrostatic weighing in accordance with BSEN772–3 (BSI, 2000). For UK type bricks the mean gross and net density shall be quoted to the nearest 10 kg/m3.

Soluble salts Most fired bricks contain soluble salts. These are commonly sulfates of sodium or potassium and to a lesser extent magnesium. Less soluble calcium sulfate is also usually present. Although soluble salts occur naturally in clays they do not normally remain in this form on firing. In any event soluble sulfates in clay are usually rendered insoluble during the manufacturing process by chemical reaction with a controlled addition of barium carbonate. Without this, salts in solution in the plastic clay are prone to be concentrated at the surface of the brick on drying and through chemical reactions on firing become insoluble to give a fixed surface stain often referred to as ‘scumming’. Soluble salts found in the body of a fired brick are developed by the reaction between sulfur oxides formed on firing and minerals present in the brick. The most common problem associated with soluble salts in fired bricks is their migration to the surface of brickwork in service and their deposition when the water in which they are dissolved evaporates. This is referred to as efflorescence. The deposit is usually white in colour but on occasions may appear pale green – 26 –

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or yellow due to vanadium salts. It can vary in intensity from a light dusting to an extensive fluffy or powder like deposit. It should be appreciated that the appearance of efflorescence on brickwork may not simply be related to the level of soluble salts in the brick. Soluble salts may be present in ground water and can be absorbed into brickwork. They are also present in mortar. Significantly efflorescence only occurs when brickwork is highly saturated and starts to slowly dry. The rate of drying and the pore structure of the brick are particularly influential in maintaining the migration process to the surface. This explains why in some cases brickwork with a relatively high level of soluble salts may effloresce less than other brickwork containing lower levels of soluble salts. In service, design details such as roof overhangs, copings, sills and the appropriate use of damp proof membranes will help to protect brickwork from high levels of saturation and hence reduce the risk of efflorescence. However, it is important that brickwork is protected as much as possible during construction when it is often fully exposed. Efflorescence is prone to appear in relatively new brickwork particularly in the springtime when brickwork, which has been saturated for the first time during the winter starts to slowly dry. Brickwork exposed to the prevailing weather conditions will be especially susceptible. Efflorescence is essentially a problem of aesthetics and in the case of the more common soluble sodium and potassium salts will normally quickly weather away perhaps returning again but to a lessening extent during subsequent years until the effect finally ceases. However, a recent study has suggested that the use of air entraining agents in modern mortars may account for an increased incidence of a calcium rich efflorescence (Bowler and Winters, 1997). Due to its lower solubility calcium sulfate efflorescence develops more slowly and tends to be more persistent. A potentially more serious effect is where crystallisation of a salt occurs near the surface of a brick which in time may cause the surface to disintegrate in a similar way to frost attack. The problem is usually associated with magnesium sulfate. Fortunately, with more selective use of clays combined with modern firing technology and the associated switch from sulfurous solid fuels to cleaner gaseous fuels, high concentrations of this potentially disruptive salt is less common in bricks produced today. High levels of soluble salts combined with ongoing saturation of brickwork can also have a detrimental effect on the durability of mortar. Sulfate in solution will react chemically with tricalcium aluminate naturally present in Portland cement or hydraulic lime to form calcium sulfoaluminate. This creates expansive effects that can cause the mortar to crumble and may even cause brickwork to bow. It is often characterised by a tension crack along the centre of a bed face mortar joint and can be confirmed by chemical analysis of a representative mortar sample. Weaker mortars are more vulnerable to sulfate attack and if necessary sulfate resisting cement should be specified. Both the BS and BSEN specifications have two categories with specific limits for active water-soluble salt contents, one of which must be declared by the manufacture where bricks in service are exposed to the elements. Table 2.4 shows – 27 –

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that the respective limits for the BS and BSEN categories are essentially the same although in some instances the limit in the BSEN is a combined value. Table 2.4

Category limits for active water soluble salts

Soluble ion

BS3921: 1985*

BSEN 771–1

Low (L)

S2

Sodium Potassium Magnesium Sulfate

0.030% 0.030% 0.030% 0.500%

Normal (N)

}

0.25% 1.6%

}

0.060% 0.030% –

S1

}

0.17% 0.08% –

S0

}

No requirement –

For bricks categorised as having a low soluble salt content, designation ‘L’ in the BS and ‘S2’ in the BSEN, the combined limit for sodium, potassium and magnesium salts is 0.09%. Similarly for bricks categorised as having higher levels of soluble salts, designation ‘N’ in the BS and ‘S1’ in the BSEN, the overall limit is 0.25%. The additional ‘S0’ category in the BSEN specification, for which there is no requirement for the level of soluble salts, is intended for bricks that will not be exposed to natural weathering in service for instance where bricks are fully rendered or used internally. It will be noted that the BS categories also include a limit for water-soluble sulfate. This will encompass calcium sulfate, which although much less soluble and therefore less active, is often present in larger quantities and can as already mentioned result in efflorescence. The BS and BSEN 772–5 (BSI, 2002) test methods are essentially the same. A sample of at least six whole bricks is crushed to provide a representative sample of up to 250 g material, ground to pass a 150 µm test sieve. The BS method also has the option to obtain the sample by representative drilling to half the depth of the units using a drill bit of up to 7 mm in diameter. A weighed sample is stirred for 1h with a known volume of distilled or de-ionised water to provide a clear solution obtained by filtration or centrifuge. The clear solution is then analysed to determine the water-soluble sodium, potassium and magnesium ion content and in the case of the BS also sulfate content. This can be done by traditional wet chemical titration or gravimetric procedures although nowadays the solution is more commonly diluted and acidified for analysis using instrumental atomic absorption spectroscopy (AAD) and flame photometry or inductively coupled plasma spectrometry (ICP) techniques. Prior to its amendment in December 1995, BS3921 included an efflorescence test. At that time, there were no limits for the ‘N’ category soluble salts although there was a 0.300% limit for calcium ions in declaring the lower ‘L’ category. The sulfate content was also determined as an acid-soluble rather than water-soluble value. The efflorescence test was originally developed to safeguard against – 28 –

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the use of bricks with excessive quantities of very soluble salts, particularly the potentially destructive magnesium sulfate. However, it has been found to be an unreliable test for generally assessing the liability of bricks containing lower levels of soluble salts to effloresce in relation to experience in service. This is perhaps not surprising, since it has been pointed out earlier that the level of soluble salts alone does not dictate the incidence of efflorescence in brickwork. Nevertheless, the test is still used by some manufacturers. The test described in Appendix C of BS3921, involves sealing an inverted wide mouthed bottle containing 300 ml distilled water against the main stretcher face of a brick and allowing the water to be absorbed into the brick body for 48 hours. If necessary, the bottle is refilled. After this time the bottle is removed and the sample left to dry in the laboratory atmosphere for 7–9 days. The procedure is then repeated, but with 14–16 days final drying time. Throughout the test, the other faces of the brick are wrapped in polythene to concentrate the drying and hence any salt deposition on the exposed stretcher face. The extent of any efflorescence on a sample of ten bricks is assessed visually as being, ‘nil’, ‘slight’ (up to 10% of the face area covered with no evidence of powdering or flaking at the surface), ‘moderate’ (as ‘slight’ but between 10% and 50% of the face covered) or ‘heavy’ (more than 50% of the face covered and or powdering or flaking of the surface). A ‘heavy’ categorisation meant that the brick did not comply with the BS specification.

Moisture expansion Bricks and hence brickwork will undergo relatively small expansive changes in service due to a combination of naturally occurring thermal and moisture effects. This is acknowledged in BS5628: Part 3 (BSI, 2001), which recommends a general design allowance for total movement in brickwork due to these effects, of up to 1 mm/m. Failure to accommodate this movement in extended runs of brickwork, usually through the use of expansion joints, can result in a build up of stress concentrations in a structure to an extent that cracking may occur. As a general rule, spacing between expansion joints should not exceed 15 m with the first joint from a corner positioned at half this distance or less. Typically therefore, expansion joints will need to be designed to accommodate up to 15 mm of movement. All clay brickwork expands and contracts with changes in temperature. The coefficient of thermal expansion of brickwork is in the range of 5–8 × 10–6/ºC. Based on a mid range coefficient and a maximum wall temperature in the UK due to solar gain, of about 50ºC, this represents a potential expansion due to thermal effects of the order of 0.3 mm/m. Two mechanisms of movement are attributed to moisture effects, a reversible component associated with variation in moisture content of masonry due to natural wetting and drying conditions, and an irreversible component associated with the adsorption of moisture from the atmosphere by the brick body. It is generally acknowledged that movement due to the former is relatively small accounting for up to 0.2 mm/m. In contrast, that due to the latter is often much larger and importantly continues throughout the life of a structure albeit at a decreasing rate. – 29 –

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Depending on the type of clay and the degree of firing, the long-term irreversible moisture expansion of a brick can itself be in excess of 1 mm/m. As a consequence, guidance on this expansion is of particular importance to the designer in assessing the likely overall movement of a brickwork structure in service. Cracking due to rotational forces associated with high levels of irreversible moisture expansion often occurs at corners particularly where there is a short brickwork return. Cracking has also been found to occur where adjacent bands of brickwork have different expansion characteristics. In this instance the higher expansion brickwork causes tension cracking in the brickwork built with lower expansion units. Against this background, there is no BS test method for determining the irreversible moisture expansion of clay bricks and hence no BS specification. Similarly there is no BSEN test method applicable to UK type bricks although the BSEN specification stipulates that in countries where a national requirement exists this should be adhered to. In the absence of a BS test method, a procedure has been investigated for accelerating the natural expansion process by exposing bricks to saturated steam at atmospheric pressure and this has been widely used in the UK (Lomax and Ford, 1988). A good correlation between the expansion obtained in steam after 4 hours and that occurring naturally over 5 years has been demonstrated for a wide range of clay bricks. Natural expansion over 5 years was also found to be broadly linearly related to the logarithm of time and extrapolation has provided a means of categorising the long-term irreversible moisture expansion characteristics. Although bricks expand very rapidly when they are first exposed to atmospheric humidity, in practice this is of no significance, since bricks are unlikely to be walled within a few hours of being fired. Nevertheless, unless the test is carried out where the bricks are manufactured, practical difficulties in initiating measurements within say 24 hours of manufacture at a remote location, has necessitated the adoption of a desorption procedure involving reheating bricks to 700 oC for 24 hours to simulate a kiln fresh condition. The use of this technique has also enabled the test to be applied to older bricks removed from a structure in investigating the cause of movement and cracking. For measurement, a reference point consisting of a shallow depression is pre-drilled in the centre of each of the two header faces of a brick and dusted clean. Where a brick has a sanded or rustic type finish, this is initially removed by a light grinding of the surface. Measurements are taken before and after steaming using a digital electronic gauge reading to 0.002 mm, via 6.5 mm diameter stainless steel balls freely located in the reference holes. The measuring arrangement is shown in Fig. 2.8. All samples are allowed to condition in a constant temperature environment for 24 hours after desorption and steaming before being measured and the resulting expansion (expressed in mm/m). Extrapolation of natural expansion data obtained over a 5-year period has indicated that the irreversible moisture expansion over the service life of a bricks, say 50 years, is likely to be about 2.5 times the expansion obtained in steam. Based on this, bricks are categorised as having long-term irreversible moisture expansion characteristics (Table 2.5). A further addition of 0.25 mm/m originally proposed is no longer applied – 30 –

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Figure 2.8

The measurement of irreversible moisture expansion.

Table 2.5

Long-term irreversible moisture expansion characteristics

Category

Expansion range (mm/m)

Low Medium High

< 0.4 0.4–0.8 > 0.8

In setting the limits for these categories, the upper limit has been set to be broadly compatible with the recommended design allowance for total movement in clay brickwork of 1 mm/m given in BS5628: Part 3. Due to the shrinkage characteristics of mortar and an element of restraint even in free standing masonry walls, the irreversible moisture expansion of brickwork is somewhat less than that of the unit and a factor of 0.6 is often applied (Smith, 1974). Applying this factor to the 0.8 mm/m upper limit and then adding the potential for reversible expansion in brickwork due to thermal and moisture effects of 0.3 mm/m and 0.2 mm/m, respectively (although these maximums are unlikely to occur at the same time), gives an overall expansion similar to the recommended 1 mm/m general design allowance for total movement. It follows therefore that where bricks are considered to have high irreversible moisture expansion characteristics an allowance for total movement in brickwork in excess of 1 mm/m may be required. – 31 –

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Although the test was developed to identify bricks having a high irreversible moisture expansion it has been requested by designers to identify bricks exhibiting low expansion characteristics. Low expansion bricks used in conjunction with movement restraining bed joint reinforcement and weaker cement-based or lime putty mortar which is more accommodating of movement, has been specified in some instances to minimise or avoid the use of unsightly expansion joints.

Frost resistance tests on clay masonry units Introduction The occurrence of frost damage to porous building materials and the difficulties of assessing their ability to resist such damage, has long been a concern for producers and users of fired clay, stone and concrete building products. The mechanism that causes frost damage is brought about by the presence of water in the pore system of masonry units which, when it freezes, expands 9% by volume and sets up stresses within the pores. If there is sufficient room for this expansion within the pore system and if the strength of the material is high enough to withstand these stresses, then damage will not occur. However, if these stresses are repeated many times by natural freeze/thaw cycling, then susceptible materials may become weakened and failure in the form of spalling or crumbling of the surface may take place. This type of frost damage, when it occurs in the outer skin of buildings between the damp proof course and eaves, is largely cosmetic and although unsightly and thus unacceptable, seldom threatens the integrity of the affected buildings. It can usually be remedied by replacing affected units. However, when frost damage occurs in more exposed situations, such as parapets, retaining walls, free standing walls and chimneys, for example, then the damage can be severe enough to necessitate demolition and re-construction. The incidence of frost failure with porous masonry materials is extremely variable, with many examples of buildings constructed with brick and stone still intact after several hundred years of exposure to the elements. On the other hand, there are cases where garden walls for instance, have been reduced to rubble after only one or two winters. The reasons for these great differences in performance, particularly with clay bricks, lie in the very variable nature of brick clays and the different methods of manufacture by which bricks are traditionally made from particular types of clay. The result of this situation is the existence of a tremendous range of bricks in terms of colour, texture, shape and physical properties and among these physical properties is frost resistance. Unfortunately, despite the implications of the term ‘resistance’, it is not a property that can be accurately measured and quantified even though the British Standard (BSI, 1985) specifies three classifications of frost resistance which a manufacturer can claim for his bricks based upon experience of the bricks in service. – 32 –

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There has been much effort expended over the years in attempts to devise laboratory tests that can accurately predict frost resistance. These tests fall into two main categories: indirect tests based upon seemingly related properties such as strength and water absorption and accelerated simulative tests where artificial freeze-thaw conditions are created which attempt to reproduce the effects caused by natural conditions. Indirect test methods The two properties linked most often to frost resistance as a means of predicting the performance of fired clay construction products in conditions of severe exposure, are water absorption and compressive strength. In the USA, the Standard Specification for bricks (American Society for Testing and Materials (ASTM), 1983) sets limits for compressive strength, water absorption and saturation coefficient for different exposure zones in the USA. The latter property saturation coefficient, is obtained by expressing the ratio of the 24-hour cold soaked water absorption, in which only the ‘open’ pores are filled, to the fully saturated 5-hour boiled water absorption, in which all the pore space is theoretically filled, including the ‘closed’ pores. A maximum value of 0.8 is specified, which is supposed to indicate the presence of sufficient closed pore space to allow room for ice to expand and thus ensure frost resistance. Bricks that do not meet these requirements have the option of a laboratory freeze/thaw test, which takes several months to complete. If they satisfy this test then the requirements for water absorption, saturation coefficient and compressive strength are waived. However, there are many cases of frost resistant bricks, so defined, which fail to live up to expectation and conversely, many bricks that fail to conform to the standard but perform satisfactorily in severe exposure conditions The problems associated with this approach can be illustrated by considering two very different bricks such as a dense, low water absorption (~ 2–3%), high strength (~150 N/mm2) Staffordshire blue engineering brick and a comparatively low strength (~20 N/mm2), highly absorbent (~20–25%), hand made brick. Both these bricks are likely to be fully frost resistant but for different reasons. The engineering brick is durable because of its low water absorption and high compressive strength and the hand made brick because of its open pore system and sponge like structure, which can accommodate the ice expansion without rupturing. It can also dry more quickly and is thus less likely to be highly saturated in normal service conditions. For example, a manufacturer who produces a fully frost resistant brick may wish to make a brick from the same clay but with a different colour. Among his options may be the reduction of the firing temperature to get a lighter colour, with the result that a brick is produced with a higher water absorption and lower strength than the original and which may prove to be not frost resistant in service. In fact in these circumstances it is possible to establish a close relationship – 33 –

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between water absorption and frost resistance which is restricted to one particular type of brick where any variation in water absorption is related to firing treatment. Because of this wide range of brick properties, most authorities do not consider it possible to predict in a general way, with a sufficient degree of confidence, the frost resistance of clay bricks by reference to easily measured physical properties such as strength or water absorption. There has been more success in relating frost resistance to properties that require more sophisticated measuring procedures such as pore size distribution by mercury intrusion porosimetry. There are also techniques whereby internal structural changes caused by freezing of individual samples can be detected by measuring small changes in dimensions or changes in Young’s Modulus as determined by indirect methods. The data obtained by these techniques is then related to frost resistance. Again, the reliability of these techniques is not sufficient to gain widespread acceptance by brick manufactures or users. The work carried out on these topics by Butterworth (1964) at the Building Research Establishment (BRE) is well documented. Accelerated laboratory test methods Salt crystallisation test method The other route to the goal of being able to predict the frost resistance of bricks has been the development of accelerated laboratory tests. The earliest of these was the salt crystallisation test in which bricks were soaked in a saturated solution of sodium sulfate. They were then dried and the cycle of soaking and drying repeated. The theory was that the salts would crystallise and grow in the pores of the bricks and cause similar disruption to that brought about by ice formation. As well as practical problems with the operation of the test, the results did not accord well with the known performance characteristics of bricks and the test has long since been abandoned. Except that the test as described by Honeyborne (1963) of the BRE is still in limited use for natural stone but is not regarded as very satisfactory. Cyclic freeze/thaw test methods With the development of controllable refrigeration, many attempts were made to devise a laboratory freeze/thaw test in most parts of the world where frost attack is a problem. The majority of these tests involved saturating single bricks by various methods and putting them through cycles of freezing in air and thawing in water. This type of test is often referred to as omni-directional because the freezing and thawing conditions are applied to all faces of the samples. The problem with these methods is that when frost damage occurs it is often in the form of cracks through the body of the brick that sometimes open up and cause the bricks to break into several pieces. Often these cracks are pre-existing firing cracks that do not cause problems when the bricks are built into walls. – 34 –

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Again these tests were found to give results which differed from known experience of service behaviour. To overcome these problems, tests were designed that attempted to mimic natural conditions more closely by exposing only the working faces of individual bricks to freezing and thawing, while protecting the other faces with insulation; or by assembling bricks together as a panel, either by using mortar joints or filling the spaces between the bricks with fine gravel or foamed rubber. These tests are known as uni-directional tests, because the freezing and thawing conditions are applied to one face only and they have been shown to give results that compare well with actual performance of bricks in natural conditions. An important feature of uni-directional tests is that they accurately reproduce the mode of failure seen in actual building situations (see Figs 2.9 and 2.10).

Figure 2.9 Typical frost damage – natural exposure.

Figure 2.10 Typical frost damage – laboratory test. Standard freeze/thaw test methods Although frost resistance is a very important property of clay bricks and as has already been stated, it is not quantifiable, there is no actual test method given in the British Standard (BSI, 1985), even though three classifications of frost resistance are described in the standard. These are ‘F’, fully frost resistant, ‘M’ moderately frost resistant and ‘O’, ordinary quality for internal use only or behind rendering. Guidance for the use of bricks with these classifications is given in the British Standard Codes of Practice (BSI, 2001). Durability, as referred to in the Standard, is a combination of frost resistance and soluble salt content and gives a classification system of F, M and O in combination with L, for a low soluble salt content and N for a normal soluble salt content. Thus FL is the highest durability classification and MN is the lowest classification for use in facing or structural brickwork. The significance – 35 –

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of the presence of salts in the bricks is that in some circumstances sub-surface crystallisation can occur which can generate internal stresses and cause surface flaking, a phenomena known as crypto-efflorescence. The presence of soluble salts in masonry can also initiate mortar damage. An FL brick can be used in any building situation where it may be subjected to freeze/thaw cycling while in a saturated condition, e.g. below d.p.c., retaining walls, free standing walls, parapets and chimneys. Bricks with ML or MN classifications can be used in most building situations such as between d.p.c. and eaves where building design offers protection against saturation by driving rain or ground water but not where they are liable to become saturated and subjected to freezing and thawing. At present a manufacturer may classify his bricks according to his experience of the bricks in service or by reference to a sample panel exposed for 3 years. In the foreword of the British Standard reference is made to the BCRL panel freezing test which at the time when the standard was issued in 1985, was a well established test method which gave results that accorded well with manufacturers’ experience. The method and its development are described by West et al. (1984). Since 1985, the method has been proven against natural exposure site results when 23 bricks, covering a wide range of UK production, were tested in the laboratory and built into specially designed panels where they were exposed to natural conditions. Fig. 2.11 shows typical panels on a severe exposure site in Scotland, belonging to BRE Scottish Laboratory. The work is reported by Peake and Ford (1988) from which Fig. 2.12 is taken, showing the relationship between frost failure in natural conditions and in the laboratory.

Figure 2.11

Severe exposure site. – 36 –

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9 8

400

7

No. of winters

6

300

5 4

200

3 2

100

1 0

No. of winter freeze/thaw cycyles

4, 11, 12, 13, 15, 16, 17, 18, 21, 22 9 7

2

10 14

5 3 1 23 8

0

19

6

20

50

100

No of laboratory freeze/thaw cycles

Figure 2.12 Calibration of the BCRL panel freezing test against exposure site results. Relation between laboratory and natural freeze/thaw cycles to produce failure. The method was eventually issued as a Draft for Public Comment (BSI, 1991), but was prevented from being adopted as a standard test when a moratorium on further development of British Standards was declared pending the introduction of European Standards. In 1991, the CEN Technical Committee 125, through its Working Group 4, which was considering test methods for clay masonry units, obtained funding for a project aimed at developing a single test method for frost resistance of clay masonry units for the proposed European Standard. Existing frost tests in use throughout the EC were considered and the test methods from four countries were selected for inclusion in a ‘round robin’ exercise. These were as follows: • From Germany, two tests were chosen, DIN 52 252: Part 1 Omnidirectional Freezing of Single Bricks and Part 3 Uni-directional Freezing of Test Walls (German Standards (DIN), 1986). • From France, AFNOR P13301 Hollow terracotta bricks and P13304 Facing Clay Bricks (French Standards (AFNOR), 1975). • From the Benelux countries, a test method was devised which combined the existing Dutch and Belgian freeze/thaw tests, NEN 2872 Stony Building Materials – Determination of Frost Resistance – single-sided freezing in a freshwater environment (Netherlands Standards (NEN), 1986) and the method given in NBN B 32-002 Facing Bricks (Belgian Standards (NBN), 1986). • From the UK, the BCRL Panel Freezing Test. The essential features of the five tests are summarised in Table 2.6. – 37 –

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Summary of test methods

Table 2.6 Country

OF

Page 38

No. of samples

Method of saturation

Freezing conditions

Thawing conditions

No. of cycles

Germany 10 (DIN) 52252

24-hour progressive immersion

OmniComplete 25 directional immersion in 0°C 110 min water for 60 min

Part 1

48-hour total 0 to –15°C immersion 200 min –15°C 60 min in a standard brick

No. of days for F/T cycles 8

Part 3

12–20 Panel As in Part 1 + 8-hour 0.25 m2 jointed with spray rubber

UniWater spray directional for 20 min 0°C 50 min 0 to –14°C 320 min –14°C 60 min in a standard brick

50

15

Netherlands (NEN) 2872 Joint

5

Full vacuum 21⁄2-hour = 931⁄2-hour immersion

UniComplete directional immersion on Sand tray for 8 hours (–5°C and –15°C) for 16 hours

24

24

Belgian Method (NBN) B23/002

5

Half vacuum (51 kPa) 21⁄2-hour + 931⁄2-hour immersion

France (NFP) 13–305

7

48-hour immersion 48 kPa vacuum

Unidirectional Face down on cold plate (–15°C) 4 hours

In air 4 hours, 25 immersion in water for 16 hours

25

UK (DD-BSI)

Panel of 30 with mortar

7-day immersion

Unidirectional –15°C in 2 hours (400 W/m2)

20 min convected heat and 2 min water spray

10

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The project involved a laboratory from each of the four countries who operated the selected frost tests. A total of 16 brick types of known frost resistance were obtained from the eight countries, which supported the project and these were tested by the five test methods. The results of this exercise are summarised in Table 2.7 and demonstrate that the UK test was the only one to correctly classify the 16 bricks according to their known durability. Table 2.7 Table of compliance of test results with suppliers rating of durability Agreement as tested by: Supplier’s specification

DIN

AFNOR

NEN

BCRL

Durable, 8 types Non durable, 8 types

6 6

6 6

4 7

8 8

The project then concerned itself with investigating some aspects of the UK test which, with some modifications, now forms the basis of prEN 772-22 Determination of Freeze/Thaw resistance of Clay Masonry units (Committee for European Standardisation, 1999). This draft test procedure is at present going forward for enquiry, following which the enquiry comments will be considered and then it will go to the formal vote when, if it is accepted, it will become the European Freeze/Thaw Test for Clay Masonry Units and will thus be referred to from within BSEN 771–1 Specification for Masonry Units, Part 1: Clay Masonry Units (Committee for European Standardisation). The durability classifications in BS3921 are changed slightly: F becomes F2, M becomes F1 and O becomes F0. The test method is different from most test methods, in one important aspect. This is the development of what is known as the ‘pinch’ effect, which is brought about by completely freezing the test sample and then limiting the thawing to a penetrated depth of 10–15 mm from the exposed face. The effect of this is that a layer of water becomes trapped between a permanent ice zone and an advancing ice front. The subsequent stresses set up as the trapped water freezes can cause delamination of the exposed face, closely reproducing the type of damage seen in natural conditions. One other important difference from other tests is that the bricks are tested in a panel of masonry, i.e. with mortar joints and not with foam rubber or gravel between them as in other types of panel test. It was considered when the test was being developed that bricks should be subjected as far as possible to conditions that compared with actual building situations. It has been shown that the type of jointing material can affect the results of the laboratory test for a significant number of brick types. – 39 –

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The BCRL panel freezing test for clay bricks The BCRL Panel Freezing Test, on which prEN 772–22 is closely based, is carried out on a panel of masonry consisting of ten courses of three bricks in stretching bond constructed with a cement mortar with tooled joints. The mortar used is typically 1:4 high alumina cement:sand mix, which allows the panels to be handled within 2 days. Portland cement mixes may also be employed, but require longer curing times. No evidence has been produced to show that the type of mortar has any influence upon the performance of the bricks. After curing, the panel is completely immersed in water at room temperature for a period of 7 days. The soaked panel is then installed vertically in the test cabinet, with the normally exposed face of the panel open to the test conditions and the remaining surfaces encased in a closely fitting jacket of expanded polystyrene. Fig. 2.13 shows a typical freeze/thaw apparatus in which two panels can be tested simultaneously.

Figure 2.13 Typical freeze/thaw apparatus: A, evaporators; B, electrical heaters; C, control thermometers; D, water supply protected with soil-warming cable; E, inspection panel; F, compressor; G, control panel; H, drain hole. The test commences with a 2-hour period of freezing at an air temperature of –15 oC, followed by a thaw period of 20 min, during which the air temperature is raised to +25 oC. This is then followed by a 2-min period in which water is sprayed across the top of the panel, forming a flood coat down its face. The cycle is completed with a 2-min period in which water is allowed to drain out of the system before the freezing cycle restarts. The test panel is examined for visible signs of frost damage and for hollowness when tapped with a steel rod, at intervals of 10, 50 and 100 cycles, at which stage it is completely thawed and dismantled brick by brick. Each brick is carefully inspected for frost damage, particularly for evidence of de-lamination – 40 –

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cracks running along the bed face, parallel to and 5–15 mm from the exposed face of the bricks. From the results of the test the bricks are classified according to the three grades given in BS3921 as follows: • If no damage is recorded after 100 cycles, then the bricks may be regarded as fully frost resistant ‘F’. • If damage, due to the action of frost, in the form of cracking, spalling and crumbling of the exposed face, or loss of material which significantly alters the appearance of the bricks is observed before 100 cycles and after 10 cycles, then the bricks may be regarded as moderately frost resistant ‘M’. • If frost damage occurs before 10 cycles, then the bricks will be regarded as not frost resistant ‘O’. The BCRL Panel Test is now universally used by the UK brick industry, with all the major manufacturers operating their own equipment. CERAM Building Technology have carried out over 4000 individual tests. It has been verified against exposure site trials and is known to correlate well with manufacturers’ experience with their own products. The draft European freeze/thaw test for clay masonry units The principles of the original BCRL Panel Freezing test have been preserved in the draft method, in particular the development of the ‘pinch effect’. The European inter comparison programme, which produced this draft test procedure resulted in the inclusion of several changes to the basic method which are summarised in Table 2.8. Table 2.8

Comparison of frost test conditions

Size of panel Jointing Saturation procedure Pre-conditioning of panel Freezing cycle Thaw cycle Water conditions Classification

UK method

Draft European method

10 courses of 10 bricks 0.42 m2 Mortar Complete panel soaked for 7 days None 2 hours at –15 ± 3oC 20 min warm air 25 ± 3oC 2 min spray 15 ± 5oC 4 l/min/n2 100 cycles – no damage – F10–100 cycles – damage – M 0–10 cycles – damage – O

Minimum 0.25 m2 Rubber or mortar Units soaked for 7 days before assembly of rubber joint panel 15 min water spray 6-hour freeze 2 hours at – 15 ± 3oC Energy input of 1.1 ± 0.4 MJ, either water or dry heat plus water 25 ± 3oC 12 ± 0.1/min/n2 100 cycles – no damage – F2 5–100 cycles – damage – F1 0–5 cycles – damage F0

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The two most significant variations are the introduction of a 6-hour long freezing period during the first cycle and the option of constructing the test panel using 10 mm thick, closed pore, foam rubber between the bricks instead of mortar. The 6-hour pre-freeze can be technically justified because it ensures that the test panel becomes frozen to a greater depth from the very first cycle, whereas with the original method the panel does not become completely frozen until at least eight cycles have elapsed. These early cycles have been shown to be less severe than the following ninety or so cycles, thus making the draft test conditions more severe. This has been shown to cause a problem with certain types of brick, which would become downgraded to ‘F0’ (O) quality with the draft test procedure requirement of no failure before 10 cycles, even though they have been successfully marketed as ‘F1’ (M) quality bricks for many decades. This issue is still under consideration. The issue of mortar or rubber joints has already been touched upon. The main concern of those who would favour rubber is one of practical convenience in the laboratory, whereas the use of mortar can be supported on technical grounds. The draft prEN includes both options as it has been demonstrated that the introduction of the 6-hour pre-freeze has led to the results from the two jointing methods being equivalent. There has been an attempt within the draft method to define the thaw conditions in terms of total heat input so that the thawing medium may either be entirely water or a combination of warm air and water. There are, however, considerable practical problems associated with water only thawing and the calculations of equivalent heat transfer for water and warm air are very uncertain. The means of identifying the types of frost damage, which can occur during the test has been refined and Table 2.9, which is reproduced from the draft standard, illustrates the criteria upon which frost failure will be determined. Table 2.9

The criteria upon which frost failure is determined

Description of damage

Type

None Crater (e.g. Lime-burst) Hair crack < 0.15 mm* Minor crack* Surface crack* Through crack Chipping, peeling, scaling Fracture Delamination

0 1 2 3 4 5 6 7 8

*If hair cracks, minor cracks or surface cracks appear to be associated with delamination and this can be confirmed by cutting the brick, then the defect is reported as delamination.

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The motivation behind some of these modifications has been the perceived need to enable the test conditions of the BCRL Panel Freeze/Thaw Test to be reproduced on other, existing designs of freeze/thaw equipment with the minimum inconvenience. These issues remain to be resolved during the lengthy process of developing harmonised European Standards but it seems likely that an accelerated, laboratory freeze/thaw test, very similar to that described, will be adopted as the standard European method for assessing the frost resistance of clay masonry units. In the meantime, it may be significant to report that Sweden have already adopted the BCRL Panel Freezing Test into their National Standard.

Standard tests on clay pavers Introduction The use of small element clay paving dates back, in particular, to Victorian times when solid square edged bricks were used for a wide range of paving applications. These ‘pavers’ were laid using rigid construction techniques with mortar providing the bedding and the joints between adjacent units. However, it was not until the latter half of the 20th century that architects and highway engineers showed an increasing interest in using small element paving in flexible construction. In order to address this form of construction, where pavers are laid on a prepared sand bed and where the joints between adjoining units are filled with sand on completion of the work, industry began to manufacture high quality, purpose made products. In particular, pavers for flexible construction were produced with nibs to create a gap between units and with chamfers around the edges of the wearing surfaces. Both these developments prevent the edge chipping known to occur when square-edged bricks or pavers are used. In order to complement the increasing use of small element clay paving, the Brick Development Association (BDA) commissioned a programme of practical and field test work and additionally formed a working party with the express purpose of producing a performance-related standard. This work resulted in the preparation of a standard specification for clay pavers together with a code of practice for flexible construction. These documents were channelled through the BSI Technical Committee resulting in the publication of BS BS6677: Parts 1, 2 and 3 (BSI, 1986). Part 1 gives a specification for the pavers, while Parts 2 and 3 provide a code of practice for the design, and a specification for the construction, of lightly trafficked pavers, respectively. Within BS6677: Part 1, test methods are given for dimensions, transverse breaking load and skid resistance of clay pavers. The standard also provides specifications for two classes of pavers designated PA and PB. As a general rule Type PA pavers comprise those pavers manufactured using soft-mud processes while the much denser pavers produced in modern extrusion/tunnel kiln plants are defined as Type PB. The classification of the two types of pavers in terms of their minimum mean transverse breaking load requirement is given in Table – 43 –

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2.10. Table 2.10, largely reproduced from the standard, includes typical applications for the two types of paver. Table 2.10

Types of paver

Type of paver

Minimum mean transverse breaking load (kN)

Predominant traffic carried

Typical applications*

PA

3.0

Pedestrians

Hard landscape, footpaths and pedestrian areas not used by heavy vehicles Car parks, driveways to private houses

Motor cars, light vans

PB

7.0

As for Type PA and also public transport and commercial road vehicles

As for Type PA and also roads in residential areas, lorry parks, factory yard areas, docks, petrol station forecourts and farm hard standings

*For further guidance on the use of pavers, see BS6677: Part 2.

While BS6677: Part 1 continues to be used, work has been carried out over a number of years to produce European Standards, prEN 1344-5 (European Committee for Standardisation, 1997), for the flexible and rigid laying of clay pavers. These are part of an initial larger family of eight draft standards now published, which also include three relating to stone products, BSEN 1341–3 (European Committee for Standardisation, 2001), and a further three to concrete paving units and accessories, BSEN 1338–40 (European Committee for Standardisation, 2003). More recently, the draft standards for flexible and rigid paving have been combined in a harmonised European Standard BSEN 1344 (European Committee for Standardisation, 2002) which will replace BS6677: Part 1. The work on the draft European Standards, strongly led by UK delegates, has resulted in a major change to the existing test method for measuring dimensions and a minor change in that for determining strength. In addition, an increasing requirement to predict the in-service pedestrian slip and vehicular skid resistance of paving units manufactured from clay, concrete and stone has resulted in experimental work to develop a test method for the durability of slip/skid resistance. The draft standards also address performance related properties absent from the original standard but considered relevant to the in-service durability of – 44 –

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clay pavers. In this context test methods have been included for abrasion and frost resistance. In considering the standard test for clay pavers, potential changes to the existing test methods will be identified and information will be given discussing the additional test methods likely to the included in the next generation of European Standards. Under an umbrella BS7533 (BSI, 1997 onwards), a series of design guides and codes of practice are being prepared for the use of clay, concrete and stone paving products over the full range of structural applications. Within this series of standards, some relate to the use of units produced from all three materials while others deal especially with clay, concrete or stone products. Although the design and construction aspects of using clay pavers are outside the brief of this publication, it is helpful to note that the production of the different parts of BS7533 will ultimately replace BS6677: Parts 2 and 3, together with the equivalent standards for laying concrete and stone paving products. Dimensions BS6677: Part 1 quotes three sets of preferred work size dimensions for rectangular pavers, namely; 215 mm × 102.5 mm, 210 mm × 105 mm and 200 mm × 100 mm. All three sizes can also be produced with thicknesses of 50 mm or 65 mm. The test method involves measuring 24 units. Following the removal of blisters or any other surface imperfections, the pavers are placed in contact in a straight line on a level surface in arrangements appropriate for each work size dimension. Using an inextensible measure sufficiently long to measure the whole row, the overall length, width and thickness are measured to the nearest millimetre. Alternatively, the standard allows the sample to be measured in two halves the sums of the pairs of separate measurements being recorded. The size limits for Type PA and PB pavers are given in Table 2.11. In Table 2.11, the length (L), width (W) and thickness (T) relate to the actual work size specified for the batch of pavers. Based on statistical principles, the 24-unit test method does provide some control over within and between batch variation. In a manner effectively the same as that being progressed in the pan-European clay brick standard, the suggested test method in BSEN1344, the harmonised European Standard for clay pavers, specifies measuring the individual dimensions of length, width and thickness of a test sample of 10 pavers. Measurements are made using callipers or other devices capable of measuring to an accuracy of 0.5 mm. Unless prevented by the positioning of the spacer nibs, the measurements are made at the mid-point of each dimension. The difference between the largest and the smallest measurement of any given dimension measured for the sample of 10 pavers is stated by reference to either Class R0 or Class R1. No determination is required for Class R0, while the range necessary to meet the R1 classification must not exceed 0.6√d rounded – 45 –

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to the nearest mm, where d is the particular work dimension in mm. Where manufacturers are able to produce pavers with tighter size tolerances appropriate declarations may be made. In practice such tighter tolerances would have the advantage of enabling more precise laying patterns to be achieved. Table 2.11

Size limits – type PA and PB pavers Limits of size of overall measurement of 24 pavers Type PA

Type PB

Work size

Maximum (mm) Minimum (mm)

Maximum (mm) Minimum (mm)

Length (L) Width (W) Thickness (T)

24.35L 24.44W 24.69T

24.26L 24.33W 24.52T

23.65L 23.56W 23.30T

23.74L 23.67W 23.48T

The tolerances for the overall measurement of 24 pavers are directly proportional to the work size, which has been accepted as a principle in other standards for fired clay units. The tolerances for type PA pavers are equivalent to those given in BS3921 for fired clay bricks with work size dimensions of 215 mm × 102.5 mm × 65 mm. The tolerances for type PB pavers are approximately three quarters of those for type PA.

Modulus of rupture Transverse breaking load In specifying a test for determining the strength of clay pavers, it was considered inappropriate to use the type of compressive test specified in the British Standard for clay bricks, BS3921 (BSI, 1985). Clay pavers do not fail in compression while, additionally, compressive strength measurements are difficult to interpret for pavers of different work thickness. As a consequence BS6677: Part 1 specifies the transverse strength test as the method for determining the strength of clay pavers. In common with other flexural strength tests the within batch variation is often quite high – approaching 30%. However, this is not a reflection on the accuracy of the test but rather that the method is able to pick up the presence of body flaws and surface imperfections. In this way the test guards against possible defects that the specifier and user would wish to eliminate from the finished paving. A prime example of a fault likely to lead to potential in-work problems is the presence of ‘dunted’ ware, ware affected by cracking generated usually by too rapid cooling of the fired pavers. Such units would be expected to fail the transverse strength test but would be unlikely to be identified during the determination of compressive strength. – 46 –

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The test method uses a three-point loading technique to determine the transverse breaking load of a sample of 10 pavers that have been pre-conditioned by soaking in water for 24 hours. Each paver is supported with its wearing surface uppermost on two self-aligning cylindrical steel bearers with their centres 175 mm apart. A load of 5 ± 0.5 kN/min is applied centrally through a third cylindrical steel bearer until failure occurs. The transverse breaking load of each paver is measured and used to calculate the mean value for the 10-unit batch. Both the individual and mean values are quoted to the nearest 0.1 kN. In order to meet the requirements of the test, the minimum mean transverse breaking loads for Type PA and PB pavers should be those given earlier in Table 2.10. In addition, there is a requirement that the minimum individual breaking loads should be 2.0 and 4.0 kN for PA and PB pavers, respectively. The above test method, with minor procedural variations, uses the same principle as that employed in many countries in mainland Europe. However, an important difference is that the results are calculated as the modulus of rupture rather than the simple breaking load used in the UK. This divergence has been highlighted in the discussions leading to the preparation of the draft European Standard. While retaining the principle embodied in the test method, these discussions have led to minor changes in the bearer alignment and loading rate. The resulting strength measurement in the draft European Standard has been changed to the breaking load per unit width (N/mm) compared with the breaking load (kN) in the British Standard. It also allows the optional calculation of the modulus of rupture. The transverse breaking load classifications being progressed in the draft European Standard are given in Table 2.12. Importantly the notes accompanying the Table 2.12 recognise the effect that raw material variation and different making methods have on the physical characteristics and hence structural applications of different types of paver. Table 2.12

Transverse breaking load Transverse breaking load not less than (N/mm)

Class

Mean value

Minimum individual value

T0 T1 T2 T3 T4

No determination 30 30 80 80

No determination 15 24 50 64

This requirement for transverse breaking load does not apply to accessories or to pavers whose overall length is less than 80 mm. Class T0 is only suitable for pavers intended for use for rigid laying where the pavers are laid with cementitious mortar joints on a similar mortar bed itself placed on a rigid base. The wide – 47 –

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variation in raw material characteristics and manufacturing techniques used in the production of clay paving units creates a natural variation in the physical characteristics on the units, which is recognised by the limits given in the Table. The manufacturer may declare a mean value and minimum individual value higher than those corresponding to class T4. The manufacturer may declare a mean and minimum bending tensile strength (modulus of rupture) value. In comparison with the results in Table 2.12, the equivalent mean and minimum individual values for a 100 mm wide Type B paver would be 70 N/mm and 40 N/mm, respectively.

Slip and skid resistance With the increased use of small element pavers in areas subjected to either or both pedestrian and vehicular traffic, the issue of in-service slip and skid resistance has become an increasing concern to manufacturers, specifiers and highway engineers. The importance of slip/skid resistance was recognised by the Brick Development Association and BSI Technical Committee resulting in the test method for skid resistance included in BS6677: Part 1, (BSI, 1986). The test method, detailed in Appendix E of the standard, uses the ‘pendulum friction tester’ described in the British Standard for testing mineral aggregates, BS812: Part 114 (BSI, 1989), to determine the skid resistance of the wetted wearing surfaces of a test sample of five pavers. The friction tester is fitted with the larger (76.2 mm) slider and standard TRRL slider rubber used over a long period by the Greater London Council (GLC). Investigational work indicated that the TRRL rubber was more appropriate for the rougher surfaces exhibited by clay paving than the 4S rubber specified for indoor floorings (James, 1989). More recently, ‘rougher surfaces’ have been broadly defined as those with roughness values >30 µm RZ measured using the Surtronic Roughness Meter (Taylor Hobson, Leicester, UK). The investigation carried out by James also found that the use of the TRRL rubber gave better discrimination under wet conditions while satisfactorily distinguishing between pavers considered safe and those assessed as likely to give in-service problems. Section E2 of BS6677: Part 1 provides details of the setting up and operational procedures for using the pendulum tester reproduced from BS812. These procedures, together with instructions for calibrating the tester, have been combined and progressed by BSI Technical Committee B/556 ‘Slip resistance for pedestrian surfaces’ by the preparation of a stand alone British Standard, BS7976 (BSI, 2002) ‘Pendulum Testers’. The work to produce the standard has been driven by the increased use of the equipment for laboratory and site investigations which in turn reflects the increased awareness of slipperiness as an issue in both pedestrianised and vehicular paver applications. The operation of the pendulum tester for testing individual pavers in the manner described in BS: Part 1 is shown pictorially in Fig. 2.14. With the paver positioned as indicated in the Fig. 2.14, the pendulum arm fitted with the wide slider is adjusted to give a sliding length on the wearing – 48 –

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surface of the paver of between 125 mm and 127 mm. After thoroughly wetting the surface the pendulum arm and pointer are released from the horizontal position and the resulting pendulum value is read from the pointer position on the ‘C’ scale of the apparatus. A minimum of five repeat determinations are carried out on each paver with the surface re-wetted prior to each measurement. Providing that the repeat determinations do not differ by more than three units, the skid resistance value of the paver is given as the mean of the five recorded readings. However, if the range exceeds three units, further tests are carried out until three successive constant readings are obtained. The constant reading is then recorded as the pendulum test value. The test is repeated for all five test specimens and a batch mean value is calculated and reported to the nearest unit.

Figure 2.14

Operation of the pendulum tester.

The test method specifies that the mean wet skid resistance value of five pavers intended for use in areas subject to vehicular traffic shall not be less than 60. In line with the guidance note included in the standard, which acknowledges that pavers are likely to polish in service, it is usual for test houses carrying out the test to add a rider to the effect that: ‘As a result of polishing by vehicular traffic, the skid resistance value of a flexible pavement constructed with clay pavers will normally decrease from the value measured on unused pavers over a period in service, eventually reaching an equilibrium value. This equilibrium value will be determined by the initial skid resistance value and by the physical properties of the fired clay’. With the added requirement that pavers shall be tested in both longitudinal directions, the test method has been incorporated into the draft harmonised European Standard for clay pavers for the determination of unpolished slip/skid resistance value (USRV). This additional requirement has been added to address the fact that the directional slipperiness of wirecut pavers can be influenced by the configuration of the cutting – 49 –

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table in relation to the extruded column. Unlike the British Standard specification of a value of 60, the draft European Standard provides for the USRV to be stated by reference to one of the classes given in Table 2.13. ‘Note: The manufacturer may declare higher values’. Table 2.13

Unpolished slip/skid resistance value (USRV)

Class Mean

USRV

U0 U1 U2 U3

No determination 35 45 55

Following the publication of BS6677, it was quickly accepted that the test method simply determined the slip/skid resistance of the freshly manufactured product but did not address the in-service durability of slipperiness. This identified the need for a test method that could predict the slip/skid resistance of pavements subjected to several years of pedestrian and/or vehicular traffic. In order to address this perceived requirement work was commissioned to develop a suitable test method (Lees, 1987). The resultant method, based on the Polished Stone Value (PSV) test for roadstones incorporates an accelerated polishing regime to simulate vehicular traffic. The technique involves positioning cut specimens with their wearing surfaces on a rubber annulus in a modified Aggregate Abrasion Testing Machine. During specified polishing periods, grades of corn emery and emery flour together with water are used as the polishing media in the manner used in the PSV test. The pendulum friction tester is used to determine the pendulum value both prior to and following the polishing process. Due to the size of the cut specimens, (92 ± 3) mm by (54 ± 3) mm, it is necessary to carry out the friction measurements using the smaller 31.75 mm slider over a shorter, 76.2 mm, swept length. Pendulum measurements are taken from an ‘F’ scale, which may be either attached to or built into the tester. The test method was subsequently published as a Draft for Development ‘Method for the determination of polished paver value of pavers’ DD 155 (BSI, 1986). In order to test the predictive accuracy of the test method laboratory measurements on a wide range of clay, concrete and stone paving units have been complemented by ongoing field measurements. The perceived need for a predictive test, together with the results of the test work, were identified by UK delegates during the discussions over the development of European Standards for all three materials. Failure to reach agreement, coupled with the necessity to replace the outdated Draft for Development, resulted in a modified test method being published as a British Standard, BS7932 (BS, 1998). However, since the durability – 50 –

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of slip/skid is a mandated requirement for the harmonised European Standards for paving products, CEN TC 178 has set up a working group with the instruction to develop a predictive test method for determining in-service slipperiness. After discounting a number of alternative test methods, a considerable volume of test work in the UK together with pan-European round-robin investigations have been carried to refine the method in BS7932. As a result a test method has been developed which updates existing British Standard BS7932 (BSI, 2003) and has been progressed within Europe and published as DDENV 12633 (European Committee for Standardisation, 2003). The ultimate aim will be to include the test method in harmonised European Standards for clay, concrete and stone paving products. A major change to the established test method enables the polishing and measurement procedures to be carried out on full sized (200 × 100 mm) paving units, or where flatness is an issue, on intermediate sized cut specimens (140 × 80 mm). Both sample sizes allow the pendulum tester measurements to be carried out using the wide slider over the full swept length obviating the need for the uncertain factor currently used to correct the values obtained using the small slider on the smaller samples specified in previous test methods. Another simplification is the removal of the curvature correction factor used to relate the results to those determined by the Polished Stone Value procedure. The changes essentially provide a stand alone test method with results expressed as pendulum values. Following the development work detailed above, test methods are being made available for determining the unpolished and polished pendulum values of clay pavers. The methods provide for a direct comparison both between the unpolished values and also with on-site measurements. However, neither BS6677 nor the more recent draft standards quantify pendulum measurements in terms of slipperiness or qualify the use of pavers for specific applications. For pedestrian areas, the UK Skid Resistance Group has produced guidelines (Rapra Technology Limited, 2000) for the interpretation of results obtained on rough surfaces using the tester fitted with a TRRL rubber slider. While the definitions of the ‘potential for slip’ are differently phrased the limits are those established earlier by the Greater London Council (GLC Bulletin, 1971). The table from the most recent guidance notes is reproduced below (Table 2.14). Table 2.14

Slip resistance limits – TRRL pendulum

TRRL pendulum value

Potential for slip

19 and below 20–39 40–74 Above 75

High Moderate Low Extremely low

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Although it is not an absolutely accurate conversion, it may be assumed that the pendulum friction tester measurements are 100 times the coefficient of friction. The limits for the coefficient of friction published in the British Standard for stairs, ladders and walkways, BS5395 (BSI, 2000), were derived from the GLC limits using this relationship. The friction limits included in BS5395 are given in Table 2.15. Table 2.15

Friction limits given in BS5395

Coefficient of friction

Condition of floor

Below 0.2 0.2 < 0.4 0.4 < 0.75 Above 0.75

Very poor Poor to fair Good Very good

Guidance notes included within Highways Authorities document HAST 8, (Highways Authorities Standard Tender Document, 1988), quantify the unpolished and polished slip and skid resistance values of paving units and make recommendations for their use in a wide range of both pedestrian and vehicular applications. Table 2.16 shows the recommended polished and unpolished values. Typical applications for the different categories are given in Table 2.17. Table 2.16

Recommended polished and unpolished values

Pendulum value

Recommended uses

Below 20 20–34 35–44 45–60 60+

Should not be used. Only suitable for aesthetic application. Satisfactory for Cat III or IV. Satisfactory for all categories, depending upon the design requirements. Suitable for special applications where risks of skidding are high.

The document also defines levels of vehicular trafficking where clay pavers would be expected to provide a satisfactory skid resistant surface. These apply to typical urban situations including traffic calming measures but exclude situations such as steep hills and the approaches to traffic signals, pedestrian crossings, etc. For these and similar high-risk sites design guidance is provided by the design manuals published and regularly updated by DoT (Department of Transport). – 52 –

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Paving applications

Category

Typical application

I

Pavement receiving severely channelised heavy vehicles, in excess of 300 cv/day (4 msa)* Adopted highways and other roads Petrol station forecourts Pedestrian precincts receiving regular heavy traffic Car parks receiving some heavy vehicles Footways regularly overridden by vehicular traffic Pedestrian precincts receiving occasional heavy traffic Car parks receiving no heavy traffic Private driveways Footpaths not likely to be overridden by vehicles Private areas receiving pedestrian traffic only

II

III IV

*cv, commercial vehicles; msa, million standard axles during design life.

Abrasion resistance In common with other flooring materials, paved areas suffer damage resulting from excessive wear. This presents a particular problem in areas of channelled pedestrian traffic such as exist at the entrances of major stores and shopping arcades in pedestrianised city centres. The problem is exacerbated when stiletto heels are in fashion. Although no test method for abrasion resistance was included in BS6677, the BSI Technical Committee investigated several methods for assessing abrasion and impact resistance and their applicability to clay paving. Ultimately the BSI Committee decided to adopt the Capon Deep Abrasion Test which had the advantage of being an established international test for clay floor tiles, BS:EN6431 (BSI, 1999). With the commencement of the work to produce European Standards, the test method was incorporated into the draft standards for flexible and rigid clay paving (prEN1344–5) and has been published (BSEN1344, 2002). The test provides a method for determining the abrasion resistance which expresses the result in terms of the volume of a groove produced in the wearing surface of the paver by means of a 10 mm wide rotating steel disc under specified test conditions. The abrasion apparatus, which essentially comprises the rotating disc, a storage hopper for dispensing abrasive, a trolley for supporting and locating the specimen and a counterweight, is shown diagrammatically in Fig. 2.15. The test method provides specifications for both the precise composition and size of the steel disc and defines the white fused aluminium oxide, which is fed between the disc and test specimen during the test. The pressure with which test specimens are held against the steel disc is determined by calibrating the – 53 –

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apparatus against transparent fused silica. The pressure is adjusted such that, after 150 revolutions, a chord of 24 mm is produced using a feed rate of abrasive of at least 100 g/100 revolutions. Hopper

Control valve Funnel Abrasive flow Steel disc

Specimen Fixing screw Clamping trolley

Counterweight

Figure 2.15

Abrasion apparatus.

The test is carried out on the wearing surfaces of a test sample for five pavers. Prior to testing, the wearing surfaces may be treated with a suitable dye or paint to facilitate the accurate measurement of the abraded chord length. Pavers with or without the surface treatment are dried prior to testing. The test procedure, described for calibrating the apparatus, is carried out in two positions at right angles on the wearing surfaces of each paver. The chord lengths (l) of the resulting grooves are measured to the nearest 0.5 mm. The resistance to deep abrasion is expressed as the volume V of the material removed where V is calculated from the measured chord length l using the equation: V = ( π . ∝ – sin∝ ) hd2 (180) 8 where sin ∝ = l 2 d and • • • •

d is the diameter of the rotating disc (mm) h is the thickness of the rotating disc (mm) l is the chord length (mm) ∝ is the angle (degrees) subtended at the centre of the rotating disc by the chord (see Fig. 2.16). – 54 –

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Diagram showing the abraded chord length.

A table is provided in the standard giving equivalent volumes for chord lengths between 20 mm and 69.5 mm. The draft European Standard specifies that the abrasion resistance of clay pavers shall be stated by reference to the three classifications given in Table 2.18. Table 2.18

Abrasion resistance

Class

Mean abraded volume mm3 (not greater than)

A1 A2 A3

2100 1100 450

With the preparation of the draft European Standards for paving products a different test method has been incorporated into the drafts dealing with concrete and stone paving products. This test, ‘The Wide Wheel Test’ BS6717 (BSI, 2000), uses the principle applied in the Capon Deep Abrasion Test using a wider (70 mm) wheel of the same radius to abrade the wearing surfaces of the concrete and stone units. The test results are quoted as the chord lengths of the abraded arcs. There is no evidence to prove that the wheel width influences the length of the abraded arc. However, changes in the test parameters, primarily the use of 75 rather than 150 revolutions and the damping effect of the increased abrasive usage, result in lower chord lengths than those obtained using the deep abrasion test. The confusion that was considered likely to occur in the minds of the specifiers and users from the use of the two outwardly similar test methods was addressed by a research project carried out by CERAM Building Technology (Beardmore – 55 –

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and Barnes, 1997). The work was carried out with the objective of establishing whether a relationship exists between abrasion resistance, measured as the abraded chord length, determined using the two test methods. Measurements were carried out on equivalent paver samples taken from 18 larger batches of different clay pavers selected to represent the range of UK production. A graphical and statistical study of the comparative results gave a poor correlation coefficient. Additionally the work indicated that the Capon Deep Abrasion Test did provide a reproducible method for determining the abrasion resistance of clay pavers. However, accurate determinations proved more difficult using the Wide Wheel Test due to the, often irregular, abraded arcs produced during testing. This effect was compounded by the lower level of abrasion achieved using the Wide Wheel Test. The results of the investigation have been published in Tile and Brick International (Beardmore and Barnes, 1997).

Frost resistance tests on clay pavers Introduction Clay paving units, by the very nature of their use are likely to be subject to extremely severe conditions of exposure, particularly in Northern Europe. They will be reach higher levels of saturation than bricks in vertical masonry, especially when covered for long periods with lying snow or when surface water is unable to drain away quickly. They will also be exposed to colder and more prolonged freezing conditions due to radiation effects giving more ground frosts than air frosts. The mechanisms that cause frost damage are brought about by the presence of water in the pore system of the pavers which, when it freezes, expands 9% by volume as ice is formed and this sets up high stress levels within the pores. If there is sufficient room for this expansion within the pore system and if the strength of the material is high enough to withstand these stresses, then damage will not occur. However, if these stresses are repeated many times by natural freeze/thaw cycling then susceptible materials may become weakened and failure in the form of spalling or crumbling of the surface may take place. When this damage occurs in paving installations, as well as looking unsightly it also presents a safety hazard to pedestrians because of the resulting unevenness of the surface. Manufacturers of clay pavers generally use the same process plant and raw materials as for their bricks and if the bricks are designated as fully frost resistant then the equivalent paver will also be fully frost resistant. They will then be suitable for paving installations. It follows, therefore, that the same laboratory tests for determining the frost resistance of bricks will also be suitable for pavers. However, the three categories of frost resistance available for bricks in the UK will not be relevant to pavers – 56 –

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which are required to be fully frost resistant and withstand any test without suffering damage. Pavers will be expected to be able to survive in the most extreme conditions of natural exposure for the lifetime of the installation. This section considers the background to the development of accelerated freezing tests for clay pavers and describes the work that led to the proposed single Pan-European freeze/thaw test for the Clay Paver Standard. Testing the frost resistance of clay pavers in the UK The long-term durability of any paving unit that is exposed to the elements refers specifically to the ability of a paver to withstand repeated freeze/thaw conditions when saturated with water. First, it is relevant to consider the durability requirements of the clay paver in relation to other clay units, in particular clay bricks. Most positions in the external envelope of a building are reasonably well protected from exposure to rainfall and from severe freezing conditions. Weathering damage in these areas is rarely a problem. On the other hand, parapet walls, copings and cappings, and chimney stacks can be subjected to severe conditions of exposure and this is taken into account in the UK Code of Practice BS5628 (BSI, 2001) concerned with the use of masonry where bricks of established good durability are recommended in such situations. Bricks are classified according to BS3921 (BSI, 1985): clay bricks as moderately frost resistant or frost resistant: Frost resistant (F). Bricks durable in all situations including those where they are in a saturated condition and subjected to repeated freezing and thawing. Moderately frost resistant (M). Bricks durable except when in a saturated condition and subjected to repeated freezing and thawing. In BS3921 the onus of classifying the durability of the brick rests with the manufacturer based on his knowledge and experience of that particular product. A similar situation exists for the clay paver where in the relevant standard BS6677 Part 2 (BSI, 1986) is to be found the statement: ‘It is essential that evidence of satisfactory performance in use, including durability, should be obtained from the manufacturer’. This gives rise to some difficulties for the manufacturer. He may for example, wish to develop a new product or be faced with the possibility of a change in the properties of his product, due to alterations in the raw material or processing conditions. The situation is made more difficult by the fact that there is no defined relation between the durability and other physical properties of the brick or paver. Strength, total porosity (or water absorption), and the general texture of – 57 –

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the fired body are the parameters which would be expected to influence the level of durability. However, the relative contribution of each of these factors has yet to be defined in sufficient detail to enable durability to be indirectly predicted. It is known that clay units of relatively low strength and high water absorption can, in fact, be extremely resistant to freeze/thaw conditions, while a denser, stronger product may fail when exposed to the same conditions. The probability of failure depends on the level of exposure of the unit to wetting and to freeze/thaw conditions. The capping course on the top of a free-standing wall has already been cited as a situation where a brick is likely to be fully exposed to freeze/thaw cycling while saturated with water. Clearly there is a general similarity between this situation and that of a clay paver in an unprotected pavement and in both cases a ‘Frost Resistant’ class of product as defined in BS3921 is called for. It is interesting to compare the level of exposure of a brick in a vertical wall with that of a clay paver or a capping course of bricks where the horizontal face is fully exposed to rain and frost. The values in Table 2.19 are based on meteorological data for the Manchester area using mean rainfall and frost figures for the winter months during 1983–1988. Table 2.19

Relative exposures of vertical and horizontal surfaces Unprotected vertical wall

Precipitation of water onto surface (1/m2) No. of frosts

South facing

North facing

Horizontal pavement

300 45

83 45

488 75+

The precipitation onto the vertical walls has been calculated from available meteorological information assuming that during the winter months, the proportion of rainfall driven onto a vertical wall by the wind is the same as for the full year. It is also assumed that freezing at the surface of a vertical wall only occurs during air-frost conditions, whereas additional freezing at a horizontal surface exposed to the sky would occur also during ground frost conditions. This simplistic comparison gives some indication of the difference in exposure conditions between for example, a brick situated between d.p.c. and eaves in a building and a clay paver situated in an unprotected horizontal pavement. The aggressive action of de-icing salts on cement based product has received considerable attention in the past. There is no practical evidence that such salts have any deleterious effect on fired clay products.

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Accelerated frost tests in the laboratory Many procedures have been used over the years for examining the durability of stone, concrete and clay building products by accelerated simulative freeze/thaw tests in the laboratory. These tests have differed mainly in the level of water saturation of the test sample, the rates of freezing and thawing and the number of cycles required to define a performance specification. There is now general agreement by workers in this field that an essential requirement of any simulative test is uni-directional freezing and thawing of the specimen. Only in this way is it possible to produce the characteristic types of failure that occur in the practical situation, where only one face of the unit is exposed to the weather. Assuming that this particular condition is met, it is reasonable to postulate that the simulative climatic conditions used in the test are not critical so long as they are closely defined and controlled during each cycle, and they lead to failures that are similar to those caused by environmental conditions in practice when susceptible products are used. This latter requirement is particularly important with regard to the degree of saturation by water of the test specimen during the test. It is a well established fact that the probability of frost failure increases rapidly as the level of saturation increases. For example, subjecting a product to freezing conditions when it is 100% saturated with water is an extremely severe test and these conditions are unlikely to be encountered in practice. However, there is unpublished data which indicates that pavers situated in conditions where they are subjected to long periods under lying water and snow during the winter months, can attain levels of saturation much closer than previously thought to full saturation, i.e. saturated under vacuum or by boiling in the laboratory. Having decided on a set of test conditions it is necessary to demonstrate that the laboratory results obtained for a range of products relate to the durability qualities which have been established for these products by experience. Alternatively the test results can be compared with the monitored behaviour of the products when placed on a purpose built exposure site. The BCRL Panel Freezing Test for clay bricks on which the clay paver test is based is a well established method which has been used in the UK for over 25 years The method and its development are described by West, Ford and Peake, British Ceramic Research Association (1984). It has been verified against manufacturers experience and during an experimental exposure site programme over a period of nine winters. The data from this programme have been published by Peake and Ford (1988). This paper shows that 100 cycles of the panel freezing test without damage are more than adequate to demonstrate that bricks will be durable under the worst conditions of exposure, e.g. in the capping course of a freestanding wall. The test method was included in a European Inter-comparison Programme aimed at establishing a single European Test Method for Clay Masonry Units and has demonstrated the importance of uni-directional freezing and thawing and the crucial part which is played by the ‘pinch effect’, brought about by partial thawing, upon the development of realistic failure patterns during laboratory – 59 –

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testing and the accuracy of matching the test results with the manufacturers declared durability of his products. The draft single test method for clay masonry units, which has now been published as prEN772–22 (European Committee for Standardisation, 2000) for enquiry for inclusion in the EN, is based firmly on these principals of unidirectional testing and the ‘pinch effect’ of the BCRL Test Method. Following an appraisal of the BCRL Test Method by the Swedish National Testing and Research Institute involving comparison with other test methods including their own, Sweden has now incorporated the BCRL test into their National Brick Standard. The case for subjecting pavers to a pinch effect test is further strengthened by the conclusions of the Brite-Project P–2085 ‘Freeze/Thaw Durability of Concrete Block Paving’, where it was reported that the only instance of actual failure of concrete pavers in natural exposure conditions occurred at the only exposure site used in the project where pinch effect conditions were recorded. In view of the brick method showing that by surviving 100 cycles of the panel freezing test a brick could be confidently expected to be durable under the worst conditions of exposure, it would be reasonable to assume that the same criterion could be applied to clay pavers. To demonstrate this, a programme of tests on a range of pavers was conducted using a modified version of the brick test. The modifications involved exposing the normal wearing surface of the paver instead of the stretcher face of the brick and assembling the pavers without mortar joints into a insulated box. The pavers were saturated individually before assembly instead of soaking a complete mortared panel. The freeze/thaw conditions were identical to those used in the brick test. Eight different samples of fired clay pavers were obtained from manufacturers. The samples were considered to have satisfactory durability by the suppliers, based on experience of use. In addition to these ‘standard’ pavers, unfired pavers were obtained and these were fired in the laboratory. Firing conditions were chosen to produce samples less well fired than the corresponding ‘standard’ works product and hence somewhat less durable. In addition to the clay pavers, two samples of concrete pavers produced by different manufacturers were obtained from a builders merchant. Sample pavers (both works and laboratory fired) were examined and any existing cracks and faults were marked prior to testing. Two methods of water saturation were used before subjecting them to freeze/thaw cycling. 1 Saturation under vacuum involving evacuation of the air at a residual pressure < 20 mmHg followed by 16 hours immersion in water at room temperature. This method is considered to produce a level of saturation approaching 100%, i.e. all accessible pores are filled with water. 2 Complete immersion of the paver in water at room temperature for 7 days. This is considered to produce a level of saturation approximating – 60 –

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to the maximum level attained under conditions of natural exposure. It is the procedure used in the BCRL panel freezing test for clay bricks. A total of 24 individual pavers of each type were subjected to freeze thaw conditions during each test viz: six works-fired pavers saturated under vacuum six works-fired pavers saturated by seven days immersion six lab-fired pavers saturated under vacuum six lab-fired pavers saturated by seven days immersion. The results of the programme are summarised below. Saturation by 7 days immersion Works quality: seven out of eight batches survived 100 freeze/thaw cycles. Lab-fired pavers: five out of eight batches failed in less than 50 cycles. The remaining three batches failed after 100 cycles. Concrete pavers: both batches survived 100 cycles. Saturation under vacuum Works quality: six out of eight batches failed in less than 50 cycles. The remaining two batches had failed after 100 cycles. Lab-fired pavers: All eight batches had failed after 50 cycles. Concrete pavers: Both samples had failed after 50 cycles. Pavers, which from experience are known to be durable in use, failed when subjected to freeze/thaw cycling after being fully saturated by water immersion under vacuum. With one exception the same pavers safely withstood 100 freeze/thaw cycles after 7 days immersion in water, although these test conditions were sufficiently rigorous to fail sub-standard pavers purposely produced by slight under-firing. The results obtained with the concrete pavers were in accordance with the results obtained on the standard manufactured clay pavers. Both concrete samples withstood 100 cycles when saturated by seven days immersion, but failed when they were saturated by immersion under vacuum. The BCRL Panel Freezing Test for examining the durability of clay bricks has been demonstrated to be suitable for the testing of clay pavers. The ability of a paver to withstand 100 freeze/thaw cycles following saturation by immersion in water for 7 days is considered to be an adequate criterion that it will perform satisfactorily in service under all conditions of exposure encountered in the UK.

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The development of a single pan-European freeze/thaw test for the clay paver standard The programme for the harmonisation of European Standards revealed that each of the member states of the EC operated their own suite of test methods. In the case of durability testing of clay based construction products the test methods in use differed significantly and each country maintained that their particular test method was best suited to the weather conditions in that country. As a result of this, when the draft standards for clay pavers were first produced, four test methods for durability were included, pending the development of a single test procedure. A ‘round robin’ project was started in 1996, partly funded by the EC, the objective of which was to study the performance of these four existing national test methods for assessing the frost resistance of clay pavers and then to develop a single harmonised test procedure for inclusion in the Draft Clay Paver Standards prENs 1344 and 1345.The four test methods on which the ones currently included in the draft clay paver standards are based, are as follows: 1 2 3 4

The The The The

German Standard DIN 18 503 August 1981: Clay Pavers French Standard NFP 13–304: Facing Clay Bricks Dutch Standard NEN 2872: Stony Building Materials UK Test Method: The BCRL Panel Freezing Test.

Each of these test methods is currently included in the Draft Clay Paver Standards. The four partners for the project were: • British Ceramic Research Limited, Ceram Building Technology (UK), Project Co-ordinator. • Royal Association of Dutch Clay Brick Manufacturers (NL) in association with sub-contractor : Stichting Technisch Centrum Voor De Keramische Industrie (NL). • Forschungsstelle des Bundesverbandes der Deutschen Ziegelindustrie e.V. (DE) in association with sub-contractor: KeramischTechnologisches Baustofflaboratorium, Hamburg e.V. (DE). • Centre Technique des Tuiles et Briques (FR). The project was organised into four Work Packages, some of which ran concurrently. The individual tasks within each work package were combined and rescheduled as the project progressed. Work package 1: Design of a draft test procedure State of the Art reports on frost testing As part of the project, each of the four partners was required to produce a ‘State of the Art Report’ on the frost testing of pavers in their particular countries. These were studied and discussed and a consolidated report was produced which brought together the individual reports and summarised the differences between the four test methods. – 62 –

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Following on from this consolidated report, the first version of the draft test procedure was devised. Comparison of test methods The details of each of the national test methods and the reasoning behind the different approaches of the National tests to each of the critical aspects of the four test procedures are now discussed and the significant features summarised in Table 2.20. Table 2.20

National frost test conditions

Method No. Saturation samples

Orientation of samples

Direction of freezing

Direction of thawing

Rate of freezing

UK

12

Immersion for Vertical panel Uni-directional Uni-directional 400 ± 7 days in water partial thaw 50 W/m2 at ambient temperatures

French

7

Saturated under Horizontal 700 mmHg individual vacuum pavers

Uni-directional Omnidirectional complete thaw

Dutch

10

Immersion in water at 80oC for 72 hours and 24 hours in water at ambient temperatures

Horizontal panel

Uni-directional Initially: 300 ± 60 Uni-directional W/m2 Finally: Omnidirectional complete thaw

Progressive immersion in water at ambient temperatures for a total 72 hours

Vertical individual pavers

Omnidirectional

German 10

Omnidirectional complete thaw

No. Time of of one cycles cycle

100

Standard 25 time/temp profile 24

Standard 25 time/temp profile

2 h, 24 min

24 h

24 h

7h

Number of samples The numbers of samples for each test are broadly similar and are sufficient to provide representative samples in each case. Saturation methods The methods by which pavers are impregnated with water prior to freeze/thaw testing – 63 –

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are substantially different for each of the four test procedures and are to some extent dictated by the damaging mechanisms which the test method is designed to induce. The UK test soaks the pavers by completely immersing them in water of ambient temperature for 7 days. Most clay articles achieve an equilibrium level of saturation during this period and experience with the test method has shown that it gives results which are in accordance with actual performance in UK conditions. Saturation under vacuum, however, has been shown to be too severe. The French method impregnates the pavers under a high level of vacuum (700 mmHg) and a total time of immersion of the pavers of 1 hour. It is considered that this technique produces nearly 100% saturation and that therefore only 25 freeze/thaw cycles are needed to be effective. It is admitted that these conditions are very severe and that some pavers, which are known to be durable in service, could fail the test. However, since climatic conditions in service are so variable, the test gives more confidence to the user. In the Dutch/Belgian method the pavers are immersed in water at 80oC for 72 hours followed by 24 hours in water at room temperature. This is considered to give the pavers an extra high moisture load consistent with the most severe climatic conditions occurring in the Netherlands. The high level of saturation in the Dutch test was chosen for a very simple reason; Dutch clay pavers in service do not suffer from frost attack. Such pavers appear to resist the Dutch test even when saturated with water using full vacuum. Concrete pavers in the Netherlands only suffer frost damage when de-icers are involved. Contrary to clay pavers, however, they do not resist the Dutch freeze/thaw test after impregnation with water using full vacuum. They do resist after impregnation using so-called half vacuum. Since in the Netherlands only highly sintered clay pavers and high quality concrete pavers are used, all pavers are tested at a level of saturation that can be obtained using half vacuum. Half vacuum, however, was replaced by the 80oC method for practical reasons. The German method consists of progressively raising the level of immersion over a 24-hour period, during which time the pavers become completely immersed and then leaving them in this state for a further 72 hours. It is considered that the progressive raising of the water level prevents most of the air from being trapped in the pore system of the pavers. The levels of saturation achieved by this method are comparable to the UK method. However, similar levels of saturation do not necessarily mean that similar results would be obtained from test methods which are based upon different principles. Freezing conditions Direction of freezing The French, Dutch/Belgian and UK test procedures all use the uni-directional method of heat extraction, i.e. only one face of the pavers is exposed to the freezing conditions, as occurs in service. The German method, in which individual pavers are frozen from all sides, has proven to be satisfactory for German pavers, which are subject to a – 64 –

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maximum water absorption limit of 6%. However, it is recognised that this test method can cause a pattern of damage to some types of paver that is not seen in practice. It is now generally accepted that the uni-directional procedure is the correct one. Rate of freezing The German test procedure defines the rate of freezing by means of a temperature/ time graph using temperatures measured at different positions within a reference test paver. In the French test, the pavers are placed face down on cold plate at –15oC and the rate of freezing is governed by the mass and geometry of the pavers. The UK and the Dutch/Belgian freezing rates are defined by measuring the heat extraction rate using a black body radiating device. The UK rate is 400 ± 50 W/m2 and the Dutch/Belgian rate is 300 ± 60 W/m2. It is considered that the freezing conditions are best defined by an absolute measuring method which is independent of the samples being tested, i.e. a dense paver will freeze at a different rate from a highly absorbent paver with a high water content. Thawing conditions The French and German thawing regimes consist of completely immersing the frozen pavers in water at ambient temperature, i.e. omni-directional. The Dutch/Belgian procedure involves spraying water onto the exposed surface of the horizontal test panel until the tray with the pavers becomes completely immersed in water at room temperature. In all three cases, the pavers are completely thawed every cycle. The UK test procedure is based upon developing the ‘pinch-effect’ where the depth of thawing is limited to 10–15 mm from the exposed face by applying a specified quantity of heat before the freezing cycle recommences. The subsequent entrapment of water between a fixed ice front and an advancing ice front is considered to cause the sort of stress levels encountered in natural conditions. This principle has been adopted for the draft European frost test for clay masonry units. Number of cycles The number of freeze/thaw cycles carried out in each test method is closely related to the length of each cycle in hours, such that the total time for a complete test is reasonably short, i.e. 2–4 weeks. The French and Dutch/Belgian test methods operate on the principle that with high levels of saturation, fewer freeze/thaw cycles are needed to achieve realistic results. The UK method uses a much shorter cycle, since only 10–15 mm thickness of the exposed face is subject to cycling, and experience with the test provides satisfactory evidence that the laboratory test results accord well with practice.

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Draft test procedure – version 1 It was agreed by the partners that the single test method should incorporate the ‘pinch-effect’ feature of the UK method to bring it into line with the proposed single European Frost Test for Clay Masonry Units prEN772–22. However, it was considered that since the service conditions for pavers are likely to be more severe than for masonry units, then the conditions of the paver test should be made more severe by raising the levels of pre-saturation of the test samples. A draft test procedure was produced with the proviso that the saturation procedure and the thaw regime should undergo more investigation within Work package 3 before a working version could be operated by the laboratories. Work package 2: Production of suitable test samples Each partner obtained two types of pavers: one known to be durable according to the appropriate national test procedure, and one known to be non-durable according to the national test procedure. The Dutch partner claimed that a non-durable paver was not available, i.e. all the pavers produced in the Benelux countries were durable according to their national test procedure. This situation was not considered to have affected the viability of the programme. The natural variation of physical properties within a sample of a particular paver type, which may influence the durabilities of individual units, is most easily assessed by the determination of the amount of water that is absorbed in 24 hours of complete immersion. Consequently each of the 2560 pavers used in the testing programme had this property measured. The data from these tests were fed into a computer program which then distributed the individual pavers within each of the eight types into lots of ten, with each lot being matched by water absorption in terms of range, mean and standard deviation. The purpose of this exercise was to ensure, as far as possible, that when each laboratory received its allocation of test samples they were identical. Work package 3: Study of the performance of the method Testing by national test methods Each of the four partners tested the eight paver types according to their national test methods and reported the results to the co-ordinator who then collated and presented them to the group for study. The results of these tests are summarised in Table 2.21, which show how the results from each laboratory comply with the declared durabilities of the eight paver types. The highlighted results in Table 2.22 are those which did not agree with the known durability classification of the tested pavers. The detailed results were discussed by the group and it was agreed that the approach that had been adopted for the draft single method was correct.

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National frost test results UK result

French results

German results Dutch results Test C

Paver

Declared

Sat

type durability (%)

GB 1 GB 2 FR 1 FR 2 NL 1 B1 DE 1 DE 2

Durable Non-durable Durable Non-durable Durable Durable Durable Non-durable

Table 2.22

3.5 7.4 5.2 12.2 9.2 2.4 5.0 10.9

Frost test result

Sat (%)

Frost test result

Pass Fail Pass Fail Pass Pass Pass Fail

3.1 8.3 9.6 17.1 15.5 6.5 6.8 11.8

Pass Pass Pass Fail Pass Pass Pass Fail

Sat (%)

3.5 6.8 4.7 11.4 8.2 1.5 4.4 10.7

Frost test result

Pass Fail Pass Pass Pass Pass Pass Fail

Sat (%)

4.1 8.0 7.0 15.1 12.8 5.1 5.8 11.9

Test D Frost test result

Pass Pass NT Pass NT NT NT Fail

Sat (%)

6.3 8.6 8.3 17.1 15.8 10.1 6.1 11.3

Frost test result

Fail Fail Pass Fail Pass Pass Pass Fail

Agreement with declared durabilities Agreement as tested by:

Supplier’s specification UK

France

Germany

Holland ‘C’ Holland ‘D’

Durable, 5 types Non-durable, 3 types

5 2

5 2

5 1

5 3

4 3

Testing with first draft test procedure Some of the eight paver types were tested according to the first version of the draft test procedure, but using three different saturation procedures. These were 7-day soak, partial vacuum (300 mmHg residual pressure) and full vacuum (20 mmHg residual). The results demonstrated that with full vacuum the draft method incorporating the ‘pinch-effect’ caused frost damage to all pavers, including fully durable types; partial vacuum increased the level of damage to non-durable pavers but did not cause damage to durable pavers. The results are summarised in Table 2.23. These results and the actual pavers tested were examined and discussed by the members of the testing group at a meeting held at the laboratories of CERAM Building Technology.

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Draft procedure – effect of saturation method

7-day soak

Partial (300 mmHg) vacuum Full (20 mmHg) vacuum

Paver type

Sat (%)

No. of Failed cycles to at 100 failure cycles (%)

Sat (%)

NL1 B1 GB1 GB2 FR2 DE2

9.2 2.4 3.5 7.4 12.2 10.9

– – – 50 50 25

0 0 0 100 83 100

No. of Failed cycles to at 100 failure cycles (%)

– – 4.3 7.3 13.8 10.9

– – 100* 48 15 17

– – 20* 83 75 100

Sat (%)

16.0 10.9 6.4 8.6 – –

No. of Failed cycles to at 100 failure cycles (%) 50* 15 27 18 – –

20* 100 100 100 – –

*Minor damage – not confirmed as frost damage.

Further development of draft method At the above meeting of the testing group a full discussion of the first draft test procedure was held which resulted in further refinements being made to the definitions of the freezing and thawing conditions. After considerable discussion about the merits of vacuum saturation, it was eventually agreed that the saturation procedure should be based upon soaking in water at 80oC, but that some work would need to be done in order to establish the optimum time of immersion. While this supplementary work was proceeding, further discussions were held regarding the thermodynamics of the freezing and thawing regimes. This resulted in much simpler and more logical definitions of the test conditions. Finally a working version of the draft test procedure was agreed and work commenced on the final stages of the project, during which time the four laboratories equipped themselves so as to be able to operate to the draft method and to test the eight paver types. Some initial equipment problems were encountered which led to delays in completing the programme. However, each laboratory produced the required data, which were collated by the co-ordinator and are summarised in Tables 2.24 and 2.25. The testing group then met at each others’ laboratories where the tested pavers were examined and the results discussed. It was noted with this method that because the test conditions are fixed, the depth of thawing is influenced by the density and water content of the pavers. This means that the pinch effect does not occur at the same distance from the exposed face for each type of paver. However, it is considered that the response of any paver to a given set of conditions is a characteristic of that paver which indicates its ability to withstand natural conditions.

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Draft test procedure – saturation levels

UK

France

Benelux

Germany

WA% (MM)

WA% (MM)

WA% (MM)

WA% (MM)

Paver

pre-test post-test 80ºC

PR 300

pre-test post-test pre-test post-test

GB 1 GB 2 Fr 1 Fr 2 NL B DE 1 DE 2 Mean

4.2 7.7 6.3 14.9 12.4 3.6 5.2 11.5 8.2

n/d 7.5 7.7 14.8 13.3 7.6 n/d 11

4.0 7.4 6.0 14.1 11.5 3.4 4.8 11.3 7.8

Table 2.25

4.3 8.2 6.6 15.3 12.2 4 5.4 n/d

4.5 8 6.5 16.9 12.5 3.7 5.2 11.6 8.6

n/d n/d n/d n/d n/d n/d n/d n/d

4.1 7.6 6.2 14.5 12 4.1 5.1 11.4 8.1

n/d n/d n/d n/d n/d n/d n/d n/d

mean WA% 80ºC 4.2 7.4 6.3 15.1 12.1 3.7 5.1 11.5

Draft test procedure – frost damage results

UK

France

Benelux

Germany

Number damaged

Number damaged

Number damaged

Number damaged

Paver

50 cycles

100 cycles

50 cycles

100 cycles

50 cycles

100 cycles

50 cycles

GB 1 GB 2 Fr 1 Fr 2 NL B DE 1 DE 2

/ 8 / 7 / / / 10

/ 9 / 10 / / / 10

/ 12* / 12* / / / 10

/ 12* / 12* / / / 10

/ n/d / n/d / / / 10

/ 10 / 10 / / / 10

/ 6 / 8 / / / 10

100 cycles / 10 / 10 / / / 10

*Sample size 12 pavers, all other sample sizes 10 pavers.

Work package 4: Summary of the study and final proposed method The four partners held meetings during the laboratory visits when the project was reviewed and the final editorial adjustments made to the draft test procedure.

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Once it had been agreed that the draft test method should be based upon the UK ‘pinch-effect’ test, the main difficulties that had arisen during the project were concerned with the establishment of an agreed set of definitions for the main parameters of the test procedure. These were as follows: 1 The level of pre-saturation of the samples and the method by which this was achieved. 2 The number and orientation of the sample pavers in the test panel. 3 The air temperature, rate of heat extraction and length of time of the freezing cycle. 4 The time, temperature and rate of flow of the water spray and the time and temperature of the air during the thawing cycle and whether these parameters could be quantified as a total heat input. 5 The description and classification of frost damage and the criteria for failure. It was considered by the testing group that these issues were satisfactorily dealt within the confines of the project and that the resulting draft test procedure had been sufficiently developed for it to be considered by CEN/TC 178 for inclusion in the European Clay Paver Standards. Current situation The two clay paver standards prEN1344 and 1345 have now been combined and published as one standard BSEN1344. After discussion at a meeting in October 2000 of the Working Group, WG3 of TC178, the single draft test procedure replaced the four separate national test procedures in the standard, and will be issued for public comment.

Standard tests on wall, floor and roofing tiles General On a world-wide scale, the manufacture of ceramic wall and floor tiles is a vast industry with the major manufacturing countries producing up to 1 billion m2 per annum. UK production is small by comparison and a large proportion of those used are imported. The UK is also a relatively small user of ceramic tiles at about 2 m2 per capita, compared with about 12 m2 per capita in Italy and Spain. A wide range of standard tests and requirements have been available in Europe for many years. Although these are not mandatory at the present time, certain of the requirements will become so in order to satisfy EU regulations and enable CE marking of tiles to be introduced. Some of the information required is not covered by the standard tests for tiles most notably that for slip resistance. Several tests for this property are in fact available but there is considerable lack of agreement in Europe as to the most appropriate and reliable test. – 70 –

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Clay roofing tiles are made from vitrified clay which may or may not have a surface coating of glaze or ‘engobe’. Engobe is a surface coating that is clay based and not glass based as in glaze. They are designed to be used on pitched roofs and usually have one or more separate, or one continuous nib at or near the head of the tile that is hooked over a timber batten on the roof. In the UK, the majority of tiles are plain, which means they are rectangular with a slight camber longitudinally and sometimes transverse and have one or more nail holes near to the head so that they can be nailed to the battens. In, for example, France and Spain, the tiles tend to be larger with a pronounced profile across the tile. There are numerous patterns of such tiles often described as pantiles or Roman tiles and these may have an interlocking profile at the side or the head of the tile or both such that they are locked into place with other tiles when laid. Plain tiles are laid on battens which typically have a spacing up the roof of 100 mm and as a common tile length is 265 mm, this means that at any particular point there may be two or three tile thicknesses above the roof pitch. It is the arrangement of plain tiles on the roof and the effectiveness of the interlock on the other types, which help to ensure the weathertightness of a roof and hence the regularity of tile dimensions is an important characteristic. Tiles made in the UK are virtually impermeable although this is not the case throughout Europe. In parts of Denmark, the practice is to fully board the sloping surfaces of the roof and to waterproof the surface allowing tiles of much greater permeability to be used than are made in the UK. The other main properties of interest are transverse breaking load, essentially will they sustain the weight of a person on the roof and frost resistance as especially with good thermal insulation if the tile is very wet it can be subjected to many cycles of freezing and thawing.

The European testing systems for wall and floor tiles The long-standing European test system for tiles is entirely encompassed in BS6431 (British Standards Institution, 1996). This comes in 23 parts as listed in Table 2.26. Also included in this Table 2.26 are the European Standard (EN) reference numbers each of the 23 parts having a separate EN reference. Table 2.26 includes a column of ISO document references, which will be discussed later in the chapter. BS6431 introduces a classification system based upon forming method and water absorption. Forming method can be by extrusion (A), granulate pressing (B), all other methods, e.g. wet pressing or hand forming, are classified as C. Water absorption (E) has the following groups: I E ≤ 3% IIa 3% < E ≤ 6% IIb 6% < E ≤ 10% III E >10% Each of these basic tile groups is covered by a requirements’ document and these are shown in Table 2.26 as Part 2 to Part 9, each with its EN equivalent reference. The tile groups are summarised in Table 2.27. – 71 –

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As can be seen in Table 2.26 tiles in group C have no requirements’ documents, but can still be tested by the same test methods as other tiles. Although there is no comment on tile use in the standard, Blll tiles are always glazed and are mostly for use on walls, Al and Bl either glazed or not would normally be used on floors. The intermediate groups are less common and usually glazed. They can be used for either application depending on other factors. Table 2.26

BS6431 European Standards (EN) for ceramic floor and wall tiles

Part

EN Title Number

1

87

2

121

3

5

186 (Parts 1 & 2) 187 (Parts 1 & 2) 188

6

176

7

177

8

178

9

159

10 11 12 13 14 15 16 17 18 19 20 21

98 99 100 101 102 103 104 105 106 122 154 155

22 23

202 163

4

ISO 10545

Ceramic floor and wall tiles – definitions, classification, characteristics and marking Extruded ceramic tiles with a low water absorption E ≤ 3%. Group AI. Extruded ceramic tiles with water absorption 3%. < E ≤ 6%. Group Alla. Parts 1 and 2. Extruded ceramic tiles with water absorption 6%. < E ≤ 10%. Group Allb. Parts 1 and 2. Extruded ceramic tiles with water absorption 3%. < E ≤ 10%. Group AllI. Dust pressed ceramic tiles with a low water absorption E ≤ 3%. Group BI. Dust pressed ceramic tiles with water absorption 3% < E ≤ 6%. Group BIIa. Dust pressed ceramic tiles with water absorption 6 < E ≤ 10%. Group BIIb. Dust pressed ceramic tiles with a water absorption > 10%. Group BIIb. Determination of dimensions and surface quality. Determination of water absorption. Determination of modulus rupture. Determination of scratch hardness of surface according to Mohs. Determination of resistance to deep abrasion. Unglazed tiles. Determination of linear thermal expansion. Determination of resistance to thermal shock. Determination of crazing resistance. Determination of chemical resistance. Unglazed tiles. Determination of chemical resistance. Glazed tiles. Determination of surface abrasion. Glazed tiles. Determination of moisture expansion using boiling water. Unglazed tiles. Determination of frost resistance. Sampling and basis for acceptance.

Note: E = Water absorption.

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Part 2: 1997 Part 3: 1997 Part 4: 1997 – Part 6: 1997 Part 8: 1996 Part 9: 1996 Part 11: 1996 Part 13: 1997 Part 13: 1997 Part 7: 1999 Part 10: 1997 Part 12: 1997 Part 1: 1997

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Grouping of tiles by water absorption and forming method

Forming Method

Group I

Group Ila

Group IIb

Group III

A B C

Group AI Group Bl Group Cl

Group Alla Group Blla Group Clla

Group Allb Group Bllb Group Cllb

Group lll Group Blll Group Clll

Tile tests Table 2.26 includes a range of test methods to be used to measure the various characteristics referred to in Parts 2–9. The test methods are described in Parts 10–22, each having an EN equivalent reference. Finally, Part 23 gives guidance on how to interpret and use the test results from each of the tests. This enables the tester to determine how a tile performs with respect to its specified criteria. For example, the strength test Part 12 or EN 100 is used to determine Modulus of Rupture level, which has to be greater than a certain level depending on tile category for compliance with the standards. The test requires that seven tiles are tested according to a precise procedure and the individual and average results compared with the minimum requirement for the particular category of tile. Tiles in each group (see Table 2.27) can be glazed or not. A slightly different range of tests is required in each case. For glazed tiles the following tests are needed: • Parts 10, 11, 12, 13, 15, 16, 17, 19, or • EN 98, 99, 100, 101, 103, 104, 105, 122 • + Part 20 (EN 154) if for floor use. For unglazed tiles the list of tests is as follows: • Parts 10, 11 ,12, 13, 14, 15, 16, 18, or • EN 98, 99, 100, 101, 102, 103, 104, 106 • + Part 21 (EN 155) is optional as there is no requirement. If the manufacturer states that the tiles (glazed or not) are suitable for external use, then the tiles must also pass the frost resistance test, Part 22, EN 202. One further specific and important point made in relation to this Standard is that it does not presuppose how tiles in any category shall be used. Hence there is no guidance or recommendation as to suitability for wall as opposed to floor use or indeed internal or external conditions. Manufacturers give this advice hence the point above regarding frost resistance and the later comment on the reflectiveness of glazed units.

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Test limitations Over the years, there have been many technical developments in tile production and to some extent, this has left the standard behind. The main development has been the vast increase in porcelain body tiles. These use a more expensive body system similar in some ways to a fully vitrified sanitaryware body. In particular, such tiles have a very low water absorption level at the bottom end of group 1. As a knock-on from this, there has also been a vast increase in polished tiles, unglazed but having the reflective qualities of high gloss glazed tiles. There has also been a big increase in texture and profiling on tile surfaces. Other developments have taken place in performance requirements as defined by the EU and other, notably safety, bodies. Thus a test of slip resistance assessment has become essential and further knowledge of the durability of tiling in service is desirable. Durability is a characteristic, which is very difficult to assess. Not only does it refer to wear, it also refers to the durability of any characteristic. For instance, how long does a polished surface continue to be reflective with use or how long does a surface continue to give the same slip resistance with use. The role of adequate cleaning and maintenance is an extra significant factor in the determination or assessment of durability.

The ISO testing system for tiles Introduction The International Standards Organisation has built on the EN system and published a series of standard test methods for tiles. These have now been accepted as European and hence British Standards so the UK Test Methods do exist as BSEN ISO 10545 Parts 1–16 (British Standards Institution, 1996–2000), but as yet there is no Specification agreed in Europe and the UK which refers to them. In the International system the requirements document is ISO 13006 (International Standards Organisation, 1998) which includes a definition document and a requirements annex for each group of tiles. The classification of tiles into groups is essentially that shown in Table 2.27 except that group Bl has two sub groups Bla and Blb, the former being for tiles with water absorptions less than or equal to 0.5%, these being the porcelain tiles. Groups Alla and Allb each also have two sub groups denoted by –1 and –2 which are intended to differentiate between natural and rustic products. The ISO Specification has not yet been accepted in Europe and is now passing through the final procedural steps with another reference number pr EN 14411 slightly modified to take account of EU requirements on building products. If approved EN 14411 will become the accepted requirements’ document in UK, which will supersede Parts 1–9 of BS6431 and the ISO 10545 tests will be referred to in place of Parts 10–23. – 74 –

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The ISO tests The final column of Table 2.26 shows the various Parts of ISO 10545 against their counterparts in the BS and EN system. There is one test which is not reproduced and that is the Test for Scratch Hardness and there are three additional ones Part 5 for Impact Resistance, Part 15 for the determination of lead and cadmium release and Part 16, which deals with small colour differences. Although attempts were made agreement could not be reached on the best method of measuring the coefficient of friction. It is not proposed to go into any detail of all the tests in either the EN or ISO lists.

Slip resistance According to EU regulations, all walking surfaces in public/commercial/industrial areas should have a slip resistance assessment. Technical Committees in CEN covering all types of hard surface have been working on this topic now for many years and this includes TC 67 for ceramic tiles. Initial work in CEN led to a draft document, which contained four alternative methods of assessing slip resistance: • Tortus® method, a drag slider • Pendulum method • Inclined platform method • Static method.

Two of these methods, the Tortus® and the Static methods are thought by many to give unsound and misleading results in wet conditions. Since wet contamination is a contributing factor in over 90% of slipping accidents, wet measurements are clearly of great importance. Therefore methods which can give wet values for slip resistance, which match or even exceed the dry values are obviously viewed with suspicion. The two methods which have most credibility in the UK are the Inclined Platform and the Pendulum.

Inclined platform This is seen in the diagram in Fig. 2.17 and is based on the German standard apparatus used in DIN 51097 and 51130 (German Standards Institution, 1992). These can give slip resistance assessment on a panel of new tiles (or any flooring) in either the shod or bare-foot condition. Two operatives are used and the corrected average inclination is then linked to categories for which there are guidance notes as to usage. The categories for shod use are shown in Table 2.28.

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Safety device Angle indicator Drive unit Test surface

Dimensions in mm

Figure 2.17

Inclined platform – test device.

Table 2.28

The categories for shod use

R Value

Category/Static friction is:

Angle of inclination

9 10 11 12 13

Low Normal Above average High Very High

3o–10o > 10o–19o > 19o–27o > 27o–35o > 35o

Usage ranges from R9 for normal walking in dry areas to R13 where excessive greasy contamination would be likely. R10 and R11 would be seen as satisfactory for water contamination. The categories for bare foot assessment are shown in Table 2.29.

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The categories for bare foot assessment

Angle of Inclination

Category

> 12o > 18o > 24o

A B C

Here, category B would be seen as satisfactory for a pool surround with adequate fall to prevent ‘ponding’. C grade would be recommended for very wet areas such as showers. The pendulum The pendulum is the method favoured in the UK for measuring slip resistance and has the great advantage of being portable and hence can be used on site, for example, where an accident has taken place. The technique can also be used to show up contamination or deficiencies in cleaning. The principle and development of the test have been described in section 2.3 and a similar relationship between pendulum value and potential for skidding has been developed based upon the use of a rubber slider appropriate to tile surfaces rather than paved ones.

Other factors Adhesives and grout A satisfactory tiled installation incorporates many other factors than just tiles and all aspects need to be of acceptable standard for the installation to work. Hence, there are also tests for tile adhesives. This has recently changed from a long-standing BS test, BS5980 (BSI, 1980). The new European document EN 12004 (BSI, 2001) specifies the requirements for tile adhesives as shown in three Tables 2.30–2.32 for the three generic types of adhesive:cementitious, dispersive and reaction resin. The relevant test methods are also listed. Design and installation The design and installation of ceramic tiles is covered in BS5385 (BSI, 1989–1995). This comes in five parts with different publication dates and covers wall and floor tiling, internal and external as well as special conditions, e.g. swimming pools. Other hard materials, e.g. granite, slate and marble, are covered, as are mosaics. Another relevant document is BS8000 Section 11.1 (BSI, 1989) which deals with on-site requirement for wall and floor tiling operations. All these standards – 77 –

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are in the form of ‘best practice’ Codes of Practice in that they try to guide the reader to avoid common pit-falls. They need to be used with manufacturers’ recommendations. In particular, specialist tile adhesive manufacturers have a wide range of products for all applications and also provide tiling specification and technical back up to ensure a high quality installation. To further ensure a quality installation, The Tile Association requires high standards from its members be they manufacturers, tile suppliers, installation companies or architects. They also provide a range of technical guidance documents on a range of important topics. In conclusion, it can be said that, as a product, the ceramic tile is covered by a wide-ranging series of tests and requirements. Furthermore, all other factors necessary to produce a tiled installation of good quality are covered in the UK by other standards and codes of practice. Table 2.30 Specification for cementitious adhesives – fundamental characteristics Characteristic 1a Normal setting adhesives Initial tensile adhesion strength Tensile adhesion strength after water immersion Tensile adhesion strength after heat ageing Tensile adhesion strength after freeze-thaw cycles Open time : tensile adhesion strength

1b Fast setting adhesives Early tensile adhesion strength

Open time : tensile adhesion strength

All other requirements as in 1a Optional characteristics 1c Special Characteristics Slip 1d Additional Characteristics High initial tensile adhesion strength High tensile adhesion strength after water immersion High tensile adhesion strength after heat ageing High tensile adhesion strength after freeze-thaw cycles 1e Additional Characteristics Extended open time:tensile adhesion strength

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Requirement

Test method

≥ 0.5 N/mm2 ≥ 0.5 N/mm2 ≥ 0.5 N/mm2 ≥ 0.5 N/mm2 ≥ 0.5 N/mm2 after not less than 20 min

8.2 of EN 8.3 of EN 8.4 of EN 8.5 of EN EN 1346

≥ 0.5 N/mm2 after not more than 24 hours ≥ 0.5 N/mm2 after not less than 10 min

8.2 of EN 1348:1997

1348:1997 1348:1997 1348:1997 1348:1997

EN 1346

EN 1348:1997

≥ 0.5 N/mm2

EN 1308

≥ 1 N/mm2 ≥ 1 N/mm2 ≥ 1 N/mm2 ≥ 1 N/mm2

8.2 8.3 8.4 8.5

≥ 0.5 N/mm2 after not less than 30 min

EN 1346

of of of of

EN EN EN EN

1348:1997 1348:1997 1348:1997 1348:1997

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Specification for dispersion adhesives (D)

Characteristic 2a Fundamental Characteristics Initial tensile adhesion strength Shear adhesion strength after heat ageing Open time:tensile adhesion strength

Optional Characteristics 2b Special Characteristics Slip 2c Additional Characteristics Adhesion strength after water immersion Adhesion at elevated temperature 2d Additional Characteristics Extended open time:tensile adhesion strength

Table 2.32

Requirement

Test method

≥ 1 N/mm2 ≥ 0.5 N/mm2 ≥ 0.5 N/mm2 after not less than 20 min

7.2 of EN 1324:1996 7.4 of EN 1324:1996 EN 1346

≥ 0.5 N/mm2

EN 1308

≥ 0.5 N/mm2 ≥ 1 N/mm2

7.3 of EN 1324:1996 7.5 of EN 1324:1996

≥ 0.5 N/mm2

EN 1346

Specification for dispersion adhesives (R)

Characteristic 3a Fundamental Characteristics Initial tensile adhesion strength Shear adhesion strength after heat ageing Open time:tensile adhesion strength Optional characteristics 3b Special Characteristics Characteristic Slip 3c Additional Characteristics Characteristic Shear adhesion strength after thermal shock

Requirement

Test method

≥ 2 N/mm2 ≥ 2 N/mm2 ≥ 0.5 N/mm2 after not less than 20 min

7.3 of EN 12003:1997 7.4 of EN 12003:1997 EN 1346

Requirement ≥ 0.5 N/mm2

Test method EN 1308

Requirement ≥ 2 N/mm2

Test method 7.5 of EN 12003:1997

Clay roofing tiles Geometrical characteristics The geometrical characteristics of clay roofing tiles that are controlled by the Standard (BSI, 1998) are individual dimensions (length and width), cover dimensions, camber and twist. The test method standard (BSI, 1997) does not – 79 –

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specify in detail the apparatus to be used, rather its accuracy. The linear dimensions i.e. length, width and cover dimensions are to be measured with a device that has a precision of 1 mm and the out of plane measurements are made with one with a precision of 0.5 mm. The principle of the test method for individual tiles is shown in Fig. 2.18, the mean length and width from a sample of ten tiles is required to be within 2% of values declared by the manufacturer. The cover dimension applied to interlocking tiles only where the actual length or width covered by the tiles is dependent on whether the interlocking features have been laid ‘tight’ or ‘loose’. For example, in the case of tiles that have a headlock, two columns of 12 tiles are laid topside down on a flat surface and interlocking at their head and side, if any. The columns are adjusted such that the overall length as measured between a point on each of the first and 11th tiles is first as small as can be and then, as great as can be. The mean of these cover dimensions divided by 20 is taken as the cover dimension of the tile and the maximum value must not exceed the mean by 2%. Similarly for tiles with a sidelock, two rows of tiles are used and the maximum transverse dimension must not exceed the mean by 2%. The camber, concavity of convexity of a tile is measured over a gauge length of two-thirds of the actual length or width of the outside surface. A simple dial gauge at the centre of the gauge length may be used. The camber is defined as the difference between actual measured height and the manufacturers declared height expressed as a percentage of the gauge length. The result is the mean value determined on ten tiles and this must be less than 1.5% for tiles larger than 300 mm and 2% for tiles less than 300 mm. In the case of plain tiles, the same criteria is applied to the transverse direction as well, although the criteria remain length dependent.

1

Figure 2.18

Principle for measuring individual dimensions.

Twist is measured as essentially the vertical distance of a fourth point on the tile from a place defined by three other points on the tile. A flat metal plate is placed horizontally and two metal bars, each 25 mm × 25 mm in cross section are taped to the plate such that their axes are two-thirds of the tile length apart. In the case of plain tiles, they are placed against one metal bar, as they would be against a batten in practice. The tile is then pressed down so that three points of the tile touch the metal bars. The vertical distance of the fourth point is then – 80 –

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measured. The coefficient of twist is this distance divided by the sum of the gauge length (bar spacing) and tile width and expressed as a percentage. The limitations are as for camber i.e. 1.5% for tiles over 300 mm in length and 2% for those that are shorter. The measurement of twist for interlocking tiles is more complex than for plain tiles and it is sometimes necessary to use more than one tile locked together to define the reference plane, the principle is however the same. The only other dimensional control is for so-called over and under tiles. These are tiles shaped liked a gutter either with parallel or tapering sides and which, when placed on the roof, consist of a series of valleys onto which a series of hips are placed. The control here is essentially on the variability of the tile width at each end. Flexural strength The principle of the flexural strength test (British Standard Institution, 1994) is the three point bending test as described, for example for clay pavers when it is known as the transverse breaking load test. Essentially the tile is placed upon two bearers of circular cross section between 15 mm and 20 mm in diameter one of which is pivoted at its centre to accommodate any small twist in the tile and is loaded through a third pivoted upper bearer at the mid span point. The lower bearers are placed at two thirds of the tile length apart. The difficulty of testing is that there is a very wide range of shapes and so although simple metal rods can be used as bearers for plain tiles a more complex arrangement is needed for interlocking tiles, where shims are used on the lower bearers to accommodate differing heights from a plane surface and a contoured upper bearer is used to spread the applied load. The contoured bearer may be rubber-faced hardwood or moulded plaster, but whichever option is chosen, a test laboratory does need to invest in a wide range of bearers. Frost resistance Despite considerable international studies, it has not yet been possible to reach the level of international agreement in Europe over the test method for roofing tiles as has been possible for bricks. Although a European test method document exists (BSI, 1998), it contains four different methods; these are essentially the traditional methods for the Netherlands, Germany, France and the UK. At the start of the programme of European drafting work in the late 1980s, efforts were made to divide Europe into zones based upon the severity of freezing and thawing when roofing tiles would be expected to be saturated. Classification of weather in this way is very difficult, CERAM Building Technology have attempted to do this and established a monthly based Freezing Index, F as follows: F = (R1 + R2) n 10 where R1 is the rainfall in the month in question (mm); R2 is the rainfall in the preceding month (mm) and n is the number of air frosts in the month in question. – 81 –

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This index has been calculated using weather records from 17 weather stations over a 45-year period (Beardmore and Ford, 1986; Beardmore, 1989) and it proves to be quite useful for comparing the relative severity of months or indeed winters. The index demonstrated quite clearly that the winter of 2000/01 was very severe in terms of frost, although the number of air frosts were about average. The key characteristic of that winter was excessive rainfall, which led to above average saturation and frost failures. However, the use of this approach for classification of weather across Europe so that perhaps the relevant test could be matched to the weather conditions was too ambitious. The first attempt placed Wales and the Algarve in the same zones. As a result, the four tests have been retained each relating to a geographic zone, for example the UK approach being relevant to the zone containing Denmark, Ireland and the UK.

1 2 3 4 5

Freezer unit Pivoted heater Fan Heat-insulated cabinet Plain tiles on 38 mm × 25 mm softwood battens at 100 mm centres on 9 mm thickness metal spacer strips

Figure 2.19

6 7 8

Thermocouple Metal sheet underlined with 50 mm thick polystyrene Water spray

Example of apparatus for determination of frost damage. – 82 –

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The UK approach has not surprisingly developed from the approach to bricks described elsewhere (section 2.2) and the apparatus illustrated in Fig. 2.19 is the UK cabinet as used for testing brickwork to which has been offered a second cabinet into which an array of tiles is placed. In the case of roofing tiles, full frost resistance is always a requirement and for a particular clay and making process there is a critical water absorption above which failures will occur. Consequently the manufacturer can, once it is evaluated, use the critical water absorption as his factory control measure to indicate adequate firing for durability. Impermeability UK tiles are virtually impermeable, so that inclusion of this test as one required to standardise tiles in Europe, is not really necessary for them. In fact, efforts are in hand to ensure water absorption can be used as a proxy test in order to reduce the amount of routine testing. It has again proven impossible to agree on a single test procedure and the standard (BSI, 1994) includes two, one involves the measurement of water passing through a piece of tile sealed to the surface of a glass vessel and submerged under a constant head of water for 48 hours, the other involves building a dam on the tile surface so that a reservoir, with the tile surface as its base can be filled with water. In the second case, the criterion is the time to first leakage as detected on a mirror resting beneath the underside of the tile. The current situation is that whichever test is used must be stated by the manufacturer and there are in each case two classes of an impermeability factor. It is surprising that so little harmonisation has been possible of such a basic property and such simple science but work continues to achieve a more satisfactory result in a later generation of European Standards.

Standard tests on vitrified clay pipes General Of all the ceramic products used in construction, the range of Standard Tests, which are available for clay pipes, is the most extensive. The reason for this is that clay pipes are required to satisfy geometric, structural and hydraulic requirements in all situations. In special circumstances, they may need to demonstrate an enhanced performance over that which is inherent to resist abnormal fatigue, abrasion or chemical loads. In addition, it is important all the joints and fittings needed to construct the complete pipeline perform to similar standards. Clay pipes used in drainage and sewerage works have traditionally been laid on the prepared bed of a trench. In some circumstances, the preparation may be simply the trimming of the trench bottom (West et al., 1972) but more commonly it is by providing a layer of natural aggregate to provide a uniform bearing layer – 83 –

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(Bland and Picken, 1973). Recent studies (Beardmore, 1999) suggest that it may be possible to use crushed demolition waste as opposed to natural aggregate for pipe bedding but as yet such a practice has yet to be proven as one to be generally recommended. In recent years, the practice of trenchless construction has been developed and clay pipes are suitable, although this introduces further requirements for their strength when loaded on end so that they resist the forces applied during jacking (BSI, 1995). Clay pipes are described as rigid in that they are stiff and when loaded are linearly elastic until failure occurs. The joints in modern clay pipes are mechanically flexible, so that they can accommodate small rotations at the joints, which may occur due to differential settlement. Flexible mechanical joints may be either a socket and spigot or sleeve design. The geometrical requirements are that the pipe is reasonably straight, so that it receives a uniform support from the bedding, that the ends are square, so that the joints can be easily made on site and there are no large changes in the level of the invert at the joints, which would enable detritus to build up and reduce the hydraulic performance. The structural requirements of clay pipes are significant in that when laid to line and level in a trench, they must be capable of resisting loads during the compaction of sidefill and backfill material and subsequently that of surface traffic. Until 1981 (BSI, 1981) the strength of clay pipes was judged solely on their ability to resist crushing forces, i.e. vertical loads that would cause tensile failures of the pipe at the inner surface of the crown and invert and the outer surface at the sides. This approach is appropriate for large diameter pipes as it is a realistic mode of failure and although clay pipes are made with internal diameters of up to 1.2 m the majority are of diameters of 300 mm or below. This fact together with the desire to reduce the cost of the completed clay pipeline by reducing the requirements for ever more expensive natural aggregate and hence the likelihood of greater longitudinal bending, meant that a bending moment resistance test was needed. In the period 1976–1981, a very extensive programme of testing was carried out at BCRA (now CERAM) to investigate what were the required bending moment resistances for smaller diameter clay pipes to be used in trenches with a range of beddings (Picken, 1980). The project involved the installation of instrumented clay pipelines 9 m long in a trench excavated from boulder clay which had been compacted into one of two concrete pits each 3 m wide and 3 m deep. The pipes were bedded, the trench backfilled and compacted and the trench surface was loaded by hydraulic jacks (see Figs 2.20 and 2.21). In this purpose-made facility, the effects of bedding type and class, backfill depth and surcharge load were extensively investigated (Edgell, 1980). As a result of this work, there are now standard requirements for the bending moment resistance of pipes of 225 mm bore and smaller for each class of crushing strength (BSI, 1991).

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Figure 2.20 Pipeline laid in trench prior to backfilling.

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Figure 2.21

Internal instrumentation.

Clay pipes are generally intended to operate partly filled under gravity. They possess a low hydraulic roughness although for special circumstances this can be evaluated. From time to time the pipeline may become hydraulically surcharged and so the pipes are designed to resist an internal overpressure. Even more critically is the impermeability of the joints for more than one reason. Joints which are not watertight will attract the ingress of tree roots which will themselves grow and extend so as to block the bore but will also act as an obstruction upon which detritus will build up making the problem worse. Such blockages can be investigated with the aid of closed circuit television cameras on mobile sledges and can be removed by the use of high pressure water jetting but clearly it would be better if this were not necessary. Another potential problem due to leaking joints is the gradual removal of the fine material in the pipe bedding leading to loss of support and possible pipe bending failure. Such failures were more common years ago when poorer quality pipes were rigidly jointed together with mortar, which could crack due to shrinkage or differential settlements. A third potential problem is the ingress of water in areas of high ground water levels, which can lead to excessive flows to be dealt with at treatment works. The clay pipe industry in Europe has been very much at the forefront in terms of the development of European Standards and has had an agreed standard in place since 1991 (BSI, 1991). This was a considerable achievement, which involved a great deal of investigation into the performance of UK pipes under new test regimes but also to address great differences in the various approaches to quality control. This progress has meant the inclusion of some test methods (BSI, 1991) that although recommended in special circumstances only were unfamiliar in the UK, for example resistance to fatigue loading and abrasion resistance. – 85 –

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The following sections deal with the principal features of some of the main tests, it is not exhaustive and reference can be made to the British Standard (1991) for the less common ones.

Geometry Although straightness of pipes and the squareness of their ends are important requirements for clay pipes for reasons described earlier the British Standard allows any suitable apparatus to be used. It does however specify the length over which straightness is to be measured and gives an example of a suitable method to measure squareness of ends and these are shown as Figs 2.22 and 2.23, respectively. Similarly, the difference in invert position at a joint is derived by calculation from measurements of the spigot and socket diameters and the maximum wall thickness at the socket including the moulding and the distance from the spigot moulding to the invert position, without being prescriptive as to how these measurements are taken. At first sight, this appears to be quite a shortcoming but in practice a combination of internal and external callipers and micrometers will give results of sufficient accuracy and perhaps all that is needed is a statement about the tolerance of measurement accuracy.

Figure 2.22

Straightness measurement.

Figure 2.23

Measurement of squareness of ends. – 86 –

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Crushing strength test The principle of the crushing strength test is that a pipe which has been saturated is placed horizontally in a test machine and supported on a bearer which has a shallow ‘vee’ section and is loaded by a line load at the crown position. As with many European Standards there are alternative details as to how the principle is achieved in order that existing practices can be allowed to continue. Consequently although the bottom bearer is of an agreed shape in metal, teak or similar hardwood, the top one can either be the width of the strip of pipe to be loaded (50 + 5 mm) or wider, see Fig. 2.24. The bearers can also be either faced with felt or with strips of elastomeric material of defined shape and hardness. These facings or strips accommodate the curvature of the pipe wall. The length of pipe to be loaded is either the overall length less 100 mm for plain-ended pipes or the length between spigot and socket less 50 mm. The bearers themselves may be rigid and rely upon the felt or strips to accommodate any longitudinal curvature in the pipe. As this measure can only accommodate limited curvature, it is restricted to pipes of less than 1.1 m in length. The alternative is to divide the bearer into segments not exceeding 300 mm in length and to either load each separately through a common hydraulic manifold or to set each onto high pressure hydraulic hoses filled with fluid, which in turn, sits in channels below the bottom and above the top bearer, respectively. There may be small gaps between the bearer segments, not greater in length than one-third of a segment length in order to match the bearer and pipe lengths for a range of lengths. Where a single load is being applied to the bearer it is important that the loading platen of the machine has adequate stiffness to ensure the bearer cannot bend significantly. It is also important that the load is applied at the centre of the loaded length and that there is some accommodation of any rotation of the loading platen in the plane of the pipe axis. Failure to provide the latter can lead to a loading piston bearing on one side of the inner of its hydraulic cylinder and giving inaccurate, high results if on a routine basis oil pressure is being measured as an indication of load. Where whole pipes are not available to be tested, a test procedure is available on cut portions in which an arc of the pipe wall with a chord length of at least five times the wall thickness is tested in three point bending. Ten such specimens are tested and a formula is provided to convert the measured failure load to an equivalent crushing strength.

Bending moment resistance test The bending moment resistance of pipes, commonly abbreviated to BMR may be determined using one of two procedures. One of them is the well-established four point loading test, which has been the subject of extensive research in the UK (Brennan, 1978; Picken, 1977). In this test, the pipe, preconditioned as for the crushing strength test, is supported by two slings, similar to lifting slings of 150 mm width and in contact with the pipe over an angle of at least 120°. Load is applied through two – 87 –

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similar slings to provide a region in the mid-length of the pipe 300 mm long between the centres of the loading slings, where the bending moment should be constant (see Fig. 2.25a). An alternative is offered, which is much simpler involving three point loading through bearing strips of similar specification to those used for crushing strength determination (see Fig. 2.25b). However, a cautionary note is included in the standard that if the simpler method leads to failures which are not caused by bending action, for example by local crushing under the bearing strips then the four point method is to be used. Load

(a) Bearer shape for bearing strips

(b) Bearer shape for bearing forcings

(c) Example pipe length ≤1.5m

(d) Example pipe length >1.5m

Figure 2.24 pipe length.

(a) and (b) Loading of pipe cross section; (c) and (d) Loading of

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Figure 2.25 Bending moment resistance test: (left) four point loading test method; (right) three point loading test method.

Fatigue strength The fatigue strength test is only used in special circumstances and is a test that was unfamiliar to the UK clay pipe industry prior to the development of Europe-wide Standards. The test derives from a concern, in particular in Germany, that pipes buried beneath railway lines are subjected to repeated and significant pulsating loads. The test procedure is similar to that for crushing strength, except that the load applied is cycled at a frequency of 12 Hz between 0.1 and 0.4 of the crushing strength of the pipe for some two million cycles. As for the crushing strength test, an option is given to test cut samples with, in this case, more attention being given to the conditions at the supports.

Abrasion resistance Vitrified clay pipes and fittings are considered to be abrasion resistant for all normal applications but the Standard does include a procedure for determining the extent of abrasion following a prescribed test regime. The Standard does not include requirements and the procedure is used in special circumstances only. This is another test that was unfamiliar to the UK pipe industry and was of greater interest to manufacturers in mainland Europe and is probably associated with the common practice of glazing pipes, a practice no longer used in the UK. The principle of the test is that a pipe is cut longitudinally to form a channel section 1 m long, the ends are sealed and a prescribed weight, which depends on pipe diameter, of natural round grained quartz gravel with defined characteristics is placed in the channel. Water is added until the depth is 38 mm, a flat top is – 89 –

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fixed to the channel and it is then tilted through an angle of 22.5° to the horizontal about the mid-point of the pipe. The channel is tilted back and forth at a frequency of 20 cycles/min for 100,000 cycles. The depth of abrasion, which has been caused, is measured over a gauge length of 700 mm in the centre of the channel. The Standard suggests that measurements be made at least at 10 mm intervals and the mean value determined. However, it has been found convenient to develop a measuring rig in which a transducer is traversed across the channel invert using a small motor and the abraded profile measured by scanning at very frequent intervals. In this approach the abraded profile can be recorded and mean depth of abrasion can be readily computed, see Fig. 2.26.

Figure 2.26

Abrasion profiles from two different pipes.

Watertightness The watertightness of both pipes and fittings are measured in a similar way. For pipes either complete pipes or sections are tested and these are preconditioned either as for the crushing strength test or by allowing them to stand full of water under the test pressure of 50 kPa (0.5 bar) for an hour prior to monitoring. With the ends of the test piece bunged the pipe is filled with water and the air vented. When filled the water is maintained at the prescribed pressure for 15 minutes and the amount of water added to maintain the pressure is noted. There should be no sign of leakage and the amount of water added should not exceed 0.07 l/m2 of internal pipe surface. This added water will replace any which enters the pore structure of the pipe wall and although during the test the outside of the pipe may darken in colour and water may evaporate from it there must be no other sign of water. Fittings are tested in a similar way, but in this case the requirement is for no leakage during a 5 min water test. Alternatively, an internal air pressure of 100 m water gauge must not fall to below 75 mm in 5 min. Pipe joints are tested at pressures of both 5 kPa and 50 kPa and must not leak in a 5 min test period when, in one case, an angular rotation has been introduced at the joint and in another, a shear displacement has been introduced by applying a shear load across the joint of 25 N/mm of nominal pipe size. Satisfying the shear load test is taken as an indication that the joint assembly will resist tree root ingress. – 90 –

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Chemical resistance Although vitrified clay pipes are regarded as resistant to chemical attack for most applications their resistance to attack by acids or alkaline solutions may be checked in special circumstances. In contrast, joint assemblies which commonly include components made of rubber, polyurethane or polypropylene, are required to be tested for resistance to chemical attack from a range of acid and alkaline solutions. In addition, these components are also required to be tested for both their short- and long-term thermal stability.

Standard tests on hollow clay pot flooring General Beam and block flooring in the UK consists of a series of parallel reinforced or prestressed concrete beams, usually with the profile of an inverted T or similar and between which precast concrete blocks are placed. The blocks may be autoclaved, aerated or aggregate concrete and are the same as those used for walling. A concrete screed is cast on to the beam and block system. In mainland Europe, there is a much greater variety in the types of block and a draft European standard was issued in 1997 (Committee for European Standardisation, 1997). The standard deals with blocks of all types to be used with precast concrete beams and the requirements for the blocks depend on whether they are considered to form a structural part of the finished floor. For example, if polystyrene blocks are used and so provide what we in the UK would regard as permanent formwork to the in situ concrete, the requirements are lower than if clay or concrete blocks are used. The general shape of the blocks is as shown in Fig. 2.27 and the manufacture is required to declare the dimensions shown. The only limiting dimension is that of the breadth of the flange bf, which sits on the beam and this should not be less than 20 mm if the blocks are being considered as loadbearing and 15 mm if only partially so. The blocks may be designed to span parallel or perpendicular to the beams and the top flange of the clay block is required to be deeper than 40 mm, see Fig. 2.28. There needs to be provision for grouting any voids in the flange of blocks acting parallel to the beams. The minimum thickness of the clay elements of the flange is 7 mm. Compliance with the standard is very much related to, in the case of strength measurements, test results from a sample of three specimens at the start of production or from a much larger sample taken over a period during production. Tests are required for punching bending strength and where relevant longitudinal compressive strength. An investigation of some of the test methods have been carried out albeit for concrete infilled blocks and this has led to some recommendations for their development (Sparks and Harris, 1998) – 91 –

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Figure 2.27 General outline shape of flooring block: h, block depth; hf, height of the block rebate; L block length; l, block width; bf, breadth of the block rebate.

transverse blocks

longitudinal blocks

Figure 2.28

Limitations on upper flange thickness (dimensions in mm).

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Punching bending test The punching bending test is a simple test in which load is applied steadily through a hardwood loading block 50 mm square, until failure occurs (see Fig. 2.29). Compared with many the test is fairly ill defined and the location or the loading block is determined by trail and error until the weakest area of the block is located.

50

Load P

50 50

Adjustable support

Fixed support

Figure 2.29

Punching bending test (dimensions in mm).

Treatment of the results is complex. The manufacturer is required to declare the block class in relation to Table 2.33. If tests are carried out at the start of production the results of three tests are used. If the average value is greater than PKN + 0.2 times the Table 2.33 value for the class concerned, where PKN is the declared value, then the first of two criteria is satisfied. If the lowest individual value is greater than PKN – 0.2 times the Table 2.33 value for the class concerned then the second criterion is satisfied. If both criteria are satisfied the declared value is confirmed. Table 2.33 Type of block

Non-resisting Semi-resisting Resisting

Block classes: punching bending strength Strength classes

Characteristic punching-bending strength (5% quartile) PKN 1. 5 kN 1. 5 kN 3 kN 1. 5 kN 3 kN

A B A B

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During production the results of over fifteen tests over the period in question are used. To satisfy the first criteria the average result must exceed PKN + 1.48σ where σ is the standard deviation results from at least 35 tests over a three month period immediately preceding the period to which the 15 results relate. However, this standard deviation can only be used if the standard deviation of the fifteen results is between 0.63 and 1.37 of it. If not, a new standard deviation must be determined from at least 35 results. The second criteria is satisfied in the same way as at the start of production except that it is now the lowest individual result from the 15 tests which is taken.

Longitudinal compression test When the manufacturer specifies a longitudinal compressive strength in the case of blocks to be used in a floor system where the longitudinal strength is utilised, this needs to be greater than 20 N/mm2. The test is carried out on an overall rectangular cross section at least 200 mm in width and the height in the test machine (its length in use) is to be greater than 170 mm. The standard allows the removal of any adhering pieces of clay and requires the block surfaces to be parallel to within 1°. The block may be capped with cardboard, mortar or sulfur or ground. This specification is really too loose, as the results from the different surface treatments will be quite different and cannot be taken into account by the floor designer. The area to be taken as the cross sectional area is the actual area of solid material although no method of measurement is given, however methods such as paper indentation are used for masonry units (Committee for European Standardisation, 1998). The confirmation that the Longitudinal Compressive Strength is being met over a period is similar to that for the Punching Bending Test in its formulation.

Dimensions The standard indicates where dimensions should be measured to within ± 0.5 mm and that the maximum tolerance on the declared value is 5 mm. A graduated threedimensional trisquare is recommended to be used to check the breadth and depth of the rebate, which sits on the beams.

Moisture expansion The test method for moisture expansion is very simply defined. Seven bars are cut from the inner ribs of blocks at least 30 days after manufacture. The following steps are then taken: 1 Measure the bars (L1) 2 Raise the temperature in an oven at 50°C/hour until 600°C is reached and leave the bars for 4 hours, let them cool for at least 20 hours and remove them when their temperature is less than 70°C. Measure the bars (L2) – 94 –

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3 Place the bars in boiling water for 5 hours, then allow them to cool. Measure the bars (L3). Prior to each measurement, the bars are kept in a climate chamber at 20°C ± 2°C and relative humidity of between 50% and 65% for a prescribed period. The conventional moisture expansion is the average value of (L2–L3). The contraction caused by the oven heating is (L1–L2), if this is negative it is taken as zero and the potential moisture expansion is taken as ((L2–L3) – (L1–L2))/L1.

Fracture energy At least once a year and when the source of clay changes, the manufacturer is required to carry out a fracture energy test. The test is a simple three point bending test on a rib taken from a block which has a pre cut notch one-fifth of the ribs depth at mid-span, see Fig. 2.30. Load is applied so that the crack opens at a rate of 1µm per second until the specimen fails. The area under the load deflection curve is the energy used in breaking the specimen and the fracture energy is the ratio of this energy to the un-notched volume of the specimen: (b × (h–a)).

Figure 2.30

Fracture energy test.

Other tests Thermal expansion Although thermal expansion of brickwork is acknowledged as an effect that needs to be considered in design, it is rarely measured by test. A designer will usually use tabulated values for the coefficients of thermal expansion of brick – 95 –

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and mortar and use a composite value from the proportion of each material in the direction of interest, usually the vertical or the horizontal. In most cases this approach will be adequate and any refinement would not be justified given the likely accuracy of the assumption about the temperature change to be considered as being relevant. Consequently no Standard Test has been developed. However, there are circumstances where further investigation is warranted as was the case for Winterton House, a multi-storey steel frame building which was reclad using off the frame brickwork (Bird, 1996). In this case it was essential to relieve the original frame of some additional load which was introduced by increased amounts of concrete in the floors. A steel space frame was placed on the top of the brickwork and jacked vertically against it, ties from the frame fixed to the columns of the original steel frame transferred load from the frame into the brickwork (see Figs 2.31 and 2.32). Winterton House is 26 storeys high and hence there was a great deal of interest in vertical movements of the brickwork due to moisture, creep and thermal effects as these would affect the efficiency of the load transfer. As part of the research into the brickwork properties to aid the design process an investigation into the thermal expansion properties was carried out.

Figure 2.31

Winterton House

Figure 2.32 Ties from the frame fixed to the columns of the original steel frame transferred load from the frame into the brickwork. – 96 –

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Figure 2.33 The test arrangement.

Two wall panels each 1130 mm long, 545 mm high and 215 mm thick were placed on either side of an electric oil filled radiator with a nominal air gap between the wall and the radiator. The top, bottom and sides of each wall were sealed with 50 mm thick thermal insulation. The test arrangement is shown in Fig. 2.33. The temperature of the radiator was increased and decreased in increments. The resultant changes in temperature of the brickwork were measured using thermocouples and the corresponding brickwork strains were measured both horizontally and vertically using a Demec strain gauge. – 97 –

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As part of this investigation, creep measurements were being made on brickwork specimens and a control wall 670 mm long, 1930 mm high and 230 mm thick was being monitored in a constant temperature environment. When the creep experiment had been completed, the opportunity was taken to measure thermal movement on this relatively large specimen. The temperature in the room was reduced by 11°C over a 25-day period and then raised back to its normal operating temperature. The thermal inertia of the room meant the change was slow and although the temperature range was relatively small vertical and horizontal movements were measured enabling another estimate of likely movement. The coefficient of thermal expansion of individual bricks was also measured (BSI, 1990) and that together with a tabulated value for mortar (BSI, 1985) were used to make another estimate. The results of the approaches are given in Table 2.34. Table 2.34

Thermal expansion coefficients from the three test methods Thermal expansion coefficient × 10–6/ºC

Test method

Horizontal

Vertical

Small panel Wall Brick

10.4 7.1 7.8

7.2 7.6 8.2

Brick measured value: 7.6 × 10–6/ºC. Mortar tabulated value: 12 × 10–6/ºC.

With the exception of the result from the small panel in the horizontal direction, the results are all close together and indicate little difference between horizontal and vertical expansion. This was very much an ad hoc investigation on one type of brickwork, but does suggest that the individual brick approach is probably sufficient in those cases where one might want to do more than estimate a relevant value from the tabulated range (4 – 8 × 10-6/°C).

Tensile testing Tensile testing of bricks is not carried out very often and is difficult to do. However, when design of brickwork to resist shear forces is carried out using DDENV1996-1-1, the characteristic shear stress is not allowed to exceed 0.065fb, where fb is the normalised compressive strength of the bricks and this is an estimate of their tensile strength. This limitation is an acknowledgement of the fact that shear failure can occur by tensile cracking through the units. In an attempt to check whether the limitation proposed was satisfactory for UK units, a programme of work was carried out at CERAM (Edward, 1993). The initial phases of the work concentrated on achieving the axial loads and failures within the – 98 –

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specimens rather than at the adhesives used to attach the specimens to end plates in the machine. The arrangement, which was eventually used, is shown in Fig. 2.34. This programme of work did not lead to definitive conclusions as to the appropriateness of the 0.065 multiplier referred to above but did make some progress in the development of a reliable test method. The degree of articulation that was provided although complicated was necessary to ensure as axial a load as possible was applied. A minimum of five specimens is probably acceptable although more replicates would probably improve further the confidence in the test. It was demonstrated that a loading rate of greater than 20N/s was likely to cause a tearing away of the end plates. Clearly more work needs to be done to demonstrate a completely reliable test method as achieving failure wholly within the test specimen was difficult especially with the stronger units although some of the important parameters have been investigated.

Figure 2.34

Articulation of the tensile loading rig.

Some further considerations are discussed in Chapter 3 by de Vekey in relation to bond strength testing. – 99 –

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References American Society for Testing and Materials 1983. Specification for building brick (solid masonry units made from clay or shale), ASTM C62–83. Beardmore, C. and Barnes, J.D. 1997. ‘The abrasion resistance of clay pavers: an assessment of two test methods’, CERAM Research Paper 809. Beardmore, C. and Barnes, J.D. 1997. ‘The abrasion resistance of clay pavers: an assessment of two test methods’, Tile and Brick International 13(5). Beardmore, C. and Ford, R.W. 1986. ‘Winter weather records relating to the potential frost failure of brickwork’, British Ceramic Research Limited, Technical Note TN371. Beardmore, C. 1999. ‘Use of recycled aggregates as clay pipe bedding’, CERAM, Research Paper 815. Beardmore, C. 1989. ‘Winter weather records relating to the potential frost failure of brickwork’, British Ceramic Research Limited, Research Paper RP 781. Belgian Standards (NBN) 1986. Facing Bricks, B23–002. Bird, B. 1996. ‘Watney Market Estate: Winterton House and Gelston Point’, Masonry International 10(2), 41–5. Bland, C.E.G. and Picken, R.N. 1973. ‘The strength of vitrified clay pipes on minimum beddings’, Public Health Engineer (5). Bowler, G.K. and Winters, N.B. 1997. ‘Investigation into causes of persistent efflorescence in masonry’, Masonry International, 11(1). Brennan, G. 1978. ‘A test to determine the bending moment resistance of rigid pipes’, TRRL Supplementary Report 348. British Standards Institution: various dates, BSEN ISO 10545 Parts 1–16, Ceramic Tiles – Test Methods. British Standards Institution (Various dates) Wall and Floor Tiling, Parts 1–5, BS5385. British Standards Institution 1974. Specification for Clay Bricks and Blocks, BS3921. British Standards Institution 1978. Specification for Calcium Silicate (Sandlime and Flintlime) Bricks, BS187. British Standards Institution 1980, 1997. Specification for Adhesives for use with Ceramic Tiles and Mosaics, BS5980. British Standards Institution 1981. Precast Concrete Masonry Units. Specification for Precast Concrete Masonry Units, BS6073, Part 1. British Standards Institution 1981. Specification for Vitrified Clay Pipes, Fittings and Joints, BS65. British Standards Institution 1985. British Standard Specification for Clay Bricks, BS3921 (amended 1995). British Standards Institution 1985. Specification for Clay and Calcium Silicate Modular Bricks, BS6649. British Standards Institution 1986. Clay and Calcium Silicate Pavers for Flexible Pavements. Part 1: Specification for pavers, BS6677. British Standards Institution 1986. Clay and Calcium Silicate Pavers for Flexible Pavements. Part 2: Code of practice for the design of lightly trafficked pavements, BS6677. British Standards Institution 1986. Clay and Calcium Silicate Pavers for Flexible Pavements. Part 3: Method for construction of pavements, BS6677. British Standards Institution 1986. Method for Determination of Polished Paver Value of Pavers, DD155. British Standards Institution 1987. British Standard Precision of Test Methods. Guide to the determination of repeatability and reproducibility for a standard test method by inter-laboratory tests, Part 1: BS5497.

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British Standards Institution 1989. Testing Aggregates: Methods for Determination of Mechanical Properties, Part 114: BS812. British Standards Institution 1989, 1995. Ceramic Tiles, Terrazzo Tiles and Mosaics, BS8000 Section 11.1. British Standards Institution 1990. Methods of Testing Refractory Materials, British Standards, 1902, Section 5.3. British Standards Institution 1990. Specification for Dimensions for Bricks of Special Shapes and Sizes, BS4729. British Standards Institution 1991. Vitrified Clay Pipes and Fittings and Pipe Joints for Drains and Sewers, Part 1, Requirements, BSEN295. British Standards Institution 1991. Vitrified Clay Pipes and Fittings and Pipe Joints for Drains and Sewers, Part 3, Test Methods, BSEN295. British Standards Institution 1992. Code of practice for use of masonry, Part 1: Structural use of unreinforced masonry, BS5628–1. British Standards Institution 1994. Clay Roofing Tiles for Discontinuous Laying – Flexural strength test, BSEN 538. British Standards Institution 1994. Clay Roofing Tiles for Discontinuous Laying – Part 1, Impermeability Test, BSEN 539-1. British Standards Institution 1995. Vitrified Clay Pipes and Fittings and Pipe Joints for Drains and Sewers, Part 7: Requirements of vitrified clay pipes and joints for pipe jacking, BSEN 295. British Standards Institution 1996. Ceramic Wall and Floor Tiles Parts 1–23, BS6431. British Standards Institution 1997 onwards. Pavements Constructed with Clay, Natural Stone and Concrete Pavers, BS7533. British Standards Institution 1997. Clay Roofing Tiles for Discontinuous Laying – Determination of geometric characteristics, BSEN 1204. British Standards Institution 1998. Method for Determination of Polished Paver Value (PPV), BS7932. British Standards Institution 1998. Methods of Test for Masonry Units, Part 3: Determination of net volume and percentage voids for clay masonry units by hydrostatic weighing. BSEN 772–3. British Standards Institution 1998. Clay Roofing Tiles for discontinuous laying – Product Definitions and specifications, BSEN 1304. British Standards Institution 1998. Clay Roofing Tiles for Discontinuous Laying – Part 2: Test for frost resistance, BSEN 539-2. British Standards Institution 1998. Methods of Test for Masonry Units, Part 2: Determination of percentage area of voids in aggregate concrete masonry units (by paper indentation), BSEN 772–2. British Standards Institution 1999. Ceramic Wall and Floor Tiles. Part 14: Method for determination of resistance to deep abrasion – unglazed tiles, BSEN 6431. British Standards Institution 1999. Methods of Tests of Mortar for Masonry. Determination of flexural and compressive strength of hardened mortar, BSEN 1015-11. British Standards Institution 2000. Part 1: Code of Practice for the Design of Straight Stairs, BS5395. British Standards Institution 2000. Concrete Paving Blocks. Part 1: Requirements and test methods, BS6717. British Standards Institution 2000. Methods of Test for Masonry Units, Part 1: Determination of compressive strength, BSEN 772–1.

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British Standards Institution 2000. Methods of Test for Masonry Units, Part 7: Determination of water absorption of clay masonry damp proof course units by boiling in water, BSEN 772–7. British Standards Institution 2000. Methods of Test for Masonry Units, Part 11: Determination of water absorption of aggregate concrete, manufactured stone and natural stone masonry units due to capillary action and the initial rate of water absorption of clay masonry units, BSEN 772–11. British Standards Institution 2000. Methods of Test for Masonry Units, Part 13: Determination of net and gross dry density of masonry units (except for natural stone), BSEN 772–13. British Standards Institution 2000. Methods of Test for Masonry Units, Part 16: Determination of dimensions, BSEN 772–16. British Standards Institution 2001. Adhesives for Tiles – Definitions and Specifications, BSEN 120004. British Standards Institution 2001. ‘British Standard Code of Practice for the use of Masonry’, Materials, Components, Design and Workmanship, BS 5628, Part 3. British Standards Institution 2001. Slabs of natural stone for external paving: requirements and test methods, BSEN 1341. British Standards Institution 2001. Setts of natural stone for external paving: requirements and test methods, BSEN 1342. British Standards Institution 2001. Kerbs of natural stone for external paving. requirements and test methods, BSEN 1343. British Standards Institution 2002. Clay pavers: requirements and test methods, BSEN 1344. British Standards Institution 2002. Methods of Test for Masonry Units. Determination of active soluble salts content of clay masonry units, BSEN 772–5. British Standards Institution 2002. Pendulum Testers. Part 1 Specification; Part 2 Method of Operation; Part 3 Method of Calibration, BS7976. British Standards Institution 2003. Method for Determination of Unpolished and Polished Pendulum Test Value of Surfacing Units, BS7932. British Standards Institution 2003. Specification for clay masonry units, Part 1: clay masonry units, BSEN 771–1. British Standards Institution 2003. Concrete Paving Blocks: Requirements and Test Methods, BSEN1338. British Standards Institution 2003. Concrete paving flags: requirements and test methods, BSEN1339. British Standards Institution 2003. Concrete kerb units: requirements and test methods, BSEN1340. British Standards Institution 2003. HD Clay Bricks – Guide to appearance and site measured dimensions and tolerance. Publicly Available Specification, PAS 70-2003. Butterworth, B. 1964. ‘The frost resistance of bricks and tiles – A review’, Journal of the British Ceramic Society, 1(2), pp.203–23. Commission of the European Communities 1993. ‘Freeze/Thaw Durability of Concrete Block Paving’, BRITE-EURAM Programme, Brite-project P-2085. Committee for European Standardisation 1999. ‘Methods of test for masonry units, Part 22: Determination of freeze/thaw resistance of clay masonry units’, prEN 772–22. Department of Transport 0000. Design Manual for Roads and Bridges, Vol. 7: Part 3. ‘Pavement Design and Maintenance; Pavement Skidding Resistance, Maintenance and Assessment’, DoT. DOE Partners in Technology 1997. ‘Brick properties, Comparative testing to British and draft European standards’, DOE Research Contract, Ref: 39/3/325 (CC 912). Edgell, G.J. 1980. ‘Methods used in Testing Clay Pipelines in Trenches’, B. Ceram R.A., Tech Note, TN300.

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Edward, P. 1993. ‘Tensile testing of masonry units’, Report from CERAM Building Technology to the Structural Masonry Steering Committee of the Department of the Environment, SMSC93(6). European Committee for Standardisation 1996. Clay pavers and complementary fittings for flexible paving: requirements and test methods, June, prEN 1344. European Committee for Standardisation 1996. Clay pavers and accessories for rigid paving: requirements and test methods, June, prEN 1345. European Committee for Standardisation 1997. Clay pavers and accessories for rigid paving: requirements and test methods, prEN 1345. European Committee for Standardisation 1999. Method of test for masonry units, Part 22: determination of freeze/thaw resistance of clay masonry units, November, prEN 772–22. European Committee for Standardisation 2003. Method of determination of unpolished and polished slip/skid resistance values, DDENV12633. European Committee for Standardisation 1997. ‘Precast concrete products – blocks for beam and block floor systems’, Draft Standard 229010–2. French Standards (AFNOR) 1975. ‘Hollow terracotta bricks and P23–304 facing clay bricks’, AFNORP13–301. French Standards (AFNOR) 1975. ‘Hollow terracotta bricks, P13–301 and facing clay bricks’, P23–304. German Standards (DIN) 1981. Testing the Frost Resistance of Clay Pavers, August. DIN 18–503. German Standards (DIN) 1986. Testing the Frost Resistance of Facing Bricks and Clinker Blocks Part 1: Freezing of single bricks on all sides and Part 2: Freezing of test walls on one side, 52–252. German Standards Institution 1992. Determination of Anti-slip Properties – Wet, Barefoot, DIN 51097. German Standards Institution 1992. Determination of Anti-slip Properties – Shod, DIN 51130. GLC 1971. ‘Slip resistance of floors, stairs and pavings’, GLC Bulletin No. 143 (second series), Item No. 5. Highways Authorities 1998. Highways Authorities Standard Tender Document 1998. Issue 8: Notes for Guidance. Hillier, R. 1981. Clay that Burns: A History of the Fletton Brick Industry, London Brick Co. Ltd., London. Hodgkinson, H.R. 1976. ‘The use of sound absorbing clay products’, B. Ceram. R.A. Tech. Note TN257. Honeyborne, D.B. 1963. ‘Frost action on natural stone’, Journal of the British Ceramic Society, 1, pp.229–30. International Standards Organisation 1998. Ceramic Tiles – Definitions, classification, characteristics and marking, ISO 13006: modified. James, D.I. 1989. ‘Assessing the pedestrian slip resistance of clay pavers’, British Ceramic Proceedings, November, 44, pp.49–60. Lees, G. 1987. Development of an Accelerated Test for Clay Pavers, The Brick Development Association, Winkfield, Berkshire. Lomax, J. and Ford, R.W. 1988. ‘A method for assessing the long term moisture expansion characteristics of clay bricks’, Proc. 8th International Brick and Block Masonry Conference, 19–21 September, Vol.1. Netherlands Standards (NEN) 1986. Stony Building Materials – Determination of Frost Resistance – single sided freezing in a fresh water environment, NEN2872.

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Netherlands Standards (NEN) 1986. Stony Building Materials – Determination of Frost Resistance – single sided freezing in a fresh water environment, Part 1: Clay masonry units, NEN 2872, prEN 771–1. Peake, F. and Ford, R.W. 1981. ‘A comparison of the vacuum and boiling methods for measuring the water absorption of bricks’, British Ceramic Research Association Technical Note No.322, July. Peake, F. and Ford, R.W. 1988. ‘Brick freeze/thaw damage: Site and lab. Results compared’, Building Technical File, 23, pp.21–4. Peake, F. and Ford, R.W. 1988. ‘Calibration of the B.C.R.L. panel freezing test against exposure site results’, CERAM Research Paper. Picken, R.N. 1977. ‘an investigation of a proposed test for the bending moment resistance of clay pipes’, B. Ceram R.A., Special Publication 92. Picken, R.N. 1980. ‘Bending moment and crushing load measurements on vitrified clay pipes in the ground – Preliminary Report, B. Ceram R.A., Tech Note, TN301. Rapra Technology Limited 2000. ‘The measurement of floor slip resistance’, Guidelines Recommended by the UK Slip Resistance Group, June, Issue 2. Smith, R.G. 1974. ‘Expansion of unrestrained Fletton brickwork’, Building Research Establishment, Current Paper 92/74, Oct. Sparks, W. and Harris, D.J. 1998. ‘An assessment of the new ISO and CEN Test methods for beam and block floors’, Proc. British Masonry Society No 8, 73–6. Taylor Hobson, 2 New Star Road, Leicester, LE4 7JQ. The British Clay Worker 1951–1952. The Directory of British Clayworkers. The British Clay Worker, London. West, H.W.H. 1970. ‘The M–G Plank: a storey-height ceramic unit’, Proc. SIBMAC, Stoke-on-Trent, p. 56. West, H.W.H., Ford, R.W. and Peake, F. 1984. ‘A panel freezing test for brickwork’, Transactions of the British Ceramics Society, 83, pp.112–5. West, H.W.H., Hodgkinson, H.R. and Davenport, S.T.W. 1968. ‘The performance of walls built with wire cut bricks with and without perforations’, B. Ceram. R.A. Special Publication No 60. West, H.W.H., Hodgkinson, H.R., Webb, W.F. and Beech, D.G. 1972. ‘The compressive strength of walls built of frogged bricks’, B. Ceram. R.A. Tech. Note TN194. West, H.W.H., Picken, R.N. and Beech, D.G. 1992. ‘Investigation of pipes laid directly on the trench bottom’, B. Ceram R.A., Special Publication No 75.

Further reading Butterworth, B. 1952. ‘The frost resistance of fired clay – A perennial problem’, Transactions of the IIIrd International Ceramic Congress, pp. 121–30. Everett, D.H. 1961. ‘The thermodynamics of frost damage to porous solids’, Transactions of the Faraday Society, 57, pp.1541–51. Litvan, G.G. 1980. ‘Freeze-thaw durability of porous building materials’, ASTM Spec. Tech. Publ., 691, pp.455–63. Ritchie, T. 1972. ‘Freeze-thaw action on brick’, NRCC Building Research Paper 555. Robinson, G.C. 1984. ‘The relationship between pore structure and durability of brick’, American Ceramics Bulletin, 63(2), pp.295. Van der Klugt, L.J.A.R. 1988. ‘Frost testing by uni-directional freezing’, British Ceramics Transactions & Journal, 87(1), pp.8–12.

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3 Element testing

Compressive strength General Brickwork is generally known as a structural material, which is good in compression and historically that characteristic has been exploited the most. Even today, when we are more interested in other characteristics than hitherto, for example, flexural strength for the design of laterally loaded panels it is still compressive strength that is of major interest. Although it was the case that engineers did some ad hoc testing at construction sites it seems that the first organised programme of testing was carried out by a committee of the American Society of Civil Engineers (ASCE) in 1887–1888. A few years later a committee of the Royal Institute of British Architects conducted its own programme of testing (Thomas, 1953). From the 1920s, a considerable amount of testing was carried out at the Building Research Station and this was reported in a comprehensive way to the Institution of Civil Engineers in 1950 (Davey and Thomas, 1950). A great deal of the earlier work was carried out on 9-inch square piers, some 3 feet high, although the 1950 report covered walls and piers of a wide range of shapes and sizes. From a structural point of view we have tried in the UK to develop economic masonry buildings and hence moved away from massive structural elements to more slender ones. Certainly in the case of cavity walls our interest is in the performance of two slender, invariably stretcher bond leaves, albeit tied together. It is probably this trend together with some influence from the continent as efforts were made to produce International Standards that has led to the situation that unless otherwise specified for particular projects or circumstances it is now more usual to test small single leaf test specimens.

Specimen format The compressive strength of clay brickwork is commonly determined on specimens, which are short enough that the result would not be influenced by slenderness effects but tall enough that there is sufficient brickwork remote enough from the loading platens so as not to be influenced by frictional restraint at the platens. The effect of the platens in resisting any lateral movement is important as the prime mode of failure – 105 –

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is by the specimen splitting vertically and this is attributed to the lateral expansion of the mortar in the bed joints which causes the bricks to be in tension horizontally. The effect of lateral restraint at the platens is therefore to enhance the strength of the brickwork and hence it is important for the specimens to be large enough that there is some brickwork not influenced by this restraint. The latest consensus on specimen size is given in Table 3.1 which is taken from BSEN 1052-1 (BSI, 1999). Small specimen sizes for testing the compressive strength of

Table 3.1 masonry Face size of unit

Masonry specimen size

lu(mm)

hu(mm)

Length (ls)

Height (hs)

≤300

≤150 >150 ≤150 >150

≥(2 x lu)

≥5hu ≥3hu ≥5hu ≥3hu

>300

≥(1.5 x lu)

Thickness (ts) ≥3ts and ≤15ts and ≥ ls

≥tu

Note: Subscript u refers to the unit and s to the specimen.

The table is necessarily complex as it needs to cover all kinds of masonry unit, i.e. calcium silicate, aggregate concrete, manufactured and natural stone as well as clay and all of the various shapes and sizes of unit used throughout Europe. For the traditional size of clay brick used in the UK the usual interpretation of the table gives a specimen five courses high with a thickness the same as that of the unit and a length of two bricks plus that of the cross joint between them. If the testing is in relation to a particular project the specimen would be built with a bonding pattern and any other specifications as for the project. It is more common for testing to be related to research or the development of a database and the specimens are built in stretcher bond. BSEN 1052-1 was primarily drafted to support the Eurocode for Masonry, DD ENV 1996-1-1 (BSI, 1996), which bases the design strength on a characteristic strength of masonry which is the strength below which no more than 5% of test results would be expected to fall. Hence, a number of specimens are required to be tested in order that the results may be treated statistically. The test method requires a minimum of three specimens to be tested and the characteristic value is taken as the mean divided by 1.2 or the lowest individual result whichever is the lower. If more than five specimens are tested then the characteristic value is taken as the 5% fractile value, based on a 95% confidence level. In the UK testing at this scale has usually been based on ten specimens, as is the case for flexural strength and a lognormal distribution of test results assumed (see Table 3.2). However, the tradition in a number of countries in mainland Europe was to test three specimens only and hence the inclusion of that possibility and the simple calculation of a lower bound strength – 106 –

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to be taken as the characteristic value rather than using a statistical calculation which would depress the result and be likely to make it unrealistically low and lead to excessively conservative designs. Table 3.2

Specimen sizes for testing the flexural strength of masonry

Direction

hu(mm)

b(mm)

Additional conditions

Flexural strength for a plane of failure parallel to the bed joints Flexural strength for a plane of failure perpendicular to the bed joints

Any

≥400 and ≥1.5 lu ≥240 and ≥3hu ≥1000

Minimum 2 bed joints within l2 Minimum 1 head joint every course within l2 Minimum 1 bed joint and minimum 1 head joint within l2

≤250 >250

In the UK, the structural design of brickwork is usually in accordance with BS5628 Part1 (BSI, 1992). The philosophy of the design for compressive strength is based on the database of strength determined on storey height walls and figures are given for the characteristic strength of walls. The approach in the Code is based on the concept that most design is of storey height walls and hence the most relevant data is that from wall tests. Nevertheless the Code was calibrated against its predecessor and hence the complete database includes the results from the piers tested at BRS. The ISO Code of Practice (ISO, 1999) for Masonry design refers to the derivation of compressive strength by test using ISO/DIS 9652-4 (International Standards Institution, 1993) and small brickwork specimens and storey height wall specimens were both included. During the last 15 years the philosophy of testing has developed and considerable research has been carried out leading to the method incorporated in BSEN 1052-1. However, although there are differences in the published documents relating to compressive strength it is expected that BSEN 1052-1 will become the generally used procedure. Testing storey height walls is expensive and consequently, although there will be circumstances where it is appropriate, smaller specimens are favoured and enable more tests to be done and hence through greater confidence lower safety factors may be appropriate for design, see Fig. 3.1. In particular, when reinforcement is used in brickwork, the bonding pattern may be unusual in order to accommodate the steel in pockets or voids and it is also less rare for the force in the brickwork to be other than normal to the bed face of the bricks. This fact is acknowledged in BS5628 Part 2 (BSI, 1995) which gives examples of test specimens together with the guidance on the use of a lognormal distribution of the results to calculate a characteristic strength. The specimen formats are reproduced in Fig. 3.2. – 107 –

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Grout Grout

Figure 3.1 Compression test on brickwork.

Figure 3.2 Typical compressive strength specimens for reinforced brickwork.

In most circumstances, it is the compressive strength of the brickwork normal to the bed faces of the unit, which is of interest to the designer. However, in some structural situations the force may be other than normal to the bed face. Common examples are in a beam where the tensile force may be resisted by reinforcement and the compressive force would be normal to the header end of the unit or in a soldier course in ordinary unreinforced loadbearing brickwork. The strength of individual bricks when loaded normal to the header end or stretcher face of the brick can be a very small fraction of the strength normal to the bed face. In the case of perforated bricks the amount can be as low as 13% or 14% the reduction being largely due to the change from an efficient structural section to a less efficient one. However, when built into the brickwork, the reduction of strength is much reduced. The data available was not all from the standardised specimens described earlier and was from a number of sources, however when reviewed (Edgell, 1990) it was demonstrated that the relationship between the strength of brickwork to that of bricks included in BS5628 Part 1 could be applied to bricks tested on end or through the stretcher face in a conservative way. Similarly the approach in BS5628 Part 2, which is to reduce the strength from that normal to the bed joint by two-thirds was shown to be reasonable. A considerable amount of work was done to develop the 9-inch brickwork cube as a quality control test for brickwork. This work, which has been referred to in more detail elsewhere, has not been used for the determination of compressive strength for design purposes because of the confining effects of platen restraint and the unrepresentative nature of the specimen, but has proved to be useful in the appraisal of existing structures.

Construction of test specimens and test procedure When testing is in relation to a specific project the project specification should be – 108 –

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followed. It is important that the bricks supplied to the laboratory are representative of those to be used on site although clearly they will not derive from a representative sampling plan of the bricks on site. It is therefore important that a sample of individual bricks supplied to the laboratory are tested for compressive strength. Subsequently if there is a question over whether the bricks supplied to site are the same as those which had been supplied to an earlier testing programme in the laboratory a check on the brick strengths will show if there is a difference. If the specification indicates any conditioning of the bricks prior to laying this should be done but in the absence of any it would be normal to ensure that the bricks do not absorb too much water from the mortar. This is achieved by ensuring that the Initial Rate of Water Absorption does not exceed 1.5 kg/m2 per minute, if necessary, by ‘docking’ the bricks in water for a few minutes prior to laying alternatively a water retaining additive may be used in the mortar. The Initial Rate of Water Absorption is measured by placing the bed face of the brick in water and measuring the increase in weight over a period of 1 minute, the procedure is given in BSEN 772-11 (BSI, 2000). The conditioning of the bricks is less critical in relation to compressive strength than to flexural or shear strength however it is important in all cases to record what is done so that any anomalies can be explained and understood. It is also important for similar reasons, to characterise the fresh and hardened mortar and as a minimum the flow value and air content of the fresh mortar should be determined and the compressive strength of specimens of hardened mortar should be determined at the same time as the brickwork is tested. The specimens should be built on a flat horizontal surface and it is convenient to use a flat metal plate on both the top and bottom of the specimen, the top one can be levelled using a spirit level and this provides specimens which are ready to load into the test machine. In some cases, the capping plate may be placed at the time the specimen is loaded into the test machine, in which case the capping medium usually mortar made with high alumina cement or a gypsum plaster must be allowed sufficient time to harden and to achieve a strength at least that of the brickwork mortar. Particular attention should be paid to the flatness of the capping plates where units greater than 100 mm in width are tested as any out of planeness can depress the results (Edgell and Saunders, 1990). In general, any capping mortars should be as thin as possible as thick layers can introduce unwanted lateral tension forces and reduce results (Shrive, 1986). It is important that the specimens do not dry out too quickly as this will affect the brickwork strength. This can be achieved in a curing room with a controlled atmosphere but a simple effective way is to cover the specimens with polyethylene sheets for the first three days of the curing period. The curing period should be as specified and there are two common approaches, either to regularly test the compressive strength of mortar specimens until the measured strength is at least the specified strength and does not exceed it by a prescribed tolerance or to cure the specimens for a specified period, usually 28 days and to record the mortar strength which is achieved. In some cases the specimens have been covered for the whole 28 days and although the magnitude – 109 –

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of the results is not very different the 3-day covering procedure leads to less variable results (Edgell et al., 1990). At the end of the curing period, the specimens are placed in the test machine. The machine should be capable of applying a uniform load to the whole of the surface of the specimens with permissible repeatability and errors prescribed in BSEN 1052-1. If the steel plates referred to above are used on the top and bottom of the specimen it is unlikely that any further means will be needed to ensure full contact between the machine platens and the specimens although if necessary a thin compensating layer should be used. It is usual for the machine to have one platen which has a ball seating so that it can accommodate any lack of parallelism between it and the specimens but it should then lock either manually or automatically. If it is required to determine the Modulus of Elasticity of the brickwork, measuring devices should be fixed to the two faces of each specimen. The gauge length should be a multiple of the brick height plus mortar joint thickness so that the result is properly representative of the brickwork. Strains should preferably be measured electronically at three dwell periods during the first half of the loading, the measuring devices may then be removed prior to the specimens being loaded to destruction. The load should be increased steadily and without any shock and the rate is usually adjusted so that failure occurs after 15–30 minutes. This rate enables the tester to observe and record the development of cracks as well as manage any strain measurement. The mean and if required characteristic strengths are determined as described earlier, each strength being simply the maximum load achieved divided by the gross cross sectional area of the specimen. Strengths are normally expressed to the nearest 0.1 N/mm2. If it is required to determine the Modulus of Elasticity the mean strain as measured by the four measuring devices at one third of the maximum load is determined and a secant modulus is calculated to the nearest 100 N/mm2. As is the case for compressive strength it may be necessary to evaluate the Modulus of Elasticity with the force other than normal to the bed joints (Hodgkinson and Davies, 1982).

Flexural strength General The flexural strength of masonry is used in the design of walls to resist lateral loading. In the case of single leaf or cavity walls subjected to wind loading the approach is based upon yield line theory and was first incorporated in a Code of Practice in 1978. Since this publication, there have been numerous discussions about the appropriateness of the approach for what is regarded as a brittle rather than ductile material, however there is considerable experimental data from full size wall tests (West et al., 1975; Haseltine et al., 1975) to suggest that the approach is suitable and a recent review has confirmed that it is probably the most accurate of those available in the world (Haseltine and Tutt, 1999). In order to use the method it is necessary to know the flexural strength of brickwork both parallel and perpendicular to the bed joints and a – 110 –

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method of doing this was incorporated in BS5628 Part 1. Table 3 of BS5628 Part 1 gives values for the characteristic flexural strength in the two directions for clay bricks, which are classified by their water absorption value, in four designations of mortar but alternatively tests may be carried out. In DD ENV 1996-1-1 the primary method of determining the characteristic flexural strength is by carrying out tests in accordance with BSEN 1052-2 (BSI, 1999) and as no agreement could be reached on values which could be used on a Europe wide basis the designer is referred to an evaluation based on the combination of units and mortar. This is not terribly helpful but it is assumed that figures will be available nationally and the National Application Document, which is published with DD ENV 1996-1-1 refers the designers to BS5628 Part 1. The same information is reproduced elsewhere (British Masonry Society, 1997), where it is related to the Europe wide mortar classification rather than the British designations. Flexural strength may also be needed in the design of walls subjected to other than wind load, for example from retained earth and the test method can be used to derive the necessary figures. The test method has been developed and improved over a number of years and this has been especially assisted by the inter laboratory comparisons (de Vekey et al., 1982) In some cases, the flexural strength of masonry perpendicular to a bed joint containing a d.p.c. (damp proof course) is of interest and is covered by DD86, Part 1 (BSI, 1983).

Specimen format Flexural strength is determined on specimens defined according to Table 3.2, which is taken from BSEN 1052-2. The effect of reducing the overlap when changing from stretching bond to quarter bond was a reduction in the bonding strength in the horizontally spanning direction of 7.5%. The reduction in overlap when simulated English bond, formed by alternate courses of stretchers and snap headers, was used was 29% of the strength of stretching bond. Nevertheless all of the values exceeded the guidance in BS5628. Some typical examples are shown in Fig. 3.3. Essentially the specimens relate to bricks of the UK format, concrete blocks of UK format and large units typically Dutch calcium silicate format. The test method is clearly based on that in BS5628 as most of the experience was in the UK but it has been extended to cover a wider range of units than is commonly used in the UK. In the UK the specimens are often referred to as wallettes (West, 1976). As is the case for compressive strength if the tests are being related to a specific project then the bonding and construction would be as specified for the project, more usually stretcher bond is used. There is a limited amount of data on the influence on flexural strength of using different bonding patterns which was derived in relation to the use of single leaf cladding to give the aesthetic appeal of thicker, traditionally bonded constructions (Edgell, 1990). If brickwork containing a d.p.c. is to be tested the d.p.c. is placed in the central bed joint (above the fifth course). – 111 –

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(a) b ≈ 21u and b ≥ 400 mm and hu ≤ 250 mm and more than two bed joints in 12 b ≈ 4hu and b ≥ 240 mm and hu ≤ 250 mm and a minimum of one head joint in 12

(b) b ≈ 1,51u and b ≥ 400 mm and hu ≤ 250 mm and two bed joints in 12 b ≈ 4hu and b ≥ 240 mm and hu ≤ 250 mm and a minimum of one head joint in 12

(c) b ≈ 1,51u and b ≥ 400 mm and two bed joints in 12 b ≥ 1000 mm and h >250 mm and one head joint and one bed joint in 12

Figure 3.3

Typical flexural strength specimens.

Construction of specimens and test procedure The construction of specimens, sampling and testing of units and mortar, conditioning of bricks is all as described for compressive strength testing. It should be noted that the flexural strength is very sensitive to the conditioning used and this must be clearly specified. Although the curing period is the same the specimens are prevented from drying out too rapidly by, for example close covering for the whole of the curing period. The specimens are also precompressed during the curing period. The specification for the precompression for a uniformly distributed vertical stress of between 2.5 × 10–3 N/mm2 and 5.0 × 10–3 N/mm2 which can be achieved as described in BS5628 Part 1 by laying three courses of dry bricks on top of the specimen. Where lime based mortars are used the curing conditions and period may need to differ from those described here and specialist advice is necessary. – 112 –

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The specimens are tested in the vertical attitude under four point loading as shown in Fig. 3.4, which generates a region of constant bending moment between the inner loading positions. If the specimen contains a d.p.c. the inner load positions are placed at the middle height of the courses above and below it. It is intended to apply a uniform load across the width of the specimen at each bearing position. This may be simply achieved by using a steel rod, typically 8 mm in diameter within a rubber tube of 10 mm bore and 7 mm wall thickness. There are other approaches, for example by the use of water filled bolsters but this is not so common as the rubber tube and steel rod, which are very convenient to use. It is usual for three of the bearers to be pivoted, so that any out of planeness of the specimens is accommodated. It is important that the base of the specimen is free of frictional restraint and this may be achieved easily by setting them on two layers of polytetrafluoroethylene (ptfe) with grease between them or by the use of ball, needle or roller bearings. There have from time to time been discussions as to whether even this is satisfactory and although theoretically as long as the conditions are standardised that should be sufficient, the body of evidence supporting the use of the yield line approach is based largely on the ptfe based specimens (West, 1976). There is some guidance in BSEN 1052-2 on the locations of the bearers so that they are not too close to the ends of the specimens or too close to mortar joints where it is possible that some localised failure might influence the result. (a)

(b)

Figure 3.4 Loading arrangement for flexural strength measurement (Wallette format). The flexural stress in the specimen is assumed to have a linear distribution with the neutral axis at the middle point of its width and the load is applied steadily, without shock such that the stress at the extreme fibre is calculated to increase at a rate between 0.03 N/mm2 per minute and 0.3 N/mm2 per minute. These recommended maximum and minimum rates are quite different but are intended to cover the wide range in flexural strengths that might be achieved in a reasonable test duration which would be 1–2 minutes. Unlike compression testing there is rarely a need to evaluate a modulus of elasticity and this is quite difficult to measure given the fairly low stresses which cause failure and consequently the time of the test can be quite short, however not too short as shock loading yields unrealistically high results. BSEN 1052-2 recommends that at least five specimens are built and tested and the results from five specimens where the failure occurred within the inner – 113 –

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bearings are needed to calculate the characteristic strength, failures which occur outside the inner bearings do so at positions where the bending moment is unknown without further measurements. In the UK it has been traditional to use 10 specimens as recommended in BS5628 Part 1 although for research purposes it has often been the case to use five. BSEN1052-2 permits the use of five in which case the characteristic value is taken as the mean divided by 1.5, which assumes that the coefficient of variation in the results is 20% and effectively limits any depression of the characteristic value caused by odd high and low results from a relatively small sample. The figure of 20% came from a study of the variability of test results from a large number of tests (de Vekey, 1994). However, if the specifier wishes to use a larger number of specimens and use a statistical approach to the results BSEN 1052-2 shows how to do this using an assumed lognormal distribution of the individual flexural strengths. It should be noted that the value which is derived is a lower bound to the strength of the weakest individual joint in the constant bending moment zone of the specimens. The tabulated values in BS5628 were based on lower bounds to the mean strengths of sets of five specimens on the grounds that in walls subjected to wind loads the mean strength in the area of greatest stress would govern the failure and not the weakest individual joint. The approach in BSEN 1052-1 and for that matter the test method in BS5628 are probably conservative but would be more appropriate in situations where flexural tension may be being relied upon at least in part at an individual joint, for example at the base of a boundary or parapet wall.

Shear strength General When brickwork walls are subjected to in plane forces, for example in cross wall structures the designer needs to check against a number of modes of shear failure. If the wall is short in relation to its height it may merely overturn if the vertical load is low. Larger walls may fail by diagonal cracking stepping down through the courses either causing failure of the bricks themselves or more commonly at the brick mortar interface at successive bed and perpend joints. It is also possible that at the base of the wall remote from the loaded edge that units may fail in compression. This last mode is not very likely for typical UK bricks but is potentially possible, certainly for the highly perforated units especially horizontally perforated clay units produced in some countries of mainland Europe, notably France. It is probably true to say that for most buildings in the UK shear is not a critical factor in the design, the same may not be true for some more adventurous structures in, for example, prestressed brickwork. As a result the development of Standard tests at full scale is less well developed than for compression or lateral load resistance although there has been useful research (Edgell et al., 1982; Cavanagh et al., 1986). Consequently at the moment apart from where seismic considerations are important reliance is based on the small-scale element test. – 114 –

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Specimen format The shear strength of a masonry joint is usually described by a Coulomb type equation, i.e. τ = τ0 + µ.σ where τ0 is the initial shear strength which is the strength at zero stress normal to the joint and µ is the coefficient of internal friction. This relationship is generally believed to apply up to a normal stress of about 2 N/mm2. If shear is critical to a particular project and tests are carried out it may be possible to achieve shear strengths in excess of the default values given in design documents, e.g. DD ENV1996-1-1. However, the generally agreed position is that µ could be taken as 0.4. Tests have shown (Greenfield, 1992) that at zero precompression the variability in the results is too high for a standard test consequently the accepted procedure is to use more than one precompression and to plot the Coulomb type relationship which has been shown to apply extremely closely at low stresses. As this test procedure is adopted there is the opportunity to use a measured value of µ rather than the agreed safe value of 0.4. Numerous different types of specimen have been used to try to measure joint strength and a very useful review has been published (Jukes and Riddington, 1997). The difficulty in developing a method is that it is very difficult to ensure a reasonably uniform distribution of both shear stress and normal stress. Most means of applying a shear stress introduce some bending and as a consequence there is variation in the normal stress. As is described in the review a method was developed which applied a non-uniform precompression to the joint, the variation in which was controlled electronically to counter the non-uniformity introduced by the moment introduced by the shear force. This approach while scientifically sound is too complex to be recommended as a standard test. BSEN1052-3 (European Committee for Standardisation, 2002) is based on the use of the triplet test, as shown in Fig. 3.5 where the intention is to apply the shear force as close as is possible to the joints. Clearly even with this arrangement some moment is introduced at the joint but efforts have been made to minimise the effect and it has been demonstrated that consistent results can be produced.

Unit height less than 200 mm

Unit height greater than 200 mm

Format A

Format B Possible saw cut

Figure 3.5 Shear strength specimen format.

Figure 3.6 Shear strength specimen format for large units. – 115 –

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Although for UK brick formats the triplet specimen as shown is to be used there is a difficulty in dealing with large continental units, for example the large Dutch calcium silicate units. As a result there is the option for units whose height exceeds 300 mm to test a couplet whereby some of the faces may have needed to be cut, as shown in Fig. 3.6 and as indicated by the limiting dimensions given in Table 3.3. Naturally the surfaces used for the mortar joint must be original ones and not cut faces. Table 3.3

Dimensions and type of shear test specimens

Face size of unit

Specimen type and dimensions

lu(mm)

hu(mm)

Type according to Fig. 3.1

Dimensions (mm)

≤300 >300 ≤300 >300

≤ 200 ≤200 >200 >200

A A B B

ls = lu ls = 300 h1 = 200; ls = lu h1 = 200; ls = 300

Construction of test specimens and test procedure The construction of the test specimens, sampling and testing of units and mortar, conditioning of bricks is all as described for flexural strength testing. At least nine specimens are constructed with the intention to test three at each of three precompressions so that the Coulomb type equation described earlier can be plotted on a graph. It is however possible that with certain types of unit the failure might be by failure of the unit either in compression or shear and so if this is felt to be a possibility having considered in particular the perforation pattern and percentage it would be prudent to build some extra specimens. The plates which are used to apply the shear forces may be fixed to it with some capping material although it must be borne in mind that this makes careful cleaning of them essential before re-use and a more practical approach is to eliminate the effects of surface irregularity using a soft board material. The load is then applied close to the joint through the steel rollers as shown in Fig. 3.5. The rollers should be at least 12 mm in diameter and there will need to be a means of restraint to stop them rolling out from between the plates, the use of simple metal lugs will achieve this. Three specimens are tested at each of three precompressions the latter being applied through large spreader beams on the bed faces of the outer units these being reasonably stiff so as to achieve as uniform precompression as possible. For units whose compressive strength is greater than 10 N/mm2 it is common to use 0.2 N/mm2, 0.6 N/mm2 and 1.0 N/mm2 as the precompression stresses, for weaker units these values would be halved. The shear stress is increased at a rate of between 0.1 and 0.4 N/mm2/min until failure occurs. – 116 –

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Failure would normally be expected to occur at either the brick/mortar interface and this may be wholly along one bed face or at one bed face for part of the length of the unit and the other for the remainder, or in the mortar itself for the whole length of the joint. While it is conceivable for the failure to be in the mortar for part of the length of the unit and then at an interface in practice this is very rare. It is also possible for failure to be in shear within the unit, parallel to the bed joint. These types of failure are all considered as valid shear failures and are illustrated in Fig. 3.7. Failure may occur in the units either by a splitting or by local crushing or splitting. In the case of weak frogged bricks the failure may often be in the bricks (see Fig. 3.8). If failures occur by these types of unit failure then it is advisable to test further specimens until three valid failures are achieved at each precompression. If this proves difficult to achieve it may be necessary to consider using alternative usually higher precompression levels. The stress at which a unit failure occurs can be used as a lower bound to the shear strength at any particular precompression but should not be used in the evaluation of the results as it will distort the outcome in an unrealistic way, it may of course have relevance to particular circumstances.

Shear failure in the unit/mortar bond area either on one or divided between two unit faces

Shear failure only in the mortar

Figure 3.8 Failure within a relatively weak frogged brick and a strong engineering brick. Shear failure in the unit

Figure 3.7

Modes of shear failure considered to be valid.

When three results are available at each chosen precompression a linear regression analysis is undertaken to determine the best straight line relationship between shear strength and precompression stress. From the intercept on the shear strength axis the shear strength at zero precompression, or initial shear strength fvo is determined the slope of the line, the angle of internal friction is α. From these two values characteristic values fvok and αk are determined by taking fvok = 0.8fvo and tanαk = 0.8tanα – 117 –

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These terms in CEN notation, i.e. fvok and tanαk are characteristic values for τ0 and µ in more classical notation.

Application of results As has been referred to in the introductory remarks there is more than one mode of failure of a shear wall and consequently despite efforts to standardise the shear strength test so as not to introduce flexure in practice in walls there will be more than one effect acting. The test merely determines the behaviour in as near to pure shear as is possible which deals with failures at the joints. There are two main consequences of this, one is that limitations need to be applied to the results from the test to ensure that tensile or compressive failure of the units do not dominate and the other is to ensure that the results accommodate the fact that in practice more than one mode of structural behaviour is occurring. It has been shown that (Mann and Müller, 1982) in walls, where both bending and shear are taking place, the equation controlling the joint failure τ = τ0 + µ.σ should be modified to: τ = τ01 + µ1.σ where τ 01 =

τ0 (1 + 2τ∆x/∆y)

and τ1 =

τ (1 + 2τ∆x/∆y) where ∆x and ∆y are the height and length of the brick. For a µ value of 0.4 and using UK brick dimensions the constants in the Coulomb equation would be reduced by some 19%. This is as yet not explicitly covered in DD ENV1996-1-1 although the reduction from the mean measured values to give a characteristic value has about the same effect. The effect will be greater for units of greater height to length ratio but for the moment these effects are absorbed into safety factors. In order that failures do not occur due to tension or compression in the units the value of fvk derived using the formula is limited by a value which is 0.065fb to control tension and a fixed value depending on the unit and mortar combination, where fb is the normalised unit strength.

Brickwork containing sheet damp-proof course materials Where brickwork contains a damp proof course this is often in the form of a sheet d.p.c. material, which can be a plane of weakness both in flexure and shear. The – 118 –

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d.p.c. is often at the base of the wall and the flexural strength may well be enhanced by the effects of self weight. The shear strength characteristics of joints containing a d.p.c. may be determined using the test method given in BSEN 1052-4, (BSI, 2000). This is essentially the same as that in BSEN 1052-3 with the exception that the specimen is typically two units long but is standardised at between 400 mm and 700 mm. The two bed joints each contain the d.p.c. bedded within the mortar joint. The method was originally developed in the UK where a specimen three bricks long was used (Hodgkinson and West, 1982), however as the test methods were developed in CEN and further work was done (Greenfield, 1990) it became clear that the stack bonded specimen gave results that were more variable than those from a two brick long (UK size brick) specimen and that the latter gave comparable variability and results to the three brick long specimen. The test method was published in the UK as DD.86 (BSI, 1983) and has been used as a way of comparing d.p.c. materials and for product development.

Bond strength testing Introduction There has long been interest in the development of reliable tests for the direct tensile and flexural tensile strength of the bond between ceramic masonry units and mortar. This has been driven by broad trends in the way masonry is used, away from the thick, sturdy loadbearing structures built with low-bond mortars towards the use of thin sheet masonry in the form of a cladding element, using portland cement mortars. These slender structures can be damaged by lateral forces from wind loads, seismic events, human activities, vehicles, hydraulic pressure on earth retaining walls and stacked granular materials and depend an a degree of flexural resistance to function. They are also more prone to tensile cracking generated by movement or differential movement than the old thicker style walls. More recent trends have led to the development of thin-joint ‘glue-style’ mortars and lightweight mortars where the bond strength is again an issue.

Direct tension testing Early research focussed on a direct tensile strength test and typical of such tests are the ASTM cross-brick test and the more conventional stack bond pair test. The problem with all direct tension tests is that it is very difficult to devise a loading system in which the fixing or gripping arrangement has no influence on the stress field and which introduces no element of bending in the test plane. Even if the loading system is perfect, if the specimen itself is not bonded homogeneously across the bond plane, a degree of bending will develop in the course of the testing. Van der Pluijm (1995) has shown that bonding is very rarely complete or homogeneous due to localised workmanship variations or differential mortar shrinkage. A typical effect, observed in – 119 –

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the author’s work (de Vekey et al., 1994) on the effects of fine sands on unit-mortar bond, is a greater shrinkage of the mortar in the frog than the surrounding mortar which causes debonding over the central area. This leads to high levels of variability and operator sensitivity in the test methods, which then engenders distrust in the users. Such was the dissatisfaction of the scientific community that the ASTM cross-brick test is no longer used even though it neatly solved the loading problem. Another drawback is that, to date, no successful in situ technique has been developed for direct tension testing. A range of such tests has been reviewed (Jukes and Riddington, 1998: see also Fig. 3.9). The main problem and Achilles heel of tension tests is how to apply significant loads both uniformly and conveniently to a wide spectrum of brick types such as frogged, multi-hole perorated, three hole perforated etc. The broad load application techniques and a brief evaluation of the advantages/disadvantage are given in Table 3.4. Riddington and Jukes (1995) and Riddington et al. (1998) carried out FE analysis of a range of tension tests and came down in favour of the bolted connection (corrected for the uneven stress distribution) but admitted some disadvantages. Most authors concede that the direct tension test may never fulfil all the requirements for all materials.

Articulated plate

Adhesive Brick Mortar joint Brick Adhesive

Articulated plate

Figure 3.9

Typical glued plate direct pull tension test.

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Direct tension test methods

No. Load transfer Author/reference method 1

2

3

4

4

5

6

Clamps

TESTING

Comments

Palmer and Hall 1931; Polyakov,1956; Jung, 1988; Ritchie, 1961

The clamps normally apply a compressive stress field to the units and this is often concentrated at clamping bolts. The most effective system seems to have been a wedge in a box used by Palmer and Hall. Rods built into Taylor-Firth and The main problem is bending of both the rods and the joints Taylor, 1990 the bricks giving a very non-uniform load distribution with very high corner stresses. Stiff stee Van der Pluijm 1993; If the plates are pulled via a central universal joint plates glued to Kuenning, 1966; Sinha there will be considerable bending forces if either the units with and Hendry, 1975; the resin or the joint are incompletely bonded in epoxy resin Chinwah, 1972; the plane. Ghazali, 1986 Very stiff steel Van der Pluijm 1993 This uses rigid plates (platens) with no universal platens glued to joint to solve the problem of bending moments the units with caused by incomplete bonding but will be very epoxy resin labour intensive and expensive. Steel bolts Murthy and Hendry, This is quite effective with the bolts at quarter or through holes 1965; Sinha and third points with some correction based on FE drilled in the Hendry, 1966; Ghazali, analysis but requires holes to be drilled and may units 1986; Jukes, 1997 fail at the holes especially for perforated units. Crossed brick ASTM, 1976; Pearson, This technique has been used widely and some couplet 1963 authors get repeatable results and modest variability but bending must occur e.g. see Kuenning 1966, and frogged units are unlikely to behave as in work. Centrifugal Hatzinikolas et al., Theoretically should produce a uniform stress field loading 1978 but one brick must still be clamped and the apparatus is too specialised for wide adoption.

Flexural tension testing As flexural bond is often crucial to the design of safe, stable thin sheet masonry there has been a need for a means of measuring or checking this parameter. The most obvious test – the building of a small representative piece of walling, which is tested in flexure is covered in Section 3.2. Other similar tests have been based on simple stack bonded brick prisms usually of between five and eleven bricks high, supported horizontally over two rollers at either end and loaded as beams with three or four point loading or UDL. The advantage of the test is of a simple specimen and a simple loading method, which can use a conventional test machine or dead weights, the disadvantages are the vulnerability of the specimen to damage during storage, handling and positioning for test and errors due to torsion induced by non-flatness of the specimen. Again, while a similar prism can be cut from masonry in situ, it will rarely be stack-bonded and – 121 –

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would be an expensive procedure prone to premature failure during cutting and handling and uncertainty about damage during preparation. The main variants of the stack prism that have been ‘standardised’ are the first three listed in Table 3.5. Table 3.5

Simple stack bonded specimens tested in flexure

Method/originator

Stack format

Load method

No. of joints tested

Coefficient of variation (%)

Pearson, 1963 ASTM E518, 1980 Australian standard AS1640, 1974 Huizer and Ward, 1978

5-high 10 high 9-high

3 point 4-point or UDL ‘4 point’ –stacked bricks 4 point

2 3 4

– 13.6% 51.8%‡ (27.6%†)

2

13.2‡

4 point

1

Huizer and Ward, 1978

3-high plus metal extns. 2-high plus metal extns.

†Australian standard specimen tested in test machine. ‡ Comparative set of results given by Huizer and Ward, 1978.

In the 1960s (Pearson, 1963) described several bond test methods that were tried to evaluate different mortars. These included two early versions of the bond wrench which he rejected as being too variable, the cross brick test which is based on a 1938 ASTM paper and a tension version of the bond wrench in which a slightly proud brick mortared to the top of a wall is levered upwards by a beam which passes over a roller support (termed the ‘wall test’). About two decades ago the bond wrench concept was further developed to allow the testing of single joints either in couplets or in succession from a stack prism. The basic procedure was probably first described in the proceedings of the 4th IBMAC in 1976 (Baker and Franken, 1976) and the developing technique was reviewed usefully in a paper published in 1980 (Hughes and Zsembery, 1980). They described a basic bond wrench in the form of a long lever, which is clamped to a brick (or block) at one end while the other end is free (Fig. 3.10). In the commoner version, an increasing force is gradually applied at the free end until the brick is rotated free from the mortar joint immediately below it. The load, or resultant moment, at which this occurs is a measure of the strength of the bond between unit and mortar. This force can and has been applied in a number of ways, of which the first three were suggested by (Hughes and Zsembery, 1980): (1) The most-common dead-weight design where a container is hung at the free end and filled with lead shot or an alternative fluid mass; (2) an alternative design where a heavy mass is permanently hung from the moment arm of the wrench but is free to move along it in the manner of a beam balance. In the test, – 122 –

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this mass starts off next to the masonry and is then gradually moved outwards (towards the free end) until failure occurs and (3) in which the force is generated using a hydraulic jack acting from a reaction point. These three systems are practical but are only really satisfactory in a laboratory situation due to the large mass or the reaction that is required.

Figure 3.10 stack prism.

Typical bond wrench clamped on to the top brick of a six-high

A disadvantage of all these wrench designs is that the dead weight of the lever arm generates a bending moment as soon as the wrench is attached which makes it impossible to measure weakly-bonded specimens. In a further elaboration of the design (Hughes and Zsembery, 1980) the loading system was arranged to counterbalance the arm mass and in a further version (Anderson, 1981), the wrench itself was counterweighted so that no initial bending moment was applied. This latter design will obviously increase, slightly, the dead weight prestress of the wrench. The bond wrench technique has been standardised in the USA (ASTM, 1986), in an annex of the Australian Code of Practice for masonry (SAA, 1988) and in an international standard (RILEM, 1994). A European standard test method is being developed. An alternative device along similar principles was developed (Huizer and Ward, 1978) and (Miltenberger et al., 1993) using two cradle clamps to apply a four-point load to only one joint of a stack prism. A further method based on an eccentric compression test is described in an American Standard (ASTM, 1976 x2) in which the failure is assumed to be in bending. A range of bond wrench variants are summarised in Table 3.6 together with data on the observed variability from investigative work. In 1986 (Anderson and Morton, 1986) discussed the question of whether a practical test for bond was required for site use. In an attempt to develop such a – 123 –

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site tool a further alternative loading method of fitting a load measuring transducer to the load application point or the arm of the wrench and applying manual force was tried at the BRE. This latter design is the basis of the ‘Brench’ (BRE, 1991) and at least one other variant. The main advantage of this system is that in situ tests become a practical option so that the technique can be applied to troubleshooting investigations of completed structures and not just used as a quality assurance tool on bespoke stack-bond specimens. In order to allow the testing in situ of one leaf of a cavity wall and, in some cases, the outer stretcher of a bonded multi-wythe wall, the BRE design has a relatively slender outer clamp and no counterweight. To try to minimise the starting applied moment, the arm is fabricated of Duralumin tube which produces a bending stress of about 0.07 N/mm2 on a 100 mm-deep stretcher unit. Other refinements of the BRENCH design are that the clamping assembly is adjustable from 100–250 mm which means that both stretcher and header bricks can be tested as can a range of block units of different thicknesses. The measuring system is set to hold the maximum applied load so it is not necessary to monitor the load. Table 3.6 Background and some example variability data for single joint test methods Reference/originator

Specimen format

Load mechanism

Coefficient of variation %

Pearson, 1963 – ‘wall test’

Bricks bedded on top of wall 9 high

Lever turning about a fulcrum Whole stack test

18.64%

2–10 high 2–10 high

25.75%† –

2–11 high

Static dead weight wrench Moving counterweighted dead weight wrench Dead load bond wrench

19.6%

2–11 high

Dead load bond wrench

18.05%

Couplet Couplet 2–10 high 7-high

Eccentric compression load Eccentric compression load Cradle clamp/4 point Hydraulic ram and wire

42% 42.0% – 26.3

7-high

Cradle clamp/4 point

25.3

2–11 high in situ

Manually loaded wrench Manually loaded wrench

17.6† 56.5

9-high stack prism tested to AS1640, 1974 Hughes and Zsembery, 1980 Hughes and Zsembery, 1980 Bond wrench to AS 3700, 1988 – Appendix. A7 Bond wrench to ASTM C1072, 1986 ASTM C149, 1976 couplet ASTM C952, 1976 couplet Miltenberger et al., 1993 Van der Pluijm and Vermeltfoort, 1995 Van der Pluijm and Vermeltfoort, 1995 BRE, 1991 ‘Brench’ BRE, 1991 ‘Brench’

†Engineering bricks in various mortars from 1:?:3 to 1:2:9 cement:lime:sand.

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Obviously manual loading cannot be perfectly axial and a degree of unavoidable shear stress must be expected, particularly at higher loads. There could also be an element of torsional shear introduced. In an investigation using an instrumented dummy couplet of aluminium blocks with a rubber ‘mortar’ (Van der Pluijm and Vermeltfoort, 1995) showed that the likely error for a careful operator was of the order of 4% reduction and, even if a deliberate thrust was applied, the error did not exceed 8%. Another, not insurmountable, problem of manual loading is that there will be some variation of loading rate. In some work at the BRE, (Lovegrove, 1990) investigated the effect of the rate of application of load using a pacing system and showed that, despite some variability, the rate had very little influence over the range assessed. The results are summarised in Fig. 3.11.

0.5 2000 Brench value

0.4 1500 0.3 1000 0.2 500

0.1

Estimated failure stress (N/mm2)

0.6

2500

0

0 0

20

40

60

80

100

Rate of loading (kg/min)

Figure 3.11 Measured mean bond strength versus loading rate for a paced hand loaded wrench.

Bond wrench structural principles Except for counterweighted or tension versions, all wrenches apply some vertical dead load and a small bending moment when first clamped to the specimen. At the start of a test the vertical load at the joint plane is the sum of the dead weight of the upper unit and the dead weight of the bond wrench, while the moment results from the dead weight of the bond wrench acting at its centre of gravity. As load is applied to the free end, both the bending moment and the vertical load are increased. The failure stress Fs may be calculated (assuming a triangular stress block), in N/mm2, as follows: Fs = M/Z - W/(b.d)

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where: M = the bending moment at failure in Newton-mm Z = section modulus of joint in cubic millimetres = b.d2/6 W = maximum compressive force applied to the joint, expressed in Newtons. b = the mean width of joint at the line of fracture, mm d = the mean depth of joint at the line of fracture, mm Where the dead weight or Brench system is used, the specimen is built from solid or perforated units laid on a full bed of mortar and a triangular symmetrical stress block is assumed the expression for the failure stress is given by: Fs = (W.Lg + W1 L1 g). 6/b.d2 - (W.L.g + W1.L1.g)/(b.d) Where additionally (see Fig. 3.10): W = load at failure applied at the extremity of the moment arm L = Lever arm for the applied load W1 = load due to the deadweight mass of the apparatus Wbw plus the clamped unit Wu acting at the centre of gravity. L1 = Lever arm for the mass of the BRENCH + unit at the centre of gravity. Although the triangular stress block is assumed, it has been shown by work by Fried (1991) and Samarasingh et al. (1997) that the effective Young’s modulus in the tension zone is lower than that in the compression zone thus the stress block is not linear and the neutral axis is displaced towards the compression face. However, the degree to which this occurs must vary with material and so it is not normally allowed for in standard bond wrench assessments.

Bond wrench calibration procedure STAGE 1 To find the mass and the position of the centre of gravity (C of G), of a bond wrench: The mass of the wrench Wbw and of the target unit type Wu are measured by weighing to the nearest 10 g. After clamping the target unit in the jaws the position of the centre of gravity is found by balancing on a knife edge. This must be carried out for each position of the jaws (i.e. each unit depth) for which a calibration is required. Both the ASTM and the Australian Code require the weight of the top brick, in an actual test, to be taken into account in the calculation of the centre of gravity and the dead weight compression load on the bed joint. This requirement means that either these measurements are carried out with a unit in the jaws and that a separate calibration has, strictly to be carried out for each density of unit catered for or that the bond wrench is calibrated empty, the unit is weighed separately and the parameters are corrected for in a separate calculation. Three separate determinations are carried out for each parameter and the mean is taken. – 126 –

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STAGE 2 First, measure the lever arm for the load point. For deadweight bond wrenches using the filled container system, further calibration is not necessary since the load is derived from the mass of the container at failure. For electronically measured bond wrenches a deadweight loading calibration of the measuring system is carried out. The device is clamped to a suitable support and, after resetting the electronics to zero, a load-hanger of known mass is suspended from the measuring handles: Known masses, up to a maximum of 70–100 kg in approximately 10 kg steps, are then added to the hanger and the reading noted. This process is then reversed to zero mass and the procedure repeated. A best-fit line is then drawn through the results and a calibration chart or spreadsheet is produced. STAGE 3 To relate bond wrench readings to stress: This is a paper exercise using the formulae given above to plot a graph of stress versus the bond wrench reading.

Variability of bond wrench measurements Such is the variability of masonry bond that failure is common with no additional applied load to a bond wrench and yet, in some specimens, with a nominally similar specification, the same device can support the whole weight of a 100 kg operative at the end of a 1 m moment arm. The bond wrench has been extensively studied and is used routinely in Australia to type-test potential unit-mortar combinations and, in the USA, to allow quality assurance checking of bond in large masonry contracts. In UK it has been used more as an investigative tool in cases where there is some doubt as to the bond being achieved in already-built or weathered masonry. In a paper by de Vekey et al. (1994), some data from bond wrenches used for these three applications was analysed (see Table 3.6) to look at the variability of the technique. The achievable running average of variability for deadweight wrenches used on bespoke stack bond specimens was under 20% (coefficient of variation). Where the wrench was used for investigation of existing structures, in both UK and Australia, the running average rose to around 50% CV. There are probably several reasons for this discrepancy the main ones being: • Commercial brickwork is likely to have a lower standard of workmanship than specimens made for laboratory tests. • The in situ samples were normally of brickwork, which was suffering from some problem which could have selectively reduced the bond, e.g. fire damage, weathering, under-specified mortars – 127 –

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• It was apparent that the coefficient of variation was influenced by the average bond strength, increasing with lower bond strength specimens, which are often encountered in these circumstances. The last factor in the list above was investigated statistically. It was found that the standard deviation was relatively constant for a population of data and that variations of the coefficient of variation were due to variations of the underlying mean. This is particularly noticeable where the wrench was used to do in situ tests on mortar joints which were at the low end of the strength range due the use of underspecified mortar where the variability climbed to above 50% compared with laboratory tests of strong joints where it could sometimes fall to around 15%. Another area of investigation has been the stack prisms used for type testing and quality control. These are usually in the form of couplets, six brick units high (five joints) or 11-brick unit-high for ten joints, although other heights have also been used. For larger units, couplets are the only practical option since anything larger would be very easily damaged. Possible reasons advanced for such differences are: • The different level of deadweight prestress during curing • Damage to subsequent joints due to bending moment applied to the joints below the one tested • Bias induced by the bond wrench itself • Inequalities in the bond, particularly for units with one large frog or different sizes of frog • Different curing conditions for different volumes of masonry. Several authors have investigated the effect of different stack heights, but there is no clear evidence that this influences the test provided that the clamping system is arranged such as not to apply any bending moment to the joints below the tested joint. In the work of Hughes and Zsembery (1980), there was a direct comparison between a 9-high, 4-high and 2-high stack prisms. Additionally a prism test was initially carried out on the 9-high prisms leaving seven remaining joints for bond wrench tests. (Note: this was a slightly questionable exercise since it could be argued that the initial beam test would fail at the weaker joint and thus give an inevitably lower value than the remainder). The results, summarised in Table 3.7, indicate that the beam test did indeed give a lower result and that there was no difference between 9-high and 4-high stack specimens. Oddly the couplets gave a similar result to the beam test but this may have been due to curing differences of the different volumes. Also the question of whether there is an overall bias such that the joints from the bottom of the stack are different to those from the top has been investigated and no systematic bias has been found. In Table 3.7, the top row of bond wrench tests were compared to the bottom row and found to be not significantly different and likewise the top two rows versus the bottom two rows. In de Vekey et al. (1994) the histograms of the failure stress of the five joints in a six-high stack were compared and found to be remarkably consistent as shown in Fig. 3.12. – 128 –

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Some variability and comparative data

Table 3.7 Test

Specimen

Number

Mean

CV%/ signifigance

Prism test Bond wrench Bond wrench Bond wrench Statistics Top row of 9-high versus bottom row Top 2 of 9-high versus bottom 2 9-high wrench versus prism test 9-high versus 4-high (both wrench) 9-high versus couplets (both wrench)

9-high stack 9-high stack † 4-high stack 2-high couplet Test Anovar

10 67 11 10 DOF 1/18

0.85 1.134 1.161 0.85 VR or t-value 0.62

18.8 25.8 35.4 15.2 Sig. Not

Anovar

1/38

0.53

Not

T-test

74

3

99%

T-test

76

0.3

Not

T-test

76

3

99%

30 Top joint

25

2nd joint

Brench value

Middle joint 4th joint

20

Bottom joint

15 10 5 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

2.25

Bond strength interval (N/mm2)

Figure 3.12 Comparison of the frequency histograms for the failure strength of the five joints in a six-high stack prism. Another issue is the difference between the average bond wrench result and data from small wallette or stack-bond tests (Chapter 3). The theoretical principles have been discussed by Lovegrove and de Vekey (1986) and Fried et al. (1988). The expectation, purely from statistical principles, is that the bond wrench should give a higher result but with greater variability. This is because most prism and wallette tests can fail at one of several joints subjected to similar bending moment and thus, logically, will fail at the weakest example of those – 129 –

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joints. With the bond wrench, however, all joints are tested and therefore a true mean should result which includes the stronger joints as well. There are other factors, however, and this means that some bond wrench designs, for example the manually operated version, give similar or even lower mean results than wallettes for some populations of specimen joints. Other useful data on variability and other factors is given by Russell and Palm (1982) and Neis and Chou (1980). In view of the increasing usage of bond wrenches, research has recently been intensified into their behaviour. Quite clearly, to a first approximation, the stress block will be assumed to be linear from tension on the remote face to compression on the near face. This is the assumption made in the conventional calculation. In practice there are a number of reasons why this is unlikely to be true: • The technique is affected by the same variation of bond in the plane as are the tension tests. • The clamping force necessary to apply the moment to the specimen will induce some localised concentrated load. • The clamping force will also induce some shear strains in the bed plane due to elastic deformation. • Because mortar joints are imperfect and contain air gaps and ‘Griffith’ cracks they tend to have a higher effective elastic modulus in compression (where gaps close and no tensile stress is applied to cracks) than in tension where the reverse occurs. A recent series of papers by Samarasingh et al. (1997 and 1999) and Riddington et al. (1995 and 1998) detail experimental work and FE analysis, which has been used to show that the stress distribution arising from the clamp in conventional wrenches is not linear and has proposed some modifications that would improve this situation. The aim is to approach more closely the ideal theoretical measured stress.

Test specimens and apparatus Laboratory tests For all wrench types used for laboratory evaluation of masonry the following are required: • A bond wrench • Stack bonded specimens • A clamping system to provide moment restraint to the brick below the top unit in the stack • A means of support for stack prisms of three or more with a resilient layer to obviate moment transfer to joints below the tested joint • A means of recording the numerical data and other items such as the mode of failure. A test using such a system is illustrated in Fig. 3.10. – 130 –

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In situ tests No clamping system or support is required, but the following are required: • A bond wrench • Access equipment such that the intended specimen is between 1 and 1.5m above a stable access platform • Both powered and hand cutting equipment • Small mechanical or hydraulic jacks (e.g. car jacks) to restrain bricks during cutting operations and adjacent bricks during testing operations • A measuring device to measure the dimensions of bricks • Sample bags • A marking device • A camera or notebook to record sampling positions.

Testing of in situ masonry: method of use It is assumed that the wall to be sampled requires openings to be made to access the sample units. At the top of walls it is possible to prepare a sample by merely cutting the perpend joints either side of a unit but this is not recommended for walls with poorly bonded units as little can be done to restrain the unit against damage from the cutting action. If an existing opening or reveal is available this can be handled normally from step 4 below. To simplify the description the preferred technique will be illustrated for a straightforward test of a single stretcher sample from the centre of a half-brick (102.5 mm thick) wall. The sequence of operations that follows is shown in Fig. 3.13.

Figure 3.13 Sequence of operations for carrying out in situ tests on masonry walls. – 131 –

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Step operation 1 Make a choice of the specimen brick 2 Make a sawcut over the chosen brick and the bricks either side with a powered abrasive saw ideally with a dust extract head (cut 1). Remember to allow for the circular shape of the ends of the cut. This isolates the specimen brick from damage from the subsequent cutting operations. Use either stitch drilling or hand saws to cut the centre perpend joint above the sample brick (cut 2). Finally break out the two units above the sample brick with a hammer and chisel 3 Next cut the two perpend joints at either end of the stretcher in the next course above (Cuts 3 and 4) and remove this brick 4 Insert car jacks and lightly prestress the sample brick and the bricks either side. Use timber packing if necessary. Now cut the perpend joints at either end of the sample brick right through into the mortar bed below. (Soft mortar can be cut easily and effectively with a short hacksaw blade mounted in a single ended holder. Harder mortar is best power cut with a rotary saw and finished off as necessary with a hacksaw or tungsten carbide tipped handsaw.) 5 Carefully remove the central jack and carry out a bond wrench test as follows (description is for a BRE Brench): a The Brench is switched on and allowed to warm up for a few minutes (A longer warm up time is preferable if it has to cope with a large temperature swing) b The jaw spacing is adjusted to suit the masonry thickness c The Brench is placed over the brick/block to be tested and the jaws are tightened (It is important that the Brench is fully but gently supported during this operation) d While still supporting the Brench, the button is pressed to reset the measuring system to zero load e The operative should then put his other hand just beneath the unit and release his grip. A failure at this point, under the self weight, indicates a bond strength below 0.07 N/mm2 and should be recorded as a zero. (Statistically, if this mode is frequent, then 50% of the minimum moment can reasonably be assumed, i.e. 0.035 N/mm2 ) f The operative then gradually presses down onto the crossbar handle until the unit breaks free g The maximum reading on the display is recorded h Measure the dimensions on the bed and the weight of the unit (or remove samples to a laboratory for further measurement) i The stress may be read from a calibration chart or calculated from a regression equation j The wall may be reinstated by mortaring the units back in place. Other variations are possible as follows: – 132 –

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1 Header samples from the centre of a 215 mm thick brick English or Flemish bonded wall can be tested in an exactly analogous manner 2 Stretcher samples from the centre of a 215 mm thick wall and any samples from the centre of thicker walls will necessitate the clearance of bricks or a wide collar joint from behind the specimen unit as well as over the specimen unit 3 Larger units can also be tested but may require extension plates to enlarge the gripping area of the clamps 4 Very large highly perforated units may be untestable because the moment required may exceed that applicable manually and the clamps may crush the side walls 5 It is not uncommon for the resistance to exceed the moment generated by the mass of an operative at a distance of 1m. In these circumstances the result can only be recorded as a proof load. However, masonry with this level of resistance would normally have adequate bond for most applications.

Accuracy To a large extent the accuracy is controlled by the characteristics of the electronic measuring system used and possibly partly by the geometry of the device. Measurements on the transducer/voltmeter system used for the BRENCH after a 30-min warm-up period gave the following results: Maximum force applied ... 375 N Resolution ... 0.4N (0.1%) Zero drift over 30 min use: 0.53% at quarter force 0.13% at maximum force Repeatability % of applied force: 0.4% of quarter force, 4.0% of maximum force.

Reporting bond wrench results for in situ tests When laboratory specimens or site QA specimens are reported then the specification and any measured properties of the units and mortar should be reported together with dates of manufacture and test and storage condition. When existing walls are sampled then the maximum amount of the available information about the units and mortar should be reported including the dimensions and masses of the units. If the materials are significantly damp or even wet then either whole units should be sealed into airtight plastic bags for subsequent moisture determinations or the drilling method (RILEM, 1997) should be used to obtain moisture samples. The condition and local sample pattern should be recorded photographically or by simple drawings. The position of the samples on plan and the storey should also be reported for larger buildings. The following checklist is a good guide to what should be reported where possible: – 133 –

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1 A reference to the method used 2 A description of the specimens including their overall size and shape, pattern of bond and joint thickness 3 The method of sampling of the units 4 The properties of the units including strength and, where appropriate, water absorption, Initial Rate of Water Absorption, density where available 5 The composition and strength of the mortar used if available 6 The date of preparation of laboratory or quality control specimens and the date of the test 7 The conditions of storage of laboratory or site specimens 8 The sampling plan in the form of a diagram or description for all samples tested in situ 9 All individual failure loads in Newtons and relevant dimensions in mm 10 The point of failure, e.g. within mortar, at upper or lower brick/mortar interface etc. 11 All individual values of bond strength calculated as specified 12 Mean bond strength 13 Sample standard deviation and coefficient of variation (%).

References American Society for Testing and Materials 1976. ‘Standard method for measurement of masonry flexural bond strength’, ASTM C149–76. American Society for Testing and Materials 1986. ‘Standard method for measurement of masonry flexural bond strength’, ASTM C1072–86. American Society for Testing and Materials 1976. ‘Test of bond strength of mortar to masonry units’, ASTM C952/76. Anderson, C. 1981. ‘Tensile bond tests with concrete blocks’, International Journal of Masonry Construction, 1(4), pp.134–8. Anderson, C. and Morton, C.C. 1986. ‘Do we need a test for practical site control of tensile bond?’ Proceedings of a Conference on Practical Design of Masonry Structures, TTL, London. Baker, L.R. and Franken, G.L. 1976. ‘Variability aspects of the flexural strength of brickwork’, Proc. 4th International Brick Masonry Conference, paper no. 2.b.4. British Masonry Society 1997. Eurocode for Masonry ENV 1996-1-1, Guidance and worked examples. Special Publication No1 SP1. British Standards Institution 1983. Draft for Development. Damp Proof Courses, Part 1: Methods of Test for Flexural Bond Strength and Short Term Shear Strength, DD 86. British Standards Institution 1992. Code of Practice for Use of Masonry, Part 1: Structural Use of Unreinforced Masonry BS5628. British Standards Institution 1996. Eurocode 6: Design of Masonry Structures, Part 1-1: General Rules for Buildings, Rules for Reinforced and Unreinforced Masonry, DD ENV 1996-1-1. British Standards Institution 1999. Method of Test for Masonry, Part 1: Determination of Compressive Strength, BSEN 1052-1.

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British Standards Institution 1999. Method of Test for Masonry, Part 2: Determination of Flexural Strength, BSEN 1052-2. British Standards Institution 2000. Code of Practice for Use of Masonry, Part 2: Structural Use of Reinforced and Prestressed Masonry, BS5628. British Standards Institution 2000. Method of Test for Masonry, Part 4: Determination of Shear Strength Including Damp Proof Courses, BSEN 1052-4. British Standards Institution 2000. Method of Test for Masonry Units, Part 2: Determination of Water Absorption of Aggregate Concrete Manufactured Stone and Natural Stone Masonry Units due to Capillary Action and the Initial Rate of Water Absorption of Clay Masonry Units, BSEN 772-11. British Standards Institution 2002. Method of Test for Masonry, Part 3: Determination of Initial Shear Strength, BSEN 1052-3. Building Research Establishment 1991. ‘Testing bond strength of masonry’, BRE Digest, No. 360. Cavanagh, C.J., Edgell, G.J. and de Vekey, R.C. 1986. ‘The racking strength of lightly loaded partition walls’, Proc. British Masonry Society No 1, p30. Chinwah, J.C.G. 1972. Shear resistance of masonry walls, PhD thesis, University of London. Davey, N. and Thomas, F.G. 1950. ‘The structural uses of brickwork’, Structural and Building Paper No 24, The Institution of Civil Engineers Session 1949–1950. de Vekey, R.C., Anderson, C., Beard, R. and Hodgkinson, H.R. 1982. ‘A Collaborative Evaluation of the BS5628 ‘Wallette’ Test for Measuring the Flexural Strength of Brickwork’, Proc. 6th IBMAC, Rome, p. 131. de Vekey, R.C., Edgell, G. J., Regan, G.D. and Southcombe, C. 1994. ‘The influence of sand constituents on the bond of mortar and lateral performance of masonry: (I) Test methodology and results’, Proc. 10th IBMAC, Calgary, V3, pp.1377, 1387. de Vekey, R.C., Page, A.W. and Hedstrom, E.G. 1994. ‘The variability of bond wrench measurements in UK, Australia and USA’, Journal of the British Masonry Society, Masonry International, 8(1), pp.21–5. de Vekey, R.C. 1994. ‘An analysis of the likely variability of the draft 1052-2 wallette flexural strength test and the implications for the derivation of the characteristic value from test results’, Private communication. Edgell, G.J. and Saunders, J.D. 1990. A comparison of masonry strengths determined at two laboratories, Masonry International 3(3), p.92. Edgell, G.J. 1990. ‘The compressive strength of bricks and brickwork when loaded in directions other than normal to the bed face of the brick’, CERAM Research Special Publication SP 130. Edgell, G.J. 1990. ‘The compressive strength of bricks and brickwork when loaded in directions other than normal to the bed faces of the unit’, CERAM Research Special Publication SP 132. Edgell, G.J., de Vekey, R.C. and Dukes, R. 1990. ‘The Compressive Strength of Masonry Specimens’, Proc. British Masonry Society No 4, p. 131. Edgell, G.J., Tellet, J. and Hodgkinson, H.R. 1982. ‘The buttressing resistance of lightly loaded partition walls’, Proc. 6th IBMAC, Rome, p.673. Edgell, G.J. 1990. ‘The flexural strength of 102.5 mm thick brickwork in different bonds’, Ceram Research Special Publication SP 131. Fried, A.N. 1991. ‘The position of the neutral axis in masonry joints’, Proc. 9th IBMAC, Bonn, DGfM, 188–95.

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Fried, A.N., Anderson, C. and Gairns, D.A. 1988. ‘A comparative study of experimental techniques for determining the flexural strength of masonry’, Proc. 1st 1986. BMS International Masonry Conference, Stoke on Trent, BMS Proc.2, p. 98. Ghazali, M.Z. 1986. Shear strength of brick masonry joints, DPhil thesis, University of Sussex. Greenfield, A. 1992. ‘Determination of the shear strength of masonry using brick ‘triplet’ prisms’, Ceram Research Paper 802. Greenfield, A. 1990. ‘The shear strength of masonry containing a damp proof course, Part 1. Ceram Research Paper 786. Greenfield, A. 1990. ‘The shear strength of masonry containing a damp proof course’, Part 2.Ceram Research Paper 789. Haseltine, B.A. and Tutt, J.N. 1999. ‘Available design methods and comparison of design methods. Seminar on the lateral loading of masonry, Department of the Environment, Transport and the Regions, London. Haseltine, B.A., West, H.W.H. and Tutt, J.N. 1975. ‘The resistance of brickwork to lateral loading. Part 2: design of walls to resist lateral loading’, The Structural Engineer 55(10). Hatzinikolas, M., Longworth, J. and Warwarak, J. 1978. ‘The use of the centrifugal force for determining the tensile bond strength of masonry’, in West, H.W.H. (ed.), Proc. British Ceramic Society, Stoke-on-Trent, No.27, p. 7. Hodgkinson, H.R. and Davies, S. 1982. ‘The stress – strain relationship of brickwork when stressed in directions other than the normal to the bed face’, Proc. 6th IBMAC, Rome, p. 240. Hodgkinson, H.R. and West, H.W.H. 1982. ‘The shear resistance of some damp proof course materials’, Proc. British Ceramic Society, No 30, p. 13. Hughes, D.M. and Zsembery, S. 1980. ‘A method of determining the flexural bond strength of brickwork at right angles to the bed joint’, Proc. 2nd Canadian Masonry Symposium, Ottawa, Canada, pp.73–86. Huizer, A. and Ward, M. 1978. A simplified flexural bond test for clay brick masonry. Proc. North American Masonry Conference, Boulder, USA. International Standards Institution 1993. Masonry Part 4: Test Methods, ISO/DIS 9652-4. International Standards Institution 1999. Masonry Part 1: Unreinforced – Code of Practice for Design by Calculation, ISO DIS 9652-1. Jukes, P. and Riddington, J.R. 1997. ‘A review of masonry joint shear strength test methods’, Masonry International 11(2), pp.37–43. Jukes, P. and Riddington, J.R. 1998. ‘A review of masonry tensile bond strength test methods’, Masonry International, 12(2), pp.51–7. Jukes, P. 1997. ‘An investigation into the shear strength of masonry joints’, DPhil thesis, University of Sussex. Jung, E. 1988. ‘The binding between mortar and brick’, in de Courcy, J.W. (ed.), Proc. 8IBMAC, 1, pp.182–93, Elsevier Applied Science, London. Kuenning, W.H. 1966. ‘Improved method of testing tensile bond strength of masonry mortars’, Journal of Materials, l(l), 180. Lovegrove, R. and de Vekey, R.C. 1986. ‘The effects of specimen format on the flexural strength of wallettes’, BMS Proceedings, M(1), 8th BCS, p. 7. Lovegrove, R. 1990. Private communication, BRE Note N18/90. Mann, W. and Müller, H. 1982. ‘Failure of shear-stressed masonry – an enlarged theory, tests and application to shear walls’, Proc. British Ceramic Society, No 30.

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Miltenberger, M.A., Colville, J. and Wolde-Tinsae, A.M. 1993. ‘A proposed flexural bond strength test method’, Proc. 6NAMC, Philadelphia, 137–48. Murthy, C. K. and Hendry, A.W. 1965. ‘Preliminary investigation of the shear strength of one-sixth scale model brickwork’, British Ceramic R.A., Stoke-on-Trent, TN65, Part l. Neis, V.V. and Chou, D.Y.T. 1980. ‘Tensile bond testing of structural masonry units’, Proc. 2nd Canadian Masonry Symposium, Ottawa, Canada, pp.381–92. Palmer, L.A. and Hall, J.V. 1931. ‘Durability and strength of bond between mortar and brick’, Bur. Stand. J.Res., RP290, 6, p.473. Pearson, J.C. 1963. ‘Measurement of bond between brick and mortar’, Proc. ASTM, 43, pp.857–67. Polyakov, S.V. (l 956) Masonry in framed buildings, Moscow. (Trans. Caims, G.L.), National Lending Library, Boston Spa. Riddington, J.R. and Jukes, P. 1995. ‘Determination of material properties for use in masonry FE analysis’, Proc. 4th International Masonry Conference, (British Masonry Society), No. 7, p. 314. Riddington, J.R., Jukes, P. and Morrell, P.J.B. 1998. ‘A numerical study of masonry tensile bond strength test methods’, Proc. 5th International Masonry Conference (Proc. British Masonry Society) No. 8, p. 157. RILEM 1997. ‘Recommendations of tests for masonry materials’, in R.C. de Vekey (ed.), Measurement of Moisture Content by Drilling, Materials and Structures, MS-D.10. 30, pp.323–8. RILEM 1994. ‘Test recommendation LUM B.3. Bond strength of masonry using the bond wrench method’, RILEM Technical Recommendations for the Testing and Use of Construction Materials, E&F Spon, London, pp.481–3. Ritchie, T. 1961. ‘A small-panel method for investigating moisture penetration and bond strength of brick masonry’, Mat. Res. and Stand., 1(5), p.360. Russell, B. and Palm, B. 1982. ‘Flexural strength of brick masonry using the bond wrench’, Proc. 2nd NAMC, University of Maryland, 1.1–1.15. Samarasinghe, W., Lawrence, S.J. and Page, A.W. 1999. ‘Numerical and experimental evaluation of the bond wrench test’, Masonry International, 12(3), pp.89–95. Samarasinghe, W., Lawrence, S.J. and Page, A.W. 1997. ‘Improvements to the bond wrench test’, Proc. 11th IBMAC, Shanghai, 347 pp. Shrive N. 1986. ‘The prism test as a measure of masonry strength: a theoretical analysis’, Proc. British Masonry Society No 1, p. 117. Sinha, B.P. and Hendry A.W. 1966. ‘Further investigations of bond tension, bond shear and the effect of precompression on shear strength of model brick masonry couplets’, Stoke-on-Trent, British Ceramic R.A., TN80. Sinha, B.P. and Hendry, A.W. 1975. ‘Tensile strength of brickwork specimens’, Proc. British Ceramic Society, Stoke-on-Trent, No.24, N91. Standards Association of Australia 1988. Masonry Code: Masonry in Buildings, Appendix A7: Flexural strength by bond wrench, AS 3700–1988. Taylor-Firth, A. and Taylor, L.F. 1990. ‘A bond tensile strength test for use in assessing the compatibility of brick/mortar interfaces’, Construction and Building Materials 4(2), pp.58–63. Thomas, F.G. 1953. ‘The strength of brickwork’, The Structural Engineer 31(2), 35. Van der Pluijm, R. and Vermeltfoort, A. 1995. ‘Bond wrench testing’, Proc. British Masonry Society M7, 1, pp.225–31. Van der Pluijm, R. 1993. ‘Shear behaviour of bed joints’, Proc. 6th North American Masonry Conference, Philadelphia, pp.125–36.

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Van der Pluijm, R. 1995. ‘Numerical evaluation of bond tests on masonry’, Masonry International, 9(1), pp.16–24. West, H.W.H. 1976. ‘The flexural strength of clay masonry determined from Wallette specimens’, Proc. 4th IBMAC, Brugge. West, H.W.H., Hodgkinson, H.R. and Haseltine, B.A. 1975. ‘The resistance of brickwork to lateral loading, Part 1: Experimental methods and results of tests on small specimens and full sized walls, The Structural Engineer 55, No. 10.

Further reading Ali, S. and Page, A.W. 1986. ‘A failure criterion for mortar joints in brickwork subjected to combined shear and tension’, Masonry International 9, pp.43–54. American Society for Testing and Materials 1938. ‘A preliminary consideration of some proposed methods of sampling and testing mortar for unit masonry’, ASTM Bulletin 94, Oct. 40. American Society for Testing and Materials 1980. ‘Flexural bond strength of masonry’, ASTM E518–80. Baker, L.R. 1979a. ‘Measurement of the flexural bond strength of masonry’, Proc. 5th International Brick/block Masonry Conference, p. 35. Baker, L.R. 1979b. ‘Some factors affecting the bond strength of masonry’, Proc. 5th International Brick/block Masonry Conference, pp.84–9. Daou, Y. and Hobbs, B. 1991. ‘Strength of brickwork loaded in different orientations’, in Glitza, H. and Gobel, K. (eds), Proc. 9th IBMAC, Bonn, DGfM, Germany, pp.157–63. de Vekey, R.C., Edgell, G.J. and Dukes, R. 1990. ‘The effect of sand grading on the performance and properties of masonry’, Proc. BMS, No. 4, p. 152. Goodwin, J.F. and West, H.W.H. 1982. ‘A review of the literature on brick-mortar bond’, Proc. British Ceramic Society, 30. Jukes, P., Bouzeghoub, M.C. and Riddington, J.R. 1997. ‘Measurement of tensile strain softening and its influence on bond tensile results’, in Middleton, J. and Pande, G.N. (eds), Computer Methods in Structural Masonry 4, E&FN Spon, London. Lawrence, S.J. and Page, A.W. 1994. ‘Bond studies in masonry’, Proc. 10th International Brick/block Masonry Conference, Calgary, pp.909–17. Lawrence, S.J. and Page, A.W. 1995. ‘Mortar bond – a major research programme’, Proc. 4th Australian Masonry Conference, pp.31–7. Lovegrove, R. 1987. ‘Testing the flexural resistance of masonry by bond wrench compared with BS5628 wallettes’, paper presented at the BMS Autumn meeting. Matthys, J.H. 1988. ‘Flexural bond strengths for Portland cement lime and masonry cement mortars’, Proc. 8IBMAC, Elsevier Applied Science, London, pp.284–91. McGinley, W.M. 1993. ‘Bond-wrench testing – calibration procedures and proposed apparatus and testing procedure modifications’, Proc. 6th NAMC, Boulder, Colorado, p. 159. McNeilly, T.H., Scrivener, J.C., Zsembery, S. and Lawrence, S.J. 1991. ‘Bond strength and the Australian masonry code’, Proc. 9IBMAC, Bonn, DGfM, Germany, 301–7. Sise, A., Shrive, N.G. and Jessop, E.L. 1988. ‘Flexural bond strength of masonry stack prisms’, Proc. 1st BMS International Masonry Conference, Stoke on Trent, 2, p. 103. Standards Association of Australia 1974. ‘Rules for brickwork in buildings’, AS 1640/1974.

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4 Prototype testing

When considering testing prototype ceramic products for use in construction, the test methods are usually those established as standard procedures. For example, from time to time what is considered to be a novel brick is produced, this may be because the perforation pattern is such that it is considered to have some useful contribution to the thermal insulation of a wall or because it incorporates some waste material such as slate waste, sewage sludge or glass cullet. Whatever the motivation it would be usual to initially test the relevant unit properties to the recognised standard e.g. BSEN771-1. If the product properties were considered to be satisfactory the next phase would be to check that brickwork made from these units meant that they could be used as if it were made from more conventional units. In this case one would need to consider strength aspects, probably at the element scale as described earlier and in addition resistance to fire probably as a small panel initially and subsequently at full scale. Despite the great importance of the standard of workmanship on the capability of brickwork to prevent rain penetration it would be necessary to confirm that the novel brick type did not lead to any unforeseen problems. During this process of building up confidence in the product one would also need to bear in mind all of the factors, which could cause it to be unacceptable in the market place. There are a variety of these which may be relevant, visual acceptability, self weight and hence ease of handling, mortar usage, level of waste and ease of cutting could all be important. Although not prototype testing in the true sense of the expression in that a final model is tested and proven and subsequently large quantities are produced there have been a number of projects where building types have been tested in order to prove that each type was safe and satisfactory. In the field of housing there have been particular projects on traditional brick and block cavity construction, timber frame housing and Single Leaf Insulated Masonry. In relation to other types both cross wall construction and medium rise timber frame construction have been tested. In each of these cases once the performance was proven and the design approach considered to be acceptable the details of subsequent buildings would differ to suit personal taste and the demands of use. Testing at this scale is expensive and hence the importance of ensuring that what is tested is both typical and towards the bottom end of the range of performance expected from the building type. Satisfying both criteria simultaneously is not easy and some compromises are usually necessary. In the period 1983–1986 a series of tests were carried out on a typical domestic house built in the laboratory (Edgell and de Vekey, 1985 and 1986; Templeton et al., 1986; de Vekey, 1986). The house was tested in a variety of ways and several – 139 –

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partial failures occurred all of which were repaired using materials drawn from the original batches, sufficient having been supplied at the start to ensure this was possible. The reason for the work was a growing concern that there had been changes to the way domestic houses were built, but that the empirical rules which were incorporated in the Building Regulations of the day and which were used in their design, were based on more traditional construction. The development of the use of trussed rafter roofs and of autoclaved aerated concrete blocks for the inner leaves of cavity walls lead to the roof load being transferred into the weaker leaf of the front and rear walls of houses which contain the predominant openings. Traditionally this load would have been transferred by purlins into the gable and party walls, which contain few or no openings and which were, at the time the rules were developed, made from 9-inch solid brickwork. Clearly on these grounds alone there was the need to confirm the safety of the empirical rules. Other developments, for example, fewer chimney breasts, more picture windows, greater use of metal connectors between vertical and horizontal elements all suggested an overall reduction in robustness and hence the need for the work.

Figure 4.1

Plan and elevation of test house. – 140 –

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Figure 4.2

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Test house under construction.

The design of the house and the selection of materials was a carefully thought through exercise. The timber floor spanned in opposite directions in the front and rear of the building so that the front and rear walls could be tested independently with differing levels of horizontal restraint applied at first floor level. The house design is shown in Fig. 4.1 and under construction in Fig. 4.2. In the tests on the rear wall, vertical load was applied by hydraulic jacks at eaves level, through spreader beams and a wall plate. The timber floor, which spanned parallel to the wall and was connected to it by steel restraint straps was not loaded. In the tests on the front wall the timber floor, which was supported by the front wall using joist hangers, was loaded by hydraulic jacks mounted on the laboratory strong floor and pulling a series of cables which passed through the floor and rested in a steel channel acting as a spreader beam on the chipboard decking (see Fig. 4.3). In both series of tests, the effect of using both reinforced concrete and steel box section lintels at eaves level was investigated. During the tests, strains were measured using Demec gauges at a number of locations, (see Fig. 4.4) and horizontal displacements of the wall were measured at the centre of the wall and of the floor at a number of locations underneath (see Figs 4.5 and 4.6). The results of these tests were very encouraging as failures tended to be localised at for example lintel bearings and beneath joist hangers (see Fig. 4.7), these were not catastrophic failures. From a more detailed analysis (de Vekey, 1986), it was confirmed that the rule of thumb allowing a minimum width of masonry between window openings of one-sixth the sum of the width of the openings was safe. The minimum factor of safety achieved was 3.2, but against rare single events such as floor overload or snow overload were often much greater. – 141 –

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Figure 4.3 Jack fixed to floor mounted cantilever applying load to tension cable

Figure 4.4 Location of Demec studs.

Figure 4.5 Location of horizontal transducers on front wall. – 142 –

Figure 4.6 Deflected profile of front wall.

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Figure 4.7 Rotational failure of brickwork at eaves level following compressive failure of inner leaf lintel bearing. When the roof had been constructed wind suction tests were carried out on the triangular section at the top of the gable wall. Instances of failure were known, in particular on exposed sites where a westerly wind could funnel between facing gable walls and cause localised suction, (see Fig. 4.8). The concern in this case was whether the strap connection at the verge was adequate. A large triangular suction box was made and supported on vertical columns some 75 mm from the wall. A polyethylene seal was made between the box and the wall and the box was depressurised by a fan via a manually operated valve. The uniformity of the pressure across the box was monitored using manometers and was good. Displacements were measured using linear voltage displacement transducers mounted on a scaffolding frame. A great deal of attention was needed in erecting the frame as it needed to be robust, quite separate from the house structure and could only access the laboratory floor through the stair well opening in the floor. The test programme showed that the plasterboard ceiling is very effective in stiffening the roof structure and enabling the straps at ceiling level to be effective. At the verge the straps were not very effective but the revised strap detail described in a Defect Action Sheet (Building Research Station, 1983) whereby the strap passes beneath the rafters and through the inner leaf was very effective. An overall positive wind pressure test on the whole of the gable wall showed that the ways in which the house resisted the load were complex. The front and rear walls acted as good buttresses despite the large openings in them and the test was terminated when a lateral load of 3 kN/m2 had been applied (see Fig. 4.9). – 143 –

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Figure 4.8 Large suction box adjacent to gable wall (author in foreground).

Figure 4.9 Reaction boards used to resist overall position pressure applied to gable wall. In the final tests on this house the floor and ceiling were loaded with kenteledge to serviceability levels and the small piece of cavity walling beneath the downstairs window openings in both front and rear walls was removed. In one case this was by using a hydraulic jack which proved to be rather slow and in the second air bags were placed inside the inner leaf, in the cavity and outside the outer leaf. Timber reaction boards were used both inside and outside and all three air bags were inflated to a pressure of 5kN/m2. The inner and cavity air bags were rapidly deflated leaving the masonry suddenly subjected to a high external pressure, which caused a rapid collapse. Clearly the load paths in the building changed as a result of the removal of the masonry but a combination of arching action, cantilevering lintels and floor and wall tie action supported the loads and there was no further collapse (see Figs 4.10–4.12). The test work on Single Leaf Insulated Masonry (SLIM) was a little closer to true prototype testing as the masonry system was novel as opposed to current. In the case of the robustness experiments SLIM was a concept in which the external wall of domestic dwellings would be built of standard UK format bricks with thermal insulation stuck to the inside face and internally lined with plasterboard. The concept is excellent in the sense that clay brickwork, which is a good structural material, was used to provide the structure and high grade insulation material was used to provide the insulation. There was no compromising in trying to make one type of material provide more than one major property of the wall. Clearly rain penetration was an issue and it was expected that the brickwork would allow some water through it. The insulation was of closed cell structure and hence unaffected itself and use was made of the uneven inside face to the brickwork due to the tolerances on brick size and the thickness of the glue fixing the insulation to provide a nominal cavity which was drained by carefully designed details. Initially a single storey dwelling was constructed using SLIM and led to no difficulties in design or construction but considerable work was needed on the details of the floor to wall connections in order to make the system suitable for use in two storey high structures. – 144 –

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Initial tests were carried out to investigate eccentricity of vertical loading and the effect of simultaneous vertical and horizontal load on a two storey height wall (West et al., 1978). (See Figs 4.13 and 4.14).

Figure 4.10 Horizontal load applied by jack to masonry inside house.

Figure 4.11 Collapsed masonry between ground floor window openings.

Figure 4.12 House after test.

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Figure 4.13 Two storey high wall test arrangement.

Figure 4.14 Crack pattern on slim wall after failure due to lateral load.

Subsequently as the design details were developed further laboratory tests were carried out (Hodgkinson, et al., 1982) and eventually a pair of semi-detached SLIM houses were built at Harriseahead on the outskirts of Stoke-on-Trent and were tested in situ (Hodgkinson and Haseltine, 1986), see Fig. 4.15. From the testing point of view much of what was done did not differ greatly from the robust house work, except of course that testing on-site meant that reaction frames for air bag loading needed to be braced by different means and in this case raking shores were used. It is interesting to note that in the early work water tanks were used to provide dead load to floors but this was quickly superseded by the use of dense clay bricks as kenteledge. There is no doubt that this was more convenient and the only real points to note are that care is needed that the bricks cannot become a structure in themselves by wedging against any of the support or load structure and that dense, low porosity bricks should be used so that changes in the moisture content are irrelevant in relation to the load being applied. – 146 –

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Figure 4.15

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Slim houses under test.

Although SLIM was not a commercial success as a concept it is good and if made from wider more highly perforated units and revised floor/wall details it could be revived. There is no doubt that for multi-layer wall construction the concept of choosing layers which each fulfil one function well and lead to a combination that meets all required performances is a valid design concept. In much the same way that the robust house experiments generated data to confirm the rules for small masonry buildings work was carried out to refine the rules for the design of cavity walls in a much larger structure (Sinha et al., 1975). In this case a five storey high structure was built using cavity brickwork walls and reinforced concrete floor slabs, the structure being built against an exposed face in a disused quarry. This structure was used to investigate among other things joint fixity and eccentricities but was also used to investigate its stability following the removal of some of a main load bearing wall (Sinha and Hendry, 1970). A programme of test work was carried out on a timber-framed house that was similar to the overall positive wind pressure test on the gable wall of the masonry house described earlier. In this programme (de Vekey, 1987) the aim was to investigate the contribution to the stability and robustness of the house that was provided by the brickwork cladding. The lateral load test was carried out on the timber frame with no cladding at all both with and without loads to simulate those from occupancy. The loads were applied by weights hanging from floor joists and from the ceiling ties of the trussed rafter roof. Snow load was simulated by laying steel chains on the tiled roof. Subsequent testing was in stages as the brickwork was added to the frame. Initially the brickwork leaf cladding the gable was added and after curing the house was retested. Similar tests were carried out when 2.5 brick long returns were added to the front and rear walls, when the brickwork was completed to first floor level and when completed to eaves level. – 147 –

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In this way the programme covered timber frame housing designs with a range of cladding finishes, for example, brickwork to first floor would represent reasonably well the case where the upper storey were clad in UPVC panels. The programme showed that the addition of the gable end brickwork reduced significantly the deformation of the building. In this case some cantilever action of the brickwork, some direct transfer of load into the base and some horizontal spanning were felt to have made the significant difference in performance. The addition of the brickwork returns made a further significant difference with the deformations reduced to 20% of the values without the brickwork. The completed front and rear walls gave the best performance but this was not a great improvement over that with the returns alone. In 1999 and 2000 work has been carried out to demonstrate the suitability of timber frame construction for medium rise buildings. A six storey high building clad in brickwork has been built at the Large Building Test Facility at BRE Cardington (see Fig. 4.16). The effectiveness of the brickwork in improving the shear stiffness of the building has been demonstrated by dynamic testing. This work is continuing, the latest structural investigation has involved the reconstruction of part of the building, using the same materials and design in the CERAM laboratory. This building was tested using simulated wind loading at various stages of construction namely, timber frame alone, timber frame plus brickwork, timber frame plus brickwork plus plasterboard. The results have emphasised the great benefit to the stiffness of the structure provided by the brickwork and the plasterboard. The reconstruction was subsequently tested to destruction and this provided the confidence to subsequently repeat the test at the Cardington building where a serviceability load of 1.5 kN/m2 was applied and no damage was observed. The results of this work when complete should enable the design of timber frame buildings to make some economies by relying more on the stiffness provided by the brickwork.

Figure 4.16

Six storey high timber frame brick clad building at BRE Cardington. – 148 –

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Experiments are also in progress at Cardington to measure the strains and relative movements between the brickwork cladding and the frame of a seven storey high reinforced concrete building. The investigation is comparing the performance of the extremes, i.e. brickwork tied to the frame but completely separate from it in the sense of any vertical movement being unrestrained and that where each storey is built tightly between the floors above and below. This work does offer the potential of reducing the cost of brickwork cladding to medium rise buildings as provision for relative movement between the frame and the cladding can be expensive. Unfortunately the Large Building Test Facility has, since the writing of this chapter, been closed down. It did offer good opportunities to experiment at full scale on what became a rebuildable test structure in much the same way as the robust house work was carried out. Results from the work here will lead to design improvements, however despite the scale of the work there remains an element of artificiality and this will require careful consideration when translating the results into design guidance.

References Building Research Station 1983. Defect Action Sheet (Design), External and Separating Walls: Lateral Restraint at Pitched Roof Level Specification, DAS 27. de Vekey, R.C. 1986. ‘An analysis of the vertical load behaviour of a typical small masonry house’, Proc. British Masonry Society 1. de Vekey, R.C. 1987. ‘Timber-framed housing: interaction between frame and cladding brickwork subject to lateral loads’, Masonry International 1(1), p.29. Edgell, G.J. and de Vekey, R.C. 1985. ‘The robustness of the domestic house. Part 2: Wind Suction Tests on Gable Walls’, British Ceramic Research Association Technical Note TN 364. Edgell, G.J. and de Vekey, R.C. 1986. ‘Robustness tests on a domestic house: compressive loading on walls’, Proc. British Masonry Society No1, p. 48. Hodgkinson, H.R., Haseltine, B.A. and West, H.W.H. 1982. ‘Structural tests on single leaf two-storey brick masonry structures’, Proc. British Ceramic Society No 30, p. 185. Hodgkinson, H.R. and Haseltine, B.A. 1986. ‘The structural testing of a SLIM house’, Proc. British Masonry Society No 1. Sinah, B.P. and Hendry, A.W. 1970. ‘The stability of a five storey brickwork cross wall structure following the removal of a section of a main loadbearing wall’, British Ceramic Research Association Technical Note TN 165. Sinha B.P., Maurenbrecher A.H.P. and Hendry, A.W. 1975. ‘An investigation into the behaviour of a five storey cavity wall structure’, Proc. British Ceramic Society No 24. Templeton, W., Edgell, G.J. and de Vekey, R.C. 1986. ‘The robustness of the domestic house part 3 positive wind pressure test on gable wall’, British Ceramic Research Association Technical Note TN 320. Templeton, W., Edgell, G.J. and de Vekey, R.C. 1986. ‘The robustness of the domestic house. Part 4: Accidental damage’, British Ceramic Research Association Technical Note TN 371. West, H.W.H., Hodgkinson, H.R. and Haseltine, B.A. 1978. ‘The Effect of floor and Wind Loads Applied Separately or Simultaneously to a Two Storey Height Wall’, Proc. North American Masonry Conference, Boulder, Colorado.

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5 Full scale testing

Wall testing Compression Walls are tested in compression for a number of reasons, all of which are related to the design of walls for buildings in some way. This statement is fairly obvious, although the rationale for the various different test programmes that have been carried out over the years vary considerably. In the work conducted at the BRS (Davey and Thomas, 1950), much of the research into the effects of brick and mortar strengths on the strength of brickwork was carried out on 9-inch piers. However, there was also a considerable amount of work done on 18-inch square piers, eight foot in height. Research to investigate the effect of slenderness and eccentricity was done on 9-inch and 13.5 inch square piers at a range of different heights. This work had either formed the basis of or was used to support revisions to the Code of Practice for the structural design of loadbearing walls, CP111, which had been first published in 1948 (BSI, 1948). Compared with the work on piers, less work was done on walls, as it was felt that the effects of slenderness were caused by variability of strength and elasticity within the brickwork and that this was likely to be of greater importance in slender piers. Although the current Code of Practice (BSI, 1992) does recommend reductions in the strength used for brickwork when small plan areas are loaded the emphasis is more towards the design of walls than piers. In the first edition of BS5628 in 1978 the opportunity was given to derive the compressive strength of brickwork from tests and these were duplicate tests on storey height walls. The philosophy at the time was that clearly the best way to derive the compressive strength for the design of a wall element was to test walls. One of the most extensive programmes of wall testing was that carried out at BCRA and reported in 1960 (West et al., 1960). The objective of this work was to investigate the performance of walls built with perforated bricks and to compare this with that of walls built with solid bricks. Although perforated bricks had been in use on mainland Europe for a long time and architects and engineers were familiar with clay blocks as an inner leaf or internal walling product there was prejudice against the use of perforated bricks. At the time manufacturers were interested in taking advantage of technological developments in the manufacturing process one of which was the move towards perforated bricks. There are considerable advantages for manufacturers using raw materials that are suitable for – 150 –

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FULL SCALE TESTING the production of perforated units. It is interesting to note that some of these would today be regarded as being of benefit to society at large, especially with its increased environmental awareness, for example reduced use of a non renewable raw material (clay) and reduced use of energy for drying and firing. However, there was at the time a perhaps understandable cynicism in some quarters that brickmakers were trying to sell their customers fresh air. A total of 24 brick types were selected which included seventeen different perforation patterns or volumes and which came from a wide range of geographical areas and geological strata. The programme consisted of three repeat walls for each combination of brick and mortar designation and these were tested with both axial and eccentric load. This programme of work did a great deal to improve confidence in the use of perforated bricks. Although statistically, differences between the performance of brickwork made with solid and perforated bricks were detected the results confirmed that the permissible stresses, which were incorporated in CP111 from the earlier work on piers (Davey and Thomas, 1950) could be used with confidence. A similar exercise was subsequently carried out for a range of frogged bricks (West et al., 1972). It was found that for axial loading, the compressive strength of walls was related to the compressive strength of the bricks in much the same way as for perforated bricks although the general strength was somewhat reduced. In the case of eccentric loading the effect of the frogs seemed to be less significant. In 1976 a programme of testing was carried out on a range of calcium silicate brickwork (West et al., 1976). A comparison of wall strengths with the permissible strengths calculated according to CP111 showed that all the results exceeded the Code requirements and that calcium silicate brickwork performed at least as well as clay brickwork

Testing machines When testing at small scale, for example compression specimens as described earlier or 9-inch cubes the availability of suitable testing machines is not really an issue. However, when in the early 1960s thought was given to the problem of generating data on the strength of brickwork walls the test equipment was not readily available. As is clear from the extent of the publications on the strength of brickwork walls the will and financial support was available from the brick manufacturing industry to do the exercise. The industry needed to develop the testing machine, carry out the programme, publish the results and most of all have them accepted as a contribution to the knowledge base as valid as the extensive work done using the Amsler Machine at the BRS. A machine was designed and built at BCRA and the original brief was: 1 That it should be capable of crushing a storey height (8 ft 4 ins) wall, 9 inches thick and 6 feet long, constructed from class A engineering bricks 2 Additionally it should be capable of applying a shear load to such a wall 3 That two such walls could be tested in one day with minimum labour requirements. – 151 –

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Not surprisingly, the cost should be a minimum, commensurate with fulfilling the first three requirements. The machine, which was designed and built, is shown in Fig. 5.1. To prevent shock waves being transmitted to adjacent houses it was mounted on a concrete block 10ft × 9ft × 5ft deep, which was cast in a 1inch thick cork-lined concrete pit. The details of the design are described in more detail elsewhere (Hodgkinson et al., 1968). At the time the machine was built (1965), two other newly commissioned machines were available, these were at Edinburgh University and Structural Clay Products Ltd. The designs of the machines were all quite different and although all were capable of satisfying the first point of the BCRA brief, bearing in mind the need to ensure the credibility of the work a comparative test programme

Figure 5.1

The BCRA wall testing machine. – 152 –

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FULL SCALE TESTING was carried out (Hodgkinson et al., 1968). Similar walls were constructed, using the same materials and craftsmen at the three laboratories mentioned and also at the Building Research Station. The plan was, as far as was possible to test walls at three ages: 1 month, 4 months and 11 months at each laboratory, although it was known that for reasons of accommodation the involvement by the Edinburgh team would be limited. The main outcome of the work was that from the results of the tests at BRS, BCRA and SCP Ltd the overall coefficient of variation was 6.8% which was described at the time as highly satisfactory. It is of tremendous credit to all concerned that so much care was taken to ensure that the basis of what proved to be such important and influential programmes of work on brickwork in the 1960s and beyond, was sound in scientific and engineering terms. The BCRA machine is in regular use perhaps most spectacularly in recent years to test the strength of masonry piers made from the granite and sandstone used for the New Parliamentary Building (Portcullis House) above Westminster Underground Station.

Construction of test specimen and test procedure When walls are to be tested in compression it is usual for this to be done in a fixed machine as described earlier. In the UK it has been traditional for the walls to be allowed to cure for 28 days prior to testing, consequently if the wall is built in the machine it cannot be used for the curing period. In any commercial laboratory the machine needs to be used efficiently and it is usual for the walls to be built out of the machine and loaded into it with a purpose made conveyor or crane. The standard way of testing walls in ISO9652-4 (ISO, 1993) is to load them axially with a uniformly distributed compressive stress in a flat-ended condition. The wall, which is typically 1.2 m–1.8 m long and 2.4 m–2.7 m high is built off a flat base often of steel or concrete. It is usually built in two lifts following the Code of Practice recommendation (BSI,1985) to not build to a height of >1.5 m in 1 day. This limitation is to prevent the mortar in the lower courses being squeezed out of the joints, but in the case of some dense bricks it may not be possible to build to this height before this happens. The part built wall should be covered with polyethylene when left overnight to prevent it drying out so that there is not an excessively dry surface off which to build the remainder of the wall. A construction joint of this sort may be unimportant but the precaution should be taken as it is in the mid-height of the wall, which is the least restrained by the platens of the test machine and consequently in the area where failures may be initiated. When the wall is built to its full height a steel plate may be bedded to its upper surface and carefully levelled so that this bears directly onto the upper, moveable loading beam of the test machine. Alternatively such a plate may be bedded onto the wall when it is loaded into the test machine, in these circumstances a rapid hardening mortar can be used, the loading beam of the machine being lowered to ensure complete contact with the plate. For low strength materials gypsum plaster may be used as an alternative to rapid hardening mortar. When the wall is complete it should be close covered with – 153 –

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polyethylene to prevent it drying out too rapidly. This is usually for the first three days and the remainder of the curing is in ambient laboratory conditions, which should be at a temperature between 15°C and 25°C and a relative humidity of 60% ± 10%. Test procedures have evolved over many years and in some cases the close covering has been for the whole period of 28 days. Although this makes little difference to the strength achieved the 3-day procedure does lead to slightly less variable results. Walls are tested for a variety of reasons and where these relate to specific projects the construction, curing and testing may differ from those described. For the development of strength data walls are usually of the same thickness as the unit and the plan area should exceed 0.125 m2. When the wall is tested the procedure is largely as for the smaller compression specimens, i.e. the wall is loaded steadily so as to cause failure to occur within 15–30 minutes. The British Standard Code of Practice refers to a loading rate of 1 N/mm2 per minute but the ISO guidance (ISO, 1993) aims to control the time to failure. Such differences may be of little significance in relation to wall strength for design purposes but it has been suggested (Knutson and Nielson, 1995) that the longer the test period leads to greater creep strains occurring which would affect the calculation of the modulus of elasticity. It is not that common to measure strains in walls being tested in compression and if it is done the measuring instruments are usually removed long before failure. In order to estimate the load to cause failure and hence calculate a suitable loading rate and load at which to remove any instrumentation ISO 9652-4 gives the formula. Fmax = 0.5(fb)0.75 (fm)0.25.A Where Fmax is the failure load fb is the normalised compressive strength of the bricks fm is the strength of the mortar A is the area of the loaded cross section In effect, this procedure is to use the design formula for a masonry wall to predict the strength of the test wall with some safety margin. Failure of a brickwork wall under axial compression is usually by vertical splitting of either the ends or faces. For bonded 215 mm thick walls the ends often show signs of failure first as the perpend joints are a source of weakness. It is often the case that some local crumbling of the mortar beneath the upper metal plate occurs prior to failure and often in the case of multiperforated units the cracking of internal webs can be heard prior to any visible signs. It is most important to realise that whatever is said about the prediction of wall strength and the signs that might occur prior to failure if one knew what was going to happen the test would not be taking place. Failure can occur unexpectedly and a brickwork wall when it collapses can be spectacular (see Fig. 5.2). Safety is a major issue and precautions must be taken to ensure that collapsing brickwork does not endanger anyone. It is important that a proper risk analysis is carried out to ensure that access is restricted and all personnel involved are wearing appropriate safety equipment, i.e. boots, glasses and helmets. – 154 –

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Figure 5.2

Brickwork wall tested to failure in compression.

The strength of the wall is simply the failure load divided by the cross sectional area and ISO9652-4 recommends that three walls be tested and the mean result used. BS5628 Part1 recommends the use of two walls and that the mean result divided by 1.2 is a reasonable estimate of the characteristic compressive strength for design purposes. Both guidance documents give advice on what to do if the batches of bricks or mortar that are used for the test, for whatever reason differ from those for which the result is to be applied. BS5628 Part 1 allows the test results to be adjusted to account for the difference between the minimum brick and mortar strengths specified for the project and the actual strengths of those used in the laboratory for the test. Although it is not that common to measure strains in walls under test the central lateral deflection of the wall is often measured. This can be done safely by the use of an extension piece to a displacement transducer some distance from the wall. The reason for doing this is that the test result may then be augmented so as to relate to the area under compression at the mid-height. So if the deflection just prior to failure is δ and the wall thickness is t the measured strength can be increased by multiplying it by t/(t–δ) although in both ISO9652-4 and BS5628 Part1 the increase is limited to 15%. – 155 –

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There are many reasons why walls have been tested with other than the standard conditions discussed so far. For example the effect of eccentric loading is of great interest in the development of design guidance. Much of the work on eccentric loading has used the approach of West et al. (1968) and shown in Fig. 5.3, although in research aimed at a more realistic representation of the situation in real buildings, eccentric load has been applied through concrete floors bedded at the head of the wall (Bradshaw and Hendry, 1968). It is possible to apply load through pin end conditions, for example by milling a groove in the plate at the head of the wall and putting a cylindrical bar into it. The bar may be greased to remove friction and some restraints should be used to stop it rolling out during the test. The same arrangement can be made at the foot of the wall and the plate kept level by wedges during construction or by the use of polystyrene which can be dissolved away by acetone immediately prior to test. Clearly the use of pin ends is difficult and less realistic than the situation in real buildings, however it does mean the effective height of the wall is clearly defined as the actual height. The effects of restraint from the platens of the test machine mean that it is somewhat less (about three-quarters) in the flat ended condition. The considerations that apply when testing single leaf walls apply also to cavity walls. The additional consideration is whether the load is to be applied to one or both leaves and this will depend on the purpose of the test. It is probably more usual for one leaf only to be loaded and this reflects the situation in the external wall of many buildings. Most design codes allow the designer to take advantage of the stiffening effect of an unloaded leaf which is tied to the loaded leaf but not its cross sectional area for resisting load. The tying arrangement is important (Fisher, 1971) and it is possible for the curvature introduced into an unloaded leaf to cause it to fail and to induce failure in the loaded leaf at a lower load than the single leaf (Davey and Thomas, 1950). As is the case for testing small specimens, it is important to characterise the bricks and mortar that were used in the test wall. As a minimum, this would be by compressive strength and water absorption for the brick and by compressive strength and consistence for the mortar. In the case of mortar the consistence has often been described by the result of the dropping ball test although it is likely that in the future the flow table will become a more popular means. Any special conditioning of the bricks, e.g. docking should be recorded as should any unusual conditions. What has been said here about the characterisation of materials applies equally to walls built for lateral load or shear testing, however, in those cases more attention may be paid to those properties more relevant to adhesion between bricks and mortar, e.g. initial rate of water absorption of the bricks or water retentivity of the mortar.

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Spreader beam Strawboard Grano-fondu capping

1 in.

1 in.

Figure 5.3

Grano-fondu plinth Concrete plinth Steel plate

Loading arrangement for eccentric compression.

Lateral load When first published in 1978, BS5628 Part 1 (BSI, 1978) was the first Code of Practice to give detailed guidance on the design of walls to resist wind loading. In the years leading up to this publication a considerable amount of wall testing had taken place largely at BCRA but also at other centres, for example the University of Edinburgh. The guidance was fairly controversial around the world as it allowed the flexural strength of the brickwork as measured in the small specimens described earlier to be used in the equations derived from the yield line theory to design wall panels. The reasons for the controversy are varied but the main ones are the use of yield line theory, which was developed for reinforced concrete, a ductile rather than brittle material and the use of the flexural strength of brickwork normal to the bed joint. In a number of countries where masonry is important, especially Germany this flexural strength is taken as zero. This caution is partly because of concerns over its reliability but also because the national practice was to use more massive walls than in the UK and for the self weight to be sufficient to resist the wind. However, the considerable amount of test work confirmed that the design method led to conservative designs and that database has been increased steadily in subsequent years. It is also significant that the crack patterns that appear in walls loaded to failure tend to follow the patterns predicted using the yield line theory (see Fig. 5.4). The use of the Yield Line Analogy as it was described was endorsed in the single most comprehensive publication on the subject at the time (Cajdert, 1980). – 157 –

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Cracking pattern on wall due to lateral loading.

The text so far has been largely concentrated on developments in the UK. Nevertheless, at a similar time in Australia the so-called strip method for reinforced concrete was being applied to masonry (Bohr, 1981) and has continued to form the basis of design since. The use of yield line analysis has many advantages for designers, especially when incorporated into computer programs as it can deal with all kinds of support conditions at the edges of the walls and readily incorporate window and door openings. Most of the original work to validate the use of the theory was on walls without openings and the 1992 edition of the Code deals with openings in a simplistic way however current developments should lead to improved guidance in this respect (deVekey et al., 1996, Edgell, G.J. and Kyaer, E., 2000). The publication of BS5628 Part 1 in 1978 was a tremendous step forward in brickwork design. The development of a useable approach for dealing with a transient load, the wind and the many variations in wall configuration and materials was a significant step. Prior to this an approximate method for sizing wall panels had been published (Bradshaw and Entwistle, 1965) but in 1973, Hendry concluded that there was not then enough test data available to determine the accuracy of the various theoretical approaches (Hendry, 1973). The earliest work was on clay brickwork (West et al., 1971, 1974) and this was later extended to calcium silicate brickwork (West et al., 1979) and (Haseltine et al., 1979). Subsequently, the work was extended to cover cavity walls with either two brickwork leaves or one brickwork and one autoclaved aerated concrete – 158 –

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FULL SCALE TESTING inner leaf with a variety of different types of wall tie (West et al., 1979). The work on cavity walls was extended further to include a wider range of concrete blockwork inner leaves (West et al., 1982) and hence the common forms of wall all construction in the UK had all been tested and compared with the design guidance. In the same systematic way that the performance of different walling materials either singly or in combination with others were tested so were the wall edge details (West et al., 1979 and Moore et al., 1979). Although it is possible to agree on standard ways to deal with the edge conditions in walls in the laboratory it is of great interest to relate the results to the performance of walls in real buildings. When buildings have become available tests have been done to compare their performance with that calculated using the Code of Practice and the results have generally been reasonable (Hodgkinson et al., 1982 and Cavanagh et al., 1982). In 1985 the first UK code of Practice for Reinforced and Prestressed Masonry was published (BSI, 1985) and this included an appendix giving an indication of the design approaches that could be used for walls where the lateral load resistance was enhanced by the introduction of bed joint reinforcement (Edgell and de Vekey, 1985).

Construction of walls and test procedure The precautions that need to be taken when constructing brickwork walls to be tested for their lateral load resistance are largely the same as for their compressive strength. However, the flexural strength of the brickwork is more sensitive than is the compressive strength to, for example, mortar consistency and so particular care is needed in the specification (Edgell, 1987; Gairns et al. 1987). The effect of the support conditions around the edges of the wall are considerable, providing moment restraint can considerably enhance the lateral load resistance. This poses a problem as in order to scientifically experiment to test a theory the support conditions should be as clearly defined as possible, however to closely simulate the conditions in real buildings requires the supports to be fairly poorly defined. In the early work (West et al., 1971, 1974) walls were built off a bituminous damp proof course laid on an inverted steel channel section. This proved to allow too much slip at the base and subsequently the d.p.c. was placed in the bed joint above the lowest course of bricks. In the case of concrete blockwork a base course of bricks was laid and the d.p.c was placed in the bed joint above before continuing to lay the blockwork wall. It is generally assumed that this sort of base to a wall can be considered to behave as a simple support. The use of polyethylene in similar tests did allow some slip at the base and led to lower lateral loads being resisted. There have been cases where considerable effort has been made to introduce a true pin ended condition at the base using a roller much as was described for compression specimens but this is the exception rather than the rule (Middleton and Drysdale, 1995). Most walls are built in some sort of frame so as to facilitate making the side supports. In order to represent typical restraint in low rise construction or infill – 159 –

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panels ties have been used to support walls at the side (West et al., 1971, 1974). Although this is realistic and ensures the walls are stable during construction the restraint is enhanced above that from a simple support without providing full moment restraint. Other approaches have been to fix a steel plate to the wall using dental plaster to allow for tolerances on the brick width and to allow this to bear onto a roundel (Regan and Southcombe, 1991) or steel rod (Middleton and Drysdale, 1995). It has also been found to be satisfactory to simply butt the rear face of the brickwork tightly against a steel flat (Cajdert, 1980) or against a steel rod faced with a 5 mm thick rubber strip (Ferguson and Edgell, 1995). However these supports are provided, it is most important that any rotation at the support does not lead to the wall becoming wedged in the testing frame as arching action can then occur within the thickness of the wall and lead to much enhanced loads being achieved (Hodgkinson et al., 1976). Where full continuity was required at a support this was achieved by extending the wall beyond the vertical steel supports of the panel in question and subsequently providing a return (Moore et al., 1979). This is clearly a more expensive approach to testing, but the provision of short returns alone does not give the effect of full continuity. Much of the data relates to walls where the head of the wall is free, although vertical ties and soft joints have been investigated. As with the sides care needs to be taken that any movement does not cause the head of the wall to come into contact with the test machine. In most lateral load tests the pressure has been applied through a single or multiple air bags so as to give a uniform distributed load. The air pressure, which is measured by manometer, is usually increased in increments and at each dwell period deflection measurements and/or strain measurements are taken and observations made. It is usual for the reaction to the air bag loading to be provided by a timber reaction board, which is supported by and attached to a steel frame. There have been other approaches for example by using a ‘Christmas Tree’ of layered steel hangers a single tensile force could be distributed among a lot of steel wires each of which passed through the wall and a steel plate which butted up to the wall face (Anderson, 1976). The idea that an incrementally applied load would lead to the same failure pressure as for a dynamic load caused by the wind was based on the assertion that the gust speeds of relevance are too slow to generate the inertia effects that occur in very fast loading, e.g. from explosions. However, the assertion has been questioned and a limited programme of testing was carried out to compare ‘static’ air bag loading with a 3 second gust applied through a suction box (Ferguson and Edgell, 1995). The conclusion was that the dynamically loaded walls exhibited no strength enhancement. When cavity walls are to be tested careful thought needs to be given to the loading arrangements as the performance of certain types of wall tie, e.g. butterfly wire can be very different in tension and compression. There may well be circumstances where it is desirable to put the ties into tension. This can be achieved by applying an air bag loading to the inside face of the outer leaf, which is reacted by boards within the cavity. In this case all that has been said – 160 –

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FULL SCALE TESTING about supports to the edge of the wall remains relevant but there is additionally the problem of ensuring the boards do not interfere with any wall movements. Ties are passed through holes in the boards and care is needed that when the air bags are inflated these do not grip the ties. The movement of each leaf can be monitored at tie positions using linear voltage displacement transducers connected to data logging equipment or indeed dial gauges and it is quite clear that for certain ties the transfer of load is unevenly distributed and varies during the test. However the assumption that the lateral strength of a cavity wall may be taken as the sum of the strengths of the two leaves is well supported by the test results.

Shear The shear strength of unreinforced walls is often not a critical factor in design as they may be quite substantial for other than structural reasons, for example the shear walls in cross wall construction are often party walls and hence are fairly massive for reasons of acoustic isolation. In this particular example they may well also be quite heavily precompressed which will aid the shear resistance. In an international sense the most important reason to rely on shear strength is for resistance to seismic effects. This is of less interest in the UK than in areas of greater seismic activity and is quite a science in its own right and will not be described further. Readers who wish to take this subject further can refer elsewhere (D.G.f.M., 1991). The shear resistance of walls is a complex subject and consideration was given as to whether such a test were needed to support the provisions of ENV 1996-1-1. It was concluded that the small-scale triplet test was sufficient provided that the adjustments to the constants in the Coulomb equation described earlier were carried out and providing upper limits were included. However in order to be in a position to make such a judgement a considerable amount of test work had been carried out. The behaviour of brickwork in shear is complex because at a very fundamental level the combination of shear and precompression causes biaxial stresses in a material whose properties are highly dependent on direction. Consequently the orientation of the principal stresses acting in the brickwork relative to the bed joints is important. In real situations there are of course practical considerations such as whether a short wall will overturn, how effective are any returns in providing shear resistance and whether the presence of a damp proof course is the controlling influence. The in plane behaviour of brickwork as a material and its relevance to the performance of shear walls has been extensively reviewed (Page et al., 1982). The importance of the orientation of the principal stresses to the bed joint has been investigated by varying the angle between the load and the bed joint. This can be done by building the specimens in special jigs so that although the joint as laid is horizontal when in the test machine it is inclined or by cutting test specimens from larger panels. These approaches are described in more detail in the review (Page et al., 1982). – 161 –

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Tests on shear walls Tests have been carried out to investigate the performance of lightly loaded (precompressed) shear walls as complete structures (Edgell et al., 1982; Cavanagh et al., 1982). In this work the programme was significant enough to warrant producing a purpose made test rig, whereas most earlier work involved using a standard compression machine to apply a vertical force. The test rig was relatively simple and is shown in Fig. 5.5. Provision was made to apply a horizontal load to the edge of the wall through a series of hydraulic jacks, that at the top having the greatest capacity to simulate wind load being transferred to the lowest storey in the building from the walls above. Vertical load was similarly applied at the head of the wall through another series of hydraulic jacks connected to a common manifold. The jacks applied the load to the wall through a series of small spreader plates above which there were rollers between them and the loading beams connected to the jacks. In this way in plane displacement was able to take place freely. The vertical jack nearest to the horizontally loaded end was greater in capacity than the others to represent a greater vertical load from the external walls of the building above. The base of this rig was reinforced concrete and the practice was to lay one course of bricks, which was prevented from sliding by an end stop and to then lay a d.p.c. in the middle of the next bed joint and build the wall up following usual good practice. Curing followed the usual procedures as described previously and tests were usually carried out 28 days after completion of construction. These particular investigations enabled storey height walls up to 4.5 m long to be built and tested and the modes of failure including sliding, diagonal cracking, crushing at the toe remote from the shear force and overturning. The improvement in behaviour when a return was added was investigated and this introduced the possibility of vertical cracking at the shear wall/return junction as part of the failure mechanism. This new possibility led to the investigation of the differences between a bonded connection between the shear wall and the return and a simple buttered up joint or tied connection.

Figure 5.5

Test rig for shear walls. – 162 –

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FULL SCALE TESTING In work at this scale and also where there are considerable variations in the stress patterns across the structure it is usual and very sensible to collect as much data as possible. It would be usual to measure sliding and any uplift at the d.p.c. and relative movement between the shear wall and any return with linear voltage displacement transducers or dial gauges. In plane displacements would be measured in a similar way as would any out of plane movement. Strains would usually be measured both linearly, for example vertically at the base remote from the shear load and by strain rosette in the central areas where the orientation of the principal strain relative to the bed joint was of interest (see Fig. 5.6). The measurement of strains in rosette formation at several locations whilst following the principle that a gauge length should be a multiple of brick plus joint dimension is quite demanding and it would be usual to use a demountable gauge to measure between studs fixed to the wall. Manual measurement is naturally quite slow and in theory some time dependent effects might have been expected but the results were apparently not affected. Although failures of shear walls in this sort of testing is usually not as explosive as in compression care is needed in deciding when to stop taking manual measurements. This is particularly so if overturning is likely or where for example there is an opening with a lintel over it which could fail or fall.

Figure 5.6

Location of strain gauge measurements. – 163 –

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Reinforced and prestressed masonry testing The use of steel to act in a composite way with masonry construction is not new, for example steel or indeed iron straps have been used extensively to ensure that masonry units and mortars behaved as if bonded together in situations of high stress. In such locations where cracking had occurred strapping was often used as a remedial measure. In similar vein prestressed wrought iron bars were often used to restrain bowing masonry walls, the bars being fitted and end plates attached when hot and which subsequently shortened and tensioned as they cooled and recovered and restricted further out of plane movement. The first recorded use of reinforcement being designed or built into brickwork in the sense that it was embedded was in 1825 when Sir Mark Brunel used it for caissons at either end of the Wapping – Rotherhithe Tunnel (Beamish, 1862). The earliest known experiments in reinforced brickwork were also by Brunel who demonstrated the way that by relying on the tensile capacity of reinforcement brickwork beams could be constructed that would carry considerable loads over long spans. His most famous experiments were in his construction yard at Nine Elms, see Fig. 5.7, and hence the results of the most successful test were reported as from the Nine Elms beam (The Civil Engineer and Architects Journal, 1938). In this century, interest in the use of reinforcement in brickwork has often been stimulated by shortages of materials for the production of reinforced concrete and consequently much of the experimental work has been to understand whether the same design principles could be used. In the 1920s, a considerable amount of testing was carried out in India by Sir Alexander Brebner (see Fig. 5.8) in order to justify a great deal of reinforced brickwork construction in Bihar and Orissa (Brebner, 1923). In the UK interest was stimulated in the 1970s especially by Donald Foster of Structural Clay Products Ltd (SCP) a company supported by four brick manufacturers to broaden the appeal of brickwork. Foster’s structures include a factory wall to a brick factory at Chesterton, Stoke-on-Trent, which is a reinforced beam spanning from one pile cap to the next in a mining subsidence area, a prestressed brickwork water tank (1975) and the slenderest piece of prestressed brickwork in the world, the obelisk built for the Stoke-on-Trent Garden Festival in 1986, which remains largely unheard of today (see Fig. 5.9). Another main reason for an increase in the use of reinforcement elsewhere in the world has been its usefulness in restricting damage due to earthquakes. Although cyclic loading is used now to demonstrate the improvement in performance when reinforcement is incorporated in less recent times it was observations of what had stood and what had not after an extreme event that led to development by trial and error. For many years architects have often recommended the use of prefabricated bed joint reinforcement at changes of section or elevation, e.g. around window openings to prevent or restrict shrinkage or settlement cracking. This recommendation by experience attracted little attention until the 1980s and today bed joint reinforcement is recommended for some structural uses, crack control or both. Another application which was popular in central Europe in the 20th century – 164 –

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FULL SCALE TESTING was in flooring where prestressed wires were concreted into grooves in ceramic units to form floor planks (Kalges,1958), sadly these are no longer produced.

Figure 5.7

The nine elms beam test.

Figure 5.8 Reinforced brickwork cantilever beam test, Brebner.

Figure 5.9 Foster’s obelisk at the Stoke-on-Trent garden festival. – 165 –

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Flexure and shear The flexural and shear behaviour of reinforced and prestressed brickwork can be investigated by testing beams. It is usual to use a simply supported beam and a central two point load as shown in Fig. 5.10. While there are no standard tests the principles are relatively simple. Rotation at the supports can be enabled by sitting a cylindrical steel bar in a ‘vee’ shaped support at the top of each support. A similar arrangement can be made at loading positions although a loading plate fixed to the face of the jack by a hinged connection is common. It is usual to stop localised crushing of the beam at the loaded supports by fixing a steel plate to the beam using a fast setting epoxy resin adhesive. Load is usually applied in increments such that failure occurs in about 15–20 minutes, This allows sufficient time for visual observations, for cracks to be highlighted in black marker and logged or photographed and for strain and deflection readings to be recorded. It is often sufficient to record deflections only and this may be by using dial gauges or electrical transducers although it is usual to remove them once deflections begin to diverge from a linear relationship with load to prevent damage. Strains in steel can be measured using electrical resistance strain gauges fixed to the bars although they do need careful protection to ensure no moisture ingress during beam construction. Where the shear span ratio defined as the shear span to effective depth ratio is greater than about six failure is usually flexural. Where the ratio is less than six, there may be an element of shear, this feature increasing as the ratio reduces. The shear span is defined as the distance between a support and the shear load, which for three point loading is half the span, the effective depth is the distance from the centre of area of the tensile reinforcement to the top of the beam. For more complex loading arrangements the shear span is taken as the maximum bending moment divided by the maximum shear force (Edgell, 1982).

Figure 5.10

Reinforced brickwork beam under four point loading. – 166 –

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FULL SCALE TESTING When flexure is the key to the design the designer’s aim is to under-reinforce the section such that failure would be by the steel yielding and hence would be relatively gradual compared with the explosive compression failure of brickwork. A theoretical analysis suggests that where the section is balanced the stress in a rectangular stress block would be 0.75fk (Beard, 1982). However, numerous experiments have shown that the value used can be fk and lead to safe results (Roberts and Edgell, 1981). For short beams or loading situations where shear is critical the behaviour is dependent on the percentage of steel and the shear strength can be enhanced for short shear span ratios. In this respect the behaviour is similar to that of reinforced concrete, although the adjustments do differ. Although brickwork can be investigated by testing beams it is primarily a walling material and in the 1980s a considerable effort was expended in investigating the performance of grouted cavity and pocket type reinforced retaining walls. The pocket type wall, first described in 1971 (Abel and Cochran) was of special interest as it had the potential to compete with reinforced concrete in price at quite significant wall heights and hence the potential for sales of bricks was good. The key issue at the time was to improve the rules for design to resist shear as this often dominated the design but the experience was that failure was in flexure. In the knowledge that several walls would be tested a re-usable steel base was designed and built at BCRL and is shown in Fig. 5.11 (Tellet and Edgell, 1983). Although it could be justified in terms of realism it was soon discovered that concrete bases that could be anchored down were quite cost effective and enabled more than one wall to be built at a time. Loading was by a series of different horizontal jacks that were anchored to a buttressing frame at different heights up the wall such that the building moment and shear force at the base were in the same ratio as for a triangular (earth pressure) distribution. In this type of testing at full scale, the BCRL walls were 3 m high and 2 m wide, the jacking forces are significant and when the wall begins to deflect significantly there must be sufficient articulation in the loading system to enable the jack to remain perpendicular to the rear of the wall. Failure to do this can lead to the jack cylinder jamming in the jack barrel or the bolts fixing the jack to the frame to fail. Loading tests on such walls typically take half to three quarters of an hour as although deflections are usually monitored using linear voltage displacement transducers mounted on an independent scaffolding frame observation of crack development can be quite time consuming. Transducers can be mounted at the rear of the wall but the scaffolding and loading arrangement became complicated. Instruments with sufficient accuracy and long enough movement are available and they can be mounted at the front. Steel strains can be measured by electrical resistance strain gauges although a flat surface needs to be filed on the surface of the ribbed bars prior to fixing. Brickwork strains can be monitored using Demec gauges or electrical resistance ‘portal’ gauges. Fig. 5.12 illustrates the loading arrangement, its articulation, the fitting of portal gauges, long throw transducers to instrument a wall on a solidly anchored concrete base. Particular care needs to be taken over safety as although failure is expected to be ductile surprises do occur. – 167 –

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In one early test a failure to properly lap reinforcement adequately at a change in wall thickness led to a quite rapid shear failure (Maurenbrecher, 1977).

Figure 5.11 Reinforced pocket type wall built off steel base under test using jack loading.

Figure 5.12 Stepped reinforced brickwork pocket type wall built off a concrete base.

Racking shear Racking shear is an issue principally in relation to design to resist seismic forces, which has not been covered here (see Section 5.1). However, grouted cavity construction is very amenable to the incorporation of both vertical and horizontal reinforcement and has been studied in the UK (Scrivener, 1982).

Compression Brickwork is strong in compression and reinforcement would rarely be considered to resist compressive forces. However in some areas of the world it is permitted to increase the compressive stress in a column by the provision of horizontal reinforcement. The rationale is that the reinforcement confines the brickwork and prevents failure by vertical splitting which is shown in Fig. 5.13. A preliminary investigation using a number of types of horizontal reinforcement did demonstrate that failure stresses were not increased greatly or by a reliable amount but that – 168 –

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FULL SCALE TESTING after failure the column retained a significant amount of loadbearing capacity. It may well be that some consideration could be given to reduction of safety factors from those used for unreinforced brickwork on the basis that failures are not catastrophic.

Figure 5.13

Hollow brickwork column after failure by vertical splitting.

Where eccentric compression is important it has been demonstrated that interaction curves similar to those used for concrete may be used and this has been investigated experimentally and shown to be reasonable (Anderson and Hoffman, 1969; Davies and Eltrify, 1982)

Prestressed brickwork Prestressed brickwork has been used in a number of buildings, the most common approach is the low technology approach of essentially tensioning the bar using a jack or using a torque wrench to tighten a nut down onto a plate on a capping beam. In some circumstances a capping beam is not needed especially if a low prestress is being introduced to prevent cracking rather than make a large increase in capacity. Despite the simple approach when associated with cellular or diaphragm wall construction some very tall walls and high overturning moments can be resisted. In work at BCRL 6 m high walls designed by Bill Curtin were tested. At this scale the number of jacks and the robustness of a reaction frame – 169 –

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make this form of loading difficult and expensive. Consequently a wide (thick) post tensioned wall 6 m high was built to act as a reaction to the load applied to a less wide test wall. The loading was by using six horizontal air bags each with a different pressure to stimulate an earth pressure load, see Figs. 5.14 and 5.15. Similar work had been carried out by Curtin with Phipps at UMIST (1982, 1986) who went on to investigate the performance of slender geometric sections under vertical load (1987). A particularly novel test was carried out on Foster’s obelisk when questions were raised by the local authority about its performance. A halter was placed about the top and a cable, anchored to a large rock so that it was at about 30° to the horizontal was tensioned using a pull-lift, a ratchet device which moved a gear wheel along a chain one link at a time. Measurements of the strain were made using Demec gauges and of displacement from curvatures. The obelisk proved to be extremely stiff and a fortuitous slip of the halter allowed an estimate of the natural frequency of the obelisk to be made. This estimate was subsequently used to demonstrate it would not resonate with wind gusts or demonstrate any of the many of the modes of aeroelasticity.

Figure 5.14 Base of post tensioned diaphragm reaction wall and first three courses of test wall.

Figure 5.15 6 m post tensioned diaphragm test and reaction walls.

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Bed joint reinforcement Prefabricated bed joint reinforcement is often specified to restrict settlement cracking or to enhance the resistance of walls against lateral load. In the latter case the methods of test are similar to those for unreinforced walls and the performance is usually such that much larger deflections can be achieved and numerous small cracks appear which are spanned by the reinforcement as shown in Fig. 5.16. The appearance of the first crack is often at a similar load to that which would cause the unreinforced wall to fail. In beam tests to simulate a settlement situation increasing the number of courses reinforced led to more distributed cracks and greater post ultimate load residual strength. The development of anchorage strength (bond) is important for all reinforced brickwork (Booth and Edgell, 2000) and the techniques are as have been standardised for reinforced concrete. In the case of bed joint reinforcement a simple pull out test in the plane of the wall has been standardised as BSEN 846, Part 2 (2000) in order that the manufacturer can use the result to declare a length of overlap needed between successive lengths of reinforcement.

Figure 5.16 Wall reinforced with prefabricated bed joint reinforcement illustrating extensive cracking at failure. – 171 –

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Arch bridges There are over 40,000 masonry arch bridges in the UK most of which are over 100 years old and all are exposed to loading regimes very different to those envisaged by their builders. It is often the case that as a consequence of changes in loading, coupled with the natural deterioration of the bridge fabric and the inadequacies of masonry arch bridge assessment methods, the assessing (or design) engineer needs to use full scale testing as a means to confirm the assessment (or design). The most obvious way of achieving this is to apply a known loading and observing the bridge’s response. There are several levels of loading, which may be considered: • Supplementary load tests apply loads that will not cause any permanent damage to the bridge, but which will inform any modifications to the mathematical model used for analysis • Proof loading is used to verify that the design, construction or repairs has been carried out satisfactorily. The load level is usually equivalent to serviceability limit state loading • Proving load testing is undertaken to check the safe load-carrying capacity. The level of loading is high and, consequently, the risk of permanent damage is also higher than the other tests above • The bridge carrying capacity is obtained by applying a suitable load factor to the test loading. Alternatively, dynamic load testing uses either ambient or forced vibration whilst the bridge’s response is monitored. This form of testing establishes a ‘fingerprint’ for the bridge, which can be used to monitor its condition over a period of time • In all the above types of load test it is very important that no permanent damage is caused to the bridge; merely restricting deflections or strain levels will not guarantee this – as will be discussed later • Finally, the bridge may be tested to collapse. This type of test is only undertaken on obsolete bridges or full-scale laboratory tests and is used for research to study their behaviour. It is very important at the outset to dispel the idea that all masonry arch bridges are of similar construction. Nothing could be further from the truth. Before embarking upon any testing programme it is vital that a comprehensive desk and field survey is undertaken to establish as accurately as possible, the dimensions and form of construction. Fig. 5.17 shows a typical arch bridge construction. Where practicable the bridge foundations were taken down to rock. This was not always possible. If good ground could be established then the piers and abutments were constructed from footings using corbelling of the brickwork or stonework to increase the foundation size and thus reduce the bearing pressures to acceptable levels (and reduce settlement and other movements). If good ground could not be found at a reasonable level, or excessive settlements or spread were expected, timber piles or grids, even faggots, were used to improve foundation performance. – 172 –

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Figure 5.17

A typical arch bridge construction.

The piers and abutments were often used to provide temporary support to the centring – this had two major beneficial effects. First, it provided some clearance to the obstacle being traversed by the bridge; and second (and perhaps more importantly), it ‘preloaded’ the piers and abutments with the dead load of the centring, which was probably not much less than the ‘normal’ live loading applied to the bridge. (In the early part of the 19th-century the heaviest load crossing a bridge would probably have been a gun carriage!) The centring was usually left in place until the arch barrel had been completed, the spandrel walls constructed up to the string course level and the backfill placed. At that stage, the centring would have been removed and the carriageway surface completed with the parapets being the final element to be constructed thus minimising the risk of settlement effects on the parapet and its coping. Most assessment and analytical methods start by determining the carrying capacity of the arch barrel – sometimes allowing for some interactions with the backfill. The engineer then considers the condition of the bridge and takes a more holistic approach to determine the allowable carrying capacity, recognising the three-dimensional, interactive nature of this type of bridge. – 173 –

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The arch barrel may take various shapes; semi-circular, parabolic, segmental, elliptical, gothic pointed, etc., and may comprise dressed stone, random rubble, brickwork or concrete. The backfill over the arch may be contained by spandrel walls that extend beyond the abutments to provide wingwalls. The backfill may be anything from ash and rubble through to concrete. Clay was sometimes used (particularly over canal bridges) as a waterproofing membrane over the extrados of the barrel. To lighten the structure, and also to eliminate horizontal soil pressures on the external spandrel walls, internal spandrel walls were sometimes used. This form of construction was often used on bridges with spans greater than about 12 metres. The proportions of these internal spandrel walls depended upon the nature of the available masonry and whether or not the over-spans took the form of transverse stone slabs or arches. Significantly, there are usually no external indications of the form of internal construction. Even the arch barrel thickness cannot be relied upon corresponding to that shown on the elevations. (The latter was frequently proportioned to comply with aesthetic demands). Alternatively, internal arches may be provided which span longitudinally and spring from the extrados of the main arch barrels. These may be totally internal, or extended through the external spandrel walls to provide an aesthetic feature and, in the case of river bridges, an escape route for flood water. The main objectives of full-scale testing are to determine the load carrying capacity of a given structure with a reasonable probability that it will not suffer serious damage, and to enhance our understanding of the behaviour of the structure. The test methodology is intimately related to our understanding of the structural behaviour. It is, therefore, appropriate that, some space is given to discuss the present state of the art of understanding of the behaviour of masonry arches. It must be remembered that most masonry arch bridges were conceived as gravity structures for which mass and geometry were the main design criteria. Certainly, the originators of the proportions passed down from antiquity had no thought of stress criteria and were probably based upon bitter experience. Barlow (1846) had demonstrated that there was no unique thrust line associated with a stable arch but that there were many possibilities. Navier (1826) had shown that for linear elastic materials, where plane sections remain plane, tensions could be avoided by ensuring that the thrust line lay within the middle third of the section. Castigiliano (1879) used the elastic continuum approach but allowed tension to develop and then, upon ‘removal’ of the tensile material, iterated the analysis to check that tensile stresses were not present in the modified structure. The main advantage of an elastic analysis is that stress levels and deflections can be calculated – how meaningful they are is open to much debate but it has to be conceded that they provide a ‘feel’ for the problem. However, it is universally accepted that masonry arch bridges crack – even before the centring is removed! This is a very important observation, which should not be obscured by the sophistication of the currently available modelling software. An alternative approach recognises the particulate nature of masonry and the observed collapse mechanism. The simplest representation of a masonry – 174 –

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FULL SCALE TESTING arch barrel with backfill is shown in Fig. 5.18. The barrel is idealised as a two-dimensional arch made up of blocks, which are unable to resist any tensile stress, are rigid, possess infinite compressive strength and do not slide relative to each other. Heyman (1982) applied plastic theorems to this idealised model and thereby allowed the collapse load to be determined.

Figure 5.18

Simplest representation of a masonry arch barrel with backfill.

This simple idealisation affords significant insight into the behaviour of masonry arches but should be applied appreciating that masonry arch bridges are complex three-dimensional structures. Consequently, it is vital that the relative significance of each of the elements of the bridge is considered. For example, in the case of a multi-span bridge, it has been found that with slender piers, it is the support offered by the unloaded adjacent spans and wingwalls that has a major influence on the capacity of the loaded span; see Fig. 5.19. Also, the stability and stiffness of the spandrel and wingwalls together with the angle of the bedding planes of the barrel together with the plan torsional stiffness of the piers and abutments which are very important in determining the behaviour of skew arch barrels. Generalisation of the model needs to take these factors into account together with the effects of sliding between voussoirs, ring separation between brickwork rings, soil pressures, load dispersion, and local crushing. Large scale laboratory tests have shown that under monotonic loading to failure, it was not unusual for the response of the structure to the loading to change as the loading increased. For example, the hinge positions moved and ring separation developed. All this makes the design of a meaningful non-destructive test very difficult. – 175 –

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Figure 5.19

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An example of a multi-span bridge with slender piers.

The determination of material properties from standard tests will not be considered in this section, as they are described and discussed elsewhere. Hendry (1990) has written a comprehensive guide to masonry properties for assessing arch bridges. It is important to remember that the initial stresses are important in any bridge structure, particularly in masonry arch bridges. Not only because the dead load is significant but also that bridges settle and distort to a greater or lesser extent with the passage of time and thus cause stress (and hence strain) redistribution. For example, spreading of the abutments, brought about by the horizontal component of the thrust, will probably have caused ‘hinges’ to occur at the crown and each abutment, thus producing a three-hinged arch barrel. Prior to fixing the testing programme, it is vital that a detailed survey of the bridge is undertaken. (This may include bore holes if ground anchors or tension piles are planned to resist the applied loading.) Foundations in water should be checked for scour. Particular attention should be given to the dimensions and geometry of the structure and its form of construction. Sound pointing may hide open joints and/or weak mortar. It is important therefore to remove intrados mortar to determine the nature and condition of the original mortar. This can be done at the same time as core samples are taken to confirm the barrel (spandrel/wing wall) construction. Cracks should be carefully mapped and measured as this will give some indication as to how the bridge is behaving. It should be noted whether or not any cracking is new or ‘historical’. It is usual for significant historical cracks to be monitored regularly using Demec – 176 –

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FULL SCALE TESTING points or ‘tell-tale’ plates. The Demec points comprise small metal discs (each with a central dimple) which are glued onto the surface of the masonry (one each side of the crack). The distance between the dimples can be monitored either simply by using dial (or digital) calipers or, more accurately, using a Demec gauge. Nowadays digital photographs, endoprobes, laser and radar techniques have been used to establish the shape and form of the construction of masonry arch bridges, (Sowden 1990). Endoprobes and radar are particularly useful if voids are suspected as both allow the ‘inside’ of the bridge to be seen. Distortion and bulging should be recorded as they may not give rise to cracks as they may have occurred over a long period of time and the mortar may have accommodated the incremental movement; additionally, regular pointing would mask any fine cracks. By tapping the masonry surface with a light hammer, (particularly the intrados of a multi-ring brickwork arch) the soundness of the masonry can be checked. A dull hollow sound usually indicates internal mechanical separation. The general condition of the surface should be mapped – wetness, cracking, spalling, friable areas etc. Water penetration ranges from damp patches to running water. (The time of year and recent weather conditions should be noted, as this may be significant if the survey was undertaken during a warm, dry period and the load test is undertaken during a cold, wet period.) Most bridges in urban areas carry a number of statutory undertakers ducts etc. These need to be located accurately if test loading is to be undertaken or if they are numerous and will affect the arch/backfill interaction. Before discussing appropriate testing methodologies it is necessary to consider the several methods of applying load and how the structural response can be maintained. The most direct way to apply loading is to use dead load or kentledge. This can be a labour intensive and slow process. Care must be taken at the Risk Assessment stage to ensure that permanent damage and failure are not likely as there will be no ‘load relief’ should things start to go wrong. If there is a requirement to apply high loading under close control then jacking systems reacting against ground or rock anchors offer a practical solution (Page, 1992). Loaded vehicles can be used, but there is little control on the range of loading although it can be positioned at several locations on the bridge thus allowing a study of a range of responses. Portable weight pads can be used for rail underbridges. Axle loading can be determined by using standard rail track equipment. It is worth noting that special single axle HB trailers are available which can apply axle loads in increments up to 45 tonnes. As with the testing of any large scale structure a risk assessment should be undertaken for the proposed test and a test brief and methodology produced. This should be clearly (and appropriately) communicated to all personnel involved in the test. Additionally the public should be protected from injury and loss. Access is often a major source of risk, although operative awareness and employer liability, together with much improved access equipment have reduced risk significantly (Sowden, 1990). Table 5.1 presents a summary of instrumentation transducers. – 177 –

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Instrumentation

Parameter

Instrumentation Type

Suitable loading

Remarks

Load

Vehicle

Dead load

Static

Kentledge

Dead load

Static

Waterbags

Dead load

Static

Load cell

All

Static, Dynamic

Flat Jack

Hydraulic

Static

Versatile loading system, ‘Hard’ loading system Labour intensive, ‘Hard’ loading system Labour intensive, water supply required, ‘Hard’ loading system Needs reactive support and loading mechanism ‘Soft’ loading system Used to check wheel loadings (also in situ pressure, see below)

LVDT

Electrical

Potentiometric

Electrical

Deflection pole

Electrical

Dial gauge

Mechanical

Static, Dynamic Static, Dynamic Static, Dynamic Static

Laser theodolite systems

Laser

Static, Dynamic

Vibrating wire gauge (VW)

Mechanical/ electrical

Static

Electrical resistance strain (ERS) gauge Demountable ERS gauge Demec gauge

Electrical

Static, Dynamic

Electrical

Static, Dynamic Static

Displacement

Strain

Temperature

Mechanical

Rigid mounting required Rigid mounting required Quick to set up Rigid mounting required. Labour intensive – manually read Good for two-dimensional displacement Dependent on quality of attachment. Large gauge lengths easily accommodated Accurate. Some restriction on gauge length. Vulnerable to local damage Accurate. Large gauge lengths possible Needs safe access. Manually read

Thermocouple

Electrical

Thermometer

Mechanical

Static, Dynamic Static

Vibration

Accelerometer

Electrical

Dynamic

Accurate if correctly used. Specialist application

Pressure

Flat jack & soil pressure cell

Hydraulic/ electrical

Static

Specialist application in conjunction with other measurements

Material deterioration

Acoustic

Acoustic/ electrical

Static, Dynamic

Specialised application – ‘listening’ to internal micro-cracking

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Easy to use Manually read

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FULL SCALE TESTING The installation of soil pressure gauges into existing backfill to measure the test pressure changes and to attempt to measure the existing soil pressure is very difficult. Specialised advice should be taken as disturbing the backfill may make the test data unreliable. The most important measurements relate to the deformation of the structural elements. Deflections are usually measured using transducers either potentiometric or linear variable differential transformers (LVDT) and (only occasionally nowadays) manually read dial gauges. They are mounted on independently supported rigid frames. This is often difficult to achieve and presents real problems for the team, and may require the use of tensioned invar wires attached to a remote secure frame. If a large support frame is used beneath the structure to directly support the gauges, then its deformation due to temperature changes, wind loading and waterflow (if its over a river) must all be considered as they will probably be significant in magnitude. Gauge accuracy can be of the order 0.01 mm, but a careful assessment of the cumulative error is important so that a greater accuracy is not claimed than that which is actually achieved. Additionally, care must be exercised that the various cables associated with the different types of instrumentation do not ‘talk’ to each other and thus corrupt the data. From this point of view mechanical devices do have some advantages. It is very important to check gauge channels to ensure that the data logging records correspond. Additionally, final checks should include the use of ‘feeler’ gauges of known thickness to ensure that correct factors have been used to convert the gauge readings to dimensions. The selection and setting of the gauge travel should take the magnitude and expected direction of movement into account. The devices for measuring strain are chosen with regard to the type of material, gauge length, type of loading and site access. Commercially available strain gauges have been used to measure deformation of stone and brickwork with varying amounts of success. It is wise for the test engineer to have a healthy scepticism towards the value of any in situ surface ‘strain’ measurement of masonry arches. Masonry is a heterogeneous material and so it is likely that any strain measuring device will be required to span over mortar bedding joints and to be secured to the stone or brick units. This presents its own problems as the surface may be weathered and/or friable; and probably wet. Adhesives are now available which can cope with damp surfaces but all too often the intrados of the arch barrel is ‘running’ wet. In such cases mechanical fastenings may be used at appropriate centres to thus present the gauge feet with a smooth dry surface for installation. This approach is usually needed for stone arches where the surface is uneven. Interpretation of the results must take the juxtaposition of the gauge relative to the intrados into account. Where cracks (which may not be visible to the naked eye) occur within or adjacent to the strain gauge location then they significantly affect the recorded changes in strain. If the cracks are already present then they may facilitate ‘rigid’ body movement – significant opening or closing of the crack with very little change in the strain in the adjacent material. The strain gauge, however, will register a change in ‘strain’, even though nearly all the change in the distance – 179 –

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between the attachment points is due to the crack opening or closing. Even in the case of uncracked masonry, it must be remembered that the Emortar