Frost and Scaling Resistance of High-Strength Concrete

FROST AND SCALING RESISTANCE OF HIGH-STRENGTH CONCRETE by Roberto C. A. Pinto and Kenneth C. Hover Research & Developmen...

Author: Roberto C. A. Pinto and Kenneth C. Hover

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FROST AND SCALING RESISTANCE OF HIGH-STRENGTH CONCRETE by Roberto C. A. Pinto and Kenneth C. Hover Research & Development Bulletin RD122

P O R T L A N D

C E M E N T

A S S O C I A T I O N

PCA RD122

Frost and Scaling Resistance of High-Strength Concrete

Keywords: air-entrained concretes, air entrainment, air void system, chloride permeability, compressive strength, deicers, freeze-thaw resistance, frost durability, high-strength concrete, hardened air content, setting, scaling resistance, surface finishing Abstract: The primary purpose of this work was to assess the effect of air entrainment and time of surface finishing operations on the frost durability and scaling resistance of high-strength concrete. The conditions under which entrained air is necessary to produce a frost-resistant mixture are explored, particularly in light of current ACI 318 provisions for air content. The laboratory program consisted of the production of six concrete mixtures with water/cement ratios of 0.50, 0.45, 0.40, 0.35, 0.30, and 0.25; each at three levels of air content: non-air entrained, 4%, and 6%. No supplementary cementing materials were used. Frost resistance was investigated as a bulk or interior concrete property, via modified ASTM C 666, and as a surface property, via ASTM C 672. Both tests were initiated at 28 days with the same curing conditions applied to the specimens. The influence of time of surface finishing on the scaling resistance was investigated by finishing the scaling specimens at two different times relative to the time of initial set as defined by ASTM C 403. Additionally for each mixture, compressive strength (ASTM C 39), rapid chloride permeability (ASTM C 1202), and microscopic analysis of the air void system (ASTM C 457) were performed. For the mixtures investigated here, it was possible to obtain frost resistance based on the modified ASTM C 666 without air entrainment for w/c = 0.35 or less, while entrained air was necessary for mixtures with w/c greater than 0.40. As far as scaling resistance is concerned, no air entrainment was necessary for mixtures with w/c of 0.25, while entrained air was necessary for mixtures with w/c greater than 0.25. It was observed that the ACI 318 provisions for frost durability are somewhat conservative. While ACI 318 requires air entrainment for all mixtures subject to freezing and thawing, mixtures studied here with w/c of 0.25 and no intentionally entrained air were shown to be frost resistant. Further, properly air-entrained mixtures with w/c of 0.50 were frost resistant, even though the w/c was in excess of the 0.45 required by the ACI 318 provisions for freeze-thaw durability. Reference: Pinto, Roberto C. A. and Hover, Kenneth C., Frost and Scaling Resistance of High-Strength Concrete, RD122, Portland Cement Association, Skokie, Illinois, 2001, 75 pages. Cover figures: (top) ASTM C 666 concrete prisms in a freeze-thaw chamber (49941); (bottom left) air-void distribution in cross section of hardened concrete (67840); (right) ASTM C 672 deicer scaling samples (44003).

This publication is intended SOLELY for use by PROFESSIONAL PERSONNEL who are competent to evaluate the significance and limitations of the information provided herein, and who will accept total responsibility for the application of this information. The Portland Cement Association DISCLAIMS any and all RESPONSIBILITY and LIABILITY for the accuracy of and the application of the information contained in this publication to the full extent permitted by the law.

©2001 Portland Cement Association All rights reserved

RD122.01

PCA R&D Serial No. 2387

Frost and Scaling Resistance of High-Strength Concrete by Roberto C. A. Pinto and Kenneth C. Hover School of Civil and Environmental Engineering Cornell University

© 2001 Portland Cement Association ISBN 0-89312-208-4

RD122 All rights reserved

TABLE OF CONTENTS

1. INTRODUCTION 1.1 OVERVIEW ...................................................................................................................................... 1 1.2 FROST DAMAGE ............................................................................................................................ 1 1.2.1 Mechanisms ........................................................................................................................... 1 1.2.2 Bulk and Interior Frost Resistance ....................................................................................... 2 1.3 REQUIREMENTS FOR AN EFFECTIVE AIR VOID SYSTEM ..................................................... 3 1.4 FROST DURABILITY OF HIGH-STRENGTH CONCRETE ....................................................... 4 1.5 LABORATORY TESTS TO ASSESS FROST RESISTANCE ......................................................... 5 1.6 PURPOSE OF THIS RESEARCH ................................................................................................... 6 2. EXPERIMENTAL PROGRAM 2.1 OVERVIEW ...................................................................................................................................... 7 2.2 MATERIALS AND MIXTURE PROPORTIONING ..................................................................... 7 2.3 MIXING PROCEDURES AND FRESH CONCRETE TESTS ..................................................... 10 2.4 CURING PROCEDURES AND HARDENED CONCRETE TESTS .......................................... 12 3. RESULTS 3.1 FRESH CONCRETE TESTS .......................................................................................................... 13 3.2 HARDENED CONCRETE TESTS ................................................................................................ 16 3.2.1 Compressive Strength......................................................................................................... 16 3.2.2 Freeze-Thaw Resistance—ASTM C 666 with Modified Curing ...................................... 16 3.2.3 Scaling Resistance—ASTM C 672 ..................................................................................... 19 3.2.4 Air Void Parameters in Hardened Concrete—ASTM C 457 ............................................. 30 3.2.5 Rapid Chloride Permeability Test (RCPT)—ASTM C 1202 .............................................. 32 4. ANALYSIS AND DISCUSSION 4.1 FRESH CONCRETE PARAMETERS............................................................................................ 34 4.1.1 Slump—Mixture Proportioning ......................................................................................... 34 4.1.2 Setting Times ....................................................................................................................... 36 4.2 AIR CONTENT AND AIR VOID PARAMETERS ........................................................................ 37 4.3 COMPRESSIVE STRENGTH ....................................................................................................... 41 4.4 RAPID CHLORIDE PERMEABILITY .......................................................................................... 44

4.5 FROST RESISTANCE ................................................................................................................... 45 4.5.1 ASTM C 666 with Modified Curing ................................................................................... 45 4.5.2 ASTM C 672 ......................................................................................................................... 46 4.6 APPLICABILITY OF ACI 318 REQUIREMENTS ....................................................................... 55 4.6.1 ACI Requirements ............................................................................................................... 55 4.6.2 Compliance of Test Mixtures with ACI 318-99 Requirements for Total Air Content ................................................................................................................. 56 4.6.3 Compliance of Test Mixtures with ACI 318-99 Requirements for Water to Cement Ratio ....................................................................................................... 56 4.6.4 Compliance of Test Mixtures with ACI 318-99 Requirements for Specified Compressive Strength, f c’ .................................................................................... 60 4.6.5 Assessing ACI 318 Requirements for Air Content, Water to Cement Ratio and Strength ................................................................................................ 60 5. SIGNIFICANT OBSERVATIONS AND CONCLUSIONS 5.1 FRESH CONCRETE—SLUMP AND SETTING TIMES ............................................................ 63 5.2 AIR CONTENT AND AIR VOID PARAMETERS ........................................................................ 63 5.3 COMPRESSIVE STRENGTH ....................................................................................................... 63 5.4 RAPID CHLORIDE PERMEABILITY .......................................................................................... 63 5.5 FROST RESISTANCE ................................................................................................................... 64 5.5.1 ASTM C 666 with Modified Curing ................................................................................... 64 5.5.2 ASTM C 672 ......................................................................................................................... 64 5.6 APPLICABILITY OF ACI 318 REQUIREMENTS TO FROST DURABILITY OF HIGH-STRENGTH CONCRETE .......................................................................................... 64 ACKNOWLEDGMENTS ..................................................................................................................... 65 REFERENCES ........................................................................................................................................ 66 METRIC CONVERSION FACTORS ................................................................................................... 69


PCA RD122

CHAPTER 1

INTRODUCTION 1.1 OVERVIEW Frost and scaling resistance are key issues in the production of high-strength/high-performance concrete, since frost resistance is achieved through incorporation of air entrainment, which in turn makes the attainment of high strengths more difficult. For example, the 100 to 140 MPa (15,000 to 20,000 psi) high-strength concrete commercially produced in one market 10 years ago was non-air entrained, while the same concrete production market in 1996 had difficulty producing a 40 MPa (6,000 psi) mixture with a consistent 6% air content (Hover 1996). This issue is further complicated since there is no consensus among researchers concerning the necessity of entrained air to produce frost-resistant high-strength concrete (Philleo 1987, Gagné et al. 1992, Aïtcin 1998a). The current state of practice, however, as described by ACI 318-99 (ACI Committee 318 1995), requires air entrainment for all concrete exposed to freezing and thawing conditions while wet, as shown in Table 1.1 (reproduced from ACI 318-99, Table 4.2.1). The values presented in Table 1.1 are a function of the maximum size of the coarse aggregate and the level of exposure. These values in ACI 318 follow the recommendation of ACI 211.1, Table 6.3.3 (ACI Committee 211 1991), and were calculated

based on the criteria that 9% air is necessary in the mortar phase of the concrete, as proposed by Klieger (1952). (Note that the same table values are also published in ACI 201.2R [ACI Committee 201 1992].) In typical mixtures, the mortar volume fraction decreases as aggregate size increases, thus decreasing the required total volume of air in the concrete. The ACI 318-99 code permits a reduction of 1% of the values in Table 1.1 when the specified compressive strength is greater than 35 MPa (5,000 psi) (paragraph 4.2.1, ACI 318-99). In addition, ACI 318-99 requires a maximum water to cementitious materials ratio (w/cm) of 0.45 by mass for concrete to be exposed to freezing and thawing in a moist condition or subjected to deicing chemicals. The research described here focuses on the study of frost resistance of high-strength concretes. Frost damage mechanisms and their applications to highstrength concrete are briefly reviewed. An extensive laboratory-testing program was conducted with mixtures with water to cement ratios (w/c) varying from 0.50 to 0.25 at several air content levels. No supplementary cementing materials were included. Freezing and thawing (ASTM C 666) and deicer salt scaling resistance tests (ASTM C 672) were performed, as well as compressive strength (ASTM C 39) and rapid chloride permeability tests (ASTM C 1202). Test results are discussed in light of the ACI 318 code provisions and the necessity of air entrainment for frost-resistant high-strength mixtures.

Table 1.1. ACI 318-99 Requirements for Total Air Content for Frost-Resistant Concrete (ACI 318 Table 4.2.1) Air content, % Nominal maximum aggregate size Severe Moderate mm (in.) exposure exposure 9.5 (3/8) 7.5 6.0 12.5 (1/2) 7.0 5.5 19.0 (3/4) 6.0 5.0 25.0 (1) 6.0 4.5 37.5 (11/2) 5.5 4.5 50.0 (2) 5.0 4.0 75.0 (3) 4.5 3.5

1.2 FROST DAMAGE IN CONCRETE 1.2.1 Mechanisms Frost damage in concrete can be a consequence of the use of non-frost-resistant aggregates, or the use of non-frostresistant paste, or both. Once properly selected frost-resistant aggregates are used in the concrete mixture, frost dam-

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age is solely related to the ability of the paste fraction to support the internal pressures generated during freezing. When subjected to freezing temperatures, concrete may be degraded due to the pressure generated by the movement of water or ice through the internal pore system of hardened concrete. As postulated by Powers (1949), upon freezing, the 9% increase in volume of freezing water in an initially saturated specimen forces a flow of water away from the regions where ice is forming towards air-filled spaces. According to the “hydraulic pressure” theory, during this flow the pore walls are subjected to hydraulic pressure proportional to: • the distance water must move to reach an air void • the rate of freezing • the inverse of the permeability of the hardened paste According to this theory, expansion of the hardened cement paste should occur during cooling at freezing temperatures, especially in non-air-entrained concrete, due to the dilating pressures in the pores. Once cooling stops but the material continues to be at freezing temperatures, the hardened cement paste should not expand any longer, since there would be no further movement of water/ice in the pores. This expected behavior of a nonair-entrained mixture, however, was not observed experimentally (Powers and Helmuth 1953). Powers and Helmuth (1953) observed that a cement paste with a water to cement ratio of 0.45 suffered expansion when subjected to nearly constant temperature of about –21oC for a period of five hours. Powers and Helmuth (1953) hypothesized that this expansion occurred due to a continuous growth of ice by transfer of water (or water vapor) from unfrozen regions. Freezing of water is a non-uniform process since there are temperature differentials and pores of various sizes exist in the concrete. As cooling initiates, water in the larger capillary pores (diameter on the order of 0.01 to 1 µm) (Mehta and Monteiro 1993) freezes before water in the smaller and adjacent gel pores (diameter less than 10 µm. Thus, since the vapor pressure over the unfrozen gel water is higher than the vapor pressure over the ice in the capillary pores, water vapor migrates from the gel pores to the capillaries. This mechanism, known as “ice accretion” or “gel water diffusion,” increases the volume of water in the larger capillaries, causing further dilation when there is no available space to accommodate such flow of water. Moreover, water in concrete is not in a pure state; it contains dissolved solids, especially when deicing chemicals have been used. As the pure water freezes, the unfrozen water becomes more concentrated with dissolved species. The differential concentration of dissolved ions between water sites leads to osmotic effects, as water at low concentration sites tends to flow toward the water at higher


ionic concentration. This uni-directional flow increases the pressure generated by ice accretion. Frost damage, however, can occur only when there is freezeable water in the pore system which cannot move or expand without putting pressure on the surrounding pore walls. The amount of freezeable water is a function of the pore system of the hardened cement paste and the degree of saturation. Mehta (1986) also suggested that water absorbed into aggregates can be available to damage the paste as well. Any factor affecting the pore system or the degree of saturation also affects frost resistance. Key factors include the water to cement ratio, air void system, presence of supplementary cementitious materials and/ or chemical admixtures, type of aggregate, type of mixing, placement, compaction, finishing, and curing procedures (Hammer and Sellevold 1990, Gagné and Marchand 1993).

1.2.2 Bulk and Interior Frost Resistance The microstructural and mechanical properties of concrete vary with depth from the concrete surface, leading to different behavior during freezing and thawing for the concrete portions on the surface and in the interior of the member. Kreijger (1990) introduced the concept of concrete “skin,” a surface outer layer that differs considerably in properties from the inner, core, or bulk concrete, as a result of bleeding, compaction, finishing, and curing conditions. This“skin”concrete usually contains a higher paste fraction than the core concrete, as shown schematically in Figure 1.1 (Meyer 1987, Pigeon 1994). Moreover, before the concrete sets, the denser components like coarse aggregate tend to settle toward the bottom, whereas water tends to flow upward (bleeding). As a result, the concrete surface has a higher water to cement ratio than the core concrete. Therefore, the bulk concrete with a greater degree of compaction, and reduced porosity, may be more resistant to damage during freezing and thawing than the

Paste volume 100%

Volume of water

Figure 1.1. Variation of paste content with concrete depth (Pigeon 1994).

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PCA RD122

surface concrete. Frost damage in the surface can be caused by the same mechanisms already reviewed. However, saturation levels on the surface layer are often greater than in the core of the member. Deicing salts used to melt ice and snow penetrate the concrete, increasing osmotic pressures. Moreover, use of deicer salts on concrete surfaces causes thermal shock, which can cause cracks leading to intrusion of still more water and salts (Pigeon 1994). Finishing and curing take on a special importance for the frost durability of the concrete surface. Since these construction procedures greatly influence the surface microstructure (Aïtcin 1998a), both affect the dur-ability of the surface layers more directly than the core concrete. McNeal and Gay (1996) reported the influence of curing and finishing on frost durability, and Falconi (1996) demonstrated that for certain high-performance mixtures the sequence and timing of finishing and curing operations were critical to the durability of the concrete surface. In studying precast concrete elements, Hover (1989) found a high incidence of surface failures due to frost action, but no penetration of this mode of deterioration to the inner core. While frost damage in the core concrete is associated with the formation of interior cracks, frost damage in the concrete surface is associated with the scaling phenomenon. Scaling is the flaking or peeling-off of the finished concrete surface (Mehta 1986).

ice, unfrozen water, or both. Upon freezing, the shorter the length of the travel path of water and/or ice toward an air-filled space, the lower the pressure resulting from such movement. A well dispersed, closely spaced system of air voids intersecting the network of capillary pores at many points is therefore required (Hover 1994). Note that the hydraulic pressures associated with freezing are reduced but not eliminated by the air voids. The maximum tolerable length of the travel path of water and/ or ice movement that does not damage the material can be calculated for a given paste, with a given porosity and degree of saturation, permeability, and tensile strength, and subjected to a given rate of cooling (Powers 1949). Therefore, an air void provides frost protection to the hardened cement paste only in a zone that exists within this maximum or critical distance, and only when the volume of freezeable water in this surrounding shell is at least 9% less than the volume of the air void. Although there are rigorous mathematical models to determine the protected portion of the hardened cement paste surrounded by an air void (Pleau and Pigeon 1996, Natesaiyer et al. 1992, Natesaiyer et al. 1993, Philleo 1983), the commonly used method in practice is a simpler, more approximate method as presented in ASTM C 457. According to this method, developed by Powers (1949), the critical length, or spacing factor ¿ is calculated on assumptions that all air voids are the same size and arranged in a simple cubic lattice where each void is at the same distance from another void, as shown in Figure 1.2. The volume of the air voids per unit volume of the cube is equal to the volume of air in the cement paste per volume of paste. The distance between the center of the cube to the surface of the nearest air void is then the spacing factor. ASTM C 457 states, “the spacing factor, ¿, is generally regarded as the most significant indicator of the durability of the cement paste matrix to freezing and thawing exposure of the concrete.” ASTM C 457 goes on to say, “The maximum value of the spacing factor for moderate exposure of the concrete is usually taken to be 0.20 mm (0.008 in.). Somewhat larger values may be adequate for mild exposure and smaller ones may be required for severe exposure, especially if the concrete is in contact with deicing chemicals.” However, ASTM recognizes that the spacing factor should be used with care in specifications, due to the high variability of this calculated value when determined at different laboratories. The ASTM C 457 method also yields the specific surface, , of the air void system, approximately indicating the average size of the air voids. In summary, an effective air void system will have sufficient volume to accommodate the freezeable water and will be distributed into a large number of small, closely spaced voids. The total volume of air required is normally considered to be as shown in Table 1.1 when measured as a fraction of the total concrete volume. Given that the air

1.3 REQUIREMENTS FOR AN EFFECTIVE AIR VOID SYSTEM An effective air void system in the concrete, both near the surface and in the core of the hardened material, is the most important parameter for frost durability in normal-strength concrete. Concrete with a properly dispersed air void system can withstand a large number of freeze-and-thaw cycles without loss in serviceability. A proper air void system is characterized by a large number of small, well-dispersed air voids in the hardened material. This system is obtained with the utilization of air-entraining admixtures, which stabilize the smaller air voids during mixing. The overall air system in air-entrained concretes consists of finer entrained air voids in addition to coarser entrapped air voids, the latter occurring in all concretes. Thus, the air void system in air-entrained concrete includes a broad gradation of void sizes, ranging from 10 µm to several milli-meters (Hover 1994). The incorporation of small voids in the cement paste fraction of the hardened material leads to a reduction of the stresses generated upon freezing. This reduction occurs since the air voids remain dry due to their much greater size (minimum size of about 0.01 mm) as compared to the capillary voids. The air voids, then, act as stress-reliever sites available to accept the intrusion of 3

PCA RD122


¿

Figure 1.2. The spacing factor from Power’s model (after Hover 1994).

In summary, an effective air void system will have sufficient volume to accommodate the freezeable water and will be distributed into a large number of small, closely spaced voids. The total volume of air required is normally considered to be as shown in Table 1.1 when measured as a fraction of the total concrete volume. Given that the air voids do not protect the aggregate against freeze-thaw damage, Klieger (1952) simplified this to 9% of the volume of mortar. More fundamentally, recognizing that air voids only protect the paste, Mielenz et al. (1958) simplified the required total air content to 18% to 20% of the paste volume. Frost resistant concrete is often associated with values of specific surface around 25 mm2 (600 in.2) and spacing factor on the order of 200 µm (0.008 in.). For further discussion of air-entrained concrete see Whiting and Nagi (1998), Hover (1994), Whiting and Stark (1983).

crete (Philleo 1987, Foy 1988, Pigeon 1994). The spacing factor required for frost durability appears to increase with decrease of water to cement ratio (Philleo 1987), with values close to the common requirement of about 200 µm (0.008 in.) for water to cement ratio greater than 0.50 (Mielenz et al. 1958). Foy et al. (1988) studied a frost resistant, non-air-entrained mixture with water to cement ratio of 0.25 and spacing factor of 750 µm. Pigeon (1994) presents data in which the spacing factor associated with frost-resistant, high-strength concrete can go up to 600 µm (0.024 in.). For scaling resistance, Gagné and Marchand (1993) indicate that a maximum water to cement ratio of 0.25 is required for non-air-entrained concrete, a conclusion shared by Foy et al. (1988). According to Gagné and Marchand, if the spacing factor is greater than 250 µm (0.01 in), the water to cement ratio should be less than 0.35 for frost resistant mixtures. Gagné, Pigeon and Aïtcin (1991) presented results for concretes with water to cement ratio = 0.30 and spacing factor = 950 µm that did not suffer significant scaling even after 150 cycles under the ASTM C 672 test method. Aïtcin (1998b) suggested maximum spacing factors as a function of water to cement ratio, based on a great number of specimens from field concrete tested by ASTM C 666 and/or ASTM C 672, according to Table 1.3.

Table 1.3. Recommended Spacing Factors as a Function of Water to Cement Ratio (after Aïtcin 1998b)

1.4 FROST DURABILITY OF HIGH-STRENGTH CONCRETE The necessity of air entrainment in high-strength concrete for frost durability has been controversial for over a decade. Researchers such as Philleo (1987) and Khalil et al. (1980) concluded that air entrainment is necessary at all levels of water to cement ratio. However, Li et al. (1994), Gagné et al. (1992), and Pigeon et al. (1991) suggested that it is possible to produce frost/scaling resistant, nonair-entrained mixtures by limiting the water to cement ratio to a maximum of 0.25, whereas air entrainment would be necessary for mixtures with water to cement ratio greater than 0.30. Whiting (1987) concluded that air contents may be reduced to levels of 3% to 4% in high-strength concrete not subjected to deicing agents. Aïtcin (1998a) states that a high-performance concrete should contain a small amount of entrained air to improve workability, placing, and finishing. He also suggests that the minimum air content should be about 4%. There has been some consensus that the spacing factor found necessary for frost durability in normal-strength concrete might not be the same for high-strength con-

Recommended Water/cement

L

> 0.40

230 µm

0.35 < w/c < 0.40

350 µm

0.30 < w/c < 0.35

450 µm

< 0.30

Permitted maximum L 260 µm (for scaling resistance) 400 µm (for scaling resistance) 550 µm (for scaling resistance)

Same criteria as for 0.30 < w/c < 0.35 due to insufficient amount of experimental data

High-strength concrete incorporating high-range water reducers (or superplasticizers) are sometimes observed to be frost resistant with spacing factors that exceed the conventional requirement of 200 µm (0.008 in.) (Kobayashi et al. 1981, Siebel 1989). Philleo (1987) has conjectured that conventional spacing factor requirements may not be applicable to mixtures with superplasticizers, observing that the original theory of air void spacing was developed for non-superplasticized concretes with relatively high water to cement ratio, in which considerable porosity and freezeable water exist. He concluded, there4


PCA RD122

fore, that requirements for normal-strength concrete might not apply to high-strength concretes.

Nonetheless, the utilization of equation 1.1 to calculate the relative dynamic modulus of elasticity is adequate for the purpose of the test (Newlon and Mitchell 1994). The durability factor is then calculated according to the expression below:

1.5 LABORATORY TESTS TO ASSESS FROST RESISTANCE Newlon and Mitchell (1994) present an historical evolution of accelerated tests to assess frost resistance of concrete. These test methods normally subject samples of concrete to a number of freezing and thawing cycles in order to obtain the degree of deterioration associated with long-term exposure. The most commonly used tests are ASTM C 666, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing, and ASTM C 672, Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals. When these test methods are used to evaluate mass loss or surface degradation due to freezing and thawing, the obtained results can be related to the frost resistance of the concrete surface. If, on the other hand, these test methods assess some change in the bulk concrete property, they are more related to the frost resistance of the concrete core. The ASTM C 666 method alternates cycles of freezing and thawing; the nominal freeze-thaw cycle consists of lowering the concrete temperature to –17.8oC (0oF) followed by raising the concrete temperature to 4.4oC (40oF) in not less than 2 hours and not more than 5 hours. The specimens can be either frozen in water (procedure A) or in air (procedure B), while thawing always takes place in water in both procedures. The standard curing condition is 14 days wet-cured unless otherwise specified. Since the development of internal microcracking is anticipated, freeze-thaw resistance is indirectly evaluated by changes in the relative dynamic modulus of elasticity, indicating the degree of internal microcrack formation. The relative dynamic modulus of elasticity is calculated from the following expression:

Pc = 100

nc2 n2

DF =

Pn N M

(1.2)

where: DF = durability factor of the specimen tested, Pn = relative dynamic modulus at N cycles (%), N = number of cycles at which the test specimen achieves the minimum specified value of Pc for discontinuing the test or the specified number of cycles of the test, whichever is less, and M = specified number of cycles of the test. As an indication of the degree of freeze-thaw resistance, Cordon (1966) suggested that a concrete of poor frost resistance would have a durability factor below 20%, while concrete with good frost resistance would have a durability factor greater than 80%. Neville (1996), on the other hand, reported modified limits of 40% as an upper bound for poor frost resistance and 60% as a lower bound for good frost resistance. The rate of freezing in this method can vary widely from 4.4 to 22.2°C/hour (8 to 40°F/hour), with a typical rate of 11.1°C/hour (20°F/hour) (Vanderhost and Jansen 1990). These rates, however, are much higher than normally encountered in field applications, in which a maximum cooling rate of 3.3°C/hour (6°F/hour) has been observed (Lin and Walker 1975). Other important parameters influencing the results are the high degree of saturation of the specimens after the first few exposure cycles and the early age at which the test is initiated (14 days after casting unless otherwise specified) (Pigeon et al. 1985). These conditions are much more severe than the natural conditions which most concretes are likely to be exposed in service. Natural freezing of concrete in service normally occurs at lower cooling rates, at later ages, and after some period of drying. Thus, it is generally concluded that the ASTM C 666 test is more rigorous than most natural exposures. In ASTM C 672, small slabs of concrete of at least 0.046 m2 (72 in.2) of surface area and a minimum thickness of 75 mm (3 in.) are used to evaluate scaling resistance. A solution of water and calcium chloride (4 g of anhydrous calcium chloride per 100 mL of solution) is placed on top of the specimens at the start date of the test. The specimens are placed in a freezing environment capable of lowering the surface temperature to –17.8 ± 2.8°C (0 ± 5°F) for 16 to 18 hours, followed by thawing at laboratory temperature of 23 ± 1.7°C (73 ± 3°F) for 6 to 8 hours, completing one freeze-thaw cycle. The

(1.1)

where: Pc = relative dynamic modulus of elasticity, after c cycles of freezing and thawing, n = fundamental transverse frequency at 0 cycles of freezing and thawing, and nc = fundamental transverse frequency at c cycles of freezing and thawing. The fundamental transverse frequency is calculated from the method described in ASTM C 215. In the calculation of Pc , it is assumed that the specimen does not change its mass or dimensions, which is not always the case due to the loss of material caused by degradation.

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solution is replaced at the end of every five cycles, when a visual evaluation is performed. The specimens are then rated from 0 to 5 varying from no scaling (0) to severe scaling (5), with values from 2 to 3 indicating moderate scaling of the surface. Generally 50 cycles are sufficient to evaluate the surface, although more cycles may be necessary in comparative tests. The standard curing procedure is 14 days moist cured followed by 14 days air-cured. (This period of air drying does not occur in the standard ASTM C 666 test, in which samples are immediately placed in the freezer after the 14-day wet cure period.) In a variation of the scaling test, the scaled debris is collected, dried, and weighed (Gagné et al. 1990, Siebel et al. 1993, Jansen and Snyder 1993, Narayanan 1997, among others). The Swedish Standard SS 13 72 44 (from Marchand et al 1996, and Jacobsen et al. 1996) correlates the scaling potential with the collected mass loss per unit area. It is suggested that scaling of less than 1.0 kg/ m2 after 50 freeze-thaw cycles in the presence of deicer salts indicates that the concrete has an acceptable scaling resistance (Gagné and Marchand 1993). The conditions applied in these two test methods rarely correspond to actual conditions in the field. The number of natural freeze-thaw cycles may differ greatly from those in the tests; a different deicer salt may be used or at a different concentration; the age of the concrete when it experiences the first freezing cycle may not be 14 or 28 days old; and the rate of freezing in the field is rarely as high as the one applied in lab tests. Therefore, these test methods only indicate potential susceptibility of a mixture to damage by freezing and thawing. Judgment is therefore required when predicting field performance based on results of these tests (Newlon and Mitchell 1994). ASTM recognizes that these methods do not quantitatively indicate the service life of a concrete structure. These tests are intended for determining the effects of variations in the properties of concrete on the freeze-thaw resistance, and are particularly useful in making comparisons of the behavior of several mixtures.


This research explores the conditions under which entrained air is necessary for frost durability in high-strength concretes insofar as the test methods define such durability. If air entrainment can be reduced or eliminated for certain classes of high-strength concrete, then cost savings will immediately result, with an expansion of the utility of highstrength concrete. In particular, the results of this study were used to examine the applicability of the current ACI 318 provisions for air content and concrete with a compressive strength greater than 35 MPa (5,000 psi).

1.6 PURPOSE OF THIS RESEARCH This study has been designed to investigate frost resistance of high-strength concrete as a bulk or interior concrete property (via ASTM C 666), and as a surface property (ASTM C 672). Further, experiments were conducted to study the influence of mixture proportions and construction operations on the frost durability of high-strength concrete. The mixture parameters studied included the w/c and air content levels. The influence of different times of surface finishing on the scaling resistance was also investigated. 6


PCA RD122

CHAPTER 2

EXPERIMENTAL PROGRAM 2.1 OVERVIEW The laboratory program included production of 6 mixtures with various water to cement ratios: 0.50, 0.45, 0.40, 0.35, 0.30, and 0.25; each at three levels of air content (nonair entrained, 4%, and 6%). Air content and water to cement ratio were primary “mixture” variables, and time of finishing was the primary “construction” variable. For each mixture, tests were performed on frost durability, setting time, air content of fresh concrete, air void parameters in hardened concrete, compressive strength, and rapid chloride permeability (Table 2.1). Frost durability was assessed as a bulk or “interior” concrete property, by ASTM C 666, and as a surface property, by ASTM C 672. Surface finishing was initiated at two different times relative to the time of initial set as defined by ASTM C 403. See a detailed description of experimental methods, equipment, and materials below.

ASTM C 33, with a fineness modulus of 2.78, specific gravity of 2.60, and absorption of 2.0%. The coarse aggregate consisted of gravel of mixed composition (mainly of sandstones and siltstones, with a small portion of limestone and dolomites), with a nominal maximum size of 13 mm (ASTM grading size #7), specific gravity of 2.68, and absorption of 1.57%. The high-range water reducer used was a naphthalene sulfonate type. This product was used in a liquid form, with solids content of 40%. Mixture proportions are shown in Table 2.2 and Figure 2.1. High dosages of superplasticizer were necessary to achieve low w/c values without increasing the cement content of the mixtures. Sample nomenclature indicates the w/c and the target level of air entrainment, with N indicating non-air-entrained mixtures, and 4 and 6 indicating the target levels of 4% and 6% air content. For example, Mixture 50-4 had a water to cement ratio of 0.50 and a target air content of 4%. Table 2.2 also presents the air content as measured by the ASTM C 231 pressure meter . Fresh properties are discussed in Section 3. Airfree paste content and air content in paste are presented in Table 2.3 for all mixtures.

2.2 MATERIALS AND MIXTURE PROPORTIONING The portland cement used was an ASTM C 150 Type I cement. Fine aggregate was concrete sand conforming to

Table 2.1. Experimental Methods Used for Each Mixture Frost durability Time of setting Air void system Strength Permeability

Tests

ASTM #

Scaling Freeze–thaw resistance Penetration resistance Air content—fresh concrete Air void parameters in hardened concrete Compressive strength Rapid chloride permeability test (RCPT)

C 672 C 666 C 403 C 231 C 457 C 39 C 1202

7

Mixture

8

Water (kg)

Fine agg. (kg)

Coarse agg. (kg)

HRWRA (1)

50-N 50-4 50-6

445 390 392

222 195 195

601 625 597

1052 1080 1059

0.0 0.0 0.0

45-N 45-4 45-6

441 411 412

198 185 185

632 629 618

1066 1078 1039

40-N 40-4 40-6

439 425 426

176 170 170

641 640 623

35-N 35-4 35-6

467 447 428

163 157 151

30-N 30-6A* 30-6B*

510 466 466

25-N 25-4 25-6

549 542 590

AEA (ml)

HRWRA (mass/mass of cement)

Air content (%)

0 29 58

— — —

1.3 3.8 5.6

0.9 0.6 0.6

0 25 46

0.25% 0.18% 0.18%

2.0 4.0 5.8

1107 1089 1057

1.8 2.1 2.0

0 27 140***

0.49% 0.59% 0.56%

2.4 4.1 5.9

647 656 629

1088 1109 1063

4.8 3.5 3.4

0 37 60

1.23% 0.94% 0.95%

3.0 3.2 6.6

153 140 140

632 633 632

1105 1071 1070

9.3 7.8 7.5

0 63 42

2.19% 2.10% 1.93%

2.1 6.2 6.3

137 135 148

626 619 570

1106 1094 1023

10.6 10.5** 14.6**

0** 0** 0**

2.32% 2.32% 2.97%

2.5 3.6 5.0

* Difficulty in achieving a 4% air content at a w/c = 0.30 precluded testing of a 30-4 mixture. ** Note that high doses of high range water reducing admixture (HRWRA) stabilized air in the absence of AEA. *** This unusually high dose of air-entraining admixture (AEA) was required to obtain the target air for this particular mixture.


Cement (kg)

PCA RD122

Table 2.2. Mixture Proportions (per m3)


1200

1000

800

600

9 400 Coarse agg. (kg) Fine agg. (kg)

200

Cement (kg) Water (kg) HRWRA (dL)

0

Air-entraining admixture (mL)

-N 0-4 50 5 Mixture -6 5-N 5-4 -6 0-N 0-4 50 4 4 45 4 4

35

-N

35

-4

35

-6

30

-N

-6A

30

30

-6B 25-N

-4

25

25

-6

PCA RD122

Figure 2.1. Mixture proportions

-6

40

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Table 2.3. Paste Content and Air in Paste

Mixture

Air-free paste content (%)*

Air content in paste (%)**

50-N 50-4 50-6

36.8 32.0 31.9

3.5 11.9 17.6

45-N 45-4 45-6

33.9 31.8 32.2

5.9 12.6 18.0

40-N 40-4 40-6

31.9 30.9 30.8

7.5 13.3 19.1

35-N 35-4 35-6

31.8 30.5 29.0

9.4 10.5 22.7

30-N 30-6A 30-6B

32.7 29.7 29.7

6.4 20.9 21.2

25-N 25-4 25-6

32.3 31.8 34.9

7.7 11.3 14.3

* Based on mix proportions in Table 2.2 ** Pressure meter air content/air-free paste volume

2.3 MIXING PROCEDURES AND FRESH CONCRETE TESTS Mix proportions were established on the basis of small trial batches to verify target air content at a target slump of 100 to 125 mm. The larger batches for test specimens were subsequently mixed in a 0.1 m3 drum mixer in the laboratory, maintaining the proportions from the trial batches, with priority given to the adjustments to achieve the target air content. The initial slump (after 8 minutes) of the batches mixed in the more powerful mixer was consequently higher than in the small trial batches. Figure 2.2 presents the sequence of mixture procedures and fresh concrete tests followed up to casting. Any exceptions are noted.

For the air-entrained mixtures, the mixer was allowed to run for 8 minutes following the introduction of all materials, after which the air content by the volumetric method (ASTM C 173) with a small meter (bowl volume of 2.8 L and the slump (ASTM C 143) were obtained. If the air content of the mixture was within ± 0.5% of the desired target air (4% or 6%), the air content by the pressure method (ASTM C 231) and the unit weight (ASTM C 138) were obtained, and the casting of the specimens initiated. Otherwise, more air-entraining admixture was introduced and the concrete was mixed for an additional 4minutes, after which the air content by the pressure method, unit weight, and slump were recorded and casting initiated. This two-step procedure helped to obtain air contents close to the target values of 4% and 6% for most

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PCA RD122

All materials in the mixer mix for 8 minutes

Measure slump

Check target air content of the mixture by volumetric air meter

Achieved

Not achieved

Add more aid-entraining admixture, mix 4 minutes more Pressure air content and unit weight Pressure air content, unit weight, and slump

Casting

Figure 2.2. Sequence of tests in fresh concrete until casting for air-entrained mixtures. of the mixtures. For the mixture with water to cement ratio of 0.30, 4% content air was not achieved. In this case only, two mixtures were produced with air content close to 6%, and labeled as 30-6A and 30-6B, as can be seen in Table 2.2. For the non-air mixtures, it was not necessary to measure the air content twice. Casting was therefore initiated after eight minutes of mixing. This sequence was followed for all mixtures except those with water to cement ratio of 0.25. These high-cement, high-superplasticizer mixtures achieved the target air contents without air-entraining admixture. The setting behavior of each concrete mixture was obtained from the penetration resistance of mortar ex-

tracted from the concrete, according to ASTM C 403. Concrete was wet-sieved in a 4.75 mm sieve, with the resultant mortar placed in 150 x 150 mm cylinders for the penetration resistance test. Penetration resistance was monitored at regular intervals, and was affected by mixture composition and temperature. When possible, all mixtures of the same w/c were produced on the same day to minimize day-to-day variations of the results. Ambient temperature, initial concrete temperature, and relative humidity of the air were recorded at the beginning of casting procedures. A total of four ASTM C 672 scaling specimens of 200 x 230 x 80 mm (two for each time of finishing) and two

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PCA RD122

ASTM C 666 freeze-thaw specimens of 406 x 102 x 76 mm were cast from each mixture. Additionally, several 100 x 200 mm cylinders, to be used for compressive strength, microscopic analysis of air system, and rapid chloride permeability test (RCPT), were also cast. Immediately after casting the scaling specimens, the samples were struck-off with a sawing motion of a magnesium float, followed by a simulated bullfloat pass 1 performed with four passes of the magnesium float. The concrete surface was later finished with a final float (four passes of the magnesium float) followed by a medium-stiff broom pass. Two types of surface-finishing were chosen: an early finish and an “on time” finish. The early finish time was arbitrarily chosen to be 20 minutes after casting the scaling samples. The “on time” finishing was performed when the mortar penetration resistance was about 0.14 MPa (20 psi). This penetration resistance was chosen according to previous research done at Cornell (Abel and Hover 2000) in which the initial set times of mortar and concrete slabs were correlated. Bleed water was present on the concrete surface of mixes with water to cement ratio of 0.50 and 0.45 at the time of the early finishing. No bleed water was observed at “on time” finishing for any mixtures.

2.4 CURING PROCEDURES AND HARDENED CONCRETE TESTS Curing of the specimens was initiated when the mortar penetration resistance achieved 27.6 MPa (4000 psi) penetration resistance at final set according to ASTM C 403. All specimens were covered with wet burlap for 24 hours at a laboratory temperature of around 27°C. On the second day, the specimens were removed from their molds and immersed in a curing tank with water temperature around 25°C. Both the ASTM C 666 and C 672 specimens were subjected to identical curing conditions. The specimens were immersed in a lime-saturated water tank for 14 days, followed by a dry curing period in laboratory conditions for 14 days. This modified curing procedure for the ASTM C 666 specimens with an extra 14-day drying period was chosen to match the curing procedures for the scaling test (ASTM C 672). Having the same curing conditions for both tests was important to compare“surface”versus “core” frost durability for samples from the same mixture. Moreover, initiating ASTM C 666 test immediately after the 14-day wet curing period results in a relatively severe test when compared with field exposures (Newlon and Mitchell 1994). The specimens are immature and completely wet as opposed to field conditions in which the specimens 1


often have dried to some extent and are subjected to the first freezing cycle at much later ages. ASTM C 666, method A (freeze and thaw cycles in water) was followed. The transverse natural frequency of the specimens was obtained at about every 30 cycles up to 300 cycles according to ASTM C 215. An accelerometer was placed on the specimen and connected to a waveform analyzer. The fundamental transverse frequency of the specimen was obtained by forcing the specimen to vibrate freely. The fundamental transverse frequency was translated to relative dynamic modulus of elasticity according to ASTM C 666. Scaling specimens were subjected to daily freezethaw cycles. According to ASTM C 672, a solution of calcium chloride and water was placed on the surface. Each 100 ml of solution contained 4 g of anhydrous calcium chloride. After every 5 cycles, the surface of each specimen was washed and the debris collected. The debris was later oven dried and weighed. A visual evaluation of the scaled surface was performed at cycles 5, 10, 15, 20, 25, 35, and 50. For uniformity of interpretation, the specimens were rated according to a photograph of a numerical evaluation of scaling produced by the Portland Cement Association and reproduced in Figure 2.3. Compressive strengths were obtained for all mixtures at 28 days and at 90 days from 100 x 200 mm cylinder specimens kept in lime-saturated water until tested. Cylinder specimens also provided samples for the rapid chloride permeability test, and for the microscopical evaluation of the air void parameters in the hardened concrete. The RCPT was performed on one sample per mixture at around 210 days after casting; thus the results indicated potential long-term chloride permeability. In the ASTM C 457 test, the modified point count method was used, with total air content, spacing factor, and specific surface values estimated for one sample per mixture.

A pass is traversing the entire concrete surface once, applying light pressure on a float.

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PCA RD122

CHAPTER 3

RESULTS

3.1 FRESH CONCRETE TESTS initial set is defined as the elapsed time after batching when the mortar penetration resistance achieved 3.4 MPa (500 psi), while final set occurs when penetration resistance is 27.6 MPa (4000 psi). The average mortar temperature until final set is also presented. Figure 3.1 shows penetration resistance development curves over time for typical mixtures at similar temperatures.

Table 3.1 shows concrete temperature, slump, unit weight, and air contents by the volumetric method (ASTM C 173), by the pressure method (ASTM C 231), and by the calculated gravimetric method (obtained from unit weight measurement). Table 3.2 summarizes the initial and final set times obtained from each mixture (the test was not performed for mixtures 25-4 and 25-6). According to ASTM C 403,

Table 3.1. Fresh Concrete Parameters Air content (%) Volumetric Pressure Gravimetric method method method (ASTM C 231) (ASTM C 173) (ASTM C 138)

Mixture

Concrete temperature (°C)

Initial slump (mm)

Unit weight (kg/m3)

50-N 50-4 50-6

29 28 28

225 220 220

2350 2300 2240

— 4.2 6.0

1.3 3.8 5.6

0.3 3.5 5.9

45-N 45-4 45-6

21 17 15

215 200 190

2340 2320 2290

— 4.0 6.1

2.0 4.0 5.8

2.2 3.5 4.3

40-N 40-4 40-6

18 17 17

110 160 145

2380 2340 2280

— 3.5 5.7

2.4 4.1 5.9

1.7 3.7 5.8

35-N 35-4 35-6

16 16 24

165 140 220

2390 2390 2220

— 5.0 7.5

3.0 3.2 6.6

2.3 2.6 7.1

30-N 30-6A 30-6B

22 22 22

130 170 200

2440 2320 2330

— 6.7 8.0

2.1 6.2 6.3

1.1 6.0 5.9

25-N 25-4 25-6

22 21 23

120 140 200

2440 2400 2340

— 4.3 5.5

2.5 3.6 5.0

2.2 3.6 5.3

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PCA RD122


Table 3.2. Initial and final set times (ASTM C 403)

Mixture

Average mortar temperature (o C)*

50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

31 31 30 18 18 17 17 19 19 17 17 23 22 26 25 22 — —

Initial set Final set (hours:min (hours:min after after batching) batching) 3:44 3:20 3:30 5:28 5:38 6:02 5:01 5:21 5:04 7:08 5:55 5:18 10:03 10:20 11:00 12:31 — —

4:33 4:22 4:39 7:08 7:06 7:30 6:52 6:56 6:22 9:06 8:03 6:34 11:55 11:54 12:26 15:24 — —

* The influence of temperature on setting time of mortar is discussed in Pinto and Hover 1999.

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PCA RD122

30 Final set

Penetration resistance (MPa)

25 40-4 35-4 30-N

20

15 10 Initial set

5 0 0:00

2:00

4:00

6:00

8:00

10:00

Elapsed time of batching (hours) Figure 3.1. Setting behavior for typical mixtures.

Table 3.3. Compressive Strength Results (MPa)

Mixture 50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

28 days 42.1 38.6 33.4 48.6 45.2 42.0 49.2 41.4 39.9 59.1 46.0 44.6 55.5 54.5 51.4 44.6 50.0 52.5

15

90 days 50.1 47.1 41.8 55.5 52.1 47.7 56.9 52.9 45.1 61.2 60.7 48.2 64.2 55.7 53.6 60.9 59.5 55.8

12:00

PCA RD122

Frost and Scaling Resistance of High-Strength Concrete 70

28 Days

Compressive Strength, MPa

60

90 Days

50 40 30 20 10 0 50-N

50-4

50-6

45-N

45-4 Mixture

45-6

40-N

40-4

40-6

Figure 3.2. Compressive strength results for mixtures with water to cement ratios of 0.50, 0.45, and 0.40. 70

28 Days

Compressive Strength, MPa

60

90 Days

50 40 30 20 10 0 35-N

35-4

35-6

30-N

30-6A 30-6B Mixture

25-N

25-4

25-6

Figure 3.3. Compressive strength results for mixtures with water to cement ratios of 0.35, 0.30, and 0.25.

3.2 HARDENED CONCRETE TESTS 3.2.1 Compressive Strength Table 3.3 shows the compressive strength results for all mixtures at 28 days and 90 days. These values were obtained from the average of individual strengths of three cylinders. These data are also shown in Figures 3.2 and 3.3.

3.2.2 Freeze-Thaw Resistance– ASTM C 666 with Modified Curing Table 3.4 presents the relative dynamic modulus of elasticity (Ed) calculated at the end of the ASTM C 666 test for all mixtures and the durability factor (DF) calculated according to Equation 1.2. The same data are graphically represented in Figure 3.4. The data are the average of the two specimens per mixture. Only mixtures without air entrain-

16


PCA RD122

ment and with high water to cement ratio suffered a decrease of the transverse fundamental frequency, and thus their final values of Ed were smaller than 100%. The de-

velopment of the relative Ed with the number of freezethaw cycles is presented in Figure 3.5.

Table 3.4. Values of Relative Dynamic Modulus of Elasticity at the End of the ASTM C 666 Test

Mixture

Pressure meter air content (%)

Relative Ed (%)

Number of cycles

Durability factor

50-N 50-4 50-6

1.3 3.8 5.6

28 101 102

98 291 291

9 98 99

45-N 45-4 45-6

2.0 4.0 5.8

34 102 102

248 298 298

28 101 101

40-N 40-4 40-6

2.4 4.1 5.9

79 102 103

306 306 306

80 104 105

35-N 35-4 35-6

3.0 3.2 6.6

98 99 104

310 310 310

101 102 107

30-N 30-6a 30-6b

2.1 6.2 6.3

97 102 101

304 304 304

98 103 102

25-N 25-4 25-6

2.5 3.6 5.0

100 102 102

303 303 303

101 103 103

17

PCA RD122

140

7

18

6

100

5

80

4

60

3

40

2

20

1

0

-N 50-4 50-6 5-N 45-4 45-6 0-N 40-4 40-6 5-N 35-4 35-6 0-N -6A -6B 5-N 25-4 25-6 50 4 4 3 3 30 30 2 Mixture

Figure 3.4. Relative dynamic modulus of elasticity at the end of ASTM C 666 test and air content of all mixtures.

0


120

Air content (%)

Durability factor

DF Air content (%)


PCA RD122

100

Relative Ed (%)

80 60 30-N 35-N 40-N 45-N 50-N

40 20 0 0

50

100

150

200

250

300

350

Number of freeze–thaw cycles Figure 3.5. Durability factors vs. number of freeze–thaw cycles for selected mixtures.

3.2.3 Scaling Resistance—ASTM C 672 When possible, the weight loss was recorded even after scaling was observed on the entire surface, representing a level 5 of the visual rating system. Eventually a level 5 specimen became so degraded that the experiment had to be terminated. Figures 3.6 to 3.11 graphically present the visual ratings, while the average weight loss is presented in Figures 3.12 to 3.17 for each water to cement ratio. Figure 3.18 shows the weight loss per cycle for mixtures without air entrainment. Also indicated in Figures 3.12 to 3.18 is the value of weight loss of 1.0 kg/m2, as discussed in Chapter 1.

Table 3.5 shows the visual rating of the scaling specimens at 5, 10, 15, 25, 35, and 50 cycles of freeze-thaw for all mixtures. An average of two values obtained from two specimens per mixture per time of finishing was used. An additional letter (E or O) was added to the mixture nomenclature to describe the time of finishing applied to the pair of specimens (E stands for early finishing, and O stands for on-time finishing). Once a specimen reached level 5, no more evaluation was recorded for subsequent cycles. Tables 3.6 and 3.7 show the average weight loss of two specimens at every five cycles for all mixtures.

19

PCA RD122


Table 3.5. Scaling Visual Ratings Scaling rating Mixture

Cycles 5

10

15

35

50

50-N-E 50-N-O

1.75 1

3.5 4.25

4.5 5

5

50-4-E 50-4-O

1 0

2 1.25

2 1.5

2.5 2

2.5 2

3 2.5

50-6-E 50-6-O

1.25 0.25

2 0.75

2 1

2.5 1.5

2.5 1.5

2.75 2

45-N-E 45-N-O

1.75 2.5

3.5 4

4 4.75

5 5

45-4-E 45-4-O

1 0.75

2 2.25

2.25 2.5

2.25 2.5

2.75 3

2.75 3.25

45-6-E 45-6-O

0.25 0.75

0.5 1.75

1 1.75

1.5 2

1.75 2.25

2 2.25

40-N-E 40-N-O

1 1.25

3 2.75

4.75 4

5 4.5

4.75

5

40-4-E 40-4-O

1 1.25

2 2.5

3.25 3.25

3.25 3.25

3.25 3.25

3.5 3.5

40-6-E 40-6-O

1.25 1.25

2 2

2 2.75

2.5 3

2.5 3

2.5 3

35-N-E 35-N-O

1.25 1.5

2.25 2.75

2.5 3

3.25 4

3.75 4.25

4 4.25

35-4-E 35-4-O

0.75 0.75

1.5 1.75

2.25 2.25

2.75 3

3 3.25

3 3.5

35-6-E 35-6-O

0.25 0

0.5 0.5

0.75 1.25

1.5 2

2 2

2.25 2.5

30-N-E 30-N-O

0.25 0.25

0.75 0.5

1.25 1.25

1.75 2

2.5 2.75

3.5 3.75

30-6A-E 30-6A-O

0 0

0.25 0.25

0.5 1

1.5 1.75

1.5 1.75

2 2

30-6B-E 30-6B-O

0 0

0.25 0.5

0.75 0.75

1.25 1

1.75 1.75

2 2.25

25-N-E 25-N-O

0.25 0.25

0.25 0.75

1.25 1

1.5 1.25

2.25 2.25

2.25 2.5

25-4-E 25-4-O

0 0.25

0.25 0.25

0.5 0.75

0.75 1

1.5 1.75

1.75 2

25-6-E 25-6-O

0 0

0.25 0

0.5 0.5

0.75 0.5

1 1.25

1 1.5

20

25

Mixture

21

50-N-E 50-N-O 50-4-E 50-4-O 50-6-E 50-6-O 45-N-E 45-N-O 45-4-E 45-4-O 45-6-E 45-6-O 40-N-E 40-N-O 40-4-E 40-4-O 40-6-E 40-6-O

5 0.14 0.08 0.04 0.01 0.04 0.03 0.07 0.12 0.05 0.05 0.04 0.06 0.14 0.10 0.10 0.16 0.09 0.10

10 0.53 1.82 0.09 0.06 0.08 0.08 0.39 0.66 0.15 0.17 0.08 0.15 0.70 0.40 0.33 0.29 0.18 0.19

Cumulative weight loss (kg/m2) cycles 15 20 25 2.13 3.51 4.95 3.68 5.08 7.07 0.14 0.15 0.20 0.12 0.16 0.21 0.11 0.13 0.18 0.11 0.16 0.16 0.84 1.69 2.09 1.39 2.20 2.67 0.23 0.34 0.38 0.24 0.33 0.40 0.12 0.15 0.18 0.19 0.23 0.26 1.33 1.91 2.45 0.78 1.13 1.42 0.50 0.62 0.72 0.45 0.57 0.64 0.25 0.29 0.34 0.26 0.30 0.35

30

35

40

45

50

0.25 0.25 0.21 0.20 3.12

0.29 0.30 0.23 0.23 3.78

0.30 0.36 0.25 0.25 4.27

0.31 0.37 0.25 0.27 4.74

0.33 0.44 0.27 0.31

0.45 0.50 0.20 0.29 2.91 1.74 0.85 0.72 0.40 0.40

0.50 0.61 0.24 0.32 3.30 1.97 0.93 0.81 0.46 0.46

0.54 0.69 0.26 0.34 3.71 2.21 1.02 0.87 0.50 0.50

0.57 0.75 0.29 0.35

0.60 0.82 0.31 0.38

2.52 1.12 0.95 0.56 0.56

2.77 1.21 1.02 0.60 0.59


Table 3.6. Weight Loss Measured for ASTM 672 Scaling Specimen for Mixtures with Water to Cement Ratios of 0.50, 0.45, and 0.30

PCA RD122

Mixture

22

30 0.86 0.97 0.71 0.64 0.22 0.20 0.26 0.23 0.22 0.18 0.25 0.16 0.26 0.14 0.11 0.13 0.07 0.09

35 0.96 1.05 0.78 0.72 0.26 0.22 0.32 0.28 0.26 0.21 0.30 0.19 0.31 0.19 0.13 0.16 0.09 0.10

40 1.11 1.19 0.90 0.82 0.29 0.24 0.39 0.36 0.32 0.26 0.36 0.22 0.36 0.21 0.16 0.19 0.11 0.12

45 1.26 1.31 0.98 0.90 0.33 0.27 0.45 0.43 0.40 0.29 0.42 0.25 0.41 0.25 0.18 0.22 0.13 0.13

50 1.42 1.46 1.08 0.99 0.36 0.30 0.58 0.51 0.48 0.34 0.51 0.29 0.45 0.28 0.20 0.24 0.14 0.15


5 35-N-E 0.12 35-N-O 0.10 35-4-E 0.13 35-4-O 0.09 35-6-E 0.03 35-6-O 0.06 30-N-E 0.03 30-N-O 0.02 30-6A-E 0.04 30-6A-O 0.04 30-6B-E 0.03 30-6B-O 0.03 25-N-E 0.02 25-N-O 0.01 25-4-E 0.01 25-4-O 0.01 25-6-E 0.01 25-6-O 0.01

Cumulative weight loss (kg/m2) cycles 10 15 20 25 0.34 0.46 0.61 0.73 0.36 0.53 0.70 0.83 0.28 0.38 0.52 0.62 0.24 0.34 0.46 0.56 0.07 0.10 0.13 0.18 0.09 0.12 0.14 0.16 0.08 0.12 0.16 0.20 0.06 0.11 0.15 0.18 0.10 0.12 0.14 0.18 0.09 0.11 0.14 0.16 0.09 0.12 0.15 0.19 0.07 0.09 0.11 0.13 0.09 0.13 0.17 0.23 0.04 0.05 0.08 0.12 0.03 0.05 0.07 0.09 0.03 0.05 0.07 0.10 0.02 0.03 0.04 0.06 0.02 0.04 0.06 0.07

PCA RD122

Table 3.7. Weight Loss Measured for ASTM 672 Scaling Specimen for Mixtures with Water to Cement Ratios of 0.35, 0.30, and 0.25


PCA RD122

50-N-E 50-N-O 50-4-E 50-4-O 50-6-E 50-6-O

5

Visual rating

4 3 2 1 0 0

10

20

30

40

50

60

Number of cycles Figure 3.6. ASTM C 672 visual rating for mixtures with water to cement ratio of 0.50.

45-N-E 45-N-O 45-4-E 45-4-O 45-6-E 45-6-O

5

Visual rating

4 3 2 1 0 0

10

20

30

40

50


23

60

PCA RD122


5

Visual rating

4 3 2

40-N-E 40-N-O 40-4-E 40-4-O 40-6-E 40-6-O

1 0 0

10

20

30

40

50

60


35-N-E 35-N-O 35-4-E 35-4-O 35-6-E 35-6-O

5

Visual rating

4 3 2 1 0 0

10

20

30

40

50


24

60


PCA RD122

5 30-N-E 30-N-O 30-6A-E 30-6A-O 30-6B-E 30-6B-O

Visual rating

4 3 2 1 0 0

10

20

30

40

50

60


5 25-N-E 25-N-O 25-4-E 25-4-O 25-6-E 25-6-O

Visual rating

4 3 2 1 0 0

10

20

30

40

50


25

60


Cumulative mass loss (kg/m2)

PCA RD122

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

50-N-E 50-N-O 50-4-E 50-4-O 50-6-E 50-6-O

0

10

20

30

40

50

60

Number of cycles


Figure 3.12. Cumulative mass loss for mixtures with water to cement ratio of 0.50.

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

45-N-E 45-N-O 45-4-E 45-4-O 45-6-E 45-6-O

0

10

20

30

40

Number of cycles Figure 3.13. Cumulative mass loss for mixtures with water to cement ratio of 0.45.

26

50



PCA RD122

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

40-N-E 40-N-O 40-4-E 40-4-O 40-6-E 40-6-O

0

10

20

30

40

50

Number of cycles



2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

35-N-E 35-N-O 35-4-E 35-4-O 35-6-E 35-6-O

0

10

20

30

40


27

50



PCA RD122

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

30-N-E 30-N-O 30-6A-E 30-6A-O 30-6B-E 30-6B-O

0

10

20

30

40

50

Number of cycles



2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

25-N-E 25-N-O 25-4-E 25-4-O 25-6-E 25-6-O

0

10

20

30

40


28

50


PCA RD122

7.0

50-N-E 50-N-O 45-N-E 45-N-O 40-N-E 40-N-O


6.0 5.0

35-N-E 35-N-O 30-N-E 30-N-O 25-N-E 25-N-O

4.0 3.0 2.0 1.0 0.0 0

10

20

30

Number of cycles Figure 3.18. Cumulative mass loss for mixtures N.

29

40

50

60

PCA RD122


3.2.4 Air Void Parameters in Hardened Concrete—ASTM C 457 Table 3.8 presents the air content, spacing factors, and specific surface values obtained from the microscopical analysis of hardened samples, according to ASTM

C 457. One sample per mixture was evaluated. Figures 3.19 and 3.20 show the spacing factor and specific surface for all mixtures.

1200

Spacing factor (µm)

1000 800 600 400 200

50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

0

Mixture Figure 3.19. Spacing factors obtained per ASTM C 457 for all mixtures.

30

31

Mixture 50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

Pressure Air air content content (C 231) (C 457) (%) (%) 1.3 1.6 3.8 4.1 5.6 6.0 2.0 1.5 4.0 3.3 5.8 5.4 2.4 2.5 4.1 3.7 5.9 6.9 3.0 3.6 3.2 3.8 6.6 7.1 2.1 2.8 6.2 5.8 6.3 5.1 2.5 3.6 3.6 3.3 5.0 4.8

Air-free Air paste content in content* paste (%) (%) 36.8 3.5 32.0 11.9 31.9 17.6 33.9 5.9 31.8 12.6 32.2 18.0 31.9 7.5 30.9 13.3 30.8 19.1 31.8 9.4 30.5 10.5 29.0 22.7 32.7 6.4 29.7 20.9 29.7 21.2 32.3 7.7 31.8 11.3 34.9 14.3

Specific surface in.-1 300 553 743 415 597 590 408 604 612 138 504 504 163 531 720 293 431 462

Spacing factor in. 0.0294 0.0101 0.0063 0.0207 0.0105 0.0082 0.0171 0.0098 0.0069 0.0428 0.0113 0.0087 0.0399 0.0084 0.0069 0.0208 0.0148 0.0122

Specific surface mm-1 11.8 21.8 29.3 16.3 23.5 23.2 16.1 23.8 24.1 5.4 19.8 19.8 6.4 20.9 28.3 11.5 17.0 18.2

Spacing factor µm 747 254 160 526 267 208 434 249 175 1087 287 221 1013 213 175 528 376 310


Table 3.8. Air Void Parameters in Hardened Concrete.

PCA RD122

PCA RD122


30

Specific surface (mm-1)

25 20 15 10 5

50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

0

Mixture Figure 3.20. Specific surface obtained per ASTM C 457 for all mixtures.

3.2.5 Rapid Chloride Permeability Test (RCPT)—ASTM C 1202 Table 3.9 shows the total charge passed after six hours for one specimen per mixture, together with the age of the specimen when tested. A graphical representation of the results is shown in Figure 3.21. Mixtures 50-N, 50-4, and 25-6 were not tested.

32


PCA RD122

Table 3.9. Results of Rapid Chloride Permeability Test

Mixture 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4

Charge (coulumbs) 3682 3478 3397 2843 2719 2642 2432 1727 1699 1161 1190 1070 913 1281 1147

Age (days) 210 206 207 208 202 203 204 208 207 202 202 203 204 199 200

4000

Change (coulumbs)

3500 3000 2500 2000 1500 1000

Mixture Figure 3.21. Rapid chloride permeability results.

33

25-4

25-N

30-6B

30-6A

30-N

35-6

35-4

35-N

40-6

40-4

40-N

45-6

45-4

45-N

0

50-6

500

PCA RD122


CHAPTER 4

ANALYSIS AND DISCUSSION

4.1 FRESH CONCRETE PARAMETERS 4.1.1 Slump–Mixture Proportioning

dosage multiplied by a K-factor, according to the following equation: slump = water content + K ⫻ (HRWRA)

The slump varied from around 120 to 220 mm, as shown in Figure 4.1. Factors influencing slump include water content, superplasticizer dosage, air content, and concrete temperature. Figure 4.2 shows the relationship between slump and water content of the mixtures. A better relationship is obtained when the slump values are plotted against the summation of water content and HRWRA

Best fit K-factors of 6.2 and 3.9 were obtained for the air-entrained mixtures and the non-air-entrained mixtures, respectively. Figure 4.3 presents such relationships. This difference in K-factor values for mixtures with and without air entrainment suggests that for a given water content, the same dosage of HRWRA causes a greater relative increase of slump for the air-entrained mixtures.

250 200

Slump (mm)

(4.1)

150 100 50

50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6A 30-6B 25-N 25-4 25-6

0

Mixture Figure 4.1. Slump values per mixture.

34


PCA RD122

240 220

Slump (mm)

200 180 160 140 Non-air entrained Air entrained

120 100 120

140

160

180

200

220

240

Water content (Liters) Figure 4.2. Slump as a function of water content.

240 outlier

220 R2 = 0.86

Slump (mm)

200 R2 = 0.74

180 160 140 Non-air entrained Air entrained

120 100 170

180 190 200 210 220 Water content + K I (HRWRA) (Liters)

Figure 4.3. Slump as a function of the water content and the superplasticizer dosage.

35

230

PCA RD122


4.1.2 Setting Times The mixtures presented a wide range of initial and final set times. Initial set for mixtures with water to cement ratio of 0.50 occurred around 3.5 hours after batching, while mixtures with water to cement ratio of 0.30 did not reach initial set until more than 10 hours after batching. Figure 4.4 shows graphically the initial set times per mixture. Set times are influenced by many factors including cement composition and fineness, cement content,

water to cement ratio, presence of admixtures, and concrete temperature (Dodson 1994, Pinto and Hover 1999). At similar temperatures (between 17°C and 23°C—mixtures 45, 40, 35, 30-N, and 25-N), initial set times were retarded for mixtures with lower w/c. These mixtures, however, had higher amounts of HRWRA, which is known to greatly affect the setting behavior (Pinto and Hover 1997). Figure 4.5 shows the observed initial set times at various dosage of HRWRA for such mixtures.

14:00

Initial set time (hours)

12:00 10:00 8:00 6:00 4:00 2:00

Mixture

Figure 4.4. Initial set times for all mixtures. 16:00

Initial set time (hours)

14:00 12:00 10:00 8:00 6:00 4:00 2:00 0:00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dosage of HRWRA (% by mass of cement) Figure 4.5. Relationship between HRWRA dosage and initial set times.

36

25-N

30-6B

30-6A

30-N

35-6

35-4

35-N

40-6

40-4

40-N

45-6

45-4

45-N

50-6

50-4

50-N

0:00


PCA RD122

In general, the total air content did not significantly affect the setting behavior, as seen from the results of Table 3.2 and Figure 4.4. Initial and final set times for mixtures with the same water to cement ratio, but different air content, did not vary significantly.

(ASTM C 231). These data are graphically presented in Figure 4.7. The paste content of the mixtures varied from 29% to around 37% of the total volume. All mixtures with air content around 6% (mixtures–6) had a corresponding air content in paste greater than 17%, while values as low as 3.5% of air in paste were observed for the non-airentrained mixtures. Figure 4.8 presents the air content in paste as a function of the dosage of air-entraining admixture per mass of cement according to values presented in Table 2.2. Higher dosages yielded higher air content in paste. The microscopical air void system results presented in Table 3.8 show high spacing factors for the mixtures without air-entraining admixture (series N), often greater than 500 µm. The mixtures with air-entraining admixture possessed much smaller spacing factors, yet the values are typically slightly higher than 200 µm (0.008 in.), a value often associated with frost resistance (Mielenz 1958, Philleo 1987). Only mixtures 50-6, 40-6, and 30-6B had a spacing factor less than 200 µm (0.008 in.).

4.2 AIR CONTENT AND AIR VOID PARAMETERS Three methods were used to evaluate the total air content in this investigation. Fresh air content by the pressure method (ASTM C 231) prior to casting, air content in fresh concrete by the unit weight measurements (ASTM C 138), and air content in hardened concrete by microscopical examination (ASTM C 457).1 Table 4.1 summarizes all data obtained. The data are graphically presented in Figure 4.6. Table 2.3 presented the air-free paste content as determined by the mixture proportions and the air content in paste (Ap) obtained from the fresh air content

Table 4.1. Summary of Sir ContentsObtained by Different Methods

1

Mixture

Fresh air content (ASTM C 231)

Fresh air content (ASTM C 138)

Hardened air content (ASTM C 457)

50-N 50-4 50-6

1.3 3.8 5.6

0.3 3.5 5.9

1.6 4.1 6.0

45-N 45-4 45-6

2.0 4.0 5.8

2.2 3.5 4.3

1.5 3.3 5.4

40-N 40-4 40-6

2.4 4.1 5.9

1.7 3.7 5.8

2.5 3.7 6.9

35-N 35-4 35-6

3.0 3.2 6.6

2.3 2.6 7.1

3.6 3.8 7.1

30-N 30-6A 30-6B

2.1 6.2 6.3

1.1 6.0 5.9

2.8 5.8 5.1

25-N 25-4 25-6

2.5 3.6 5.0

2.2 3.6 5.3

3.6 3.3 4.8

In fact, a fourth method, the volumetric method was also used for air-entrained mixtures, as a first method to evaluate the air content of the mixture. Later, for the same mixture the pressure air meter was used, and its value regarded as the air content in fresh concrete.

37

PCA RD122

Fresh air content (unit weight) Fresh air content (pressure) Hardened air content

8 7

Air content (%)

6 5 4 3

38

1

Mixture Figure 4.6. Air content measurements.

25-6

25-4

25-N

30-6B

30-6A

30-N

35-6

35-4

35-N

40-6

40-4

40-N

45-6

45-4

45-N

50-6

50-4

50-N

0


2


Air-free paste concent Air content in paste

40 35

Air content (%)

30 25 20 15

39

10 5

25-6

25-4

25-N

30-6B

30-6A

30-N

35-6

35-4

35-N

40-6

40-4

40-N

45-6

45-4

45-N

50-6

50-4

50-N

0

Mixture

PCA RD122

Figure 4.7. Air-free paste content and air content in paste for all mixtures.

PCA RD122


25

Ap (%)

20 15 10 5 0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Air-entraining admixture (mL/kg cement) Figure 4.8. Influence of air-entraining admixture dosages on air content in paste.

Figure 4.9 represents the relationship between fresh air content by the pressure method and the spacing factor in the hardened concrete. As the total air content

increased for the air-entrained mixtures, the spacing factor decreased. The same data are presented in Figure 4.10 as a function of air content in paste.


1100 1000

Non-air-entrained mixtures Air-entrained mixtures

900 800 700 600 500 400 300 200 100 1

2

3

4

5

Air content in concrete (%) Figure 4.9. Spacing factor as a function of air content in concrete.

40

6

7


PCA RD122

1100 1000

Non-air-entrained mixtures Air-entrained mixtures


900 800 700 600 500 400 300 200 100 0

5

10

15

20

25

Air content in paste (%) Figure 4.10. Spacing factor as a function of air content in paste.

4.3 COMPRESSIVE STRENGTH pressive strength for the same mixtures. As the air content increased, the compressive strength decreased. Figure 4.11 shows the compressive strength at 90 days for each water to cement ratio mixture as a function of the air content.

The results in Table 3.3 indicate an increase in compressive strength at lower water to cement ratio. However, the lowest water to cement ratio mixtures (mixture 25) did not achieve the highest compressive strength. The results also indicate the influence of total air content on the com-

90-day compressive strength (MPa)

Water to cement ratio 0.25 0.30 0.35 0.40 0.45 0.50

70

60

50

40 0

1

2

3

4

5

6

7

Fresh air content (%) Figure 4.11. Relationship between compressive strength at 90 days and air content.

41

PCA RD122


Multiple regression was performed with compressive strength at 90 days as a function of water to cement ratio and air content for all mixtures with the exception of mixture 25. An equation of the following form was obtained. The corresponding r2 is 0.92. fc90 = 92.0 – 71.0 w/c – 2.58 A (MPa)

(4.2)

where: A = air content in concrete (%) fc90 = compressive strength at 90 days (MPa)

90–day compressive strength (MPa)

This equation indicates that an increase of 1% in the air content corresponds to a decrease in the compressive strength of 2.6 MPa; similarly a decrease of 0.05 in the w/c represents an increase of 3.6 MPa. Thus, for the same water to cement ratio, an increase in 4% of the air content (from 2% in non-air entrained to 6%) would mean a decrease of about 10 MPa in the compressive strength. This decrease in strength would have to be counteracted by a decrease in the water to cement ratio of about 0.14, which would necessitate a greater amount of cement, superplasticizer, and air-entraining admixture. For example, mixture 45-N with 2% air content, and mixture 30-6A with 6.2% air content, achieved compressive strength at 90 days of 55.5 and 55.7 MPa, respectively. Looking at their mixture proportions (Table 2.2), it would be necessary to add 25 kg of cement, 6.9 L of superplasticizer, and 63 mL of AEA per cubic meter in the amount of materials for mixture 45-N to arrive at mix-

ture 30-6A. This represents an increase of 5.7% of cement weight and 733% of superplasticizer, plus inclusion of the necessary air-entraining admixture. Compressive strength can also be interpreted as a function of the total porosity, or void content, in hardened concrete. Data from the Minnesota Department of Transportation (1998) indicates that the compressive strength for concrete mixtures with water to cement ratio greater than 0.35 is almost a linear function of the socalled “cement-voids ratio,” calculated by MINNDOT for a given volume of concrete as the volume of cement divided by the sum of the volumes of air and water. Figure 4.12 investigates this relationship for the mixtures studied here, showing the linear regression curve obtained for the data from mixtures with water to cement ratio of 0.35 and higher. Figure 4.12 suggests that there is a linear relationship, as indicated by MINNDOT data, between compressive strength and the cement-voids ratio for mixtures with water to cement ratio of 0.35 and higher. For the mixtures with water to cement ratio of 0.30 and 0.25, however, the linear relationship obtained overestimates the compressive strength at 90 days. For these lower w/c mixtures, the estimation of the volume of voids as being the volume of water plus the volume of air may be oversimplified, since in these mixtures the cement particles achieve a reduced degree of hydration. A relationship between compressive strength and total porosity, adjusted for degree of hydration, would be more appropriate.

70 60

R2 = 0.87

50 40 30 20

water to cement ratio 0.25, 0.30 0.35 or greater

10 0 0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Cement–voids ratio Figure 4.12. Compressive strength at 90 days as a function of the cement-voids ratio.

42


PCA RD122

Total porosity is the air volume (air content) plus the capillary porosity. A method for estimating capillary porosity as a function of water to cement ratio and degree of hydration was proposed by Powers (1958):

various mixtures had to be estimated. A previous study (Pinto et al. 1999) showed a longterm degree of hydration of about 0.85 and 0.64 for mortar mixtures with a water to cement ratio of 0.49 and 0.33, respectively. Using this information and a simple linear relationship between water to cement ratio and degree of hydration at 90 days, the capillary paste porosity was estimated from Equation 4.3, and presented in Table 4.2. Table 4.2 also shows the total estimated porosity and assumed degree of hydration for all the mixtures. (To obtain the estimated capillary porosity of the concrete, Vp was multiplied by the paste content from Table 2.3.) Figure 4.13 shows the relationship between compressive strength at 90 days and estimated total porosity. Compressive strength consistently increased with decreasing total porosity for all mixtures, including those with water to cement ratio lower than 0.35.

(4.3)

w/c - 0.36α Vp = 0.317 + w/c where: Vp = capillary porosity ␣ = degree of hydration (fraction of total amount of cement that has hydrated)

An exact relationship between compressive strength and porosity for the mixtures studied cannot be obtained, since there was no measurement of the degree of hydration of the mixtures. In order to estimate this relationship, the degree of hydration at 90 days for the

Table 4.2. Estimated Total Porosity of Concrete for All Mixtures.

Mixture

Assumed degree of hydration

50-N 50-4 50-6 45-N 45-4 45-6 40-N 40-4 40-6 35-N 35-4 35-6 30-N 30-6a 30-6b 25-N 25-4 25-6

0.86 0.86 0.86 0.80 0.80 0.80 0.73 0.73 0.73 0.67 0.67 0.67 0.60 0.60 0.60 0.54 0.54 0.54

Estimated concrete capilary porosity (%) 8.5 7.4 7.3 7.2 6.8 6.8 6.1 5.9 5.8 5.2 5.1 4.9 4.4 4.0 4.0 3.2 3.2 3.6

43

Air content (%)

1.3 3.8 5.6 2.0 4.0 5.8 2.4 4.1 5.9 3.0 3.2 6.6 2.1 6.2 6.3 2.5 3.6 5.0

Estimated total concrete porosity (%) 9.8 11.2 12.9 9.2 10.8 12.6 8.5 10.0 11.7 8.2 8.3 11.5 6.5 10.2 10.3 5.7 6.8 8.6


90–day compressive strength (MPa)

PCA RD122

70 60 50 40

R2 = 0.85

30 20 10 0 4

6

8

10

12

14

Total pore volume (% of concrete volume) Figure 4.13. Compressive strength at 90 days as a function of estimated total porosity.

4.4 RAPID CHLORIDE PERMEABILITY The results presented in Table 3.9 indicate a decrease in the total charge passed through the specimen at lower water to cement ratio. Figure 4.14 shows the data of Table 3.9; a simple linear relationship between the observed cu-

mulative charge and water to cement ratio is suggested for water to cement ratios between 0.30 and 0.50. The results also consistently show a lower charge passed at higher air contents for mixtures from the same water to cement ratio.

Cumulative charge (coulombs)

4000

mixtures N

3000

mixtures 4 mixtures 6

2000

1000

0 0.2

0.3 0.4 Water to cement ratio

0.5

Figure 4.14. Total charge at the end of the RCPT as a function of water to cement ratio.

44

Durability factor

100

35-N

25-N

120

PCA RD122

30-N


80 60

non-air-entrained mixtures air-entrained mixtures

40 20 0 0

200

400

600

800

1000

1200

Spacing factor (µm) Figure 4.15. Relationship between durability factor and spacing factor.

The influence of water to cement ratio and air content on the charge passed after six hours for mixtures with 0.30 water to cement ratios between 0.30 and 0.50 was tested through a multiple regression analysis in which the variables tested were: water to cement ratio, air content, interaction between water to cement ratio and air content, and the quadratic terms of w/c2 and air2. The results of this multiple regression showed that only the coefficients for water to cement ratio and air content were statistically significant. An equation of the following form was obtained with an R2 = 0.97. Charge = - 2800 – 100 air + 14300 w/c

(4.4)

where: Charge = cumulative charge after 6 hours (coulombs) air = total air content (%) Thus at lower water to cement ratio, the total charge decreases, while at lower air contents the charge increases. The effect of air content on the total charge passed after six hours is not as significant as water to cement ratio, as can be seen from the constants in Equation 4.4, and from Figure 4.14. According to ASTM C 1202, values of cumulative charge less than 2,000 coulombs are indicative of a specimen with low chloride ion penetrability. For the mixtures studied here, which did not incorporate any supplementary cementitious materials, a water to cement

ratio less than or equal to 0.35 would be necessary to achieve such a low chloride ion penetrability.

4.5 FROST RESISTANCE 4.5.1 ASTM C 666 with Modified Curing As shown in Table 3.4, only specimens 50-N, 45-N, 40N and suffered a decrease of the transverse fundamental frequency, and thus their relative dynamic moduli Ed after 300 cycles or fewer were significantly smaller than 100%. The relative Ed is calculated as the squared ratio of the fundamental transverse frequency at the beginning of the test to the fundamental transverse frequency at the end of the test. For 50-N, 45-N, and 40-N mixtures, as the number of cycles increased, their transverse frequency decreased. For all other mixtures, the transverse frequency measured at the end of the test had either increased or remained approximately the same. None of the intentionally air-entrained mixtures suffered frost deterioration, no matter the water to cement ratio, total air content, spacing factor, or compressive strength. Figure 4.15 shows the relationship between the durability factor and the spacing factor. As can be noticed, the spacing factor is not a clear discriminator of frost resistance over the breadth of this study, since durability factors of about 100 are associated with spacing factors from about 150 to 1100 µm. Excluding the very low water to

45

PCA RD122


cement ratio non-air-entrained mixtures, however, a high durability factor was associated with spacing factors less than about 400 µm. This substantiates the observations of others that higher strength, superplasticized concretes can exhibit frost resistance with values of ¿ greater than 200 µm. For mixtures with non-air-entraining admixture, the results as presented in Figure 3.4 indicated that a water to cement ratio of 0.35 or less was necessary for frost resistance. An important consideration in evaluating these results is the 14-day period of air drying that preceded initiation of the ASTM C 666 testing. According to Newlon and Mitchell (1994), even a brief period of drying greatly improves frost durability as measured by ASTM C 666. Since it is difficult to resaturate concrete after a period of drying, less freezeable water would be present in the pores. Further, the influence of the predrying may be even more significant, the lower the permeability of the concrete. The decreased permeability of the low w/c mixtures (as suggested by the RCPT results) would make it difficult to resaturate the concrete, thus reducing the amount of freezeable water present. On the other hand, the ASTM C 666 test applies conditions much more severe than natural conditions concrete is likely to be exposed to in service (Newlon and Mitchell 1994, Vanderhost and Jansen 1990, Philleo 1987, Pigeon et al. 1985, Lin and Walker 1974). Setzer (1996) has concluded that the freeze-thaw cycles accelerate the saturation of immersed specimens according to his “micro-pump” theory. Based on Setzer’s statement that concrete exposed to freezing and thawing“especially in contact with water, could reach critical saturation very fast,”the effects of the 14-day drying period may be nullified after a number of freeze-thaw cycles (Auberg and Setzer 1998). Nevertheless, one can conclude from these results that it is necessary to incorporate air in mixtures with w/c

5

Visual rating

4

greater than 0.35 to obtain frost resistance. Mixtures with w/c less or equal to 0.35 were frost resistant regardless of the air void system, for the pre-dried condition of the test.

4.5.2 ASTM C 672 Two methods were used to evaluate the surface scaling of each specimen: the visual scaling as described in ASTM C 672, and the weight of debris collected after each five cycles. Figure 4.16 shows the correlation between the measured individual weight loss and the ASTM visual rating for each specimen over the entire period of the test. A best-fit power curve was also obtained and is presented in the graph with the 95% prediction interval. These data suggest, for example, that a 1 kg/m2 of weight loss of the surface material corresponds to visual evaluations of the surface between 3 and 4.5. All mixtures with surfaces rated 2.5 or less lost less than 1.0 kg/m2 of surface material. The scaling resistance is related to the quality of the concrete on the surface. In this regard, the time of application of finishing operations could affect the scaling resistance. However, the results presented in Tables 3.5,3.6 and 3.7 do not indicate an influence of such parameters for the water-cured conditions studied here. The visual scaling ratings or weight loss of debris do not differ considerably or follow any consistent pattern from the specimens finished early or on time, as can be seen from Figures 4.17 through 4.22. Scaling resistance should also be a function of the air void system of the mixture. Figures 4-23. and 4-24. show the weight loss after 50 freeze-thaw cycles as a function of spacing factor ( L ) for all mixtures. The weight loss was obtained as the average of the mass of debris collected for the on-time and early finishing specimens. The line representing a total loss of 1 kg/m2 (as discussed in Chapter 1) of material is also presented.

y = –1.06 + 4.68x0.31 r2 = –0.86

3 2 1 0 0.01

0.10

1.00

10.00

2

Mass loss (kg/m ) Figure 4.16. Correlation between mass loss and visual rating for each specimen by ASTM C 672

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3

Time of finishing

Visual rating

early on-time

2

1

0

50-6

45-6

40-6 35-6 Mixture

30-6A

25-6

Figure 4.17. Visual rating for mixtures around 6% air content.

0.6

Time of finishing

Mass loss (kg/m2)

0.5

early on-time

0.4 0.3 0.2 0.1 0.0

50-6

45-6

40-6 35-6 Mixture

30-6A

25-6

Figure 4.18. Mass loss for mixtures around 6% air content.

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5

Time of finishing

Visual rating

4

early on-time

3 2 1 0

50-4

45-4

40-4 Mixture

35-4

25-4

Figure 4.19. Visual rating for mixtures around 4% air content.

1.4

Time of finishing

Mass loss (kg/m2)

1.2

early on-time

1.0 0.8 0.6 0.4 0.2 0

50-4

45-4

40-4 Mixture

35-4

25-4

Figure 4.20. Mass loss for mixtures around 4% air content.

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5

Time of finishing

Visual rating

4

early on-time

3 2 1 0

50-N

45-N

40-N 35-N Mixture

30-N

25-N

Figure 4.21. Visual rating for non-air-entrained mixtures.

2

Time of finishing

Mass loss (kg/m2)

1.6

early on-time

1.2 0.8 0.4 0

50-N

45-N

40-N 35-N Mixture

30-N

Figure 4.22. Mass loss for non-air-entrained mixtures.

49

25-N


Mass loss after 50 cycles (kg/m2)

PCA RD122

2.0 water to cement ratio 0.50 0.45 0.40

1.5

1.0 0.5 0.0 0

200

400 Spacing factor (µm)

600

800

Mass loss after 50 cycles (kg/m2)

Figure 4.23. Mass loss as a function of ÷ for mixtures with water to cement ratio of 0.50, 0.45, and 0.40.

2.0 water to cement ratio 0.35 0.30 0.25

1.5

1.0 0.5 .00 0

200

400 600 800 Spacing factor (µm)

1000

1200

Figure 4.24. Mass loss as a function of ÷ for mixtures with water to cement ratio of 0.35, 0.30, and 0.25.

Figures 4.23 and 4.24 show that all mixtures with a spacing factor less than 200 µm can be considered scaling resistant (as defined by a weight loss at the end of the test of less than 1.0 kg/m2). On the other hand, mixtures with ¿ around 250 µm showed mixed results; some were scaling resistant while others were not. When the water to cement ratio decreases to a 0.30 level, it seems that the requirement for an ¿ of around 200 µm is no longer necessary to achieve scaling resistance, as shown in Figure

4.24. In fact, no air entrainment was necessary for mixtures with water to cement ratio of 0.30 or less. However, weight loss on the surface is not the only indicator of scaling resistance. The visual quality of the surface is important as well. In this sense, Figures 4.23 and 4.24 were transformed using the visual rating values for each mixture, and are presented in Figures 4.25 and 4.26. A visual rating between 2 and 3 corresponding to moderate scaling, as seen from Figure 2.3, may be considered

50


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Again, a spacing factor of around 200 µm was necessary to achieve a minimum visual rating of approximately 2.5 for all mixtures but those with water to cement ratio of 0.25. These latter mixtures achieved a visual rating below 2.5, even though the spacing factors were much higher.

satisfactory for scaling resistance, resulting in the scaling rating of 2.5 shown in Figures 4.25 and 4.26. Moreover, according to Figure 4.16, all mixtures that suffered scaling at levels below 2.5 in the visual rating had weight loss less than 1.0 kg/m2.

5

Visual rating

4 3 2

water to cement ratio 0.50 0.45 0.40

1 0 0

200

400 Spacing factor (µm)

600

800

Figure 4.25. Visual rating as a function of ÷ for mixtures with water to cement ratio of 0.50, 0.45, and 0.40.

5

Visual rating

4 3 2 water to cement ratio 0.35 0.30 0.25

1 0 0

200

400 600 800 Spacing factor (µm)

1000

Figure 4.26. Visual rating as a function of ÷ for mixtures with water to cement ratio of 0.35, 0.30, and 0.25.

51

1200

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Mixture parameters include the water to cement ratio and the total air content. The effect of water to cement ratio on scaling behavior can be seen from Figure 4.27. This figure represents the dependence of weight loss at 40 cycles (average of on-time and early finished specimens) with water to cement ratios for all mixtures. The weight loss of scaled material observed at 40 cycles was chosen instead of 50 cycles since there were no values recorded for mixture 40-N after 40 cycles. In general, as the water to cement ratio increased, the average weight loss increased as well. However, it is interesting to notice that for mixtures with air entrainment (mixtures-4, and mixtures-6), the mass loss at water to cement ratio of 0.45 and 0.50 was less than that occurring at water to cement ratio of 0.35 and 0.40. A review of values of spacing factors associated with these data leaves the better performance of the 0.45 and 0.50 water to cement ratio mix-

tures unexplained. It is noted, however, that mixture 50 did not have superplasticizer, while mixture 45 had a dosage around 0.2% of mass of superplasticizer as mass of cement. On the other hand, mixtures 40, 35, 30, and 25 had dosages from 2 to 10 times that of mixture 45. It is unclear whether this had an effect on scaling resistance. The effect of air content can be better seen from the data of mixtures 40, 35, 30, and 25, as presented in Figure 4.28 and in Figure 4.29. The relation between weight of scaled material and total air content, and air content in paste are presented in Figures 4.28 and 4.29, respectively. It can be seen that for such mixtures, as the total air content and air content in paste increased, the average weight loss decreased. Figure 4.30 shows the average mass loss after 40 cycles as a function of the total porosity of the mixture, as calculated from Equation 4.3 and the total air content.

Average mass loss (kg/m2)

5 mixtures – N mixtures – 4 mixtures – 6

4 3 2 1 0 0.2

0.25

0.3 0.35 0.4 Water to cement ratio

0.45

0.5

Figure 4.27. Measured mass loss at various water to cement ratio at 40 cycles.

3.0

Average mass loss at 40 cycles (kg/m2)

Water to cement ratio 0.25 0.30 0.35 0.40

2.0

1.0

0.0 1

2

3 4 5 Total air content (%)

6

Figure 4.28. Measured mass loss at various air contents after 40 cycles .

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3.0 Water to cement ratio 0.25 0.30 0.35 0.40

2.0

1.0

0.0 0%

5%

10% 15% Air content in paste

20%

25%

Figure 4.29. Measured mass loss after 40 cycles at various air contents in paste.


5.0 Water to cement ratio 0.25 0.30 0.35 0.40 0.45 0.50

4.0 3.0 2.0 1.0 0.0 0

2

4 6 8 10 12 14 Total pore volume (% of concrete volume)

16

Figure 4.30. Measured mass loss after 40 cycles at various total porosities. A multiple regression analysis was then performed to obtain an equation relating the average weight loss, the w/c and the air content in paste. An equation of the following form was seen to express such relationship.

W=C1e

C2(w/c) C3(Ap) e

where: W = weight of scaled material C1, C2, C3 = constants w/c = water/cement ratio of the mixture Ap = air content in paste

(4.5)

Using Equation 4.5 for the superplasticized highstrength mixtures (w/c = 0.40 and less) with the results presented in Table 3.6 at 50 cycles, the best-fit surface was obtained and is presented in Figure 4.31. The best-fit curve indicates that in order to obtain an average weight loss less than 1 kg/m2 after 50 cycles of freeze-thaw (per ASTM C 672), the minimum air content in the paste fraction at various w/c are those presented in Table 4.3. Table 4.3 also presents the minimum air content in concrete, assuming a paste content of 30%. Thus, a mixture with w/c of 0.30 or less would not require entrained air based on the weight loss of the concrete surface.

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Table 4.3. Minimum Air Content at Various Water to Cement Ratios

Minimum air content in paste (%) 15.8 10.5 5.2 —

Water to cement ratio 0.40 0.35 0.30 0.25

Minimum air content in concrete (%) 4.8 3.2 1.6 —

C2(w/c) C3(Ap)

W=C1e

e

r2=0.89 C1=-.04 C2=12.8 C3=0.12

W (kg/m2)

3 2

1 0 0.4

10 0.35

0.3

15 0.25

Water to cement ratio

20 25

0.2

Air in paste (%)

Figure 4.31. Best-fit surface relating average weight loss (W), air content in paste, and water to cement ratio.

However, as discussed before, a visual evaluation of the surface is also important to assess scaling resistance. Toward this end, mixture 30-N (without air entraining) suffered surface deterioration equivalent to a level 3.5 (from Table 3.5), which corresponds to moderate to severe scaling. Such a level of deterioration is, of course, not satisfactory for most applications. Therefore, a more

conservative conclusion from both analyses (weight loss of scaled material and visual evaluation) would be that the maximum water to cement ratio to achieve scaling resistance without the benefits of air entraining would be 0.25. This conclusion agrees with findings from other researchers (Li et al. 1994, Gagné et al. 1992).

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4.6 APPLICABILITY OF ACI 318 REQUIREMENTS 4.6.1 ACI Requirements The second line of Table 4-4. shows the code requirements for the mixture used in this study, all of which incorporated a coarse aggregate with a nominal maximum size of 12.5 mm. With the exception of mixture 50-6, all of the mixtures studied could be classified as qualifying for the 1% reduction in air content for complying with a specified strength requirement of 34.5 MPa (5000 psi). This would be valid as long as the standard deviation of the production facility was no greater than about 3.1 MPa (450 psi) (ACI 318 Section 5.3.2). For the same standard deviation, mixture 50-6 would meet a specification requirement for only about 29 MPa. Required air content for mixture 50-6 thus comes from the ACI 318 category for ˘ 34.5 MPa.

ACI 318-99 includes minimum durability requirements for freezing and thawing exposures based on the watercementitious ratio, compressive strength, and total air content. These requirements depend on the level of exposure, which can be severe or moderate. Severe exposure is defined as the cases in which concrete may be in almost continuous contact with moisture prior to freezing, or where deicing salts are used. Moderate exposure, on the other hand, is where concrete will be only occasionally exposed to moisture prior to freezing, and where no deicing salts are used. ACI 318 requirements for total air content are summarized in Table 4.4, which incorporates not only the ACI 318 tabular data (ACI 318 Table 4.2.1), but also the ACI provisions that the “tolerance on air content as delivered shall be ± 1.5%.” Further, ACI 318 states that total air content may be reduced by 1% for “specified compressive strength ˘ greater than 34.5 MPa (5000 psi).”

Table 4.4. ACI 318-99 Requirements for Frost-Resistant Concrete

Air content (%)

Nominal maximum aggregate size (mm)

Severe exposure

Moderate exposure

Severe exposure

Moderate exposure

9.5 12.5 19.0 25.0 37.5 50.0 75.0

6.0–9.0 5.5–8.5 4.5–7.5 4.5–7.5 4.0–7.0 3.5–6.5 3.0–6.0

4.5–7.5 4.0–7.0 3.5–6.5 3.0–6.0 3.0–6.0 2.5–5.5 2.0–5.0

5.0–8.0 4.5–7.5 3.5–6.5 3.5–6.5 3.0–6.0 2.5–5.5 2.0–5.0

3.5–6.5 3.0–6.0 2.5–5.5 2.0–5.0 2.0–5.0 1.5–4.5 1.0–4.0

fc' < 34.5 MPa (5000 psi)

55

fc' >34.5 MPa (5000 psi)

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4.6.2 Compliance of Test Mixtures with ACI 318-99 Requirements for Total Air Content Table 4.5 shows a summary of the results obtained in the present study. Data are presented for water to cement ratio, average compressive strength at 28 days, total air content in the concrete, and the expected severity of exposure that could be tolerated on the basis of ACI 318 requirements for air content. The table also includes the actual performance in ASTM C 666 freeze-thaw and ASTM C 672 scaling tests. On the basis of total air content requirements alone, mixtures 50-6, 45-6, 40-6, 356, 30-6a, 30-6b, and 25-6 would be acceptable under the code for severe exposure, while mixtures 50-4, 45-4, 404, 35-N, 35-4, and 25-4 would meet code requirements for moderate exposure. On the basis of total air content, mixtures 50-N, 45-N, 40-N, 30-N, and 25-N did not meet ACI 318 requirements for frost durability. This result is graphically presented in Figure 4.32. Figures 4.33 and 4.34 are graphic comparisons of ACI 318 allowable exposure with ASTM C 666 and ASTM C 672 test results, respectively. Mixtures that complied with ACI 318 air content requirements for severe exposure (50-6, 45-6, 40-6, 35-6, 30-6A, 30-6B, and 256) performed well in both ASTM C 666 and ASTM C 672 tests. For these mixtures, the durability factor (ASTM C 666) ranged from 99 to 107, while the weight loss of surface material was 0.3 to 0.6 kg/m2, and the visual rating of the scaled surface was 1.2 to 2.7 (ASTM C 672). The mixtures that complied with ACI 318 air content requirements for moderate exposure (50-4, 45-4, 40-4, 35N, 35-4, and 25-4) also performed well in the ASTM C 666 test, with durability factors ranging from 98 to 104. According to ACI 318, deicing salts are not present in a moderate exposure, and thus these mixtures do not need to be deicing-salt scaling resistant. Nevertheless, the results from the ASTM C 672 test showed that mixture 25-4 performed as well as the mixtures qualifying for severe exposure, suffering weight loss of only 0.2 kg/m2 and with an average visual rating of 1.9. The other mixtures qualifying for moderate exposure under ACI 318 air content requirements had scaling weight losses of 0.4 to 1.4 kg/ m2, and visual scale ratings from 2.7 to 4.1. Even though mixture 25-N would not have qualified as frost resistant by ACI 318 requirements for air content, this mixture performed as well as those qualifying for severe exposure. Similarly, mixture 30-N behaved as well as those qualifying for moderate exposure. Only mixtures 50N, 45-N, and 40-N can be considered non-frost resistant on the basis of both the ACI air content criteria and the standard test results.


It would appear, therefore, that the ACI 318 requirements for air content conservatively define frost-resistant concrete, even when the 1% reduction is taken into account for concrete complying with a specified strength of 34.5 MPa, and even when the 1.5% tolerance is taken into account. In fact, were it not for the 1.5% code tolerance, only three mixtures from the entire study would have qualified for a severe exposure, even though all but three mixes scored 80 or above in the ASTM C 666 test. It is suggested that application of the ACI 318 requirement for air content without recognition of the 1% reduction for higher strength concrete, and without full recognition of the 1.5% tolerance would be uneconomically conservative. As a final note on air content, it is clear that in a general discussion of frost resistance it is insufficient to consider total air content without regard to air void size and spatial distribution, even though ACI 318 contains no requirements for either specific surface (a), or spacing factor ¿. In this study, however, there was a general relationship between air content and spacing factor for all but two of the mixtures studied (see Figure 4.9). This is a result of a reasonably consistent specific surface coupled with the effects of paste volume. For that reason, conclusions can be drawn on the basis of air content within the context of this study. Refer also to the discussions in sections 4.5.1 and 4.5.2 concerning spacing factor, durability factor, and scaling resistance.

4.6.3 Compliance of Test Mixtures with ACI 318-99 Requirements for water to cement ratio Section 4.2.2 of the code requires that concrete exposed to freezing and thawing in a moist condition or exposed to deicing chemicals have a maximum water-cementitious materials ratio of 0.45. By that standard, none of the mixtures with a water to cement ratio of 0.50 would have complied with code requirements, although it was observed that concrete with a water to cement ratio of 0.50 was highly resistant to freeze-thaw damage when tested by ASTM C 666 for air contents of 3.8% and 5.6%. Thus, concrete that does not comply with the code requirements for water to cement ratio can be frost resistant. All airentrained mixtures at a water to cement ratio less than or equal to 0.45 proved to be frost resistant when tested under ASTM C 666. Mixtures 40-4 (4.1% air) and 35-4 (3.2% air) exhibited objectionable scaling (ASTM C 672 with 1.0 kg/m2 weight loss criterion imposed). Mixtures with w/c of 0.50 performed well in scaling tests at air contents of 3.8% and 5.6%.

56

Mixture

w/c

Air content Average fc in at 28 days concrete (MPa) (%)

Tolerable exposure severity per ACI 318

ASTM C 666 durability factor (%)

ASTM C 672 average average weight loss visual 2 (kg/m ) rating

57

50-N 50-4 50-6

0.50 0.50 0.50

42.1 38.6 33.4

1.3 3.8 5.6

None Moderate Severe

9 98 99

> 6.0 0.4 0.3

5.0 2.7 2.4

45-N 45-4 45-6

0.45 0.45 0.45

48.6 45.2 42.0

2.0 4.0 5.8


28 101 101

> 3.7 0.7 0.4

5.0 3.0 2.1

40-N 40-4 40-6

0.40 0.40 0.40

49.2 41.4 39.9

2.4 4.1 5.9


80 104 105

> 3.2 1.1 0.6

5.0 3.5 2.7

35-N 35-4 35-6

0.35 0.35 0.35

59.1 46.0 44.6

3.0 3.2 6.6

Moderate Moderate Severe

101 102 107

1.4 1.0 0.3

4.1 3.2 2.4

30-N 30-6A 30-6B

0.30 0.30 0.30

55.5 54.5 51.4

2.1 6.2 6.3

None Severe Severe

98 103 102

0.5 0.4 0.4

3.7 2.0 2.1

25-N 25-4 25-6

0.25 0.25 0.25

44.6 50.0 52.5

2.5 3.6 5.0


101 103 104

0.4 0.2 0.1

2.4 1.9 1.2


Table 4.5. Compliance of Mixtures with ACI 318 Air Content Requirements for Frost Durability

PCA RD122

none

Mixture

Figure 4.32. Level of allowable exposure for all mixtures according to ACI 318 requirements for air content. Frost and Scaling Resistance of High-Strength Concrete

25-6

25-4

25-N

30-6B

30-6A

30-N

35-6

35-4

35-N

10-6

10-4

10-N

45-6

45-4

45-N

50-6

50-4

50-N

58

Level of allowable exposure by ACI 318 requirements for air content

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severe

moderate


Level of Exposure by ACI 318 for Air Content

None

Moderate

Severe

120

Durability factor

100 80 60 40

59

25-6

30-6B

30-6A

35-6

40-6

45-6

50-6

25-4

35-4

35-N

40-4

45-4

50-4

25-N

30-N

40-N

45-N

0

50-N

20

Mixture

Figure 4.33. Comparison between level of exposure according to ACI 318 requirements for air content and actual performance in the ASTM C 666 tests.

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Level of Exposure by ACI 318 for Air Content

Visual rating Weight loss Moderate

Severe

5

2.5

4

2.0

3

1.5

2

1.0

1

0.5

0

0.0

Figure 4.34. Comparison between level of exposure according to ACI 318 requirements for air content and actual performance in the ASTM C 672 tests.

25-6

30-6B

30-6A

35-6

40-6

45-6

50-6

25-4

35-4

35-N

40-4

45-4

50-4

25-N

30-N

40-N

45-N

50-N

Mixture


3.0

60

6

Weight loss (kg/m2)

Visual rating

None


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4.6.4 Compliance of Test Mixtures with ACI 318-99 Requirements for Specified Compressive Strength

the 211.1 guidelines are based on average 28-day strength, more or less the same relationship is used by 318 for specified 28-day strength. Figure 4.35 also displays the average 28-day strength values obtained for the mixtures studied. In each case the actual average strength obtained exceeded the values from both ACI 211.1 and ACI 318, indicating that the ACI 211.1 values underestimate the average strengths for the materials used, and indicating that ACI 318 values appropriately account for the difference between specified and average strength for this set of materials.

In addition to the maximum limit of 0.45 on water to cement ratio, the code sets a minimum specified compressive strength ˘ of 31.0 MPa (ACI Table 4.2.2) for concrete exposed to freezing and thawing in a moist condition, or exposed to deicing chemicals. As discussed earlier, all of the mixtures had a sufficiently high average 28-day compressive strength to comply with the specified value of 34.5 MPa (5000 psi) for the 1% reduction in the air content requirements, with the exception of 50-6. That mixture, with an average 28-day strength of 33.4 MPa, could not meet a 34.5 MPa specification at all, and could meet the 31.0 MPa specification only with a producer’s standard deviation of 1.8 MPa or less. This would require a coefficient of variation of about 5%. Although mixture 506 is therefore out of compliance with the ACI 318 code for frost-resistant concrete on the basis of water to cementitious materials ratio, and probably out of compliance on the basis of strength, this mix did perform well in actual tests. While discussing the ACI water to cementitious materials ratio and strength requirements, it is of interest to note that ACI Table 4.2.2 associates a water to cement ratio of 0.50 with a specified 28-day compressive strength of 27.6 MPa, a water to cement ratio of 0.45 with 31.0 MPa, and a water to cement ratio of 0.40 with 34.5 MPa. As shown in Figure 4.35, this implied relationship closely corresponds to that included in the ACI 211.1 mixture proportioning guidelines (ACI Committee 211 1991). Whereas

4.6.5 Assessing ACI 318 Requirements for Air Content, Water to Cement Ratio and Strength Table 4.6 shows the exposure conditions for which all concretes tested would comply with all ACI 318 criteria for frost resistance, along with actual performance data. It is observed that concrete meeting the code requirements for severe exposure consistently correlates with observed durability factors (ASTM C 666) in the range of 99 to 107, weight loss below 1.0 kg/m2, and visual scaling rating below 2.7 (ASTM C 672). Similarly, concrete meeting code requirements for moderate exposure correlates with observed durability factors (ASTM C 666) in the range of 98 to 104, with generally poorer scaling results (up to 1.4 kg/ m2 weight loss and up to 4.1 visual rating). This appears to be in agreement with ACI 318 definitions where severe exposure anticipates the presence of deicing salts but moderate exposure does not.

28-day compressive strength (MPa)

70 60 50 40 30 20 10

High air-entrained mixtures Low air-entrained mixtures ACI 318-95—Specified stength ACI 211.1—Average strength

0 0.25

0.30

0.35

0.40 0.45 0.50 Water to cement ratio

0.55

0.60

Figure 4.35. Relationship between average 28-day compressive strength and ACI 318, and ACI 211.1 recommendations.

61

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On the other hand, ACI 318 requirements are conservative, particularly in regard to lower strength mixtures with reasonable air contents. Mixtures with a water to cement ratio in excess of 0.45, and perhaps not meeting a strength of 31 MPa were shown to be frost-resistant when properly air entrained (mixture 50-6). Similarly, code provisions are also conservative in regard to air content required for low water to cement ratio mixtures, where concretes considered under code criteria to be non-frost resistant and those considered to be only moderately resistant performed as well as those considered to be resistant to a severe environment (mixtures 25-N and 25-4). One must recall, however, that these observations are based in part on a modified ASTM C 666 test in which the speci-

mens were allowed a 2-week drying period before freezing (see section 4.5.1). Further, all tests were based on the relatively rapid freezing rates of ASTM C 666 and C 672, which tend to favor the hydraulic pressure damage mechanism. Concrete more susceptible to damage by ice accretion might perform differently in tests with a slower freezing rate and a longer, sustained freeze cycle. However, to the degree to which these test results simulate an actual exposure condition, this study suggests that to obtain frost resistance as defined by both ASTM C 666 and ASTM C 672, air entrainment is not necessary for mixtures with a water to cement ratio of 0.25. When scaling resistance is not required, air entrainment may not be necessary for mixtures with a water to cement ratio ≤0.35.

Table 4.6. Compliance of Mixtures with ACI 318 Air Content Requirements for Frost Durability, Including Total Air Content, Water to Cementitious Material Ratio, and ¯

Air Average ¯ content in at 28 days concrete (MPa) (%)

Tolerable exposure severity per ACI 318

ASTM C 666 durability factor (%)

ASTM C 672 Average Average weight loss visual 2 (kg/m ) rating

Mixture

w/c

50-N 50-4 50-6

0.50 0.50 0.50

42.1 38.6 33.4

1.3 3.8 5.6

None None None

9 98 99

> 6.0 0.4 0.3

5.0 2.7 2.4

45-N 45-4 45-6

0.45 0.45 0.45

48.6 45.2 42.0

2.0 4.0 5.8


28 101 101

> 3.7 0.7 0.4

5.0 3.0 2.1

40-N 40-4 40-6

0.40 0.40 0.40

49.2 41.4 39.9

2.4 4.1 5.9


80 104 105

> 3.2 1.1 0.6

5.0 3.5 2.7

35-N 35-4 35-6

0.35 0.35 0.35

59.1 46.0 44.6

3.0 3.2 6.6

Moderate Moderate Severe

101 102 107

1.4 1.0 0.3

4.1 3.2 2.4

30-N 30-6A 30-6B

0.30 0.30 0.30

55.5 54.5 51.4

2.1 6.2 6.3

None Severe Severe

98 103 102

0.5 0.4 0.4

3.7 2.0 2.1

25-N 25-4 25-6

0.25 0.25 0.25

44.6 50.0 52.5

2.5 3.6 5.0


101 103 104

0.4 0.2 0.1

2.4 1.9 1.2

62


PCA RD122

CHAPTER 5

SIGNIFICANT OBSERVATIONS AND CONCLUSIONS

5.1 FRESH CONCRETE—SLUMP AND SETTING TIMES

5.3 COMPRESSIVE STRENGTH • For the mixtures studied, in the water to cement ratio range of 0.30 to 0.50, an increase of 1% in the air content corresponded to a decrease in compressive strength of about 2.6 MPa. A corresponding decrease of 10 MPa in the compressive strength would be expected when 4% of air is incorporated in the total volume of concrete. • A decrease of the water to cement ratio of about 0.14 corresponded to an increase in compressive strength of the order of 10 MPa. • There was an observed linear relationship between the cement-voids ratio (as calculated according to MINNDOT) and the 90-day compressive strength for mixtures with water to cement ratio greater than or equal to 0.35. For mixtures with water to cement ratio of 0.30 and 0.25, this linear relationship overestimated compressive strength. • A consistent linear relationship between total porosity (based on the Powers model) and compressive strength was obtained for all mixtures.

• For a given water content, the same dosage of highrange water reducer produced a greater relative slump increase for air-entrained mixtures than for non-air-entrained mixtures. • Slump may be predicted as a function of water content and dosage of HRWRA. • Higher dosages of the HRWRA retarded setting. • There is a consistent relationship between set time and HRWRA dosage. • Setting did not appear to be affected by air content.

5.2 AIR CONTENT AND AIR VOID PARAMETERS • The three methods used to evaluate air content: pressure method (ASTM C 213) and gravimetric method (ASTM C 138) in fresh concrete, and microscopic evaluation (ASTM C 457) in hardened concrete, yielded similar values. The maximum difference between the three results for any sample was 1.5%. No one method was consistently the highest or lowest. • An approximate relationship was found to exist between the dosage of air-entraining admixture and the air content of the paste. • The microscopic evaluation of the air void system (ATM C 457) showed spacing factors often greater than 500 µm for the non-air-entrained mixtures, and values generally slightly higher than 200 µm for the air-entrained mixtures. • Given more or less consistent paste content and specific surface of air-entrained mixtures, an approximately consistent relationship exists between air content in the concrete and spacing factor.

5.4 RAPID CHLORIDE PERMEABILITY • An expression was developed to correlate charge passed in coulombs to air content and water to cement ratio. • Both air content and water to cement ratio affected the rapid chloride permeability results, with water to cement ratio having a much more pronounced effect. • For the mixtures studied here, a maximum water to cement ratio of 0.35 would be necessary to achieve a maximum charge of 2000 coulombs in the RCPT, indicating a mixture with a potentially low chloride ion penetrability.

63

PCA RD122

5.5 FROST RESISTANCE 5.5.1 ASTM C 666 with Modified Curing • The ASTM C 666 tests were intentionally conducted with a 14-day drying period prior to the initiation of freezing in water. The effect that this drying period may have had on the results is discussed in this report. • The ASTM C 666 test, when evaluated via resonant frequency, assesses damage in the core or “bulk” of the specimen. • It was possible to obtain durability factors close to 100 for mixtures with entrained air at water to cement ratio of 0.50 or lower. • Durability factors close to 100 were also achieved without air entrainment at water to cement ratio equal to or less than 0.35. • Durability factors of about 100 were obtained for spacing factors varying from 150 to 1100 µm. • Excluding non-air-entrained mixtures with very low water to cement ratio, a high durability factor was associated with spacing factors less than about 400 µm. • The higher strength, low water to cement ratio, superplasticized concretes tested here exhibited frost resistance based on the modified ASTM C 666 test with spacing factors greater than 200 µm.

5.5.2 ASTM C 672 • The ASTM C 672 test evaluates the scaling resistance of the surface of the concrete, i.e., the zone which is most affected by finishing and curing operations. • A relationship between the visual scaling rating and the measured weight loss of the scaled material was obtained. The visual rating was found proportional to the weight loss to the power 0.3. • All mixtures with a visual rating of 2.5 or less lost less than 1.0 kg/m2 of surface material. • Time of finishing did not consistently affect the scaling resistance of the mixtures studied, which were generally characterized by low rates of bleeding. All specimens were wet cured, however, which may have mitigated any effects of finishing (Falconi 1996). • A maximum spacing factor of 200 µm ensured scaling resistance. • Mixtures with a spacing factor of around 250 µm showed mixed results, as some were scaling resistant while others were not. • Non-air-entrained mixtures with water to cement ratio of 0.25 were scaling resistant regardless of the spacing factor. • Scaling resistance is affected by both the water to cement ratio and the air void system.


• An exponential relationship was found between the mass of scaled material and the water to cement ratio and air content of the paste for the superplasticized mixtures. • Entrained air was necessary to provide scaling resistance for mixtures with water to cement ratio of 0.30 or greater.

5.6 APPLICABILITY OF ACI 318-99 REQUIREMENTS TO FROST DURABILITY OF HIGH-STRENGTH CONCRETE • Mixtures were evaluated against ACI 318 criteria, taking into account the 1% reduction in required air content for mixtures complying with a specified 28-day compressive strength of at least 34.5 MPa. • Mixtures were evaluated against ACI 318 criteria, also taking into account the 1.5% tolerance on total air content as delivered. • It would appear that ACI 318 requirements for total air content conservatively define frost-resistant concrete, even with the 1% reduction for higher strength, and with the 1.5% tolerance. • Were it not for the 1.5% tolerance, only three mixtures from the entire study would have qualified for a severe exposure, even though all but three mixtures performed well in ASTM C 666 testing. • The requirement that water to cement ratio be less than or equal to 0.45 for frost resistance is conservative. Airentrained mixtures with a water to cement ratio of 0.50 were frost and scale resistant. • The requirement that concrete comply with a specified 28-day compressive strength of at least 31 MPa is conservative. One mixture studied would not comply with such a specification, but was nevertheless frost resistant. • The relationship between water to cement ratio and f c' implied in ACI 318 closely matches the relationship between water to cement ratio and average 28day strength found in ACI 211.1. The ACI relationship between water to cement ratio and fc' appears to account for the difference between specified and average compressive strength. • Concrete meeting the code requirements for severe exposure consistently correlated with observed durability factors (ASTM C 666) in the range of 99 to 107, weight loss below 1.0 kg/m2, and visual scaling rating below 2.7 (ASTM C 672). Similarly, concrete meeting code requirements for moderate exposure correlated with observed durability factors (ASTM C 666) in the range of 98 to 104, with generally poorer scaling results (up to 1.4 kg/m2 weight loss and up to

64


PCA RD122

4.1 visual rating). This appears to be in agreement with ACI 318 definitions where severe exposure anticipates the presence of deicing salts but moderate exposure does not. • This study suggests that to obtain frost resistance as defined by both ASTM C 666 and ASTM C 672, air entrainment is not necessary for mixtures with water to cement ratio of 0.25. When scaling resistance is not required, air entrainment may not be necessary for mixtures with water to cement ratio < 0.35. • These observations apply to concrete that is wet cured and incorporates only portland cement, water, frostresistant aggregates, and chemical admixtures. No supplementary cementing materials were used. The ACI 318 limits apply to concretes with and without fly ash, ground granulated blast furnace slag, silica fume, or other pozzolans. These findings may therefore not apply to mixtures incorporating such supplementary cementing materials.

ACKNOWLEDGEMENTS The research reported in this paper (PCA R&D Serial No. 2387) was conducted by Cornell University, with the sponsorship of the Portland Cement Association (PCA Research Fellowship No. F18-96). The contents of this paper reflect the views of the author, who is responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the views of the Portland Cement Association

65

REFERENCES Abel, J. H., and Hover, K. C. (2000), “Field Study of Measuring Setting Time of Fresh Concrete and its Influence on Flatwork Finish,” Submitted for Publication to Concrete International. ACI Committee 201 (1992), Guide to Durable Concrete (ACI 201.2R-92), American Concrete Institute, 41 pages. ACI Committee 211 (1991), Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91), American Concrete Institute, 38 pages. ACI Committee 318 (1999), Building Code Requirements for Structural Concrete (ACI 318-99) and Commentary (ACI 318R99), American Concrete Institute, 391 pages. Aïtcin, P. C. (1998a),“High-Performance Concrete,” E & FN Spon, London, 591 pp. Aïtcin, P. C. (1998b), “The Influence of the Spacing Factor on the Freeze-Thaw Durability of High-Performance Concrete,”International Symposium on High-Performance and Reactive Powder Concretes, Sherbrooke,Vol .4, pages 419 to 433. Auberg, R., and Setzer, M. J. (1998),“Freeze Thaw Testing of Concrete,” Proceedings of the 5th Bolomey Conference, Essen, Germany. Cordon, W. A. (1966),“Freezing and Thawing of Concrete—Mechanics and Control,” ACI Monograph No. 3, American Concrete Institute. Dodson, V. H. (1994), “Time of Setting,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C, American Society for Testing Materials, edited by P. Klieger and J. P. Lamond, pages 77 to 87. Falconi, M. I. (1996), Durability of Slag Cement Concretes Exposed to Freezing and Thawing in the Presence of Deicers, M. Sc. Thesis, Cornell University, 306 pages. Foy, C., Pigeon, M., and Bathia, N. (1988),“Freeze-Thaw Durability and Deicer Salt Scaling Resistance of a 0.25 WaterCement Ratio Concrete,” Cement and Concrete Research, Vol. 18, No. 4, pages 604 to 614. Gagné, R., Pigeon, M., and Aïtcin, P. C. (1990), “Deicer Salt Scaling Resistance of High-Performance Concretes,” Paul Krieger Symposium on Performance of Concrete, ACI Special Publication SP-122, edited by David Whiting, pages 29 to 44. Gagné, R., Pigeon, M., and Aïtcin, P. C. (1991),“Deicer Salt Scaling Resistance of High Strength Concretes Made with Different Cements,” Durability of Concrete Second International Conference, ACI Special Publication SP-126, edited by V. M. Malhotra, pages 185 to 199. Gagné, R., Aïtcin, P. C., Pigeon, M., and Pleau, R. (1992),“Frost Durability of High Performance Concretes,” High Performance Concrete – From Material to Structure, edited by Y. Malier, Chapter 16, E & F.N. Spon, London, pages 239 to 251. Gagné, R., and Marchand, J. (1993),“La Résistance à L’écaillage des Bétons à Haute Performance: État de la Question,” International Workshop on the Resistance of Concrete to Scaling due to Freezing in the Presence of Deicing Salts, Centre de Recherche Interuniversitaire sur le Béton (CRIB), Université de Sherbrooke – Université Laval, Sainte-Foy, pages 23 to 47. Hammer, T. A., and Sellevold, E. J. (1990),“Frost Resistance of High-Strength Concrete,” High Strength Concrete – Second International Symposium, ACI Special Publication SP-121, edited by W. T. Hester, Detroit, pages 457 to 487. Hover, K. C. (1989), Report to Fort Miller Precast Concrete Co. on the probable cause for aggregate pop-outs in precast concrete. Hover, K. C. (1994),“Air Content and Unit Weight of Hardened Concrete,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C, American Society for Testing Materials, edited by P. Klieger and J. P. Lamond, pages 296 to 314.

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Hover, K. C. (1996), Personal correspondence, Washington Department of Transportation. Jacobsen S., and Sellevold, E. J. (1996),“Frost Testing of High Strength Concrete: Scaling and Cracking,” 4th International Symposium on the Utilization of High Strength/High Performance Concrete, Paris, pages 597 to 606. Janssen, D. J., and Snyder, M. B. (1993), “Mass Loss Experience with ASTM C 666: With and Without Deicing Salts,” International Workshop on the Resistance of Concrete to Scaling due to Freezing in the Presence of Deicing Salts, Centre de Recherche Interuniversitaire sur le Béton (CRIB), Université de Sherbrooke – Université Laval, pages 137 to 152. Khalil, S. M., Ward, M. A., Morgan, D. R. (1980), “Freeze-Thaw Durability of Non-Air-Entrained High-Strength Concretes Containing Superplasticizers,” Durability of Building Materials and Components, ASTM STP 691, edited by P. J. Sereda and G. G. Litvan, American Society for Testing and Material, pages 509 to 519. Klieger, P. (1952),“Effect of Entrained Air on Strength and Durability of Concrete Made with Various Sizes of Aggregates,” Proceedings, Highway Research Board, Vo. 3, pages 177 to 201. Kobayashi, M., Nakamuro, E., Kodama, K., and Negami, S. (1981),“Frost Resistance of Superplasticized Concrete,”ACI Special Publication SP-68, pages 233 to 252. Kreijger, P. C. (1990),“Inhomogeneity in Concrete and its Effect on Degradation: a Review of Technology,” Protection of Concrete, edited by R. K. Dhir and J. W. Green, E & F.N. Spon, London, pages 31 to 52. Li, Y., Langan, B. W., and Ward, M. A. (1994),“Freezing and Thawing: Comparison Between Non Air-Entrained and AirEntrained High-Strength Concrete,” Proceedings ACI International Conference High-Performance Concrete, ACI Special Publication SP-149, edited by V. M. Malhotra, pages 545 to 560. Lin, C., and Walker, R. D. (1975),“Effects of Cooling Rates on the Durability of Concrete,” Transportation Research Record, 539, pages 8 to 19. Marchand, J., Gagné, R., Jacobsen, S., Pigeon, M. and Sellevold, E. J. (1996), “La Résistance au Gel des Bétons a Haute Performance,” Canadian Journal of Civil Engineering, Vol. 23, No. 5, pages 1070 to 1080. McNeal, F., and Gay, F. (1996),“Solutions to Scaling Concrete,” Concrete Construction, March 1996, pages 250 to 255. Mehta, P. K. (1986), Concrete: Structure, Properties and Materials, Prentice Hall, Inc., Englewood Cliffs, 450 pages. Metha, P. K. and Monteiro, P. J. M. (1993), Concrete: Microstructure, Porperties, and Materials, 2nd edition, The McGraw-Hill Companies, Inc., 548 pages. Meyer, A. (1987),“The Importance of the Surface Layer for the Durability of Concrete Structures,” Concrete Durability – Katharine and Bryant Mather International Conference, ACI Special Publication SP-100, Volume 1, edited by J. M. Scanlon, Detroit, pages 49 to 61. Mielenz, R. C., Wolkodoff, V. E., Backstrom, J. E., and Burrows, R. H. (1958),“Origin, Evolution, and Effects of the Air Void System in Concrete. Part 4 – The Air Void System and Job Concrete,” Proceedings, American Concrete Institute, Vol. 55, pages 507 to 517. Mindess, S., and Young, J. F. (1981), Concrete, Prentice Hall, Inc., Englewood Clifts, 671 pages. Minnesota Department of Transportation (1998), Concrete Manual, Developed by Pavement Engineering Section – Concrete Engineering Unit, Douglas Schwartz, Concrete Engineer, St. Paul, Minnesota. Narayanan, G. (1997), “Further Investigations into the Deicer Scaling of Slag Cement Concrete with Emphasis on Surface Properties,” M. Sc. Thesis, Cornell University, 199 pages. Natesaiyer, K. C., Hover, K. C., and Snyder, K. A. (1992),“The Protected Paste Volume of Air Entrained Cement Paste; Part I,” Journal of Materials in Civil Engineering, Vol. 4, No. 2, pages. 166 to 184. Natesaiyer, K. C., Hover, K. C., and Snyder, K. A. (1993),“The Protected Paste Volume of Air Entrained Cement Paste; Part II,” Journal of Materials in Civil Engineering, Vol. 5, No. 2, pages. 170 to 186. Neville, A. M. (1996), Properties of Concrete, John Wiley & Sons, Inc., 4th edition, London, 844 pages. Newlon, H., and Mitchell, T. M. (1994), “Freezing and Thawing,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C, American Society for Testing Materials, edited by P. Klieger and J. P. Lamond, pages. 153 to 163. Perenchio, W. F., and Klieger, P. (1978),“Some Physical Properties of High-Strength Concrete,”Research and Development Bulletin RD056.01T, Portland Cement Association, 7 pages. Philleo, R. E. (1983),“A Method for Analyzing Void Distribution in Air-Entrained Concrete,” Cement, Concrete, and Aggregates, Vol. 5, pages. 128 to 130. Philleo, R. E. (1987),“Frost Susceptibility of High-Strength Concrete,” Concrete Durability – Katharine and Bryant Mather International Conference, ACI Special Publication SP-100,Volume 1, edited by J. M. Scanlon, Detroit, pages 819 to 842.

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Pigeon, M., Prevost, J., and Simard, J. (1985),“Freeze-Thaw Durability Versus Freezing Rate,” ACI Materials Journal, Vol. 82, pages 684 to 692. Pigeon, M., Gagné, R., Aïtcin, P. C., and Banthia, N. (1991),“Freezing and Thawing Tests of High-Strength Concretes,” Cement and Concrete Research, Vol. 21, No. 5, pages 844 to 852. Pigeon, M. (1994), “Frost Resistance, A Critical Look,” Concrete Technology Past, Present, and Future – Proceedings of V. Mohan Malhotra Symposium, ACI Special Publication SP-144, edited by P. K. Mehta, Detroit, pages 141 to 158. Pinto, R. C. A., and Hover, K. C. (1997), “Effect of Silica Fume and Superplasticizer Addition on Setting Behavior of High-Strength Mixtures,” Transportation Research Record, No. 1574, pages 56 to 62. Pinto, R. C. A., Hobbs, S. V., and Hover, K. C. (1999),“The Development of Non-Evaporable Water Content and Hardened Properties of High-Performance Mixtures,” High Performance Concrete, SP-189, American Concrete Institute, Detroit, 1999, pages 351 to 366. Pinto, R. C. A., and Hover, K.C. (1999), “The Application of the Maturity Approach to Setting Times,” ACI Materials Journal, Vol. 96, No.6, Nov.–Dec. 1999, pages 686 to 691. Pleau, R., and Pigeon, M. (1992),“Precision Statement for ASTM C 457 Practice for Microscopical Determination of AirVoid Content and Other Parameters of the Air-Void System in Hardened Concrete,” Cement, Concrete, and Aggregates, Vol. 14, pages 118 to 126. Powers, T. C. (1949),“The Air Requirement of Frost-Resistant Concrete,” Proceedings, Highway Research Board, Vol. 29, Washington, pages 184 to 202. Powers, T. C., and Helmuth, R. A. (1953), “Theory of Volume Changes in Hardened Portland Cement Paste During Freezing,” Proceedings, Highway Research Board, Vol. 32, pages 285 to 297. Powers, T. C. (1958), The Physical Structure and Engineering Properties of Concrete, Research and Development Bulletin No. 90, Portland Cement Association. Setzer, M. J. (1996), “Micro Ice Lense Formation in Concrete,” Pore Solution in Hardened Cement Paste, edited by F. Wittmann, M. J. Setzer, and J. Adolphs, Proceedings of the 3rd Bolomey Conference, Essen, Germany. Siebel, E. (1989), “Air-Void Characteristics and Freezing and Thawing Resistance of Superplasticized Air-Entrained Concrete with High Workability,” Proceedings of the 3rd International Conference on Superplasticizers and Other Chemical Admixtures, Ottawa, pages 297 to 319. Vanderhost, N. M., and Jansen, D. J. (1990), “The Freezing-and-Thawing Environment: What is Severe?” Paul Krieger Symposium on Performance of Concrete, edited by David Whiting, SP-122, American Concrete Institute, pages 181 to 200. Whiting, D., and Stark, D. (1983), Control of Air Content in Concrete, National Cooperative Highway Research Program Report 258, Transportation Research Board, 84 pages. Whiting, D. (1987),“Durability of High-Strength Concrete,” Concrete Durability – Katharine and Bryant Mather International Conference, ACI Special Publication SP-100, Volume 1, edited by J. M. Scanlon, Detroit, pages 169 to 186. Whiting, D. A., and Nagi, M. A. (1998), Manual on Control of Air Content in Concrete, EB116, Portland Cement Association, 42 pages.

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PCA RD122


METRIC CONVERSION FACTORS

To convert trom

The following list provides the conversion relationship between U.S. customary units and SI (International System) units. The proper conversion procedure is to multiply the specified value on the left (primarily U.S. customary values) by the conversion factor exactly as given below and then round to the appropriate number of significant digits desired. For example, to convert 11.4 ft to meters: 11.4 X 0.3048 = 3.47472, which rounds to 3.47 meters. Do not round either value before performing the multiplication, as accuracy would be reduced. A complete guide to the SI system and its use can be found in IEEE/ASTM SI-10, Metric Practice.

Mass (weight) per length kip per linear foot kilogram per meter (kif) (kg/m) pound per linear foot kilogram per meter (pif) (kg/m)

To convert trom

to

Length inch (in.) inch (in.) inch (in.) foot (ft) yard (yd)

micron (µ) centimeter (cm) meter (m) meter (m) meter (m)

Area square foot (sq ft) square inch (sq in.) square inch (sq in.) square yard (sq yd)

multiply by 25,400 E* 2.54 E 0.0254 E 0.3048 E 0.9144

square meter (sq m) square centimeter (sq cm) square meter (sq m) square meter (sq m)

Volume cubic inch (cu in.)

cubic centimeter (cu cm) cubic inch (cu in.) cubic meter (cu m) cubic foot (cu ft) cubic meter (cu m) cubic yard (cu yd) cubic meter (cu m) gallon (gal) Can. Iiquid liter gallon (gal) Can. Iiquid cubic meter (cu m) gallon (gal) U.S. Iiquid** liter gallon (gal) U.S. Iiquid cubic meter (cu m) fluid ounce (fl oz) milliliters (ml) fluid ounce (fl oz) cubic meter (cu m) Force kip (10001b) kip (10001b) pound (lb) avoirdupois pound (lb) Pressure or stress kip per square inch (ksi) kip per square inch (ksi) pound per square foot (psf) pound per square foot (psf) pound per square inch (psi) pound per square inch (psi) pound per square inch (psi) Mass (weight) pound (lb) avoirdupois ton, 2000 lb grain

kilogram (kg) newton (N) kilogram (kg)

0.09290304 E 6.452 E 0.00064516 E 0.8361274 16.387064 0.00001639 0.02831685 0.7645549 4.546 0.004546 3.7854118 0.00378541 29.57353 0.00002957 453.6 4,448.222 0.4535924

newton (N)

4.448222

megapascal (MPa)

6.894757

kilogram per square 70.31 centimeter (kg/sq cm) kilogram per square 4.8824 meter (kg/sq m) pascal (Pa)† 47.88 kilogram per square 0.07031 centimeter (kg/sq cm) pascal (Pa)† 6,894.757 megapascal (MPa)

0.00689476

kilogram (kg)

0.4535924

kilogram (kg) kilogram (kg)

907.1848 0.0000648

to

multiply by 0.001488 1.488

Mass per volume (density) pound per cubic foot kilogram per cubic (pcf) meter (kg/cu m) pound per cubic yard kilogram per cubic (lb/cu yd) meter (kg/cu m) Temperature degree Fahrenheit (°F) degree Fahrenheit (°F) degree Kelvin (°K) Energy and heat British thermal unit (Btu) calorie (cal) Btu/°F hr • ft2 kilowatt-hour (kwh) British thermal unit per pound (Btu/lb) British thermal unit per hour (Btu/hr) Power horsepower (hp) (550 ft-lb/sec) Velocity mile per hour (mph) mile per hour (mph) Permeability darcy feet per day (ft/day)

degree Celsius (°C) degree Kelvin (°K) degree Celsius (C* )

16.01846 0.5933

tC = (tF – 32)/1.8 tK = (tF + 459.7)/1.8 tC = tK – 273.15

joule ( J) joule ( J) W/m2 • °K joule ( J) calories per gram (cal/g) watt (W)

1055.056 4.1868 E 5.678263 3,600,000. E 0.55556 0.2930711

watt (W)

745.6999 E

kilometer per hour (km/hr) meter per second (m/s) centimeter per second (cm/sec) centimeter per second (cm/sec)

1.60934 0.44704 0.000968 0.000352

*E indicates that the factor given is exact. **One U.S. gallon equals 0.8327 Canadian gallon. †A pascal equals 1.000 newton per square meter. Note: One U.S. gallon of water weighs 8.34 pounds (U.S.) at 60°F. One cubic foot of water weighs 62.4 pounds (U.S.). One milliliter of water has a mass of 1 gram and has a volume of one cubic centimeter. One U.S. bag of cement weighs 94 lb.

The prefixes and symbols listed below are commonly used to form names and symbols of the decimal multiples and submultiples of the SI units. Multiplication Factor 1,000,000,000 = 109 1,000,000 = 106 1,000 = 103 1 =100, 0.01 = 10-2 0.001 = 10-3 0.000001 = 10-6 0.000000001 = 10-9

69

Prefix giga mega kilo — centi milli micro nano

Symbol G M k — c m µ n

P O R T L A N D

C E M E N T

A S S O C I AT I O N

5420 Old Orchard Road Skokie, Illinois 60077-1083 Phone: 847.966.6200 Fax: 847.966.9781 Internet: www.portcement.org An organization of cement companies to improve and extend the uses of portland cement and concrete through market development, engineering, research, education, and public affairs work.

RD122.01

Frost and Scaling Resistance of High-Strength Concrete

Chloride Resistance of Concrete

Frost durability of roller-compacted concrete pavements

Scaling and Disordered Systems

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Scaling, Fractals and Wavelets

Frost

Frost

Silver Frost (Bitter Frost #3 of The Frost Series)

Scaling and Disordered Systems

Scaling, Fractals and Wavelets

Frost

Frost

Frost

A Touch of Frost (Jack Frost series)

Scaling CouchDB

Multidimensional Scaling

Jack Frost 2 A Touch of Frost

Frost (Saga of Frost, Vol 1)

Frost and Scaling Resistance of High-Strength Concrete

Chloride Resistance of Concrete

Frost durability of roller-compacted concrete pavements

Scaling and Disordered Systems

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Frost

Scaling, Fractals and Wavelets

Frost

Frost

Silver Frost (Bitter Frost #3 of The Frost Series)

Scaling and Disordered Systems

Scaling, Fractals and Wavelets

Frost

Frost

Frost

A Touch of Frost (Jack Frost series)

Scaling CouchDB

Multidimensional Scaling

Jack Frost 2 A Touch of Frost

Frost (Saga of Frost, Vol 1)

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