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Concrete Masonry Units

Materials and Manufacturing of Concrete Masonry Units

Concrete masonry units are formed from zero-slump concrete, sometimes using lightweight aggregate. The concrete mixture is usually vibrated under pressure in multiple-block molds. After stripping the molds, the units are usually cured under atmospheric conditions in a chamber that is maintained at warm and humid conditions by the presence of the curing units. Atmospheric steam or high-pressure steam (autoclaving) can also be used for curing. Concrete units normally have a much higher void ratio than clay units, making determination of (c/b) ratios unnecessary.

Visual and Serviceability Characteristics of Concrete Masonry Units

The following visual and serviceability characteristics are addressed by ASTM C90 (Standard Specification for Hollow Load-Bearing Concrete Masonry Units):
1. Dimensional tolerances: ASTM C90 prescribes maximum dimensional tolerances of ± 1/8 in. Thicknesses of face shells and webs are specified.
2. Chippage: According to ASTM C90, up to 5 percent of a shipment may contain units with chips up to 1 in. in size.
Other visual and serviceability characteristics, such as color, are not addressed by ASTM C90. Color is gray or white, unless metallic oxide pigments are used.

Mechanical Characteristics of Concrete Masonry Units The following mechanical characteristics are covered by ASTM C90, ASTM C140, and ASTM C426:

1. Compressive strength is typically 1500 to 3000 psi on the net area (actual area of concrete). ASTM C90 requires a minimum compressive strength (average of 3 units) of 1900 psi, measured on the net area.

2. Absorption (used to measure void volume) is evaluated in the following manner. The unit is immersed in cold water for 24 h. It is weighed immersed (weight F), and weighed in air while still wet (weight E). It is then dried for at least 24 h at a temperature of 212 to 239°F, and again weighed (weight C). Absorption in lb/ft3 is calculated as [(E − C)/(E − F)] × 62.4. Maximum permissible absorption is 18 lb/ft3 for light-weight units (less than 105 lb/ft3 oven-dried weight), 15 lb/ft3 for medium-weight units (105 − 125 lb/ft3), and 13 lb/ft3 for normalweight units (more than 125 lb/ft3).

3. Shrinkage of concrete masonry units due to drying and carbonation is 300−600 με. In general, shrinkage is controlled by controlling the concrete mix used to make the units, and by limiting the moisture content of the units between the time of production and when they are placed in the wall.

The concrete masonry industry formerly produced Type I (moisturecontrolled) units, which due to a combination of inherent characteristics and packaging were designed to shrink less, and Type II units (nonmoisture-controlled). This distinction was not as successful as originally hoped, because it was difficult to control the condition of Type I units in the field. As a result, ASTM C90 now does not refer to Type I and Type II units. All C90 units must demonstrate a potential drying shrinkage of less than 0.065 percent (650 με), which was the shrinkage requirement that formerly applied to Type II units.

Other Characteristics of Concrete Masonry Units

The following characteristic are not covered by ASTM specifications:

1. Surface texture can be smooth, slump block, split-face block, ribbed block, various patterns, or polished face.

2. Tensile strength is about 10 percent of compressive strength.

3. Tensile bond strength (strength between mortar and CMU) is typically about 40 to 75 psi when portland cement-lime mortar is used, and about 35 psi or less when masonry-cement mortar is used.

4. Initial rate of absorption (IRA) is typically 40 to 160 g/min per 30 in.2 of bed area. It is much less than this in units with integral water-repellent admixtures. In contrast to clay units, the tensile bond strength of concrete masonry units is not sensitive to initial rate of absorption. For this reason, specifications for concrete masonry units do not require determination of IRA. 5. Modulus of elasticity is typically 1 − 3 × 106 psi. 6. Coefficient of thermal expansion is typically: 4 − 5 με/°F.

Polymer impregnated concrete

A polymer impregnated concrete is produced via a process whereby an existing hardened concrete (structure or factory component) is dried at around 150ëC and then impregnated with a low viscosity monomer, usually methyl methacrylate, which is subsequently cured in situ. Impregnation is often achieved by soaking at atmospheric pressure although the process may be facilitated by evacuation and/or higher pressures. Polymerisation is accomplished by a thermal catalytic or promoted catalytic process or is radiation induced. The resulting polymer fills, or lines the surface, of the pores of the concrete modifying the properties of the hardened mortar. Extensive reviews of early work (Swamy, 1979; Shaw, 1989) showed that increases in modulus of elasticity, strength (compressive, flexural and tensile) and improvements in durability could be effected, although the extent of these improvements does depend on the depth of impregnation achieved and porosity of the original concrete. The process shows the most marked improvements when starting with high porosity, low compressive strength concretes. Since these reviews were written, there has been only a little additional investigative work reported (Ohama, 1997; Fowler, 1999). More recent work (Chen, 2005) has compared soaking time and polymerisation tem- perature on the mechanical properties of PIC in a systematic way and confirmed the possible improvements. Decreases in surface absorption compared with normal concrete are attributed to significant decreases in total porosity with maximum pore diameters of <50 nm. Despite the much improved properties and early application of PIC for remedial treatment of road bridge decks in the US, their high processing costs has generally precluded further use apart from limited precast operations in Japan, (Ohama, 1997).

However, one developing area where polymer impregnation may prove beneficial is the wear resistance of concrete used in floor construction. Treat- ments have been found to significantly increase the abrasion resistance of all types of concrete (Sadegzadeh, 1988), although improvements are greatest with low compressive strength concrete (Sebok, 2004).

Polymer-modified cement, mortar and concrete Nature of systems

Widespread international use is made of these materials, e.g. as grouts and mortar patches for finish and repair work and concretes for bridge deck overlays. Basically, unreactive polymer latex is added to the water of a fairly conventional cementitious mix, which is then cured. The cement begins to hydrate, whilst the polymer particles from the latex coalesce to give a film which then binds the hydrated cement phases and aggregate. There are numerous proprietary polymer emulsions and redispersible powders available. Polymer type and composition vary widely and all dispersions include other constituents. The manufacturers of these systems are often reluctant to disclose their complete make-up and characteristics for commercial reasons. Specific properties of the hardened material depend very much on the formulation of the polymer dispersion, mortar mix design, and curing regime used. Unfortunately, in much of the considerable published information, the exact nature of the systems and the mixing and curing procedures used are not very well documented, making an understanding of their behaviour and a comparison between different systems difficult.

The most commonly used latexes are aqueous suspensions of styrene- butadiene-rubber (SBR) and various acrylics (Ac) containing 45±50% polymer solids. In SBR, the ratio of styrene to butadiene governs the properties of the polymer, with 60±65% styrene giving a good balance. Higher styrene contents would improve compressive and tensile strengths but reduce adhesion and raise the minimum film-forming temperature (MFT). Usually ~1% carboxylic acid is chemically bound onto the polymer particle surface. These groups ionise in the high pH environment of the fresh cement, Fig. 10.1(a) (Chandra, 1987) and this generally results in improved stability of the latex and adhesion of the PMC to existing substrates (Dennis, 1985). Similarly there are a wide variety of co- and ter-acrylic polymers with some of them being available in redispersible powder form. Ethylene-vinyl acetate (EVA) was one of first redispersible powders on the market. Powders which can be pre-bagged with the cement and aggregate are preferable from a practical and environmental view, compared to latexes which are supplied in plastic containers and require batching to be carried out on site.

In addition to the polymer, a sufficiency of surfactant is added which is then adsorbed onto the surfaces of the polymer particles and helps maintain their dispersion. This dispersion should be preserved during mixing and transfer operations in the high pH environment of the cement and over a practical range of temperatures. They may be anionic, cationic or non-ionic and can affect latex- latex or latex-cement interactions. Any free surfactant in the mix water tends to stabilise air bubbles and so a de-foaming agent may be added to counter this. Other additions may also be present, e.g. anti-oxidants and bactericides.

Little detailed information is available on mix procedures although a typical mix design consists of a sand:cement ratio in the range of 3:1 to 2:1 with a polymer solids: cement ratio (p/c) of 0.10±0.20 by weight of cement and a w/c of about 0.3 compared to around 0.5 for the unmodified mortar (Dennis, 1985).

Whilst there is a wide range of latex systems available, they all tend to influence behaviour in the same manner in that: (a) when added to the fresh mix, the polymer particles improve workability to such an extent that, for a given workability, the w/c can be significantly reduced, which in turn reduces porosity and leads to improvements in compressive strength and durability, and (b) after curing, the polymer must be in the form of a film which leads to improved flexural strength, toughness and adhesion.

Concrete and aggregate Specification issues in Standard EN 206-1

The prescriptive approach to durability in EN 206-1 maintains the durability grade principle of Deacon and Dewar (1982) that underpins the advice in concrete design standards such as those published in the UK since the 1980s. This approach is based on the principle that a clear relationship exists between durability expectation and minimum concrete grade, minimum binder content and maximum water/binder ratio. Exposure classifications based on a range of deterioration mechanisms are used and these are further sub-divided to take account of varying degrees of severity in each major class. The subclasses relevant to freeze/thaw attack are presented in Table 8.3. The recommended limiting values of concrete composition and properties are presented in Table  F.1 of the Standard, which is generally adapted in locally determined national annexes.

The recommended limiting values are based on the following assumptions:
· an intended working life of 50 years
· concrete made with cement type CEM I
· maximum nominal upper size of aggregate in the range of 20 to 32 mm
· cover to reinforcement in compliance with the minimum requirements of European design standard EN 1992-1-1:2004 (Eurocode 2).

Research on deterioration rates is being conducted in many institutes to better inform requirements for service lives in excess of 50 years. However, in the case of freeze-thaw resistance, Hobbs et al. (1998) argue that the limiting values for a 100-year life should be the same as for 50 years. This is due to the event- dependence of the process and the fact that concrete capable of resisting freeze- thaw events on an on-going basis should be durable, irrespective of age.

Section 4 of Eurocode 2 is concerned with durability and cover to reinforcement. It is harmonised with EN206-1 in respect of exposure classes. Although a clear trade-off table between cover and strength, a feature of earlier codes, does not appear the concept survives in the form of different values for different `structural classes’. The recommended structural class (design working life of 50 years) is `Structural Class S4′ but this aspect is not very significant in the case of freeze-thaw, since it is the quality of the pore structure of the concrete that determines durability in cold climates.

The set of four exposure subclasses differentiated in respect of freeze-thaw attack, designated XF1, XF2, XF3, and XF4, is based on a matrix of two degrees of saturation (`moderate’ and `high’) and the absence or presence of deicing agent or seawater. Harrison (2000) explained that CEN intended the terms `moderate saturation’ and `high saturation’ to imply a moderate and high risk of damage, respectively. However, Hobbs et al. (1998) noted that the differentiation is not entirely clear and could be on the basis of a lower number of freeze-thaw cycles per annum or a lower likelihood of freeze-thaw events whilst saturated, when distinguishing the exposure classes XF1 and XF2 from XF3 and XF4.

Control of the pore distribution and permeability of the concrete is approached in one of two ways. One approach involves keeping the free water content so low that the amount of expansion will not be deleterious. This is achieved by minimising the permeability and porosity of the concrete. The second approach involves increasing the porosity of the concrete in a controlled manner through air entrainment so that the pores can act as safety valves, providing space for the freezing water to expand without stressing the concrete. In either case the amount of free water in the concrete is minimised by specifi- cation of moderately low water/cement ratio concrete. This prevents inclusion of excessive amounts of mix water and limits ingress from external sources during service.

The exposure classes in EN 206-1 take account of the degree of saturation and the presence, if any, of external sources of salt from deicing agents and seawater. As described in Section 8.4 the presence of salt increases the risk of damage due to the detrimental influence of solute concentration gradient.

The informative annex in EN 206-1 presents indicative limits for maximum water/cement ratio, minimum cement content, minimum strength class, minimum air content (except for class XF1) and a requirement for freeze-thaw resisting aggregates for each exposure condition, as presented in Table 8.4. It cannot be over-emphasised that these are informative values only, based on the mean of values representative of European practice; specifiers must consult the appropriate advice on limiting values in national annexes or complementary standards valid in the place of use of the concrete. For example, the informative annex to EN 206-1 shows a requirement for freeze-thaw resisting aggregates in accordance with the recommendations of the harmonised aggregate standard. Unanimity on this requirement is not apparent in national annexes and complementary standards in respect of exposure class XF1. Exposure class XF1 covers the case of concrete exposed to significant attack by freeze-thaw cycles whilst wet and having `moderate water saturation’. Examples include the vertical surfaces of members exposed to rain and freezing.

Exposure class XF2 covers concrete exposed to significant attack by freeze- thaw cycles whilst wet, having `moderate water saturation’ in the presence of deicing agent. Examples therefore include the vertical surfaces of road structures exposed to freezing and airborne deicing agents. The values for class XF2 in the informative table in Annex F of EN 206-1 indicate typical requirements that are the same as in class XF1 but with the inclusion of a minimum air content of four per cent and consequently the minimum strength class is lower. A higher minimum strength class is also implied as an alternative to an air-entrained concrete, through performance testing. Experience in some countries indicates that the use of non-air-entrained mixes of minimum strength class C40/50 can provide adequate resistance.

Exposure class XF3 covers concrete exposed to significant attack by freeze- thaw cycles whilst wet and described as having `high water saturation’. Examples of concrete in this context include horizontal surfaces of members exposed to rain and freezing.

Exposure class XF4 covers the case of concrete exposed to significant attack by freeze-thaw cycles whilst wet and having `high water saturation’ in the presence of deicing agent or seawater. Examples of concrete covered by class XF4 include bridge decks, surfaces exposed to spray containing deicing agents, and surfaces in the splash zone of marine structures. Some initial difficulty may arise in categorising the exposure of an element in a highway structures as either class XF2 or XF4. If the consequences of failure are significant, in terms of access for repair or threat to passing traffic, it may be prudent to opt for XF4, the severest class.

Testing of fresh concrete

The most commonly employed tests on fresh air entrained concrete measure the total air content. The pressure method, described in ASTM C231, is widely used and is based on Boyle’s Law. It is assumed that solid constituents in fresh concrete and water are incompressible so that volume change under pressure is due to the contraction of air voids. Volume change and difference in pressure are directly related to air content in two air meter designs, described as `Type A’ and `Type B’ respectively. Another approach, the volumetric method, such as that described in ASTM C173, uses an apparatus that allows water to replace air voids in agitated concrete while monitoring the displaced air volume. A third technique, the gravimetric method, described in ASTM C138, compares the difference between actual and theoretical unit weights to estimate the volume represented by air.

Specifications typically define a target mean volume of air to be entrained as a percentage of the concrete and these test methods are therefore adequate. However, the requirement in service is for a certain volume of air as a percentage of the cement paste, properly distributed through a myriad of bubbles conforming to limitations on diameter and spacing. Thus the air content test is useful as a quality control check but it does not provide any indication of the distribution of the air content.

The need for a test method better related to the performance requirements of entrained air led to the development of an alternative method that can measure air content, spacing factor and specific surface in a short period of time. The principle of the test is that the rate of rise of an air bubble in water is related to its size. The test method, reported by Price (1996), exploits the ability to monitor the change in buoyancy of a plate supported by a liquid and air bubbles. The test involves injecting a sample of fresh concrete into a viscous liquid at the base of a column of water. Entrained air bubbles are released and rise through the column where they strike a plate. The change in buoyancy of the plate with time is monitored. The buoyancy change can be related to air content, specific surface and spacing factor.

Degradation of concrete in cold weather conditions Introduction

Concrete and cement composites are forgiving materials and the expectation of a long service life at an extreme ambient temperature is not unreasonable. Extreme temperatures in hot and cold climates do not in themselves present a threat because dry concrete has an acceptably low coefficient of thermal expansion and moderate movements can be taken into consideration in design. A durability threat does, however, exist when moist concrete in cold climates is exposed to repeated temperature cycles that cause freezing and thawing of pore water. The number of cycles is more significant than the absolute lowest temperature. Expansion of wet concrete can be considerable and stresses induced within the concrete may be unacceptable.

This chapter explores the fundamental issues involved in producing freeze- thaw resistant concrete. The phenomenon is reviewed, factors of influence are examined and the particular effects of deicing agents are considered. Air entrainment has a role to play and this is considered in detail. Test methods and related specification issues, current and future, are examined. The prospect of moving to performance-based specifications and design for durability is also addressed.

Limiting the alkali content of the concrete

Stanton’s (1940) formative work on ASR indicated that expansive reaction is unlikely to occur when the alkali content of the cement is below 0.60% Na2Oe. This value has become the accepted maximum limit for cement to be used with reactive aggregates in the United States, and appears in ASTM C 150 Standard Specification for Portland cement as an optional limit when concrete contains deleteriously reactive aggregate. However, this criterion takes no account of the cement content of the concrete which, together with the cement alkali content, governs the total alkali content of concrete, and is considered to be a more accurate index of the risk of expansion when a reactive aggregate is used in concrete. Some national specifications take cognizance of this fact by specifying a maximum alkali level in the concrete; this limit is reported (Nixon and Sims, 1992) to range from 2.5 to 4.5 kg/m3 Na2Oe in the UK. In some countries (e.g., Canada), the limit may vary, depending on the reactivity of the aggregate.

There is currently no test method available that is suitable for determining the `safe level’ of alkali for a particular aggregate. Most test methods require an artificially high alkali content to accelerate the rate of reaction and conducting the test with low-alkali cement may fail to produce expansion in the laboratory even though the same combination of materials could result in long-term expansion under field conditions. In addition, small laboratory specimens are more prone to leaching of the alkalis during test and higher levels of alkali are required under laboratory conditions (e.g., concrete prism tests) to compensate for this phenomenon.

Aggregates that normally are not reactive when used in concrete with low- alkali cement may be deleteriously reactive in concrete of higher alkali content. This may occur through alkali concentration caused by drying gradients, alkali release from from aggregates, or the ingress of alkalis from external sources, such as deicing salts or seawater. Stark (1978) reported increases in soluble alkali from 1.1 to 3.6 kg/m3 Na2Oe close to the surface of some highway structures. Migra- tion of alkalis due to moisture, temperature, and electrical gradients has also been demonstrated by laboratory studies.

A number of workers have demonstrated that many aggregates contain alkalis that may be leached out into the concrete pore solution, thereby increasing the risk of alkali-aggregate reaction. ,

Stark and Bhatty (1986) reported that, in extreme circumstances, some aggregates release alkalis equivalent to 10% of the Portland cement content. Supplementary cementing materials (SCM), such as fly ash, silica fume, slag and natural pozzolans may also contain significant quantities of alkali, and this is discussed in the next section.

Alkalis may penetrate concrete from external sources such as brackish water, sulfate-bearing groundwater, seawater, or deicing salts. Nixon et al. (1987) showed that seawater (or NaCl solutions) present in the mixing water elevatesthe hydroxyl-ion concentration and increases the amount of expansion of concrete. Several researchers have also shown that exposure of concrete to saline environments, from which NaCl (and other alkali metal salts) penetrate into the material, can enhance expansion and cracking due to ASR (Chatterji et al., 1987; Oberholster, 1992; Kawamura et al., 1996; Sibbick and Page, 1998). Deicing salts, such as potassium acetate or sodium formate, typically used on airfield pavements as less corrosive alternatives to NaCl, may also be expected to exacerbate ASR, although there is little or no information about this in the literature.

Test methods for identifying aggregate reactivity

The first step in assessing the potential of an aggregate for AAR expansion and cracking is the performance of a petrographic analysis by a trained petrographer, the methodology being as recommended by a RILEM technical committee (Sims and Nixon, 2003). However, a petrographic analysis may not identify certain reactive materials (some may not be readily identified by optical microscopy), and the results of the analysis should not be used summarily to reject or accept an aggregate for use in concrete. Nevertheless, valuable insight into potential AAR reactivity can be gained through a petrographic analysis of a given aggregate, and information can also be obtained through petrography on physical, chemical, and mineralogical properties of aggregates that may affect other concrete properties. Although there are various rapid chemical test methods available for evaluating the potential for either the alkali-silica reactivity or the alkali- carbonate reactivity of an aggregate source, in the authors’ opinion, the most reliable means of assessment is to test the potential for expansion in concrete.

Concrete expansion tests for identifying aggregate reactivity have been standardized in many countries and, although the tests may differ in details, they typically involve the fabrication of concrete prisms using the aggregate under test and specific concrete mixture proportions. In most cases, either the cement content of the mixture or the alkali content of the cement, or both, are increased above normal values to increase the rate of reaction. The prisms are stored in conditions of high humidity (e.g. over water in sealed containers) and, usually, at elevated temperature (e.g. 38ëC/100ëF is widely used) to further accelerate the rate of reaction. The length change of the prisms is monitored during storage and the expansion is used to determine the reactivity of the aggregate. Figure 7.4 shows typical expansion curves for concrete prisms manufactured using a non- reactive, moderately-reactive and highly-reactive aggregate. Although expan- sion limits and test durations vary between different guidelines and specifica- tions, a widely-used limit for identifying reactive aggregates is 0.04% after 12 months storage at 38ëC. In other words, aggregates that produce concrete expansion less than this value are considered innocuous (non-reactive) and those that produce expansion above this value are considered to be potentially reactive. Concrete prism tests are generally considered to be suitable for evaluating both alkali-silica and alkali-carbonate reactivity. Potentially reactive aggregates should either be avoided or only used in concrete with appropriate preventive measures (see Section 7.7).

Various mortar bar tests have been developed to evaluate the potential for alkali-silica reaction (ASR). Such tests usually employ one or more methods for accelerating expansion including: augmentation of cement alkalis, addition of alkali to the mortar mix, immersion of mortar bars in alkaline solution, elevated temperature and even autoclaving. At this time the most commonly used mortar bar test, often referred to as the accelerated (in North America) or ultra accelerated (in Europe) mortar bar test, involves the immersion of mortar bars in a solution of 1M NaOH at 80ëC. Length change is monitored during immersion and the test is capable of identifying most reactive aggregates after just 14 days immersion in the hot alkaline solution. The test conditions are very aggressive and there are many aggregates with satisfactory performance in the field or in concrete prism tests that fail the accelerated mortar bar test. Consequently, this test should not be used to reject aggregates and the potential reactivity of aggregates that fail this test should be confirmed by more reliable concrete prism testing. The (ultra) accelerated mortar bar test is not suitable for evaluating some alkali-carbonate reactive rocks and an additional test method has been proposed recently by a RILEM technical committee for use in such cases (Sommer et al.,2005). Further consideration is beyond the scope of this chapter and readers interested in the ongoing international efforts aimed at developing improved screening procedures for AAR-susceptible aggregates are directed to the work of RILEM Technical Committee 191-ARP. A review of the work of this com- mittee’s activities and those of the earlier RILEM Technical Committee 106- AAR was presented at the 12th International Conference on Alkali-Aggregate Reaction in Concrete (Sims et al., 2004).

Manifestations of aggregate-related damage in field concrete

The extent of damage caused by frost-susceptible aggregates in field concretes is dependent on the exposure conditions of the concrete, the proximity of such aggregates to the exposed surface, and the quality (especially permeability) of the mortar layer above and surrounding the aggregates. Although the distress can be throughout the concrete in extreme cases, many instances of aggregate- related problems are manifested in surface deterioration, such as D-cracking or pop-outs, as described next.

D-cracking, also referred to as durability cracking or D-line cracking, appears as a series of closely spaced, crescent-shaped cracks that occur along joints or pre-existing cracks in concrete flatwork, such as pavements or sidewalks. The cracking and staining often appear in an hourglass shape on the pavement surface at affected joints and cracks (see Fig. 7.1). Cracking tends to initiate near joints or cracks due to the ingress of water, thereby increasing the degree of saturation of the concrete, as well as the amount of freezable water.

D-cracking is caused when water in susceptible aggregates freezes, leading to expansion and cracking of the aggregate and/or surrounding mortar. The rapid expulsion of water from the aggregates may also contribute to dissolution of soluble paste components (Van Dam et al., 2002). D-cracking generally takes 10 to 20 years (or more) to develop, with deterioration often beginning at the bottom of the slab where free moisture is available. The length of time necessary for D-cracking to occur is a function of the aggregate type and pore structure, climatic factors, availability of moisture, and concrete quality, especially its permeability.

The coarse aggregate type clearly plays a role in the development of D- cracking. Most D-cracking-susceptible aggregates are of sedimentary origin and are most commonly composed of limestone, dolomite, or chert (Stark, 1976). Key aggregate properties related to D-cracking susceptibility are mineralogy, pore structure, absorption, and size. In addition, the potential for D-cracking is also exacerbated in the presence of deicing salts, especially so for certain carbonate aggregates (Dubberke and Marks, 1985).

When D-cracking-susceptible aggregates must be used in cold weather applications, the options for ensuring durability are somewhat limited. Crushing certain aggregates below their critical size can help to minimize the risk of D- cracking by reducing the escape distance for freezing water, thereby reducing hydraulic pressure and subsequent damage. However, not all aggregates, when crushed to relatively small sizes, are immune to D-cracking; some carbonate aggregates still show poor durability when their particle size is reduced. Another method reported to improve the frost resistance of concrete containing certain aggregates prone to D-cracking is to increase the entrained air content of the mixture, which helps to alleviate hydraulic pressure developed near aggregate particles (Schlorholtz, 2000). Although decreasing aggregate particle size or increasing the entrained air content of concrete have been identified as potential methods of mitigating D-cracking, the most common practical approach is to identify non-durable aggregates and preclude their use in key applications.

Another common manifestation of frost-susceptible aggregates in field concrete (especially pavements, bridge decks, etc.) is pop-outs. Pop-outs occur at the surface of concrete, where near-surface aggregates either fail or cause the mortar above the aggregates to fail. It should be noted that there are other causes for pop-outs in concrete (e.g., alkali-silica reaction or presence of soft aggregates like shale), which are discussed elsewhere in this chapter.

Pop-outs typically range in diameter from 25mm to 100mm and depth from 13mm to 50 mm, leaving behind a conical depression at the top surface of concrete (Miller and Bellinger, 2003). Aggregates with high porosities tend to be the most prone to pop-outs (due to the large amount of water that can be expelled), and larger aggregate particles are worse than smaller aggregates (due to the increased water stored in larger aggregates and the longer escape path for freezing water) (Pigeon and Pleau, 1995). The tendency for pop-outs is influ- enced not only by the aggregate pore structure, but also by the characteristics of the ITZ and mortar layer above the aggregate. As in the general case for frost resistance of aggregates, pop-outs are exacerbated by the presence of deicing salts, which tend to increase the degree of saturation in this critical near-surface region of concrete. Air-entrained concrete mixtures with lower w/cm ratios and SCMs, when properly designed, placed, and cured, can be helpful in reducing pop-outs, mainly by reducing the ingress of water and deicing salts into the upper portions of slabs. Aggregates that tend to cause pop-outs in concrete often also cause D-cracking, but the opposite is not necessarily typical (Pigeon and Pleau, 1995).

General requirements of aggregates for use in concrete

Although most issues related to frost resistance of concrete are associated with damage emanating from the paste (e.g., due to internal disruption or surface scaling), there are cases where the aggregates, themselves, either exhibit distress or cause distress of the surrounding paste. This section discusses the basic mechanisms responsible for this aggregate-related distress and describes the manifestations of such distress in field concrete, such as pop-outs and durability cracking (D-cracking). Particular emphasis is placed on discussing the aggregate characteristics, such as pore size distribution, total porosity, and size, that have most impact on frost resistance. Some information is also provided on how to identify those aggregates that may lead to poor frost resistance.