Durability of concrete and cement composites

Factors affecting frost resistance of aggregates

Air-entrainment is an accepted method of ensuring durability of concrete in a cold environment, with regard to both internal distress and external damage from salt scaling (see also Chapter 9). However, even when concrete is properly air-entrained, certain types of coarse aggregates can cause distress. The mechanisms responsible for this distress are discussed next.

Frost damage in hydrated cement paste has been attributed to several potential mechanisms, including hydraulic pressure, osmotic pressure, ice lens formation, and others. When the source of the distress is from within the aggregates, the most likely mechanism is related to hydraulic pressure, where water undergoes approximately a 9% increase in volume as it freezes to form ice (Powers, 1955). Hydraulic pressure is more likely to be the main culprit within aggregates because of their coarser pore structure, compared to that of hydrated cement paste (Pigeon and Pleau, 1995).

The tendency for an aggregate to generate internal stresses upon freezing is primarily a function of its internal pore structure. The ability for a given aggre- gate to withstand the resultant stress is a function of its strength (and stiffness), permeability (to water as it is forced out under pressure), and size (due to effects on distance for water to reach the escape boundary). In addition, the structure of the surrounding hydrated cement paste (or more accurately, mortar) has an important impact as it will influence the flow of water as it is being expelled by the aggregate upon freezing. Both the aggregate and hydrated cement paste phases are influenced by the degree of saturation (Fagerlund, 1976). The following is a discussion on what properties of aggregates most influence frost resistance; this is followed by discussion on the role of the surrounding mortar.

Perhaps the single most important feature of an aggregate that affects frost resistance is the pore size distribution, a fact highlighted by several researchers (Kaneuji et al., 1980; Hudec, 1989). Aggregates with a significant number of pores smaller than 5 um typically do not remain completely filled with water (at modest relative humidities) and so do not result in frost damage, while the water in pores finer than 0.1 um tends not to freeze readily because of freezing point depression (ACI, 1996).

In addition to the importance of pore size distribution on frost resistance of aggregates, the total porosity of aggregates plays a role. In a comprehensive study involving a range of aggregates with different pore size distributions and porosities, Kaneuji et al. (1980) showed that, for aggregates with similar pore size ranges, the aggregate with higher total porosity was the less frost resistant. In the same study, the impact of total porosity was further recognized by the fact that, regardless of pore size distribution, if the total porosity of a given aggregate was low enough (e.g., aggregates with absorption capacities less than 1.5%), durability was not a problem.

Thus, for aggregates, both the pore size distribution and total porosity affect frost resistance. However, for convenience, it is possible to group aggregates into three categories, based solely on total porosity (Pigeon and Pleau, 1995) ± low porosity, intermediate porosity, and high porosity. Aggregates with low porosity are generally durable because the amount of freezable water is so low. Aggregates with intermediate porosity are the least durable because, upon full saturation, they contain appreciable freezable water, and the pore structure is restrictive enough to impede the expulsion of water upon freezing. Aggregates with high porosity are generally durable because they tend to have a coarse pore system, which is easily drained as the relative humidity drops below 100% and, when water does freeze within the aggregate, the porous structure (and inherent high permeability) allows for the freezing water to be expelled without causing significant hydraulic pressure (Pigeon and Pleau, 1995).

Because hydraulic pressure is the prevalent mechanism governing aggregate- related distress, it is logical that the size of aggregates affects the development (and release) of pressure within aggregates as water is expelled upon freezing. According to Powers (1955), the extent of the hydraulic pressure is a function of several parameters, including permeability, tensile strength, freezing rate, and distance to escape boundary. When the source of distress is from within an aggregate, the distance to an escape boundary is directly related to the aggregate size. For a given aggregate, there exists a critical size above which frost damage may occur because the distance to an escape boundary is too large for flow of water to relieve the internal stress generated around the freezing sites (Verbeck and Landgren, 1960). For fine-grained aggregates with low permeability, such as chert, the critical particle size may be within the range of normal aggregate sizes (Kosmatka et al., 2002). In fact, many non-durable coarse aggregates can be rendered durable by crushing them, thereby reducing the aggregate dimension below the critical size. This is discussed in more detail later in this chapter with regard to D-cracking. The critical particle size for coarse-grained aggregates or those with pore systems interrupted by numerous macropores (voids too large to hold moisture by capillary action) may be sufficiently high to be of no consequence for typical coarse aggregates used in construction (Kosmatka et al., 2002).

In the previous paragraph, the concept of critical size of aggregate was introduced, with the explanation that the aggregate size relates to the distance to an escape boundary, as per the hydraulic pressure theory. This was somewhat of a simplification in that the exterior surface of an aggregate does not actually constitute an escape boundary; rather, it represents an interface between the coarse aggregate and the surrounding mortar. Thus, the properties of the surrounding mortar will also have a significant impact on the ability of a given aggregate to expel water without damage to the aggregate or surrounding area. The structure of the interfacial transition zone (ITZ) and the hydrated cement paste will both influence this behavior. The ITZ is typically more porous than the surrounding bulk cement paste and tends to have more calcium hydroxide (with a preferred crystal orientation) (Mehta and Monteiro, 1993). In particular, the permeability, availability of entrained air bubbles, and presence of cracking within the ITZ and/or hydrated cement paste will determine whether some or all of the expelled water from aggregates can be accommodated without generating excessive hydraulic pressure. The quality of the hydrated cement paste, especially as influenced by w/cm and presence of supplementary cementing materials (SCMs), will impact on the frost resistance of aggregates significantly. Lower w/cm concrete containing SCMs tends to reduce the ingress of external water, thereby reducing the degree of saturation of aggregates.

Although it is not possible to accurately predict which aggregates will exhibit or cause damage due to freezing and thawing based solely on mineralogy, there are some relevant trends in behavior that can be identified. Most non-durable aggregates are sedimentary, can be calcareous or siliceous, and can be gravel or crushed rock (Neville, 1996). Some examples of susceptible aggregates include sandstones, shales, limestones, and especially cherts (Cordon, 1966). Of the above susceptible aggregates, chert is considered the most damaging and prone to D-cracking and pop-outs (Larson and Cady, 1969). Typically, igneous or metamorphic rocks are not frost susceptible due to their inherent low porosity; examples of durable aggregates include granites and basalts (Pigeon and Pleau, 1995).

Although it is not possible to accurately predict which aggregates will exhibit or cause damage due to freezing and thawing based solely on mineralogy, there are some relevant trends in behavior that can be identified. Most non-durable aggregates are sedimentary, can be calcareous or siliceous, and can be gravel or crushed rock (Neville, 1996). Some examples of susceptible aggregates include sandstones, shales, limestones, and especially cherts (Cordon, 1966). Of the above susceptible aggregates, chert is considered the most damaging and prone to D-cracking and pop-outs (Larson and Cady, 1969). Typically, igneous or metamorphic rocks are not frost susceptible due to their inherent low porosity; examples of durable aggregates include granites and basalts (Pigeon and Pleau, 1995).

Just as is the case with frost action on hydrated cement paste, the importance of degree of saturation should not be overlooked when considering aggregate- related frost damage. For an aggregate to be damaged internally or to exude freezing water to damage the surrounding paste, sufficient water must be present within the aggregate. As per the hydraulic pressure theory, aggregates must have at least 91.7% internal moisture content to generate appreciable pressure (Fagerlund, 1976). The ability of an aggregate to reach this degree of saturation is, of course, a function of the exposure condition that the subject concrete encounters, the permeability of the concrete, and the pore structure of the aggregate. As discussed earlier, the porosity and pore size distribution of a given aggregate will influence the rate of water ingress into the aggregate, and the size of the pores will influence the freezing point of the water within the aggregate. It is important to note that the presence of deicing salts will tend to increase the degree of saturation within the concrete (as well as within the aggregates near the exposed surface), and this can exacerbate damage caused by frost- susceptible aggregates. The penetration of deicing salts (or other sources of dissolved salts) can also lead to osmotic pressures within the concrete, and the extent to which this plays a role within aggregates depends on the porosity or permeability of aggregate particles and their proximity to the salt front.

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