Durability of concrete and cement composites

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).


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