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Deterioration of Concrete - Freezing and thawing

(1) Mechanism. As the temperature of a critically saturated concrete is lowered during cold weather, the freezable water held in the capillary pores of the cement paste and aggregates expands upon freezing. If subsequent thawing is followed by refreezing, the concrete is further expanded, so that repeated cycles of freezing and thawing have a cumulative effect. By their very nature, concrete hydraulic structures are particularly vulnerable to freezing and thawing simply because there is ample opportunity for portions of these structures to become critically saturated. Concrete is especially vulnerable in areas of fluctuating water levels or under spraying conditions. Exposure in such areas as the tops of walls, piers, parapets, and slabs enhances the vulnerability of concrete to the harmful effects of repeated cycles of freezing and thawing. The use of deicing chemicals on concrete sur-faces may also accelerate damage caused by freezing and thawing and may lead to pitting and scaling. It involves the development of osmotic and hydraulic pressures during freezing, principally in the paste, similar to ordinary frost action. (2) Symptoms. Visual examination of concrete damaged by freezing and thawing may reveal symptoms rang-ing from surface scaling to extensive disintegration. Laboratory examination of cores taken from structures that show surficial effects of freezing and thawing will often show a series of cracks parallel to the surface of the structure. (3) Prevention. (a) Designing the structure to minimize the exposure to moisture. For example, providing positive drainage rather than flat surfaces whenever possible. (b) Using a concrete with a low w/c. (c) Using adequate entrained air to provide a satisfactory air-void system in the concrete, i.e., a bubble spacing factor of 0.008 in. or less, which will provide protection for the anticipated service conditions and aggregate size. (d) Selecting suitable materials, particularly aggregates that perform well in properly proportioned concrete. (e) Providing adequate curing to ensure that the compressive strength of the concrete is at least 3,500 psi before the concrete is allowed to freeze in a saturated state.

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