Technical Questions

Resistance to cycles of freezing and thawing

Q. How is concrete made resistant to cycles of freezing and thawing?

A. Air is entrained in concrete to increase its resistance to disintegration when exposed to cycles of freezing and thawing in a critically saturated condition and to minimize the scaling that results from applications of chemicals for ice removal (ACI 212.3R). Entrained air bubbles of diameters larger than 3 micrometer provide resistance to damage caused by freezing and thawing. Bubbles should be distributed in the cement paste at a spacing not greater than 0.2 mm, achieved by the use of air-entraining admixtures or air-entraining hydraulic cement (ACI 201.2R). These closely spaced air bubbles provide relief from the pressure built up by the freezing of water in the capillary cavities in the cement paste and thus minimize damage to the hardened paste.

The number of freezing and thawing cycles that can be resisted by air-entrained concrete is improved several hundred percent compared with concrete not containing entrained air. On freezing, water expands in volume by about 9%. Concrete rarely contains more than 10% freezable water by volume, resulting in approximately a 1% expansion. An air content of 9% by volume of the mortar fraction is usually sufficient to protect the concrete.

References:   SP-1(02); E4-22; ACI 201.2R-16; ACI 212.3R-16

Topics in Concrete: Admixture; Concrete Fundamentals; Durability; Quality Assurance; Testing of Concrete