Air-entrained concrete is more resistant to disintegration and scaling caused by freezing and thawing cycles and by the application of deicing salts. After an air-entraining agent is added to concrete at the batch plant, billions of extremely small bubbles are produced during mixing; it's this air void system that protects the concrete. Experience has shown that air-entrained concrete is more durable that non-air-entrained concrete and that durability increases, up to a point, as the air content increases. Too much air, though, will significantly reduce strength, so specifications generally require the air content to be within a fairly narrow range--around 4 to 7 percent of the total concrete volume. Measurements of the size and number of bubbles in hardened concrete can be used to calculate a spacing factor, which is a measure of the adequacy of the bubble system. The smaller the spacing factor, the more durable the concrete will be.
Air-entraining admixtures should conform to requirements given in Standard Specifications for Air-entraining Admixtures for Concrete (ASTM C 260) published by the American Society for Testing and Materials. Probably the most widely used air-entraining material is neutralized vinsol resin, a product obtained from pine wood stumps.
Water-soluble alkalies in portland cement or fly ash can have a harmful effect on the performance of some air-entraining agents. This alkali-induced breakdown of the air void system makes concrete more vulnerable to freezing and thawing damage.