Air-entrained concrete is ordinary concrete that contains controlled amounts of air in the form of microscopic bubbles. These intentionally entrained air bubbles are extremely small. They range in size from a few thousandths of an inch in diameter to a few hundredths. There are literally billions of these air bubbles in a single cubic foot of air-entrained concrete, and their presence dramatically changes the nature of both the fresh and hardened concrete. The primary benefit of entrained air in hardened concrete however is the resistance it offers to freeze-thaw damage and scaling caused by de-icing salts or chemicals. Most concrete contains some moisture which expands during freezing temperatures. Without room for this expansion, large forces develop that can rupture the surface causing what is commonly called surface scaling. The small, entrained air bubbles serve as reservoirs or expansion chambers to relieve these pressures. Research has shown that the spacing and size of air bubbles are important in assuring their proper action. To be effective, bubbles must be spaced not more then .01 inches apart throughout the cement paste. Using today's methods of entraining air, it is not difficult to obtain this bubble spacing. Some common questions about air-entrained concrete are: does air entrainment reduce strength; does it affect the proportions of a concrete mixture; and can it be used in mild climates. The strength of air-entrained concrete depends on the water/cement ratio as it does in non-air-entrained concrete. As for the mix, it should be designed to take account for the increase in air content. The water content for an air-entrained mix will be 3 to 5 gallons per cubic yard less than for a non-air-entrained mix having the same slump. The sand content will also be less by about 90 to 125 pounds per cubic yard. As for climate, air-entrained concrete is recommended and used considerably in southern climates.
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