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Air-entrained concrete was developed in the mid-1930s. Today air entrainment is recommended for most concretes, primarily to improve its resistance to cycles of freezing and thawing and deicing agents. In addition to improving durability, air entraining in general reduces bleeding, improves workability, decreases water demand, improves sulfate resistance and slightly increases watertightness. Testing air entrainment straight out of the back of the truck should yield a measurement of about 5%. This test is important to reveal the adequacy of air-entrainment and should be performed on a regular basis. This test reveals the amount of air in the concrete, but it is also important to know the size of air voids and distribution throughout the concrete. Laboratory testing is the only viable way to test these factors, but once the initial expense is covered, the benefits are many. Once you know you can produce concrete with a good air-void system, you should watch the following factors to alert you to changes in the air entrainment: increases in cement fineness and content; aggregate gradation changes; increased amounts of water; concrete temperature; admixtures; mixing action; and placing methods. All of these can have effects on size and spacing of voids.