Air entrainment

Air entrainment is the intentional creation of tiny air bubbles in concrete. A concrete maker introduces the bubbles by adding to the mix an air entraining agent, a surfactant (surface-active substance, a type of chemical that includes detergents). The air bubbles are created during mixing of the plastic (easy flowing, not hardened) concrete, and most of them survive to be part of the hardened concrete. The primary purpose of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze-thaw; the secondary purpose is to increase workability of the concrete while in a plastic state in concrete.Though hardened concrete appears solid, it is actually highly porous, having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction. A water to cement ratio (w/c) of approximately 0.20 to 0.25 (this means 20 to 25 lbs. of water for every 100 lbs. of cement) is required for all the cement particles to hydrate. Water beyond that is surplus and is used to make the plastic concrete more workable or easily flowing or less viscous . Most concrete has a w/c of 0.45 to 0.60, which means there is substantial excess water that will not react with cement. Eventually the excess water evaporates, leaving little pores in its place. Environmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates stresses which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually the concrete spalls - chunks break off. The failure of reinforced concrete is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel. Steel expands when it rusts, and these forces create even more cracks, letting in more water.