While hardened concrete appears solid, it is porous, having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction. A water:cement ratio (w/c) of approximately 0.25 (this means 25 parts water for every 100 parts 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 flowable. 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.
The air bubbles are typically 10 to 500 micrometres in diameter (0.0004 to 0.02 in) and are closely spaced. The air bubble can be compressed a little, and so the bubbles act to reduce or absorb stresses from freezing. Air entraining was introduced in the 1930s and most modern concrete, especially if subjected to freezing temperatures, is air-entrained. The bubbles contribute to workability by acting as a sort of lubricant for all the aggregates and large particles in a concrete mix.
In addition to entrained air, hardened concrete also contains entrapped air. These are larger bubbles, and are typically less evenly distributed than entrained air. Entrapped air is considered to not make a positive contribution to durability and is undesirable though not entirely avoidable.