Air entrainment, which may be defined as the intentional generation of small air bubbles in the freshly mixed concrete, is often employed to improve the set concrete’s resistance to frost damage. Specifications often stipulate air entrainment, but since it is impossible to measure entrained air separately from the other air contained in the freshly mixed concrete, a total air content percentage for the fresh concrete is usually specified.
Air entrainment improves the workability of a mix and so decreases the required free water content of a mix. However air entrainment also makes concrete weaker and so the minimum free water/cement ratio required for a given concrete strength is reduced. These two effects counterbalance each other. The net result of air entrainment is generally a small increase in the required cement content of a mix but a small percentage of entrained air may actually decrease the required cement content.