It is stated on here that:
Air-entraining admixtures cause small stable bubbles of air to form uniformly through a concrete mix. The bubbles are mostly below 1 mm diameter with a high proportion below 0.3 mm. The benefits of entraining air in the concrete include increased resistance to freeze-thaw degradation, increased cohesion (resulting in less bleed and segregation) and improved compaction in low-workability mixes.
This does not make sense to me. By introducing more voids into the concrete there are more paths of ingress for water which can then expand and crack the concrete.
Why do air entrainment admixtures improve the freeze-thaw resistance of concrete?