Why air- Entraining admixture is used in concrete?

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.

i agree with Eng. MUHAMMAD SARFRAZ SHAMIM

Actually Air Entraining Admixture is a special type of admixture  that use to produce microscopic millions of bubbles in fresh and hardened concrete and this concrete is called air entrained concrete because such concrete containing more than 2% air and this this different concrete as compare to normal concrete.

These added air bubbles increases  the concrete resistance against the freezing and thawing affect. such type of concrete containing air can reduce the possibility of bleeding and segregation and increase workability. 

High cement contain concrete usually require more  air contain and resulting 5% strength decrease when we add 1% air in concrete this is a drawback of this admixture  so more consideration is required when adding air in concrete but less cement contain concrete may increase strength slightly.