Concrete always contains air bubbles. During concrete mixing and placement, these bubbles are first formed, then ruptured, merged, and floated out of the mix. But once the concrete has hardened, the bubbles that remain are fixed in place forming the air-void system in hardened concrete.
Sometimes small air bubbles are intentionally incorporated (entrained) into the mix using admixtures; other times larger bubbles are entrapped during mixing. When the bubbles are smaller than 0.04 inch, the air is called entrained; larger, and it's called entrapped.
Entrapped air voids are usually irregular in shape while smaller entrained air bubbles are spherical. Air-entraining admixtures stabilize bubbles at a smaller size.
Concrete with an air-void system of mostly smaller bubbles loses very little strength and has much better workability. The bubbles act as cushions between aggregate particles, reducing friction and interlock and allowing a lower water content. Bleeding and segregation are reduced.
Air voids improve concrete's ability to resist freeze damage. Hardened cement paste contains a network of flow channels called capillaries. Water absorbed into these capillaries expands when it freezes, stressing the concrete. This pressure relaxes when the ice thaws. Each cycle of freezing and thawing stresses concrete in tension