Air Content of concrete is the air present inside concrete in its voids. The air content might have come naturally, as the concrete might not have been compacted to full extent or this air content might have been injected intentionally.
Air entrainment is a necessary component of concrete mixtures exposed to freezing and thawing environments.Amount of Air content in concrete depends on:
Maximum coarse aggregate size and exposure environment.
In fresh concrete, the air bubbles reduce the water demand of the concrete and make the mix stickier, which helps to reduce segregation and reduces bleeding when air content is around 3%. If the air content is higher, the increased stickiness makes the concrete more difficult to finish, and the bubbles can lead to blisters and delaminations on steel-troweled surfaces. As the concrete hardens, the cement paste sets around these bubbles, leaving bubble-shaped voids in the hardened concrete.