Why is there air in concrete?

Air is always present in concrete mixes. It is intentionally or unintentionally trapped in fresh concrete as a result of mixing and placing. Because air and water do not mix, air trapped in fresh concrete normally is in the form of gas bubbles surrounded by a thin liquid film and suspended in the mix water. Once concrete hardens, the air bubbles are fixed in place. The hollow space formed by the last position and shape of a bubble is an air void. It is shown that small, well-dispersed air voids in concrete can improve workability, reduce bleeding and segregation, and improve resistance to freezing and thawing.