Air is always present in concrete mixes. It is intentionally or unintentionally trapped in fresh concrete as a result of mixing and placing. About the only way to avoid trapping some air would be to mix, transport, and place concrete in a vacuum.
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 hollow space formed by the last position and shape of a bubble is an "air void."
The most important effect of air voids is to reduce the pressure experienced by hardened cement paste when absorbed water turns to ice. An air void system having a total value of about 18% to 20% of the cement paste volume can generally accommodate expanding ice and water.