Definition EN 934 Part 2:

Admixture which allows a controlled quantity of small, uniformly distributed air bubbles to be incorporated during mixing which remain after hardening.

Air entrainers are used to develop a large number of small air bubbles in the concrete (diameter less than 300 micron) which are homogeneous and stable after the mixing process. The incorporated air bubbles have several effects on the fresh and the hardened concrete.

In the fresh state, the concrete mix is stabilised and by the “ball bearing effect” of the bubbles the workability may be increased, especially in low cement content or dry mixes. The air can also significantly reduce any tendency for the mix to bleed.

In the hardened state the remaining air bubbles interrupt the porous system of the concrete, reducing the capillary suction (water adsorption).

The bubbles act as an expansion area for freezing water in the pore system leading to increased freeze-thaw resistance.