Air in concrete??

 I am not talking about voids, or honeycomb.  I am talking about air entrainment.  Air entrainment is a way to purposely create tiny little air bubbles in the concrete.  Air entrainment is what is called an admixture, or chemical added to concrete.  It is added during the mixing process and stays in the concrete during and after the curing process.

Why do you want air in your concrete?  Durability.  It is used in most concrete, especially concrete that is exposed to cold temperatures to help keep it from cracking.

When concrete cures, the water in the mix comes out of the concrete through tiny little pores which are too small to see with the naked eye.  When it rains, water can enter the concrete and freeze.  Water expands when it freezes and acts as a hydraulic jack.  This can cause the concrete to crack.  More water enters the cracks and damages the steel reinforcement and continues to freeze and make the cracks bigger.  Bigger cracks eventually spall, or chunk off.

The air entrainment serves as tiny little cushions to help keep the concrete from cracking.  They are like shock absorbers to help reduce the stress caused by the freezing and thawing process.

If you live in a place that freezes in the winter, you better have air entrainment.  It is cheap insurance, and you can’t add it after the concrete has cured.