Air bubbles added to fresh concrete improve workability and frost resistance. Frost-resistant concrete requires an adequate number and volume of closely spaced, microscopic air voids. This is best accomplished with a minimum total air content and minimum impact on strength provided by a fine gradation of air voids.
Many mistakenly think that air-entraining admixtures create air bubbles just by being added to the mix. However, these inexpensive but effective chemicals are detergents with no ability to make air bubbles. Instead, they stabilize bubbles created and trapped in the mixing.
By including air-entraining admixtures in a good concrete mix and following sound batching, mixing, and transporting practices, it's possible to deliver concrete with an acceptable air bubble system to the jobsite. This is verified by sampling the concrete as delivered, before modification of the material by adding water, placing, or handling. After the concrete has been discharged, fine, effective air voids are rarely created. However, construction operations can create coarser, less-effective bubbles and lose small and large bubbles.