Air entrainment is adding an admixture at the plant that fills concrete with microscopic, evenly spaced air bubbles, which give water in the hardened concrete room to expand when it freezes.
Without those bubbles, water that freezes in the concrete's pores has nowhere to go. Every freeze and thaw pushes on the paste, and a driveway that sees winters and deicing salt starts to scale and flake within a few seasons. Air-entrained concrete holds up.
How much air depends on the size of the stone and how severe the exposure is, and the spec or local code sets the number. Exterior flatwork in freeze-thaw country commonly lands somewhere around 4½ to 7½ percent total air. Air costs some strength, and the plant's mix design makes up for it, usually with a lower water-cement ratio.
Two things to handle on the job:
- Check it. Air is measured on the fresh concrete, usually with a pressure meter, and loads can come in high or low. If the spec calls for air, test the truck.
- Don't hard-trowel it. Steel-troweling air-entrained concrete can seal the surface and lead to blisters and delamination. Exterior flatwork gets a float and a broom finish instead. ACI 301 says not to use entrained air in slabs that need a dense, machine-troweled surface, so settle the finish and the air content together before you order.
Put "air-entrained" on the proposal and the ready-mix order for any exterior slab that will freeze. The concrete flatwork estimating guide covers the rest of what goes on the proposal.