Tiny bubbles, put there on purpose
Air entrainment is an admixture added at the plant that creates a network of microscopic, evenly spaced air bubbles through the mix, not the larger, uneven air pockets that get worked out of concrete during placement and finishing. Those two things look similar on paper and do opposite jobs. Trapped air is a flaw to be removed. Entrained air is engineered in.
The bubbles stay in the hardened concrete permanently, spread through the paste rather than floating to the surface, because they are sized and stabilized by the admixture to do exactly that.
The job those bubbles do
Concrete is porous, and it holds water in the paste even after it has cured. When that water freezes, it expands, and in ordinary concrete it has nowhere to go, so the pressure pushes outward against the surface. Repeated over enough freeze and thaw cycles, that pressure is what causes scaling and pop outs, the flaking and pitting that show up on old, unprotected concrete after a run of hard winters.
The air bubbles give that expanding water somewhere to go instead. Each one acts as a small relief valve, so the pressure has an escape route and the surface does not take the damage.
- Scaling: flaking off the top surface layer of the slab
- Pop outs: small, cone shaped craters where a piece breaks loose
- Both come from water freezing inside concrete with nowhere to expand
Where it belongs on the order
Air entrainment is specified as a percentage of the mix and it goes on the ticket the same way strength does, ordered as air entrained rather than assumed. On most of our own exterior work in this climate we spec a 4,000 PSI air entrained mix, because a driveway, a patio or a walkway is going to sit outside through every winter it is ever going to see.
Not every slab needs it
A floor inside a heated or enclosed building that never sees a hard freeze is not fighting the same battle, and air entrainment does trade off a small amount of strength for that protection. Interior work is a different conversation with the plant than exterior flatwork is, and it is one we have before the mix is ordered rather than after.
The safer default around here still leans toward using it. A garage floor or a shop slab that is unheated for part of the year, or that sits close to an open bay door, sees more of the outside climate than the walls suggest, and that is worth talking through at the estimate rather than assuming either way.