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Glossary: Materials and mix

Air entrainment

Definition

Air entrainment is the practice of mixing microscopic air bubbles into concrete with an admixture, giving water inside the slab somewhere to expand into when it freezes instead of pushing the surface apart.

What the bubbles are actually doing

Water expands when it turns to ice, and concrete that has taken on moisture is full of water sitting in tiny pores. Without anywhere for that expansion to go, ice pushes outward against the surrounding paste, and enough freeze-thaw cycles crack and flake the surface from the inside. The microscopic bubbles an air-entraining admixture distributes through the mix give that expanding ice a place to go that is not the surface of your slab.

This part of Virginia goes through freeze and thaw repeatedly from November on, and that is exactly the condition air entrainment is specified against. It is not an optional finish detail on exterior flatwork here, it is part of what the mix is built to survive.

A driveway, a patio, a sidewalk, a garage slab with the door open to the weather, all of it goes through the same repeated cycle every winter it sits outside. None of it is a special case that needs a conversation before ordering it air-entrained. It is closer to the default assumption for anything exterior in this climate, the same way a 4,000 PSI strength spec is.

It has to survive the trip through a line pump too

Entrained air is a property of the fresh mix, not something added at the end, and pressure moving concrete through a hose is not gentle with fresh mix. That is one more reason the plant needs to know a load is being pumped rather than chuted straight into a form. Designed with the trip through line in mind, the air comes out the other end accounted for. Treated as a surprise, it is a question nobody thought to ask before the truck was loaded.

The short version stays simple. Order it as air-entrained, say the load is going through a line pump, and both facts travel to the plant on the same ticket.

Good questions

Do I need air entrainment on an interior slab?

Usually not to the same degree. A slab that will never see freezing and thawing, a basement floor under a heated house for instance, is not fighting the condition air entrainment is specified against, so it is a different mix decision entirely.

Can you tell if a mix has entrained air just by looking at it?

Not reliably by eye. It is confirmed at the plant when the mix is designed and ordered, which is the reason it has to be said out loud on the ticket rather than assumed from how a load looks in the chute.

What happens to exterior concrete that was not air-entrained?

It is more likely to show surface damage after repeated winters, since the water inside it has nowhere to go when it freezes. That is a separate topic from air entrainment itself, but it is the reason the spec exists on exterior flatwork here in the first place.

Not sure if your mix was ordered air-entrained?

Ask us before the concrete is booked. It has to be on the ticket, not assumed after the fact.

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