Flaking and scaling concrete

Why the top layer comes off

Concrete gains most of its surface durability from a dense, well-cured top layer. Several things weaken it: working excess water into the surface during finishing, finishing while bleed water is still present, insufficient curing so the surface dries too fast, and — in freezing climates — inadequate air entrainment in concrete that will be saturated and frozen.

Once that layer is weak, freeze–thaw cycling and de-icing salts do the rest. The damage often appears in the first few winters, which is a clue that its origins are in the pour rather than in the years since.

What can actually be done

Scaling that has already happened cannot be reversed. The realistic options are to leave it, to resurface where the concrete underneath is sound, or to replace where it is not.

Sealing sound concrete that has not yet started scaling is genuinely worthwhile in freezing climates, because it reduces how much water the surface absorbs. Sealing concrete that is already scaling does not stop it.

Reducing further damage

Minimising de-icing salt on vulnerable concrete helps. Sand provides traction without the chemical effect. Keeping water from pooling on the surface helps. Where a new slab is going in, the pour and cure decisions are the ones that matter most, which is a reason to ask about curing when getting quotes for new flatwork.

The first winter matters more than the ones after it

Concrete continues to gain strength for a long time after it is placed, and a surface exposed to freezing and to de-icing chemicals in its first season is being tested before it is ready. A great deal of the scaling that shows up on flatwork in its second or third year traces back to what happened in its first.

Where new concrete has gone in late in the season, it is worth asking the contractor directly what they recommend for that first winter, and whether they would rather you kept salt off it entirely. Sand gives traction without the chemical effect and is a reasonable substitute for one season.

This is also a fair thing to raise before agreeing to a late-season pour at all. Some contractors will tell you honestly that waiting until spring is better for the concrete, and that answer is worth more than the earlier start date.

What to ask a contractor

• How deep does this go — is the concrete underneath still sound?
• Is this a resurfacing candidate, and what preparation would it need?
• Would sealing the areas that have not started help, and with what product?
• For new concrete: how will you cure it, and for how long?

Common questions