Spalling is when the surface of concrete flakes, chips, or peels away, exposing the aggregate underneath. It is usually caused by water freezing inside the concrete, de-icing salt, corroding rebar, or finishing mistakes. Unlike efflorescence, spalling is actual surface damage — and it tends to spread if the cause is not addressed.
Most spalling traces back to water getting into porous concrete and then freezing, or to salt accelerating the damage. The fixes are about keeping water out and using the right concrete in the first place.
What causes spalling
The big ones: freeze-thaw — water soaks into the surface, freezes, expands, and pops off flakes; de-icing salt, which drives more water in and worsens the cycle; corroding rebar set too close to the surface, which rusts, expands, and blows off the cover; and finishing errors like overworking the surface or troweling in bleed water, which leaves a weak top layer that flakes.
How to prevent it
Use air-entrained concrete in freeze-thaw climates — the tiny air bubbles give freezing water room to expand. Keep rebar at proper depth (about 3 inches of cover from earth). Do not overwork the surface or trowel in bleed water. Cure the slab properly, and seal it so water and salt cannot soak in. Avoid de-icing salt on the first-winter concrete.
How to repair spalling
For shallow surface spalling, clean out the loose material, then patch with a polymer-modified concrete resurfacer or overlay that bonds to the old slab. Deeper or spreading spalling — especially over rusting rebar — needs the damaged concrete removed back to sound material and a structural repair, which is a job for a pro.
Common questions
What causes concrete to spall?
Water freezing inside the concrete, de-icing salt, rebar rusting too near the surface, or finishing mistakes that leave a weak top layer.
How do you stop concrete from spalling?
Use air-entrained concrete, keep rebar at proper depth, don't overwork or trowel in bleed water, cure it, and seal it. Avoid salt on new concrete.
Can spalled concrete be repaired?
Shallow spalling can be patched with a resurfacer or overlay. Deep spalling over rusting rebar needs the bad concrete removed and a structural repair.