Problems
Spalling concrete
The surface is breaking away and you can see the stones underneath.
Spalling is concrete breaking away from the surface, usually in pieces rather than thin flakes, often leaving the aggregate exposed. It looks alarming and frequently is not — but it is a signal worth reading, because it can also be the visible end of something worse.
The useful question is how deep it goes and what is driving it.
What usually causes it
Water absorbed into the concrete, freezing, and expanding is the classic driver in cold climates. De-icing salts accelerate it by increasing the number of freeze–thaw cycles the surface goes through and by drawing more moisture into the concrete.
Other contributors include a mix with too much water, inadequate curing when the concrete was placed, finishing that sealed water into the surface, and insufficient air entrainment in concrete exposed to freezing. Where reinforcement sits too close to the surface and corrodes, it expands and pushes the concrete off from within — that variety is more serious and looks different, often with rust staining.
How deep is it?
Surface spalling that stops within the top layer, on concrete that is otherwise sound and sitting where it should, is a candidate for resurfacing. The slab is doing its structural job; the wearing surface has failed.
Spalling that reveals reinforcement, that goes deep into the slab, or that appears across most of the area alongside soft or crumbling material, is a different situation. At that point the concrete itself is the problem, and an overlay bonded to failing material fails with it.
What not to do
Sealing over actively spalling concrete does not stop the spalling. The surface underneath continues to come apart and takes the sealer with it. Sealing is a preventive measure for sound concrete, not a treatment for damaged concrete.
The same applies to coatings and overlays applied without proper preparation and repair. Preparation is the job.
Telling spalling from delamination
These look similar and mean different things. Spalling is material breaking away from a surface that is otherwise attached. Delamination is a layer that has separated from the concrete below and is still sitting there, held in place by nothing much.
You can often hear the difference. Tap across the area with the handle of a screwdriver or a length of metal. Sound concrete returns a hard, dead note. A delaminated area sounds hollow and slightly drummy, and the hollow region usually extends well beyond whatever has already come off.
It matters because it changes what any surface treatment is bonding to. Applying an overlay over a delaminated layer bonds it to something already detached, and the whole assembly releases together. Mapping the hollow areas by sounding is a normal part of a proper assessment, and worth asking whether a contractor has done.
When resurfacing is plausible
- The damage is confined to the surface layer.
- No reinforcement is exposed.
- The slab has not settled and is not cracking widely.
- The remaining concrete is hard and sound when probed.
When it points toward replacement
- Reinforcement is exposed and corroding.
- The concrete beneath the spalled surface is soft or crumbly.
- Spalling covers most of the area and is still progressing.
- The slab has also settled or is cracking widely.
What to ask a contractor
- How deep does this go — did you probe it?
- Is the concrete underneath sound enough to bond an overlay to?
- What is driving this, and does anything need to change about drainage or salt use?
- If we resurface, what is the realistic life given the exposure here?
Common questions
Will spalling keep spreading?
Often, yes, where the driver is still present — continued freeze–thaw cycling on a surface that absorbs water, or continued salt exposure. Addressing water absorption on sound adjacent concrete can slow it. Once a surface has started coming apart, though, sealing that area does not reverse it.
Is rock salt the culprit?
De-icing chemicals are a well-established contributor to surface scaling and spalling, particularly on concrete that was not air-entrained or was poorly cured. That does not mean salt caused any specific case. Reducing salt use on vulnerable concrete is sensible; it is not a repair.
Work it out, or judge the quote
Related guides
Flaking and scaling
Thin layers lifting off the surface — a finishing, curing and freeze–thaw story.
Read more →Resurfacing and overlays
A new wearing surface over sound concrete — and the movement problems it cannot fix.
Read more →Sealing and coatings
Sealers, epoxy and garage-floor systems: what they protect against and what they do not.
Read more →Resurfacing vs replacement
When a new surface is genuine value and when it is a cosmetic delay.
Read more →Garage floors
Interior slab, exterior abuse — coatings, cracks and the threshold step.
Read more →