Spalling Concrete

Spalling is the term for concrete surface failure that shows up as flaking, chipping, pitting, or scaling. The concrete loses its top layer in patches, sometimes exposing aggregate or rebar underneath. Spalling is almost always caused by one of four mechanisms:

1. Freeze-thaw cycling. Water absorbed into the concrete pore structure freezes, expands, and fractures the surrounding matrix. Common in colder climates; less of an issue in coastal San Diego but still a factor in inland and elevation-prone areas.
2. Chloride-induced rebar corrosion. Salt (from de-icing, ocean spray, or chemical exposure) penetrates the concrete, reaches the reinforcing steel, and corrodes it. The corroded steel expands and pops the concrete cover off. Common on parking decks, ocean-adjacent structures, and any concrete with shallow rebar.
3. ASR — alkali-silica reaction. A chemical reaction between cement alkalis and certain reactive aggregates that causes the concrete to expand and crack from within. Slow to develop; shows up years after pour.
4. Improper finishing. Concrete that was over-troweled, over-worked at the surface during finishing, or finished too wet, develops a weak surface skin that flakes off under normal wear.

The reason this comes up in commercial cleaning conversations is that pressure washing can expose existing spalling but doesn’t usually cause it. A 4,000 PSI pressure washer applied at a reasonable working pressure (1,500-2,000 PSI effective) and standoff distance won’t damage sound concrete. What it will do is dislodge already-spalled material that the surface debris was hiding — flakes that were going to come off anyway, just not today.

When we encounter visible spalling during a parking structure or sidewalk cleaning job, the protocol is: stop, document with photos, notify the property manager, and continue the cleaning only on the unaffected areas. Spalling is a structural conversation for a concrete engineer or a structural consultant, not a cleaning crew. We’re often the people who first put eyes on a parking deck condition that needs structural attention; the documented condition report is part of that handoff.

For a property manager: if your parking deck or sidewalk shows visible flaking or rust staining before the wash starts, expect some material to come off during cleaning that was going to come off anyway.