Concrete spalling is when the surface of concrete flakes, chips, or breaks away, leaving a rough, pitted, or hollowed area behind. It can be a shallow layer peeling off a slab or a large piece falling from a balcony edge or ceiling. Spalling is a symptom. Something is pushing the concrete apart from within or weakening its surface, and the repair depends on what that something is.
What causes concrete spalling?
Rusting reinforcement
The most common cause in buildings and structures is corroding steel inside the concrete. Rust takes up several times the volume of the steel it replaces. As it builds up, it pushes off the concrete covering the bars. This is concrete cancer, and spalling is its later stage. If you can see rusted steel in the broken area, this is almost certainly the cause.
Poor finishing or curing
Overworking the surface when the concrete was placed, adding water to make it easier to finish, or letting it dry too quickly can leave a weak surface layer. That layer can flake or scale away under traffic and weather, usually without any steel being involved. Engineers often call this shallow form scaling or delamination, and keep the word spalling for deeper breakouts.
Fire
Intense heat can cause concrete to spall as moisture inside it turns to steam and the surface layers expand faster than the core. Concrete exposed to a fire should be assessed before it is relied on again.
Impact and movement
Vehicle impact, heavy point loads, and restrained movement at joints can chip and break slab edges, columns, and kerbs. Expansion joints that have failed or filled with debris can cause the concrete either side to break up as it moves.
Water and chemicals
Repeated wetting and drying, salt crystallisation, and exposure to chemicals or sewage gases can break down the surface of concrete over time. Stormwater pits, plant rooms, and industrial floors are common examples.
Why spalling shouldn’t be ignored
- Safety. Falling concrete from balconies, facades, and ceilings can injure people below.
- It speeds up. Each piece that breaks away lets more water in, and where steel is involved, exposes more of it.
- Structural strength. Where steel is corroding, the element can lose capacity over time.
- Cost. Small spalls are a small repair. Widespread spalling can mean scaffolding, engineering, and much larger repairs.
How is spalled concrete repaired?
The loose and damaged concrete is removed back to sound material. If steel is exposed and rusting, it is cleaned, treated, or replaced, as in a concrete cancer repair. The area is then rebuilt with a repair mortar and finished to match. Finally, the cause is dealt with, whether that is waterproofing, a protective coating, or fixing a failed joint. At the Lennon Mills Silos, AWS repaired the spalled concrete silos for 2Construct, then coated them with polyurea in fade-resistant colours to protect the repaired concrete.
Can I fix spalling myself?
Shallow surface scaling on a path or driveway, with no steel involved, can sometimes be resurfaced with the right product. Spalling that shows rusted steel, sits on a balcony, ceiling, or facade, or is above where people walk should be handled by a remedial contractor, and in some cases assessed by an engineer. Patching over rusting steel hides it without stopping it.
About AWS Services
AWS Services is a remedial, civil, protective coatings, and construction contractor founded in 2000. We repair concrete cancer, spalling, and cracking for councils, builders, asset owners, and strata across Melbourne and Perth.