Spalling
Spalling is the breaking away or flaking of concrete from the surface, typically caused by corroding reinforcement, freeze-thaw cycles, or insufficient cover.
Ask Chalkline about this →Spalling is the breaking away, cracking, or flaking of concrete from its surface. In Australian residential construction the most common cause is corrosion of embedded steel reinforcement: as reo corrodes it expands and forces the surrounding concrete to fracture and separate. Insufficient concrete cover is the primary risk factor; where cover is too thin, moisture and oxygen reach the steel more quickly and corrosion begins sooner. Cover errors usually trace back to displaced or missing bar chairs at the pre-pour stage.
Spalling is also caused by impact damage, fire (explosive spalling from steam pressure in the cement paste), and aggressive chemical exposure. Slab edges are a common location because cover at the edge is often the smallest and most exposed.
Once spalling exposes reinforcement, the rate of corrosion accelerates. The standard repair sequence is: cut back to sound concrete, clean the reo (and treat with a passivating primer where section loss has occurred), apply a bonding agent, and reinstate with a cementitious repair mortar. Simple patching without reo treatment will spall again.
Also known as: concrete spalling, spall, concrete breakout (in fastening contexts, a different failure mode).
Category: Defects.
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Last updated: 2026-05-08. Verified: 2026-05-08. Quarterly review for currency.