Concrete spalling is a form of concrete degradation where chunks or flakes of concrete break away from the surface, typically caused by corrosion of the steel reinforcement embedded within it. When the steel bars rust, they expand to several times their original volume, generating internal pressure that cracks and eventually pushes off the surrounding concrete.
The process usually begins when moisture and carbon dioxide penetrate the concrete cover — the layer of concrete protecting the reinforcement. Over time, a chemical reaction called carbonation lowers the alkalinity of the concrete, removing the passive protective layer around the steel. Once this protection is gone and moisture is present, the steel begins to corrode.
Application
Concrete spalling is one of the most common structural defects in ageing buildings, bridges, parking garages, and balconies. You can recognise it by rust stains on the concrete surface, hairline cracks running along the line of the reinforcement, or areas where concrete has visibly broken away to expose corroded steel bars.
For DIY builders and homeowners, spalling most often appears on older concrete balconies, retaining walls, lintels, and garage structures. It is not merely a cosmetic problem — once the reinforcement is exposed and corroding, the structural capacity of the element is being reduced.
Repair involves removing all loose and damaged concrete back to sound material, cleaning the exposed reinforcement (or replacing bars if they have lost significant cross-section), treating the steel with a corrosion inhibitor or zinc-rich primer, and then rebuilding the concrete cover with a polymer-modified repair mortar. For large-scale damage, a structural engineer should assess the extent before any work begins.
Prevention is always better than repair. Ensuring adequate concrete cover during construction (typically 25 to 50 mm depending on the exposure class), using quality dense concrete, and applying surface sealers or coatings in aggressive environments all help to delay the onset of spalling.
Related terms
- Concrete cover
- Reinforcement
- Carbonation
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