In short
When reinforcement corrodes, the rust takes up more space than the steel, cracks the concrete and pushes it off: spalling. Common causes are thin cover, carbonation, chlorides and long-term moisture. A lasting repair removes damaged concrete, cleans and protects or replaces the steel, rebuilds the section with a compatible repair mortar, and deals with the moisture that started it.
Key points
- Rust expands, so corroding steel cracks the concrete around it from the inside.
- Thin cover, carbonation, chlorides and persistent damp are the usual causes.
- Patching over rust fails: the damaged concrete must be removed back to sound material.
- Steel is cleaned, assessed and protected, or supplemented where it has lost section.
- The source of moisture must be stopped, or the corrosion returns.
Spalling is concrete breaking away from a beam, column, slab or balcony, often with rusty steel visible behind it. It looks like a surface problem. It is usually a reinforcement problem.
Why it happens
Concrete normally protects steel: its alkaline environment keeps the bars from rusting. That protection can be lost:
- Thin cover: too little concrete between the steel and the surface.
- Carbonation: carbon dioxide from the air slowly reduces the alkalinity of the concrete from the surface inwards.
- Chlorides: from sea air, groundwater or contaminated materials, they attack the steel directly.
- Moisture: corrosion needs water; leaks, condensation and damp keep it going.
When steel rusts, the rust occupies more space than the steel. It pushes the surrounding concrete apart, cracks it, and eventually breaks it off.
Assessing the damage
An engineer maps the affected areas, checks cover and carbonation, and looks for hidden damage beyond what has already spalled, often with non-destructive tests. The question is how much steel section has been lost and whether the element still does its job.
A lasting repair
- Make safe: remove loose concrete that could fall.
- Break out damaged concrete back to sound material and behind the corroded bars.
- Clean the steel of rust; assess the remaining section.
- Supplement bars that have lost too much section, as the engineer specifies.
- Protect the steel with a suitable primer or system.
- Rebuild the section with a compatible repair mortar or concrete, and cure it.
- Protect the surface with a coating where appropriate.
Stop the water
None of this lasts if the moisture continues. Leaks are traced and fixed as part of the repair; see finding the source of water ingress. This is the approach of Nation’s maintenance and rehabilitation service: structural rehabilitation, water ingress control, and improved safety and durability.
Questions
Is spalling dangerous?
Falling pieces of concrete can injure, and lost steel section reduces strength. Spalling should be inspected and made safe promptly.
Why does it happen near the sea?
Chlorides from sea air, groundwater or contaminated materials break down the protection concrete normally gives to the steel.