Rust stains and falling chunks under a balcony, sunshade or bathroom are the classic signs. The repair is well established — and plastering over it is the one thing that does not work.
Why It Happens
| Cause | Effect |
|---|---|
| Thin cover | Moisture reaches steel quickly |
| Porous concrete | Water and carbon dioxide penetrate |
| Leaks from terrace or bathroom above | Constant moisture at the steel |
| Poor curing | Weak, permeable surface |
| Cracks | Direct paths to the steel |
Repair Steps
- Assess the extent; call an engineer if many bars are badly corroded or the member is structural and heavily affected.
- Break out loose and contaminated concrete, going behind the bars so they can be cleaned all round.
- Clean the steel to bright metal by wire brushing or blasting.
- Check section; add new bars where steel has lost significant thickness.
- Prime the steel with a rust-protective coating.
- Bonding coat on the old concrete.
- Rebuild with polymer-modified repair mortar in layers, or micro-concrete in formwork for larger areas.
- Cure properly.
- Protect with a coating, and fix the source of water.
The Short Version
Spalling is a rust problem. Break out, clean, prime, bond, rebuild, cure and protect — and stop the water that started it.
