What causes it
Steel inside concrete is normally protected by the alkaline concrete around it. Over time, carbon dioxide and moisture reduce that protection (carbonation), and in coastal or polluted settings chlorides speed corrosion. Thin cover, poor compaction and leaking roofs make it worse. Rust takes more space than steel, which pushes the concrete cover off.
Warning signs
- Brown rust stains on beams, columns or slab soffits.
- Cracks running along the line of the reinforcement.
- Hollow sound when tapped.
- Pieces of concrete falling, with exposed rusted bars.
- Repeated leakage in the same area.
Repair process
- Survey and mark the damaged areas. Where significant steel is lost or cracks are wide, involve a structural engineer.
- Remove loose and cracked concrete until sound material is reached.
- Expose the corroded bars and clean off rust.
- Apply an anti-corrosion primer to the steel.
- Apply a bonding agent and repair with a polymer-modified repair mortar in layers.
- Finish with a protective or waterproof coating to slow future damage.
Why waterproofing is part of the fix
If water keeps entering, the new repair will deteriorate too. Fix the leak source on the terrace, wall or joint alongside the structural repair.
When strengthening is needed
Where load-carrying capacity has dropped, systems such as jacketing or carbon fiber strengthening may be specified by a structural engineer. Repair mortar alone is not a substitute.
Prevention
- Keep roofs and drains in good condition.
- Seal cracks early.
- Maintain protective coatings on exposed concrete.
- Inspect older buildings periodically.