"Concrete cancer" is the everyday name for spalling caused by corroding steel reinforcement. As the steel rusts it expands, cracking and pushing off the concrete around it. It's common in older strata buildings and in coastal areas, and it gets worse and more expensive the longer it's left.
Warning signs
- Rust-coloured stains running from cracks or on the underside of balconies
- Cracks that follow straight lines, often along the line of the reinforcement
- Concrete that sounds hollow when tapped, or bulges
- Pieces of concrete falling, exposing rusted steel
Falling concrete is a safety risk. If pieces are coming off, keep people clear of the area and get an engineer to inspect urgently.
Why it happens
Steel in concrete is normally protected. Over time water, salt and carbon dioxide can get through, especially where the concrete cover is thin, cracked or poor quality. Once the protection breaks down, the steel starts to corrode.
How it's fixed properly
- Investigation: find out how widespread the damage is. It's often more than you can see.
- Scope of works: specify the repair method, materials and extent so builders price the same job.
- Repair: remove damaged concrete, treat or replace the steel, and reinstate with suitable repair materials.
- Protect: deal with the cause, such as waterproofing or coatings, so it doesn't come back.
- Inspect and certify: check the work at key stages before it's covered up.
Patching over the surface without treating the steel usually fails within a few years. Proper engineering advice up front saves money over the life of the building.