"Concrete cancer" is the common name for spalling caused by corrosion of the steel reinforcement inside concrete. As the steel rusts it expands, cracking and pushing off the concrete around it — which exposes more steel, and the process accelerates. It's a structural problem, not a cosmetic one, and it doesn't stop on its own.
If you've seen cracked concrete with rust stains or exposed steel, here's what's happening and what to do.
What causes it?
Reinforced concrete works because the concrete protects the steel inside it. That protection can break down when:
- Carbonation — atmospheric carbon dioxide gradually reduces the concrete's alkalinity, removing the chemical protection around the steel
- Chlorides — salt, particularly in coastal environments, reaches the steel and drives corrosion
- Insufficient cover — the steel sits too close to the surface, with too little concrete protecting it
- Water ingress — through cracks, failed waterproofing or poor drainage
- Original construction issues — poorly compacted concrete or inadequate mix leaving the steel vulnerable
Because rust occupies more volume than the steel it replaces, the expansion is what physically breaks the concrete apart.
What to look for
Typical signs include cracking that runs in lines (often following the reinforcement beneath), rust staining bleeding through the surface, sections of concrete sounding hollow when tapped, bulging or delaminating surfaces, and in advanced cases, exposed and visibly corroded steel with concrete having fallen away.
Where it shows up most: balconies, car parks, basements, façades, and any concrete exposed to weather or water — which is why apartment buildings and commercial structures see it more often than houses.
Why it matters structurally
Two things are happening at once. The reinforcement is losing section as it corrodes, reducing the capacity of the element. And the concrete is losing its bond with the steel, so the two stop working together as they were designed to.
There's also an immediate safety dimension: spalling concrete on a balcony or façade can fall. Where that's a risk, it needs prompt attention rather than monitoring.
What an assessment involves
Diagnosing it properly means establishing how far it's progressed and why — not just where it's visible. An engineering assessment typically looks at the extent of affected areas, the condition of the reinforcement, the likely cause (carbonation, chlorides, water ingress or cover), and what's driving the water or salt exposure in the first place.
That last part matters: repairing spalled concrete without fixing the water ingress causing it means it comes back.
Our team investigates structural deterioration across residential and commercial buildings in Melbourne, and can advise on remediation requirements.
Owners corporations and apartment buildings
For apartment buildings, spalling on balconies and in car parks is a common and significant maintenance issue, and it sits with the owners corporation where it affects common property. Independent engineering assessment gives owners a clear picture of the extent, the cause and the scope of remediation — useful both for planning works and for informed decisions about funding them.
Seeing cracking, rust stains or spalling concrete? Talk to a Melbourne structural engineer for an assessment.
Frequently asked questions
What is concrete cancer? The common name for concrete spalling caused by corrosion of the steel reinforcement inside. The rusting steel expands and breaks the surrounding concrete apart.
Is concrete cancer serious? Yes. It reduces structural capacity as the reinforcement corrodes, and spalling concrete on balconies or façades can fall. It doesn't resolve on its own.
What causes it? Carbonation, chloride (salt) exposure, insufficient concrete cover over the steel, water ingress, and original construction defects.
Where does it usually appear? Balconies, car parks, basements and façades — concrete exposed to weather or water. It's common in apartment and commercial buildings.
Can it be repaired? Yes, but the cause must be addressed as well as the damage. Repairing spalled concrete without stopping the water or salt ingress means the problem returns.
