What concrete cancer is and what causes it
Concrete cancer is corrosion of the steel reinforcement inside concrete. Here is the mechanism, the causes, and where it shows up in Australian buildings.
Concrete cancer, known technically as concrete spalling, is not a disease of the concrete itself. It is the corrosion of the steel reinforcement embedded inside it. Reinforced concrete works because steel bars carry tension while the concrete carries compression, and the alkaline concrete forms a protective film that normally keeps the steel from rusting. When that protection breaks down, the steel corrodes, and the trouble begins.
The mechanism: rust that expands
The key fact is that rust takes up much more space than the steel it forms from, expanding to several times the original volume of the bar. As the reinforcement rusts, it swells and pushes outward on the surrounding concrete with enormous force. The concrete cracks, then bulges, then breaks away in chunks, a process called spalling, often exposing the rusting bar underneath. Once the steel is exposed, corrosion accelerates, so concrete cancer gets worse over time and does not repair itself.
What breaks down the protection
Two processes destroy the concrete's protective film. The first is carbonation: carbon dioxide from the air slowly penetrates the concrete and neutralises its alkalinity, and once that neutral front reaches the depth of the steel, the bar is no longer protected. The second is chloride attack, where salts, most importantly from a marine or coastal environment but also from some admixtures, reach the steel and trigger corrosion directly. Both are made far worse by water, because moisture is what carries the reaction along and lets the rust form.
Poor construction speeds it all up. Insufficient concrete cover over the steel, porous or cracked concrete, and failed waterproofing all let air, water and salt reach the reinforcement much sooner than they should.
Where it shows up
In Australian buildings concrete cancer concentrates where concrete is exposed to weather and water: balconies and their edges, suspended concrete floors and car park decks, basement and retaining walls, concrete cancer on older apartment blocks, and almost anywhere within reach of coastal salt spray. The early signs are rust-coloured staining bleeding through render or paint, fine cracking that follows the line of the reinforcement below, bubbling or drummy render that sounds hollow when tapped, and eventually pieces of concrete flaking or falling away. Because a spalling balcony or facade can drop material, active concrete cancer is a safety issue as well as a durability one, and it is worth getting a remedial specialist or engineer to assess it promptly.
Common questions
Why is it called concrete cancer if the concrete is fine?
It is a nickname, not a technical term. The concrete is the victim rather than the cause: the real problem is the steel reinforcement inside it rusting and expanding, which cracks and breaks the concrete apart from within. The technical name is concrete spalling due to reinforcement corrosion.
Does concrete cancer spread?
It progresses rather than spreads like an infection, but the effect is similar. Once reinforcement starts corroding and concrete spalls away, the exposed steel corrodes faster and the affected area grows. It does not stop or reverse on its own, so early treatment is cheaper and safer than waiting.
Sources
- Cement Concrete and Aggregates Australia, durability guidance
- Standards Australia, AS 3600 Concrete structures
General information only. Confirm details for your property with a licensed structural engineer.
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