Advice & Information

What is "concrete cancer"?

"Concrete cancer" is the common name for what happens when the steel reinforcement inside concrete starts to corrode. As the steel rusts it expands — up to several times its original volume — and that pressure cracks and blows out the concrete around it (spalling). Once it starts, it tends to accelerate: more steel is exposed, more moisture gets in, and the damage spreads.

The important part: by the time you can see it on the surface, there's usually more going on underneath, along the reinforcement. That's why an honest inspection looks past the cosmetic symptom to the cause.

Why coastal buildings are most at risk

Salt is the accelerant. Airborne chlorides from the surf settle on and soak into concrete, and chloride is exactly what kicks off corrosion in the reinforcing steel. A building within a few hundred metres of a surf beach lives in an aggressive environment its whole life.

• Constant salt-laden air and spray
• Wind-driven rain finding its way into cracks and joints
• Rooftop ponding and tired waterproofing keeping concrete wet
• Sun, heat and movement opening up hairline cracks over time

AS3600, in plain English

AS3600 is the Australian "Concrete Structures Code." For our purposes it does two useful things: it sorts environments into exposure classifications, and it sets the minimum concrete strength and cover (how much concrete sits between the surface and the steel) for each one.

A typical exposed coastal slab is classed B1. Under the current code that points to around a 32 MPa slab with a minimum of 40 mm cover to the reinforcement. When the real cover is less than that — common in older buildings — the steel is far more exposed to chloride, and corrosion starts sooner.

First signs to watch for

You don't need to be an expert to spot the early warnings. If you see any of these, it's worth a look:

• Rust staining — brown streaks weeping from the concrete
• Cracking — especially fine cracks running in straight lines (often tracing the steel beneath)
• Spalling — concrete flaking, bubbling or breaking away, sometimes with steel visible
• "Drummy" concrete — areas that sound hollow when tapped
• Ponding water — water sitting on rooftops, balconies or walkways instead of draining

Why a quick fix often fails

Patching the surface can make a problem disappear for a while. But if the corroding steel behind it isn't dealt with, the corrosion keeps going — and the patch simply pops off again, usually taking a bit more concrete with it. Done thoroughly, by qualified people, repairs last. Done as a cosmetic cover-up, they don't. Knowing which is which before you spend money is the whole point of an independent report.