Concrete cancer in apartment buildings, and how it is diagnosed

Concrete cancer in apartment buildings, and how it is diagnosed

Every strata committee in Sydney meets this eventually. Rust stains bleeding down a balcony soffit, render sounding hollow when you tap it, a patch of concrete that has fallen away and left steel showing.

The common name is concrete cancer. What is actually happening is corrosion of the reinforcement inside the slab, and understanding that changes how it gets fixed.

The mechanism

Reinforcing steel inside concrete sits in a highly alkaline environment, and that alkalinity forms a passive layer on the steel that stops it rusting. Concrete protects steel chemically, not just physically.

Two things destroy that protection. Carbon dioxide from the air slowly reacts with the concrete and lowers its alkalinity, working inward from the surface. That is carbonation. Separately, chlorides from sea spray or from the original mix migrate to the bar and break the passive layer directly.

Once the steel starts corroding, the rust occupies several times the volume of the original steel. That expansion has nowhere to go, so it cracks the concrete along the line of the bar and pushes the cover off. The falling concrete is the last stage, not the first.

What the symptoms tell you

Rust staining means corrosion is already active and has been for a while.

Cracks running in straight lines parallel to each other, usually at regular spacing, are following the reinforcement. That is a strong indicator, because shrinkage and structural cracks do not follow that pattern.

Drummy or hollow sounding areas mean the concrete has already delaminated from the steel even though nothing has fallen yet. Those areas are the immediate safety concern, particularly on a soffit above a walkway.

How it gets tested properly

A visual inspection tells you where to look. It does not tell you how far it has gone.

A hammer or drag chain survey maps the delaminated areas across the whole element rather than the ones you can see.

A cover meter measures how much concrete sits between the surface and the steel. Low cover is the single most common reason a building in Sydney has this problem at forty years old while its neighbour does not.

Carbonation depth testing on a freshly broken face shows how far the alkalinity has been lost, using an indicator that changes colour where the concrete is still alkaline. If the carbonation front has reached the bar, that explains the corrosion without needing chlorides.

Chloride sampling, taken as dust at increasing depths, shows whether salt is driving it. That matters because the repair strategy is different.

Why patch repair on its own fails

Break out the loose concrete, clean the steel, fill with repair mortar, and it looks solved. Twelve months later the concrete immediately beside the patch starts to spall.

That happens because the new patch is highly alkaline and the surrounding old concrete is not. The steel inside the patch becomes passive again while the steel just outside it stays active, and the difference between them accelerates the corrosion at the boundary. The repair has moved the problem rather than removed it.

Fixing that properly means breaking out behind the bar rather than just to it, cleaning the full circumference of the steel, and dealing with the boundary condition, sometimes with sacrificial anodes set into the repair. It also means treating the whole element rather than the failed patch.

The bit that stops it recurring

Water. Almost every case I inspect has a water path feeding it, a failed balcony membrane, a blocked drainage outlet, a planter box sitting against a wall, a handrail fixing that penetrates the slab and has never been sealed.

Repair the concrete and leave the water path, and you have bought a decade at most. Fix the water first and the repair lasts.

What a strata committee should ask for

A condition assessment across the whole building, not a quote to fix the visible patch. A scope that names the testing done and the results. A repair specification rather than a description. And a clear statement of what is driving it, because a chloride-driven problem on a coastal building and a low-cover carbonation problem inland need different answers.

We carry out this assessment work under structural engineering, and the report exists to give a committee something it can tender against rather than a set of photographs.