The name is dramatic, but the problem is real. Concrete cancer is one of the most common and misunderstood forms of building deterioration in Sydney, and it has a habit of progressing quietly for years before it becomes impossible to ignore.
If you manage a commercial building or sit on a strata committee, understanding what concrete cancer actually is — and more importantly, what it looks like before it gets serious — is genuinely useful. The difference between catching it early and catching it late is usually thousands of dollars, sometimes tens of thousands.
What concrete cancer actually is
Concrete is strong in compression but weak in tension. That’s why reinforced concrete — concrete with steel bars (rebar) embedded through it — was such a building revolution. The steel handles the tension, the concrete handles the compression, and together they’re enormously strong.
The problem is that steel and water don’t get along.
Over time, concrete naturally becomes more porous as it ages. Carbonation — a chemical process where carbon dioxide from the air reacts with the concrete — lowers the pH inside the concrete, neutralising the alkaline environment that protects the steel from rusting. Once that protection is gone, moisture and oxygen penetrate to the steel and corrosion begins.
Here’s where the name makes sense. Rust takes up significantly more volume than the original steel. As the steel corrodes and expands, it exerts tremendous outward pressure on the surrounding concrete — pressure the concrete can’t absorb. The concrete cracks. Then chunks break off. Then you’ve got spalling concrete falling from balconies and car park ceilings, which is a safety issue, not just an aesthetic one.
In coastal and high-humidity environments like Sydney, the process is accelerated by chloride ions from the salt air, which attack the steel through a different mechanism but with the same result.
The warning signs — what to look for
You don’t need to be an engineer to spot early indicators of concrete cancer. What you need is to know what you’re looking at.
Rust staining on concrete surfaces. Brown or orange streaks running down from joints, bolt holes, or cracks are often the first visible sign. The rust is leaching out from the corroding steel inside. By the time you can see it on the surface, the process is already underway.
Surface cracking that follows the line of the reinforcing. If you see cracks that run in a linear pattern — particularly parallel to edges or in a grid pattern that matches the likely layout of the rebar beneath — that’s worth having looked at. Random map cracking from shrinkage is less concerning. Cracking that tracks the steel is a different story.
Spalling and delamination. This is where chunks of concrete are starting to separate from the surface, or where you can hear a hollow sound when you tap the concrete with a hammer. That hollow sound means there’s a void behind the surface — the concrete has separated internally even if it hasn’t fallen off yet. In a car park ceiling or underside of a balcony, that represents a real safety risk.
Exposed steel. If you can actually see the reinforcing steel because the concrete has broken away around it, you’ve moved well past early-stage. That said, it’s not too late to repair — it just becomes a more involved job.
Water staining and efflorescence. White powdery deposits on concrete surfaces (efflorescence) indicate water is moving through the concrete and evaporating, leaving mineral deposits behind. On its own this isn’t always a sign of concrete cancer, but it does indicate water infiltration, which accelerates the process.
Where does concrete cancer tend to appear?
Not all parts of a building are equally at risk. In Sydney, the highest-risk areas are consistently the ones with the most moisture exposure.
Car parks are at the top of the list. Water gets in from vehicles, from drainage, from the building above. The underside of car park slabs is a classic location for advanced concrete cancer, and the consequences of allowing it to progress are serious — concrete falling from overhead in an occupied car park is not a theoretical risk.
Balconies are another common problem area, particularly where the waterproofing membrane on the balcony surface has failed or was never properly installed. Water sits on the surface, finds its way through cracks or failed joints, and gets to work on the steel beneath.
Facades, columns, and structural beams in older buildings (pre-1990s construction is particularly common in our work) often have lower concrete cover over the steel than modern standards require under AS 3600, making them more vulnerable. The concrete cover — the thickness of concrete between the surface and the steel — is what buys time against carbonation and chloride ingress.
Swimming pool surrounds, planter boxes, and any area where water collects or sits are also higher-risk zones.
Does age matter?
Yes, but it’s not the only factor.
Concrete that was poured in the 1960s, 70s, and early 80s often has lower concrete cover, was mixed to lower strength standards, and may have been placed with less rigorous curing practices than we’d use today. These buildings make up a large proportion of our Sydney strata stock, and they warrant regular inspection.
That said, concrete cancer isn’t exclusive to old buildings. Poor waterproofing, inadequate concrete cover, a construction defect, or simply an environment with high chloride exposure can put a much newer building in the same position. We’ve worked on buildings less than fifteen years old with significant concrete cancer because the original waterproofing membrane failed early and nobody noticed.
What to do if you see the signs
Have it properly assessed. Not by a painter, not by a general builder who’s walked past and had a look, but by a specialist concrete repair contractor or a structural engineer. The reason is that what you see on the surface rarely tells you the full extent of the damage underneath.
A proper assessment involves sounding (hammer testing) the suspect areas, inspecting any exposed steel, and in some cases core sampling to check the depth of carbonation. Only then can you get an accurate picture of the scope of the problem — and an accurate picture is essential before you commit to a repair.
The cost of a concrete cancer repair increases significantly the longer it’s left. Early-stage treatment — while the steel is still mostly intact and the concrete cover is cracked but not yet spalling widely — is manageable. Late-stage repairs involving widespread breakout, steel replacement, and full surface reinstatement are considerably more expensive and disruptive.
Act early. Get it assessed. It’s a straightforward thing to say, but in our experience, the jobs that end up costing the most are almost always the ones where someone saw the signs two years before they called.
Stakat Building specialises in concrete cancer repair across commercial buildings, strata complexes, and industrial properties in Sydney.
If you’ve spotted something that concerns you, we’re happy to arrange a site inspection and give you a straight assessment of what you’re dealing with.