Concrete cancer: what it is, what causes it, and what to do about it.
If you have spotted rust-coloured stains, cracking, flaking, or worse...crumbling concrete, on a balcony, car park ceiling, or facade, there is a good chance someone has mentioned the words “concrete cancer”. It is an alarming term, and in Perth’s coastal environment it is a common one. This article explains what concrete cancer actually is, what causes it, how to recognise it early, and, most importantly, why the way it is repaired matters far more than most building owners realise.
For strata committees, building managers, and property owners in Perth and regional WA, understanding concrete cancer before you get quotes matters on two fronts. Fix it properly once and you avoid paying far more to do it again in a few years. Leave it too long and it stops being just a repair bill: spalling concrete can fall from balconies, soffits, and facades, putting residents and the public at risk.
What concrete cancer actually is.
Concrete cancer is the common name for the damage that happens when the steel reinforcement inside concrete begins to corrode. Reinforced concrete works because steel bars sit inside the concrete and give it tensile strength. The concrete around the steel is normally alkaline, and that alkalinity protects the steel from rusting. When that protection breaks down and the steel starts to corrode, the rust expands to several times the volume of the original steel. That expansion pushes the surrounding concrete apart from the inside, causing it to crack, flake, and eventually break away. That breaking away is called spalling.
Once it starts, it does not stop on its own. The more the concrete cracks, the more air and moisture reach the steel, and the faster the corrosion spreads. Left alone, concrete cancer moves from a cosmetic blemish to a structural problem.
What causes concrete cancer.
There are a few common drivers, and in Perth they often work together.
Carbonation. Over time, carbon dioxide from the air penetrates the concrete and gradually lowers its alkalinity. Once that protective alkalinity drops far enough at the depth of the steel, the steel loses its protection and corrosion can begin. This is a slow process, which is why concrete cancer is more common in older buildings.
Chloride attack, and why Perth is exposed. Salt is one of the most aggressive causes of reinforcement corrosion, and Perth’s coastal setting means salt-laden air reaches a great many buildings. Salt can also be present from the original construction or from marine environments. Coastal and near-coastal buildings across the Perth metro area are particularly prone to chloride-driven concrete cancer, especially on exposed balconies, facades, and balustrades.
Water ingress. Water is the vehicle that carries oxygen and chlorides to the steel. A failed balcony membrane, a cracked slab, blocked drainage, or a leaking planter box will accelerate corrosion wherever the water travels. This is why concrete cancer and waterproofing failure so often appear together.
Insufficient concrete cover. If the original construction left too little concrete between the surface and the steel, the steel was never well protected to begin with, and problems appear sooner.
You may also hear “concrete cancer” used to describe alkali-silica reaction, a chemical reaction within the concrete itself. It is less common than reinforcement corrosion but produces similar cracking. A proper assessment identifies which mechanism is at work, because the repair approach differs.
What does concrete cancer look like?
The earlier it is caught, the cheaper and simpler it is to address. The signs to look for:
• Rust-coloured stains bleeding through the concrete surface
• Cracking that runs in straight lines, often following the path of the reinforcement below
• Concrete that sounds hollow or “drummy” when tapped
• Flaking, bubbling, or blistering of the surface or any coating
• Sections of concrete breaking away and exposing rusted steel underneath
• Bulging or swelling in slabs, columns, soffits, or balcony edges
On balconies and facades, the first sign is often a stain or a hairline crack that owners dismiss as cosmetic. By the time concrete is visibly breaking away and steel is exposed, the corrosion is well advanced.
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Concrete spalling versus concrete cancer: what is the difference?
The two terms get used interchangeably, but they are not quite the same thing. Spalling is the visible symptom: concrete flaking, chipping, or breaking away from the surface. Concrete cancer usually refers to the underlying process driving that spalling, most often the corrosion of the reinforcement.
The distinction matters because not all spalling is caused by corroding steel, and the correct repair depends on the cause. This is exactly why a surface inspection alone is not enough, and why a strata or building manager is right to ask what is actually driving the damage before accepting a quote.
Why you cannot just patch it over.
This is the single most important thing to understand about concrete cancer, and it is where a lot of money gets wasted.
If a contractor patches one area, this can actually accelerate corrosion in the concrete immediately around the patch, a well-documented effect where the newly repaired section shifts the electrochemical balance and drives new corrosion at the edges of the old repair. The building owner sees a tidy repair, pays for it, and watches fresh cracking appear next to it within a year or two.
Proper concrete cancer repair treats the cause, not just the symptom. That is the whole point of a remedial approach, and it is why diagnosis has to come before pricing. That's where we come in!
Concrete cancer in Perth strata and apartment buildings.
In multi-owner buildings, concrete cancer most often shows up on balconies, facades, car park ceilings, and exposed structural elements. Because these are usually common property, the cost of repair typically falls to the strata company and is funded through levies or, where reserves fall short, a special levy. That makes early detection and planned repair a financial decision as much as a technical one: catching it early and budgeting for it through a capital works reserve is far cheaper than an emergency response after a piece of concrete has fallen.
It also makes independent assessment valuable. A documented scope, prepared before contractors are approached, means every quote prices the same properly diagnosed job, rather than a range of guesses at what the problem might be.
What to do if you think you have concrete cancer.
If you have noticed any of the signs above, the right first step is not to get three quick quotes. It is to get the problem properly assessed so you understand the cause and the true extent before anyone prices a repair. A cosmetic quote against a structural problem, or a structural quote against a cosmetic one, both cost you.
MRO inspects and diagnoses concrete cancer across Perth strata, commercial, and residential buildings, and the inspection and diagnosis are free. You get a clear picture of what is actually happening in the concrete and what a proper repair involves, before any commitment. If repair is needed, MRO Go is a registered WA building contractor with the remedial expertise to fix the cause, not just the surface.
If you are seeing stains, cracking, or spalling on your building, get in touch with the MRO team for a free site inspection.
Frequently asked questions about concrete cancer.
What does concrete cancer look like in the early stages?
Early concrete cancer usually shows as rust-coloured or white, "salty" looking bleeding through the surface, fine cracks that follow straight lines, or small areas of flaking or blistering. At this stage it is often mistaken for a cosmetic issue, which is why it frequently goes untreated until concrete starts breaking away.
Is concrete cancer dangerous?
It can become dangerous. As it progresses, it weakens the structural element it affects, and spalling concrete can fall from balconies, ceilings, soffits, and facades. It is not usually an immediate emergency in its early stages, but it does not resolve on its own and gets more expensive and more serious the longer it is left.
Can concrete cancer be fixed permanently?
Yes, when it is repaired properly. A repair that removes all affected concrete, treats or replaces the corroded steel, reinstates correctly, and addresses the underlying cause, such as water ingress, can last for many years. Repairs that only patch the surface tend to fail and can accelerate corrosion nearby.
Who pays for concrete cancer repair in a WA strata building?
Where the affected concrete is common property, which balconies, facades, and structural elements usually are, the strata company is generally responsible and funds the repair through levies. Where the element sits within a lot boundary, the position can differ. Your strata manager can confirm responsibility for your specific scheme.