Concrete Cancer
Rusting steel inside concrete walls and balconies — the most common structural problem in Sydney apartment blocks.
Concrete cancer is when steel reinforcement inside concrete rusts and expands, cracking the concrete off from the inside. It is widespread in Sydney apartment blocks built between the 1960s and 1990s, especially near the coast. A proper repair means cutting back to sound concrete, treating the exposed steel with a corrosion inhibitor, and patching with a salt-resistant repair mortar — anything less fails within a few years. Romans Building Services assesses concrete cancer across Sydney before recommending repair, so the visible damage and the cause are both dealt with.
Last updated: 2026-05-29
What is concrete cancer?
Concrete cancer is what happens when the steel reinforcement inside concrete starts to rust. As the steel rusts, it expands — up to seven times its original size. That expansion cracks the concrete from the inside out, pushing it off in chunks. Left alone, the structural strength of the concrete drops and the damage accelerates.
It is by far the most common structural problem we see in Sydney apartment blocks, especially the ones built between the 1960s and the 1990s. Coastal suburbs — Bondi, Manly, Dee Why — get it worst, because salt air speeds everything up. But we also see plenty of it inland, on older car parks, balcony slabs, and commercial buildings.
The good news: it is fixable, as long as you catch it before the structural steel is too far gone. The bad news: poorly done repairs fail quickly and make the problem worse, because you are trapping moisture and salt against steel that is already compromised.
Signs to look for
- Rust stains bleeding out of concrete walls or balconies
- Cracks running in straight lines (usually following the steel reinforcement)
- Concrete flaking off in chunks, exposing brown rusty steel underneath
- Hollow sound when you tap the concrete surface
- Bulging or swelling patches on balcony soffits or walls
- Paint or render bubbling and peeling off in sheets
Why it happens
- Salt in the air crystallising inside the concrete (biggest cause in coastal Sydney)
- Water ingress through cracks, failed waterproofing or poor drainage
- Concrete cover over the steel that was too thin when originally built
- Carbonation — the concrete losing alkalinity over decades, letting steel rust
- Previous repairs done with the wrong materials, sealing moisture against the steel
How urgent is this?
If you can see exposed rusty steel, the damage is already into the structural layer. For balconies, parapets or anything overhead, get it looked at quickly — concrete can fall. For walls at ground level, you have more time, but every year you wait the repair gets bigger and more expensive.
How we fix it properly
Full assessment
We probe the concrete with sounding hammers to find all the damage, including hidden sections that are hollow behind intact-looking surface. You need to find every bit of compromised concrete before you start cutting, or the repair will fail at the edges.
Cut out damaged concrete
We cut back to sound, well-bonded concrete — usually behind the steel. All the rusty, spalled or delaminated concrete comes out. Halfway measures do not work.
Clean and treat the steel
Every bit of exposed steel gets wire-brushed or sandblasted back to clean metal, then treated with a corrosion inhibitor and a protective coating. This is the step that decides whether the repair lasts 5 years or 30.
Apply a proper patch mortar
We use a salt-resistant, polymer-modified repair mortar matched to the original concrete. Applied in layers with proper curing. We never use builders mix — it shrinks and cracks off.
Protective coating
The repaired area gets a breathable protective coating that keeps water and salt out while letting moisture inside the concrete escape. Without this, the next round of damage starts immediately.
Typical cost range
$2,500 – $15,000+ per balcony, depending on damage. Whole-building programs typically $30k–$300k+ staged over 1–3 years.
Every job is different. We give a firm quote after inspection.
Common questions
Can concrete cancer spread to other parts of the building?
Yes, absolutely. The conditions that caused it in one spot usually exist across the building. If one balcony has concrete cancer, the others are probably at different stages of the same process. A full inspection gives you the real picture.
How long does a proper concrete cancer repair last?
Done properly — full removal, steel treated with a corrosion inhibitor, salt-resistant patch mortar, breathable protective coating — 20 to 30 years is realistic in a coastal environment. Done badly, repairs can fail in 3 to 5 years.
We are a strata committee — how do we budget for this?
Most strata jobs get staged over 2 or 3 years to spread cost. We inspect the whole building, rank the damage by urgency, and set up a program that deals with critical areas first and less urgent ones in later stages. Happy to work with your strata manager on this.
Is painting over concrete cancer a real fix?
No. Painting hides the damage and can make it worse by trapping more moisture. You have to cut out the damaged concrete, treat the steel, and patch properly. Paint is a last step, not a fix on its own.
Services that fix this
Concrete Cancer in your area
The causes and right fix for concrete cancer vary with local housing stock and exposure. Read the version closest to where you are:
Where we see concrete cancer most often
Some suburbs have more of this problem than others — the local housing stock, age, and coastal exposure all play a part. Click through for the local context.
Think you might have concrete cancer?
Send a photo or call Minas directly. We will tell you straight whether it needs doing now, or whether it can wait.