“Concrete cancer” is the everyday name for concrete spalling caused by corroding steel reinforcement. It shows up as cracked, flaking or broken-away concrete, usually with rust stains nearby, and it is common on Melbourne’s older apartment blocks, balcony slabs and coastal buildings.
It matters because it gets worse on its own, and because the pieces that break away fall. On a building with balconies, walkways or a footpath below, spalling is a safety issue first and a maintenance issue second.
What causes it
Reinforced concrete relies on the concrete around the steel bars to protect them. Two things break down that protection over time.
Carbonation. Carbon dioxide from the air slowly reacts with the concrete from the surface inward, lowering its alkalinity. When that front reaches the steel, the steel loses its protection and can start to rust.
Chlorides. Salt from sea air, or from some older construction materials, penetrates the concrete and attacks the steel directly. Buildings near the bay, such as those around St Kilda and Geelong, are more exposed.
Both happen faster where the concrete cover over the steel is thin, where the concrete is poor quality, or where water keeps getting in through cracks and failed waterproofing.
Rust takes up more space than the steel it came from. As the bar corrodes it expands, the concrete around it cracks, and eventually a section breaks away, which is the spall you can see.
How to spot it early
- Rust stains running down from a slab edge, balcony soffit or column, often before any concrete has broken
- Hairline cracks running along the line of the reinforcement, frequently at balcony edges and slab ends
- Drummy or hollow-sounding concrete when tapped, which means it has already delaminated from the steel behind
- Flaking, bulging or broken concrete, sometimes with the bar visible
- Debris on balconies, footpaths or landscaping below.
By the time the bar is exposed, the corrosion has usually spread well beyond the visible area. That is why an inspection maps the extent of damage rather than just the obvious spots.
Why the repair method matters
The quick fix, chipping off loose concrete and patching over it, often fails within a few years and can even speed up corrosion nearby. A durable concrete repair generally involves:
- breaking out all the damaged and contaminated concrete, not just the loose piece
- exposing the reinforcement fully and cleaning it back to bright steel
- replacing or supplementing bars that have lost too much section, on an engineer’s advice where it is structural
- treating the steel and the repair area with a compatible protective system
- reinstating with a repair mortar suited to the exposure
- sealing or coating the surface to slow further carbonation or chloride ingress, and fixing the water path that caused the problem.
Skip the last step and the same area, or the one next to it, will spall again.
Responsibility on strata buildings
In many Victorian strata plans, external walls, balcony slabs and structural concrete are common property, which puts repair and maintenance with the owners corporation. Plans differ, so check the plan of subdivision for your building. Either way, the owners corporation will want a clear record that defects were identified and acted on once it knew about them.
What to do if you see it
- Make it safe. If concrete is loose above a walkway, balcony or footpath, restrict access below it until it has been assessed.
- Get it inspected. A facade inspection maps every affected area, rates severity, and separates urgent make-safe items from work that can be planned.
- Scope the repair properly. Larger or structural areas should be scoped with input from an engineer.
- Plan the rest. Early-stage cracking and staining can be scheduled into a maintenance cycle rather than left until the next spall.
Summit Facades inspects concrete elements, including balcony slabs, soffits and slab edges, as part of every facade inspection. Each defect is photographed, rated and given a recommended action and timeframe in our facade defect report, so the owners corporation knows exactly what is urgent and what can be budgeted through a maintenance program.
