Concrete spalling is when pieces of the concrete surface crack, flake or break away, leaving rough craters and sometimes exposed steel. You'll see it on balcony edges, slab soffits, car park ceilings, columns, stairs and coastal façades.
It can look cosmetic at first. But spalling is usually a sign that something is happening inside the concrete, and falling pieces can be a real safety risk. This guide explains what causes it, how it relates to concrete cancer, and what a proper repair involves.
Spalling vs concrete cancer: what's the difference?
The two terms get used interchangeably, but they aren't quite the same thing.
Concrete cancer
Steel reinforcement inside the concrete rusting. Rust takes up several times more space than the original steel, so it pushes the concrete apart from the inside. Read our concrete cancer guide
Spalling
The concrete surface breaking away: cracking, flaking and chunks falling off, sometimes leaving the steel exposed.
Most spalling in Sydney buildings is caused by concrete cancer. But spalling can also come from other things, so the cause needs to be confirmed before the repair is planned.
What causes concrete to spall?
Most causes come back to one thing: water, salt and air reaching the steel inside the concrete.
Corroding reinforcement (concrete cancer) Most common
Water and oxygen reach the steel, it rusts and expands, and the concrete cover cracks off.
Salt and chlorides
Coastal air carries salt that soaks into concrete and speeds up steel corrosion. That's why Northern Beaches buildings within a few kilometres of the ocean are especially prone.
Northern Beaches remedial builderCarbonation
Over decades, carbon dioxide from the air slowly lowers the alkalinity of concrete. Once that reaches the steel, the steel loses its natural protection and starts to rust.
Too little concrete cover
If the steel was placed too close to the surface when the building went up, water reaches it sooner. This is common in older buildings.
Water getting in
Failed waterproofing, blocked drains and cracks let water into the slab.
Waterproofing explainedOther causes
Impact damage, fire, poor original concrete and, less often in Sydney, contaminated materials like magnesite floor toppings.
Is magnesite flooring dangerous?Warning signs to look for
- Rust stains bleeding through concrete or render
- Cracks running along the line of a balcony edge, beam or column
- Bulging or drummy concrete that sounds hollow when tapped
- Flakes, chips or chunks of concrete on the ground below
- Exposed, rusty steel bars
- White deposits (efflorescence) on the underside of slabs
Safety first: if pieces are falling, or steel is exposed on a balcony or over a walkway, fence off the area below and get it inspected promptly.
How concrete spalling is repaired
A proper repair treats the cause, not just the surface. A typical process looks like this.
- 1
Inspect and test
Map the damage, tap-test for drummy areas, and where needed test chloride levels, carbonation depth and concrete cover. Larger or structural jobs involve a remedial engineer.
- 2
Break out the damaged concrete
Remove all loose and contaminated concrete, going behind the corroded steel so it can be treated all the way round.
- 3
Treat or replace the steel
Clean the rust off. Bars that have lost too much section are replaced or supplemented.
- 4
Protect the steel
Apply a corrosion-inhibiting primer. In salt-affected areas, sacrificial anodes may be installed to keep protecting the steel after the repair.
- 5
Reinstate the concrete
Rebuild the area with a polymer-modified repair mortar to restore the original profile and cover.
- 6
Seal and protect
Finish with an anti-carbonation or protective coating, and fix any water entry points, such as the balcony membrane above.
Why patch jobs fail: simply patching over the crater with mortar, without treating the steel, usually fails within a few years, often cracking around the edges of the patch. This is sometimes called the "incipient anode" effect.
Is spalling concrete dangerous?
It can be. Falling concrete from balconies, façades and car park soffits can injure people below. Advanced corrosion also reduces the strength of the steel, which matters for balconies, beams and columns.
The cost difference is significant too. Small, early repairs are far cheaper than large-scale remediation once corrosion has spread through a slab.
Early vs advanced spalling
Concrete spalling FAQ
Can I fix spalling concrete myself?
Small cosmetic chips in non-structural concrete, maybe. But if steel is exposed or rust stains are present, the steel needs treating properly, and on balconies or in strata buildings the work should be done by a licensed remedial builder.
How long does a spalling repair last?
Done properly, with the steel treated, the cover reinstated and the water source fixed, repairs are designed to last many years. Patch-only repairs often fail quickly.
Will spalling get worse if I leave it?
Yes. Once corrosion has started, it keeps going and spreads along the steel, so the repair area and cost grow over time.
Who pays for spalling repairs in an apartment building?
Concrete slabs, balconies and façades are usually common property, so the owners corporation is generally responsible. Check your strata plan and by-laws.
Seeing cracks, rust stains or falling concrete?
Stamm Remedial inspects, diagnoses and repairs concrete spalling on homes and strata buildings across Sydney and the Northern Beaches.
