Concrete Cancer vs Surface Damage: What a Coating Can and Can't Fix

Two Problems That Get Mistaken for Each Other

"Concrete cancer" is the phrase people reach for whenever a slab looks past saving - stained, cracked, flaking and generally tired. In the trade, though, the term means one specific thing, and it isn't cosmetic: corroding steel reinforcement breaking the slab apart from within. Most of the tired-looking concrete we assess across South-East Queensland doesn't have it.

The distinction matters because it decides everything about the fix. Surface damage can be repaired and coated over for tens of dollars per square metre. Genuine structural failure can't be coated over at any price - a new surface on a failing slab just hides the problem while it gets worse. This guide covers how to tell the two apart from what you can see, what each fix costs at our published rates, and where the honest limits of a coating sit.

Surface Damage Lives in the Top Few Millimetres

Staining, faded colour, surface wear, pitting, chips and hairline cracks that stopped moving years ago are all problems with the slab's face, not its bones. The concrete underneath is still doing the structural work it was poured to do - it just looks rough doing it.

Concrete Cancer Attacks the Slab From the Inside

When moisture gets deep enough into reinforced concrete, the steel inside starts to corrode. Rust takes up more space than the steel it grew from, so the corroding reinforcement cracks and pushes off the concrete around it - the slab is being broken apart from within. That process is structural by definition, and it doesn't stop on its own.

The Signs That Point to Concrete Cancer

Because the damage starts at the steel, the visible signs are different in kind from surface wear - they're evidence of something happening inside the slab rather than on it. The surface symptoms arrive late, after the corrosion has already been running for a while.

These are the signs worth taking seriously:

• Rust-coloured staining bleeding out of cracks or joints - the corroding steel announcing itself
• Concrete breaking away in chunks to expose rusted reinforcement underneath
• Sections of slab that have come loose, dropped, or sit stepped against their neighbours
• Cracking that keeps opening and moving with the seasons instead of settling

Two close relatives belong on the same list even though they aren't corrosion: significant subsidence, and a base that has failed underneath the slab. Different cause, same verdict - the problem is under or inside the concrete, not on it.

A coating can't fix any of these, because an overlay bonds to the slab underneath and is only ever as good as that slab. Coating over genuine structural failure just gives the failure a new colour. When we find these problems at the quote stage we say so and recommend replacement, cost aside - there's no version of resurfacing that makes a structurally failed slab sound again.

The Grey Area: Cracks

Cracks sit in both camps, which is why they cause the most second-guessing. Much of Brisbane sits on reactive clay soil that swells when wet and shrinks when dry, and every wet season the ground moves. Older slabs poured without decent control joints crack along whatever line the soil chooses.

The test isn't the crack - it's the movement. Hairline cracks that have stopped moving are cosmetic: they're repaired during surface preparation and disappear under the new finish. Cracks that are still opening, closing or stepping with the seasons mean the movement has to be sorted first. We assess this at the quote stage, and we won't resurface a slab that's still moving - an overlay applied over live movement will simply crack along the same line.