Concrete Cancer: 7 Warning Signs Building Owners Should Never Ignore

Concrete cancer is the common name for reinforcement corrosion. Steel bars inside concrete are normally protected by the high alkalinity of the surrounding cement paste. When carbonation or chloride contamination strips that protection away, the steel begins to rust. Rust occupies several times the volume of the original steel, so the corroding bar pushes outward against the concrete around it until the concrete cracks and breaks away.

The process is slow, progressive and largely hidden until it surfaces. By the time concrete is falling from a soffit, corrosion has usually been active for years. The good news is that the early signs are visible to anyone who knows what to look for.

The 7 warning signs

1. Rust staining

Brown or orange streaks running down from cracks, joints, balustrade fixings or slab edges are the most reliable early indicator. The stain is iron oxide being washed out of the concrete by rain. Staining around a fixing may only mean a corroding bolt, but staining along a beam or slab edge usually means the reinforcement itself is rusting.

2. Cracks that run in straight lines

Shrinkage cracks tend to wander. Corrosion cracks follow the reinforcement, so they appear as straight lines that mirror the bar layout, often parallel to slab edges or running along beams. A straight crack with rust staining is corrosion until proven otherwise.

3. Drummy, hollow-sounding concrete

Tap suspect concrete with a hammer or even a coin. Sound concrete rings; delaminated concrete sounds hollow or drummy because a fracture plane has already formed at the level of the reinforcement. Drummy areas are the next pieces to spall, and on a building facade that makes them a falling object hazard.

4. Spalling

Spalling is the end result: pieces of concrete breaking away, typically exposing rusted reinforcement behind. On commercial buildings the highest-risk locations are balcony edges, window heads, awnings and car park soffits, anywhere a fragment can fall onto people below.

5. Exposed or visibly corroded reinforcement

Any bar you can see has lost its cover concrete and is corroding in open air. Section loss accelerates from this point because there is nothing slowing the supply of oxygen and moisture. Exposed bars also indicate the original cover may have been below what AS 3600 requires for the exposure classification, which means neighbouring areas are likely to follow.

6. Bubbling paint or render over concrete

Coatings blister when something underneath is expanding or releasing moisture. On painted concrete, a patch of bubbling or lifting paint often sits directly over a corroding bar or a delaminating layer. Painting over it without investigation hides the symptom and traps moisture against the steel.

7. White deposits and persistent damp patches

Efflorescence, the white crystalline deposit on concrete surfaces, shows that water is moving through the concrete and carrying dissolved salts with it. Water movement is the engine of corrosion. A soffit or wall that never quite dries out is telling you the concrete is saturated, and saturated concrete with chlorides present corrodes fast.

Why the damage accelerates

Carbonation advances from the surface inward at a rate that depends on concrete quality and exposure. Once the carbonation front reaches the steel, corrosion begins, cracks open, and those cracks become highways for more oxygen, water and chlorides. The deterioration curve is not linear: each year of delay tends to cost more than the year before it.

Coastal buildings have it worst. Airborne salt drives chloride-induced corrosion, which is more aggressive and harder to arrest than carbonation. Buildings within a few kilometres of the surf, common across the Gold Coast, can show corrosion damage decades earlier than identical structures inland.

How fast does it progress?

Owners usually want a number of years, and the honest answer is a range that testing narrows. Carbonation in average-quality concrete advances slowly, and the years it takes to reach the steel depend mostly on how much cover the original builders achieved; buildings with shallow cover lose their protection decades early. Once corrosion starts, the visible damage cycle, staining to cracking to drummy concrete to spalling, commonly plays out over a handful of years rather than decades, faster in marine air and on weather-exposed elevations. Two practical consequences follow. First, a building showing early signs on one elevation almost certainly has the same chemistry advancing quietly on the others. Second, the difference between repairing at the staining stage and repairing after widespread spalling is commonly a multiple of the cost, not a percentage. Testing turns this from guesswork into a dated forecast the sinking fund can plan against.

What a proper diagnosis involves

Patch-and-paint quotes prepared from the ground are guesswork. A defensible diagnosis maps the cause and the true extent before anyone prices the repair. Expect a combination of:

• Hammer or tap survey to map drummy concrete beyond what is visibly cracked
• Cover meter survey to measure how much concrete protects the steel
• Carbonation depth testing on freshly exposed cores or breakouts
• Chloride sampling at depth increments where salt exposure is suspected
• Targeted breakouts to inspect bar condition and measure section loss

This is the same discipline that underpins a good facade audit: classify each defect, establish cause, then scope the repair. The standards that govern the work, including AS 3600 for the structure and the EN 1504 series for repair methods and materials, are summarised on our standards page.

What to do if you see these signs

First, do not paint over them. Second, deal with falling-object risk: drummy overhead concrete near foot traffic should be made safe promptly. Third, commission an inspection while the options are still wide open. Caught early, concrete remediation can be a planned, budgeted programme of patch repairs and protective coatings. Left late, it becomes structural strengthening with engineering input, access costs and disruption to match.

As a guide, early-stage repairs identified from an annual inspection often cost a fraction of what the same building spends after its first major spall closes a footpath. The difference is rarely the repair rate per square metre; it is the quantity of concrete that has been allowed to fail.

Allied Commercial diagnoses and repairs concrete cancer on commercial and strata buildings across Brisbane and the east coast. If any of these warning signs look familiar, get in touch for an inspection.