Balconies, car parks and 1960s–80s unit blocks
Concrete cancer is the reinforcement inside the concrete rusting and pushing the cover off from within. It does not stop on its own and the repair grows with time. We arrange an inspection and a written scope so a committee has something real to price and vote on — not a number with no working behind it.
Six things owners and building managers describe when they ring. The first three are early; the last three mean the repair has already grown.
Usually the first sign, and the one most often painted over. A brown stain on a soffit or a balcony edge means the steel inside has already begun to corrode and the rust is finding its way out through the pores.
Cracks that follow a line rather than wandering are following the reinforcement bar beneath them. That is the expanding rust splitting the concrete along the steel, and it is quite different from shrinkage cracking.
Drummy concrete. The cover has already separated from the steel behind it but has not fallen yet. A committee should treat this as urgent, because the next stage is material landing on somebody.
Spalling. Once the cover comes off, the steel is exposed to the weather directly and the rate of corrosion increases sharply. This is the point at which a repair stops being optional.
Visible steel, usually orange with rust and sometimes visibly reduced in diameter. Section loss is what turns a durability problem into a structural one.
Fixings set into concrete that is losing its integrity. On a balcony this is a safety matter before it is a maintenance one, and it should be isolated rather than scheduled.
It is corrosion of the steel reinforcement inside the concrete, not damage to the concrete surface. Steel cast into sound concrete is protected by the concrete's high alkalinity, which forms a microscopic passive film on the bar and stops it rusting. Lose that protection and the steel corrodes.
The critical part is what rust does to volume. Iron oxide occupies up to six times the space of the steel it formed from, and that expansion happens inside a material that cannot stretch. It cracks the concrete from within, the cracks let in more water and oxygen, and the process accelerates itself. By the time a piece of concrete falls off, the corrosion has been running for years.
Overwhelmingly on reinforced concrete built between the 1960s and the 1980s, and worst where salt air reaches the steel.
The core of this problem in Brisbane. Built when concrete cover requirements were lower than AS 3600 now demands and the concrete itself was more permeable. Balconies, spandrels and access walkways are exposed on every face, so they carbonate from both sides and reach the steel sooner.
The most common single repair on this site. A balcony has thin cover, sits in the weather, and usually relies on a membrane that has long since failed. Water gets to the top reinforcement and the failure appears on the underside as staining and then spalling.
Chlorides arrive by vehicle in coastal suburbs, ponding is common, and ventilation is poor. Soffit spalling over parking bays is both a durability problem and a falling object risk, which is why building managers treat it as urgent.
Anything within reach of Moreton Bay air, plus the river frontage. Airborne chloride attacks the passive layer far faster than carbonation does, and it pits the steel locally rather than degrading it evenly.
Small elements with very thin cover, frequently forgotten in maintenance plans until a piece drops. Cheap to fix early, disproportionately disruptive when they fail over a footpath.
Less common and much more serious. Corrosion of a post-tensioning tendon is a structural matter, not a cosmetic one, and it needs a structural engineer rather than a patching contractor. Any inspection should establish early whether a slab is post-tensioned.
Three things compound here. Heat and humidity: corrosion needs moisture and oxygen, and reaction rates roughly double for every ten degrees, so a subtropical climate runs the process faster all year rather than pausing over winter.
Salt air: anything within reach of Moreton Bay or the river collects airborne chloride, and chloride is far more aggressive than carbonation — it breaks the steel's protection locally and pits the bar rather than degrading it evenly, so damage concentrates and arrives sooner.
The building stock: Brisbane built a great many walk-up unit blocks between the 1960s and 1980s, when concrete cover requirements were lower than AS 3600 now demands and the concrete itself was more permeable. Those buildings are now at exactly the age where carbonation reaches the steel on its own, without any help from the coast.
Published ranges. Committees cannot budget from silence, and almost nobody in this trade puts a number on a page.
| Work | Indicative range | What it covers |
|---|---|---|
| Initial inspection and written scope | Free | Arranged through us. A defect schedule and a scope your committee can send out to several contractors on the same basis. |
| Independent engineer's report | $1,800 – $4,500 | Where the extent is unclear, a slab is post-tensioned, or the committee needs an independent opinion before committing to a levy. |
| Concrete repair, patch method | $650 – $1,400 per square metre | Cut back to sound concrete, expose and treat the steel, then reinstate with a repair mortar. The standard remedial approach for localised spalling. |
| Crack injection | $180 – $420 per linear metre | Epoxy or polyurethane injection where cracking is the route in and the steel is not yet badly affected. |
| Sacrificial anodes, per anode | $95 – $180 | Zinc anodes set into the repair to protect the surrounding steel. Worth it where chloride is present, because a patch alone can accelerate corrosion at its edges. |
| Balcony repair and re-membrane, typical unit | $4,500 – $16,000 | Concrete repair plus a new waterproof membrane and finish. Doing the concrete without the membrane simply schedules the next repair. |
| Protective anti-carbonation coating | $55 – $110 per square metre | Applied after repair to slow further carbonation. It is maintenance, not a cure, and it does nothing for steel that is already corroding. |
| Full remediation, small walk-up block | $60,000 – $400,000 | The range is enormous because access, extent and whether scaffolding or swing stages are needed dominate the cost. This is why a scope comes before a number. |
Prices are indicative ranges only. Remedial concrete work is priced from a scope prepared after inspection, and a fixed quote follows that.
Step 1
A description and photographs of the worst area. For a body corporate, any previous reports or a sinking fund forecast help us judge what stage this is at.
Step 2
Hammer tap survey to find drummy areas, cover meter readings, and where warranted half-cell potential testing to map where corrosion is actually active rather than where it is merely visible.
Step 3
Quantified: how many square metres of patch repair, how many linear metres of injection, how many anodes, what access. This is the document that lets several contractors quote the same job, which is the only way a committee can compare prices honestly.
Step 4
Two or three contractors pricing identical work. Without a common scope, cheaper quotes usually mean less work rather than better value, and that is very hard to see from a committee meeting.
Step 5
General meeting, sinking fund or special levy. We can supply what the committee needs to put the resolution together, including the consequence of deferring.
Step 6
Repair, protection and reinstatement, and then a plan for what to inspect and when. Concrete cancer is a durability problem, and durability problems recur where nothing changes about how the building is maintained.
Written for someone who has to explain a recommendation to other people.
Who pays, how to get comparable quotes, and how to put a levy resolution together that survives a general meeting.
For committeesThe question that decides whether you need an engineer or a remedial contractor, and how it is answered.
Structural or notThe honest version, with the cost of deferring attached — because a committee will always ask.
Deferring the workHammer tap survey, cover meter, half-cell testing, and the defect schedule that lets contractors quote the same job.
What an inspection involvesCut back, treat the steel, reinstate. Why the method matters more than the price per square metre.
Repair methodThe most common single job on this site, and why concrete without a new membrane just schedules the next one.
Balcony workVehicle-borne chloride, ponding and poor ventilation — plus a falling object risk over occupied bays.
Car park workTwo different mechanisms with different speeds, patterns and repairs. Bayside buildings usually have both.
Telling them apartThe earliest visible sign, how to tell it from harmless tie-wire spotting, and why painting over it costs more.
Rust stainingHollow-sounding cover that has already separated from the steel — usually two to three times the visible area.
Drummy concreteSinking fund, special levy, staged programme or borrowing, and what makes a resolution survive the meeting.
Sinking funds and leviesCorrosion of the steel reinforcement inside the concrete. Steel in sound concrete is protected by the concrete's high alkalinity, which forms a passive film on the bar. When that protection is lost the steel rusts, and rust occupies several times the volume of the steel it came from. That expansion cracks the concrete from the inside and eventually pushes the cover off. The visible damage is the last stage, not the first.
It depends entirely on how much steel section has been lost and where. Early carbonation damage on a non-structural spandrel is a durability problem. Significant section loss in a cantilevered balcony slab or corrosion of a post-tensioning tendon is a structural problem and needs an engineer, not a patching contractor. Establishing which you have is the single most useful outcome of an inspection.
It gets worse and it gets more expensive, in that order. Once the cover spalls, the steel is exposed directly to Brisbane's humidity and corrosion accelerates. The repair grows from patching to section replacement, and falling concrete becomes a genuine liability question for the body corporate. Deferring is a decision with a price, and the honest way to present it to a committee is with that price attached.
Warmth and humidity. Corrosion needs moisture and oxygen and reaction rates roughly double for every ten degrees, so a subtropical climate runs the process faster year round. Add airborne chloride anywhere near Moreton Bay and the river, and a large stock of 1960s to 80s buildings constructed to lower cover requirements than apply now.
No, and it is an expensive mistake. Painting traps moisture against concrete that is already corroding and hides the evidence while the steel continues to lose section. Render over spalling concrete falls off in sheets within a few years and takes the cover with it. Coatings have a real place, but only after the repair and never instead of it.
In Queensland, common property is generally the body corporate's responsibility and lot property is the owner's, but balconies and their boundaries are a frequent grey area and it turns on the plan and the by-laws. Get this established before scoping the work, because it decides who is funding it. We are not lawyers and this is not legal advice — a strata lawyer or your body corporate manager should confirm it.
A properly executed patch repair with anodes where chloride is present, protected and maintained, should give decades. A patch repair with no protection on a chloride-affected building can start failing at its edges within a few years — the repair itself changes the electrochemistry and can accelerate corrosion in the steel immediately around it. That effect is why the method matters more than the price per square metre.
Concrete Cancer Brisbane is a lead referral service. We arrange inspections and quotes with independent remedial contractors and engineers working across Brisbane. We do not carry out the work ourselves, and any contractor we refer holds the QBCC licence the work requires.
Most of the stock affected is the 1960s–80s walk-up belt and the bayside strip, where salt air reaches the steel faster.
Tell us what the concrete is doing. We will arrange an inspection and a written scope your committee can actually price.
