Concrete Cancer Repair — Chatswood
A North Shore strata apartment block with concrete cancer on the balcony columns. Case study: full chloride remediation, sacrificial concrete replacement, and anti-carbonation coating.
What was wrong when we arrived
A 1980s walk-up strata block in Chatswood had visible spalling on the balcony support columns on the north and east elevations. The Owners Corporation had been told by an engineer that the cause was chloride-induced concrete cancer — salt had got into the concrete through the failed waterproofing on the balcony decks above and rusted the reinforcing.
Two of the four affected columns were showing signs of structural concern — visible cracking, displacement of the cover concrete, and rust staining running down the face.
The strata committee needed a builder who could work with the engineer they had already appointed, give a fixed-price quote the committee could take to the AGM, and start within the financial year.
How we fixed it
We broke out all the loose and spalled concrete back to sound substrate. Where the reinforcing was reduced by more than 20%, we cut back further and spliced in new bar. The engineer was on site for each splice sign-off.
The patch was rebuilt with a polymer-modified repair mortar — not a standard cement patch, which would have failed again in two winters. The repair mortar is designed to be compatible with the original concrete's thermal movement.
After a 28-day cure, the entire column face was coated with an anti-carbonation coating. This stops CO2 getting into the concrete and starting the corrosion cycle again.
The balcony decks above were re-waterproofed by a separate contractor we introduced. The Owners Corporation had been told for years the decks needed it — we made it part of the same scope.
What the client got
The engineer signed off the work and the strata committee lodged the long-term maintenance plan revision. The repair is expected to last 15+ years before any maintenance is needed.
The Owners Corporation has since asked us back to quote on the south elevation, which was showing early signs of the same problem. They want it fixed before it gets to the same stage.
The AGM quote was approved on the first vote. The committee chair said it was the first remediation quote the owners had seen in five years that was a real number, not a guess.
Questions about concrete cancer repair
How do you know if concrete spalling is structural?
Visual signs are cracking, displacement of the cover concrete, and rust staining. The real test is a structural engineer's hammer tap and covermeter survey — that tells us how much reinforcing has been lost and whether the column can still carry the load. We do not quote concrete cancer repairs without an engineer's report.
How long does concrete cancer repair last?
A properly specified concrete cancer repair — full chloride remediation, polymer-modified patch, anti-carbonation coating — should last 15 to 25 years before any maintenance is required. Cheap patches (just a cement fill over the rust) fail in two to five years.
Can strata owners get funding help for concrete cancer repairs?
Yes. The NSW Strata Building Bond Inspection Scheme applies to new buildings, but most strata schemes have a sinking fund that can be drawn on for major repairs. We help the strata committee prepare the scope and the quote for the AGM.
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