Carbonation is one of the two main drivers of reinforcement corrosion in concrete buildings. Atmospheric carbon dioxide reacts with the alkaline pore fluid in the concrete and gradually converts calcium hydroxide to calcium carbonate. This drops the pH of the concrete from around 13 towards 8 or 9. At that lower pH the thin passive oxide film that normally protects the embedded steel breaks down — the steel is said to be depassivated — and once moisture and oxygen are present, the reinforcement begins to corrode.
The difficulty is that carbonation is invisible from the surface. It advances as a front from the face of the concrete inwards, and the structure can look perfectly sound right up until the carbonation front reaches the depth of the reinforcement. When it does, corrosion starts, the steel expands, and the first visible symptoms — cracking along the line of the bars, rust staining, and eventually spalling — appear. By that stage the corrosion is already established, which is why diagnosis before failure is so valuable.
In Sydney this matters most for concrete apartment buildings from the 1960s through the 1990s, where low cover to reinforcement was common and where decades of exposure have given the carbonation front time to advance. Unlike chloride attack, carbonation is not confined to the coastline — it affects sheltered, inland, and courtyard-facing elements just as readily. For strata schemes and owners corporations, catching carbonation while it is still a protection problem rather than a spalling problem is the difference between a coating programme and a full concrete repair.
How We Diagnose & Treat Carbonation
- Carbonation Depth Testing
- Break out or core small test locations and spray the freshly exposed concrete with phenolphthalein indicator.
- Sound, alkaline concrete turns bright pink; carbonated concrete stays colourless, giving a direct read of how far the carbonation front has advanced.
- Compare carbonation depth against measured cover to reinforcement to determine whether the steel is still passive or already at risk.
- Cover & Condition Survey
- Cover meter readings across representative elements to map how much protective concrete sits over the bars.
- Half-cell potential surveys where required to identify areas of active corrosion before they become visible.
- Realkalisation of Carbonated Concrete
- Where the steel is still sound but the surrounding concrete has carbonated, restore alkalinity so the passive film can re-form.
- Electrochemical realkalisation drives an alkaline electrolyte into the concrete using a temporary anode and applied current, raising pH back around the reinforcement.
- For shallower carbonation, realkalising or highly alkaline repair mortars re-establish a protective environment at the bar.
- Repair of Affected Areas
- Where carbonation has already triggered corrosion and spalling, break out to sound concrete, clean the steel, and reinstate with a compatible structural repair mortar in line with AS 3600 and EN 1504 repair principles.
- Anti-Carbonation Protection
- Apply an anti-carbonation coating to the finished surface to slow further CO2 ingress and keep the carbonation front away from the steel.
- Integrate the coating with the building’s wider facade and waterproofing strategy so protection is continuous, not piecemeal.
Why Atomic Projects
- Diagnosis first: We test carbonation depth against real cover rather than guessing from surface appearance, so the treatment matches the actual condition.
- Root-cause focus: Realkalisation and anti-carbonation protection address why the steel is corroding, not just the concrete that has already failed.
- Standards-based: Repairs are carried out to AS 3600 and recognised EN 1504-class repair principles, with proprietary systems applied to specification for warranty validity.
Frequently Asked Questions
How do we know if our building has a carbonation problem?
Often the first clue is cracking that follows the line of the reinforcement, fine rust staining, or isolated spalling on soffits, balcony edges, and facades. But carbonation can be well advanced before any of that shows. The only reliable way to know is a phenolphthalein depth test compared against cover to the steel — that tells us whether the carbonation front has reached the bars or is still safely short of them. For older concrete buildings, this is worth checking as part of routine condition assessment rather than waiting for symptoms.
Is carbonation the same thing as concrete cancer?
They are closely related but not identical. Carbonation is one of the mechanisms that causes reinforcement corrosion; concrete cancer (spalling) is the visible damage that results once that corrosion is underway. Treating carbonation early — while the steel is still passive — is what stops you ending up with widespread spalling later. If spalling has already started, that is addressed through our concrete spalling and steel corrosion repair work.
Can carbonated concrete be treated without replacing it?
In many cases, yes. If the reinforcement is still sound and the concrete is intact, realkalisation and anti-carbonation coatings can restore a protective environment around the steel without breaking out large areas. Full breakout and reinstatement is only needed where corrosion and spalling have already damaged the concrete. This is exactly why early diagnosis is valuable — it keeps the works in the protection category rather than the reconstruction category.
How long does anti-carbonation protection last?
A properly specified and applied anti-carbonation coating system is designed to provide many years of service and can be maintained and recoated over time as part of a building’s cyclical maintenance plan. As an indicative guide, anti-carbonation and protective coatings typically require re-coating every 10–15 years depending on exposure and the specified system, with the exact interval confirmed against the specified manufacturer system and substrate condition. We set out the expected maintenance cycle in the scope so the owners corporation can budget for it.
Does this need a strata committee decision?
Carbonation treatment usually forms part of planned capital works or a facade maintenance programme, which in most schemes requires a general meeting resolution above the relevant spending threshold. Where testing reveals active corrosion that presents a safety concern, urgent works can often be authorised by the committee under urgent maintenance provisions. We provide condition assessment reports and scopes of works to support committee decisions and tendering.
Related Services
Carbonation is a slow, quiet process — and that is exactly what makes it dangerous. By the time the concrete is falling, the corrosion has been running for years. Treating it early, while the steel is still passive, is the most cost-effective concrete decision an owners corporation can make.
As a Class 2 Registered Builder with over 10 years of experience in remedial works across Sydney, Atomic Projects delivers carbonation diagnosis and treatment grounded in real repair practice and specified to recognised standards. Call us on 0410 515 509 or email hello@atomicprojects.com.au to arrange an assessment.
— Ben Tran, General Manager, Atomic Projects
