Concrete Does Not Last Forever: Understanding Carbonation, Chloride Attack, and ASR

Priya had managed the same commercial building on Adelaide Street for eleven years. In that time, she had dealt with leaking roofs, failing lifts, and a heritage facade dispute that ran for two years. Concrete was not something she thought about. It was just there: solid, grey, permanent.

Then the car park ceiling started dropping chunks.

The first piece was about the size of a dinner plate. The second, a week later, was bigger. By the time she called an engineer, there were six impact marks on the floor below and a growing sense that the building had been keeping a secret for a long time.

The secret, it turned out, had a name. Actually, it had three possible names. And working out which one applied would determine everything that happened next.

The Assumption That Concrete Is Permanent

Concrete is the most widely used construction material on the planet. It is strong in compression, relatively cheap to produce, and when well-designed, genuinely durable. But it is not inert. From the day it is poured, a slow chemical conversation begins between the concrete and its environment. In most cases, that conversation is quiet and manageable. In some cases, it ends badly.

The three mechanisms responsible for the majority of concrete deterioration in Australian buildings are carbonation, chloride-induced corrosion, and alkali-silica reaction (ASR). Each operates differently, produces different symptoms, and demands a different response. Treating them as interchangeable is one of the more expensive mistakes a building owner can make.

Mechanism One: Carbonation

Fresh concrete is highly alkaline, with a pH typically above 12.5. This alkalinity is not incidental. It creates a passive oxide layer on the steel reinforcement embedded within the concrete, protecting it from corrosion the way a coat of paint protects bare metal. As long as that alkalinity is maintained, the steel sits quietly inside the concrete and does its job.

Carbonation is the process by which atmospheric carbon dioxide diffuses into the concrete and reacts with calcium hydroxide to form calcium carbonate. This reaction is chemically straightforward and physically slow, but its consequence is significant: it reduces the pH of the concrete, typically to below 9. At that level, the passive layer on the reinforcement breaks down. Corrosion begins.

The rate at which carbonation progresses depends on concrete quality, cover depth, and exposure conditions. In well-compacted, low water-cement ratio concrete, the carbonation front might advance only a millimetre or two per year. In older, higher water-cement ratio mixes, or in concrete that was poorly cured, the front can move considerably faster. Inner-city commercial buildings from the 1960s through the 1980s are particularly susceptible, partly because mix designs of that era prioritised workability over durability, and partly because atmospheric CO2 concentrations in urban environments are higher than in rural ones.

The visual symptoms of carbonation-induced corrosion are familiar: rust staining on the concrete surface, longitudinal cracking along reinforcement lines, and eventually spalling as the expanding corrosion products force the cover concrete away. By the time spalling is visible, the process has been underway for years.

How Carbonation Is Detected

The standard field test for carbonation is phenolphthalein indicator solution. When applied to a freshly broken or drilled concrete surface, phenolphthalein turns pink-purple in zones where pH remains above approximately 9, and stays colourless in carbonated zones. The boundary between the two zones is the carbonation front.

Measuring the depth of that front relative to the cover depth over the reinforcement tells you something important: how much protection remains. A carbonation depth of 20mm in a structure with 40mm cover means the steel is still protected. The same carbonation depth in a structure with 15mm cover means the passive layer has already been compromised.

This is not a difficult test to run, but the interpretation requires judgment. A single core tells you about one location. A systematic grid of cores across a facade or soffit tells you about the pattern of deterioration, which is where the real information lies.

Mechanism Two: Chloride Attack

If carbonation is the slow drift of urban air into concrete, chloride attack is the ocean asserting itself. Chloride ions, whether from seawater, sea spray, or deicing salts, penetrate concrete through diffusion and capillary absorption. When chloride concentrations at the reinforcement level exceed a threshold, typically around 0.4 percent by mass of cement for ordinary reinforcing steel, the passive oxide layer breaks down and corrosion initiates.

Unlike carbonation, which produces a relatively uniform front, chloride penetration is more variable. It concentrates in cracks, at construction joints, in areas of low cover, and wherever the concrete surface has been damaged. The result is often pitting corrosion rather than the uniform surface corrosion associated with carbonation, and pitting corrosion can be more aggressive in terms of section loss per unit time.

Coastal Queensland and New South Wales have produced some of the most instructive examples of chloride-induced deterioration in the country. Marine structures, waterfront boardwalks, and buildings within a few hundred metres of the ocean face a fundamentally different exposure environment than their inland counterparts. The Marina Mirage assessment on the Gold Coast is a case that illustrates this well: a 37-year-old boardwalk structure with 120 piles, where chloride profiling revealed that the deterioration was not uniform across the structure and that a blanket remediation approach would have been both technically unnecessary and financially unjustifiable. The full details are at [/preview/trsc/projects/marina-mirage](/preview/trsc/projects/marina-mirage).

How Chloride Penetration Is Measured

Chloride profiling involves drilling cores or taking dust samples at incremental depths, typically at 5mm or 10mm intervals, and submitting them for laboratory analysis. The result is a chloride concentration profile: a curve showing how chloride content varies from the surface inward.

From this profile, engineers can calculate the apparent diffusion coefficient for the concrete, which describes how quickly chlorides are moving through the material. Combined with the current chloride front depth and the cover depth, this allows a service life projection: how many years until chloride concentrations at the reinforcement level reach the corrosion threshold.

This is genuinely useful information. It converts a visible symptom into a timeline, and a timeline into a budget. A structure with chloride concentrations approaching threshold at the reinforcement level needs attention now. A structure where the chloride front is still 15mm short of the reinforcement, in concrete with a low diffusion coefficient, may have another decade before intervention is warranted. Those are very different conversations to have with a building owner or a strata committee.

NATA-accredited laboratory testing is the standard for this work. Results from non-accredited sources should be treated with caution, particularly when they are being used to justify significant expenditure.

Mechanism Three: Alkali-Silica Reaction

ASR is the least well-known of the three mechanisms among building owners, and in some ways the most insidious. It is a chemical reaction between the alkalis in cement paste and certain reactive silica minerals present in the aggregate. The reaction produces a hygroscopic gel that absorbs water and expands, generating internal pressures that can crack the concrete from within.

The characteristic symptom of ASR is a map cracking pattern on the concrete surface, sometimes called crazing or crocodile cracking, often accompanied by a white gel exudate at crack faces. In severe cases, the expansion causes measurable deformation of structural elements. In infrastructure, particularly bridges, dams, and pavements, ASR has caused significant structural problems. In buildings, it tends to be less dramatic but can still compromise durability and, in some cases, structural capacity.

ASR is not uniformly distributed across Australia. The reactivity of aggregates varies significantly by region and by quarry source. Some aggregate sources that were widely used in Queensland and New South Wales construction during the 1970s and 1980s have since been identified as reactive. Buildings constructed with those aggregates may now be showing the consequences.

The mechanism is also moisture-dependent. ASR requires water to proceed. Structures in permanently dry environments may have reactive aggregates but show little progression. The same structure in a humid coastal environment, or one subject to regular wetting and drying cycles, may deteriorate much faster.

How ASR Is Diagnosed

Field observation of map cracking is suggestive but not diagnostic. Several other mechanisms, including plastic shrinkage cracking and thermal cycling, can produce superficially similar patterns. Definitive diagnosis requires petrographic analysis of concrete cores.

Petrography is the examination of concrete under polarised light microscopy by a specialist concrete petrographer. It can identify reactive aggregate types, the presence of ASR gel, the extent of reaction, and secondary effects such as ettringite formation. It is not a cheap test, but it is the only way to confirm ASR with confidence.

The Victory Hotel investigation in Brisbane provides a useful reference point here. That project involved a 170-year-old building where material science and petrographic analysis were central to understanding what was actually happening within the fabric of the structure, rather than what appeared to be happening from the outside. The case study is at [/preview/trsc/projects/victory-hotel](/preview/trsc/projects/victory-hotel).

For ASR, there is no simple remediation in the way that there is for carbonation or chloride attack. You cannot remove the reactive aggregate. Management strategies focus on controlling moisture ingress, monitoring expansion rates, and assessing structural implications if deformation is occurring. In some cases, surface coatings or crack injection can slow progression. In others, structural assessment of affected elements is the more pressing priority.

Why Getting the Diagnosis Right Matters

These three mechanisms can coexist in the same structure. A coastal commercial building might have carbonation-induced corrosion on its sheltered faces and chloride-induced corrosion on its exposed faces, with ASR contributing to surface cracking that accelerates both. Treating only one mechanism while ignoring the others produces incomplete results.

More importantly, the treatment for each mechanism is different. Carbonation-induced corrosion that has not yet reached the reinforcement may be managed with surface coatings that slow further CO2 ingress. Chloride-induced corrosion may require electrochemical chloride extraction, cathodic protection, or localised patch repairs depending on the extent and pattern of attack. ASR requires a moisture management strategy and ongoing monitoring.

The case of 12 Creek Street in Brisbane is instructive in the other direction. Chloride and carbonation testing on that external wall demonstrated that the concrete was not in the condition that a visual inspection suggested, and that the proposed remediation programme was not warranted by the actual condition data. The investigation saved the building owner a significant sum by providing evidence that contradicted the remediation contractor's assessment. That case study is at [/preview/trsc/projects/12-creek-street](/preview/trsc/projects/12-creek-street).

This is the core of what systematic testing provides: not just a description of what is wrong, but a quantified picture of how far the deterioration extends and how severe it actually is. Without that data, remediation contractors price for the worst case. With it, building owners can make decisions based on evidence.

What Testing Actually Looks Like

For a typical commercial building investigation, the testing programme for concrete deterioration might include:

• Phenolphthalein carbonation testing: on cores or drilled holes at representative locations across the structure, with measurements at a grid spacing determined by the size and complexity of the asset
• Chloride profiling: at locations selected to capture the range of exposure conditions, including sheltered and exposed faces, areas near expansion joints, and zones where visual deterioration is already apparent
• Half-cell potential mapping: to identify zones of active corrosion in reinforced concrete, providing a spatial picture of corrosion activity that complements the chemical data
• Cover depth surveys: using Ferroscan or GPR to establish the actual cover depth distribution across the structure, which is the other half of the equation when interpreting carbonation and chloride data
• Petrographic analysis: where map cracking or other symptoms suggest ASR may be a contributing factor

The results from these tests are not individually definitive. They are pieces of a picture. The engineer's job is to assemble that picture, identify what it means for the structural condition of the asset, and translate it into decisions: what needs attention now, what can be monitored, what can wait, and what the consequences of waiting are.

The Practical Takeaway

Concrete deterioration is not a crisis that arrives without warning. It is a process that unfolds over years or decades, and it leaves evidence at every stage. The buildings that end up with falling car park ceilings and emergency make-safe works are almost never buildings where the deterioration was genuinely unpredictable. They are buildings where the warning signs were not read, or not read correctly.

For building owners, the practical implication is straightforward. If your structure is more than twenty years old, is located within a few kilometres of the coast, or is showing any of the visual symptoms described here, a condition assessment that includes targeted material testing is a reasonable investment. Not because remediation is necessarily required, but because knowing what you are dealing with is always better than not knowing.

For engineers and construction professionals, the implication is about the quality of the diagnosis. Visual inspection identifies symptoms. Testing identifies mechanisms. And mechanisms determine treatment. Skipping from symptom to treatment without the diagnostic step in between is how buildings end up with remediation programmes that address the wrong problem.

TRSC works with building owners and asset managers across Queensland, New South Wales, and Victoria to investigate concrete deterioration through systematic testing, root-cause analysis, and evidence-based recommendations. More information is available at [https://trsc.com.au](https://trsc.com.au).