Concrete Does Not Last Forever: Understanding Carbonation, Chloride Attack, and ASR
Priya had owned the Fortitude Valley office block for eleven years before anyone mentioned the word carbonation. The building was 1970s construction, solid enough on the surface, and the spalling on the lower carpark columns had been patched twice by a local concreter. Each time, the patches held for eighteen months or so before cracking again. When she finally called a structural engineer, the answer was not what she expected.
The patches were failing because nobody had asked why the concrete was deteriorating in the first place.
This is not an unusual story. Concrete is the most widely used construction material in Australia, and it carries a reputation for permanence that its chemistry does not fully support. Given the right conditions, three mechanisms will degrade concrete from the inside: carbonation, chloride-induced corrosion, and alkali-silica reaction. Each operates differently, each produces different symptoms, and each demands a different response. Treating them interchangeably is how remediation budgets blow out.
Why Concrete Corrodes at All
Reinforced concrete works because steel and concrete have complementary properties. Steel carries tension; concrete carries compression. The concrete also provides a chemical shield for the steel. Fresh concrete has a pH of around 12.5 to 13, and at that alkalinity, a passive oxide layer forms on the steel surface and resists corrosion almost indefinitely.
The three deterioration mechanisms described below all attack this protection in different ways. Two of them destroy the alkaline environment. One bypasses it entirely by expanding the concrete matrix itself. Understanding which mechanism is active in a given structure determines every decision that follows.
Carbonation: The Slow Advance from the Surface
Carbonation is the most common deterioration mechanism in Australian urban buildings, particularly structures built between 1950 and 1985 when concrete cover depths were often specified at 20 to 25 mm rather than the 40 mm or more required by modern standards.
The chemistry is straightforward. Atmospheric carbon dioxide diffuses into the concrete pore network and reacts with calcium hydroxide to form calcium carbonate. This reaction reduces the pH of the concrete from around 12.5 down to 8 or 9. At that pH, the passive oxide layer on the steel breaks down. Corrosion begins, rust expands in volume, and the concrete cracks and spalls.
Carbonation advances as a front, moving inward from the exposed surface. The rate depends on concrete porosity, water-cement ratio, and environmental humidity. Concrete with a high water-cement ratio, common in older construction, carbonates faster. The process is also accelerated in sheltered locations where the concrete stays dry enough to allow CO2 diffusion but wet enough to sustain the reaction.
This is why Priya's carpark columns were failing. The lower levels were sheltered from rain, which meant the concrete dried enough to allow carbonation to advance, but the humidity from vehicle movement and ground moisture was sufficient to drive the electrochemical corrosion once the passive layer was gone.
How Carbonation Is Detected
The standard field test uses phenolphthalein indicator solution. A fresh concrete break is sprayed with the indicator, which turns pink or purple in alkaline concrete (pH above 9.5) and remains colourless in carbonated zones. The depth of the colourless zone is measured directly.
This is a simple test, but its interpretation requires care. A carbonation depth of 18 mm in a column with 20 mm of cover means the steel is already at risk. The same depth in a column with 50 mm of cover means the structure has decades of remaining service life. The number only has meaning when set against the actual cover depth, which requires either as-built drawings or electromagnetic cover measurement using a Ferroscan or equivalent device.
In Priya's building, phenolphthalein testing showed carbonation depths averaging 28 mm. Cover measurement confirmed steel at 22 to 25 mm depth. The steel had been exposed to carbonated concrete for years. The patches had been applied over active corrosion without treating the underlying chemistry.
Chloride Attack: The Coastal Threat That Travels Inland
Chloride-induced corrosion is the dominant deterioration mechanism in marine and coastal structures, but it is not limited to them. Chlorides enter concrete from seawater, sea spray, tidal splash, and in some inland environments, from deicing salts or contaminated groundwater.
Unlike carbonation, chloride attack does not destroy the alkaline environment. Instead, chloride ions penetrate the pore network and, once they reach a critical threshold concentration at the steel surface, they break down the passive oxide layer directly. Corrosion initiates in localised pits rather than across a broad front, which makes it harder to detect visually until significant section loss has occurred.
The critical chloride threshold for embedded steel is generally taken as 0.4% by mass of cement, though this varies with concrete quality and the electrochemical conditions at the steel surface. Once corrosion initiates, the expansion of rust products generates internal tensile stresses that crack and delaminate the cover concrete.
Marina Mirage on the Gold Coast is a case that illustrates the complexity of chloride assessment in marine infrastructure. TRSC conducted a condition assessment of the 120-pile boardwalk structure, which was 37 years old at the time of inspection. The challenge was not identifying that chlorides were present, they obviously were, but determining how far they had penetrated and at what concentration. Without that data, any remediation scope would have been a guess. You can read more about that project at [/preview/trsc/projects/marina-mirage](/preview/trsc/projects/marina-mirage).
How Chloride Penetration Is Measured
Chloride profiling involves extracting concrete cores or dust samples at incremental depths, typically 10 mm intervals, and submitting them for acid-soluble chloride analysis at a NATA-accredited laboratory. The results produce a chloride concentration profile from the surface inward.
This profile serves two purposes. First, it shows whether chloride concentrations at the current steel depth exceed the critical threshold, confirming whether corrosion has initiated. Second, the shape of the profile can be fitted to a diffusion model (typically Fick's second law) to project how long before chlorides reach critical levels at greater depths. This is how engineers estimate residual service life, not by looking at the surface, but by modelling the chemistry.
Half-cell potential mapping is used alongside chloride profiling to assess the probability of active corrosion at the steel surface. Readings more negative than minus 350 mV (CSE) indicate a greater than 90% probability of active corrosion per ASTM C876. Combined with the chloride data, this gives a clear picture of where corrosion is active and where it is not yet initiated.
The practical value of this approach is that it identifies which sections of a structure need immediate intervention and which can be monitored. In a large marine structure, the difference between treating 30% of the piles and treating 100% of them is a significant cost difference, and the data is what makes that distinction defensible.
Alkali-Silica Reaction: The Expansion from Within
ASR is less common than carbonation or chloride attack, but it is arguably the most destructive when it occurs, and it is the most frequently misdiagnosed. It produces a characteristic map cracking pattern on concrete surfaces that is sometimes mistaken for shrinkage cracking or thermal movement.
The mechanism involves a reaction between alkalis in the cement paste (sodium and potassium hydroxides) and certain reactive silica minerals in the aggregate. The reaction produces an alkali-silica gel that absorbs water and expands. Because the gel forms within the aggregate particles and at aggregate-paste interfaces, the expansion is distributed through the concrete matrix. The result is internal tensile stresses that crack the concrete in a three-dimensional network.
ASR requires three conditions simultaneously: reactive aggregate, sufficient alkali content in the cement, and moisture. Remove any one of these and the reaction stops or never initiates. This is why ASR is often found in structures exposed to wetting and drying cycles, where moisture supply is sustained over decades.
In Australian infrastructure, ASR has been documented in dams, bridges, and pavements, particularly in regions where locally sourced aggregates contain reactive silica phases such as chert, opaline silica, or strained quartz. The problem is that reactive aggregates were used extensively before ASR was well understood, and structures built in the 1960s through 1980s are now reaching the stage where the reaction has had decades to develop.
How ASR Is Confirmed
Visual inspection can suggest ASR, particularly when map cracking is accompanied by gel exudation or a white crystalline residue at crack faces. But visual evidence alone is not sufficient for a diagnosis, because the same surface pattern can result from other causes.
Confirmation requires petrographic analysis of concrete cores at a NATA-accredited laboratory. A petrographer examines thin sections under polarised light microscopy, looking for reactive aggregate types, gel deposits in cracks and voids, and the characteristic reaction rims around aggregate particles. This is the same approach used in the Victory Hotel investigation, where petrographic analysis was central to understanding the material condition of a 170-year-old structure. Details of that project are at [/preview/trsc/projects/victory-hotel](/preview/trsc/projects/victory-hotel).
Ultrasonics (UPV testing) can also be used to assess the degree of internal cracking. A reduction in pulse velocity relative to baseline or to unaffected sections indicates that the concrete matrix has been disrupted, though UPV alone cannot distinguish ASR from other forms of internal damage.
Once ASR is confirmed, the engineering question shifts to rate of progression and structural consequence. Some structures with ASR remain serviceable for decades with appropriate monitoring. Others require intervention to control moisture access or, in severe cases, to address section loss and reinforcement corrosion that has developed as a secondary consequence of the cracking.
What the Test Results Actually Mean
This is where the gap between investigation and decision-making most often appears. A laboratory report showing a chloride concentration of 0.6% at 30 mm depth, or a phenolphthalein test showing 25 mm carbonation, does not by itself tell a building owner what to do. The data needs to be interpreted against the specific geometry, loading, and service life expectations of the structure.
The approach TRSC uses is to quantify not just whether a defect is present, but how far it extends and how severe it actually is across the structure. This is what we call addressing the extent and severity gap. A condition assessment that identifies spalling on a carpark column is a starting point. A condition assessment that maps carbonation depth against cover depth across every column, identifies which ones have active corrosion and which do not, and produces a risk-ranked remediation scope is what enables a building owner to make a budget decision grounded in evidence.
The 12 Creek Street external wall assessment is an example of this in practice. Chloride and carbonation testing across the facade produced data showing that the concrete was not at risk, and that proposed remediation was unnecessary. The investigation paid for itself many times over by avoiding work that the evidence did not support. That project is documented at [/preview/trsc/projects/12-creek-street](/preview/trsc/projects/12-creek-street).
The Decision Framework After Testing
Once the mechanism is identified and the severity quantified, the decision framework follows a logical sequence.
For carbonation: Is the carbonation front at or beyond the steel? If yes, is corrosion active (confirmed by visual evidence or electrochemical testing)? If corrosion is active, what is the extent of section loss and is structural capacity affected? The answers determine whether intervention is urgent or whether monitoring with defined trigger levels is appropriate.
For chloride attack: Are chloride concentrations at the steel above the critical threshold? Is corrosion active per half-cell potential mapping? What is the projected time to critical chloride concentration at the next layer of steel? These answers determine the urgency of cathodic protection, electrochemical chloride extraction, or conventional patch repair with a barrier coating.
For ASR: Is the reaction still active (assessed by gel fluorescence testing or expansion monitoring)? What is the structural consequence of the cracking pattern? Is reinforcement corrosion a secondary concern? The answers determine whether moisture control, structural monitoring, or reinforcement intervention is required.
In each case, the sequence is the same: make the structure safe if there is an immediate risk, then monitor to gather evidence before committing to remediation that may not be warranted.
A Note on Timing
Concrete deterioration is not an emergency until it is. Carbonation advances at roughly 1 mm per year in average urban conditions, which means a building with 25 mm cover has perhaps 25 years before the steel is at risk, assuming carbonation begins at construction. But that window is often already partly consumed by the time anyone looks, and the rate accelerates in porous concrete.
The structures most at risk in Australia right now are those built between 1960 and 1985, when cover depths were lower, water-cement ratios were higher, and coastal construction was expanding rapidly. Many of these buildings have never had a systematic condition assessment. Their owners are making maintenance decisions based on what they can see, which is almost never the full picture.
If you own or manage a concrete structure built before 1990, particularly one in a coastal environment or with a history of patch repairs, the question is not whether deterioration is occurring. It is which mechanism is active, how far it has progressed, and what the evidence says about timing.
TRSC works with building owners, asset managers, and construction professionals across Queensland, New South Wales, and Victoria to answer those questions with data rather than assumptions. More information is available at [https://trsc.com.au](https://trsc.com.au).