Why Concrete Fails: A Plain-English Guide to Carbonation, Chloride Attack, and ASR

Priya had owned the building for eleven years before anyone mentioned the word carbonation to her. The eight-storey commercial block in inner Brisbane had been a reliable asset, low maintenance, steady tenants, no drama. Then a routine facade inspection flagged rust staining on the soffit of the level-three balcony, and a structural engineer used a word she had never heard before.

She asked what it meant. The engineer explained. She asked what it would cost to fix. The engineer said it depended on how far it had spread and how severe it was. She asked how you find that out. The engineer said: you test.

That conversation is where most building owners enter the world of concrete deterioration. It is rarely a dramatic moment. It is usually a rust stain, a crack, a flake of cover concrete sitting on a balcony tile. But behind that small visible sign, one of three processes has usually been at work for years, sometimes decades.

Understanding those three processes, how they are detected, and what the results actually mean, is the difference between a targeted repair and an open-ended remediation bill.

The Problem With Concrete's Reputation

Concrete has a reputation for permanence that it does not entirely deserve. It is durable, certainly. But it is also porous, chemically reactive, and vulnerable to the environments Australian structures routinely inhabit: coastal salt air, urban CO2, and reactive geology.

The reinforcing steel inside concrete is protected by a thin layer of cover concrete and by the concrete's naturally high alkalinity, which sits around pH 12 to 13. At that pH, steel forms a passive oxide layer that resists corrosion. The three primary deterioration mechanisms all work, in different ways, to undermine either that alkalinity or the physical integrity of the concrete itself. Once the protection is gone, the steel corrodes, expands, and cracks the cover from the inside out.

The visible result, spalling, rust staining, cracking, is the end of a long process. The process itself is invisible until testing reveals it.

Mechanism One: Carbonation

Carbonation is the most common deterioration mechanism in urban Australian buildings, and the most misunderstood.

Atmospheric carbon dioxide reacts with calcium hydroxide in the concrete's cement paste to form calcium carbonate. That reaction is not inherently destructive. The problem is that it reduces the concrete's pH, typically from around 12.5 down toward 8 or 9. At that pH, the passive oxide layer on the reinforcing steel breaks down. Corrosion begins.

The process advances as a front moving inward from the concrete surface. In well-compacted, low water-to-cement ratio concrete, the front moves slowly, perhaps 1 to 2 millimetres per decade. In older concrete, or concrete placed with a higher water content, the front can advance several millimetres per year. The critical question is always whether the carbonation front has reached the steel.

How Carbonation Is Detected

The standard field test uses phenolphthalein indicator solution. A core or drilled sample is taken, the freshly exposed face is sprayed with phenolphthalein, and the result is read visually. Concrete that remains alkaline turns bright pink-purple. Carbonated concrete, where the pH has dropped, stays colourless.

The depth of the colourless zone is the carbonation depth. That measurement is then compared against the cover depth, either measured directly or taken from drawings. If the carbonation front has not reached the steel, the risk is low. If it has reached or passed the steel, corrosion is likely already active.

At TRSC's 12 Creek Street project, chloride and carbonation testing on an inner-city commercial facade produced results that told a different story from what the visible condition suggested. The data showed carbonation depths that had not reached the reinforcement across the majority of the facade. That finding changed the remediation scope entirely, and the evidence supported a decision to monitor rather than immediately repair. The full assessment is documented at [12 Creek Street](/preview/trsc/projects/12-creek-street).

For building owners, the key number from a carbonation test is not the carbonation depth in isolation. It is the ratio of carbonation depth to cover depth. A 20mm carbonation depth in concrete with 40mm cover is a very different situation from the same depth in concrete with 22mm cover.

Mechanism Two: Chloride Attack

If carbonation is the urban deterioration mechanism, chloride attack is the coastal one. It is also, in many ways, the more aggressive of the two.

Chloride ions penetrate concrete through the same capillary pore network that allows carbonation. In coastal environments, airborne sea spray deposits chlorides on exposed surfaces. In marine structures, elements in direct contact with seawater face continuous chloride loading. In some older structures, chlorides were introduced during construction through the use of calcium chloride as an accelerant, a practice now prohibited.

Chlorides do not reduce concrete's alkalinity the way CO2 does. Instead, they attack the passive oxide layer on the steel directly, breaking it down locally and initiating pitting corrosion. Pitting corrosion is particularly damaging because it concentrates attack at discrete points, which can cause significant section loss in the reinforcing bar before any surface sign appears.

The threshold chloride content at which corrosion initiates is typically cited at around 0.4% by weight of cement, though this varies with concrete quality and exposure conditions. Australian Standard AS 3600 sets prescriptive requirements for cover depth and concrete quality in different exposure classifications, precisely because chloride ingress is predictable and manageable when designed for correctly.

How Chloride Attack Is Detected

Chloride profiling involves taking core samples at multiple depths and analysing the chloride concentration at each depth interval, typically in 10mm or 20mm increments. The result is a concentration profile from the surface inward.

That profile serves two purposes. First, it identifies whether chloride concentrations at the depth of the reinforcement have exceeded the corrosion threshold. Second, it allows engineers to model the rate of ingress using Fick's second law of diffusion, which gives a projection of when the threshold will be reached if it has not been already.

This is where the data becomes genuinely useful for asset management. A chloride profile taken today, combined with a projected ingress rate, gives a building owner a defensible estimate of the remaining service life before active corrosion initiates. That is not a guess. It is a calculation based on measured material properties.

TRSC's Marina Mirage assessment involved chloride profiling across a 37-year-old marine boardwalk structure with 120 piles. The profiling data informed a risk classification that allowed the asset owner to prioritise intervention on the piles with the highest chloride concentrations at the steel interface, rather than treating all 120 piles as equally urgent. The project is documented at [Marina Mirage](/preview/trsc/projects/marina-mirage).

The Half-Cell Potential Test

Chloride profiling tells you about the environment the steel is in. Half-cell potential testing tells you about the state of the steel itself. By measuring the electrochemical potential of the reinforcing bar through the concrete cover, engineers can assess the probability that active corrosion is occurring. The technique is standardised under ASTM C876 and is a routine part of any thorough chloride investigation.

The two tests together, profiling for ingress, half-cell for corrosion state, give a complete picture of where the structure is in its deterioration timeline.

Mechanism Three: Alkali-Silica Reaction

ASR is the least well understood of the three mechanisms among building owners, and arguably the most complex to manage.

Alkali-silica reaction is a chemical reaction between alkali hydroxides in the cement paste and certain forms of reactive silica present in some aggregates. The reaction produces a gel that absorbs moisture and expands. That expansion generates internal pressure that cracks the concrete from within, producing a characteristic pattern of map cracking, sometimes called crazing or crocodile cracking, often accompanied by a white gel exudate at crack faces.

ASR does not corrode steel directly. Its primary damage is to the concrete matrix itself, reducing strength, stiffness, and durability. In structures where the cracking then allows chloride or moisture ingress, the secondary effects can be severe.

Australian aggregates vary considerably in their reactivity. Some Queensland and New South Wales aggregate sources have documented ASR histories. The reaction requires three conditions simultaneously: reactive aggregate, sufficient alkali content in the cement, and moisture. Remove any one of those three, and the reaction stops or slows significantly.

How ASR Is Detected

Field identification of ASR begins with the crack pattern. Map cracking that follows no structural logic, often with gel staining at crack edges, is a strong indicator. But visual identification is not diagnosis.

Confirmation requires petrographic analysis of core samples. A petrographer examines thin sections of the concrete under polarised light microscopy, looking for gel deposits in cracks and at aggregate interfaces, reaction rims around aggregate particles, and evidence of aggregate mineralogy consistent with known reactive types.

TRSC's Victory Hotel investigation involved petrographic analysis as part of a broader material science programme on a 170-year-old structure. The analysis provided definitive identification of deterioration mechanisms that visual inspection alone could not have resolved. The full case study is at [Victory Hotel](/preview/trsc/projects/victory-hotel).

For structures where ASR is confirmed, the management question becomes one of moisture control and load assessment. Sealing exposed surfaces to limit moisture ingress, combined with structural monitoring to track crack progression, is often the first response. Major intervention is warranted only when the structural capacity is demonstrably affected.

What the Test Results Actually Mean

Building owners often receive test reports containing numbers and classifications without a clear explanation of what those numbers mean for their building and their budget. That gap between data and decision is where poor outcomes originate.

A carbonation depth of 28mm means nothing without knowing the cover depth. A chloride concentration of 0.6% by weight of cement means nothing without knowing whether that measurement was taken at the steel interface or at the surface. An ASR diagnosis means nothing without a structural assessment of whether the cracking has compromised load-carrying capacity.

The translation from test result to actionable decision requires an engineer who understands both the testing methodology and the structural context. It also requires an approach that resists the default to worst-case assumptions.

TRSC's approach to concrete investigations follows a systematic hierarchy: make the structure safe first, then monitor its behaviour, then investigate root causes with appropriate testing, then remediate based on what the evidence actually shows. Remediation before investigation is how building owners end up replacing concrete that did not need replacing.

The Cost of Not Testing

The alternative to testing is estimation. And estimation in concrete remediation defaults to the conservative end, because contractors pricing without data have no choice but to assume the worst.

A facade with visible rust staining might have active corrosion at 15% of the reinforcement nodes or 80%. Without testing, a remediation contractor cannot know. With testing, an engineer can map the extent and severity of active corrosion, identify which areas require immediate intervention, and identify which areas can be monitored and addressed in a later phase.

The difference between those two approaches is not marginal. On a mid-size commercial building, the gap between a worst-case remediation scope and a tested, evidence-based scope can run to hundreds of thousands of dollars. The testing programme that enables that distinction typically costs a fraction of the saving.

That is not an argument for testing as an end in itself. It is an argument for understanding what you are actually dealing with before committing to a course of action.

A Starting Point

Priya's building, back to where we started, turned out to have carbonation depths that had reached the steel in two discrete areas on the level-three balcony soffit. The rest of the facade showed carbonation at depths well short of the reinforcement. The repair scope was targeted, the cost was defined, and the timeline was planned around the body corporate's budget cycle rather than driven by an emergency.

None of that was possible without the testing. The rust stain alone told her something was wrong. The phenolphthalein test told her where, and how far, and what it would take to address it.

That is what concrete investigation is for. Not to produce a report. To produce a decision.

For building owners and engineers dealing with concrete deterioration in Queensland, New South Wales, or Victoria, TRSC provides structural investigation programmes that translate test data into actionable recommendations. More information is available at [trsc.com.au](https://trsc.com.au).