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

Concrete is not inert. From the day it is placed, it begins reacting with its environment. Those reactions are slow, often invisible, and frequently misunderstood until the damage is already advanced. Three mechanisms account for the majority of premature concrete deterioration in Australian buildings and infrastructure: carbonation, chloride-induced corrosion, and alkali-silica reaction. Each follows a different pathway, produces different symptoms, and demands a different investigative response.

Understanding how these processes work is not an academic exercise. It directly affects how much remediation costs, when it needs to happen, and whether targeted intervention is possible or wholesale replacement becomes unavoidable.

Carbonation: The Slow Neutralisation of Inner-City Concrete

Fresh concrete has a highly alkaline pore solution, with a pH typically above 12.5. That alkalinity is what protects embedded steel reinforcement. A passive oxide layer forms on the steel surface in this environment, and corrosion cannot take hold while the pH remains high.

Carbonation changes that. Atmospheric carbon dioxide diffuses into the concrete and reacts with calcium hydroxide in the cement paste to form calcium carbonate. The process is not destructive to the concrete matrix itself, but it progressively lowers the pH of the pore solution. Once the carbonation front reaches the depth of the reinforcement and pH drops below approximately 9, the passive layer breaks down. Corrosion begins.

The rate of carbonation follows a square-root-of-time relationship. Early progress is faster; it slows as the carbonated zone thickens and CO2 must diffuse further. In practice, carbonation depth in a 40-year-old inner-city commercial building might be 15 to 30 mm depending on concrete quality, cover depth, and exposure conditions. Buildings constructed in the 1960s and 1970s, when water-to-cement ratios were higher and cover requirements less stringent, are particularly susceptible.

Sheltered facades, car park soffits, and internal structural elements are common sites. Outdoor surfaces that are regularly wetted and dried tend to carbonate more slowly than sheltered ones because moisture in the pores impedes CO2 diffusion.

How Carbonation Is Detected

The standard field test uses a phenolphthalein indicator solution sprayed onto a freshly broken or cored concrete surface. Uncarbonated concrete turns bright pink or purple. Carbonated concrete remains colourless. The boundary between the two zones marks the carbonation front, and its depth is measured directly.

This test is simple and low-cost, but it is a snapshot. Combining phenolphthalein testing with cover meter surveys and half-cell potential mapping gives a far more complete picture: where the carbonation front is, how much cover remains between it and the steel, and whether corrosion activity is already present. For a building owner, the key number is the difference between carbonation depth and actual cover depth. If cover is 25 mm and carbonation has reached 22 mm, intervention is overdue.

Chloride Attack: The Coastal and Marine Threat

Chloride ions do not lower concrete's pH. They attack the passive oxide layer on steel directly, disrupting it locally and allowing corrosion to initiate even in highly alkaline concrete. This is why chloride-induced corrosion can be more aggressive than carbonation-induced corrosion, and why it is the dominant deterioration mechanism for structures within a few kilometres of the coast.

Chlorides reach reinforcement by two pathways. External chlorides, from seawater splash, airborne salt spray, or de-icing salts, diffuse inward through the concrete cover. Internal chlorides are incorporated during construction, typically from contaminated aggregates or mixing water, though this is far less common in modern construction.

Australia's coastline creates extensive exposure. Structures in the tidal and splash zones of ports, wharves, and coastal car parks face the most severe conditions. The Queensland coast, with its warm temperatures accelerating ionic diffusion, compounds the problem. A reinforced concrete wharf structure in Moreton Bay or along the Gold Coast waterway network faces chloride exposure that a comparable inland structure simply does not.

Corrosion products from chloride attack occupy a greater volume than the original steel. The expansive pressure cracks the concrete from within, producing the characteristic rust staining, longitudinal cracking along reinforcement lines, and eventually delamination and spalling.

How Chloride Ingress Is Measured

Chloride profiling involves extracting concrete dust samples at incremental depths, typically 10 mm intervals from the surface to beyond the reinforcement level, and submitting them to a NATA-accredited laboratory for acid-soluble chloride content analysis per AS 1012.20. The results produce a chloride concentration profile through the cover zone.

Fitting a diffusion model to that profile, using Fick's second law of diffusion, allows an engineer to back-calculate the surface chloride concentration and the apparent diffusion coefficient for that particular concrete. From those parameters, it is possible to project when the chloride threshold at the reinforcement depth will be reached, or to determine whether it already has been.

The threshold for corrosion initiation is typically cited as 0.4% chloride by mass of cement, though this varies with cement type and concrete chemistry. For a building owner, the practical output of this analysis is a remaining service life estimate and a ranked list of locations by risk severity, which is exactly the kind of data needed to plan phased maintenance rather than reacting to each new spall as it appears.

Alkali-Silica Reaction: The Expansion from Within

Alkali-silica reaction, commonly called ASR, is a chemical reaction between alkali hydroxides in the cement paste and certain forms of reactive silica present in some aggregates. 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 internal and distributed throughout the concrete mass.

The result is a characteristic map cracking or crazing pattern on the surface, sometimes accompanied by a white gel exudate at cracks. In severe cases, the concrete expands enough to cause structural distortion, closing of joints, and spalling at edges and corners.

ASR requires three conditions simultaneously: reactive silica in the aggregate, sufficient alkali content in the cement paste, and moisture. Remove any one of those and the reaction stops or does not initiate. This is why ASR tends to be a problem in infrastructure exposed to wetting and drying rather than in well-drained or protected elements.

In Australia, ASR has been documented in a range of infrastructure assets including bridges, dams, and pavements. Certain Queensland and New South Wales aggregate sources have been identified as potentially reactive. Structures built before the current Australian Standard AS 3600 and the guidance in AS 1141.60 series addressed reactive aggregate selection are at higher risk.

Detecting and Characterising ASR

Field identification starts with the visual pattern: map cracking that does not follow structural stress patterns, gel deposits, and surface discolouration. But visual assessment alone cannot confirm ASR or quantify its severity.

Petrographic examination of concrete cores by a specialist petrographer is the definitive diagnostic tool. Under polarised light microscopy, the analyst can identify reactive aggregate types, observe gel deposits within cracks and voids, assess the extent of reaction, and distinguish ASR from other deterioration mechanisms that produce similar surface cracking, such as drying shrinkage or delayed ettringite formation.

Expansion testing on cores, per ASTM C1293 or the accelerated mortar bar test, can confirm reactivity. Structural monitoring using precision survey or embedded sensors can quantify ongoing movement rates, which is critical for deciding whether the reaction is still active or has largely run its course.

For a building owner or asset manager, the distinction matters enormously. An ASR-affected bridge that has consumed most of its reactive silica and is no longer expanding requires a different management response than one still in active expansion. Monitoring resolves that question with measured data rather than assumptions.

What the Three Mechanisms Have in Common

All three processes are time-dependent, progressive, and detectable before they become structurally threatening. None of them are visible in their early stages. By the time spalling, cracking, or rust staining is apparent, the underlying process has typically been active for years or decades.

This is why the sequence of investigation matters. A condition assessment that records visible defects is a starting point, not a conclusion. The next step is quantifying the extent and severity of the underlying process: how deep has carbonation penetrated, what is the chloride concentration at reinforcement depth, is ASR still active. Without that data, a remediation scope is a guess, and contractors will price accordingly.

The approach that produces defensible, cost-effective outcomes starts with making the structure safe, then gathering the evidence needed to understand what is actually happening. Remediation designed around measured data consistently costs less than remediation designed around worst-case assumptions.

What Building Owners Should Do

If a concrete structure is more than 20 years old, in a coastal or humid environment, or showing any surface distress, a targeted investigation is warranted. That investigation should include:

• Carbonation depth testing using phenolphthalein indicator on cores or break-outs
• Cover depth survey using a calibrated cover meter
• Chloride profiling at representative locations, with laboratory analysis
• Half-cell potential mapping to assess corrosion activity
• Petrographic examination if ASR is suspected

The output should be a condition classification by location and mechanism, a remaining service life estimate where the data supports one, and a prioritised remediation scope tied to evidence rather than visual impression alone.

For Queensland structures, Form 15 certification requirements and the obligations under the Building Act 1975 add a compliance dimension to what is also a safety and asset management question.

TRSC works with building owners, strata managers, and asset managers across Queensland, New South Wales, and Victoria to investigate concrete deterioration, quantify its extent, and develop remediation strategies grounded in measured data. For more information, visit [https://trsc.au](https://trsc.au).