Carbonation testing in concrete — what it is and when you need it

Carbonation is one of the slow, quiet processes that decides how long a reinforced concrete structure lasts. It does no visible damage on its own, but it strips away the chemical protection that keeps embedded reinforcement from corroding. By the time the consequences are visible — cracking, spalling, rust staining — the carbonation front has usually been working for years. Carbonation testing tells you where that front has reached before the damage starts. Here is how it is done and when it is worth commissioning.

What carbonation actually is

Fresh concrete is highly alkaline, typically around pH 13. That alkalinity forms a passive oxide layer on the surface of the reinforcement, and that layer is what stops the steel corroding even in a damp environment. Carbonation is the gradual reaction between carbon dioxide in the air and the alkaline compounds in the cement paste. As the reaction progresses, it lowers the pH of the concrete from the surface inwards.

Once the pH near the reinforcement drops below roughly 9, the passive layer breaks down. The steel is no longer protected. If moisture and oxygen are present — and in most UK structures they are — corrosion begins. Carbonation itself does not weaken the concrete. The damage comes afterwards, when corroding steel expands and cracks the cover concrete from within.

How carbonation testing is done

The standard test is simple and quick. A fresh surface of concrete is exposed — either by drilling a small hole, breaking out a small area, or splitting a core — and a phenolphthalein indicator solution is sprayed onto it. Phenolphthalein turns bright pink where the concrete is still alkaline and stays colourless where it has carbonated.

The result is a clear visible boundary. The surveyor measures the distance from the concrete surface to the pink boundary, and that distance is the depth of carbonation at that point. The test is governed by recognised guidance and is normally carried out at several locations across an element so the variation can be understood.

The test is fast, low-cost, and only minimally destructive. It is often run alongside other concrete testing, particularly cover depth measurement, because the two numbers only mean something together.

Reading the results

A carbonation depth on its own tells you very little. The question that matters is how that depth compares with the depth of cover over the reinforcement. If carbonation has reached 15mm and the cover is 40mm, the steel is still protected. If carbonation has reached 35mm and the cover at the same point is 25mm, the carbonation front has already passed the steel and corrosion risk is real.

This is why a carbonation survey should always be read together with cover depth data. A structure with deep, consistent cover can tolerate a surprising amount of carbonation. A structure with shallow or variable cover can be in trouble while the carbonation depth still looks modest. The two datasets together let an engineer estimate how much protective life is left and where the weak points are.

Carbonation also progresses at a predictable, slowing rate over time, so depths measured now can be used to project forward. That projection is approximate, but it is useful for deciding whether a structure needs intervention soon or can be monitored.

When to commission it

Carbonation testing is worth commissioning in a few clear situations:

• Older reinforced concrete structures, particularly those built before modern cover and durability standards, where the original construction quality is uncertain.
• Structures showing early signs of distress — fine cracking along the line of reinforcement, rust staining, or isolated spalling.
• Before refurbishment or change of use, where an engineer needs to know how much service life the existing structure has left.
• As part of a wider condition survey, where carbonation is one of several factors being assessed together.

Sheltered concrete in dry, carbon-dioxide-rich environments — internal car parks, plant rooms, enclosed structures — tends to carbonate faster than people expect, so internal elements are not automatically lower risk than external ones.

What it does not tell you

Carbonation testing identifies the loss of chemical protection. It does not, on its own, tell you whether the reinforcement is already corroding. Where carbonation has reached the steel, the next questions are whether corrosion is active and how far it has progressed, and those are answered by other methods such as half-cell potential mapping and, where needed, breaking out to inspect the bar directly.

It is also a point test. Each reading describes one location. Carbonation depth varies across a structure with exposure, surface finish, and original concrete quality, so a sensible survey takes enough readings to capture that variation rather than relying on one or two.

Practical advice

Carbonation testing is cheap, fast, and genuinely informative — but only when it is paired with cover depth data and interpreted by someone who understands what the combination means. Commissioned on its own, a set of carbonation depths is just numbers. Commissioned as part of a properly scoped condition assessment, it is one of the clearest early indicators of how much durable life a reinforced concrete structure has left, and it gives an engineer the time to plan an intervention rather than react to a failure.