Most reinforcement corrosion comes down to one of two causes: carbonation or chlorides. Knowing which is at work is the key to choosing the right repair.

Why steel in concrete doesn't normally rust

Concrete is highly alkaline, and this forms a protective film on the steel. Carbonation and chlorides both break that film down.

Carbonation

Carbon dioxide from the air slowly lowers the concrete's alkalinity, moving inwards from the surface over time. Once it reaches the steel, corrosion can begin. It's fastest in porous concrete with shallow cover and usually causes cracking and spalling along the line of the bars.

We test it by spraying a fresh concrete surface with phenolphthalein indicator. Healthy concrete turns pink and carbonated concrete stays clear, showing how close the carbonation front is to the steel.

Chlorides

Chlorides attack the steel directly, even in alkaline concrete. They come from de-icing salts, seawater and, in older structures, calcium chloride added to speed up setting. Chloride corrosion causes deep, localised pitting that can seriously weaken bars with little visible warning.

We test it by drilling dust samples at increasing depths to build a chloride profile, showing how far chlorides have penetrated and whether levels at the steel are high enough to cause corrosion.

Why it matters

The cause decides the repair. Carbonation damage can often be treated with patch repairs and coatings, while chloride contamination may mean removing affected concrete well beyond the visible damage. Combined with cover surveys and half-cell mapping, these tests show where corrosion is happening, why, and what to do about it.

For a full picture of how half-cell potential surveys fit into a corrosion investigation, see our article on half-cell potential surveys. For information on the full range of corrosion investigation services, visit our corrosion surveys service page. Where concrete quality is also in question, our UPV testing service can assess compressive strength and internal cracking without damage.

Concrete corrosion testing in London and across the UK

ACUO Structural Insights provides carbonation testing, chloride profiling, cover surveys and half-cell potential mapping across London, Kent, Essex and the South East.

Concerned about concrete deterioration? Call 07340 549892 or email [email protected].