Corrosion of steel reinforcement is one of the biggest threats to concrete structures, and one of the hardest to see. By the time rust staining, cracking or spalling appears on the surface, corrosion has usually been active for years. A half-cell potential survey helps find it much earlier, so repairs can be planned before damage spreads.

What is a half-cell potential survey?

Steel reinforcement in healthy concrete is protected by the alkaline environment around it. When that protection breaks down, usually through carbonation or chloride contamination, the steel can begin to corrode. Corroding steel has a different electrical potential from protected steel, and that difference can be measured from the surface.

In a half-cell survey, a connection is made to the reinforcement at one small point. A reference electrode, the "half-cell", is then placed on the concrete surface at regular grid points, and the potential between the steel and the electrode is recorded at each one. The readings are plotted as a colour-coded contour map showing where corrosion is most likely to be active.

How are the results interpreted?

ASTM C876 gives general guidance on reading the results. Using a copper/copper sulphate reference electrode, readings more positive than -200mV suggest a high probability that no corrosion is taking place. Readings more negative than -350mV suggest a high probability of active corrosion. Readings in between are uncertain.

In practice, interpretation needs more care than simple thresholds. Moisture, concrete cover, coatings and temperature all affect the readings. That's why UK guidance, including Concrete Society Technical Report 60, places emphasis on the pattern of the results: steep changes in potential across a small area are often a stronger sign of active corrosion than any single value.

Why combine it with other tests?

A half-cell survey shows where corrosion is likely. It doesn't explain why. That is why we combine it with:

• Cover surveys, because shallow cover leaves steel vulnerable
• Carbonation testing, to see whether carbonation has reached the steel
• Chloride profiling, to measure chloride levels at increasing depths
• Targeted break-outs, to inspect the reinforcement directly and confirm what the survey shows

Together these give a reliable picture of the extent, cause and severity of corrosion.

When do you need one?

Half-cell surveys are particularly valuable for:

• Multi-storey car parks, where de-icing salts carried in by vehicles drive chloride-induced corrosion
• Bridges and highway structures
• Coastal and marine structures
• Balconies and exposed concrete frames
• Condition assessments before acquisition, refurbishment or change of use
• Planning and prioritising concrete repair programmes

What do you get?

A good survey report should include the half-cell contour maps, supporting test results, and clear commentary on which areas are at highest risk and should be dealt with first. The goal is to target repairs where they're needed, rather than replacing large areas of sound concrete unnecessarily.

Half-cell potential surveys in London and across the UK

ACUO Structural Insights carries out half-cell potential surveys and full corrosion investigations for car parks, bridges and concrete structures in London, Kent, Essex and the South East.

For information on the full range of corrosion investigation services, visit our corrosion surveys service page. To understand the underlying causes of reinforcement corrosion, see our article on carbonation and chloride attack. Where concrete quality assessment is also needed, our UPV testing service can be carried out alongside half-cell mapping.

Need a corrosion survey? Call 07340 549892 or email [email protected].