Half-cell potential testing — detecting rebar corrosion

When reinforcement inside concrete starts to corrode, the damage stays hidden until it is well advanced. Cracking and spalling appear late, often years after corrosion has begun. Half-cell potential testing is the standard non-destructive method for finding active corrosion before it reaches that visible stage. It does not measure how much steel has been lost — it identifies where corrosion is happening, which is exactly the information an engineer needs to plan an investigation or a repair. Here is how it works and how to read the results.

What the test measures

Corroding steel behaves electrochemically. Where reinforcement is actively corroding, it sets up a measurable electrical potential relative to the surrounding concrete. Half-cell potential testing measures that potential across the surface of an element and uses it to map where corrosion is and is not occurring.

The method is governed by recognised standards and is well established across bridges, car parks, marine structures, and any reinforced concrete exposed to the conditions that drive corrosion. It is non-destructive in the sense that it does not damage the structure, although it does require localised access to the reinforcement to establish an electrical connection.

How the survey is done

The surveyor first makes a direct electrical connection to the reinforcement, usually by exposing a small area of bar at one point. A reference electrode — the half-cell itself — is then moved across the concrete surface on a grid, and the potential is recorded at each grid point. The concrete surface usually needs to be pre-wetted so the readings are stable and consistent.

The output is a grid of potential readings, normally expressed in millivolts, across the whole surveyed element. From that grid the surveyor produces a contour map or colour-shaded plan showing how the potential varies across the structure. A typical survey covers a defined element — a deck, a soffit, a section of wall — at a grid spacing chosen to suit the size of the element and the level of detail required.

Interpreting the results

The single most important point about half-cell data is this: it is the pattern of readings that matters, not the individual numbers. Recognised guidance offers broad bands — more negative potentials indicating a higher probability of corrosion, less negative potentials a lower probability — but those bands are probabilities, not verdicts. The readings are influenced by moisture content, cover depth, concrete resistivity, and surface conditions, so an absolute threshold applied blindly will mislead.

What a good interpretation looks for is the gradient. Sharp changes in potential across a short distance — strong negative “hot spots” surrounded by more positive concrete — are the clearest indicator of active corrosion cells. A uniform field of moderately negative readings across an entire element usually means something else, such as consistently damp concrete, rather than widespread corrosion.

For this reason, half-cell results are most useful when read alongside other data:

• Cover depth, because shallow cover skews readings and also flags where corrosion is most likely.
• Carbonation depth or chloride content, because these explain why corrosion is occurring where the map shows it.
• Visual inspection, because existing cracking and spalling should correlate with the worst potential readings.

What the survey tells you, and what it does not

Half-cell potential mapping tells you where corrosion is active and lets you prioritise. On a large structure, it turns a vague concern into a specific list of locations: this bay needs breaking out and inspecting, that bay is sound. That prioritisation is the real value of the survey, because it lets an investigation and any repair programme be targeted rather than blanket.

What it does not tell you is the rate of corrosion or how much section has already been lost. A strongly negative hot spot confirms corrosion is occurring; it does not say whether the bar has lost five per cent or fifty per cent of its section. That question is answered by breaking out at the worst locations the map identifies and inspecting the steel directly, or by complementary techniques such as resistivity and corrosion rate measurement.

When to commission it

Half-cell potential testing is worth commissioning when:

• A reinforced concrete structure shows signs of corrosion-related distress and the extent needs to be established.
• Carbonation or chloride testing has confirmed the conditions for corrosion exist, and you need to know whether it has actually started.
• A repair programme needs to be scoped and priced, and blanket repair is not affordable or justified.
• A structure exposed to de-icing salts or a marine environment is being assessed for remaining service life.

It is most informative on car park decks, bridge structures, marine and coastal structures, and balconies — anywhere the combination of reinforcement, moisture, and chlorides or carbonation makes corrosion a live concern.

Practical advice

Half-cell potential mapping is a mature, reliable technique, but it rewards a careful surveyor and punishes a careless one. The connection to the steel must be sound, the surface conditions must be controlled, and — most importantly — the results must be interpreted as a pattern, in context, alongside cover and exposure data. Commission it as part of a properly scoped condition assessment, ask for a contour map rather than a table of raw numbers, and treat the output as a guide to where to investigate further, not as a final verdict on the structure.