Chloride ingress testing — what it reveals and why it matters

Chlorides are the single most aggressive driver of reinforcement corrosion in UK structures. They come from de-icing salt on roads and car park decks, from sea spray and groundwater in coastal environments, and occasionally from contaminated aggregates in older concrete. Unlike carbonation, chlorides can trigger corrosion even where the concrete is still highly alkaline. Chloride ingress testing measures how far this contamination has penetrated and how much has reached the steel. Here is how it is done and what to do with the results.

Why chlorides are so damaging

Chloride ions break down the passive protective layer on reinforcement directly. They do not need to lower the pH of the concrete first, which is what sets them apart from carbonation. Once enough chloride has accumulated at the level of the steel, corrosion begins — and chloride-driven corrosion tends to be localised and aggressive, producing deep pitting rather than uniform loss. Pitting concentrates section loss in a small area, which makes it particularly serious for the structural performance of a bar.

The chloride does not have to be cast into the concrete. In most affected structures it has migrated in from the surface over years, drawn deeper by repeated wetting and drying. That is why a car park deck exposed to salt-laden vehicles, or a structure within reach of sea spray, is at far higher risk than the same concrete in a sheltered, dry location.

How chloride ingress is tested

The standard approach is to take samples of concrete at known depths and measure the chloride content of each. The sampling is done either by drilling and collecting concrete dust in depth increments, or by taking a core and sectioning it into slices once it is in the laboratory. Each sample is analysed for chloride content, which is reported as a percentage by mass of cement or of concrete.

The result is a chloride profile: a curve showing chloride concentration plotted against depth. High chloride near the surface, falling away with depth, is the typical pattern for a structure that has absorbed chlorides from its environment. A flatter profile with chloride present throughout can indicate chlorides that were cast into the original mix.

Sampling is normally carried out at several locations so the variation across an element can be understood, and it is often combined with cover depth measurement, because — as with carbonation — the chloride profile only means something once it is compared with the depth of the steel.

Reading the chloride profile

The number that matters is the chloride concentration at the depth of the reinforcement. There is a broadly recognised threshold above which corrosion risk becomes significant, but it should be treated as a guide rather than a hard line. The actual risk depends on the moisture state of the concrete, the presence of oxygen, the concrete quality, and how the chloride is bound within the cement matrix.

A few patterns are worth understanding when you read a profile:

• High surface chloride, low chloride at rebar depth. The structure is being exposed to chlorides, but ingress has not yet reached the steel. This is the time to act on protection, before corrosion starts.
• Chloride above the threshold at rebar depth. The conditions for corrosion are present. The next question is whether corrosion is already active, which is answered by half-cell potential mapping and direct inspection.
• A flat profile with chloride throughout. This suggests cast-in chlorides rather than ingress, which changes the diagnosis and the options.

A chloride profile is also predictive. Because chloride penetrates at a slowing rate over time, a profile measured now can be used to estimate when the threshold will be reached at the steel — useful for deciding whether intervention is urgent or can be deferred.

What to do with the data

Chloride testing rarely sits on its own. It is most useful as the diagnostic layer of a wider assessment, answering the question of why a structure is deteriorating. A typical sequence is to use chloride profiling to establish the cause and extent of contamination, half-cell potential mapping to find where corrosion is actually active, and cover depth data to tie the two together and explain the pattern.

The data then drives the decision on intervention. Where chloride has reached the steel and corrosion is confirmed, the options range from localised repair to cathodic protection, surface barriers, or — in severe cases — replacement of affected elements. The chloride profile is what tells the engineer which of those options is proportionate, and how long there is before the problem worsens.

When to commission it

Chloride ingress testing is worth commissioning for:

• Car park decks and structures exposed to de-icing salts.
• Coastal and marine structures within reach of salt spray or seawater.
• Older structures where cast-in chlorides are a possibility.
• Any structure where corrosion is suspected and the cause needs to be confirmed before a repair strategy is chosen.

Practical advice

Chloride ingress testing is the test that explains the cause of corrosion rather than just confirming its symptoms — and that makes it valuable, because a repair strategy chosen without understanding the cause is a strategy likely to fail. Commission it as part of a structured condition assessment, ask for a proper depth profile rather than a single bulk figure, and make sure the sampling locations are chosen to capture the real variation across the structure. Read alongside cover depth and corrosion data, a good chloride profile gives an engineer a defensible basis for deciding what to do and when.