Mapping Corrosion in Concrete: Chain Drag and Half-Cell Potential

May 15, 2018
One issue over time with the use of concrete as a building material is the use of steel rebar to increase the tensile strength of the structure. When adding steel to concrete, problems can be created from corrosion and deterioration of the steel.

This can be a very costly problem and one that, if gone unseen for a long time, can cause major issues and damage to the structure. As such it is vastly important to check for potential corrosion and damage by mapping areas of the structure to ensure its integrity over time.

How can you check for corrosion?

There are a number of methods to check for corrosion in concrete including Chain Drag, Impact Echo, Ultrasonic Pulse-Echo, GPR, Electrical Resistivity Measurement, and Half-Cell Potential Mapping. In this article we’re going to talk about two of the most popular methods along with Xradar’s own techniques for mapping corrosion within concrete.

All of these methods are non-destructive, meaning they do not cause any damage to the structure during testing.

Chain Drag

Chain drag (or dragging) is a widely used technique to detect delamination and corrosion, however, there are a huge number of drawbacks to this method.

In terms of its advantages it is incredibly simple, with no special equipment required whatsoever – the process is that of dragging a chain across the concrete structure while listening for hollow points beneath the surface.

The immediate point of contention with this method is that the process can be very subjective, and relies heavily on the experience of the person doing it. An even bigger disadvantage is that it can only provide results if the deterioration process is well in the advanced stages. Early diagnosis and detection is not possible when using this technique.

Half-Cell Potential

Another popular and widely used method for detecting corrosion is Half-Cell Potential Mapping. This is done by using a portable half-cell (normally copper or copper sulphate) as a standard reference electrode, connecting the negative end to a voltmeter and connecting the positive end to the reinforcement steel. Depending on the measured half-cell corrosion potential value, the probability of active corrosion is determined.

While this method definitely has its merits, the results can very easily be skewed due to factors such as humidity, cover thickness, and the quality of the concrete materials. The main disadvantage is that the accuracy of the test relies heavily on the electrode being placed directly over the surface of the concrete – something which is not possible in a huge number of cases (especially on roads with asphalt overlay).

Xradar and Ground Penetrating Radar (GPR)

Developed in Quebec, Xradar offer a breakthrough and innovative approach to mapping corrosion in concrete that has greatly improved the accuracy and speed of the service.

Utilizing computer-assisted image analysis of our Xradar technology, this advanced form of GPR scans the subsurface and detects areas of corrosion within the concrete. This method works on all surfaces such as roads, runways, bridges, and car parks. The data is then processed by our trained technicians, who then overlay the results onto real-life drawings, allowing you to see exactly where the corrosion might be and the extent of the damage.

Get in touch to find out more about the process and how it can benefit you.