Half cell potential
Corrosion of reinforcing steel is an electro-chemical process and the behaviour of the steel can be characterised by measuring its half-cell potential. The greater the potential the higher the risk that corrosion is taking place. An electrode forms one half of the cell and the reinforcing steel in the concrete the other. The preferred reference electrode for site use is silver/silver chloride in potassium chloride solution although the copper/copper sulphate electrode is still widely used.
The survey procedure is firstly to locate the steel and determine the bar spacing using a covermeter. The cover concrete is removed locally over a suitable bar and an electrical connection made to the steel. It is necessary to check that the steel is electrically continuous by measuring the resistance between two widely separated points. The reinforcing bar is connected to the half-cell via a digital voltmeter, see diagram. Readings of half-cell potential are taken over a regular grid of points (say ˝ m apart) to give a potential map of the area.
Contour lines may be plotted between points of equal potential to indicate those areas that have the greatest risk of corrosion. Locally exposing and inspecting the reinforcement in areas where both high and low risks of corrosion are indicated can be used to approximately calibrate the potential readings for the structure with respect to its present corrosion and the need for further investigation or repair.