Corrosion of reinforcing steel is an electro-chemical process. It is commonly known that for corrosion of the steel to occur, a current must pass between the anodic and cathodic regions of the concrete. Consequently, the electrical resistivity of the concrete affects the flow of ions and the rate at which corrosion can then occur.

A higher concrete resistivity decreases the flow; a relationship between corrosion rate and resistivity has been determined through research from measurements conducted on structures giving a criteria where there is risk of corrosion occurring at specific resistivity levels.

There are a number of methods for measuring resistivity available, however the Wenner four-probe test instrument is the method most commonly used in industry with modern versions making on site testing quick and easy. There are a number of other factors that can affect the resistivity measurements, therefore through the use of Empirical qualified engineers ensure that the testing is undertaken accurately with observations of site conditions.  The Wenner four probe applies a current between the two outer probes and the potential difference measured between the two inner probes.

Resistivity measurement is useful for identifying areas of reinforced concrete at risk of corrosion. It should not be considered in isolation but used in conjunction with other techniques such as half-cell potential. There are a number of papers and standards available for guidance on testing such as Concrete Society Technical Report TR 60, and BRE Digest 434, Corrosion of reinforcement in concrete: electrical monitoring.