Corrosion of reinforcing steel is an electro-chemical process. For corrosion of the steel to occur a current must pass between the anodic and cathodic regions of the concrete. The electrical resistivity of the concrete affects the flow of ions and the rate at which corrosion can occur. A higher concrete resistivity decreases the flow; an empirical relationship between corrosion rate and resistivity has been determined from measurements on actual structures.
Various approaches for measuring resistivity are available but the four-probe device is the most suitable. Modern devices are spring-loaded and are applied directly to the surface. A current is applied 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 from corrosion. It should not be considered in isolation but used in conjunction with other techniques such as half-cell potential.