Resistivity of concrete and mortars
Corrosion of reinforcing steel is an electrochemical 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 the 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.
Resistivity is affected by a number of factors including cement content, mix design, the use of polymers and microsilica, hence permeability, but is very dependant on the moisture content of the concrete.