measuring the resistivity of bulk concrete in situ. The resistivity is given by: V is the voltage measured between the inner two probes (measured in volts, V) I is the current injected in the two outer probes (measured in amps, A) a is the equal distance of the probes (measured in metres, m). Rebar The presence of rebars disturbs electrical resistivity measurement as they conduct current much better than the surrounding concrete. This is particularly the case when the concrete cover depth is less than 30 mm. In order to minimize the effect, placing the electrodes above a rebar is usually avoided, or if unavoidable, then they are placed perpendicular to the rebar. However, measurement of the resistance between a rebar and a single probe at the concrete surface is sometimes done in conjunction with electrochemical measurements. Resistivity strongly affects corrosion rates and electrochemical measurements require an electrical connection to the rebar. It is convenient to make a resistance measurement with the same connection. The resistivity is given by: R is the measured resistance, D is the diameter of the surface probe. Relation to corrosion Corrosion is an electro-chemical process. The rate of flow of the ions between the anode and cathode areas, and therefore the rate at which corrosion can occur, is affected by the resistivity of the concrete. To measure the electrical resistivity of the concrete a current is applied to the two outer probes and the potential difference is measured between the two inner probes. Empirical tests have arrived at the following threshold values which can be used to determine the likelihood of corrosion. These values have to be used cautiously as there is strong evidence that chloride diffusion and surface electrical resistivity is dependent on other factors such as mix composition and age. The electrical resistivity of the concrete cover layer decreases due to: Increasing concrete water content Increasing concrete porosity Increasing temperature Increasing chloride content Decreasing carbonatation depth When the electrical resistivity of the concrete is low, the rate of corrosion increases. When the electrical resistivity is high, e.g. in case of dry and carbonated concrete, the rate of corrosion decreases. Standards ASTM Standard C1202-10: Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration AASHTO TP 95 (2011), “Standard Test Method for Surface Resistivity of Concrete’s Ability to Resist Chloride Ion Penetration.” American Association of State Highway and Transportation Officials,