Assessment of the electrochemical state of steel reinforcement in water saturated concrete by resistivity measurement

The surface resistivity of concrete with and without reinforcement was measured in the laboratory. Uncarbonated and carbonated concrete specimens were investigated corresponding to a passive and active state of corrosion, respectively. The concrete was water saturated, so the measurement medium was considered to be homogeneous. The measurements were performed directly over the rebar on reinforced specimens in order to minimize boundary effects. The presence of a rebar, with the resulting short-circuit effect due to the large differences of resistivity between concrete and steel, strongly reduced the measured resistivity. The magnitude of this reduction increased as the rebar reached a more active state. Other parameters influencing the short circuit effect, particularly concrete cover depth and concrete resistivity, were studied experimentally as well as numerically. The measurement results were inverted through a numerical simulation using COMSOL Multiphysics software. The Butler-Volmer relation was used to characterise the charge exchange at the steel-concrete interface. This inversion process determined the real resistivity of concrete as well as the electrochemical parameters of the embedded reinforcing steel bar. The methodology combining both measurement and inversion is relevant to evaluate the corrosion state of reinforcement. By comparing the electrochemical parameters of a passive and active rebar, the corrosion state can be determined with the proposed methodology.