Under severe conditions reinforced concrete (RC) structures may suffer from reinforcement corrosion. To evaluate the risk of corrosion different measurement methods are available. The concrete resistivity is a significant parameter to evaluate the durability of a RC structure. It has a significant influence on the corrosion rate of the reinforcement and provides information about the water content and the porosity of the concrete. By means of resistivity mapping areas may be located where water and depending on the exposure chlorides penetrate into the concrete. In these areas a depassivation of the reinforcement due to corrosion will be probable over the time. The resistivity of concrete can be determined non-destructively by using a 4-electrode setup established by Wenner. Reinforcement bars as well as several concrete layers with different resistivities have a significant influence on the readings. At the Institute of Building Materials Research of RWTH Aachen in Germany the 4-electrode methodology was improved to minimize these influences in order to determine the real concrete resistivity.