Since 1990, a special macrocell system, the so-called anode-ladder-system, has been used world-wide to monitor the corrosion risk of new concrete structures, besides other systems. This sensor-system indicates the depth of the critical chloride content initiating corrosion, i.e. the critical depth of the reinforcement with respect to corrosion. Subsequently, the time-to-corrosion can be determined, enabling the owners of buildings to initiate preventive protection measures before cracks and spalling occur. By measurement of the potentials and the electrical resistance of the concrete around the sensors, an estimation of the humidity, the availability of oxygen and the corrosion behaviour after depassivation is possible. To be able to monitor the corrosion risk for existing structures as well, a new sensor-system has been developed called expansion-ring-system. It consists of six measuring rings (‘anodes’) separated by sealing rings as parts of the main sensor and a ‘cathode-bar’, which are installed in small holes, to be drilled into the concrete.