The corrosion of reinforcement can be detected and its evolution quantified through the polarization resistance, Rp, which enables to calculate the instantaneous corrosion rate. In large structures, the determination of the Rp method uses a small counter electrode and the current applied attenuates with the distance, which makes unknown the area polarized. Two main solutions have been applied to overcome this limitation: to use a guard ring (GR) to confine the current to a prefixed area or to measure the bar length polarized by the current. In present work, a 3D numerical simulation is presented using the program COMSOL Multiphysics. Three cases are studied: no confinement, non-modulated confinement, and modulated confinement of the current. In the results, it is illustrated that only when the GR is correctly modulated, the correct homogeneous distribution is achieved and the critical length is well confined in the area below the circle passing underneath the “confinement controller electrodes”. In a second part, the paper presented the manner to obtain an annual representative value of the corrosion rate through the integration of the instantaneous Icorr values.