1. SCOPE This Recommendation covers the description of non- destructive electrochemical test methods for the estimation in large size concrete structures of the instantaneous corrosion current density, ioorr, expressed in gA/cm 2, by means of the so-called Polarization Resistance technique, R o, in order to assess the condition of embedded steel reinforcement related to its corrosion. The values of i~orr, Can be used to assess the rate of degradation of concrete structures affected by reinforcement corrosion. However, they cannot give information on the actual loss in steel cross section which, at present, only can be assessed by means of direct visual observation. Values of the free corrosion potential or half-cell potential, Ecorr (V), of the embedded reinforcing steel and of the electrical concrete resistance, Re (f)), are obtained as preliminary steps of the Rp measurements. Values of the concrete resistivity, P (~m), can be calculated from Re values providing the geometrical arrangement of the electrodes enables this calculation. Both parameters, Ecorr and Re (or P) may be used to complement the reliability of the ico~r measurements.