A large experimental investigation was conducted in order to assess the capability of electrochemical techniques to detect the effective onset of carbonation-induced corrosion. Specimens made with CEM I, CEM II and CEM III cement-based concrete with embedded steel bars were subjected to several exposure conditions (wet/dry cycles, partial saturation and outdoor conditions) and tested. At regular time intervals, the polarisation resistance and free corrosion potential were measured, as was the amount of corrosion product. From the data acquired, and considering the experimental uncertainties, the probabilities of right or wrong alarm were estimated depending on the alarm thresholds for the electrochemical measurements and the threshold of the effective onset of corrosion. Optimum values of these thresholds were proposed on the basis of the receiver-operating-characteristic curve concept.