A new approach is presented to evaluate corrosion of steel bars in concrete by 1.5- and 2.6-GHz ground-penetrating radar (GPR) and a modified half-cell potential method. Changes in time-lapsed travel times, amplitudes, and peak frequencies that are associated with short-time Fourier transform spectrograms of the bar reflections were continuously measured. The year-long corrosion process of the reinforcement bar rapidly accelerated within a few days by impressing direct current across a pair of embedded reinforcement bars, which served as the anode and cathode. When corrosion started, the travel times, amplitudes, and frequency spectra of the bar reflection changed. The results were analyzed by dividing the material’s response into three phases (NaCl contamination, depassivation, and corrosion). The writers attribute the phenomena of the first two phases to the ionic conduction and interfacial polarization effect, described in the low-frequency regime of complex dielectric permittivity outlined in ...