Owing to the presence of loops related to the time constants originating from the structure and interfacial reactions, it is difficult to select a suitable equivalent circuit for curve fitting of the impedance spectrum of reinforced concrete. To investigate the time constants observed in the impedance spectrum of reinforced concrete and propose an appropriate equivalent circuit, electrochemical impedance measurements of reinforced concrete with different cover thicknesses were performed using a two-electrode system. In this case, two parallel reinforcing bars were embedded in the concrete, and a cyclic wet-dry test was conducted to accelerate the corrosion of the reinforcing bars. It was confirmed that part of the large loop in the low-frequency range was related to the reinforcing bar/ concrete interface, a distorted loop in the middle-frequency range was associated with rust formation on the reinforcing bars, and the small loop in the high-frequency range was attributed to the water distribution and pore structure in concrete.

Electrochemical impedance spectroscopy (EIS) is effective tool to investigate degradation of concrete and corrosion of reinforcing steel bar. The impedance spectrum of concrete reinforced by steel bar shows many capacitive loops on the Nyquist plane. McCarter et al. [1] reported that capacitive loop in the high frequency range originated form the dielectric property of concrete absorbing water. According to the paper by McCarter et al. [1], the EIS allows the information concerning the amount of water and pore structure in concrete. We measured impedance spectra of concrete sandwiched between two stainless steel electrodes, and found that the capacitance is very large when the concrete absorbed much water. We observed the variation of capacitance during the drying process of concrete, and proposed a complicated pore structure in concrete. Reference: [1] W. J. McCarter and S. Garvin, J. Phys. D: Appl. Phys., 22 (1989) 1773-1776.