Non-contact wide-frequency-range electrochemical impedance measurements were employed to detect the corrosion of a steel rebar embedded into concrete. The measurements were performed by placing two Cu sensing electrodes at a distance of 50 mm from each other on the concrete surface immediately above corroded and non-corroded rebar portions. The low-frequency impedance related to the steel rebar/concrete interface was found to be lower in the former case, leading to the detection of corroded portions of steel rebar embedded in concrete. Such discrimination was only possible at cover thicknesses of up to 50 mm (cf. inter-electrode distance), as at higher thicknesses, a large fraction of current flowed directly from one electrode to the other one. This demonstrates the importance of selecting the proper distance between the sensing electrodes for optimal detection of corrosion portions when the cover thickness of reinforced concrete is known.

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.

We developed a probe electrode for impedance measurement to investigate of rebar corrosion in concrete. Impedance measurements of the electrode in an aqueous solution simulating the transition zone of concrete and rebar interface were carried out. The charge transfer resistance R ct obtained from the two point frequency measurement results by the method developed by Haruyama and Tsuru almost agreed with R ct estimated by spectral measurement. It was also confirmed that R ct was drastically decreased by de-passivation following the decrease in pH. Furthermore, impedance measurement of the probe electrode in concrete specimen was carried out.It was confirmed that R ct was small when the depth of setting the probe electrode was shallow. It was concluded that the state of the rebar/concrete interface can be evaluated by R ct of the probe electrode installed in the concrete.

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