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.

The durability of steel-reinforced concrete is a serious problem that concerns the whole world. It is well known that the corrosion of steel rebar in concrete is strongly influenced by the conditions of cover concrete, namely, water absorption, neutralization and chloride ion permeation. There are some literatures regarding the electrochemical tests, which are mainly measurements of corrosion potential and polarization curves, to evaluate corrosion of rebar in concrete [1,2]. Sagues et al. [3] reported the operation of a macroscopic Kelvin Probe (KP) for contactless measurement of concrete surface potentials. Dawson et al. [4] applied an electrochemical impedance spectroscopy (EIS) to the investigation of corrosion of rebar in concrete because the charge transfer resistance can be determined from the impedance spectrum. Figure 1 shows the scheme of Nyquist plot of electrochemical impedance of steel-reinforced concrete. For example, a capacitive loop related to the time constant of charge transfer resistance and electric double layer capacitance at rebar/concrete interface is observed in the low frequency range. The capacitive loop in the middle frequency range is related to the electric nature of rust or oxide layer formed on rebar surface [5].  McCarter et al. [6] reported that capacitive loop in the high frequency range originated form the dielectric property of concrete absorbing water. In this presentation, we introduce some configurations of electrodes (or probes, sensor) to measure an impedance spectrum of steel-reinforced concrete. And we will discuss the corrosion behavior of rebar in concrete by analyzing the conditions of concrete and rebar/concrete interface by EIS. References: [1] ASTM C 876-91 (Reapproved 1999): Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, Annual Book of ASTM standards, Vol. 03.02, pp. 457-462, 1999. [2] M.J. Dugarte, A.A. Sagues, Corrosion, 70 (2014) 303-317. [3] A.A. Sagues, M.T. Walsh, Corrosion Science, 56 (2012) 26-35. [4] J. L. Dawson, L. M. Callow, K. Hladky and J. A. Richardson, Corrosion/78, No-125 (1978). [5] J. M. Deus, B. Diaz, L. Freire and X. R. Novoa, Electrochimica Acta, 131 (2014) 106-115. [6] W. J. McCarter and S. Garvin, J. Phys. D: Appl. Phys., 22 (1989) 1773-1776. Figure 1

A reinforced concrete has been used for the civil engineering and construction in the whole world. In order to obtain the real-time information of reinforced concrete, Dawson et al. 1) attempted to investigate the electrochemical behavior of reinforced concrete by an electrochemical impedance spectroscopy (EIS). John et al. 2) proposed the equivalent circuit to interpret the impedance spectrum of reinforced concrete. Though these methods can be applied to the investigation of conditions of reinforced concrete, an electrical connection of reinforced concrete to the measurement device is required. Keddam et al. 3) proposed the concept of floating electrode for contact-less electrochemical measurement of reinforced concrete. In this method, a four-electrode probe was set on the reinforcement, and the capacitive loop related to the double-layer capacitance at the cement-rebar interface was measured 3). In the present study, a new method to detect the corrosion site of reinforced concrete was proposed by using two sensing electrodes. The sensing electrodes were placed on the surface of reinforced concrete as working and counter electrodes. The electrochemical impedance measurement was performed in this setup. The impedance related to the corrosion site of steel rebar in concrete with different cover thickness is measured by proposed method. In addition, the current and voltage distributions between sensing electrodes set on the concrete surface were discussed by a finite element method. Reference: 1. J. L. Dawson, L. M. Callow, K. Hladky and J. A. Richardson, Corrossion/78, No-125(1978). 2. D. G. John, P. C. Searson, J. L. Dawson, Br Corros J, 16, 102-106 (1981). 3. M. Keddam, X. R. Nòvoa, V. Vivier, Corros. Sci., 51, 1795-1801 (2009).

A reinforced concrete has been used for the building structures and bridge in the world. However, the degradation of reinforced concrete related to the corrosion of the reinforcing steel in concrete is now serious problem. It is important to establish the method to evaluate the durability of reinforced steel in concrete. Dawson et al. [1] applied the electrochemical impedance spectroscopy to investigation of the corrosion resistance of reinforced steel in concrete. Koleva et al. [2] investigated the durability of the steel rebar casted mortal cylinder by the rest potential, polarization curve and electrochemical impedance measurements. The present study developed the method to determine the corroded reinforcing steel in concreate by two electrode system. Figure 1 shows the schematic of developed system in the present study. Two copper electrodes are placed on the surface of the concrete in which the steel bar was embedded. The cover thickness of concrete was 10 mm. The half side of the steel bar corroded to simulate the degradation of the steel bar. Schematic of the impedance spectrum of reinforcing steel in concrete measured by two electrode system is shown in Fig. 2. Two loops were observed in the impedance spectrum when the copper electrodes are placed on the concrete in which corroded steel was embedded. It indicated that the current flowing though the copper electrodes was flowed into the corroded region of the steel bar. The loop in the high frequency range was related to concrete and the loop in the low frequency range was related to interface of concrete between steel rebar. Reference: [1] J. L. Dawson, L. M. Callow, K. Hladky and J. A. Richardson, Corrossion/78, 125 (1978). [2] D. A. Koleva, K. van Breugel, J. H. W. de Wit, E. van Westing, N. Boshkov and A. L. A. Fraaij, J. Electrochem. Soc., 154, E45 (2007). Figure 1