The degradation of steel-reinforced concrete is serious problem in infrastructures. It is well known that the rebar corrosion in concrete is strongly influenced by the conditions of cover concrete. Figure 1 shows schemes of interface between steel rebar and concrete. Thin water layer is formed at this interface due to water absorption in the concrete which covers the steel rebar. And this water layer is called “transition zone” in the field of civil engineering. The pH of water at transition zone is high when the concrete is good health status, and the passive film is formed on steel rebar. The breakdown of passive film occurs by neutralization and chloride concentration in the water layer with the degradation of concrete. Corrosion resistance of steel rebar is influenced by the water quality in the transition zone. Thus, we can conclude that rebar corrosion in concrete is electrochemical process at metal/water interface. Electrochemical measurements are useful to investigate the corrosion mechanisms. There are some literatures regarding the electrochemical tests, which are mainly measurements of corrosion potential [1] and polarization curves, to evaluate the stability of passive film formed on rebar in concrete. Dawson et al. [2] applied an electrochemical impedance spectroscopy (EIS) to the investigation of rebar corrosion in concrete because the charge transfer resistance at rebar/concrete interface can be determined from the impedance spectrum.

The authors developed an electrode as a sensor probe to monitor concrete degradation and rebar corrosion. A couple of electrodes, which were made of same material as rebar, can be embedded in concrete structure and complex impedance can be measured continuously on the basis of two-electrode method. In this presentation, the authors report the results of three impedance measurements as follows.

(1) Impedance between two electrodes in aqueous solution that simulates the water quality of transition zone (laboratory test).

(2) Impedance between two electrodes embedded in the concrete which was fabricated in the laboratory.

(3) Field test at bridge pier of the highway in coastal marine environment.

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] J. L. Dawson, L. M. Callow, K. Hladky and J. A. Richardson, Corrosion/78, No-125 (1978).