Investigation of the local response of the steel–concrete interface for corrosion measurement

Abstract An equivalent circuit representation for the steel–concrete interface is determined from direct potential measurements at the steel surface. The local response of steel–concrete interface to a given polarization applied at the concrete surface is investigated using an Ag/AgCl embedded reference electrode and a test system which allows simultaneous measurements at the steel–concrete interface and on the concrete surface. It is shown that the impedance spectrum on Nyquist plot for the steel–concrete interface comprises of a single arc. The equivalent circuit representation of the steel–concrete interface comprising of a parallel combination of a constant phase element (CPE) and a resistance was found to be suitable for representing the observed frequency response above 10 mHz. The parameters for the equivalent circuit obtained from the frequency-domain impedance measurements are shown to provide close prediction of the transient time-domain response from a linear polarization resistance measurement. The equivalent circuit was found to be suitable for interpreting the transient response of the steel–concrete interface during the linear polarization measurements. Available results indicate that while the response of steel undergoing active corrosion exhibits a distinctive CPE behavior, the passive steel approaches a pure capacitor. The value of resistance when the measured current increases linearly with time during a linear polarization measurement from the concrete surface provide acceptable measurements of the charge transfer resistance, and the Ohmic resistance of the concrete.