An a.c. impedance study of steel in concrete

Abstract

Impedance measurements executed between both steel and platinum electrodes embedded in both porous and non-porous concretes were used to explore the physical characteristics of the system. A simple method is described for measuring high impedances (up to 500 MΩ) in which the unknown impedance is compared with the input impedance of the measuring instrument, previously calibrated. Impedance measurements on concrete immersed in an electrolyte were used to quantify concrete quality, and the dynamics of wetting, drying and electrolyte exchange were explored: the dynamics of such processes are considered to be amongst the factors determining the rate of corrosion of steel reinforcement under some conditions since intermittent wetting is an efficient method for transporting oxygen and salt into the concrete. The existence of cracks along the electrode-concrete interface was inferred. Impedance changes consequent on the initiation of corrosion of steel electrodes were noted, but the changes were small and not clear-cut.