AC impedance study of sealer- and coating-treated mortar specimens

Chloride diffusion resistance is an important property of surface treatments on concrete in relation to chloride-induced reinforcement corrosion. Currently there is no rapid and accurate method available to measure it. This paper reports on a laboratory investigation of AC impedance measurement as a means of assessing the chloride diffusion resistance of sealers and coatings on mortar. Five surface treatments are studied. It was found that due to the difference in dielectric properties of the surface treatment materials and cementitious mortar, the impedance spectrum of a surface-treated mortar specimen consists of two individual spectra located at high and medium frequencies corresponding to the mortar substrate and the surface treatment layer, respectively. An equivalent circuit for surface-treated mortar/concrete is proposed. Hence the dielectric properties of a surface treatment layer and substrate can be analysed and their individual properties can be studied. The electrical resistance of a surface treatment obtained by impedance analysis was found to correlate well with the chloride diffusion resistance measured by a long-term diffusion cell test. The results suggest that the rapid and non-destructive AC impedance test has great potential in performance monitoring and deterioration mechanism studies of surface treatments.