Features of immittance spectra as performance indicators for cement-based concretes

Abstract

Design for durability and performance-based standards and specifications for reinforced concrete infrastructure is limited by the lack of rapid, science-based test methods for characterising the deterioration resistance of concrete. In this paper, this issue is addressed though the application of two-point electrical impedance measurements taken within the frequency range 100 Hz–10 MHz. Data are presented for a range of industry-standard cement-based concrete mixes with and without supplementary cementitious materials. Nyquist plots (−iZ″(ω) against Z′(ω)) and Bode plots (Z*(ω) and θ against frequency) clearly highlight the frequency dependence of the electrical response. However, when presented in the form of permittivity and conductivity, a region of dispersion was evident over the entire frequency range for all concretes. The features of this response, which could be gainfully exploited as durability indices for assessing the long-term performance of concrete, are identified and discussed. A range of formalisms is presented and it is shown that, within this frequency range, conductivity was found to obey Jonscher's universal power law. Two novel durability parameters are presented based on features of Jonscher's model; from a practical viewpoint, the power-law model can be evaluated using conductivity measurements obtained at three, easily measured, spot frequencies (10 kHz, 1 MHz and 10 MHz).