Features of Immittance Spectra as Performance Indicators for Cement-Based Concretes

The 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 characterizing 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 100Hz-10MHz. Data are presented for a range of industry-standard cement-based concrete mixes with and without supplementary cementitious materials (SCM's). The Nyquist (-iZ’’(ω) vs Z’(ω) and Bode (Z*(ω) and θ vs frequency) formats 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. 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 the conductivity was found to obey Jonscher's universal power-law. Two novel durability parameters are presented based on features of Jonscher's model and, from a practical viewpoint, the power-law model can be evaluated using conductivity measurements obtained at three, easily measured, spot frequencies (viz. 10kHz, 1MHz and 10MHz).