Electrical Property-Based sensing of concrete: Influence of material parameters on dielectric response

Electrical property based sensing of the properties of cement based materials is becoming increasingly popular. This paper reports the results of a systematic investigation carried out to understand the influence of material parameters on the dielectric response of cement pastes and concretes. A simple mathematical model that links the effective conductivity and time has been proposed, which can provide the setting time of the pastes. The variation in conductivity between plain and fly ash modified pastes is brought out. A fractional factorial experimental design coupled with range analysis is used to extract the significant parameters that influence the electrical conductivity of concretes. It is found that the aggregate-to-cementing materials ratio (a/cm) influences the very early age conductivity of concretes while the combinations of a/cm and water- to-cementing materials ratio (w/cm), and fly ash content and curing condition influences the final conductivity significantly. The changes in dielectric constant and conductivity spectra are attributed to the polarization phenomena. The dispersion in electrical conductivity starts at lower frequencies for fly ash modified pastes. There is an observed dielectric enhancement also for fly ash modified pastes. The dielectric response of concrete is very similar to that of pastes, and the effect of dilution by the aggregates in evident. The relaxation times of the cement pastes were determined from a geometric analysis of the bulk arc in the Nyquist plot. This method facilitates the determination of relaxation times and depression angle of the bulk arc using limited frequency sweep. The relaxation times decrease with increase in hydration time and conform to a power law. Though only a single relaxation time is considered in this study, the relaxation time distributions as represented by the depression angle, indicate increasing material heterogeneity with increasing hydration. Representing the dielectric constant at particular frequencies as a function of the capillary porosity in the cement pastes, it is possible to detect the percolation threshold of cement pastes. The observed percolation thresholds agree well with those reported for cement based materials.