Effects of coarse aggregates on the electrical resistivity of Portland cement concrete

Electrical resistivity of concrete is an inherent material parameter associated with the composition, condition, and deterioration of concrete. Over the past few decades, studies have focused on the applicability of employing concrete resistivity as a self-monitoring indicator. This study attempted to investigate the effects of coarse aggregates (CAs) on the electrical resistivity of Portland cement concrete. Concrete mixtures with variations in CA content, size, and type were designed under identical water to cement ratios and curing conditions, and tested at 28 days. Bulk resistivity (BR) was measured using two-probe methods under saturated surface dry (SSD) conditions. Backscattered electron images (BEI) obtained using scanning electron microscopes (SEM) and X-ray computed tomography (CT) image analysis were also employed to study the capillary porosities and mesoscale voids that might affect BR. A mesoscale current conduction model was also employed for interpretation. The experimental results suggested that concrete resistivity is sensitive to the inclusion of CAs. Instead of altering the cement hydration, CAs were observed to serve as direct electrical obstacles that resulted in the enhancement of BR. Higher CA content increased the existence of obstacles, thereby increasing BR. Variations in CA size and type were observed to exert no effects on BR, which further validated the adopted mesoscale current conduction model, and further indicated that interfacial transition zones do not exert a substantial influence on concrete resistivity. It was also observed that capillary porosities and mesoscale voids were not influenced by CA content, indicating that cement pastes had similar quality among all concrete mixtures.