Influence of the aggregate on the electrical conductivity of Portland cement concretes

The electrical conductivity and compressive strength of several high-performance Portland concretes with different amounts of crushed aggregate and sand have been measured at early age in isothermal conditions (20 °C). The total aggregate volume fraction varied from 0 (plain paste) to 0.75 and a constant weight ratio (1.2) between crushed aggregate and sand was used. The w/c ratio was 0.37 and microsilica (in slurry form) and a superplasticizer in water solution were used.The time taken before the electrical conductivity began to drop correlated very well with the induction period. The drop of conductivity was slightly delayed by the aggregate. The analysis of the electrical data, by means of different numerical and analytical models [hard core soft shell model (HCSS), differential effective medium theory (DEMT), Lu-Torquato, Maxwell], allowed an estimate of the properties of the interfacial transition zone (ITZ). In particular, an ITZ thickness of about 9 μm and an ITZ to bulk conductivity ratio of ∼2.5 were found. The existence of a percolating pathway through the interfacial regions was found by both electrical measurement and modeling when the aggregate volume fractions exceeded 60%. Finally, a new relationship among electrical conductivity, compressive strength, and aggregate amount was derived.