Electrical resistivity monitoring and characterisation of early age concrete

The electrical resistivity of concretes with various aggregate volume fractions (V-a) at water/cement (W/C) ratios of 0.4 and 0.5 over 24 h were measured by a non-contact electrical resistivity apparatus. The four structure formation periods are the dissolution and precipitation period, the setting period, the accelerating period, and the decelerating period, which are identified from three critical peak points on the differential electrical resistivity curve. A logarithmic equation (rho(t) = K-c ln(Dt) = KmKa ln(Dt)) is proposed to express electrical resistivity development with time after the decelerating period, where K-c is the slope of the electrical resistivity for the concrete system after hardening, K-a is a function of V-a and represents the impact factor of the aggregate in a unit volume concrete, and K-m is firstly defined as the structure formation factor of the matrix and represents the rate of densification of the matrix during hydration, which is independent of V-a, and D is the average reaction rate of cement particle hydration during a particular period. The mixture with the lower W/C ratio had a higher K-m. An increase either in aggregate fraction or in cement particles per unit volume of concrete results in a higher rate of electrical resistivity development in the concrete.