Prediction of concrete compressive strength based on early-age effective conductivity measurement

Prediction of compressive strength of concrete at early ages has attracted a lot of attention, but several defects still remain in previous research using nondestructive techniques such as the use of electrical surface resistivity. A true prediction model for the 28-day compressive strength of concrete via effective conductivity at microwave frequency is presented for the first time in this paper. The open-ended coaxial probe was used to undertake the loss factor measurement, in the frequency range of 0.1 GHz-10 GHz, of ordinary concrete with four water-cement ratios (0.40, 0.45, 0.49, 0.60) at first seven days. Traditional destructive test was used to obtain the concrete compressive strength at 3, 7, 14, and 28 days of curing. The reduction of conductivity versus curing age was modeled by using exponential decay function whereas the development of compressive strength curing age was modeled by using a nonlinear raising function. By taking curing age as an intermediate variable, a linear prediction model as a function of effective conductivity was developed. A minimum effective conductivity was discovered where the frequency of about 3 GHz was considered as the dominated loss mechanism transits from ionic conduction loss to dipole polarization loss. Our results demonstrated that the 28-day compressive strength of concrete having close water-cement ratios could be accurately predicted using effective conductivity. Moreover, the magnitude of the slope of strength-conductivity curves could give an indication on the grade of standard concrete during early ages. The model was evaluated by using data from open literature with DC conductivity and the accuracy was obtained with about 10% error in underestimation.