About electrical resistivity variation during drying and improvement of the sensing behavior of carbon fiber-reinforced smart concrete

International audienceThe addition of conductive fibers to a cementitious material reduces its electrical impedance, for monitoring purposes, based on the relationship between external stresses (mechanical or thermal) and changes in electrical properties. In this context, variations in the electrical properties due to drying prevent the application of this technique, unless such variations are negligible or precisely predictable. The aim of this paper, therefore, is to study the variation of the complex electrical impedance during drying, for measurement frequencies between 4 Hz and 1 MHz. Shrinkage, weight loss and electrical impedance spectrum were measured on three mortars with carbon fiber volume fractions (FVF) of 0, 0.1 and 0.5%. The results show that in the absence of conductive fibers, the real and imaginary parts of the impedance increase due to water loss. On the other hand, in the presence of a percolated carbon fiber network, the real impedance of the material decreases at low frequencies and increases at high frequencies during drying; for intermediate frequency ranges, quasi-constant values can be observed. In addition, as the material dries, the capacitive behavior of the material wanes, and the imaginary impedance values tend towards 0. The electrical behavior then approaches ideal resistive behavior with a real impedance value independent of the measurement frequency. Blind frequencies have been identified for fibrous mortars around 1 kHz and 40 kHz for 0.1% and 0.5% FVF, respectively. For monitoring purposes, coupling the presence of fibers with an appropriate measurement frequency would minimize the effects related to the variation of impedance as a function of time – i.e., allowing more precise monitoring of mechanical loads with time