Determination of concrete water content by coupling electromagnetic methods: Coaxial/cylindrical transition line with capacitive probes

This paper deals with the development and the validation of an innovative, easy-to-use and on site technique for determination of concrete water content. The on-site technique is the capacitive probe, able to characterize in situ dielectric media in the 30-35 MHz frequency band (around 33 MHz). For the evaluation of water content in various civil engineering structures, a calibration methodology has to be developed and is presented herein. It is based on the complex permittivity estimation of various dispersive concretes, which is carried out by a cylindrical coaxial electromagnetic (EM) transition line allowing the characterization in laboratory of material samples in a large GPR frequency bandwidth [50-600 MHz]. This methodology consists then on a coupling between the results of both the capacitive probes and the coaxial EM cell extrapolated at low frequency (33 MHz). The extrapolation procedure used to link physically the results of the two techniques is provided by the 4parameter-variant of Jonscher's model which is parameterized to obtain dispersion curves of the complex permittivity for very wide frequency bands. The methodology is checked by a parametric study that associates the 4p-variant of Jonscher's model with the physical and hydric characteristics of the six concrete mix designs representing high performance and ordinary concretes at various hydric states. The surface testing results measured by capacitive probes on slabs are successfully compared to the extrapolated results obtained on cores.