The objectives of this paper is twofold: firstly, to introduce an innovative technique for extracting and exploiting the Electromagnetic (EM) waveguide dispersion of civil engineering materials with GPR; secondly, to monitor the water ingress front during an absorption process of various concrete mixtures. This technique is based on a new inversion procedure that relies on the Electromagnetic Waveguide Model (WGM) to invert the phase velocity dispersion curves in their modal form. A homogenization Parallel model, derived from the well-known Lichtenecker-Rother equation is employed to develop the waveguide from one-layer medium to a multi-layer medium. The WGM outputs are then used to estimate the dielectric and geometric parameters of the propagation medium and offer as a primary application the monitoring of hydric transfers in concrete through water ingress cycles by capillarity. The validation is performed on GPR data derived from measurements on homogeneous materials. The results on concrete obtained by GPR measurements and then analyzed by the WGM enable us to monitor the water ingress front during an absorption process. These results are compared to the moisture gradients generated on cores by means of gammadensimetry, which is set as the reference. This procedure yields trends providing key information on the conditioning state of the studied concretes.