This paper presents a new method to characterize the change of geometric and dielectric properties inside a material, such as concrete from civil engineering structures, with the application of GPR (ground penetrating radar). The principle of this method is to consider the dielectric medium with changing properties as a waveguide, composed by several layers of media with different thicknesses and dielectric constants. Then we can derive the phase velocity dispersion curve from the guided waves to extract the information of each layer. As we know from some former studies, the penetration of water and chlorides can significantly change the complex permittivity of concrete. Under this condition, the new method using waveguide can be applied to monitor the chloride ingress into concrete. A dielectric mixing homogenization model found by the authors to efficiently implement the multi-layered waveguide is the parallel model. It is originally from the well-known Lichtenecker-Rother equation, combining the dielectric properties from different parts of a composite with its global dielectric. A general procedure of this method starts by a CMP (common mid-point) measurement using GPR on the material. Then the picked dispersion curve describing several propagation modes is used to invert the dielectric constant and thickness of each layer. Our discussion focuses on the monitoring of salty water penetrating into 2 different concretes due to the capillary effect. The concrete slab is considered as an equivalent medium composed by two layers: dry layer and wet layer. The multi-layered waveguide inversion results are compared to the semi-destructive gammadensimetry results, considered as reference. They show that this NDT method makes it possible to monitor the sea water ingress front in concrete over time.