The objective of this paper is threefold. First, a parametric study is performed to evaluate the absorption-dispersion effect on GPR waveform spectra. Reference data for this evaluation correspond to synthetic GPR signals generated by one of the frequency power law variants. The result of this study justifies the need for processing within the spectral domain in order to calculate either propagation velocities or complex permittivities. Second, an innovative technique for extracting the electromagnetic dispersion of hydraulic concretes using GPR is presented. This technique is based on a coupling between modified forms of the two-dimensional Fourier transform and quality factor estimation methods; moreover, it allows identifying dispersion phenomena as a variation in the complex permittivity as a function of frequency. For each method, an analytical validation is carried out on simulated propagation signals. For the third objective, an experimental study is conducted in order to correlate the electromagnetic dispersion indicator with both physical and hydrical characteristics of the various concrete mixtures. The complex permittivities obtained according to this "inter-method coupling" (applied to GPR measurements) are compared with those obtained from semi-destructive measurements on cores using a cylindrical transmission line considered as the reference. This procedure makes it possible to derive trends that provide information on the conditioning state of the studied media.