Applications of Ground Penetrating Radar (GPR) in civil engineering context are popular and it has been widely used to characterize the media in terms of structural arrangement and pathologies. This work presents some tests carried out under laboratory conditions on a concrete slab. The aim of this study is to evaluate the moisture content in a homogeneous medium (concrete) using data obtained with a 1.6 MHz GPR centre frequency antenna. The evaluation process includes detailed analysis of the signal in both time and frequency domains. The key parameters are amplitude of surface reflection, changes in the centre frequency and attenuation of signal. The central frequency decreases in the A-scans obtained from zones with high water content. The amplitude of the first signal (reflection on the surface of the specimen) exhibits an increment. These results could be useful in the assessment of foundations and slabs, as well as in the evaluations of buried geological channels.