The paper presents a study on the use of the ground penetrating radar (GPR) method in diagnostics of a footbridge. It contains experimental investigations and numerical analyses of the electromagnetic field propagation using the finite difference time domain method (FDTD). The object of research was a reinforced concrete footbridge over a railway line. The calculations of the GPR field propagation were performed on a selected cross-section of the bridge. Three different damage scenarios (i.e. one missing rebar and two air voids with different shapes and locations) were investigated in FDTD simulations. Numerical GPR maps were compared with results of in-situ GPR surveys for the healthy footbridge.