Radar survey of concrete elements: Effect of concrete properties on propagation velocity and time zero

Ground-penetrating radar (GPR) is widely used to detect and locate steel reinforcement in concrete. However, there are many factors which need to be considered, if accurate depth measurements are to be made. Knowledge of the reference time zero and the propagation velocity of radar waves is essential. This paper presents experimental results on the effect of the physical properties of concrete on the time zero and the propagation velocity of both direct and reflected waves radiated by a ground-coupled antenna. Laboratory experiments were conducted on reinforced concrete slabs and involved various porosities, water contents and depths of steel reinforcing bars. The results of this research demonstrate clearly that the direct wave, which is radiated laterally, propagates at the same velocity as the reflected waves. The implication of such a result is that the direct wave time position is not constant and is affected by the concrete porosity and water content. Therefore, in cases where the direct wave is used as the time reference, this paper proposes to improve delay measurements by applying a delay correction corresponding to the travel time of the direct wave and taking account of the transmitter–receiver offset and of the propagation velocity of radar waves in concrete.