NUMERICAL SIMULATIONS AND LABORATORY TESTS TO EXPLORE THE POTENTIAL OF GROUND-PENETRATING RADAR (GPR) IN DETECTING UNFILLED JOINTS IN BRICK MASONRY STRUCTURES

The objective of this work is to define the sensitivity of the ground-penetrating radar (GPR) signal to detect deep unfilled joint defects in the inspection of brick masonry structures and, in particular, to look for deep unfilled joint defects. This definition will help the manager to quantify the volume of mortar to be reinjected in case of reinforcing work. As a first approach, a numerical modeling of a GPR antenna with a central frequency of 1.5 GHz is used to define the sensitivity of radar waves to detect unfilled joint defects. The simulations are carried out in a separated bistatic configuration. For each transmitter position, several signal acquisitions are implemented using a regularly spaced crescent pattern for the receivers. A specific algorithm for the processing of the simulated signals has been developed that uses the inverse methods applied in the time domain and specifically a method of phase focusing to locate the defects. The processing analyzes the travel times of the reflected signals by making the assumption that each point of the modeled space is a scattering point. The calculation of the travel time, which helps to identify the signal corresponding to each point of space, is made by using an estimated speed of the direct wave between transmitter and receivers in the material, which is then regarded as representative of the whole of the simulated environment. A parametric study allowed limits to be set in terms of size, orientation and depth of the defect. Early results are promising and show that unfilled joints can be detected in the depth of the masonry structure with good accuracy. The last stage of this work is to test the validity of these algorithms on a full scale model with different kinds of unfilled joint defects before using them on real structures.