Radar Testing of a Masonry Composite Structure with Sand and Water Backfill

As part of an ongoing program into the noninvasive geometrical characterization of masonry arch bridges, a laboratory study of radar propagation through masonry has been undertaken. A 2.4 m long × 1 m wide × 1.5 m high experimental test rig was built in the laboratory with brick walls on three sides and a wooden gate on the fourth narrow side. The objective was to construct a composite test rig that would replicate certain of the environments that ground-penetrating radar might encounter when being used to test a masonry arch bridge. Tests were undertaken using 500, 900, and 1,000 MHz bow-tie antennas with the masonry box filled with fresh water and then saline water with different concentrations of sodium chloride. Further tests were undertaken with the masonry box filled with dry sand and wet sand. Finally the test rig was modified to simulate a composite masonry structure and tests were performed both on the hollow- and sand-filled models. The results from this work are reported herein indicating the reliability and ease of identifying various targets. It was found that radar signals were severely attenuated when the saline concentration exceeded 0.05% by mass. Better penetration of the radar signals was obtained when the masonry box was filled with dry sand rather than wet sand. In the case of the hollow cellular masonry structure, refraction effects were more evident.