Quantitative Study of Concrete-Embedded Voids by Using Ground-Penetrating Radar at Various Frequencies

River levees in Taiwan are exposed to typhoons, earthquakes, and long-term erosion and scour, which often cause subsurface voids of varying severity within the levee body. This study conducted a quantitative physical analysis of 0.15 m-thick concrete specimens containing voids of different dimensions (widths of 0.10–0.40 m and sizes of 0.06–0.15 m). The specimens were scanned using ground-penetrating radar (GPR) antennas with center frequencies ranging from 750 MHz to 2.3 GHz. Variations in electromagnetic-wave reflection amplitude within the material were used to determine void size along the X-axis, whereas the depths corresponding to the reflection points were quantified along the Y-axis. The void area was then estimated based on the X-Y coverage. The results showed that absolute amplitude differentiation provided distinct quantitative features that reflected the presence of voids of various sizes. The proposed method was further validated using an actual river-levee scour case. The findings of this study offer a practical reference for the inspection, maintenance, and repair of river levees.