Use Of A Ground-Coupled Monostatic Antenna For Determining Deterioration Of Concrete Structures

A 1.5 GHz ground-coupled antenna was used with a digital ground penetrating radar system to<br>evaluate the amount of deterioration within an aboveground concrete holding tank. Because of<br>the corrosiveness of the solution held within and the structural design of the 72 year-old tank,<br>deterioration could manifest itself by delamination and/or by surface cracking. On the GPR<br>record, potential areas of deterioration appear as zones of attenuation. Delamination is most<br>likely to occur at the inner wall where the tile and concrete meet, but can also occur at either the<br>bottom or top rebar schedules within the concrete. Surface cracking can indicate both a suticial<br>stress problems, caused by the elliptical shape of the structure, as well as more severe<br>voiding/delamination problem. Over 50 vertical profiles were conducted on the 17-foot high<br>walls using GPR to more accurately assess the deterioration associated with failure of the<br>structure’s integrity.<br>Attenuation, measured as dB loss relative to the transmitted pulse when the antenna is coupled to<br>the concrete surface, was mapped and contoured for the tile/concrete boundary and the upper<br>rebar schedule. Deterioration at the concrete surface was achieved by calculating the real<br>concrete dielectric permittivity (i.e. dielectric “constant”) from the reflection coefficient of the<br>surface reflector when the antenna was mounted on a 12” thick foam block. Attenuation and<br>dielectric information were then compared with visual observations of the data to determine the<br>overall deterioration of the structure. Overall, the curved walls revealed more deterioration, over<br>25%, while the straight sections of wall had about 10 to 14% deterioration.