Accuracy of Ground Penetrating Radar (GPR) in locating post-tensioned cables

The inspection and assessment of post-tensioned (PT) bridges is a complex issue, first involving the evaluation of the conservation status of the cables. One of the most crucial pieces of information to be confident with is the cables' location since it allows the success of all subsequent tests in the tendons (both non-destructive and destructive) and is critical in the bearing capacity of the beam in bending and shear as well. Many non-destructive techniques (NTDs) are studied and employed by the scientific community and practitioners for the cables' trace determination in PT beams; one of the most promising is the Ground Penetrating Radar (GPR). This technique is commonly employed in the location of metallic elements, both under the ground (pipes, utilities, etc.) and inside the concrete (reinforcements, ducts, etc.). The GPR technology exploits electromagnetic waves typically in the range of 1000-5000 MHz in the inspection of structures and tunnel lines. The variation of electromagnetic properties of the materials results in changes in the speed of travel, attenuation, and redirection of signals. The present research deals with the determination of the GPR accuracy in locating cables in PT structures. A laboratory testing campaign on specially designed samples was conducted for this aim. The tested walls contain both metallic and plastic ducts.