GPR: a new tool for structural health monitoring of infrastructure
Recent developments in acquisition methodology, data acquisition systems, post-processing software, and the availability of user friendly high-frequency antennas have enabled the GPR user to generate accurate three dimensional images of the interior of concrete structures.
Four case studies are discussed with emphasis on output that is readily understood by the non-specialist engineer. In the first two the results of traditional applications such as mapping reinforcing steel and post-tensioning cables are demonstrated.
The third case shows the results of a challenging void detection investigation, whilst the last one demonstrates for the first time that GPR can detect fibre reinforced polymer bars (FRP) embedded in concrete.
The application of ground penetrating radar (GPR) for the investigation of concrete structures is well known and has been in widespread use for a number of years [1, 2, 3]. However, it has only become mature with the advent of digital data processing in the 1990s. Early applications included exploration and mining, archaeology, mine detection and military uses. Most commercially available systems were slow and cumbersome to operate and their use required a trained and experienced geophysicist. However, starting in 2001, when faster microprocessors become available, new and user friendly dedicated radar systems came to the market. The manufacturers rolled out task specific units creating a virtually new non-destructive testing (NDT) technique for engineering, construction, and infrastructure monitoring and management applications. Despite the relative obscurity of GPR outside of geophysics or electrical engineering departments, its use has been gaining ground steadily.
GPR is a real-time, non-destructive testing technique using high frequency radio waves that can yield data with very high spatial resolution (on the order of centimeters) and the data can be acquired rapidly. It uses high frequency radio waves to inspect the interior of concrete structures. Data collection is continuous, allowing scanning of a two-foot by two-foot (60 cm by 60 cm) area in 15 minutes or less, or capturing several kilometers of continuous data in a few hours.
Current applications for structural engineers most commonly include locating spacing and depth of reinforcing steel, post tensioning cables or anchors, measuring rebar cover, mapping voids, and clearing areas prior to cutting, coring and trenching . GPR is a useful tool for seismic upgrades, road and bridge deck condition surveys, mapping delamination, or locating “lost” footings and/or utilities. Bungey  provides a comprehensive review on GPR testing of concrete.
Structural applications include addressing the integrity of the concrete itself, such as the presence of voids, cracks or chemical alteration. Due to the less well defined character of such features, GPR applicability is not always predictable on these projects and interpretation of the results depends on the specific site conditions and on the experience of the technical personnel . Intrusive testing, such as drilling or coring, often accompanies GPR investigations in order to draw definitive conclusions [3, 4].