Ground-penetrating radar (GPR) is a high-frequency electromagnetic method that GEOVision commonly for the NDE and NDT of bridge decks, asphalt pavements, concrete pavements, and other concrete structures. GPR is an ideal tool for conduit detection, pavement thickness measurements (concrete and asphalt), delamination surveys, conrete deterioration surveys, rebar mapping and rebar corrosion surveys.
GPR has been used for many years to map the condition of aging bridge deck structures by measuring surface dielectric properties, signal attenuation through the deck thickness, and reinforcement reflection amplitude, as well as a number of other variables. Although this method is moderately successful in terms of its ability to estimate the percentage of deck area that is deteriorated, it has had very mixed results in terms of its ability to accurately locate deterioration quantities on a deck and define their boundaries.
New bridge deck evaluation techniques have been developed over the past four years which have markedly improved the ability of GPR to locate and define the boundaries of deterioration significant enough to require removal. Along with the big improvement in pinpointing deteriorated zones on a deck has been an additional improvement in GPR’s ability to estimate total deterioration quantity (historically expressed as % of total deck area).
Initially, these high-resolution surveys were performed exclusively with high-frequency ground-coupled antennas, which are still used in situations where their deployment is simpler and more effective. The ground-coupled antenna’s size and higher resolution gives it the unique ability to accurately measure top rebar reflection amplitude without interference from other signals returning from within the deck. Any single air-coupled antenna cannot match the ability to measure this parameter, which has the single, greatest influence on accurately estimating deck deterioration quantities.
However, continued investigation into the use of air-coupled horn antennas (1.0 GHz) resulted in the development of a novel tech-nique enabling two independently polarized antennas to effectively simulate the higher-resolution ground-coupled antenna. When two air-coupled antennas, mounted in-line and deployed so that one is sensitive to transverse steel in the deck and the other is rotated 90° so that it is most sensitive to longitudinal steel, the signals from both antennas can be measured and evaluated in a unique manner.
The signal from one antenna is subtracted from the other, taking the offset between the two antenna’s positions into account, and the resulting signal yields a top rebar reflection which is similar to the one measured by a high-frequency (1.5 GHz) antenna. Removed from this signal are unwanted interference from either a thin overlay (less than 2 inches), shallow concrete cover (less than 2.5 inches), or both. Additionally, the negative influence of the signal from the longitudinal steel, which is randomly measured along every GPR profile running the deck’s length because the antennas are sometimes directly over a bar, but often are not—is removed from the top bar signal, as well. The result is a survey which matches the accuracy of a ground-coupled inspection, but can be performed at speeds of up to 45 mph.
In addition to mapping deterioration quantities, this method allows us to determine:
GEOVision has the most advanced data acquisition unit, antennas and processing software available for this particular application, as well as many others.