Comparison of Ground-Coupled and Air-Launched Ground Penetrating Radar Methods for Bridge Deck Condition Evaluation

Ground Penetrating Radar (GPR) has been increasingly adapted by highway agencies for bridge deck condition evaluation. Recent research has shown a strong correlation between GPR rebar amplitudes, chloride content, and deck corrosion. Most of this research has been carried out with small 1.6 and 2.6 GHz ground-coupled antennas, since these are the most suitable for research studies. For practical field evaluations, however, the collection of data at driving speed using 1 and 2 GHz non-contact air-launched "horn" antennas is much more attractive, since closures are not required and coverage is much more efficient and economical. While ground-coupled antennas detect the individual reinforcing steel bars, the air-launched antennas, due to their position above the surface and the speed of the survey, detect multiple bars simultaneously which appear as a “layer” in the deck cross section. They key question for practitioners is whether or not the data and results obtained with these air-launched antennas replicate what would have been generated using the lower speed ground-coupled antennas, and to what extent. This paper will present case studies of bridge deck condition evaluations in which both approaches have been utilized for the same structure. The paper discusses the relationship between the deck structure and the signal differences between the two antenna types, and also the comparison of the GPR predictions of overall deterioration quantities and spatial distribution of deck deterioration predicted used each antenna type. The results show that while there are some differences in the spatial distribution of deck deterioration using the two methods, the overall deterioration quantities are fairly close. From the owner's perspective of making deck maintenance and rehabilitation decisions, the overall quantity is the key parameter, and the air-coupled data is shown to be adequate for that purpose.

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