Accurate knowledge of pavement layer thicknesses is important for accurate backcalculation of pavement layer properties as well as for network-level pavement condition inventories. Often this information is unknown, and records are inaccurate, or difficult to access. This paper describes a self-calibrating Ground penetrating Radar (GPR) system for highway speed pavement layer thickness evaluation. The radar system incorporates air-coupled horn antennas, PC based digital data acquisition, and software for computing pavement properties from raw waveforms. A software system called PAVLAYER~ has been developed and extensively tested for pavement thickness evaluation. This system has demonstrated an accuracy ranging from 5 to 10 percent for asphalt layer thickness, and 10 to 15 percent for base layer thickness. These accuracy figures are based on test data from 100 pavement sections and correlation with data from 594 cores and test pits. The paper describes the details of the hardware and software components and the analytic methods used in the pavement thickness evaluation system. Also presented are descriptions of, and results from, field evaluation programs in which the radar results were correlated with ground truth. These programs have been carried out with federal and local highway agencies in 20 different states in the U.S., in the U.K. and in Germany. Each of these programs have involved radar surveys of pavement sections representing different ages, pavement designs and pavement condition. Extensive coring has been carried out to confirm the radar predictions, and the correlation between core and radar data are presented. The reported results also show how the accuracy of the layer thickness calculations are affected by different variables and how other pavement structure and condition information is revealed in the radar data. Specific results are presented showing repeatability, and the influence of the survey speed, moisture on the pavement surface, surface treatments and thin overlays, and pavement age and structure.


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