TRANSPORTATION RESEARCH DIGEST
ARIZONA TRANSPORTATION INSTITUTE
Ground-Penetrating Radar Calibration at the Virginia Smart Road and Signal Analysis to Improve Prediction of Flexible Pavement Layer Thicknesses by Imad L. Al-Qadi, Samer Lahouar, and Amara Loulizi, Virginia Tech Transportation Institute 3500 Transportation Research Plaza Blacksburg, VA 24061 (Virginia Department of Transportation, 1401 E. Broad Street, Richmond, VA 23219; ) (Jan 2005)
Use of GPR to nondestructively estimate the layer thicknesses of aggregate, HMA, and concrete layers is validated.
GPR can successfully be used as a quality control tool during pavement construction.
Ground-Penetrating Radar (GPR) technology has been used for the past 20 years for a variety of applications to assess pavement performance. The main issue after all these years remains to be "how well GPR works and under what conditions?" Results show that GPR works well for some situations, but is not an appropriate tool for other situations. Currently, it is not used on a routine basis by the U.S. Department of Transportation (DOT). This is due mainly to difficulties encountered in data interpretation, as well as the expenses involved for conducting GPR surveys. It is expected that there will be a growing interest and demand for GPR surveys. However, a number of limitations exist that are mainly related to interpreting the results of GPR testing:
The images obtained from the reflected signals (using signal-processing packages) are not photographs of the features that are beneath the surface being investigated. The images show the amplitude of the radar-reflected signals from the interfaces with different dielectric properties. These amplitudes are plotted in colors using
user defined color codes. Therefore, a considerable amount of experience and operator skill may be required to interpret sub-surface GPR results correctly.
Extensive amount of data.
Determination of the exact location of a reflecting feature beneath the surface relies upon a prior knowledge of the dielectric properties of the material. These dielectric properties are also frequency dependent; therefore, a frequency domain method of analysis is required to accurately measure thicknesses.
Change of the dielectric constant with depth mainly because of the presence of moisture.
Losses in the pavement materials, especially with the presence of moisture or conducting sub grade soils. . Reflections from thin layers may overlap, depending upon the GPR system resolution. . Insufficient dielectric contrast between layers may hinder the detection of the underneath layers.
This report presents findings from the GPR measurements performed between June 1999 and December 2002 over the pavement sections of the Virginia Smart Road. The first part of the Results and Discussion