Ground Penetrating Radar for Measuring Thickness of an Unbound Layer of a Pavement

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Ground Penetrating Radar (GPR) is a widely used technology in the United States of America for measuring pavement layer thickness. GPR consists of an antenna that produces short duration electromagnetic pulses that penetrate pavement materials. The radar pulses are reflected at interfaces where the dielectric constants of material layers change. The reflected amplitude depends on the change in dielectric constant, while the arrival time of a reflected wave at a detector also depends on the depth at which the discontinuity is encountered. Layer material’s dielectric property is used for pulse velocity and thickness calculation. Most of the GPR software performs the pulse velocity calculation using surface reflection method, where dielectric constant is assumed to be 4.0 for unbound materials, which may not be field representative. Simply, field moisture content may vary at different months over a year, which may lead to variations in the dielectric constant of an unbound layer. This study examines whether the default dielectric value, which is a common practice in USA, should be used for determining thickness of an unbound layer. In this study, using a percometer, the dielectric constant of an unbound layer material collected from an instrumented pavement section at the Interstate I-40, near Rio Puerco, New Mexico (milepost 141) was measured. GPR tests were conducted at the same instrumented section using 400 MHz and 900 MHz antennas. From analysis, it is observed that laboratory measured dielectric constant resulted in an accurate determination of layer thickness. Additional GPR and percometer tests were conducted on two other pavements: US 54 and US 285. Results confirmed that the accuracy level of the unbound layer thickness prediction drops significantly if the default dielectric constant value of 4.0 is used, as opposed to actual values. It is therefore recommended to use actual/measured dielectric constant values to determine unbound layer thickness using GPR technology.