Approach to Determining In Situ Dielectric Constant of Pavements: Development and Implementation at Interstate 81 in Virginia

Abstract

A major problem in using ground penetrating radar (GPR) for estimating pavement layer thickness is assuming the dielectric properties of that layer. Pavement dielectric properties may vary significantly due to aggregate type, moisture presence, and other conditions. Therefore, uncertainties in the dielectric constant, which may vary from 3 to 15, will result in misleading thickness determination. Obtaining cores for calibration may reduce the error, but the variation in the dielectric constant along the roadway often leads to errors in the thickness determination. A method was developed to determine the dielectric constant, and therefore the thickness, of the hot-mix asphalt (HMA) layer of a pavement using GPR. Because of the different compositions and ages of the layers forming HMA in older pavements, dielectric constant estimation based on the surface reflection may not be accurate and may lead to wrong thickness estimations. The developed method uses a modified common midpoint technique (usually used in seismic testing) to estimate the dielectric constant, based on the reflections from a common point at the bottom of the layer. Data were collected from a 27-km portion of Interstate 81 and processed with this technique. Comparison between the thickness estimated by this method and that measured on cores extracted from the highway revealed a mean error of 6.8%.

Characterising moisture within unbound granular pavements using multi-offset Ground Penetrating Radar

“…A common use is to estimate the depth of pavement layers (Al-Qadi and Lahouar, 2005, Plati and Loizos, 2012. These estimates need to be calibrated to account for variations in pavement permittivity and the EM wave velocity to achieve accurate results (Lahouar et al, 2002, Evans et al, 2007. The permittivity of pavement layers is strongly influenced by water content Loizos, 2013, Rmeili and Scullion, 1997).…”

“…While the use of multi-offset methods has been somewhat limited for pavement moisture quantification, this approach has been used by many authors to determine the depth and permittivity of pavement layers , Mesher et al, 1995, Edwards and Mason, 2011, Hamrouche and Saarenketo, 2014, Zhao and Al-Qadi, 2016, Liu and Sato, 2014, De Pue et al, 2016, Hugenschmidt, 2000, Lahouar et al, 2002. To enable efficient data collection using off-the-shelf equipment, some authors have combined the …”

“…A disadvantage is that data collection and analysis is more involved compared to the conventional common-offset GPR approach, which involves collecting measurements using one antenna pair with a fixed separation. While a number of studies have used pairs of common-offset antennas to enable rapid collection of a simplified multi-offset dataset , Hamrouche and Saarenketo, 2014, Lahouar et al, 2002 or have used 3D GPR systems to achieve a greater number of antenna offsets or wider coverage across the road lane (Zhao and Al-Qadi, 2016, De Pue et al, 2016), most have used air coupled antennas. This is problematic as surface-refraction issues limit calibration sensitivity .…”

“…This in turn enables calibration of layer depth estimates and determination of the bulk relative permittivity of pavement layers, from which the moisture content or other pavement parameters or their variation may be inferred, for example changes in density for new asphalt pavements (Al-Qadi et al, 2010). As the approach accounts for material inhomogeneity within layers, it enables more accurate layer depth estimates for older and more variable pavement materials compared to the surface reflection coefficient approach (Lahouar et al, 2002) as that approach is better suited to new or defect-free pavements that are non-attenuative .Another advantage of the multi-offset approach is that permittivity estimates can also be determined based on the change in groundwave arrival times measured at different offsets, provided ground-coupled antennas are used. There are a number of other advantages of using ground coupled antennas for these measurements, including the maximisation of penetration depth and the minimisation of surface refraction issues.…”

“…The measurements are analysed using geophysical techniques similar to those used for seismic reflection to determine the depth of pavement layers and the velocity of EM waves within these materials based on the change of arrival time of layer interface reflections measured at different offsets. As the approach involves measuring through the full depth of layers, permittivity estimates using this approach represent an average and therefore account for material inhomogeneity (Al-Qadi et al, 2003, Lahouar et al, 2002. A disadvantage is that data collection and analysis is more involved compared to the conventional common-offset GPR approach, which involves collecting measurements using one antenna pair with a antennas.…”