Toward Core-Free Pavement Compaction Evaluation: An Innovative Method Relating Asphalt Permittivity to Density

Abstract

Asphalt pavement compaction quality control and quality assurance (QC/QA) are traditionally based on destructive drilled cores and/or nuclear gauge results, which both are spot measurements representing significantly less than 1 percent of the in-service pavement. Ground penetrating radar (GPR) is emerging as a tool that can be used for nondestructive continuous assessment of asphalt pavement compaction quality through measuring the pavement dielectric constant. Previous studies have established that asphalt pavement dielectric constant measurements are inversely proportional to the air void content for a given asphalt mixture. However, field cores are currently required to calibrate the measured dielectric constant to the pavement density. In this paper, a method is proposed to eliminate the need for field calibration cores by measuring the dielectric constant of asphalt specimens compacted to various air void contents. This can be accomplished with a superpave gyratory compactor (SGC), which is routinely used in the pavement industry to fabricate 6 in. (15.2 cm.) diameter specimens. However, this poses difficulties with the GPR antenna height, direct coupling, and the Fresnel zone in relation to the asphalt specimen dimension limitation. These challenges are overcome by employing a plastic spacer with a known dielectric constant between the SGC specimen and the antenna. The purpose of the spacer is to reduce GPR wave speed so that the signal reflected from the specimen is separated from the direct coupling effects at an antenna height where the Fresnel zone of the GPR is not affected by the specimen dimension. The specimen dielectric constant can then be measured using the reflection coefficient-based surface reflection method (SR) or the pulse velocity-based time-of-flight method (TOF). Also, The Hoegh–Dai model (HD model) is demonstrated to reasonably predict pavement density based on the results of field measurements and corresponding core validation, especially as compared to the conventional exponential model. Results are presented from multiple days of paving on one project, as well as a single paving day on a project with significantly different mix properties. The agreement between the HD model, coreless prediction, and field cores shows the promise for implementation of dielectric-based asphalt compaction evaluation without the need for destructive field core calibration.

Minnesota Department of Transportation Case Studies for Coreless Asphalt Pavement Compaction Assessment

“…Until recently, conversion of dielectric to air void content has required calibration to field cores cut in the newlyl placed asphalt pavement. To establish a conversion from dielectric constant of the pavement to air void content without field cores, a procedure as detailed in Hoegh et al is completed ( 10 ). Gyratory pucks are created from mix collected from the asphalt plant on the project.…”

“…To determine the dielectric constant of the gyratory specimens, the procedure, as detailed in Hoegh et al, is completed using the DPS ( 10 ). The procedure allows for calculation of the dielectric constant of the asphalt material two ways including the surface reflection and time-of-flight method.…”

“…The surface reflection method calculates the reflection coefficient between the Delrin and asphalt surface as the basis of the dielectric measurement. As detailed in Hoegh et al ( 10 ), although this method is more analagous to the field measurements, it exhibits too much variability, presumably as a result of the effect of asphalt heterogeneity when the signal origin is so close to the asphalt surface. The time-of-flight method is based on a velocity measurement of the signal through the asphalt specimen, which can be calculated using the time of flight as the specimen dimensions are known, and Fermat’s principle is an appropriate assumption to estimate the signal path.…”

“…The conventional method for accomplishing the mix-specific conversion requires multiple field cores, which are destructive, and the process is not accommodating to projects in which mix components are changing. To complete this task without requiring field cores from the placed pavement, a method has been proposed using pucks manufactured using a Superpave Gyratory Compactor (SGC) ( 10 ). Asphalt specimens compacted across a range of observable air void contents are created from the production mix.…”

“…Hoegh et al show that by using a known dielectric spacer material, the wave from the GPR antenna can be slowed down enough to increase the time gap between the direct coupling and the asphalt specimen response that an accurate measurement of the dielectric constant of the 6 in. specimens created from the SGC can be completed ( 10 ).…”