The Ontario Ministry of Transportation (MTO) retained Stantec Consulting to complete a Ground Penetrating Radar (GPR) survey with the intent of identifying the thickness of the surface course on Highway 401, from 500 m west of the Trent River Bridge, in Trenton Ontario, westerly 11.7 km. The GPR survey was completed in the eastbound and westbound travel lanes to determine the pavement layer surface course profiles. The surface course on this section of Highway 401 consisted of a Dense Friction Course (DFC) which was delaminating at a number of locations. Conventional GPR surveys identify the entire thickness of a bituminous material over a granular base or concrete pavement. The application of GPR technology used on this project was considered nonconventional since the survey equipment was used to isolate the surface layer from the rest of the bituminous pavement layer. Additionally, core data was used to calibrate and validate the GPR thickness data. The GPR data was checked for quality and processed using RADAN 6.5, an advanced GPR data reduction software developed by GSSI. GPR data processing involves identifying reflections caused by changes in the electrical properties (dielectric, electrical conductivity, etc.) of a material. The data technician digitized the measured reflection and the software was used to convert the digitized reflection into layer thicknesses. Once the layers were identified with RADAN 6.5, the layer and thickness data was exported as an ASCI or Excel file. The exported GPR data was summarized and formatted as per the specifications outlined by MTO. The GPR layer statistics including the minimum, maximum, average, and standard deviation were reported and are presented in this paper. The GPR data was calibrated using ground truth information obtained by cores that were extracted along Highway 401 within the project limits. This process involved inputting a known layer thickness (core information) at a given point along the GPR survey, into the RADAN software which allowed it to calculate the electrical properties for the specific asphalt material that was present on site. In total, 28 cores were used to calibrate the GPR data. By default, the RADAN software will use an assumed average value for the electrical properties of the pavement materials if no ground truth information is available. The RADAN software will typically select the nearest core to calculate the electrical properties at each GPR scan. This paper will discuss the specialized GPR equipment setup and survey/analysis methods used for this case study project. Ultimately, a near continuous depth profile on the surface course asphalt was determined. Considering deficient lift thickness as the primary causation, this data was used to determine the extent of pressing repair needs which in turn was used to structure a preventative pavement intervention.
