ROADBUILDING & REHABILITATION
SEEING THE BIG PICTURE
Ground Penetrating Radar provides critical insights for maintaining & rehabilitating Canada’s aging roads and bridges
By Kevin Vine
M
unicipalities across Canada are burdened with aging transportation infrastructure, much of which was constructed in the post-war era and is nearing the end of its useful life. The 2016 Canadian Infrastructure Report Card (CIRC) found that one third of municipal infrastructure across the country is in fair, poor or very poor condition, and this includes 39 percent of roads and 26 percent of bridges. Cold weather and the use of corrosive de-icing salts further threaten the reliability of the transportation infrastructure we rely on every day. The estimated replacement value of these roads and bridges is upwards of $380 billion, and municipalities are responsible for covering 60 percent of their own costs. As such, finding cost-effective, efficient condition assessment tools and techniques has become vitally important. Municipalities are now looking beyond traditional methods to assess road and bridge conditions and strategically prioritize maintenance and rehabilitation activities.
Gaining insights with GPR
From 2006 to 2015, the second Strategic Highway Research Program (SHRP2) was launched south of the border as a mechanism to find strategic solutions to three national transportation challenges: highway safety, congestion, and bridge and road renewal. The report concluded that non-destructive condition assessment significantly reduces the overall resources and expenditures required for bridge and road renewal. It also concluded that capturing data via non-destructive methods helps to prevent premature, unexpected failure. Ground Penetrating Radar (GPR) ranked highest in overall value for assessing bridge deck deterioration based on accuracy, precision, speed, ease of 18
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use and cost. GPR is a technology that transmits high frequency radio waves into the ground or structure and analyzes the reflected velocity and energy to create a profile of the subsurface features. The reflections are caused by a contrast in the electrical properties of subsurface materials which can be indicative of changes in water content, void spaces in the ground or structure, rebar or post tension cable corrosion, asphalt deterioration and other factors. By revealing the complex substructure of a road or bridge, GPR can detect anomalies between coring locations, and allow project stakeholders to achieve a continuous profile of subsurface conditions. As an added benefit, fewer core samples are required because they can be strategically placed based on the GPR results - for example, in areas where the road or bridge appears to be in the worst condition. Dan Conte, Senior Bridge Engineer for URS Canada (an AECOM company) explains: “GPR evaluation provides an additional dataset that can be cross-compared and correlated with results from traditional condition survey techniques. This makes it possible to readily identify locations of suspected deterioration and therefore target areas where destructive or intrusive sampling should be carried out during follow-up condition surveys.” Once a GPR survey is complete, reports illustrating the internal condition of a road or structure can be generated in many formats, including plan maps, colour contour maps and 3D. This information can then be integrated into a GIS or CADD-based system for reference years down the line, so that patterns of deterioration can be evaluated over time.
Rehabilitating roads in our nation’s capital
GPR has been applied on multiple highways and roads across the City of Ottawa and surrounding area, including Highway 417 which connects Montreal with Ottawa and is considered the backbone of
GPR data is captured at posted speeds, often eliminating the need for traffic disruption or road closures. the transportation system in the National Capital Region. The technology has been applied to assess road structure, pavement, concrete and asphalt thickness in order to inform strategic decision-making around improved driving surfaces, localized repairs, and accessibility improvements and updates. GPR surveys were carried out using a groundcoupled, high-speed system that allows for surveying up to 1.5 metres in depth and incorporates real-time GPS synchronization. Survey profiles and reference stations for the data acquisition were established according to the customer’s base maps and project requirements. Data was collected at posted travel speeds, eliminating the need for traffic disruption or road closures. Data collected in the field was digitally recorded and backed up on site. The data was then processed and analyzed to determine the average thickness of pavement, concrete and asphalt subgrade layers. Results were presented in scaled and referenced colour contour maps along with plan and sectional views of the road and bridge structures. Statistic summary tables were also provided that depicted average, maximum and minimum estimated thicknesses.
Preserving Toronto’s main artery
The City of Toronto’s Gardiner Expressway serves as the main artery into the City, stretching 18 kilometres from the Don Valley Parkway to the Queen Elizabeth Way (QEW). In operation since 1955, structural concerns have required sections of the Highway to be rehabilitated and, in some cases, replaced. To get an accurate picture of the Expressway’s condition, GPR was used to map bridge deck deterioration over a vast, geometrically complex area.