A big chunk of the cost of maintaining the U.S. highway system is determining the remaining life of pavements and bridge decks, including asphalt-overlaid bridge decks. Departments of Transportation not only have to determine their useful life, but what is causing them to deteriorate and how to fix them.
Asphalt pavements are good for bridge decks because they prolong their life. They are not affected by deicing salts and, if properly constructed, can waterproof the bridge floor, which prevents water and salts from damaging it. Asphalt surfaces are also highly skid-resistant, providing a safe bridge deck surface.
Traditional testing of asphalt overlaid bridge decks involves drilling core samples from the pavement. The samples measure layer thickness and show what conditions beneath the pavement surface are causing deterioration.
But drilling cores for pavement condition information is labor intensive, requires lane closures and creates potential safety hazards for highway workers and motorists. Also, extracting cores and evaluating them at offsite laboratories is time consuming and costly.
The solution to the problem of efficiently assessing asphalt-overlaid bridge decks is Ground Penetrating Radar (GPR). GPR can survey bridge deck pavements and structures quickly and economically.
“You can actually scan a bridge deck without coring or testing,” says Evan Guarino, Field Engineer for INFRASENSE Inc., a GPR consulting firm in Arlington, Massachusetts. “We find GPR to be better than coring or infrared thermography. It’s the fastest and most efficient option to get an accurate condition of the bridge deck.”
Highway engineers can assess sub-surface conditions at a fraction of the cost of conventional methods. GPR systems can survey pavements quickly, with minimal traffic disruption and safety costs.
What is GPR?
Simply stated, GPR is a non-destructive technique to measure the subsurface condition of bridge decks. It creates a cross-sectional image of the pavement subsurface — a pulse-echo technique that measures pavement layer thickness and a number of other bridge deck conditions.
Originally developed for geotechnical evaluations and mine detection, GPR was introduced for highway applications in the early 1980s. Work by the New England Transportation Consortium, Ontario Ministry of Transportation and the Strategic Highway Research Program (SHRP) helped to establish GPR capabilities for bridge deck condition assessment.
There is now an ASTM specification for evaluating asphalt-covered bridge decks with GPR, “ASTM 6087-08: Evaluating Asphalt-Covered Concrete Bridge Decks Using GPR.”
A typical GPR system has antennas mounted on a moving vehicle. The antennas transmit short pulses of radio wave energy into the pavement. As the energy travels through the pavement structure, it creates echoes when it hits different materials, such as asphalt pavement. The strength of the echoes, and the time it takes to travel through the pavement can be used to calculate pavement layer thickness and other properties.
The advantages of GPR are many. GPR surveys can be conducted anywhere. The survey equipment is mounted on a vehicle that can travel at normal highway speeds, which means lane closures aren’t necessary. Traffic isn’t interrupted and highway workers aren’t exposed to safety hazards.
GPR can assess freeze-thaw damage, evaluate pavement deterioration, measure overlay thickness and maintain quality control of steel reinforcing bar placement. Bridge deck information like the presence of high chlorides, delamination activity, moisture, corrosion potential and voids can be revealed with GPR evaluation.
SHRP, the Federal Highway Administration (FHWA), and State DOTs have conducted studies that demonstrate the advantages of GPR technology. Florida, Louisiana, Michigan, North Carolina and Texas all use GPR in their pavement evaluation programs.
The Arizona DOT used GPR to survey 135 bridge decks as part of a bridge inspection program. The program provided data on deck conditions on more than 1.5 million square feet of bridge decking. GPR made it possible for the State DOT to test as many as 12 bridges a day without lane closures and traffic disruptions.
Guarino says that GPR does well when there is a mix of materials on the bridge deck or when a project includes a number of bridges with bridge decks that have different materials.
Evaluating GPR efficiency
A 2006 study by the FHWA and the South Dakota DOT (SDDOT) on the “Feasibility of Using GPR for Pavements, Utilities and Bridges, the Findings and Conclusions” stated that for SDDOT and other DOTs the most common application of GPR was evaluation of pavement thickness and assessment of bridge deck deterioration.
The feasibility study found that the primary purposes of GPR were to provide layer thicknesses for back calculation; determine layer thicknesses and representative sections using fewer cores; categorize pavement type (thick vs. thin); determine pavement thickness for the design of in-place rehabilitation; determine salvage quantities; detect bridge deck deterioration at reasonable speed and reduce exposure of coring crews; and identify pavement stripping, air voids and reinforcement location.
FHWA and SDDOT’s study recommended that the equipment most suited to both pavement and bridge deck applications was a vehicle-mounted horn antenna coupled with a vehicle-based data acquisition and storage system. The study recommended that SDDOT use a horn antenna GPR system and that data analysis software such as “BridgeScan” and “RoadScan” be used with the equipment.
The study concluded that for SDDOT and other DOTs, using GPR for pavement evaluation, pavement thickness and assessment of bridge deck deterioration, is both efficient and economical.