Better Roads Staff
“As recently as 10 years ago we used more harmful chemicals, such as trichloethylene, that could be cancer-causing,” Clyne says. “But we’ve gone away from those to use much safer, much more environmentally friendly chemicals that still extract the asphalt, but don’t come with all the health and safety risks.”
Either way, workers are protected by safety garb, gloves, protective eye wear and fume hoods that pull fumes away from the work area.
Non-destructive testing or evaluation of pavements in the field avoids coring and section-cutting, which can compromise the long-term performance of a pavement if not done right, and certainly affect ride quality. New technologies make this possible.
“These tools have their limitations but they are very good tools all the same,” Clyne says. “If we can run equipment over the road surface without cutting a core or a trench, and it can tell you what you need to know, that equipment is a very helpful thing.”
The falling weight deflectometer (FWD) is a nondestructive testing device that evaluates physical properties of pavement, including structural capacity for overlay design, or determines if a pavement is being overloaded; a load pulse is imparted that simulates the load produced by a rolling vehicle wheel.
The trailer-mounted FWD will use a plate about a foot in diameter, which drops a load of a known weight on the pavement. Sensors at various spacings around the load plate measure the deflection of the pavement surface from the impact.
On concrete, the FWD can indicate load-transfer efficiency across joints. For all pavements it can test the entire structural capacity of the road, or by using backcalculation, use the raw load and deflection data to determine the stiffness of each of the layers in the pavement system.
“The FWD will not tell you depth,” Clyne says. “You either have to know that from the plans, take cores, or use ground penetrating radar.”
Ground penetrating radar (GPR) is a relatively new, non-invasive, nondestructive pavement testing procedure that will reveal pavement structure data. GPR is an alternate to FWD testing but also may supplement it.
Oklahoma DOT uses GPR to reduce the number of cores required
Antennae mounted on a moving vehicle transmit short pulses of radio wave energy into the pavement structure, and echoes are created at boundaries of dissimilar materials (such as the asphalt–base interface), reports the Federal Highway Administration. The arrival time and strength of these echoes can be used to calculate pavement layer thickness and other properties, such as moisture content.
“Coring may have some degree of effectiveness for specific projects, but at a network level it is costly, intrusive to traffic, and provides very limited samples of the actual pavement structure,” says Dr. Ken Maser, P.E., president of Infrasense, Arlington, Mass.