Better Roads Staff
The dynamic modulus test is not unlike the compression test for portland cement concrete. With the compression test, though, the lab is looking for a failure strength, in which the specimen is broken to bits; the dynamic modulus is tested at much lower loads, in a low strain range, and not tested to failure.
The Asphalt Pavement Analyzer (APA) Rut Tester may predict rutting by exposing mix samples to repetitive loads. It consists of a rubber hose resting on a beam of asphalt, or cores, with a steel wheel that passes over the hose repetitively, replicating the impact of a tire. It is not unlike the Hamburg Loaded Wheel Tester, which uses the steel wheel only. “There is a lot of debate as to which rut tester is more accurate, and they each have their own advantages,” Clyne says. “Most of the time they will rate mixtures similarly.”
The solvent extraction method is one way of measuring asphalt content. The ignition oven method, as developed by the National Center for Asphalt Technology, is a quicker, less labor-intensive method, but it has a limitation.
“If you are looking just for asphalt content in the mix, either one will give you an accurate measure,” Clyne says. “If you want to run an aggregate gradation test afterwards you can run a gradation on either sample. The advantage to the solvent extraction method is that you can test the asphalt as well as the aggregate; the ignition oven quickly burns off all the asphalt so you can’t test it afterwards.”
There is a “green” element to the NCAT ignition oven: The ignition oven does not involve use of polyaromatic hydrocarbons to dissolve the asphalt from the sample, which are perceived to be atmospheric pollutants.
“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.
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