Infrared Ovens Make Quick Work of Asphalt Testing
Michael D. Wheeler
FAYETTEVILLE, Ark. -- No one likes driving on a road pitted with ruts and potholes. One way to ensure smooth roads is to use good asphalt, and the key to high-quality asphalt is achieving the proper proportions of aggregate, air and binder. Too much of any ingredient and the asphalt is liable to break down easily.
A team at the University of Arkansas has determined that infrared ignition ovens test the binder content of asphalt blends as effectively as standard ovens. Stacy Williams, supervisor of the university's asphalt laboratory, and her colleague, Kevin Hall, compared the performance of a standard ignition oven with that of a Model 4730 infrared oven on loan from Troxler Electronic Laboratories Inc. of Research Triangle Park, N.C.
Researchers have determined that infrared ovens are as effective as conventional ignition ovens for testing the binder content of asphalt blends, and they offer added benefits. Courtesy of Stacy Williams.
A standard ignition oven heats an asphalt sample to 1000 °F using convection, which causes the binder to burn away from the aggregate. The sample is weighed before and after ignition, enabling an engineer to calculate the loss of mass and, thus, the binder content. An aggregate correction factor also is applied to account for mass changes caused by impurities and other characteristics of the aggregate.
The Troxler oven, by contrast, uses infrared radiation generated by its patented elements to initiate the burn, with a peak wavelength of 2.25 µm for the 240-V model and 2.8 µm for the 120-V unit. It also features an integrated scale to measure the lost mass. Experimental data are available in hard copy via a built-in printer, or information may be downloaded to a computer using an RS-232 interface.
The researchers tested 53 samples of known asphalt content and 77 samples of unknown asphalt content in the ovens. Each oven effectively burned away the binder. The results indicated that the aggregate-correction-factor values are not interchangeable between the ovens, but that the ovens produced comparable results when the researchers applied the appropriate correction factors.
The IR oven has some benefits over standard ignition ovens, which are comparable in price, Williams noted. In the study, the IR oven required a shorter preparation time, and she was able to retrieve the sample without using a face shield. An operator must wear face protection and cumbersome elbow-length gloves to guard against the intense heat of a standard oven. She cautioned, however, that risks remain when operating an IR oven.
"The infrared oven is slightly safer from the standpoint of having less radiant heat escape from the oven when the door is open," Williams said. "Obviously, the internal parts of the oven are still very hot and could cause a severe burn to the careless operator. I think it is an exaggeration to claim that the infrared is so much safer, because injury is possible in either case."
Williams and Hall presented their findings at the annual meeting of the Association of Asphalt Paving Technologists in March.
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