IF YOU CAN’T STAND THE HEAT

FAYETTEVILLE, Ark. - UA researchers have determined that an infrared ignition oven developed to test asphalt binder content is safer and more efficient than the standard ignition oven. They presented their findings Monday at the Association of Asphalt Paving Technologists' Annual Meeting.

"The ovens are similar in accuracy, speed and cost," said civil engineering research assistant Stacy Williams. "But the infrared oven is easier to use." They conclude that the infrared oven can make testing more efficient because it requires less safety equipment and fewer steps in the testing process.

Taking some little rocks and some bigger rocks and adding some binder and makes asphalt. But add too much of any one ingredient, and the asphalt may break down too easily, leaving a road with ruts and potholes. That makes testing the binder content of hot mix asphalt (HMA) a critical step.

Recently new testing tools, including the infrared oven, have been introduced. University of Arkansas researchers Williams and Kevin Hall, associate professor of civil engineering, compared the infrared oven with the standard ignition oven to determine its accuracy and ease of use.

"A lot of methods have been used in the past," Hall explained. "The nuclear and solvent extraction methods have environmental concerns. Currently, the standard ignition oven, which was developed in the 1960s, is the most commonly used ignition method."

A standard ignition oven heats an asphalt sample to 1000 F (538 C) by radiant heat. The binder catches fire and burns away from the rocks. By weighing the sample before and after ignition, an engineer can calculate the loss of mass and then use an aggregate correction factor (ACF) to calculate the binder content. In addition, the ignition oven leaves clean aggregate that can be tested for gradation.

The radiant-heat ignition ovens are effective, but they pose some problems for the user. Because of the intense heat that emerges when the oven door is opened, the operator must wear a full-face shield and other cumbersome personal protective equipment. The samples must be pre-heated to prevent warping of the test vessels and the intense heat of the ovens can sometimes degrade the aggregate particles.

A new type of ignition oven uses infrared radiation to heat the sample. It is supposed to be faster and operate at lower temperatures than the standard oven, thus reducing sample degradation.

The researchers compared the two ignition ovens by using matched pairs of samples of Superpave HMA. HMA calibration samples were used to determine the ACF for each oven.

"Every oven has a different ACF," Williams explained. "You can’t use the ACF for a standard oven with an infrared oven or vice versa."

After calibrating the ovens, Williams and Hall compared the measured values of the ovens by using 77 sample pairs with unknown content. They then tested 53 sample pairs with known content to determine oven accuracy.

Although similar to the standard ignition oven in accuracy, the infrared ignition oven sometimes had a larger ACF. The researchers examined the oven’s printout and found that, contrary to expectations, the infrared oven sometimes had higher temperatures than the standard oven. They also found that the infrared oven took the same amount of time to process a sample, although it did eliminate the preheating step required for the standard oven.

"Because it doesn’t produce a blast of heat when you open the door, it doesn’t require so much safety equipment," Williams explained. "And not having to preheat the sample can make testing more efficient."