A "Better Mousetrap" for Asphalt Mix Quality Control

Looking for a fast, easy, and environmentally acceptable way to determine the asphalt content and aggregate gradation of a Superpave mix sample? According to a growing number of contractors and materials engineers, the new binder ignition oven may be the answer.

The binder ignition oven is very simple to operate. A sample of a mix is weighed and then placed on a tray inside the oven. The oven heats up to the preset level of 538 ºC (1,000 ºF), at which point the asphalt binder ignites and burns off. This process takes just 30 minutes to 1 hour, depending on the volume of asphalt. After the sample cools, it is weighed again to determine the asphalt content. Some models weigh the sample automatically at the beginning and end of the test. The remaining aggregate is almost completely free of asphalt and is ready for gradation testing.

The binder ignition oven has several major advantages over the old solvent extraction method. The biggest is speed. The solvent extraction methods required that a mix sample be washed free of asphalt and dried for weighing-a process that could take well over 2 hours.

The binder ignition test's big edge in speed may get even bigger. "We're now looking at a procedure that would take just 10 to 15 minutes," says Ray Brown at the National Center for Asphalt Technology (NCAT) at Auburn University in Alabama, where the binder ignition procedure was developed.

The binder ignition oven is also preferable to the chemical bath for environmental and cost reasons. The chemicals most commonly used are chlorinated solvents, which are carcinogenic and detrimental to the ozone layer. Manufacturers have been barred from producing many of these chlorinated solvents in the United States since the beginning of the year, causing prices and disposal costs to soar.

Tests by NCAT found that the binder ignition oven is more accurate than solvent methods and the nuclear gauge, another procedure for determining asphalt content.

Because of its speed, convenience, and accuracy, the binder ignition oven is ideal for quality control. "The test is extremely useful for verifying mix designs for quality control purposes," says John D'Angelo of FHWA's Office of Technology Applications. "It allows us to ensure that Superpave mixes are what they are supposed to be."

More than half of the States now allow contractors to use the test, according to NCAT's Brown.

Among the first was Virginia, where the switch was prompted by the ban on production of chlorinated solvents. Brian Prowell, from the Virginia Department of Transportation's (DOT) Virginia Transportation Research Center (VTRC), says Virginia began evaluating the binder ignition oven in 1994 and found it to be as accurate as solvent methods and the nuclear gauge. Contractors had the option of using the device for quality control in 1995, and about half did so. "Now, all but three or four contractors are using it," Prowell says. Virginia also allows contractors to use the binder ignition oven for acceptance procedures, which are used to determine whether mixes have the specified asphalt content and thus whether the contractor will be penalized for failing to meet the specifications for a project.

At Virginia DOT, the oven is used for quality assurance, the procedures highway agencies use to ensure the validity of contractors' quality control methods. The VTRC also uses the binder ignition oven (set at a lower than normal temperature) instead of solvents to remove asphalt from glass bottles used in the rolling thin-film oven.

The Florida DOT is beginning to use the new test. Contractors can use the binder ignition oven for quality control, and the DOT is evaluating the oven for use in acceptance procedures, says Gale Page, materials engineer with the Florida DOT. "Florida began using safer nonchlorinated solvents about 5 years ago. We're changing because the ignition oven is faster, and unlike the solvent procedure you can set it and forget it. We're going to require the use of automated weighing and printing features.

"The ignition oven is a better mousetrap, and we are always open to a better mousetrap," Page continues. "I think it will benefit the DOT and the industry."

Contractors agree. Randy West of APAC calls the binder ignition oven "one of the best things to come along in testing equipment in a long time."

The binder ignition oven isn't suited to all applications, D'Angelo warns. "The oven slightly changes the aggregate's properties. You can still run the gradations, but it can change the specific gravity."

NCAT's Brown notes that while the device is great for quality control, "it is somewhat less useful for forensic studies to identify what caused a pavement to fail."

Tom Harman, materials engineer with FHWA, warns of another obstacle. "One of the challenges is determining how to calibrate the test for materials in your area." The VTRC's Prowell says his organization overcame this problem through familiarity with local aggregates and a very rigorous calibration procedure.

Procedures for using the binder ignition oven are currently being reviewed by the American Association of State Highway and Transportation Officials and the American Society for Testing and Materials and could be published early next year.

The binder ignition oven is commercially available from several manufacturers. Prices range from $3,500 to $12,000. It is also used in FHWA's mobile asphalt laboratories.

For more information, contact John D'Angelo at FHWA (telephone: 202-366-0121; fax: 202-366-7909).

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