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Federal Highway Administration > Publications > Focus > October 2001 > How Well Do You Know Your Ashpalt Mix Design?
October 2001Publication Number: FHWA-RD-01-068

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How Well Do You Know Your Ashpalt Mix Design?

You've chosen an asphalt mix design and you're ready to pave. But how will the mix perform under the traffic and climate conditions at the pavement site? According to a canvass of State highway agencies conducted earlier this year by the Federal Highway Administration (FHWA), 29 agencies now use some sort of a test prior to paving to indicate how their mix design will hold up. Of those highway agencies not currently conducting tests, 13 are gathering information and considering implementing a test. Research is being conducted nationally to develop a simple performance test to be used with the Superpave volumetric mix design procedures as a way of addressing States' need for a test tied to field performance (see sidebar).

As many agencies await the results of this national research into the simple performance test, they continue to use a variety of other test methods. Some of the evaluation methods used include the Hveem Stabilometer, Asphalt Pavement Analyzer (APA), and Marshall Criteria tests. Some of the other tests either being used or evaluated include the Static Creep, Nottingham Asphalt Tester, French Rut Test, and the Hamburg Wheel-Track Test.

Seventeen highway agencies report they have altered their decision to place a mix because of test results. Oklahoma, for example, says that "Hveem stability tests have been used on as-produced mixtures as a quality measurement. Production is stopped if tests on field mixtures consistently fail." And Washington State reports that it "runs stability tests on the first 5 days of production and then once a week thereafter. We use this information to adjust the mixture or to halt production or to be able to use this design for future projects out of the same aggregate source and the same asphalt source and grade."

Only four States currently compare the actual field performance of their mixes with the testing results. Georgia notes that it "did a limited study but was unable to get a direct correlation." The State has seen, however, "that mixtures that meet our APA test requirements do not cause rutting problems in the field." Thirteen States indicated they are working to set up systems for comparing results to field performance.

The review was conducted at the request of the Pacific Coast Conference on Asphalt Specifications, which serves as the asphalt user/producer group for the West Coast. "The Conference's Task Force on Performance Testing realized the need for using a performance test and wanted to see the direction agencies around the country are taking," says Katherine Petros of FHWA, who conducted the review. As a result of the review, the conference is now looking more closely at the simple Superpave performance tests being evaluated under National Cooperative Highway Research Program (NCHRP) Project 9-19 and gathering more information on the APA test.

For more information on the review or the individual State responses, contact Katherine Petros at FHWA, 202-493-3154 (fax: 202-493-3161; email: katherine.petros@fhwa.dot.gov).

One of the tasks of NCHRP Project 9-19, "Superpave Support and Performance Models Management," which is the continuation of an FHWA-sponsored project of the same name originally begun in 1994, is to identify simple performance tests for permanent deformation and fatigue cracking for incorporation in the Superpave volumetric mix design method. In the initial phase of the task, the project team selected three candidate tests (dynamic modulus, static creep, and triaxial repeated load permanent deformation) from among 33 test/parameter combinations. The three candidates were selected on the basis of good statistical correlation with pavement distress, as measured in tests at the Mn/ROAD test road in Minnesota, WesTrack pavement test track in Nevada, and FHWA's accelerated loading facility at the Turner-Fairbank Highway Research Center in McLean, Virginia. In the current phase of the project, which is expected to be completed in February 2002, the ability of these tests to provide an accurate measure of potential field performance is being validated through testing of hot-mix asphalt materials from the Long Term Pavement Performance Specific Pavement Study -1, -5, and -9 experiments, as well as from several State field experiments. For more information, contact Ed Harrigan at NCHRP, 202-334-3232 (email: eharriga@nas.edu).

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Updated: 04/07/2011

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