Binder Expert Task Group Considers Improved Models for Fatigue, Critical Cracking of Modified Binders

At its meeting in February, the Binder Expert Task Group (ETG) proposed a detailed plan for the development of a new low-temperature specification for modified asphalt binders and a new specification for fatigue criteria.

Data from pavement test sections have been used to determine the critical cracking temperature of neat (unmodified) asphalt binders, but not modified binders. As a result, the Superpave binder specification relies on a model based on neat asphalt, and thus does not correlate well with the performance of modified binders. The redesigned direct tension test device will be used to determine all binders' critical cracking temperature. The Binder ETG is developing a plan for validating the use of the direct tension test in this application.

Similarly, the Superpave binder criteria for fatigue, which is based on dissipated energy, works well only for thick pavements built with neat asphalts. It does not adequately describe the performance of thin pavement sections and/or modified binders when evaluating asphalt alone, as opposed to a mix. The ETG is now evaluating a new procedure based on a binder's response to changes in traffic loading frequency, a more complex variable that should better explain the behavior of modified binders and thin pavements, says John D'Angelo, chairman of the ETG.

D'Angelo says these proposed changes will be reviewed over the next several months. He predicts that it will probably be late fall before the group has any strong recommendations on either specification.

The ETG continues to be an excellent example of the partnership between experts from the State and Federal governments. Binder ETG member Ron Reese of the California Department of Transportation says, "There's been a cooperative effort at the national level on the validation of standards."

The Binder ETG will meet again on May 1 and 2 in Washington, D.C. For more information, contact John D'Angelo at FHWA, 202-366-0121
(fax: 202-366-7909).

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