Publication Number: FHWA-SA-96-022
Date: November 1996

Mix and Binder Expert Task Groups Meet

The expert task group (ETG) on the Superpave binder specification and the ETG on the Superpave mix procedures both met in September. Here's a recap of some of the key items discussed at the meetings.

Binder ETG

• A closer look is being given to how fatigue influences the binder characteristics in the Superpave specification. Selecting the proper climate or test temperature may be the most critical aspect in evaluating a binder's resistance to fatigue cracking. The ETG has assigned a task group to review findings on this issue developed by the California Department of Transportation (DOT) and others.
• A prototype of the direct tension device, which is used to measure a binder's low-temperature tensile and fracture properties, is now in the final stages of design. Preliminary test results indicate that the binder elongation rate for the test should be changed from 1.0 mm/min to 0.1 mm/min. This change brings the data from the direct tension device more in line with results from the bending beam rheometer. Once all necessary changes have been made to the prototype and ruggedness testing has been completed, FHWA will buy direct tension test devices for the State highway agencies, as part of the Superpave pooled-fund purchase.
• An ongoing National Cooperative Highway Research Program project (NCHRP 9-10) is developing protocols for evaluating modified binders with the Superpave tests. The ETG has recommended that the American Association of State Highway and Transportation Officials (AASHTO) review those results, when available, to determine changes that need to be made to the provisional standard, "Standard Practice for Grading or Verifying the Performance Grade of an Asphalt Binder" (PP6).
• The ETG is investigating why there is a 0.01 difference between measurements taken by the bending beam rheometer manufactured by ATS and that manufactured by Cannon. Until the reason can be found, the ETG recommends that laboratories outfitted with a bending beam rheometer manufactured by ATS increase their test results by 0.01.

Mix ETG

• A guide on using reclaimed asphalt pavement in Superpave mixes will be completed this winter.
• Guidelines for the evaluation of solid additives incorporated into asphalt mixes during construction are being developed.
• The ETG is evaluating the fine-aggregate angularity test to see if test procedures or criteria should be modified.
• The ETG is investigating some relatively simple tests, such as the rolling wheel tester and a simplified shear tester, as additional Superpave mix design tools.
• The ETG's recommendations on the final version of the "Standard Specification for Superpave Volumetric Mix Design" (MP2) and the "Standard Practice for Designing Superpave Hot-Mix Asphalt" (PP28) have been forwarded to the AASHTO Subcommittee on Materials.

For more information on the Binder ETG, contact John D'Angelo at FHWA (202-366-0121; fax: 202-366-7909; email: john.d'angelo@fhwa.dot.gov).

For more information on the Mix ETG, contact John Bukowski at FHWA (202-366-1287; fax: 202-366-7909; email: john.bukowski@fhwa.dot.gov).

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