Changes Make Superpave Mix Design Easier To Use

To make the Superpave mix design system simpler, more practical, and more user friendly, more than 50 changes to 4 Superpave mix design standards were recently published by the American Association of State Highway and Transportation Officials (AASHTO). Proposed by a 5-State task force established at the August 1998 meeting of the AASHTO Subcommittee on Materials, the changes were approved by the States in February 1999 and published in the May 1999 AASHTO Provisional Standards. Some of the more significant changes include the following:

• The table for determining the number of gyrations a mix must undergo in testing (N-design table) has been dramatically simplified, and the design levels are now based on 20-year design equivalent single-axle loads (ESALs);
• In the use of the gyratory compactor, a relaxed criterion of density at the initial number of gyrations (N-initial) will allow the use of more finely graded mix designs for low-volume roads-of likely interest to local governments;
• Pavement layers are defined more precisely;
• Mixture aging procedures are modified for both times and temperature;
• Clarifications to the table used to tell how far an asphalt binder's high temperature grade should be "bumped up," depending on the speed or volume of traffic (binder grade bumping tables), should improve the table's ease of use for occasional users and result in cost savings on rural and low-volume roads, where the use of unnecessarily stiff asphalt can now be avoided; and
• The determination of mixture properties are now calculated directly on the mixture at N-design instead of being back-calculated from N-maximum.

The relatively short time between the changes proposal and passage is just one aspect of "a real success story of cooperation and synergy among many stakeholders in moving standards through a complicated process quickly," says Paul Mack of the New York State Department of Transportation (DOT). Those stakeholders, the source of many of the proposed changes, include the National Cooperative Highway Research Program (NCHRP), the Lead States Superpave Team, the Superpave Mix Expert Task Group (ETG), and the AASHTO Subcommittee on Materials. Ron Sines, also of New York State DOT and a member of the task force, notes that "Some on the task force were also members of the Lead States team and the Mix ETG. We had no trouble getting the technical input we needed from those two groups. The result was a very cooperative effort."

John Bukowski of the Federal Highway Administration (FHWA) adds, "The Mix ETG and the Lead States team worked in parallel in accumulating needed changes that grew out of experience with the Superpave system. But the biggest changes grew out of studies by Auburn University (NCHRP Project 9-9) and the Asphalt Institute (N-design II Experiment), which resulted in simplification and refinement of N-design procedures."

The State balloting, reports Bukowski, was "overwhelmingly in favor of approval, but there were valid critical comments as well. Those comments were referred by AASHTO to the Superpave Mix ETG for further clarification. At their September 1999 meeting, the ETG members addressed those concerns. Overall, the refinements are a step forward-the procedures are much improved and simplified."

"In any new system," Sines says, "there are things that need to be worked out. With Superpave, we've learned over time and have adapted the mix design process to make it more user friendly. In particular, the reduced number of design levels makes it much easier for the contractor to use it in the field. The overall effect of these changes is to make the Superpave system simpler and more practical."

For more information, contact Ron Sines at New York State DOT (phone: 518-457-4582; fax: 518-457-8171; email: rsines@gw.dot.state.ny.us) or John Bukowski at FHWA (phone: 202-366-1287; fax: 202-493-2070; email: john.bukowski@fhwa.dot.gov).

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