The National Academies

NCHRP 09-09(1) [Completed]

Verification of Gyration Levels in the Ndesign Table

  Project Data
Funds: $625,447
Research Agency: National Center for Asphalt Technology - Auburn University
Principal Investigator: E. R. Brown
Effective Date: 2/3/2000
Completion Date: 9/30/2006

The Superpave gyratory compaction procedure is the method required for all SUPERPAVE mix designs. It provides a density in the compacted laboratory specimen that closely approximates the ultimate density of the asphalt concrete mixture obtained in the pavement when subjected to in-service traffic loads and climatic conditions, so that an appropriate optimum asphalt content may be selected during the laboratory mix design process.

SHRP established, based on a limited set of data, a procedure for determining Ndesign for dense-graded mixtures that is now solely based on the project traffic level. During the development of the research project statement for the recently completed NCHRP Project 9-9, a decision was made not to include an objective to verify the relationship of the compactive effort (Ndesign) to the traffic level (ESALs) expected over the service life of the project as this issue was under study at the time in two internal FHWA projects. However, these projects did not provide a conclusive answer. Therefore, at the conclusion of Project 9-9, its panel decided to start a follow-up project to provide a definitive field verification of the current Ndesign table. This would answer the key question continually raised about the Superpave gyratory compactor: How do we know that the gyratory compaction level assigned to each project traffic level in the table is correct?

The objective of this project was to verify through a series of field project evaluations that the gyration levels in the Ndesign table in AASHTO PP28 are correct for the stated project traffic levels and to modify the levels as necessary.

The final report has been published as NCHRP Report 573. The appendixes of the final report, which are not published in NCHRP Report 573, are available online as NCHRP Web-Only Document 96.

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