Resumen de Mechanistic Characterization of Aggregate Packing to Assess Gyration Levels During HMA Mix Design
Some state DOTs are moving away from the standard Superpave Ndesign tables and volumetric criteria and some researchers continue to suggest lower Ndesign values. The concept of a gyration Locking Point (LP) in the Superpave gyratory compactor was developed as an alternative to the Superpave Ndesign that would avoid over-compaction and possible damage to the aggregate skeleton.
This paper investigates the changes in aggregate packing that occur during Superpave mix design by utilizing mechanical performance testing to explore and define a �locking� phenomenon rather than a simplistic pattern of heights in SGC. Four asphalt mixtures, which were designed by the Superpave mix design method, are studied in detail. Gyration heights, air void content, VMA and axial, radial and volumetric strains measured during the flow number performance test were used to identify the presence of a locking point.
Test specimens were evaluated at nine gyration levels from 8 to 125 gyrations.
Locking points for each mixture were obtained from the compaction curves and the second instance of two consecutive gyrations resulting in the same sample height was found to produce a locking point measurably lower than the Ndesign. A three-stage description of distinct aggregate packing phenomena during gyratory compaction is proposed ranging from rapid packing to aggregate reorientation. Aggregate degradation was detected in varying degrees depending on the aggregate softness and type of mixture.
Recommendations were made regarding the best practical definition of locking point using only gyration heights. Fine aggregates should be included along with coarse aggregates when evaluating susceptibility to degradation during compaction. Finally, utilizing performance characterization tests for a follow-up study is necessary to develop appropriate insight as to the consequence of reducing design gyration levels.
Key Words: asphalt mixture, locking point, mix design, flow number test, compaction, design gyration, Superpave ______________________________________________________ 1Asphalt Mixture Scientist, ESCINC/FHWA, McLean, VA 2Research Engineer, FHWA, McLean, VA The oral presentation was made by Dr. Gibson.
