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Implementation of the Superpave Mix Design Method
AuthorMontenegro, Daniela Maria
AdvisorHand, Adam J.T.
Civil and Environmental Engineering
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The objective of this study was to develop a guide for the Washoe County Nevada, Regional Transportation Commission (RTC) to implement the Superpave mix design method. A key component of the effort was focused on finding a correlation between the number of blows applied with the Marshall hammer and the number of gyrations with the Superpave gyratory compactor for mixes commonly used in Washoe County Nevada. This was important since RTC has historically used the Marshall method. Eight mixtures were evaluated with performance tests to validate the relationship observed and develop a guide for the RTC to implement the Superpave mix design method. Materials for eight different mixtures including virgin aggregate, binder, and reclaimed asphalt pavement (RAP) were sampled from the three producers in the Reno-Sparks area. The producer mix designs were verified, the number of gyrations correlating to Marshall blows were determined, and performance tests were ultimately conducted on gyratory compacted specimens to validate the correlation. Moreover, plant mix from the same producers and mixtures was sampled to verify if the density of the plant produced mix was similar to laboratory mix densities, and for performance testing.Densities of the majority of the field mix laboratory compacted samples, were found to be higher than the mix design target density, which lead to the conclusion that changes from laboratory mix design to plant production caused these variations. In addition, the number of gyrations needed to reach the same density of the compacted Marshall samples and the mix design target air voids on the plant produced materials did not present a clear trend between the mixes. However, the number of gyrations for mixtures incorporating PG 64-28NV binder were close to the mix design Marshall blows, to reach the same density for the plant produced mix. The optimum asphalt content found in the UNR laboratory was close and sometimes equal to the producer, but the number of gyrations to reach the same optimum asphalt content was determined to be lower than the minimum compaction level in the AASHTO R35 Standard Practice for Superpave Volumetric Design for Asphalt Mixtures . For these mixes, the locking point was not reached under the number of gyrations required to obtain the same optimum asphalt content as observed using for the Marshall mix design method.Mixes with PG 64-28NV binder were compacted at 50 gyrations to find the optimum asphalt content. The optimum asphalt content from the Marshall Mix designs were higher as expected. Observed differences in optimum asphalt contents between the Marshall and Superpave methods were 0.5% to 1.5%, with the optimums for the Superpave method always being lower than for the Marshall method. Volumetric properties of these mixes were evaluated and for some the voids in mineral aggregates (VMA) did not meet the requirements at the design air void level of 4%. This indicates that when using 50 gyrations, the gradation needed to be adjusted for those mixtures to meet all Superpave volumetric criteria. Interestingly the volumetric properties for Superpave designed mixtures at a design air void level of 3% air voids met the specifications.The performance tests completed were dynamic modulus, tensile strength ratio (TSR), and Ideal CT. The first two, were performed on field mix and laboratory mix specimens. The dynamic modulus was similar for both laboratory and field produced mixes. The TSR results showed higher values for some lab mixes; however, it was possible to determine that the compaction method does not significantly affect TSR. The ideal CT was evaluated on the field mix and it is recommended to correlate the results with field performance. The Hamburg Wheel-Track test was not evaluated in this study, but based on the volumetric results this test should be used to validate the number of gyrations selected for the mix design.