Update of Design Number of Gyrations

By Bob McGennis, Navajo Western Asphalt Company
     Weng Tam,Superpave Asphalt Research Program
     Mike Anderson, Asphalt Institute

Working with the Asphalt Institute, the South Central Superpave Center (SCSC) participated in a study to evaluate and update the design number of gyrations for use with the Superpave Gyratory Compactor. Personnel from the Heritage Research Group and Advanced Asphalt Technology were also active in this study. Most of the project was conducted between May 1997 and August 1998. The study was initiated at the request of the FHWA and employed a five-task approach:

1. evaluate in-place air void content of existing Superpave mixtures,
2. evaluate and select an engineering property that indicates mixture performance,
3. measure the sensitivity of the engineering property to number of gyrations,
4. measure the sensitivity of mixture volumetric properties to number of gyrations, and,
5. recommend a revised table for the number of design gyrations.

The principal goal of this study was to validate, update, and simplify the N-design table using a research approach that differed from that used by the SHRP researchers. The central premise of the study was that the design number of gyrations could be established on the basis of an engineering stiffness property, in addition to mix volumetrics. SCSC was primarily responsible for Tasks 1 and 3.

Task 1, conducted at SCSC, involved surveying state DOTs and LTPP contractors involved in SPS-9 projects for data on in-place air void content of Superpave mixtures. Surprisingly, not a great amount of data was forthcoming. However, Arizona, Arkansas, Louisiana, Texas, Florida and New York responded to a personal appeal to state DOTs for data. The results of this task generally showed that the in-place air void content of Superpave mixtures was in reasonable proximity to four percent, which suggested that the number of gyrations was approximately accurate.

Task 2, conducted by Heritage Research, involved mechanical property tests on cores from WesTrack, a test facility in Nevada. Cores were taken from Superpave mixtures exhibiting good and bad rutting performance. The results of this task showed that the complex shear modulus (G*) arising from the shear frequency sweep test adequately discriminated between good and bad rutting performance.

Task 4, originally intended as part of this study, was abandoned because a similar and more extensive evaluation was concurrently being conducted at Auburn University as part of NCHRP Project 9-9. That study indicated that a change of 30 gyrations resulted in change in VMA and air voids of about one percent.

Task 3, conducted at SCSC, involved measuring the sensitivity of G* to the number of gyrations. Three crushed limestone mixtures (A, B, and C) and three crushed gravel mixtures (D, E, and F) were designed at three levels of design gyrations: 70, 100, and 130. Their aggregate structure was varied to achieve approximately the same VMA and asphalt content. Table 1 indicates the properties of the six mixtures designed.

Table 1. Summary of Mixture Designs for Task 3

Once the six mixtures were designed, nine specimens were fabricated for each mix — three replicates for each level of gyrations. For example, Mix A was designed at 130 gyrations but nine specimens were compacted, three replicates each at 70, 100, and 130 gyrations. The Superpave Shear Tester (SST) was used to measure G* while volumetric tests were conducted to measure VMA and air void content. The volumetric properties observed in this experiment agreed very closely with the findings of NCHRP 9-9 study in that a change of 30 gyrations resulted in approximately one percent change in air void content and VMA. The frequency sweep test results showed that a change of 30 gyrations resulted in a change in G* of approximately 30 percent for both the limestone and gravel mixtures.

Based on the results and combined principals of this study (Tasks 1, 2, 3) and NCHRP 9-9 study, a recommended N-design table was developed and submitted to the FHWA Superpave Mixtures Expert Task Group, for the ultimate purpose of recommending it to AASHTO for adoption.

Table 2. Revised N-Design Table

For more information pertaining to this project, contact Weng Tam (512-232-1929), Mike Anderson at the Asphalt Institute (606-288-4984), or Bob McGennis at Navajo Western Asphalt Company (602-939-3311).