Determining the Optimum Compaction Level for Designing Stone Matrix Asphalt Mixtures

Abstract

As Stone matrix asphalt (SMA) becomes more widely used in the United States, there is a need to further refine its mix design procedure. Some states have found that 100 gyrations with the Superpave Gyratory Compactor (SGC) suggested in current design guides are excessive for their materials and have specified the use of lower compaction levels. However, the use of these low compaction levels had little research support. The objective of this study was to determine the optimum compaction level for SMA mixtures that will provide increased durability and acceptable rutting resistance. The study is also needed to determine if the same compaction effort is applicable for SMA mixes of various nominal maximum aggregate sizes (NMAS). The study was carried out by conducting SMA mixture designs using different compaction levels, and comparing these different compaction levels in terms of volumetric properties and rutting performance. Five aggregates with a wide range of Los angles (L.A.) abrasion values were selected. For each aggregate, three NMAS (19 mm, 12.5 mm, and 9.5 mm) mixtures were designed by at least three compaction efforts (50 blows Marshall, 65 and 100 gyrations with the SGC. A further lower gyration level (40 gyrations) was also used to design two mixtures, to show the effect of further reduction in number of gyrations. A total of 47 mixture designs were conducted in this study. Both vacuum seal (CoreLok) and saturated surface dry (SSD) methods were used for measurement of air voids. Aggregate breakdown was evaluated for all compaction efforts. Permeability, wheel load tracking (Asphalt pavement analyzer, APA), dynamic modulus, static creep, and repeated load tests were conducted on all mixtures designed with the different gyration levels. The CoreLok and SSD method provided a significant difference in air void results for lab compacted SMA mixtures. The correction factor embedded in the CoreGravityTM software is not acceptable for determining the bulk specific gravity of SMA mixtures. The error potentials for both methods were analyzed and suggestions were made to properly use these two methods for determining air voids of SMA mixtures. SMA mixtures designed with 65 gyrations should provide improved durability than those designed with 100 gyrations due to the increased optimum asphalt content. SMA mixtures designed with 65 gyrations were generally had similar or lower permeability than those designed with 100 gyrations at similar air voids. Marshall compaction resulted in significant higher aggregate breakdown than gyratory compaction. The aggregate breakdowns for both 65 and 100 gyrations were very similar to that observed in the field. All designed SMA mixtures achieved stone-on-stone contact as indicated by the VCA ratio, and had acceptable asphalt draindown. For the APA rutting test, 13 of 15 SMA mixtures designed with 65 gyrations performed well when 5.0 mm was used as the maximum allowable rut depth. The dynamic modulus test results indicated that reducing the compaction level from 100 gyrations to 65 gyrations only resulted in a small difference, and the ability to use E*/sin? term for predicting rutting resistance is questionable at high temperature for SMA mixtures due to the erroneous trend shown. Due to high test variability and long testing time, static creep test was not recommended to be used for evaluating SMA rutting resistance. Most (14 of 15) mixtures designed with 65 gyrations met the suggested 5 percent cumulative strain criterion after 10,000 cycles in the repeated load test. The rutting resistance indicated by the APA rut depth and cumulative strain from repeated load test becomes marginal when the gyration level reduced to 40 gyrations. The findings of this study indicated that 65 gyrations (the SGC used had an internal gyration angle of 1.23 degrees) can be used to design a more durable SMA mixture, while still maintaining the good ruttin