Comparison of 9.5 mm SuperPave and Marshall Wearing I Mixes in ...

54percentage of aggregate from the SKID stockpile into the mix. After more trial blends with thisadjusted gradation it was found that the highest allowable amount of natural sand stockpile inthe mix was 64%.The final Superpave light mixture developed was one that was exactly like the 64% natural sandmix but with the sand portion consisting of limestone mixed at the same gradation as thenatural sand. This mix was developed to allow for a direct comparison of identical gradationsusing different material. It was decided that volumetric requirements for this mix design wouldnot be considered. This was done so the gradation curves could remain identical. However, thismix design only had a VMA of 13.9.5.2.3 Specimen FabricationSuperpave mix design is covered and outlined by two standards published by AASHTO: StandardPractice for Superpave, PP28-99, and Specification for Superpave, MP2 –99. The design processstarted by first developing trial blends for determining the proper aggregate structure or testingone that has previously been shown to be a good blend for other traffic volumes. Next,estimated asphalt content was calculated using the same process used with the Marshallsamples. The trial blends were then tested at the estimated asphalt content and tested for eachspecimen’s volumetric properties. The trial blend that yielded the best result, based onvolumetric properties, was selected as the design aggregate blend.The next step in the design process was to determine the design asphalt content, which yields 4percent air voids. Using the newly determined design aggregate structure and estimatedasphalt content as a starting point, samples also were prepared at 0.5 percent above and belowestimated asphalt content and also 1.0 percent above. Duplicate samples at each asphaltcontent percentage were also made along with maximum specific gravity samples for eachasphalt content. These samples were made up by first mixing the proper proportions ofaggregates for each blend, then heating them to the mixing temperature of 162 C. Theaggregate and corresponding weight of asphalt for each asphalt content percentage was thencombined and mixed in the large bucket mixer until all aggregate was thoroughly coated. Thisloose mixture was then placed in an oven at the compaction temperature of 149 C for twohours. The mix was stirred at half hour intervals.After the proper soak time, the mix was removed from the oven and placed in the gyratorycompactor mold, placed in the compactor that has been set to the correct number of gyrationsfor the type of sample being compacted and the compactor activated. Once the specimen wascompacted, the mold was removed from the compactor and specimen removed from the moldwith an extracting jack. The specimens were cooled to room temperature. After the specimenscooled they were ready for volumetric determination and testing.