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188 A Manual for Design of Hot Mix Asphalt with Commentary viscosity of 17020 cSt. Compaction temperature will be the temperature required to provide an asphalt binder viscosity of 28030 cSt. However, although these temperatures work for neat asphalt binders, the selected temperatures may need to be changed for polymer-modified asphalt binders. The asphalt binder supplier's guidelines for mixing and compaction temperatures should be used. When preparing GGHMA in the laboratory, a mechanical mixing apparatus should be utilized. The gap-grading and use of stabilizing additives make GGHMA very difficult to hand-mix. Aggregate batches and asphalt binder are heated to a temperature not exceeding 28C above the temperature established for mixing temperature. The heated aggregate batch is placed in the mechanical mixing container. Asphalt binder and any stabilizing additive are placed in the container at the required masses. Mix the aggregate, asphalt binder, and stabilizing additives rapidly until thoroughly coated. Mixing times for GGHMA should be slightly longer than for dense-graded HMA to ensure a good distribution of the stabilizing additives. After mixing, the GGHMA mixture should be short-term aged in accordance with AASHTO R 30, Mixture Conditioning of Hot Mix Asphalt. For aggregate blends having water absorption less than 2% the mixture should be aged for 2 hours. If the water absorption of the blend is 2% or higher, the mixture should be aged for 4 hours. Number of Samples Twelve samples are initially required--four samples for each of the three trial gradations. Each sample is mixed with the trial asphalt binder content and three of the four samples for each trial gradation are compacted. All 12 samples should be short-term aged. The remaining sample of each trial gradation is used to determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209. Sample Compaction Specimens should be compacted at the established compaction temperature after laboratory short-term aging in accordance with AASHTO R 30. Laboratory samples of GGHMA are com- pacted using 75 gyrations of the Superpave gyratory compactor (SGC). Some agencies have used 100 gyrations with success; however, GGHMA is relatively easy to compact in the laboratory and exceeding 100 gyrations can cause aggregate breakdown and may result in an unacceptably low asphalt content. Step 3--Selection of Optimum Gradation After the samples have been compacted and allowed to cool, they are tested to determine the bulk specific gravity, Gmb (AASHTO T166). Using Gmb, Gmm, and the bulk specific gravity of the coarse aggregate fraction (Gca), the percent air voids (VTM), voids in mineral aggregate (VMA), and VCAMIX are calculated. The VTM, VMA, and VCAMIX are calculated as shown below: 1 - Gmb VTM = 100 Gmm (10-2) G P VMA = 100 - mb s (10-3) Gsb G P VCAMIX = 100 - mb ca (10-4) Gca