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6 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 5. Recommended nominal maximum aggregate sizes for dense-graded HMA mixtures. Application Recommended Minimum Lift Thickness, mm NMAS, mm Fine-Graded Coarse-Graded Mixtures Mixtures 4.75 15 to 25 20 to 25 Leveling course mixtures 9.5 30 to 50 40 to 50 4.75 15 to 25 20 to 25 Wearing course mixtures 9.5 30 to 50 40 to 50 12.5 40 to 65 50 to 65 19.0 60 to 100 75 to 100 Intermediate course mixtures 25.0 75 to 125 100 to 125 19.0 60 to 100 75 to 100 Base course mixtures 25.0 75 to 125 100 to 125 37.5 115 to 150 150 9.5 30 to 50 40 to 50 Rich base course mixtures 12.5 40 to 65 50 to 65 Table 6. VMA requirements for dense-graded mixtures. Aggregate Minimum Maximum Target NMAS VMAA VMAA VMA (mm) (%) (%) (%) 4.75 16.0 18.0 17.0 9.5 15.0 17.0 16.0 12.5 14.0 16.0 15.0 19.0 13.0 15.0 14.0 25.0 12.0 14.0 13.0 37.5 11.0 13.0 12.0 A The specifying agency may increase the minimum and maximum values for VMA by up to 1.0% to obtain mixtures with increased asphalt binder content, which can improve field compaction, fatigue resistance, and general durability. Care should be taken to ensure that the resulting HMA mixtures maintain adequate rut resistance for their intended application. design binder content may also be necessary to account for differences between the initially assumed binder absorption and the actual absorption in the trial mixtures. For the design of WMA mixtures, the same minimum, maximum, and target VMA values discussed in Chapter 8 for HMA should be used. These values are reproduced in Table 6. Higher design VMA will increase the binder content of the mixture, thereby improving compactability, durability, and resistance to fatigue damage, but decreasing the resistance to rutting. Decreas- ing the design VMA will have the opposite effect on compactability, durability, resistance to fatigue damage, and resistance to rutting. The target air void content for WMA mixtures should be 4.0% with an acceptable range of 3.5 to 4.5%. Lower design air voids will increase the design binder content of the mixture, thereby improving compactability, durability, and resistance to fatigue damage, but decreasing the resistance to rutting. Higher design air voids will have the opposite effect on compactability, durability, resistance to fatigue damage, and resistance to rutting. Step 6. Calculate Target Binder Content The target binder content by volume for WMA is calculated in the same manner as described in Chapter 8 for HMA: target VMA minus design air voids plus volume of binder absorbed. The lower temperatures for WMA mixtures result in less binder absorption compared with HMA. In NCHRP Project 9-43, the binder absorption for WMA mixtures was about 90% of that for HMA