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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) Part I of this report describes recommended procedures for designing dense-graded, asphalt concrete mixtures that will be produced using any one of several currently available WMA processes. These WMA mix design recommendations are based on research conducted in National Cooperative Highway Research Program (NCHRP) Project 9-43, "Mix Design Practices for Warm Mix Asphalt," which concluded that only minor modification of current mix design practice is needed to address WMA. Although the procedures described have been specifically selected for use in designing dense-graded mixtures, most can be applied to the design of other mix types with little or no modification. The following sections of this Part II of the report are a commentary that presents supporting information from the NCHRP 9-43 research report for the recommendations included in Part I. Many of these sections also include recommended additional research, because NCHRP Project 9-43 was the first major study addressing WMA mixture design, and some of the findings require further validation through additional research. Both Parts I and II are organized around the eleven steps described in Chapter 8 of NCHRP Report 673: A Manual for the Design of Hot Mix Asphalt with Commentary for the design of dense-graded HMA. Table 1 summarizes the differences between WMA and HMA design for each of the eleven steps. Step 1. Gather Information For the design of WMA, additional information must be collected on the WMA process that will be used, additive rates, and planned production and compaction temperatures. The reason that this information is needed is the design of WMA uses process-specific specimen fabrication procedures. These specimen fabrication procedures were designed to simulate, in an approxi- mate manner, the WMA process in the field. For the purposes of mixture design, the various WMA processes can be grouped into four generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. Specimen fabrication techniques are somewhat different for each of these categories. Given that viscosity-based mixing and compaction temperatures are not applicable to many WMA processes, the planned production and compaction temperatures are used in the WMA mixture design process to evaluate coating and the compactability/workability of the WMA. It should be emphasized that the optimal production and compaction temperatures are different for the various WMA processes and should be carefully considered when selecting production and compaction temperatures to be used in the WMA design process. C-1