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2 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 1. Steps in design of dense-graded HMA and WMA. Step Description Major WMA Differences 1. WMA process, 2. Additive rates, 1 Gather Information 3. Planned production temperature, 4. Planned compaction temperature. 1. Recommended limit on high-temperature stiffness of recycled binders. 2 Select Asphalt Binder 2. May consider low-temperature grade improvement when using blending charts. 3 Determine Compaction Level Same as HMA Select Nominal Maximum 4 Same as HMA Aggregate Size Determine Target VMA and 5 Same as HMA Design Air Voids Value 1. Lower asphalt absorption due to lower 6 Calculate Target Binder Content temperatures. 7 Calculate Aggregate Volume Same as HMA Proportion Aggregate Blends for 8 Same as HMA Trial Mixtures Calculate Trial Mixture 9 Proportions by Weight and Check Same as HMA Dust/Binder Ratio 1. WMA process specific specimen fabrication procedures, Evaluate and Refine Trial 2. Lower short-term aging temperature, 10 Mixtures 3. Evaluate coating and compactability in lieu of viscosity-based mixing and compaction temperatures. 11 Compile Mix Design Report Same as HMA Specimen fabrication in Step 10, Evaluate and Refine Trial Mixtures, is the primary differ- ence for the design of WMA compared with HMA. Procedures for specimen fabrication are process specific; therefore, information on the WMA process that will be used and the planned production and compaction temperatures must be collected in Step 1 at the beginning of the WMA mix design process. Given that binder absorption is lower in WMA mixtures, the lower absorption should be accounted for when estimating the target binder content in Step 6. Another important difference between WMA and HMA design occurs in the selection of binders in Step 2. The high-temperature grade of the recycled binders should be lower than the planned WMA compaction temperature to promote mixing of the new and recycled binders. When using blending charts, the low-temperature grade of the new binder may be improved due to the lower aging that occurs at WMA temperatures. The following sections provide step-by-step discussions of the similarities and differences between WMA and HMA. These are followed by an example WMA design. Step 1. Gather Information The design of WMA requires the same information about the design traffic level, the climate at the place of construction, available aggregates and binders, anticipated lift thickness, and pavement type (that is, surface, intermediate, or base course) as the design of HMA. In addition, WMA design requires information on the WMA process and the planned mixing and compaction temperatures because the fabrication of WMA specimens in the laboratory is process specific, simulating in an approximate manner, the production of the mixture in the field. Table 2 summarizes the informa- tion that should be collected for designing WMA mixtures and compares this information to that required for designing HMA mixtures.
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 3 Table 2. Information required for WMA and HMA design. Type of Information Detail WMA HMA Geographic Location X X Climate Relating to Binder Grade X X Site Design Traffic Level X X Design Life X X Unusual Performance Requirements X X Lift Thickness X X Haul Time X X Construction Construction Temperatures X X Unusual Specification Requirements X X Unusual Construction Requirements X X Mix Type X X Pavement Distance from Pavement Surface X X Nominal Maximum Size X X Gradation X X Aggregate Specific Gravity and Absorption X X Specification Properties X X Performance Grade X X PG Plus Properties, if applicable X X Type of Modification, if applicable X X Binder Continuous Performance Grade for Blending Chart Analysis X X Mixing and Compaction Temperatures NA X Binder Content X X Continuous Performance Grade X X Nominal Maximum Size X X RAP Gradation X X Specific Gravity and Absorption X X Specification Properties X X Type X X Anti-Strip Additives Dosage Rate X X WMA Process X NA Additive Type X NA WMA Additive Dosage Rate X NA Production Temperature X NA Compaction Temperature X NA WMA process selection is best made by the producer in consultation with the specifying agency and WMA process suppliers considering (1) available performance data, (2) cost of the required warm mix additives, (3) planned production and compaction temperatures, (4) planned produc- tion rates, (5) existing plant capabilities, and (6) plant and laboratory modifications required to successfully use the WMA process. 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 empha- sized 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.