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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-7 using PG 64-22 virgin binder and a WMA production temperature of 250F, the virgin binder low-temperature continuous grade would be improved 0.6C to account for the lower WMA production temperature. This would allow approximately 10% additional RAP binder to be added to the mixture through the blending chart analysis. The ability to use 10% additional RAP binder without changing the grade of the virgin binder may be significant in some areas of the United States. These recommended binder grade improvements are reasonable, based on the recovered binder grading data presented earlier in Tables 29 and 30. Recovered binder tests on WMA with RAP should be conducted to verify the suggested improvements in low-temperature properties for blending chart analyses. Step 3. Determine Compaction Level The same compaction levels are recommended for designing WMA and HMA mixtures. Step 4. Select Nominal Maximum Aggregate Size The same aggregate requirements are recommended for designing WMA and HMA mixtures. Step 5. Determine Target VMA and Air Voids Values The same target VMA and air voids values are recommended for designing WMA and HMA mixtures. Step 6. Calculate Target Binder Content Asphalt absorption is somewhat lower in WMA compared to HMA. NCHRP Project 9-43 included a mixture design study designed to assess the difference in volumetric and engineering properties between WMA and HMA mixtures. Six combinations of binder and aggregate were designed as HMA and then again as WMA using three different processes. The HMA and WMA mixtures prepared at mixing/compaction temperatures of 270/260F were made using a PG 64-22 binder, while the WMA mixtures prepared at mixing/compaction temperatures of 225/215F were made using a PG 70-22 binder. The RAP content was 25% for mixtures made with RAP. All mixtures were prepared using short-term conditioning for 2 hours at the compaction temperature. Table 6 summarizes the design of this experiment. The experimental design for the NCHRP mix design study was a paired difference experiment. This design is commonly used to compare population means, in this case the properties of prop- erly designed WMA and HMA mixtures for the same traffic level, using the same aggregates with the same gradation. In this design, differences between the properties for WMA and HMA are computed for each mixture included in the experiment. If the two design procedures produce mixtures with the same properties, then the average of the differences will not be significantly different from zero. The difference for an individual mixture may be positive or negative, but the average difference over several mixtures should be zero. A t-test is used to assess the statistical significance of the average difference as summarized below: Null hypothesis: WMA - HMA = 0 Alternative hypothesis: WMA - HMA > 0 or WMA - HMA < 0 (as appropriate)