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4 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Step 2. Select Asphalt Binder The grade of binder used in WMA mixtures with less than 15% recycled binder is the same as that for an HMA mixture designed for the same conditions. The change in the high- and low- temperature properties of the binder due to lower WMA temperatures is not sufficient to warrant a change in the grade of the binder used in the mixture. The binder grade used in WMA should be either (1) the grade required by the specifying agency for HMA or (2) selected as discussed in Chapter 8 considering 1. The climate at the project location, 2. High-temperature grade adjustment required for traffic level and speed, and 3. High-temperature grade adjustments for temporary construction. When a recycled binder is used in WMA, it is recommended that the continuous high- temperature grade of the recycled binder be equal to or lower than the planned compaction tem- perature to ensure adequate mixing of the new and recycled materials. This recommendation will generally not affect the use of recycled asphalt pavement (RAP) in WMA. The high-temperature grade of RAP in the United States ranges from about 82C in colder climates to about 100C in hotter climates. Planned compaction temperatures for most warm mix processes are greater than 212F (100C). This recommendation will, however, limit the use of recycled asphalt shingles (RAS) in many WMA processes. Many RAS binders have high-temperature grades exceeding 124C, indicating that these materials should not be used in WMA processes where the planned compaction temperature is less than about 255F (124C). When adding more than 15% recycled binder to WMA, the blending chart analysis described in Chapter 9 for HMA should also be used. If permitted by the specifying agency, the low-temperature continuous grade of the new binder may be improved somewhat to account for the lower WMA process temperatures. The recommended improvement depends on the new binder grade and the production temperature. Table 3 presents recommended low-temperature binder grade improvements developed in NCHRP Project 9-43 for some common binder grades. For a WMA process having a production temperature of 250F, the low-temperature grade improvement ranges from 0.3 to 0.7C. For a typical blending chart analysis this translates to 5 to 10% additional RAP based on low-temperature binder grade considerations. As discussed in Chapter 9, binder grade is one of several considerations affect- ing the amount of RAP that can be added to a mixture. In summary, the grade of binder used in most WMA mixtures will be the same as that used in a comparable HMA mixture. When the WMA mixture incorporates a recycled binder, it is rec- ommended that the high-temperature grade of the recycled binder be equal to or lower than the planned compaction temperature to ensure adequate mixing of the new and recycled materials. If permitted by the specifying agency, it is reasonable to improve the low-temperature grade of the new binder somewhat when performing blending chart analyses for higher RAP content mix- tures. Although small, this improvement may permit 5 to 10% additional RAP to be added, based on low-temperature binder grade considerations. Step 3. Determine Compaction Level Although it is well documented that WMA mixtures are generally easier to compact than HMA, the same design compaction level should be used with WMA and HMA. In NCHRP Project 9-43, several mixtures with less than 1% binder absorption designed as WMA and HMA following the procedures given in this manual had optimum binder contents and volumetric properties that were essentially the same. The recommended design compaction levels for both WMA and HMA