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C-16 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 9. Summary of average difference in flow number of WMA compared to HMA for the NCHRP 9-43 field validation sections. Process Number Average Average Difference in Difference Compaction in Flow Temperature, Number, F % Advera 3 -46.7 -39 Evotherm 2 -50.0 -38 LEA 1 -80.0 -50 Sasobit 3 -48.3 +38 recommended WMA flow numbers are approximately 55% of those recommended for HMA. The different criteria are needed because of the different short-term conditioning used for WMA compared to HMA. WMA flow number specimens are conditioned 2 hours at the planned field compaction temperature while HMA flow number specimens are conditioned 4 hours at 275°F (135°C). NCHRP Project 9-43 included comparisons of flow number data for 10 pairs of WMA and HMA sections. Table 9 summarizes the difference in flow numbers obtained for field validation mixtures. The Sasobit process increases the rutting resistance because it increases the high-temperature grade of the binder. Additional research is needed on the development of a short-term conditioning procedure for specimens used for the evaluation of moisture sensitivity and rutting resistance that is equally applicable to both WMA and HMA. Research completed in NCHRP Project 9-43 concluded that 2 hours of oven conditioning at the compaction temperature reasonably reproduces the binder absorption and stiffening that occurs during construction for both WMA and HMA mixtures. Current criteria for evaluating moisture sensitivity and rutting resistance are based on mixtures that have been aged to a greater degree. The conditioning originally specified in AASHTO T 283 for moisture sensitivity testing was 16 hours at 140°F (60°C). Additionally, most rutting criteria are based on 4 hours of conditioning at 275°F (135°C). In NCHRP Project 9-13, mixtures were conditioned for 2 hours at 275°F (135°C), 4 hours at 275°F (135°C), and 16 hours at 140°F (60°C). Analysis of these data in NCHRP Project 9-43 concluded that 16 hours at 140°F (60°C) resulted in somewhat more aging than 4 hours at 275°F (135°C). The difference in aging between 2 and 4 hours at 275°F (135°C) was not statistically significant. To simulate both WMA and HMA, a two-step conditioning process should be considered for specimens used for evaluation of moisture sensitivity and rutting resistance. In the first step, the mixture would be conditioned for 2 hours at the compaction temperature to simulate the binder absorption and stiffening that occurs during construction. In the second step, the mixture would be further conditioned for an extended time at a representative high in-service pavement temperature to simulate a short period of time in service. Only specimens used to evaluate moisture sensitivity and rutting resist- ance would receive the second conditioning step. Volumetric design would be based on only the first step. Step 11. Compile Mix Design Report This step is the same as that for HMA with some additional information provided. The additional information for WMA is that needed in Step 1 of the mix design process: