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C-8 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 6. Mix design experiment. Mixing/Compaction Temperature, F, Mixture Identification for Process: Aggregate No. Evotherm Water Advera Sasobit Ndesign RAP HMA 3G Absorption, WMA WMA WMA % 1 50 1.5 Yes 320/310 225/215 225/215 270/260 2 50 0.8 No 320/310 270/260 270/260 225/215 3 75 1.0 Yes 320/310 270/260 225/215 270/260 4 75 1.6 No 320/310 225/215 270/260 225/215 5 100 1.2 Yes 320/310 270/260 270/260 225/215 6 100 1.3 No 320/310 225/215 225/215 270/260 d Test statistic: t= sd n Rejection region: Reject the null hypothesis and accept the alternative hypothesis if t > t for n-1 degrees of freedom. where WMA = population mean for WMA mixtures _ = population mean for HMA mixtures HMA d = average of the differences between WMA and HMA mixtures sd = standard deviation of the differences n = number of mixtures compared One way to present the results is to develop 95% confidence intervals for the mean differ- ence in the properties for WMA compared to HMA. If the 95% confidence intervals capture zero, the properties are statistically the same for WMA and HMA. The paired difference com- parisons for design binder content and binder absorption are shown in Figures 3 and 4, respectively. Figure 3 shows that the average design binder content for the WMA mixtures was 0.05% lower than that for HMA mixtures made with the same aggregates and binder. This difference, however, was not statistically significant. Figure 4 shows that the binder absorption was sig- nificantly less for the WMA mixtures. The average difference was 0.1%. The average absorp- tion for the mixtures tested was approximately 1.0%. Thus, absorption for WMA was about 90% of that for HMA. Based on these data it was recommended to use 45% of the water absorption as the initial estimate binder absorption in WMA compared to 50% of the water absorption for HMA. Step 7. Calculate Aggregate Content by Volume These calculations are identical to those for HMA. Step 8. Proportion Aggregates for Trial Mixtures These calculations are identical to those for HMA.

OCR for page 35
II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-9 Average Difference in Design Binder Content, wt. % 0.40 0.30 0.20 0.10 0.00 Advera Evotherm Sasobit All WMA -0.10 -0.20 -0.30 -0.40 Mixture Figure 3. Average difference in design binder content (WMA-HMA) from the NCHRP 9-43 mix design study (error bars are 95% one-sided confidence intervals). 0.40 Average Difference in Binder Absorption, wt % 0.30 0.20 0.10 0.00 Advera Evotherm Sasobit All WMA -0.10 -0.20 -0.30 -0.40 Mixture Figure 4. Average difference in binder absorption (WMA-HMA) from the NCHRP 9-43 mix design study (error bars are 95% one-sided confidence intervals).