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C-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 Gather Information 1. WMA process, 2. Additive rates, 3. Planned production temperature, 4. Planned compaction temperature. 2 Select Asphalt Binder 1. Recommended limit on high-temperature stiffness of recycled binders. 2. May consider low-temperature grade improvement when using blending charts. 3 Determine Compaction Level Same as HMA 4 Select Nominal Maximum Same as HMA Aggregate Size 5 Determine Target VMA and Same as HMA Design Air Voids Value 6 Calculate Target Binder Content 1. Lower asphalt absorption due to lower temperatures. 7 Calculate Aggregate Volume Same as HMA 8 Proportion Aggregate Blends for Same as HMA Trial Mixtures 9 Calculate Trial Mixture Same as HMA Proportions by Weight and Check Dust/Binder Ratio 1. WMA process-specific specimen fabrication procedures, 10 Evaluate and Refine Trial 2. Lower short-term aging temperature. Mixtures 3. Evaluate coating and compactability in lieu of viscosity-based mixing and compaction temperatures. 11 Compile Mix Design Report Same as HMA Step 2. Select Asphalt Binder Performance Grade The same grade of binder should be used with WMA and HMA designed for the same envi- ronmental and traffic conditions. This recommendation is based on recovered binder grading data collected during NCHRP Project 9-43. These data showed only small differences in the grade of the binder for WMA and HMA sections. Table 2 summarizes the recovered binder data from NCHRP Project 9-43. Table 3 presents average differences in the continuous grade between HMA and WMA. Excluding Sasobit, which increases the high-temperature grade of the binder, an approximately 50°F (28°C) reduction in production temperature resulted in a small average decrease in the high-temperature grade of -0.2°C, while an approximately 100°F (56°C) reduction in production temperature resulted in approximately a one-half grade decrease for one low energy asphalt (LEA) project. For the low-temperature grade, again excluding Sasobit, an approximately 50°F (28°C) reduction in production temperature resulted in an average improvement in the low-temperature grade of the binder of 1.5°C, while an approximately 100°F (56°C) reduction in production temperature resulted in 2.9°C improvement for one LEA project. The differences in the high- and low-temperature binder properties between WMA and HMA are not large enough to warrant changing the grade of the binder when WMA is used. For WMA processes with very low production temperatures, it may be necessary to increase the high-temperature performance grade of the binder to meet rutting resistance requirements. Additional recovered binder grade data should be collected and analyzed to verify the conclusion from NCHRP Project 9-43 that binder grade changes are not necessary for WMA.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-3 Table 2. Summary of continuous grading of recovered binders. Production Continuous Grade Temperature, C Project Process Temperature, High Intermediate Low F Specified NA 58.0 19.0 -28.0 Control 280 59.3 14.2 -30.6 Colorado I-70 Advera 250 60.0 13.7 -31.6 Evotherm 250 61.3 14.1 -31.1 Sasobit 250 63.9 15.1 -29.9 Specified NA 58.0 16.0 -34.0 Yellowstone Control 325 60.0 11.1 -34.1 National Park Advera 275 56.3 8.9 -36.2 Sasobit 275 60.7 10.1 -35.6 New York Specified NA 64.0 22.0 -28.0 Route 11 LEA 210 60.5 14.0 -31.1 Specified NA 64.0 25.0 -22.0 Pennsylvania Control 320 67.7 22.0 -24.6 SR2007 Evotherm 250 67.2 22.0 -24.9 Specified NA 64.0 25.0 -22.0 Control 310 66.6 24.1 -22.5 Pennsylvania Advera 250 67.0 22.9 -24.1 SR2006 Gencor 250 67.5 21.7 -25.7 LEA 210 63.2 21.6 -25.4 Sasobit 250 72.9 23.3 -22.5 Monroe, North Specified NA 70.0 28.0 -22.0 Carolina Astec 275 71.5 23.7 -23.9 Maximum RAP Stiffness Research completed in NCHRP Project 9-43 found that recycled asphalt pavement (RAP) binders and new binders do mix at WMA process temperatures. Therefore, it is appropriate to design WMA mixtures containing RAP in the same manner as HMA, accounting for the contri- bution of the RAP binder to the total binder content of the mixture. From the research com- pleted in NCHRP Project 9-43, the RAP and new binders continue to mix while the mix is held at elevated temperature. To ensure that adequate mixing of RAP and new binders occurs, a limit is placed on the maximum stiffness of RAP binders for WMA. That limit is based on the com- paction temperature of the mixture given that this temperature will govern the temperature of the mix during storage and transport. The RAP binder should have a high-temperature grade that is less than the compaction temperature for the WMA. This limit will have little effect on the use of RAP in WMA. RAP binders typically range from PG 82 to PG 100, resulting in corresponding minimum WMA compaction temperatures ranging Table 3. Summary of average differences in continuous grade temperatures for WMA compared to HMA. Average Average Difference in Continuous Grade Difference in Temperature, C Process Number Production Temperature, High Intermediate Low F Advera 3 -46.7 -0.9 -1.3 -1.6 Evotherm 2 -50.0 0.8 0.0 -0.4 LEA 1 -100.0 -3.4 -2.5 -2.9 Plant Foaming 1 -60.0 0.9 -2.4 -3.2 Sasobit 3 -46.7 3.9 -0.3 -0.3
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C-4 Special Mixture Design Considerations and Methods for Warm Mix Asphalt from 180 to 212°F (82 to 100°C). The limit will, however, restrict the use of recycled asphalt shin- gles (RAS) in WMA. RAS binders have high-temperature grades exceeding 125°C, limiting the use of these binders in WMA to the highest temperature WMA processes. NCHRP Project 9-43 included a laboratory mixing study where WMA and HMA mixtures incorporating RAP were prepared in the laboratory and stored for various lengths of time at the compaction temperature. The degree of mixing of the RAP and new binders was evaluated by comparing dynamic moduli measured on mixture samples with dynamic moduli estimated using the properties of the binder recovered from the mixture samples. The dynamic modulus test is very sensitive to the stiffness of the binder in the mixture, and adding RAP increases the dynamic modulus significantly when the RAP is properly mixed with the new materials. The measured dynamic modulus values represent the as-mixed condition. The dynamic modulus for the fully blended condition was estimated using the Hirsch model from the shear modulus of binder recovered from the dynamic modulus specimens. If the measured and estimated dynamic moduli are the same, there is good mixing of the RAP and new binders. The findings of the laboratory mixing experiment are shown in Figure 1. At conditioning times of 0.5 and 1.0 hours, there is little blending of the new and recycled binders. For all processes and temperatures, the ratios of the measured to estimated fully blended moduli ranges from about 0.35 to 0.55. At the 2-hour conditioning time, the ratios of the measured to estimated fully blended moduli reach values approaching 1.0 for the Control HMA, Advera WMA, and Sasobit WMA. The effect of temperature is also evident for these processes, with the higher conditioning temperature resulting in somewhat improved blending. The ratios of the measured to estimated fully blended moduli for the Evotherm WMA remained low, even at the 2-hour conditioning time. This suggests that either the particular form of Evotherm used in this study retards the mixing of the new and recy- cled binders or that the extraction and recovery process stiffened the Evotherm modified binder. Further evidence of the mixing of new and RAP binders at WMA process temperatures was obtained from a mixture design study completed in NCHRP Project 9-43. In this study, six mix- tures were designed as HMA and as WMA and various volumetric and engineering properties were compared. Three of the mixtures included RAP. Table 4 summarizes the optimum binder Control 255 Control 230 Advera 230 Advera 212 Evotherm 230 Evotherm 212 Sasobit 230 Sasobit 212 1.20 Average Ratio of Measured Modulus to Estimated Fully Blended Modulus 1.00 0.80 0.60 0.40 0.20 0.00 0.0 0.5 1.0 1.5 2.0 2.5 Conditioning Time, Hours Figure 1. Comparison of the ratios of measured to fully blended dynamic moduli.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-5 Table 4. Optimum binder contents for RAP mixtures from the NCHRP 9-43 mixture design study. Advera Evotherm Sasobit Mixture HMA WMA WMA WMA 50 gyrations, 25% RAP 6.4 6.5 6.1 6.3 75 gyrations, 25% RAP 5.5 5.3 5.2 5.3 100 gyrations, 25% RAP 6.0 6.1 5.8 6.2 content for the three mixtures containing RAP. As shown, the optimum binder content is the same or lower for the WMA compared to the HMA, further supporting the conclusion that RAP and new binders do mix at WMA process temperatures. In this study, the Evotherm mixtures do not have higher optimum binder contents than the HMA or the other WMA processes, suggesting that the RAP and new binder do mix in Evotherm mixtures and that the differences shown in Figure 15 for this process are due to the extraction and recovery process used in the mixing study. Plant mixing studies similar to the NCHRP 9-43 laboratory mixing study are needed to con- firm that RAP and new binders mix at WMA process temperatures for field conditions. NCHRP Project 9-43 included one field project that used 30% RAP, the Astec Double Barrel Green WMA process, and mixing and compaction temperatures 275 and 260°F (135 and 127°C). For this project, the mixing analysis showed good mixing of the RAP and new binders. Additional stud- ies of this type are needed. Blending Chart Analysis The NCHRP Project 9-43 recovered binder data (shown in Table 29) confirmed that binders from WMA mixtures have improved low-temperature properties, probably due to the lesser amount of aging that occurs during production. Although the improvement in low-temperature properties is not large enough to warrant changing the low-temperature grade, the improvement is large enough to affect the amount of RAP that can be added to a mixture when blending chart analyses are used. NCHRP Project 9-43 included a binder grade study where the Rolling Thin Film Oven Test (RTFOT) was used to simulate the effect on binder properties of changes in production temper- atures. Figure 2 shows that there appears to be a weak relationship between the rate of change in low-temperature grade with RTFOT temperature and the low-temperature grade of the binder. Binders with better low-temperature properties tend to show more improvement in low- temperature properties when the RTFOT temperature is decreased. For the binders tested, decreasing the production temperature by 95°F (53°C) only improved the low-temperature grade of the binder by 1 to 2°C which is only 1/6th to 1/3rd of a grade level. As discussed earlier this change is not sufficient to warrant changing the low-temperature grade for WMA mixtures; however, this low-temperature grade improvement can be significant when considering mix- tures incorporating recycled asphalt pavement (RAP). When RAP blending charts are used, the low-temperature continuous grade of the binder changes approximately 0.6°C for every 10% of the total binder in the mixture replaced with RAP binder. Thus, improving the low-temperature properties of the virgin binder in the mixture 0.6°C by lowering the production temperature will allow 10% additional RAP binder to be added to the mixture. Using the relationship shown in Figure 16, for the middle of the low-temperature binder grade temperature range, recommended improvements in virgin binder low-temperature continuous grades for RAP blending chart analysis were developed as a function of WMA production temper- ature for mixtures incorporating PG XX-16, PG XX-22, and PG XX-28. These recommended improvements are summarized in Table 5 for some common binder grades. For a mixture
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C-6 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 0.045 Rate of Change of Low Temperature Grade 0.040 With RTFOT Temperature, oC/oC 0.035 y = -0.0019x - 0.024 0.030 2 R = 0.52 0.025 0.020 0.015 0.010 0.005 0.000 -34.0 -28.0 -22.0 -16.0 o Low Temperature Grade, C Figure 2. Effect of low-temperature binder grade on the rate of change of low-temperature grade with RTFOT temperature. Table 5. Recommended improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures. Virgin Binder PG Grade 58-28 58-22 64-22 64-16 67-22 Average HMA Production Temperature, oF 285 285 292 292 300 Rate of Improvement of Virgin Binder Low- Temperature Grade per oC Reduction in Plant 0.035 0.025 0.025 0.012 0.025 Temperature Recommended Improvement in Virgin Binder Low- WMA Production Temperature, oF Temperature Continuous Grade for RAP Blending Chart Analysis, oC 300 NA NA NA NA 0.0 295 NA NA NA NA 0.1 290 NA NA 0.0 0.0 0.1 285 0.0 0.0 0.1 0.0 0.2 280 0.1 0.1 0.2 0.1 0.3 275 0.2 0.1 0.2 0.1 0.3 270 0.3 0.2 0.3 0.1 0.4 265 0.4 0.3 0.4 0.2 0.5 260 0.5 0.3 0.4 0.2 0.6 255 0.6 0.4 0.5 0.2 0.6 250 0.7 0.5 0.6 0.3 0.7 245 0.8 0.6 0.7 0.3 0.8 240 0.9 0.6 0.7 0.3 0.8 235 1.0 0.7 0.8 0.4 0.9 230 1.1 0.8 0.9 0.4 1.0 225 1.2 0.8 0.9 0.4 1.0 220 1.3 0.9 1.0 0.5 1.1 215 1.4 1.0 1.1 0.5 1.2 210 1.5 1.0 1.1 0.5 1.3 205 1.6 1.1 1.2 0.6 1.3 200 1.7 1.2 1.3 0.6 1.4