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Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 30
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 31
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 32
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 34
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 35
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Page 36
Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
Page 42
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
Page 43
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Suggested Citation:"II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Part I of this report describes recommended procedures for designing dense-graded, asphalt concrete mixtures that will be produced using any one of several currently available WMA processes. These WMA mix design recommendations are based on research conducted in National Cooperative Highway Research Program (NCHRP) Project 9-43, “Mix Design Practices for Warm Mix Asphalt,” which concluded that only minor modification of current mix design practice is needed to address WMA. Although the procedures described have been specifically selected for use in designing dense-graded mixtures, most can be applied to the design of other mix types with little or no modification. The following sections of this Part II of the report are a commentary that presents supporting information from the NCHRP 9-43 research report for the recommendations included in Part I. Many of these sections also include recommended additional research, because NCHRP Project 9-43 was the first major study addressing WMA mixture design, and some of the findings require further validation through additional research. Both Parts I and II are organized around the eleven steps described in Chapter 8 of NCHRP Report 673: A Manual for the Design of Hot Mix Asphalt with Commentary for the design of dense-graded HMA. Table 1 summarizes the differences between WMA and HMA design for each of the eleven steps. Step 1. Gather Information For the design of WMA, additional information must be collected on the WMA process that will be used, additive rates, and planned production and compaction temperatures. The reason that this information is needed is the design of WMA uses process-specific specimen fabrication procedures. These specimen fabrication procedures were designed to simulate, in an approxi- mate manner, the WMA process in the field. For the purposes of mixture design, the various WMA processes can be grouped into four generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. Specimen fabrication techniques are somewhat different for each of these categories. Given that viscosity-based mixing and compaction temperatures are not applicable to many WMA processes, the planned production and compaction temperatures are used in the WMA mixture design process to evaluate coating and the compactability/workability of the WMA. It should be emphasized that the optimal production and compaction temperatures are different for the various WMA processes and should be carefully considered when selecting production and compaction temperatures to be used in the WMA design process. C-1 II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

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. C-2 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 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 Aggregate Size Same as HMA 5 Determine Target VMA and Design Air Voids Value Same as HMA 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 Trial Mixtures Same as HMA 9 Calculate Trial Mixture Proportions by Weight and Check Dust/Binder Ratio Same as HMA 10 Evaluate and Refine Trial Mixtures 1. WMA process-specific specimen fabrication procedures, 2. Lower short-term aging temperature. 3. Evaluate coating and compactability in lieu of viscosity-based mixing and compaction temperatures. 11 Compile Mix Design Report Same as HMA Table 1. Steps in design of dense-graded HMA and WMA.

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 II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-3 Project Process Production Temperature, F Continuous Grade Temperature, C High Intermediate Low Colorado I-70 Specified NA 58.0 19.0 -28.0 Control 280 59.3 14.2 -30.6 Advera 250 60.0 13.7 -31.6 Evotherm 250 61.3 14.1 -31.1 Sasobit 250 63.9 15.1 -29.9 Yellowstone National Park Specified NA 58.0 16.0 -34.0 Control 325 60.0 11.1 -34.1 Advera 275 56.3 8.9 -36.2 Sasobit 275 60.7 10.1 -35.6 New York Route 11 Specified NA 64.0 22.0 -28.0 LEA 210 60.5 14.0 -31.1 Pennsylvania SR2007 Specified NA 64.0 25.0 -22.0 Control 320 67.7 22.0 -24.6 Evotherm 250 67.2 22.0 -24.9 Pennsylvania SR2006 Specified NA 64.0 25.0 -22.0 Control 310 66.6 24.1 -22.5 Advera 250 67.0 22.9 -24.1 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 Carolina Specified NA 70.0 28.0 -22.0 Astec 275 71.5 23.7 -23.9 Table 2. Summary of continuous grading of recovered binders. Process Number Average Difference in Production Temperature, F Average Difference in Continuous Grade Temperature, C High Intermediate Low 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 Table 3. Summary of average differences in continuous grade temperatures for WMA compared to HMA.

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 C-4 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 0.00 0.20 0.40 0.60 0.80 1.00 1.20 0.0 0.5 1.0 1.5 2.0 2.5 Conditioning Time, Hours A ve ra ge R at io o f M ea su re d M od ul us to Es tim at ed F ul ly B le nd ed M od ul us Control 255 Control 230 Advera 230 Advera 212 Evotherm 230 Evotherm 212 Sasobit 230 Sasobit 212 Figure 1. Comparison of the ratios of measured to fully blended dynamic moduli.

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 II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-5 Mixture HMA Advera WMA Evotherm WMA Sasobit 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 Table 4. Optimum binder contents for RAP mixtures from the NCHRP 9-43 mixture design study.

C-6 Special Mixture Design Considerations and Methods for Warm Mix Asphalt y = -0.0019x - 0.024 R2 = 0.52 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 -34.0 -28.0 -22.0 -16.0 Low Temperature Grade, oC R at e of C ha ng e of L ow T em pe ra tu re G ra de W ith R TF O T Te m pe ra tu re , o C/ o C Figure 2. Effect of low-temperature binder grade on the rate of change of low-temperature grade with RTFOT temperature. 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 Temperature 0.035 0.025 0.025 0.012 0.025 WMA Production Temperature, oF Recommended Improvement in Virgin Binder Low- 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 Table 5. Recommended improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures.

using PG 64-22 virgin binder and a WMA production temperature of 250°F, the virgin binder low-temperature continuous grade would be improved 0.6°C 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 270/260°F were made using a PG 64-22 binder, while the WMA mixtures prepared at mixing/compaction temperatures of 225/215°F 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: µWMA − µHMA = 0 Alternative hypothesis: µWMA − µHMA > 0 or µWMA − µHMA < 0 (as appropriate) II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-7

Test statistic: Rejection region: Reject the null hypothesis and accept the alternative hypothesis if t > tα for n-1 degrees of freedom. where µWMA = population mean for WMA mixtures µHMA = population mean for HMA mixtures 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. t d s n d = ⎛⎝⎜ ⎞⎠⎟ C-8 Special Mixture Design Considerations and Methods for Warm Mix Asphalt No. Mixture Identification Mixing/Compaction Temperature, °F,for Process: Ndesign Aggregate Water Absorption, % RAP HMA AdveraWMA Evotherm 3G WMA Sasobit 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 Table 6. Mix design experiment.

II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-9 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in D es ig n Bi nd er C on te nt , w t. % 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 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in B in de r A bs or pt io n, w t % 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).

Step 9. Calculate Trial Mixture Proportions by Weight and Check Dust-to-Binder Ratio These calculations are identical to those for HMA. Step 10. Evaluate and Refine Trial Mixtures This step involves the preparation and evaluation of laboratory specimens of WMA. The pro- cedure follows that for HMA with slight modification. Table 7 summarizes the steps for WMA and HMA design. The modifications required for WMA design are 1. For some processes, the WMA additive must be calculated. 2. Viscosity-based mixing temperatures are not used with WMA. Laboratory mixing is done at the planned production temperature. 3. Process-specific specimen fabrication procedures are used to prepare laboratory mixtures. 4. The short-term conditioning temperature for WMA is the planned compaction temperature. 5. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction is done at the planned compaction temperature. 6. WMA design includes an evaluation of coating and compactability using the planned pro- duction and compaction temperatures. Supporting data from NCHRP Project 9-43 for these modifications are discussed in the sections that follow. Additive Dosage The computation of WMA additive dosage rates is straightforward. The amount of additive needed may be specified by the WMA process supplier as percent by weight of binder or total C-10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Step Description HMA WMA Comment 1 Calculate batch weights X X Must calculate WMA additive content for some processes 2 Batch aggregates X X Must batch WMA additive for some processes 3 Heat aggregates and asphalt binder X X Use planned production temperature for WMA 4 Mix aggregates and binder X X Procedure is WMA process specific 5 Short-term oven conditioning X X WMA uses lower temperature. 6 Compact laboratory specimens X X WMA uses lower temperature 7 Calculate volumetric composition of laboratory specimens X X 8 Adjust aggregate proportions to meet volumetric requirements X X 9 Evaluate coating and compactability NA X Used in WMA design in place of viscosity- based mixing and compaction temperatures 10 Conduct performance testing X X Moisture sensitivity for all mixtures, rutting resistance for design traffic levels of 3 m ESALs or greater Table 7. Comparison of trial specimen fabrication procedures for WMA and HMA design.

mixture. For wet aggregate processes, water is added to a portion of the fine aggregate, and then this wet, fine aggregate is added cold to the mixture during the mixing process. The proportion of the aggregate that is added wet and the moisture content are provided by the WMA technol- ogy provider. Mixing Temperatures Viscosity-based mixing temperatures cannot be used with the wide range of WMA processes currently available. Laboratory specimens are mixed at the planned production temperature, and coating is evaluated to determine the acceptability of the WMA process. Process-Specific Specimen Fabrication Procedures For mixture design, the various WMA processes were grouped into four generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. The procedures in the report address laboratory mixing. These were developed based on recommendations from various WMA technology providers and verified during the mix design experiment completed in NCHRP Project 9-43. Once mixing is complete, specimen fabrication for all processes continues with short-term conditioning and specimen compaction. These steps are the same for all processes and the same as done with HMA. WMA mixture designs will require additional equipment. Since coating is used in lieu of viscosity-based mixing and compaction temperatures, a mechanical mixer is required. During NCHRP Project 9-43, it was observed that planetary mixers and bucket mixers do not have the same mixing efficiency. Planetary mixers are more efficient. The specimen fabrication proce- dures were developed in NCHRP Project 9-43 using a planetary mixer. For WMA processes where the additive is blended in the binder, a mechanical stirrer is needed. For designing mixtures for plant foaming processes, a laboratory foamed asphalt plant that can produce foamed asphalt at the moisture content used by the field equipment is also needed. NCHRP Project 9-43 demon- strated that it is feasible to perform foamed asphalt WMA mixture designs in the laboratory. In NCHRP Project 9-43, a modified Wirtgen WLB-10 laboratory foaming plant was used to simu- late the Gencore Ultrafoam GX process using 1.25% water by weight of binder and the Astec Double Barrel Green process using 2.0% water by weight of binder. The modification that was required was to the replace the flow controller with a smaller, more precise flow controller to accommodate the water contents used in WMA mixtures. Short-Term Conditioning Short-term conditioning for WMA was set at 2 hours at the planned compaction temper- ature to represent the absorption and binder stiffening that occurs during construction. This level of conditioning is used for the volumetric design and for the moisture sensitivity and rutting evaluation. These conditions were selected based on comparisons of properties of laboratory-mixed, laboratory-compacted specimens with those from field-mixed, laboratory- compacted specimens. II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-11

Figures 5 and 6 summarize the results of comparisons of maximum specific gravity and indirect tensile strength for the field sections tested in NCHRP Project 9-43. The error bars shown in Figure 5 are the single operator D2s precision from AASHTO T 209. These data show that the maximum specific gravity of the lab and field mixtures are the same, indicating that the binder absorption is the same for the lab and field mixtures. The aggregate water absorption ranged from 0.5% for the Pennsylvania SR2007 mixtures to 2.5% of the Yellowstone National Park mixtures. Figure 6 shows average differences in indirect tensile strength for the field mixtures minus the laboratory mixtures. The error bars in this figure are 95% confidence intervals for a paired t-test comparison. If the error bars do not capture zero, then the difference in the tensile strength of the field- and laboratory-mixed specimens is different from zero. Figure 6 shows that several mixtures have significantly different tensile strengths. The differences are not consistently in one direction except for the Pennsylvania SR2006 project, where the field-mixed specimens always have significantly higher tensile strengths compared to the laboratory-mixed specimens. Given that one-third of the mixtures were from this project, this difference biased the results. The average difference for all projects was 7 psi (48 kPa); not considering the Pennsylvania SR2006 project, the average difference was essentially zero. Short-term conditioning for performance evaluations, moisture sensitivity, and rutting was one of the areas where additional research was recommended in NCHRP Project 9-43. This addi- tional research was recommended because it appears that the current HMA short-term condition- ing procedure for performance evaluation, 4 hours at 275°F (135°C), represents the stiffening that occurs during construction and some short time in service. C-12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 2.300 2.400 2.500 2.600 CO I-70 Control CO I-70 Advera CO I-70 Evotherm CO I-70 Sasobit YNP Control YNP Advera YNP Sasobit PA SR2007 Control PA SR2007 Evotherm PA SR2006 Control PA SR2006 Advera PA SR2006 Gencor PA SR2006 LEA PA SR2006 Sasobit Monroe NC Astec Maximum Specific Gravity M ix tu re /P ro ce ss Compaction Temp 2 Hours Field Mix Figure 5. Comparison of maximum specific gravity between field mixes and laboratory mixes short-term conditioned 2 hours at the compaction temperature.

Compaction Temperatures Viscosity-based compaction temperatures cannot be used with the wide range of WMA processes currently available. Laboratory specimens are compacted at the planned compaction temperature. Additionally a compactability evaluation is conducted to ensure that the mixture is compactable at the planned compaction temperature. WMA Evaluations Four evaluations are conducted on WMA mixtures at the design binder content: (1) coating, (2) compactability, (3) moisture sensitivity, and (4) rutting resistance. The sections below describe the supporting information from NCHRP Project 9-43 for these evaluations. Coating Coating is one way to evaluate proposed WMA production temperatures that is relevant to all WMA processes. In NCHRP Project 9-43, coating was evaluated on a number of HMA and WMA mixtures using AASHTO T 195. AASHTO T 195 counts the percentage of the coarse aggregates in the mixture that are fully coated. This is a strict criterion. When a planetary mixer was used, coat- ing was always found to be nearly 100 percent for both WMA and HMA. When a bucket mixer was used with a smaller number of WMA mixes, the coating was much lower. This indicates that the bucket mixer is less efficient than the planetary mixer. The criterion of 95% was based on the plan- etary mixer data. Though bucket mixers are less efficient than planetary mixers, they are signifi- cantly less expensive and likely more readily available in mix design laboratories. Until additional research is conducted to develop appropriate mixing times for bucket mixers, technicians and engi- neers will have to develop mixing times for their WMA mixtures based on coating evaluations for HMA mixtures produced using the traditional viscosity-based mixing temperatures. II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-13 -25 -15 -5 5 15 25 35 45 55 CO I-7 0 Co n tro l CO I-7 0 Ad ve ra CO I-7 0 Ev o th e rm CO I-7 0 Sa so bi t YN P Co n tro l YN P Ad ve ra YN P Sa so bi t PA SR 20 07 C o n tro l PA SR 20 07 E vo th e rm PA SR 20 06 C o n tro l PA SR 20 06 A dv e ra PA SR 20 06 G e n co r PA SR 20 06 L EA PA SR 20 06 S a so bi t M on ro e N C As te c Av e ra geID T St re n gt h D iff er en c e s , ps i Mixture/Process Figure 6. Differences in indirect tensile strength between field mixes and laboratory mixes short-term conditioned 2 hours at the compaction temperature.

Compactability The compactability evaluation is used in lieu of the viscosity-based mixing temperature used for HMA. Compactability is evaluated by compacting specimens to Ndesign at the planned field compaction temperature and again at 54°F (30°C) below the planned field compaction temper- ature. The number of gyrations to reach 92% relative density is then calculated from the height data. The ratio of the gyrations to 92% relative density at the lower temperature to the higher temperature should be less than 1.25. The methodology for the compactability evaluation resulted from a workability study conducted in NCHRP Project 9-43. The workability study evaluated the feasibility of using various workabil- ity devices and the gyratory compactor to measure WMA workability during the mixture design process. The workability study demonstrated that it is possible to measure differences in the workability and compactability of WMA compared to HMA. The differences, however, were only significant at temperatures that are below typical WMA discharge temperatures. Figures 7 and 8 show the effect of WMA process and temperature on workability and compactability. Given that the workability devices were not able to discriminate more precisely than compaction data obtained from a standard Superpave gyratory compactor, the method for evaluating the tem- perature sensitivity of the compactability of WMA was developed for assessing WMA workability and compactability. The method involves determining the number of gyrations to 8% air voids at the proposed compaction temperature and a second temperature that is approximately 54°F (30°C) lower than the proposed compaction temperature. A tentative limit allowing a 25% increase in the number of gyrations when the temperature is decreased was developed. This limit was inves- tigated using data from nine WMA field mixture projects sampled in NCHRP 9-43. The increase in gyrations for the WMA processes ranged from 0 to 20%. Workability and compactability was not reported to be a problem on any of the projects. Moisture Sensitivity Moisture sensitivity is evaluated using AASHTO T 283, the same as HMA. The criterion for AASHTO T 283 is the same as that for HMA. C-14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 0 50 100 150 200 250 300 350 400 450 300 250 190 150 Temperature, F To rq ue , i n- lb Control Advera Sasobit Figure 7. Effect of temperature and WMA additive on torque measured in the UMass workability device.

Tests conducted during NCHRP Project 9-43 showed that the moisture sensitivity will likely be different for WMA and HMA mixtures designed using the same aggregates and binder. WMA processes that included antistrip additives improved the tensile strength ratio of some of the mix- tures included in the NCHRP Project 9-43 testing and analysis. Of the nine WMA mixtures that used a WMA process that included an antistrip additive, the tensile strength ratio remained the same or improved in 67% of the mixtures. For WMA mixtures produced using processes that do not include antistrip additives, the tensile strength ratio never improved and decreased in 79% of the mixtures. Rutting Resistance Rutting resistance is evaluated using the flow number test, AASHTO TP 79. The same testing conditions that are used for HMA flow number testing are used with WMA: • Air voids of 7.0 ± 0.5% • 50% reliability high pavement temperature from LTPPBind 3.1 for the project location, 20 mm below the pavement surface, or 20 mm below the top of the sub-surface pavement layer of interest • Unconfined • Repeated deviator stress of 87 psi (600 kPa), contact deviator stress of 4.4 psi (30 kPa), Minimum flow numbers as a function of traffic level are provided and these are lower than those for HMA. Table 8 compares the recommended flow numbers for WMA and HMA. The II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-15 0 5 10 15 20 25 30 35 40 45 300 250 190 Temperature, F G yr at io ns Control Advera Sasobit Figure 8. Effect of temperature and WMA additive on gyrations to 92% relative density. Traffic Level, Million ESALs Minimum Flow Number WMA HMA < 3 NA NA 3 to < 10 30 50 10 to < 30 105 190 30 415 740 Table 8. Flow number criteria for WMA and HMA mixtures.

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: C-16 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Process Number Average Difference in Compaction Temperature, F Average Difference in Flow Number, % Advera 3 -46.7 -39 Evotherm 2 -50.0 -38 LEA 1 -80.0 -50 Sasobit 3 -48.3 +38 Table 9. Summary of average difference in flow number of WMA compared to HMA for the NCHRP 9-43 field validation sections.

• WMA process, • WMA additive rate, • Planned production temperature, and • Planned compaction temperature. References Bonaquist, R., “Mix Design Practices for Warm Mix Asphalt,” NCHRP Report 691, National Cooperative Highway Research Program, Washington, D.C., 2011. Christensen, D. W., “A Manual for the Design of Hot Mix Asphalt with Commentary,” NCHRP Report 673, National Cooperative Highway Research Program, Washington, D.C., 2010. Prowell, B. D., and Hurley, G. C., “Warm-Mix Asphalt: Best Practices,” Quality Improvement Series 125, National Asphalt Pavement Association, Lanham, MD, 2007. II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-17

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Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 714: Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary presents special mixture design considerations and methods used with warm-mix asphalt.

NCHRP Report 714 is a supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt. All references to chapters in NCHRP Report 714 refer to the corresponding chapters in NCHRP Report 673.

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