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Mix Design Practices for Warm-Mix Asphalt (2011)

Chapter: Chapter 3 - Findings and Applications

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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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23 This chapter presents the major findings from the various studies conducted in NCHRP Project 09-43 and discusses how these findings shaped the draft appendix to AASHTO R 35, Spe- cial Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA), which is contained in Appendix A herein. Detailed results and supporting analyses for these findings are included in Appendix E. Conclusions and recommendations drawn from these findings are presented in Chapter 4. 3.1 Phase I Findings 3.1.1 Sample Reheating Study The sample reheating study was conducted to determine whether sample reheating significantly affects the mechanical properties of WMA mixtures. HMA samples are often reheated for a variety of acceptance and performance tests. When the WMA process includes an irreversible component, such as foamed asphalt or some of the chemical additives, it may not be possible to use reheated samples for volumetric accept- ance. However, reheated samples can be used to evaluate the mechanical properties of WMA mixtures for pavement analy- sis provided the effect of reheating on WMA samples is similar to the effect of reheating on HMA. The sample reheating study found that reheating has a sim- ilar effect on the mechanical properties of WMA and HMA. Details of this analysis are presented in Section E2 of Appen- dix E. Figures 6 through 10 show the effect of sample reheating on the dynamic modulus master curve for a control HMA and four WMA processes: Aspha-min, Evotherm ET, Sasobit, and LEA. The error bars in these figures represent 95 percent con- fidence intervals for the mean based on a typical coefficient of variation for the dynamic modulus test of 14 percent and the number of samples that were tested. When the confidence intervals do not overlap, there is a significant difference in the dynamic modulus for the various conditions. Reheating has an effect on the stiffness of WMA that is similar to the effect it has on the stiffness of HMA. There is a stiffening of the middle por- tion of the dynamic modulus master curve, which is most sen- sitive to changes in binder stiffness. Table 19 quantifies the stiffening caused by reheating. This table presents the ratio of the reheated modulus to the imme- diate modulus and the delayed modulus for tests at 68°F (20°C), 0.1 Hz loading, which corresponds to a reduced fre- quency of 0.1 Hz in Figures 6 through 10, and is near the max- imum difference between the master curves. The modulus after reheating is 60 to 150 percent higher than the immediate modulus and 30 to 80 percent higher than the delayed modu- lus. When the immediate modulus is used as the basis, the Aspha-min mixture was more sensitive to reheating effects than the HMA control and the Evotherm and Sasobit mixtures. When the delayed modulus is used as the basis, the WMA mix- tures and the HMA control have similar sensitivity to reheat- ing. The reheating effect is probably the result of the additional aging that occurs when field samples are reheated to tempera- tures high enough to allow proper compaction. As with HMA, reheating times and temperatures for WMA should be limited to minimize this effect. 3.1.2 Binder Grade Study The lower production temperatures used with WMA pro- duce less aging of the binder during construction. This reduced aging may result in increased rutting of pavements produced using WMA processes, and it may also result in improved resistance to fatigue and low-temperature cracking. NCHRP Project 09-43 included analysis of an experiment conducted by the FHWA where the effects of WMA production tempera- tures were simulated using the RTFOT—AASHTO T 240. This section presents key findings from this analysis. The detailed analysis is presented in Section E3 of Appendix E. Analysis of the data from the RTFOT study showed that the high-temperature grade of the binder is affected more by changes in the RTFOT temperature than the low-temperature C H A P T E R 3 Findings and Applications

24 Figure 6. Effect of sample reheating on the dynamic modulus of the St. Louis HMA control mixture. Figure 7. Effect of sample reheating on the dynamic modulus of the St. Louis Aspha-min mixture. 10 100 1000 10000 100000 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Dy na m ic M o du lu s, M Pa Immediate Delayed Reheat 10 100 1000 10000 100000 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Dy na m ic M o du lu s, M Pa Immediate Delayed Reheat

25 Figure 8. Effect of sample reheating on the dynamic modulus of the St. Louis Evotherm mixture. Figure 9. Effect of sample reheating on the dynamic modulus of the St. Louis Sasobit mixture. 10 100 1000 10000 100000 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Dy na m ic M o du lu s, M Pa Immediate Delayed Reheat 10 100 1000 10000 100000 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Dy na m ic M o du lu s, M Pa Immediate Delayed Reheat

grade. The high-temperature continuous grade of the binder decreased approximately one grade (6°C) when the RTFOT aging temperature was reduced from 325°F to 230°F (163°C to 110°C). For the same change in aging temperature, the low-temperature grade improved only about one third of a grade level (2.0°C). The additional aging from the PAV that is included in the characterization of the low-temperature properties of binders is the likely cause of this difference. Apparently, improvements in low-temperature binder prop- erties resulting from lower short-term aging temperatures are offset by the simulated long-term aging from the PAV, resulting in little change in the low-temperature grade of the binder. The high-temperature grade change was sufficient to con- sider high-temperature grade bumping when WMA produc- tion temperatures are low enough to result in a half grade (3°C) change in the high-temperature grade. The high-temperature grade bumping limits were developed by relating the change in the high-temperature grade of the binder to the aging index of the binder as shown in Figure 11. The aging index of the binder is defined by Equation 1 and can be obtained from normal binder testing. It is a measure of the aging susceptibility of the binder. Binders with higher aging indices stiffen more in the RTFOT test, and, as shown in Figure 11, are affected more by changes in the RTFOT aging temperature. where AI = aging index, (G/sin δ)RTFOT = RTFOT high-temperature stiffness at grade temperature, and (G/sin δ)TANK = Tank high-temperature stiffness at grade temperature. The relationship shown in Figure 11 was used to estimate the effect of WMA production temperature on the high- temperature properties of the binder. To limit the change in high-temperature grade to one-half of one grade level, divide 3°C by the slope obtained from the binder aging index and the relationship shown in Figure 11. Equation 2 presents the allow- able temperature changes to limit the high-temperature grade change to less than one-half of one grade level. For a typical AI G G = ( ) ( )   sin sin ( ) δ δ RTFOT TANK 1 26 10 100 1000 10000 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 D yn am ic M od ul us , k si Reduced Frequency, Hz Delayed Reheat Figure 10. Effect of sample reheating on the dynamic modulus of the New York LEA mixture. Table 19. Comparison of reheated to immediate and delayed dynamic moduli for 68°F (20°C), 0.1 Hz Loading. Dynamic Modulus Ratio 68°F (20°C) 0.1 Hz Mixture Reheat to Immediate Reheat to Delayed Control 1.89 1.80 Aspha-min 2.52 1.48 Evotherm 1.59 1.32 LEA NT 1.49 Sasobit 1.59 1.60

binder with an aging index of 2.4, the maximum allowable temperature change is −54°F (−30°C). where ΔT1/2 grade = maximum temperature change for a 1⁄2 grade change, °C; and AI = binder aging index at the performance grade temperature. Plant mixing temperatures below which consideration should be given to increasing the binder grade were obtained using Equation 2 and typical HMA production temperatures. Table 20 summarizes typical plant mixing temperatures rec- ommended by the Asphalt Paving Environmental Council (19) based on the high-temperature performance grade of the binder. WMA production temperatures below which consider- ΔT AI 1 2 0 495 35 3 1 2/ . . ( )grade = − −( ) ation should be given to increasing the performance grade were obtained by combining the temperature change from Equa- tion 2 with the typical mixing temperatures from Table 20. The results rounded to the nearest 5.0°F (2.8°C) are presented in Table 21 for various binder grades and levels of the aging index of the binder. If the planned plant mixing temperatures are lower than those listed in Table 21, consideration should be given to increasing the high-temperature performance grade of the binder one grade level above that normally used for HMA. The relatively small effect of RTFOT temperature on the low-temperature binder grade did not warrant recommended changes in low-temperature binder grade selection for WMA. The low-temperature grade improvement, however, can be significant when considering mixtures incorporating RAP. When RAP blending charts are used, the low-temperature con- tinuous grade of the binder changes approximately 0.6°C for every 10 percent of the total binder in the mixture replaced with RAP binder (20). Thus, improving the low temperature properties of the virgin binder in the mixture 0.6°C by lower- ing the production temperature will allow 10 percent addi- tional RAP binder to be added to the mixture. The data collected in the RTFOT study indicated that the low-temperature grade improvement was approximately 0.035°C per °C reduc- tion in RTFOT temperature for PG XX-28 binders; 0.025°C per °C reduction in RTFOT temperature for PG XX-22 binders; and 0.022°C per °C reduction in RTFOT temperature for PG XX-16 binders. Using these values and typical HMA production temperatures for various binder grades given in Table 20, low-temperature grade improvements for RAP blend- ing chart analyses were developed for some common grades 27 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Binder Aging Index R at e of C ha ng e of R TF O T Hi gh 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 Rate of high-temperature grade change (oC/oC) = 0.085(AI-1)0.495 Figure 11. Effect of short-term aging susceptibility on the rate of change of RTFOT high-temperature grade with RTFOT temperature. PG High- Temperature Grade Recommended Mid-Point HMA Mixing Temperature (°F) 52 270 58 285 64 292 67 300 70 302 76 308 82 315 Table 20. Typical HMA mixing temperatures (19).

of binder. These improvements are summarized in Table 22. For a mixture 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. 3.1.3 Short-Term Oven Conditioning Study An important step in mixture design and analysis is short- term oven conditioning of laboratory-prepared loose mix prior to compaction. Short-term oven conditioning simulates the binder absorption and aging that occurs during construc- tion. Comparisons of properties of plant- and laboratory- prepared mixtures for the Colorado I-70 WMA project were used to select an appropriate short-term conditioning. For con- venience, the short-term conditioning temperature was selected to be the compaction temperature. Conditioning times of 2 h and 4 h were investigated. This section presents the findings from the short-term oven conditioning study. The detailed analysis is presented in Section E4 of Appendix E. 28 Aging Index 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 PG High- Temperature Grade Minimum WMA Mixing Temperature Not Requiring PG Grade Increase (°F) 52 170 190 200 205 210 215 220 220 225 225 230 230 58 185 205 215 220 225 230 235 235 240 240 245 245 64 190 210 220 230 235 235 240 245 245 250 250 250 67 200 220 230 235 240 245 250 255 255 255 260 260 70 200 220 230 240 245 245 250 255 255 260 260 260 76 210 225 235 245 250 255 260 260 265 265 265 270 82 215 235 245 250 255 260 265 265 270 270 275 275 Table 21. Minimum WMA production temperatures not requiring a high-temperature PG grade increase based on the RTFOT experiment. Virgin Binder PG Grade 58–28 58–22 64–22 64–16 67–22 Average HMA Production Temperature, °F 285 285 292 292 300 Rate of Improvement of Virgin Binder Low- Temperature Grade per °C Reduction in Plant Temperature 0.035 0.025 0.025 0.012 0.025 WMA Production Temperature, °F Improvement in Virgin Binder Low-TemperatureContinuous Grade for RAP Blending Chart Analysis, °C 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 22. Anticipated improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures.

The short-term oven conditioning study confirmed the practice suggested by some WMA process developers of using 2 h of oven aging at the WMA compaction temperature. Fig- ure 12 compares maximum specific gravity measurements for the mixtures used in the Colorado I-70 WMA project. The error bars shown in Figure 12 are the acceptable range of max- imum specific gravity measurements based on the single oper- ator precision statement given in AASHTO T 209. Figure 12 shows that for the aggregate used in the Colorado I-70 mix- tures, the maximum specific gravity is essentially the same for all processes and all short-term aging conditions. The reported water absorption for the Colorado I-70 job mix formula was 0.8 percent. Figure 13 compares the indirect tensile data for plant-mixed, laboratory-compacted specimens and laboratory-mixed, laboratory-compacted specimens. The error bars in Figure 13 are 95-percent confidence intervals for the average indirect tensile strength. The graphical analysis shown in Figure 13 sug- gests that 2 h of conditioning at the compaction temperature provides tensile strengths for laboratory-prepared specimens that are approximately equal to those for field mixtures. Sup- porting statistical analyses for this finding are presented in 29 2.410 2.420 2.430 2.440 2.450 2.460 2.470 Control Advera Evotherm Sasobit Process M ax im um S pe ci fic G ra vi ty Field Mix Compaction Temp 4 Hours (test not done for Evotherm) Compaction Temp 2 Hours Figure 12. Maximum specific gravity for Colorado I-70 mixtures. 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Control Advera Evotherm Sasobit Process ID T St re ng th , p si Field Mix Compaction Temp 4 Hours Compaction Temp 2 Hours (test not done for Evotherm) Figure 13. Comparison of tensile strengths for Colorado I-70 mixtures.

Section E4 of Appendix E. Section E4 of Appendix E also includes graphical and statistical analysis of dynamic moduli that also supports the use of 2 h at the compaction temperature for short-term conditioning of WMA mixtures. 3.1.4 RAP Study The primary concern when using RAP in WMA is whether the RAP and new binders mix at the lower temperatures used in WMA. The RAP study included two experiments designed to assess the mixing of RAP and new binders at WMA process temperatures. This section presents the findings from the RAP study. Detailed analysis of the data from the RAP study is pre- sented in Section E5 of Appendix E. The first experiment was an interfacial mixing study that used atomic force microscope (AFM) imaging of “film-on- film” interface contact lines. A thin film of WMA binder was cast onto a film of binder that was previously aged in the PAV to simulate an aged RAP binder. These samples were imaged via AFM in the center of the new binder film, at the contact line between the two films, and toward the edge of the RAP binder film where the new binder had not coated this film. All sample films were imaged after casting, then periodically imaged after being thermally conditioned in a 266°F (130°C) oven. The interfacial mixing study found that the binder films mixed under thermal cycling. This is shown in the AFM images pre- sented in Figure 14. The lower left image is the surface of the RAP binder. The surface of the WMA binder, in this case Saso- bit modified, is shown in the upper right image. Finally, the upper left image is that of the thermally mixed interfacial con- tact line. It depicts a transition in structuring between the RAP binder surface and the new binder surface, indicating that the two binders are mixing during thermal conditioning at 266°F (130°C). This finding was confirmed through a laboratory mixing experiment. In the laboratory mixing experiment, HMA and WMA mixtures incorporating RAP were prepared at different temperatures and short-term conditioned for periods ranging from 0.5 h to 2.0 h. The mixing of the new and recycled binders was quantified by comparing dynamic moduli measured on 30 Figure 14. AFM Scans at the interfacial contact line (upper left), at the new binder surface (upper right), and at the RAP binder surface (lower left) after annealing.

samples of the mixtures with dynamic moduli estimated using binder recovered from the mixtures. The measured dynamic moduli represented the “as mixed” condition; the estimated moduli represented the “fully blended” condition. A measured- to-estimated dynamic modulus ratio approaching one indi- cated a high degree of mixing of the RAP and new binders. The findings of the laboratory mixing experiment are shown in Figure 15. At conditioning times of 0.5 h and 1.0 h, there is little blending of the new and recycled binders. For all processes and temperatures, the ratio of the measured-to-estimated fully blended moduli range from about 0.35 to 0.55. At the 2 h con- ditioning time, the ratio of the measured-to-estimated fully blended moduli reach values approaching 1.0 for the Control, Advera, and Sasobit. The effect of temperature is also evident for these processes, with the higher conditioning temperature resulting in somewhat higher ratios. The ratio of the measured- to-estimated fully blended moduli for the Evotherm WMA remained low even at the 2 h conditioning time. This sug- gests that either the particular form of Evotherm used in this study retards the mixing of the new and recycled binders or that the extraction and recovery process stiffens the Evo- therm modified binder. Other findings from the mixture design study show that RAP and new binders do mix well for the Evotherm G3 process for 2 h of conditioning time. Mix- tures with 25-percent RAP designed using Evotherm G3 had the same optimum binder content as 25-percent RAP mixtures designed as HMA. The findings from the two experiments in the RAP study show that RAP and new binders mix at WMA process tem- peratures in a manner that is similar to the way that they mix in HMA at higher temperatures. Clearly the mixing is time dependent, indicating that the new binder coats the virgin aggregate and RAP, and then, during storage at elevated tem- perature, the two binders continue to mix. The RAP used in this study was very stiff, having a continuous performance grade of PG 105.8 –2.3, and likely represents a worst-case sce- nario. Since the binders continued to mix during oven con- ditioning at the compaction temperature, the compaction temperature was probably the critical temperature in the mixing study. It is likely that the minimum temperature that can be used is related to the viscosity of the RAP binder at that temperature. The RAP binder used in this study had a viscos- ity of approximately 22,000 P (220 Pa•s) at the average com- paction temperature of 221°F (105°C) used in this study, suggesting that a reasonable tentative requirement for RAP in WMA is that the RAP have a viscosity less than 22,000 P (220 Pa•s) at the planned field compaction temperature. This is approximately equivalent to requiring the planned field compaction temperature to be greater than the temperature where the as-recovered RAP binder meets the AASHTO M 320 requirement of G*/sinδ = 2.20 kPa. 3.1.5 Workability Study The workability study was conducted to identify a worka- bility test to be used in place of viscosity-based mixing and compaction temperatures to directly evaluate workability and compactability of WMA mixtures. Three devices that measure either the torque or the force required to move a blade through the loose mixture were evaluated as potential workability devices: (1) UMass Workability Device, (2) Nynäs Workabil- ity Device, and (3) University of New Hampshire Workability 31 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, h 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 15. Comparison of the ratio of measured to fully blended dynamic moduli.

Device. Additionally, various parameters that could be obtained during gyratory compaction, including the gyratory shear stress, were evaluated as measures of compactability. This section, presents the findings from the workability study. Detailed analysis of the data from the workability study is presented in Section E6 of Appendix E. The primary finding from the workability study for the three workability devices was that these devices could only discrim- inate between HMA and WMA at temperatures that are much lower than the temperatures normally associated with the pro- duction of WMA. This is illustrated in Figure 16, which pre- sents torque measurements at different temperatures for the UMass Workability Device. The workability study further found that the number of gyrations to reach 92-percent rela- tive density in the gyratory compactor, shown in Figure 17, had similar sensitivity to temperature and WMA additives. The workability study demonstrated that it is possible to measure differences in the workability and compactability of WMA as compared to HMA. The differences, however, are only significant at temperatures that are below typical WMA 32 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 16. Effect of temperature and WMA additive on torque measured in the UMass workability device. 0 5 10 15 20 25 30 35 40 45 300 250 190 Temperature, °F G yr at io ns Control Advera Sasobit Figure 17. Effect of temperature and WMA additive on gyrations to 92-percent relative density.

discharge temperatures. This suggests that it is not necessary to evaluate workability at the planned production temperature. The evaluation of coating at the planned production temper- ature should suffice. It appears that workability and com- pactability can be evaluated by using the gyratory compactor to determine the gyrations to 8-percent air voids at the planned field compaction temperature and a second temperature that is approximately 55°F (30°C) lower than the planned field compaction temperature. This will permit an assessment of the temperature sensitivity of the workability and compactability of the mixture. 3.2 Preliminary Mixture Design Procedure Revisions The preliminary mixture design procedure was modified based on the findings of the Phase I studies. Table 23 summa- rizes how the findings from each of the Phase I studies were used to revise the preliminary mixture design procedure. The key finding from the sample reheating study was that reheating has a similar stiffening effect on WMA and HMA. This finding was not directly incorporated into the revised pre- liminary mixture design procedure. It was used in analysis of data from the other Phase I studies, particularly the selection of a short-term conditioning time for WMA. The binder grade study yielded three key findings. First, the high-temperature grade of the binder was significantly affected by changes in the RTFOT aging temperature. A relationship among the short-term aging temperature, the aging index of the binder, and the change in high-temperature grade of the binder was developed and used to provide preliminary guid- ance on production temperatures below which the high- temperature binder grade should be increased one grade level. This guidance was included in the revised preliminary proce- dure. The second key finding from the binder grade study was that the changes in the low-temperature grade of the binder that resulted from changes in the RTFOT aging temperature were small and did not warrant recommended changes in low- temperature binder grade selection for WMA. The third key finding was that small improvements in the low-temperature properties could result in increased RAP usage when a blend- ing chart analysis is conducted. The key finding from the short-term conditioning study was that 2 h of conditioning at the compaction temperature reasonably reproduced the absorption and binder aging that occurred during construction. The revised preliminary proce- dure specified 2 h of conditioning at the compaction tempera- ture for volumetric design and performance testing. The RAP study found that RAP and new binders do mix at WMA production and compaction temperatures in a manner 33 Phase I Study Key Findings Preliminary Design Procedure Revisions Sample Reheating 1. Reheating has similar stiffening effect for WMA and HM A 1. Not directly incorporated. Used in selection of tentative short-ter m conditioning tim e. Binder Grade 1. The high-tem perature binder grade is significantly affected by the RTFOT te mp erature. The tem perature effect was greater for binders with greater short-ter m aging susceptibility. 2. The effect of RTFOT tem perature on the low-tem perature binder grade was sm all. 3. The effect of RTFOT tem perature on the low-tem perature grade could affect RAP contents when blending charts are used. 1. Preliminary recommendations were included in the binder grade selection to increase the high-tem perature perform ance grade based on the planned production temperature and Table 21. 2. No change required. 3. Preliminary recommendations for improving the low-tem perature grade of the virgin binder for blending chart analyses were added based on Table 22. Short-Ter m Conditioning 1. 2 h of oven conditioning at the co mp action te mp erature reasonably reproduced the binder absorption and binder stiffening that occurs during construction. 1. 2 h at the co mp action tem perature was specified for volum etric design and perform ance testing. RAP 1. RAP and new binders mix at WMA production and com paction te mp eratures in a manner similar to HMA. The mixing depends on the tim e at elevated te mp erature and probably the stiffness of the RAP binder. 1. Note added that the “as recovered” high - temperature grade of the RAP binder should be less than the planned field com paction temperature. Also, if the mix time at elevated tem perature was expected to be less than 2 h, plant mixing studies should be conducted to verify the degree of mi xing of the RAP and new binders. Workability 1. Differences in workability and compactability of WMA could only be measured at te mp eratures that are below WMA production tem peratures. 1. A workability test was not included in the revised prelim inary procedure. Coating is evaluated at the planned production temperature. Limits on the gyrations to 92 - percent relative density at the planned field compaction temperature and 54°F below the planned field compaction temperature were added to control compactability. Table 23. Summary of preliminary design procedure revisions.

that is similar to the way RAP and binders mix in HMA pro- duction and compaction at higher temperatures. The mixing depends on the time at elevated temperature and probably the stiffness of the RAP binder. In the preliminary mixture design and analysis procedure for WMA, it was envisioned that the amount of RAP that could be added to WMA would be limited by the planned WMA production temperature and the com- patibility of the new and RAP binders. Since the Phase I RAP study showed that substantial mixing of the RAP and new binders does occur at WMA process temperatures, the limi- tations on RAP usage included in the preliminary procedure were removed. The appendix describing compatibility testing for blended binders was also removed because the compatibil- ity tests completed during Phase I showed blends of RAP and new binders had compatibility values within the range of typ- ical unmodified binders. A requirement that the planned field compaction temperature for WMA incorporating RAP should exceed the temperature where the recovered RAP binder has a G*/sinδ value of 2.2 kPa was added. A note indicating that the effectiveness of the RAP in a mixture will depend on the total time that the mixture is exposed to elevated temperatures was added. This note also recommends that dynamic modulus tests should be conducted on samples of plant mix if the mix will be exposed to temperatures above the compaction temperature used in design for less than 2 h. The workability study found that it was possible to measure differences in the workability and compactability of WMA as compared to HMA. The differences, however, were only signif- icant at temperatures that are below typical WMA production temperatures. This indicated that it is not necessary to evaluate workability at the planned production temperature. The eval- uation of coating at the planned production temperature is suf- ficient. Workability and compactability can be evaluated by using the gyratory compactor to determine the gyrations to 92-percent relative density at the planned field compaction temperature and a second temperature that is approximately 54°F (30°C) lower than the planned field compaction temper- ature. This will permit an assessment of the effect of tempera- ture on the workability and compactability of the mixture. The preliminary mixture design and analysis procedure for WMA was modified accordingly. Evaluation of workability and com- pactability at the planned production temperature was elim- inated from the “design aggregate structure” and “design binder content” sections of the procedure. The measure of gyrations to 92-percent relative density was used to assess the compactability of the mixture. This is evaluated in the “design binder content” section of the procedure. A tentative limit of 35 percent of the design gyrations was included based on research reported by NCAT (21). To evaluate workability and compactability, two additional specimens at the optimum binder content are compacted at 54°F (30°C) below the planned field compaction temperature and the number of gyrations to 92-percent relative density is determined. A tenta- tive limit of 125 percent of the value at the planned field com- paction temperature was included based on the limited testing performed during the Phase I workability study. 3.3 Phase II Findings Phase II of NCHRP Project 09-43 was directed at evaluating and validating the revised preliminary WMA mixture design procedure. Phase II included three studies: (1) a laboratory mixture design study, (2) a field validation study, and (3) a fatigue study. Findings from each of these studies are presented in the following sections. Detailed analysis of the Phase II studies is included in Appendix E. 3.3.1 Laboratory Mixture Design Study In the laboratory mixture design study, the revised pre- liminary WMA mixture design procedure was successfully applied to several WMA mixtures. For the mixtures tested, the volumetric properties were not sensitive to the WMA process type or the WMA process temperature. Mixture compactabil- ity, moisture sensitivity, and rutting resistance were sensitive to the WMA process type and the WMA process temperature. Specific findings are presented and discussed below. Details of the analysis leading to these findings are presented in Section E7 of Appendix E. 3.3.1.1 Volumetric Properties For a specific combination of aggregates and binder, the paired difference statistical analysis presented in Section E7 of Appendix E found little difference in the volumetric prop- erties of properly designed WMA and HMA when the binder absorption for the HMA was 1.0 percent or less. These find- ings are shown graphically in Figures 18 through 21. Figure 18 shows the difference in binder absorption for WMA compared to HMA for the mixtures included in the mix- ture design study. The absorption in the WMA mixtures was on average 0.1 percent less than the absorption in the HMA mix- tures. This difference in binder absorption was statistically sig- nificant. It resulted in an average increase in the design VMA for the WMA of approximately 0.2 percent as shown in Figure 19, which was also statistically significant. However, it had little effect on the design binder content and the effective volume of binder (VBE) for the mixtures as shown in Figures 20 and 21. The average design binder content for the WMA was less than 0.1 percent lower than the average design binder content for the HMA while the average design VBE was 0.1 percent higher for WMA as compared to HMA. Neither of these was statistically significant. The design VBE for the Advera mixtures was signif- icantly higher than the design VBE for the HMA. 34

35 Figure 18. Average difference in binder absorption (WMA-HMA) from the mix design study (error bars are ± 95-percent one-sided confidence intervals). Figure 19. Average difference in design VMA (WMA-HMA) from the mix design study (error bars are ± 95-percent 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 % -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in D es ig n VM A, v ol %

36 Figure 20. Average difference in design binder content (WMA-HMA) from the mix design study (error bars are ± 95-percent one-sided confidence intervals). Figure 21. Average difference in design VBE (WMA-HMA) for the mix design study (error bars are ± 95-percent 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 D es ig n Bi nd er C on te nt , w t % -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in D es ig n VB E, v ol %

3.3.1.2 Compactability The revised preliminary procedure uses the gyratory com- pactor to evaluate the compactability of the mixture by mea- suring the number of gyrations required to reach 92-percent relative density at the planned field compaction temperature and again at 54°F (30°C) below the planned field compaction temperature. The paired difference statistical analysis pre- sented in Section E7 of Appendix E found that the com- pactability of WMA mixtures (as measured by the increase in the gyrations to 92-percent relative density when the com- paction temperature is decreased 54°F [30°C]) was sensitive to the process temperature, presence of RAP in the mixture, and the WMA process. Figure 22 shows the effects of temperature and RAP on the number of gyrations to reach 92-percent rela- tive density at the planned field compaction temperature. Fig- ure 22 shows that the compactability of the WMA at 260°F and 215°F (126°C and 102°C) was no different than that for HMA at 310°F (154°C) indicating that the WMA processes are effec- tive even with 25-percent RAP added. Figures 23 and 24 show the effects of temperature, WMA process, and RAP on the increase in the gyrations to reach 92-percent relative density when the compaction temperature is decreased 54°F (30°C). Figure 23 shows that the combination of low process temperature and RAP significantly decreases the compactability of WMA. The proposed limit in the revised preliminary mixture design procedure was 25 percent, and this was exceeded by the RAP mixtures at the lower compaction temperature of 215°F (102°C). Figure 24 shows that different WMA processes have different effects on compactability when the compaction temperature decreases. The Evotherm WMA with RAP was more sensitive to reductions in the compaction temperature compared to the other processes. It should be noted that because the Evotherm was blended in the binder at the terminal, the Evotherm concentration as a percentage of the total binder in the mixture was reduced for the RAP mix- tures, and this may have affected the compactability of the Evotherm WMA with RAP mixtures. 3.3.1.3 Moisture Sensitivity Moisture sensitivity is evaluated in both the revised pre- liminary WMA mixture design procedure and AASHTO R 35, Standard Practice for Superpave Volumetric Design for Hot Mix Asphalt (HMA), using AASHTO T 283, Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced Damage. The test is performed on samples that have been short-term conditioned for 2 h at the compaction temperature. The paired difference statistical analysis presented in Section E7 of Appendix E found the dry tensile strength, conditioned tensile strength, and tensile strength ratio to be significantly lower for WMA as compared to HMA. The analysis also found that the WMA process affected the tensile strength ratio. Fig- ure 25 shows the effect of the WMA process on dry tensile strength. The dry tensile strengths of the WMA mixtures averaged 25 psi (172 kPa) less than the strength of the HMA 37 Figure 22. Average difference in gyrations to 92-percent relative density at the compaction temperature (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals). -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 260 215 Compaction Temperature, oF A ve ra ge D iff er en ce in G yr at io ns to 9 2- Pe rc en t R el at iv e De ns ity Without RAP 25-Percent RAP All Mixtures

38 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 260 215 Compaction Temperature, oF Without RAP 25-Percent RAP All Mixtures A ve ra ge D iff er en ce in In cr ea se in G yr at io ns to 92 -P er ce nt R el at iv e De ns ity fo r a 5 4o F D ec re as e in C om pa ct io n Te m pe ra tu re , % Figure 23. Effect of temperature on the average difference in increase in gyrations to 92-percent relative density for a 54°F decrease in compaction temperature (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals). -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 Advera Evotherm Sasobit All WMA Process A ve ra ge D iff er en ce in In cr ea se in G yr at io ns to 92 -P er ce nt R el at iv e De ns ity fo r a 5 4o F D ec re as e in C om pa ct io n Te m pe ra tu re , % Without RAP 25-Percent RAP All Mixtures Figure 24. Effect of WMA process on the average difference in increase in gyrations to 92-percent relative density for a 54°F decrease in compaction temperature (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals).

39 Figure 25. Effect of WMA process on the average difference in dry tensile strength at 25°C (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals). -40.0 -35.0 -30.0 -25.0 -20.0 -15.0 -10.0 -5.0 0.0 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in D ry T en si le S tre ng th , p si mixtures. This reduction was consistent for all WMA processes and similar for the 260°F and 215°F (127°C and 102°C) com- paction temperatures. Figure 26 shows the effect of the WMA process on the tensile strength ratio. There was no reduction in the tensile strength ratio for the Evotherm process, which uses an anti-strip addi- tive, even though the tensile strength was reduced significantly due to the reduced aging of the WMA mixture. Table 24 summarizes the tensile strength ratios for all of the mixtures included in the study. Most of the WMA mixtures had tensile strength ratios below the AASHTO M 323 mini- mum of 80 percent. Only mixtures produced with Evotherm Figure 26. Effect of WMA process on the average difference in tensile strength ratio (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals). -50.0 -40.0 -30.0 -20.0 -10.0 0.0 10.0 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in T en si le S tre ng th R at io , %

consistently had tensile strength ratios exceeding 80 percent. Mixture 2, made with Virginia limestone and having a very high binder content, was highly resistant to moisture damage with tensile strength ratios exceeding 92 percent for HMA and all WMA processes. 3.3.1.4 Rutting Resistance Rutting resistance in the revised preliminary mixture design procedure is evaluated using the flow number, AASHTO TP 79, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Perfor- mance Tester (AMPT). The flow number has also been pro- posed for evaluating the rutting resistance of HMA in NCHRP Project 09-33 (6). Because the flow number is significantly different for different gyration levels, the paired difference statistical analysis used normalized differences defined by Equation 3. Normalized differences were used so that the mix- tures with the higher flow numbers would not dominate the analysis. where ND = normalized difference, NfWMA = flow number for the WMA mixture, and NfHMA = flow number for the HMA mixture. The paired difference statistical analysis presented in Sec- tion E7 of Appendix E for the flow number showed the flow numbers to be significantly lower for the WMA in compari- son to the HMA. Figure 27 shows the effect of WMA process on the flow number. The average difference was approxi- mately 40 percent and it was similar for all WMA processes. It was also similar at both compaction temperatures, as shown in Figure 28. The rutting resistance was similar for all WMA processes and both temperatures because the high-temperature binder grade for the non-RAP Advera and Evotherm mixtures was increased one grade level for the 215°F (102°C) com- paction temperature based on the findings from the Phase I ND Nf Nf Nf = −⎛ ⎝⎜ ⎞ ⎠⎟WMA HMAHMA 100 3( ) 40 Table 24. Summary of tensile strength ratios. TSR, % TSR, % Compaction Temp., ºF Compaction Temp., ºF TSR, % Compaction Temp., ºF TSR, % Compaction Temp., ºF 1 50 < 0.3 Yes 88.3 310 74.5 215 83.0 215 81.7 260 2 50 <0.3 No 92.3 310 95.2 260 93.9 260 94.9 215 3 75 <3 Yes 81.4 310 34.5 260 89.8 260 68.4 260 4 75 <3 No 91.8 310 66.7 215 83.6 260 71.5 215 5 100 <10 Yes 94.7 310 70.6 260 83.7 260 74.0 215 6 100 <10 No 69.8 310 17.9 215 81.5 215 57.8 260 HMA Advera Evotherm Sasobit Gyration Level Design Traffic MESAL RAPMixture -80.0 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 -10.0 0.0 Advera Evotherm Sasobit All WMA Mixture A ve ra ge D iff er en ce in F lo w N um be r, % Figure 27. Average normalized difference in flow number (WMA-HMA)/HMA for the mix design study (error bars are ± 95-percent one-sided confidence intervals).

binder grade study. Increasing the binder stiffness for these conditions increased the measured flow numbers, making the measured flow number difference smaller. In NCHRP Project 09-33, the following relationship be- tween the flow number and the allowable traffic to a rut depth of 0.5 in. (12.5 mm) was developed (6): where MESAL = estimated traffic to 12 mm rutting, million equiv- alent single axle loads (MESAL); and Fn = flow number per NCHRP 09-33 test conditions, cycles. Table 25 summarizes the allowable traffic from Equation 4 for all of the mixtures included in the study. Based on the HMA data, the rutting resistance of the two 50 gyration mixtures is sig- nificantly higher than required because both of these mixtures MESAL Fn = 0 873 6 222 4 . . ( ) used aggregates that far exceeded the angularity requirements in AASHTO M 323 for this traffic level. The rutting resistance of the HMA design for Mixture 4 is slightly less than the design traffic level, while the rutting resistance for Mixture 6 is only one-half of the design traffic level. The rutting resistance of the mixtures with RAP is significantly higher than the rutting resistance of the mixtures without RAP. Analysis of the data in Table 25 suggests that it will be difficult for WMA mixtures designed for 10 MESAL or greater to meet the flow number rutting resistance criteria developed in NCHRP Project 09-33. 3.3.2 Field Validation Study The field validation study addressed several parts of the revised preliminary mixture design procedure including (1) binder grade selection, (2) RAP, (3) short-term oven conditioning, (4) specimen fabrication, (5) coating and com- pactability, (6) moisture sensitivity, and (7) rutting resistance. Specific findings for each of these parts are presented and 41 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 -10.0 0.0 260 215 ALL WMA Compaction Temperature, oF A ve ra ge D iff er en ce in F lo w N um be r, % Figure 28. Effect of compaction temperature on the average normalized difference in flow number (WMA-HMA)/HMA for the mix design study (error bars are ± 95-percent one-sided confidence intervals). 1 50 < 0.3 Yes 6.1 310 2.4 215 2.0 215 3.5 260 2 50 <0.3 No 2.2 310 1.0 260 1.8 260 1.6 215 3 75 <3 Yes 13.5 310 4.7 260 5.9 260 9.5 260 4 75 <3 No 2.8 310 2.6 215 2.2 260 1.6 215 5 100 <10 Yes 12.3 310 3.5 260 5.0 260 4.1 215 6 100 <10 No 4.9 310 3.9 215 3.9 215 5.9 260 MESAL MESAL Compaction Temp., ºF Compaction Temp., ºF MESAL Compaction Temp., ºF MESAL Compaction Temp., ºF HMA Advera Evotherm Sasobit Gyration Level Design Traffic MESAL RAPMixture Table 25. Summary of NCHRP 09-33 rutting resistance from flow number testing.

discussed below. Details of the analysis leading to these find- ings are presented in Section E8 of Appendix E. 3.3.2.1 Binder Grade Selection Recovered binder grading and estimates of rutting using dynamic modulus test data from plant mixtures and the MEPDG rutting model were used to validate the high- temperature grade bumping table developed from the RTFOT experiment (1). Table 26 summarizes the continuous grades for the recovered binders from each of the validation mixtures. Table 26 includes the specified binder grade as well as the recovered grade. In all cases, the low and intermediate temper- ature properties for the WMA processes comply with the binder grade specified for the project. There are three cases where the high-temperature grade was lower than specified: Advera for the Yellowstone National Park project was 1.7°C lower, LEA for the NY Route 11 project was 3.5°C lower, and LEA for the Pennsylvania SR2006 project was 0.8°C lower. Table 27 summarizes the average difference in continuous grade temperatures for WMA as compared to HMA. The high- temperature grade changes are significantly less than estimated from the RTFOT experiment. From the RTFOT experiment, the estimated reduction in high-temperature grade for 50°F and 100°F (28°C and 56°C) reductions in production temper- ature for a typical asphalt binder having an aging index of 2.4 are 2.8°C and 5.6°C, respectively. For the field data—excluding Sasobit, which increases the high-temperature grade of the binder—an approximately 50°F (28°C) reduction in pro- duction temperature resulted in less than a 1°C decrease in high-temperature grade, while an approximately 100°F (56°C) reduction in production temperature resulted in approximately 42 Table 26. Summary of continuous grading of recovered binders. Table 27. Summary of average difference in continuous grade temperatures for WMA compared to HMA. Continuous Grade Temperature (°C)Project Process Production Temperature (°F) High Intermediate Low 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 Colorado I-70 Sasobit 250 63.9 15.1 29.9 Specified NA 58.0 16.0 34.0 Control 325 60.0 11.1 34.1 Advera 275 56.3 8.9 36.2 Yellowstone National Park Sasobit 275 60.7 10.1 35.6 Specified NA 64.0 22.0 28.0New York Route 11 LEA 210 60.5 14.0 31.1 Specified NA 64.0 25.0 22.0 Control 320 67.7 22.0 24.6PennsylvaniaSR2007 Evotherm 250 67.2 22.0 24.9 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 Pennsylvania SR2006 Sasobit 250 72.9 23.3 22.5 Specified NA 70.0 28.0 22.0Monroe, North Carolina Astec 275 71.5 23.7 23.9 Average Difference in Continuous Grade Temperature (°C)Process Number Average Difference in Production Temperature (°F) 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

a one-half grade decrease in the high-temperature grade for one LEA project. The low-temperature grade changes, on the other hand, are greater than estimated from the RTFOT exper- iment. From the RTFOT experiment, the estimated improve- ment in the low-temperature grade for 50°F and 100°F (28°C and 56°C) reductions in production temperature are 0.5°C and 1.0°C, respectively. For the field data—excluding Sasobit, which increases the low-temperature grade of the binder— 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 approxi- mately 100°F (56°C) reduction in production temperature resulted in a 2.9°C improvement in the low-temperature grade for one LEA project. Based on the recovered binder testing, it does not appear that the binder grade should be changed when using WMA as long as the production temperature is not decreased by more than 100°F (56°C). Rutting for the Colorado I-70, Yellowstone National Park, and New York Route 11 projects was predicted using the Excel spreadsheet, E*Rutting.xls, developed by Arizona State University for the dynamic modulus simple performance test (22). This spreadsheet rapidly performs asphalt layer rutting predictions using the calibrated rutting model contained in the MEPDG (1). The mixture dynamic modulus master curve is the required material property for this analysis. Master curves were developed for plant mixtures in accordance with AASHTO PP 61. The rutting estimates for these three proj- ects are shown in Figures 29, 30, and 31. These figures show the following: 1. The predicted rut depths for the control mixtures are rea- sonable for the design traffic levels. The design traffic level of the Colorado project was 10 million equivalent single axle loads (MESAL), and the estimated rut depth is 0.11 in. (2.8 mm). The design traffic level of the Yellowstone National Park and New York projects was 3 MESAL, and the estimated rut depth was 0.09 in. (2.3 mm) in both cases. 2. For the Colorado I-70 project, the predicted rutting for the Advera and Evotherm mixtures was slightly greater than the control while the predicted rutting for the Saso- bit mixture was slightly less than the control. The pre- dicted rutting for the Advera and Evotherm mixtures was only 0.13 in. (3.3 mm). 3. For the Yellowstone National Park project, the predicted rutting of the Sasobit and Advera mixtures was essentially the same as the control at the design traffic level. 4. For the New York project, the predicted rutting for the PG 64-28 LEA mixture was 0.11 in (2.8 mm), while the predicted rutting for the PG 70-22 LEA mixture is 0.05 in. (1.3 mm). The differences in estimated rutting resistance from field- mixed WMA do not support the binder grade bumping rec- ommendations developed from the RTFOT experiment. For production temperature decreases as large as 100°F (56°C), the estimated rutting for a mixture produced as WMA is only approximately 25 percent greater than that for the same mix- ture produced as HMA. 43 Figure 29. Predicted rutting for the Colorado I-70 project. Control Sasobit Advera Evotherm 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.1 1 10 100 Traffic, Million ESALs Pr ed ic te d Ru t D ep th , i n

44 Figure 30. Predicted rutting for the Yellowstone National Park project. Figure 31. Predicted rutting for the New York project. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.1 1 10 100 Traffic, Million ESALs Pr ed ic te d Ru t D ep th , i n Control Sasobit Advera 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.1 10 1001 Pr ed ic te d Ru t D ep th , i n Traffic, Million ESALs PG 64-28 PG 64-28 LEA PG 70-22 LEA

3.3.2.2 RAP Only one of the validation mixtures—the Monroe, North Carolina, mixture—included RAP. This mixture used PG 64-22 binder with 30-percent RAP to produce a mixture meeting the requirements for PG 70-22 binder. The mixture was pro- duced at 275°F using the Astec Double Barrel Green process. For this mixture, the mixing analysis—based on dynamic modulus testing of the plant mixture described earlier in the laboratory RAP study (see Section 3.1.4)—was conducted to validate that RAP and new binder mix in field-produced WMA. The results of this analysis are summarized in Table 28 and shown in Figure 32. The error bars in Figure 32 are 95-percent confidence intervals for the measured data and 95-percent prediction intervals for the Hirsch model predictions. Since the averages of the measured data fall within the prediction intervals for the Hirsch model, the plant-mixed modulus is not significantly different from the fully blended modulus, indicating that the mixing of the RAP and new binders is acceptable. 3.3.2.3 Short-Term Oven Conditioning For WMA and HMA, short-term oven conditioning of 2 h at the compaction temperature was determined by comparing properties of field-mixed, laboratory-compacted specimens with properties of laboratory-mixed, laboratory-compacted specimens for the mixtures from the Colorado I-70 project. The properties that were compared were maximum specific 45 Table 28. Measured and estimated fully blended dynamic modulus for the Monroe, North Carolina, mixture produced with the Astec Double Barrel Green process and 30-percent RAP. Figure 32. Comparison of measured and estimated fully blended dynamic modulus for the Monroe, North Carolina, mixture produced with the Astec Double Barrel Green process and 30-percent RAP. Temperature ( °F) Frequency (Hz ) Recovered Binder G* (psi) Hirsch Estimated E* (ksi) Measured E* (ksi) Ratio of Measured to Estimate d 39.2 10.0 15,681 2,145 2,344 1.09 39.2 1.0 7,339 1,755 1,785 1.02 39.2 0.1 2,839 1,281 1,216 0.95 68.0 10.0 2,014 1,123 1,083 0.96 68.0 1.0 596 663 626 0.94 68.0 0.1 145 328 316 0.96 104.0 10.0 100 270 201 0.74 104.0 1.0 19 114 80 0.70 104.0 0.1 3 51 38 0.74 1 10 100 1000 10000 39 .2 °F , 1 0 Hz 39 .2 °F , 1 H z 39 .2 °F , 0 .1 H z 68 °F , 1 0 Hz 68 °F , 1 H z 68 °F , 0 .1 H z 10 4° F, 1 0 Hz 10 4° F, 1 H z 10 4° F, 0 .1 H z Testing Condition D yn am ic M od ul us , k si Measured Estimated for Complete Mixing

gravity, indirect tensile strength, and dynamic modulus. To validate this short-term conditioning, maximum specific grav- ity and indirect tensile strength measurements were made on all of the validation sections. Figures 33 and 34 compare the maximum specific gravity and tensile strength data for all of the validation mixtures. The error bars shown in Figure 33 are the single operator d2s pre- cision from AASHTO T 209. These data show that the maxi- mum specific gravity of the laboratory and field mixtures is the same, indicating that the binder absorption is the same for the laboratory and field mixtures. The aggregate water absorption ranged from 0.5 percent for the Pennsylvania SR2007 mixtures to 2.5 percent for the Yellowstone National Park mixtures. Figure 34 shows differences in indirect tensile strength for the field mixtures minus the laboratory mixtures. The error bars for the average difference in this figure are 95-percent con- 46 Figure 33. Comparison of maximum specific gravity for all validation mixtures. Figure 34. Differences in indirect tensile strength between field mixes and laboratory mixes short-term conditioned for 2 h at the compaction temperature. 2.300 2.350 2.400 2.450 2.500 2.550 2.600 M ax im um S pe ci fic G ra vi ty Mixture/Process Field Mix Compaction Temp 2 h -25 -15 -5 5 15 25 35 45 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 o n ro e N C As te c Av e ra ge ID T St re ng th D iff er en ce s, p si Mixture/Process

fidence intervals for a paired t-test comparison. Since the error bars for the average difference do not capture zero, the tensile strength of the field-mixed specimens is statistically higher than the tensile strength of the laboratory-mixed specimens. This indicates that short-term conditioning of 2 h at the com- paction temperature provides less aging on average than the field mixtures. The findings from this analysis appear to have been biased by the data from the Pennsylvania SR2006 project. This project provided one-third of the data for the analysis, and the field mixtures for this project had consistently higher tensile strength than the laboratory-prepared mixtures. The average difference for all projects was 9 psi (48 kPa); not con- sidering the Pennsylvania SR2006 project, the average differ- ence was only 1.4 psi (9.8 kPa). Considering the bias from this project, the recommended short-term oven conditioning in the final WMA mixture design procedure was kept at 2 h at the compaction temperature. 3.3.2.4 Specimen Fabrication In the validation study, the WMA specimen-fabrication procedures were used to fabricate specimens for several WMA processes including Advera, Astec Double Barrel Green, Evotherm DAT, Gencor Ultrafoam, LEA, and Sasobit. Fig- ure 35 shows the difference in air voids at Ndesign between the WMA mixture and either the HMA job mix formula or the cor- responding HMA control mixture. The average difference over all projects is less than 0.03 percent. The error bars shown for the average are 95-percent confidence levels. Since the error bars capture zero, this indicates that the WMA mixtures are very similar to the HMA that they were based on. These find- ings confirm the findings of the mixture design study that the volumetric properties of properly designed WMA and HMA mixtures are very similar. The WMA mixture design procedure uses process-specific, specimen-fabrication procedures to simulate the WMA process. For plant foaming systems this requires the production of foamed asphalt in the laboratory. At the time NCHRP Project 09-43 was completed, the Wirtgen WLB-10 laboratory foaming machine was the only commercially available laboratory foam- ing equipment. An evaluation of the feasibility and practicality of designing foamed asphalt WMA mixtures in the laboratory using the Wirtgen WLB-10 was conducted at the University of Wisconsin-Madison. The evaluation was conducted for the Gencor WMA process from the Pennsylvania SR2006 project and for the Astec WMA process from the Monroe, North Carolina, project. The following describes the process used to fabricate foamed asphalt using this equipment. Operation of the Wirtgen WLB-10 foaming machine requires asphalt binder temperatures above 320°F (160°C), thus the mixing temperature of the foamed asphalt mixture is controlled by the temperature of the aggregates. It is assumed that the asphalt binder will quickly revert to the mixing tem- perature when it comes in contact with the aggregate. The mass of foamed asphalt that is required is calculated based on the weight of the aggregates. The aggregate and mixing bucket are placed under the foaming head, and the foamed asphalt is shot into the bucket as shown in Figure 36. The flow of foamed 47 Figure 35. Differences in air voids at Ndesign between WMA and corresponding HMA mixtures. -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 CO I-7 0 Ad v er a CO I-7 0 Ev o th er m CO I-7 0 Sa s o bi t YN P Ad v e ra YN P Sa s o bi t PA SR 20 07 E v o th er m PA SR 20 06 A dv er a PA SR 20 06 G e n c o r PA SR 20 06 L EA PA SR 20 06 S a s o bi t N C As te c Av er a ge A ir Vo id D iff er en ce (W MA -H MA ), % Project/Mixture

asphalt into the mixing bucket is metered using a flow con- troller. Based on a known flow rate, the user prescribes the time required to obtain the appropriate quantity of asphalt binder. The mixing bucket, with the foamed asphalt sitting on top of the aggregate, is immediately transferred to the laboratory mixer, mixed for 90 s, and transferred to a shallow pan. Illus- trations of the foamed asphalt mixture before and after the 90 s mixing time are provided in Figures 37 and 38, respec- tively. After mixing, the foamed mix is short-term aged at the compaction temperature for 2 h and compacted. The operation of the foaming machine presents some practical concerns for WMA laboratory mixture design. • The machine is intended for preparation of samples of foam- stabilized asphalt base course and cold in-place recycling. These applications require foamed asphalt with water con- tents above 10 percent by weight of the asphalt binder. In contrast, the water content for WMA applications ranges from 1.0 to 3.0 percent. To accommodate this difference, the existing flow controller was replaced with one that was smaller and more precise. Operation of the machine for WMA applications was possible; however, due to the low percentage of water required for WMA, the operation of the flow controller was approaching its minimum control tol- erance. The more precise flow controller was selected with the intent of delivering a more consistent foam at the water content used in the WMA field production. • The machine is designed to produce large quantities of material. This was an issue especially in trying to prepare samples for evaluation of the maximum specific gravity. The sample size to conduct the maximum specific gravity test for 9.5-mm mixtures is 1,000 g. A timer is used to control the amount of foamed asphalt shot into the bucket. Because of the flow rate of the foaming head, the machine provides the required 50 to 60 g of foamed asphalt in a fraction of a sec- ond. This amount of time is insufficient for the machine to produce a consistent asphalt foam, introducing potential reproducibility issues into the results. Use of this machine for small batches of aggregate is not recommended; instead, large batch sizes should be produced and then split for the various tests required. • At times the air line in the machine becomes clogged, so instead of foamed asphalt, an asphalt/water mix is produced. This problem has been encountered with both the neat PG 64-22 binders from the Pennsylvania and North Car- olina projects and Styrene-Butadiene-Styrene (SBS) mod- ified PG 76-22 binder used in another project. The valve that controls the flow of the air at the foaming head becomes clogged regularly, requiring disassembly and cleaning of the head. This issue occurred on three separate occasions while preparing samples for this project, each time resulting in a 48 Figure 36. Introducing foamed asphalt to mixing bucket. Figure 37. Foamed asphalt before mixing. Figure 38. Foamed asphalt mixture after 90 s mixing time.

delay of 2 to 3 h. The regularity with which this problem occurs suggests that redesign of the foaming head of the machine may be needed for continuous use as a mix design tool. The problem was more severe with the SBS modified binder. After production of approximately 20 samples, the machine clogged and had to be taken apart and fully cleaned before further use. • Finally, the preparation of the foamed mixes requires a sig- nificant amount of technician time and expertise. Each mix design evaluated for this project required three separate days for foamed mix production. 3.3.2.5 Coating and Compactability As required by the revised preliminary WMA mixture design procedure, coating and compactability were measured on all of the WMA mixtures in the field validation study. Coating was evaluated using AASHTO T 194, which counts the number of fully coated coarse aggregate particles in the mix- ture. Compactability was evaluated on the basis of the number of gyrations necessary to achieve 92-percent relative den- sity at the planned field compaction temperature and again at 54°F (30°C) below the planned field compaction temperature. Table 29 summarizes the results of the evaluation of coating and compactability. Coating was 100 percent for all of the mixtures that were mixed using a planetary mixer with a wire whip. The percent- age of coating was lower for the two mixtures mixed with a bucket mixer and particularly low for the North Carolina mix- ture, which had about 16 percent of its total binder content con- tributed by the RAP. All of the mixtures were prepared using 90 s of mixing. Apparently, the bucket mixer is less efficient and requires longer mixing times for equivalent coating. There were no reported coating issues for any of the field mixtures. The compactability data in Table 29 indicate that the Colorado I-70, Pennsylvania SR2007, and Monroe, North Carolina, mixtures were easy to compact. National Center for Asphalt Technology (NCAT) reported average gyrations to 92-percent relative density of 35 and 20 percent of Ndesign for dense graded HMA mixtures with coarse and fine grada- tions (21). At the compaction temperature, the gyrations to 92-percent relative density for these three projects ranged from 20 to 40 percent of Ndesign. For the other two projects, Yellowstone National Park and Pennsylvania SR2006, the mixtures were less compactable. The Yellowstone National Park mixtures were designed using the Hveem method and therefore had much lower binder content than they would have if they had been designed using AASHTO R 35 or the revised preliminary WMA mixture design procedure. The air void content at 75 gyrations for the HMA control mixture for this project was 6.8 percent. The Pennsylvania SR2006 con- trol HMA mixture could not be verified by the research team. At the optimum binder content from the approved mix design, the air void content at 75 gyrations was 6.2 percent, well above the design value of 4.0 percent. The effect of temperature on the compactability of the mix- tures is quantified by the percent increase in the number of gyrations to 92-percent relative density. The revised prelimi- nary WMA mixture design procedure limits this increase to 25 percent. All of the WMA mixtures included in the field val- idation study met this criterion. There were no reported work- ability issues for any of the field mixtures. 3.3.2.6 Moisture Sensitivity Moisture sensitivity was evaluated for all of the validation mixtures using AASHTO T 283. Specimens were compacted to a target air void content of 7.0 percent ± 0.5 percent using 49 Temperature, °F Gyrations to 92% of Gmm Project Process Mix Compact Coating Compact At Temp Compact At Temp –54°F Gyration Increase, % Advera 250 230 1001 15 18 20 Evotherm DAT 250 230 1001 20 23 15 Colorado I-70 Sasobit 250 230 1001 19 22 16 Advera 275 250 1001 69 — — Yellowstone National Park Sasobit 275 245 1001 > 75 — — PA SR2007 Evotherm DAT 250 230 1001 20 24 20 Advera 250 230 1001 47 48 2 Gencor Ultrafoam GX 250 230 81 2 35 38 9 LEA 210 195 1001 50 52 4 PA SR2006 and PA SR2012 Sasobit 250 230 1001 51 59 16 Monroe, North Carolina Astec Double Barrel Green 275 260 65 2 16 16 0 1 Mixed with Blakeslee planetary mixer with wire whip. 2 Mixed with bucket mixer. Table 29. Coating and compactability of field validation mixtures.

the binder content from the job mix formula or the binder content determined from the mix design verification. Per the preliminary WMA mixture design procedure the mixture was conditioned 2 h at the compaction temperature. Table 30 summarizes the results. Nine of the 11 WMA mixtures and 2 of the 4 HMA control mixtures have tensile strength ratios less than 80 percent. The effect of the WMA process on moisture sensitivity is mixture and process specific. For the Colorado I-70 project, the tensile strength ratio was reduced by all of the WMA processes. For this project, the Advera specimens failed during the condition- ing processes. The WMA processes had no effect on the tensile strength ratio for the Yellowstone National Park and Pennsyl- vania SR2007 projects. For the Pennsylvania SR2006 project, the Advera, Gencor, and Sasobit WMA processes reduced the tensile strength ratio, while the LEA process increased it. The LEA process includes an anti-strip that is added to the binder at the plant. For the plant foaming processes, the AASHTO T 283 results may have been adversely affected by the poorer coating obtained with the bucket mixer when simulating these processes. 3.3.2.7 Rutting Resistance Rutting resistance was evaluated for all of the field validation mixtures using the flow number test, AASHTO TP 79. Speci- mens were compacted to a target air void content of 7.0 per- cent ± 0.5 percent using the job mix formula binder content or the binder content determined from the mix design verifica- tion. All of the specimens were within this tolerance except for the Gencor mixture for the Pennsylvania SR2006 project, which was compacted to 4.5 percent. Per the preliminary WMA mixture design procedure, the mixture was conditioned 2 h at the compaction temperature. The flow number test was conducted at the 50-percent reliability high pavement temper- ature from LTPPBind 3.1 for the project location. As recom- mended in NCHRP 09-33, the flow number testing used unconfined specimens with repeated deviator stress of 87 psi (600 kPa) and contact deviator stress of 4.4 psi (30 kPa). Table 31 summarizes the results. The allowable traffic in Table 31 was calculated using the relationship between flow number and allowable traffic to an estimated rut depth of 0.5 in. (12.5 mm) developed in NCHRP Project 09-33 (see Equation 4) and discussed earlier in the mix- ture design study (see Section 3.3.1.4). Three of the mixtures do not meet the rutting resistance criteria: the Advera and LEA mixtures for the Pennsylvania SR2006 project and the Monroe, North Carolina, mixture. The North Carolina mixture has a very high design VMA of 17.6 percent, indicating that the rut- ting resistance of this mixture could be improved by decreas- ing the design VMA. In NCHRP Project 09-33, a maximum VMA of 17 percent has been recommended for 9.5-mm mix- tures to limit the effective binder content of the mixture and provide adequate rutting resistance. The rutting resistance of the Hveem designed mixtures from the Yellowstone project is very high. Also, the rutting resistance of the 50-gyration mix- tures from the Pennsylvania SR2007 project is high consider- ing the design traffic level. These mixtures were produced with highly angular manufactured sand and crushed stone. Table 32 compares the rutting resistance of the WMA mix- tures to that of the HMA control mixtures. The Gencor mix- ture from the Pennsylvania SR2006 project was not included in this analysis because the air void content of the specimens for this mixture was much lower than the air void content of all of the others. The rutting resistance for all WMA processes except Sasobit is less than the HMA control due to the lower 50 Project Process Production Temperature (°F) Compaction Temperature (°F) Dry Tensile Strength (psi) Conditioned Tensile Strength (psi) Tensile Strength Ratio (%) Control 280 260 88.3 80.4 91 Advera 250 230 80.3 0.0 0 Evotherm 250 230 71.9 32.4 45 Colorado I-70 Sasobit 250 230 82.8 58.8 71 Control 325 315 110.2 87.1 79 Advera 275 250 86.4 65.7 76 YellowstoneNational Park Sasobit 275 245 95.4 72.5 76 Control 320 300 102.3 92.1 90 Pennsylvania SR2007 Evotherm 250 230 86.0 79.1 92 Control 310 275 104.6 65.6 63 Advera 250 230 98.3 34.8 35 Gencor 250 230 97.3 42.1 43 LEA 210 195 103.7 86.1 83 Pennsylvania SR2006 Sasobit 250 230 97.1 51.8 53 Monroe, North Carolina Astec 275 260 164.0 127.7 78 Table 30. Summary of AASHTO T 283 results.

short-term conditioning temperatures. The rutting resis- tance decreases approximately 6 percent for every 10°F (5.5°C) reduction in compaction temperature. Sasobit increases the high-temperature stiffness of the binder, resulting in improved rutting resistance. 3.3.3 Feasibility of Using a Two-Step Aging Process for Performance Testing Criteria for evaluating rutting resistance using the flow number and other tests are generally based on mixtures that have been laboratory conditioned for 4 h at 275°F (135°C) in accordance with AASHTO R 30. Although it is generally accepted that this conditioning represents the binder stiffening that occurs during construction, it appears from the short- term conditioning study that this level of conditioning is more representative of the stiffness of the binder after some short period in service. The findings from the mix design study and the field validation study show that the rutting resistance of WMA mixtures that are conditioned 2 h at the compaction temperature, which represents the stiffness of WMA mixtures at the time of construction, generally fail criteria that are based on 4 h of conditioning at 275°F (135°C). To extend existing performance criteria to WMA, a two-step loose mix condition- ing procedure should be considered. This two-step procedure would include 2 h of conditioning at the compaction temper- ature to simulate the absorption and binder stiffening that occurs during construction, followed by aging at a representa- tive high in-service pavement temperature to simulate early stiffening during the service life of the pavement. The represen- tative in-service pavement temperature should be in the range of 120°F to 150°F (50°C to 65°C) depending on the project location and based on the 50-percent reliability high pave- ment temperature from LTPPBind 3.1. The conditioning time should be selected such that typical HMA mixtures reach approximately the same stiffness after the two-step condition- ing procedure as they reach after 4 h of conditioning at 275°F (135°C). This additional study was beyond the scope of NCHRP Project 09-43, but an analysis of the feasibility was performed using loose mix aging data collected under NCHRP Project 09-13 (23). NCHRP Project 09-13 included data that could be analyzed to investigate the effect of aging at in-service pavement temper- atures compared to HMA mixing and compaction tempera- tures. With the database reported for NCHRP Project 09-13, dry tensile strengths were collected on Superpave gyratory- compacted samples for five mixtures prepared for four loose mix aging conditions: • Unaged, • 2 h at 275°F (135°C), • 4 h at 275°F (135°C), and • 16 h at 140°F (60°C). 51 Project Design Traffic Level (MESAL) Process Production Temperature (°F) Compaction Temperature (°F) Test Temperature (°F) Flow Number NCHRP 09-33 Allowable Traffic (MESAL) Control 280 260 321 24.8 Advera 250 230 165 13.9 Evotherm 250 230 154 13.0 Colorado I-70 < 10 Sasobit 250 230 101 409 30.7 Control 325 315 687 48.2 Advera 275 250 459 33.9 Yellowstone National Park < 3 (estimated) Sasobit 275 245 106 1089 72.2 Control 320 300 124 10.8 Pennsylvania SR2007 < 0.3 Evotherm 250 230 126 93 8.4 Control 310 275 42 4.2 Advera 250 230 27 2.8 Gencor 250 230 1041 9.31 LEA 210 195 21 2.3 Pennsylvania SR2006 < 3 Sasobit 250 230 121 54 5.2 Monroe, North Carolina < 10 Astec 275 260 136 38 3.9 1 Specimens compacted to 4.5-percent air voids instead of 7.0 percent. Table 31. Summary of flow number and rutting resistance results. Process Number Average Difference in Compaction Temperature (°F) Average Difference in Allowable Traffic (%) Advera 3 46.7 35 Evotherm 2 50.0 35 LEA 1 80.0 45 Sasobit 3 48.3 +32 Table 32. Summary of average difference in allowable traffic WMA compared to HMA.

Dry tensile strengths were measured after two compacted sample conditioning periods: 0 h and 96 h at room tempera- ture. The database extracted from this study is presented in Section E9 of Appendix E. In analyzing this data, the data for the two compacted mix aging conditions were combined. Figure 39 shows plots of the ratio of the average strength of the conditioned specimens to the unaged specimens. From Figure 39, it appears that there is an error in the unaged data for the Maryland mixture because the ratios of the conditioned to unaged tensile strengths are always less than one, indicating that the mixture softens upon loose mix conditioning, which is not rational. Individual spec- imen air voids were not reported, but the text stated that the air void tolerance for specimen fabrication was 7.0 ± 1.0 percent. Because of the questionable unaged data for the Maryland mixture, the unaged data were eliminated from the analysis. Figure 40 shows the average tensile strength for the remaining three loose mix aging conditions: 2 h at 275°F (135°C), 4 h at 275°F (135°C), and 16 h at 140°F (60°C). The error bars in Fig- ure 40 are 95-percent confidence intervals based on the mea- sured data for each mixture. Figure 40 shows that the tensile strengths for 16 h at 140°F (60°C) are somewhat higher than the other aging conditions, indicating that this aging condition stiffens the mixture somewhat more than the shorter aging times at the higher temperatures. This was confirmed by a two- way analysis of variance that is presented in Section E9 of Appendix E. This analysis shows that it is possible to reach the level of binder stiffening caused by 4 h of loose mix oven condition- ing at 275°F (135°C) through loose mix oven conditioning at representative in-service temperatures. Since the suggested two-step procedure would include 2 h of conditioning at the compaction temperature to simulate the absorption and binder stiffening that occurs during construction, the in-service aging step will require less than 16 h of loose mix aging at the repre- sentative in-service temperature. 3.3.4 Fatigue Study One of the potential benefits of WMA mixtures is improved fatigue characteristics in comparison to HMA mixtures due to the reduced aging that occurs during plant mixing at the lower WMA process temperatures. The fatigue study was designed to evaluate the fatigue resistance of WMA in comparison to HMA. The study was conducted on the two mixtures summa- rized in Table 33. For each mixture, specimens were prepared as HMA and WMA using three processes: Advera, Evotherm G3, and Sasobit. The fatigue resistance of the eight mixtures was then characterized using continuum damage theory. Continuum damage theory is a new, powerful tool for characterizing the fatigue behavior of asphalt concrete in a thorough and rational way with relatively limited amounts of testing. Continuum damage theory has recently been applied to the fatigue response of asphalt concrete mixtures by several researchers (24, 25). Recently a practical approach for using continuum damage theory to quickly and accurately characterize the fatigue resis- tance of asphalt concrete mixtures was developed (26). In this 52 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2 h at 135°C 4 h at 135°C 16 h at 60°C Aging Condition A ge d/ Un ag ed T en si le S tre ng th R at io Nevada Alabama Colorado Maryland Texas Figure 39. Effect of loose mix aging on tensile strength (23).

53 0 50 100 150 200 250 300 D ry T en si le S tre ng th , p si Nevada Alabama Colorado Maryland Texas 2 h at 135°C 4 h at 135°C 16 h at 60°C Aging Condition Figure 40. Comparison of tensile strengths for loose mix aging conditions (23). 4 6 75 100 1.6 1.3 No No 9.5 mm 9.5 mm Coarse PA Gravel VA Diabase Fine PA Limestone PA Gravel VA Diabase Natural Sand RAP None None Sieve Size, mm 12.5 100 100 9.5 98 98 4.75 63 53 2.36 44 40 1.18 32 31 0.6 22 22 0.3 12 12 0.15 5 7 0.075 3.0 4.8 FAA 43.5 48.3 CAA 98/95 100/100 Flat & Elongated 7.4 7.6 Sand Equivalent 80.2 76.7 6.3 5.7 5.3 4.7 4.3 3.7 16.3 15.1 12.0 11.4 73.6 75.5 0.6 1.0 Effective Binder Content, vol % Voids Filled With Asphalt, % Dust to Effective Asphalt Ratio Gradation Binder Content, wt % Effective Binder Content, wt % Air Voids, vol % Voids in Mineral Aggregate, vol % Aggregate Sources Aggregate Properties Mix Number Design Gyrations Aggregate Water Absorption, % RAP NMAS Table 33. Design properties for fatigue study mixtures.

approach, cyclic direct tension fatigue tests are performed at two strain levels and temperatures. For this study, the cyclic fatigue tests were performed at 39.2°F and 68°F (4°C and 20°C) using a low strain level of approximately 150 µstrain, and a high strain level of approximately 250 µstrain (peak-to-peak). The resulting data were analyzed using the concept of reduced cycles (26). In this approach, the damage ratio, C (damaged modulus divided by the linear viscoelastic modulus), for each specimen tested is plotted as a function of reduced cycles, NR, at the reference temperature of 39.2°F (4°C) and the reference strain of 200 µstrain using Equation 5. where NR = reduced cycles; NR-ini = initial value of reduced cycles, prior to the selected loading period; N = actual loading cycles; f0 = reference frequency; f = actual test frequency; |E|LVE = initial (linear viscoelastic or LVE) dynamic mod- ulus under given conditions; |E|LVE/0 = reference initial (LVE) dynamic modulus (the LVE modulus at 4°C was used); α = continuum damage material constant; ε = applied strain level; ε0 = reference effective strain level (0.0002 suggested); and a(T/T0) = shift factor at test temperature T relative to refer- ence temperature T0. The values of the continuum damage material constant, α, and the shift factor, a(T/T0), are then varied until the C versus NR plots for the tests at different temperatures and strain lev- els converge into a single continuous function. Experience has shown that the damage ratio, C, follows the following function of NR: where C = damage ratio, K1 = cycles to 50% damage at the reference effective strain, and K2 = a model constant. C N K K = +( ) 1 1 6 1 2 R ( ) N N N f f E E R R-ini LVE LVE = + ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ 0 0 2   α ε ε0 2 0 1 5 ⎛⎝⎜ ⎞⎠⎟ ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ α a T T ( ) The values of α, a(T/T0),K1, and K2 are best determined using numerical optimization. Figure 41 presents typical results of the analysis. This figure shows the shifted fatigue test data, the reduced cycles damage relationship, and comparisons of the measured and predicted damage ratio. The reference temper- ature for the analysis was 39°F (4°C), and the reference strain was 200 µstrain. The detailed analysis is included Section E10 of Appendix E. Table 34 summarizes the parameters from the reduced cycles continuum damage analysis for all of the mixtures. The parameter K1 is the number of cycles at the reference tem- perature and strain level to reach a 50-percent reduction in the modulus of the mixture, the fatigue half-life. The WMA mix- ture fatigue half-lives range from approximately 70 percent to 170 percent of the fatigue half-life of the control HMA for the 100-gyration mixture and 70 percent to 92 percent for the 75-gyration mixture. This indicates that the fatigue resist- ance of WMA and HMA mixtures produced from the same aggregates and binders are essentially the same. Figures 42 and 43 provide further evidence of the similarity of the WMA and HMA fatigue resistance. These figures compare the fitted reduced cycles damage curves for the 100- and 75-gyration mixtures, respectively. The reduced cycles damage curves are very similar for the WMA processes and the HMA controls, providing further evidence that the fatigue performance of WMA and HMA mixtures produced from the same aggregates and binders will essentially be the same. 3.4 Draft AASHTO Standards Table 35 summarizes the major findings of the studies con- ducted during NCHRP Project 09-43 and the final disposition of each finding in the draft AASHTO standards that are the pri- mary products of NCHRP Project 09-43. Perhaps the most important finding from the laboratory studies was that the vol- umetric design of WMA mixtures does not differ substantially from that of HMA. Therefore, a separate mixture design pro- cedure for WMA is not needed. The mixture design portion of the revised preliminary procedure was reformatted to be in the form of an appendix to AASHTO R 35 highlighting special mixture design considerations and procedures for addressing WMA during mixture design. This document is included as Appendix A of this report. Appendix B is a commentary that provides supporting information for use in adoption and future revision of the mix design considerations and methods for WMA. Training materials for introducing the recommended WMA methods are included in Appendix C. The mixture analysis portion of the procedure was reformatted to be a stan- dard practice for measuring properties of WMA for perfor- mance analysis using the MEPDG (1). This document is 54

55 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 D am ag e Ra tio Reduced Cycles Low Strain 20 C High Strain 20 C Low Strain 4 C High Strain 4 C Fit 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Pr ed ic te d C Measured C Low Strain 20 C High Strain 20 C Low Strain 4 C High Strain 4 C Equality Figure 41. Continuum damage analysis for 100-gyration Advera WMA. Process Mixture Gyration Level Reference Temperature Reference Modulus (ksi) α Continuum Damage Shift Factor Fatigue Half-Life K 1 (10 7 ) K 2 Control 100 39 2,155 3. 5 2 ,098 6.07E+07 0.23 Advera 100 39 2,000 2.5 3,137 1.03E+08 0.25 Evotherm 100 39 1,941 3. 1 4 ,370 5.95E+07 0.25 Sasobit 100 39 2,135 3. 6 3,731 4.41E+07 0.25 Control 75 39 774 2.8 2,183 3.06E+08 0.21 Advera 75 39 773 2.0 5,000 2.75E+08 0.23 Evotherm 75 39 1,697 3. 6 5 ,183 2.21E+08 0.21 Sasobit 75 39 1,667 3. 8 3 ,573 2.83E+08 0.21 Table 34. Summary of continuum damage fatigue parameters.

56 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 D am ag e Ra tio Reduced Cycles Control Advera Evotherm Sasobit Figure 43. Comparison of continuum damage fatigue curves for the 75-gyration mix. 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 D am ag e Ra tio Reduced Cycles Control Advera Evotherm Sasobit Figure 42. Comparison of continuum damage fatigue curves for the 100-gyration mix.

57 Topic Major Finding Disposition Sample Reheating Sam ple reheating has a sim ilar effect on WM A and HMA. It further stiffens the binder of the mi xture. Not included directly in the products of NCHRP 09 - 43. Considered during analysis of co mp arisons of field and laboratory data. High - Temperature Binder Grade Selectio n The RTFOT study showed a major effect of temperature on high- temperature binder grade. This finding was not confirm ed by the recovered binder testing from the field validation mi xtures except for the LEA process where the mi xture temperature is approximately 10 0 °F (5 6 °C) lower th an typical HMA production temperatures. Draft Appendix to AASHTO R 35 does not include a recomm endation for high-tem perature binder grade bum ping. A note is included that high-te mp erature grade bu mp ing may be needed if the mixture rutting resistance is inadequate and cannot be im proved through reductions in VMA or increase in the filler content of the mixture. Low- Temperature Binder Grade Selectio n Both the RTFOT study and the recovered binder testing from the field validation mi xtures showed a minor im provem ent in low-tem perature grade for WM A co mp ared to HMA. Draft Appendix to AASHTO R 35 does not include recomm endations for changes in low-te mp erature binder grade selection. Low- and interm ediate- temperature binder grade improvements may be considered for RAP blending chart analysis. A table of recommended im provem ents as a function of production temperature was included. Short-Term Conditioning Short-ter m conditioning of 2 h at the planned field co mp action temperature reasonably reproduces the binder absorption and stiffening that occurs during WMA production. The Draft Appendix to AASHTO R 35 recommends 2 h of conditioning at the planned field compaction temperature for volumetric design, moisture sensitivity testing, and flow number testing. The draft standard practice for measuring properties of WMA for perform ance analysis using the MEPDG recomm ends 2 h of conditioning at the planned field co mp action te mp erature for dyna mi c m odulus testing for structural design. RAP RAP and new binders do mix at WMA process te mp eratures when conditioned 2 h at the com paction te mp erature. The Draft Appendix to AASHTO R 35 recommends limiting the high-temperature grade of the recovered RAP binder to the planned field compaction temperature of the WMA to ensure adequate mixing of the RAP and new binders. The optim um bi nder content of WM A mi xtures incorporating RAP should be deter min ed using the proposed RAP source, and the total binder content of the mixture is the sum of the binder content of the RAP and new binder added. Specimen- Fabrication Procedures All of the WMA process, including plant foam ing processes, could be reasonably reproduced in the laboratory for mixture design and perform ance evaluation. The Draft Appendix to AASHTO R 35 includes process-specific specim en-fabrication procedures for the maj or categories of WMA processes. Coating The type of mi xer used to prepare laboratory mixtures of WMA significantly affects the coating of coarse aggregate particles. The Draft Appendix to AASHTO R 35 includes a note that the mixing ti me s included in the appendix were developed using a m echanical planetary mi xer with a wire whip. Mixing time for bucket mixers should be determined by preparing HMA mi xtures using the viscosity-based mi xing te mp erature from AASHTO T 312, and evaluating coating. Workability Devices that measure the torque duri n g mi xi n g or the force to move a The Draft Appendix to AASHTO R 35 does not include an evaluation of workability. blade though loose mi x could not detect differences between HMA and WMA mi xtures at norm al WMA production temperatures. Differences could be detected at lower te mp eratures associated with co mp action. Compactability A primary benefit of WMA is im proved co mp actability at lower te mp eratures. The change in the gyrations to reach 92-percen t relativ e density when th e compaction temperature was reduced 54 °F (30 °C) provides a simple procedure to evaluate the co mp actability of WMA. The Draft Appendix to AASHTO R 35 includes evaluating the compactability of WMA mixtures by determ ining the number of gyrations to 92-percent relative density at the planned field compaction te mp erature and 54 °F (30 °C) below the planned field co mp action tem perature. A ma xi mu m increase in gyrations of 25 percent when the compaction te mp erature is reduced is recommended. Table 35. Major findings of NCHRP Project 09-43 studies. (continued on next page)

58 Volumetric Properties For mixtures with 1.0-percent binder absorption or less, the volumetric properties of properly designed WMA and HMA mixtures using the same aggregates and binders are very similar. The Draft Appendix to AASHTO R 35 states that volumetric properties of WMA for mixtures with 1.0 percent or less binder absorption will be the same as those for HMA. Evaluation of compactability, moisture sensitivity, and rutting resistance at the optimum binder content should be conducted using the WMA procedures. Moisture Sensitivity Moisture sensitivity, as measured by AASHTO T 283, will likely be different for WMA compared to HMA. Some WMA processes improve the resistance to moisture damage because they include anti-strip additives. Anti-strip dosage rates may be different for WMA compared to HMA. The Draft Appendix to AASHTO R 35 recommends that moisture sensitivity be evaluated and that appropriate anti-strip additives be used if needed. Rutting Resistance The rutting resistance of all WMA processes except Sasobit, as measured by the flow number test on mixtures conditioned for 2 h at the planned field compaction temperature, is lower compared to HMA. Current criteria for the flow number test are based on mixtures that have been short-term conditioned for 4 h at 275°F (135°C). This conditioning represents the aging that occurs during construction as well as some time in service. A two-step conditioning process that includes 2 h at the compaction temperature followed by further loose mix aging at a representative service temperature appears feasible. The Draft Appendix to AASHTO R 35 recommends performing flow number tests on laboratory-prepared mixtures that have been conditioned 2 h at the planned field compaction temperature to simulate the effect of construction. The flow number criteria included in the Draft Appendix to AASHTO R 35 were adjusted to be 56 percent of the values recommended in NCHRP Project 09-33. This adjustment was made to account for the fact that the standard aging of 4 h at 275°F (135°C) used with HMA accounts for the stiffening that occurs during construction as well as some time in service. Fatigue Resistance The fatigue resistance of WMA and HMA are similar for mixtures made from the same asphalt binders and aggregates and having the same volumetric properties. The draft standard practice for measuring properties of WMA for performance analysis using the MEPDG does not include a fatigue test since the calibrated fatigue relationship in the MEPDG should also apply to WMA mixtures. Topic Major Finding Disposition Table 35. (Continued). included as Appendix D of this report. The sections that follow describe the two draft AASHTO standards. 3.4.1 Draft Appendix to AASHTO R 35: Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) One of the major findings of the mixture design study conducted in Phase II of NCHRP Project 09-43 was that the volumetric properties of HMA and WMA mixtures having 1 percent or less binder absorption were very similar. It is, therefore, not necessary to have a separate design proce- dure for WMA because the major differences in the way WMA and HMA mixtures are designed are the specimen- fabrication procedures and the evaluation of coating and compactability. These differences can easily be included in AASHTO R 35 by adding an appendix addressing special considerations and procedures for design of WMA mixtures. The draft appendix titled, Special Mixture Design Consider- ations and Methods for Warm Mix Asphalt (WMA), addresses the following: 1. WMA Process Selection. The draft appendix includes a limited discussion of items to be considered when selecting one of the 20 or so WMA processes currently available. It advises that WMA process selection be done in consultation with the specifying agency and technical assistance person- nel from WMA process suppliers. This section alerts users that when selecting a WMA process, consideration should be given to a number of factors, including (1) available performance data, (2) the cost of any warm mix addi- tives, (3) planned mixing and compaction temperatures, (4) planned production rates, (5) plant capabilities, and (6) modifications required to successfully use the WMA process with available field and laboratory equipment.

2. Binder Grade Selection. The draft appendix explains to users that it is not necessary to change the grade of the binder in WMA from that normally used in HMA. If a mix- ture does not have adequate rutting resistance and volumet- ric properties and gradation cannot be altered, then the high-temperature grade of the binder may be increased to provide acceptable rutting performance. 3. RAP in WMA. The draft appendix explains that designing WMA mixtures with RAP is essentially the same as design- ing HMA mixtures with RAP. The only additional require- ment for WMA is that the high temperature of the “as recovered” RAP binder should be less than the planned field compaction temperature of the WMA mixture in order to ensure adequate mixing of the RAP and new binders. A table for low-temperature grade improvement of the virgin binder for RAP blending chart analysis is provided. 4. Process-Specific Specimen-Fabrication Procedures. The draft appendix includes process-specific specimen- fabrication procedures for several common WMA pro- cesses that were used in NCHRP Project 09-43, including (1) WMA additives added to the binder, (2) WMA additives added to the mixture during production, (3) WMA pro- duced using wet aggregate, and (4) plant foaming. 5. Evaluation of Coating and Compactability. The draft appendix describes the procedures for evaluating coat- ing and compactability of WMA mixtures. Both of these evaluations are made on the mixture at the design binder content. Coating is evaluated at the planned production temperature using AASHTO T 195. Compactability is eval- uated using the gyrations to 92-percent relative density at the planned field compaction temperature and 54°F (30°C) below the planned field compaction temperature. 6. Evaluation of Rutting Resistance. The draft appendix explains how to use the flow number test, AASHTO TP 79, to evaluate the rutting resistance of WMA. Recommended criteria as a function of traffic level are provided. These cri- teria are 56 percent of the values recommended in NCHRP Project 09-33 for HMA. This adjustment was made to account for the fact that the standard conditioning of 4 h at 275°F (135°C) used with HMA accounts for the binder stiffening that occurs during construction as well as some time in service while the standard conditioning of 2 h at the planned field compaction temperature for WMA only addresses the stiffening that occurs during construction. 7. Adjusting the Mixture to Meet Specification Require- ments. The draft appendix expands this section of AASHTO R 35 to address the following: (1) coating, (2) compactabil- ity, (3) moisture sensitivity, and (4) rutting resistance. 3.4.2 Proposed Standard Practice for Measuring Properties of WMA for Performance Analysis Using the Mechanistic-Empirical Pavement Design Guide Software The evaluation of the performance characteristics of WMA should not differ from the evaluation of the performance char- acteristics of HMA. NCHRP Project 09-33 included an evalua- tion of various performance tests and concluded that only moisture sensitivity and rutting resistance need to be evaluated as part of the mixture design process; fatigue and thermal cracking can be effectively controlled by controlling the effec- tive binder content of the mixture and the low-temperature binder grade, respectively (6). The WMA mixture design process discussed above includes testing to evaluate moisture sensitivity and rutting resistance. Predicted rutting and cracking from the MEPDG can be used to evaluate the performance of HMA and WMA mix- tures in specific pavement structures (1). In addition to in- place volumetric properties for the mixtures, the following engineering properties are needed for a Level 1 analysis using the MEPDG (1): • Dynamic modulus master curve, • Low-temperature creep compliance, and • Low-temperature strength. The dynamic modulus master curve is used in the stress- strain calculations as well as the rutting and fatigue-cracking models. The low-temperature creep compliance and strength properties are used in the thermal-cracking model. These models were field calibrated as part of the MEPDG develop- ment (1). The permanent deformation and fatigue tests on WMA conducted during NCHRP Project 09-43 indicate that permanent deformation and fatigue characteristics of WMA mixtures are similar to HMA mixtures; therefore, the cali- brated MEPDG models should provide a reasonable estimate of the expected performance of pavements constructed with WMA mixtures. The mixture analysis portion of the revised preliminary mixture design and analysis procedure was reformatted to be a standard practice for measuring properties of WMA for performance analysis using the MEPDG (1). The draft stan- dard practice describes how to prepare WMA performance test specimens and conduct dynamic modulus and low- temperature creep compliance and strength tests to obtain material properties for analysis using the MEPDG. This pro- posed standard practice is presented in Appendix D of this report. 59

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Mix Design Practices for Warm-Mix Asphalt Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 691: Mix Design Practices for Warm-Mix Asphalt explores a mix design method tailored to the unique material properties of warm mix asphalt technologies.

Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 50°F (28°C) or more cooler than typically used in the production of hot mix asphalt (HMA). The goal of WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures.

There are important environmental and health benefits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes.

Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving.

Appendices to NCHRP Report 691 include the following. Appendices A, B, and D are included in the printed and PDF version of the report. Appendices C and E are available only online.

• Appendix A: Draft Appendix to AASHTO R 35: Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

• Appendix B: Commentary to the Draft Appendix to AASHTO R 35

Appendix C: Training Materials for the Draft Appendix to AASHTO R 35

• Appendix D: Proposed Standard Practice for Measuring Properties of Warm Mix Asphalt (WMA) for Performance Analysis Using the Mechanistic-Empirical Pavement Design Guide Software

Appendix E: NCHRP Project 09-43 Experimental Plans, Results, and Analyses

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