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

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24 Immediate Delayed Reheat 100000 10000 Dynamic Modulus, MPa 1000 100 10 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Figure 6. Effect of sample reheating on the dynamic modulus of the St. Louis HMA control mixture. Immediate Delayed Reheat 100000 10000 Dynamic Modulus, MPa 1000 100 10 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Figure 7. Effect of sample reheating on the dynamic modulus of the St. Louis Aspha-min mixture.

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25 Immediate Delayed Reheat 100000 10000 Dynamic Modulus, MPa 1000 100 10 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Figure 8. Effect of sample reheating on the dynamic modulus of the St. Louis Evotherm mixture. Immediate Delayed Reheat 100000 10000 Dynamic Modulus, MPa 1000 100 10 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Reduced Frequency, Hz Figure 9. Effect of sample reheating on the dynamic modulus of the St. Louis Sasobit mixture.

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

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27 0.160 Rate of Change of RTFOT High Temperature Grade 0.140 With RTFOT Temperature, C/ C 0.120 o o 0.100 0.080 0.060 0.040 0.020 Rate of high-temperature grade change (oC/oC) = 0.085(AI-1)0.495 0.000 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Binder Aging Index Figure 11. Effect of short-term aging susceptibility on the rate of change of RTFOT high-temperature grade with RTFOT temperature. binder with an aging index of 2.4, the maximum allowable ation should be given to increasing the performance grade were temperature change is -54F (-30C). obtained by combining the temperature change from Equa- tion 2 with the typical mixing temperatures from Table 20. The -35.3 results rounded to the nearest 5.0F (2.8C) are presented in T1/2 grade = (2) ( AI - 1)0.495 Table 21 for various binder grades and levels of the aging index of the binder. If the planned plant mixing temperatures where are lower than those listed in Table 21, consideration should be given to increasing the high-temperature performance T1/2 grade = maximum temperature change for a 1/2 grade grade of the binder one grade level above that normally used change, C; and for HMA. AI = binder aging index at the performance grade The relatively small effect of RTFOT temperature on the temperature. low-temperature binder grade did not warrant recommended Plant mixing temperatures below which consideration changes in low-temperature binder grade selection for WMA. should be given to increasing the binder grade were obtained The low-temperature grade improvement, however, can be using Equation 2 and typical HMA production temperatures. significant when considering mixtures incorporating RAP. Table 20 summarizes typical plant mixing temperatures rec- When RAP blending charts are used, the low-temperature con- ommended by the Asphalt Paving Environmental Council (19) tinuous grade of the binder changes approximately 0.6C for based on the high-temperature performance grade of the every 10 percent of the total binder in the mixture replaced binder. WMA production temperatures below which consider- with RAP binder (20). Thus, improving the low temperature properties of the virgin binder in the mixture 0.6C by lower- ing the production temperature will allow 10 percent addi- Table 20. Typical HMA mixing tional RAP binder to be added to the mixture. The data temperatures (19). collected in the RTFOT study indicated that the low-temperature PG High- Recommended Mid-Point grade improvement was approximately 0.035C per C reduc- Temperature HMA Mixing Temperature tion in RTFOT temperature for PG XX-28 binders; 0.025C Grade (F) 52 270 per C reduction in RTFOT temperature for PG XX-22 58 285 binders; and 0.022C per C reduction in RTFOT temperature 64 292 for PG XX-16 binders. Using these values and typical HMA 67 300 70 302 production temperatures for various binder grades given in 76 308 Table 20, low-temperature grade improvements for RAP blend- 82 315 ing chart analyses were developed for some common grades

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28 Table 21. Minimum WMA production temperatures not requiring a high-temperature PG grade increase based on the RTFOT experiment. Aging Index PG High- 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Temperature Minimum WMA Mixing Temperature Not Requiring PG Grade 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 of binder. These improvements are summarized in Table 22. prior to compaction. Short-term oven conditioning simulates For a mixture using PG 64-22 virgin binder and a WMA the binder absorption and aging that occurs during construc- production temperature of 250F, the virgin binder low- tion. Comparisons of properties of plant- and laboratory- temperature continuous grade would be improved 0.6C to prepared mixtures for the Colorado I-70 WMA project were account for the lower WMA production temperature. 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 3.1.3 Short-Term Oven Conditioning Study and 4 h were investigated. This section presents the findings An important step in mixture design and analysis is short- from the short-term oven conditioning study. The detailed term oven conditioning of laboratory-prepared loose mix analysis is presented in Section E4 of Appendix E. Table 22. Anticipated improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures. Virgin Binder PG Grade 5828 5822 6422 6416 6722 Average HMA Production Temperature, F 285 285 292 292 300 Rate of Improvement of Virgin Binder Low- Temperature Grade per C Reduction in 0.035 0.025 0.025 0.012 0.025 Plant Temperature Improvement in Virgin Binder Low-Temperature WMA Production Temperature, F Continuous 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

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

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

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31 samples of the mixtures with dynamic moduli estimated using dependent, indicating that the new binder coats the virgin binder recovered from the mixtures. The measured dynamic aggregate and RAP, and then, during storage at elevated tem- moduli represented the "as mixed" condition; the estimated perature, the two binders continue to mix. The RAP used in moduli represented the "fully blended" condition. A measured- this study was very stiff, having a continuous performance to-estimated dynamic modulus ratio approaching one indi- grade of PG 105.8 2.3, and likely represents a worst-case sce- cated a high degree of mixing of the RAP and new binders. nario. Since the binders continued to mix during oven con- The findings of the laboratory mixing experiment are shown ditioning at the compaction temperature, the compaction in Figure 15. At conditioning times of 0.5 h and 1.0 h, there is temperature was probably the critical temperature in the little blending of the new and recycled binders. For all processes mixing study. It is likely that the minimum temperature that and temperatures, the ratio of the measured-to-estimated fully can be used is related to the viscosity of the RAP binder at that blended moduli range from about 0.35 to 0.55. At the 2 h con- temperature. The RAP binder used in this study had a viscos- ditioning time, the ratio of the measured-to-estimated fully ity of approximately 22,000 P (220 Pas) at the average com- blended moduli reach values approaching 1.0 for the Control, paction temperature of 221F (105C) used in this study, Advera, and Sasobit. The effect of temperature is also evident suggesting that a reasonable tentative requirement for RAP for these processes, with the higher conditioning temperature in WMA is that the RAP have a viscosity less than 22,000 P resulting in somewhat higher ratios. The ratio of the measured- (220 Pas) at the planned field compaction temperature. This to-estimated fully blended moduli for the Evotherm WMA is approximately equivalent to requiring the planned field remained low even at the 2 h conditioning time. This sug- compaction temperature to be greater than the temperature gests that either the particular form of Evotherm used in this where the as-recovered RAP binder meets the AASHTO M 320 study retards the mixing of the new and recycled binders requirement of G*/sin = 2.20 kPa. or that the extraction and recovery process stiffens the Evo- therm modified binder. Other findings from the mixture 3.1.5 Workability Study design study show that RAP and new binders do mix well for the Evotherm G3 process for 2 h of conditioning time. Mix- The workability study was conducted to identify a worka- tures with 25-percent RAP designed using Evotherm G3 had bility test to be used in place of viscosity-based mixing and the same optimum binder content as 25-percent RAP mixtures compaction temperatures to directly evaluate workability and designed as HMA. compactability of WMA mixtures. Three devices that measure The findings from the two experiments in the RAP study either the torque or the force required to move a blade through show that RAP and new binders mix at WMA process tem- the loose mixture were evaluated as potential workability peratures in a manner that is similar to the way that they mix devices: (1) UMass Workability Device, (2) Nyns Workabil- in HMA at higher temperatures. Clearly the mixing is time ity Device, and (3) University of New Hampshire Workability Control 255 Control 230 Advera 230 Advera 212 Evotherm 230 Evotherm 212 Sasobit 230 Sasobit 212 1.20 Average Ratio of Measured Modulus to Estimated Fully Blended Modulus 1.00 0.80 0.60 0.40 0.20 0.00 0.0 0.5 1.0 1.5 2.0 2.5 Conditioning Time, h Figure 15. Comparison of the ratio of measured to fully blended dynamic moduli.

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32 Control Advera Sasobit 450 400 350 300 Torque, in-lb 250 200 150 100 50 0 300 250 190 150 Temperature, F Figure 16. Effect of temperature and WMA additive on torque measured in the UMass workability device. Device. Additionally, various parameters that could be obtained duction of WMA. This is illustrated in Figure 16, which pre- during gyratory compaction, including the gyratory shear stress, sents torque measurements at different temperatures for the were evaluated as measures of compactability. This section, UMass Workability Device. The workability study further presents the findings from the workability study. Detailed found that the number of gyrations to reach 92-percent rela- analysis of the data from the workability study is presented in tive density in the gyratory compactor, shown in Figure 17, had Section E6 of Appendix E. similar sensitivity to temperature and WMA additives. The primary finding from the workability study for the three The workability study demonstrated that it is possible to workability devices was that these devices could only discrim- measure differences in the workability and compactability of inate between HMA and WMA at temperatures that are much WMA as compared to HMA. The differences, however, are lower than the temperatures normally associated with the pro- only significant at temperatures that are below typical WMA Control Advera Sasobit 45 40 35 30 Gyrations 25 20 15 10 5 0 300 250 190 Temperature, F Figure 17. Effect of temperature and WMA additive on gyrations to 92-percent relative density.