Mix Design Practices for Warm-Mix Asphalt (2011)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

83 A P P E N D I X B Commentary to the Draft Appendix to AASHTO R 35

84 B1. Introduction One of the products of National Cooperative Highway Research Program (NCHRP) Project 09-43 was a draft appen- dix to AASHTO R 35 titled, Special Mixture Design Consider- ations and Methods for Warm Mix Asphalt (WMA). The draft appendix addresses the following aspects of WMA mixture design: • Equipment for Designing WMA; • WMA Process Selection; • Binder Grade Selection; • RAP in WMA; • Process Specific Specimen Fabrication Procedures; • Evaluation of Coating, Compactability, Moisture Sensitiv- ity, and Rutting Resistance; • Adjusting the Mixture to Meet Specification Requirements; and • Additional Reporting Requirements for WMA. This commentary to the draft appendix provides support- ing information taken from the NCHRP Project 09-43 Final Report for each of the major sections of the draft appendix. It is intended for those who are responsible for the adoption and future revision of the draft appendix. Each section of the commentary has the following structure: General Comments Description of general contents of the section and the underlying philosophy. Basis for Critical Content Provides engineering justification for the critical content contained in the section. It includes a summary of the analyses and findings from NCHRP Project 09-43 that support the critical content. Need for Further Research Describes additional research that is needed to improve the section. B2. Section 1. Purpose General Comments This section describes the purpose of the Appendix. Basis for Critical Content There is no critical content in this section. Need for Further Research There is no need for additional research. B3. Section 2. Summary General Comments This section lists the major topic covered by the appendix. Basis for Critical Content There is no critical content in this section. Need for Further Research There is no need for additional research. B4. Section 3. Additional Laboratory Equipment General Comments This section describes the additional equipment needed for designing WMA mixtures in the laboratory. Since coating is used in lieu of viscosity-based mixing temperatures, a mechan- ical mixer is required. For WMA processes where the additive is blended in the binder, a mechanical stirrer is needed. For designing mixtures for plant foaming processes, a laboratory foamed asphalt plant that can produce foamed asphalt at the moisture content used by the field equipment is also needed. Basis for Critical Content The design of WMA mixtures includes an evaluation of coat- ing using AASHTO T 195. To standardize the mixing process, a mechanical mixer is required. During NCHRP Project 09-43, it was observed that planetary mixers and bucket mixers do not have the same mixing efficiency. The mixing times in the spec- imen fabrication procedures in Section 7 of the draft appendix were developed in NCHRP Project 09-43 using a planetary mixer. Mixing times for bucket mixers will likely be longer. NCHRP Project 09-43 demonstrated that it is feasible to perform foamed asphalt WMA mixture designs in the labo- ratory. In NCHRP Project 09-43, a modified Wirtgen WLB-10 laboratory foaming plant was used to simulate the Gencor Ultrafoam GX process using 1.25 percent water by weight of binder and the Astec Double Barrel Green process using 2.0 percent water by weight of binder. The modification that was required was to replace the flow controller with a smaller, more precise flow controller to accommodate the water con- tents used in WMA mixtures. Need for Further Research Bucket mixers are significantly less expensive and likely more readily available in mix design laboratories than plane- tary mixers. Additional research should be conducted to develop appropriate mixing times for bucket mixers.

85 Manufacturers of plant foaming equipment should be en- couraged to develop laboratory foaming equipment that can be used to design foamed asphalt WMA mixtures in the lab- oratory. The laboratory foaming equipment that was used in NCHRP Project 09-43 was designed for preparing laboratory samples of foamed stabilized bases, not WMA. Although it is feasible to design WMA mixtures for plant foaming processes using this equipment, devices specifically designed to repli- cate the WMA foaming process and produce the smaller quantities of foamed asphalt used in mix design batches with- out extensive cleaning are needed to make the design process efficient. B5. Section 4. WMA Process Selection General Comments This section lists factors to be considered when selecting a WMA process. Basis for Critical Content There is no critical content in this section. Need for Further Research There is no need for additional research. B6. Section 5. Binder Grade Selection General Comments The same grade of binder should be used with WMA and HMA. For WMA processes with very low production temper- atures it may be necessary to increase the high-temperature performance grade of the binder to meet rutting resistance requirements. Basis for Critical Content Performance grading data for binders recovered from sev- eral WMA projects sampled during NCHRP Project 09-43 showed only small differences in the grade of the binder for WMA and HMA sections. Table 1 summarizes the recovered binder data from NCHRP Project 09-43. Table 2 presents average differences in the continuous grade between HMA and WMA. Excluding Sasobit, which increases the high- temperature grade of the binder, an approximately 50°F (28°C) reduction in production temperature resulted in less than a 1°C decrease in the high-temperature grade, while an approximately 100°F (56°C) reduction in production temper- ature resulted in approximately a one-half grade decrease for one low energy asphalt (LEA) project. For the low-temperature grade, again excluding Sasobit, an approximately 50°F (28°C) reduction in production temperature resulted in an average improvement in the low-temperature grade of binder of Continuous Grade Temperatur e ( °C) Project Process Production Temperature ( °F) High Intermediate Low Specified NA 58.0 19.0 Control 280 59.3 14.2 30.6 28.0 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 28.0 New 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 24.6 Pennsylvania SR2007 Evotherm 250 67.2 22.0 24.9 Specified NA 64.0 25.0 22.0 Control 310 66.6 24.1 22.5 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 22.0 Monroe, North Carolina Astec 275 71.5 23.7 23.9 Table 1. Summary of continuous grading of recovered binders.

86 1.5°C, while an approximately 100°F (56°C) reduction in production temperature resulted in 2.9°C improvement for one LEA project. Need for Further Research Additional recovered binder grade data should be collected and analyzed to verify the conclusion from NCHRP Project 09-43 that binder grade changes are not necessary for WMA. B7. Section 6. RAP in WMA General Comments Research completed in NCHRP Project 09-43 found that recycled asphalt pavement (RAP) binders and new binders do mix at WMA process temperatures. Therefore, it is appropriate to design WMA mixtures containing RAP in the same man- ner as HMA, accounting for the contribution of the RAP binder to the total binder content of the mixture. From the research completed in NCHRP Project 09-43, the RAP and new binders continue to mix while the mix is held at elevated temperature. To ensure that adequate mixing of RAP and new binders does occur, a limit is placed on the maximum stiffness of RAP binders for WMA. That limit is based on the planned field compaction temperature of the mixture since this tem- perature will govern the temperature of the mix during stor- age and transport. The limit is the RAP binder should have a high-temperature grade that is less than the planned field compaction temperature for the WMA. RAP binders typi- cally range from PG 82 to PG 94 resulting in corresponding minimum field compaction temperatures ranging from 180°F to 200°F (82°C to 94°C). Binders from WMA mixtures have improved low- temperature properties due to the lower amount of aging that occurs during production. Although the improvement in low- temperature properties is not large enough to warrant changing the low-temperature grade, it is large enough to affect the amount of RAP that can be added to a mixture when blending chart analyses are used. Basis for Critical Content NCHRP Project 09-43 included a laboratory mixing study where the WMA and HMA mixtures incorporating RAP were prepared in the laboratory and stored for various lengths of time at the compaction temperature. The degree of mixing of the RAP and new binders was evaluated by com- paring dynamic moduli measured on mixture samples with the dynamic moduli estimated using the properties of the binder recovered from the mixture samples. The dynamic modulus test is very sensitive to the stiffness of the binder in the mixture, and adding RAP will increase the dynamic modulus significantly when the RAP is properly mixed with the new materials. The measured dynamic modulus values represent the as-mixed condition. The dynamic modulus for the fully blended condition was estimated using the Hirsch model from the shear modulus of binder recovered from the dynamic modulus specimens. If the measured and estimated dynamic moduli are the same, there is good mixing of the RAP and new binders. The findings of the laboratory mixing experiment are shown in Figure 1. At conditioning times of 0.5 and 1.0 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 conditioning time, the ratio of the measured to estimated fully blended moduli reach values approaching 1.0 for the Control HMA, Advera WMA, and Sasobit WMA. The effect of temperature is also evident for these processes, with the higher conditioning temperature resulting in somewhat improved blending. The 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 Evotherm modified binder. Further evidence of the mixing of new and RAP binders at WMA process temperatures was obtained from a mixture 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 Table 2. Summary of average difference in continuous grade temperatures for WMA compared to HMA.

87 design study completed in NCHRP Project 09-43. In this study, six mixtures were designed as HMA and as WMA and various volumetric and engineering properties were compared. Three of the mixtures included RAP. Table 3 summarizes the optimum binder content for the three mixtures containing RAP. As shown, the optimum binder content is the same or lower for the WMA compared to the HMA, further supporting the conclusion that RAP and new binders do mix at WMA process temperatures. In this study, the Evotherm mixtures do not have higher optimum binder contents than the HMA and the other processes suggesting that the Evotherm does mix and that the differences shown in Figure 1 for this process are due to the extraction and recovery process used in the mixing study. NCHRP Project 09-43 included a binder grade study where the Rolling Thin Film Oven Test (RTFOT) was used to simu- late the effect on binder properties of changes in production temperatures. Figure 2 shows that there appears to be a weak relationship between the rate of change in low-temperature grade with RTFOT temperature and the low-temperature grade of the binder. Binders with better low-temperature prop- erties tend to show more improvement in low-temperature properties when the RTFOT temperature is decreased. The relatively small effect of RTFOT temperature on the low- temperature binder grade does not warrant recommended changes in low-temperature binder grade selection for WMA. For the binders tested, decreasing the production tempera- ture by 95°F (53°C) only improved the low-temperature grade of the binder by 1°C to 2°C which is only 1⁄6 to 1⁄3 of a grade level. The low-temperature grade improvement, however, can be significant when considering mixtures incorporating recycled asphalt pavement (RAP). When RAP blending charts are used, the low-temperature continuous grade of the binder changes approximately 0.6°C for every 10 percent of the total binder in the mixture replaced with RAP binder. Thus, improving the low-temperature properties of the virgin binder in the mix- ture 0.6°C by lowering the production temperature will allow 10 percent additional RAP binder to be added to the mixture. Using the relationship shown in Figure 2, for the middle of the low-temperature binder grade temperature range, recom- mended improvements in virgin binder low-temperature con- tinuous grade for RAP blending chart analysis can be made as a function of WMA production temperature for mixtures 0.00 0.20 0.40 0.60 0.80 1.00 1.20 0.0 0.5 1.0 1.5 2.0 2.5 Conditioning Time, Hours A ve ra ge R at io o f M ea su re d M od ul us to Es tim at ed F ul ly B le nd ed M od ul us Control 255 Control 230 Advera 230 Advera 212 Evotherm 230 Evotherm 212 Sasobit 230 Sasobit 212 Figure 1. Comparison of the ratio of measured to fully blended dynamic moduli. Mixture HMA AdveraWMA Evotherm WMA Sasobit WMA 50 gyrations, 25 % RAP 6.4 6.5 6.1 6.3 75 gyrations, 25% RAP 5.5 5.3 5.2 5.3 100 gyrations, 25 % RAP 6.0 6.1 5.8 6.2 Table 3. Optimum binder contents for RAP mixtures from the NCHRP 09-43 mixture design study.

88 incorporating PG XX-16, PG XX-22, and PG XX-28. These recommended improvements are summarized in Table 4 for some common binder grades. 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. Need for Further Research Plant mixing studies similar to the laboratory mixing study are needed to confirm that RAP and new binders mix at WMA process temperatures for field conditions. NCHRP Project 09-43 included one field project that used 30-percent RAP, the Astec Double Barrel Green WMA process, and field mixing and compaction temperatures of 275°F and 260°F (135°C and 127°C). For this project, the mixing analysis showed good mixing of the RAP and new binders. Additional studies of this type are needed. Recovered binder tests on WMA with RAP should be conducted to verify the suggested improvements in low- temperature properties for blending chart analyses. B8. Section 7. Process-Specific Specimen Fabrication Procedures General Comments This section describes specimen fabrication procedures for several common types of WMA processes. Basis for Critical Content The specimen fabrication procedures were designed to rea- sonably reproduce the WMA process. Procedures are pro- vided for: • WMA additives that are added to the binder. • WMA additives that are added to the mixture. • WMA processes incorporating wet fine aggregate and se- quential mixing. • Plant foaming processes. These procedures were developed from guidance provided by WMA process developers and verified through laboratory testing in NCHRP Project 09-33. Need for Further Research Developers of new WMA processes should be encouraged to prepare specimen fabrication procedures in a similar for- mat so that they can be added in the future to the appendix to AASHTO R 35. B9. Section 8. WMA Mixture Evaluations General Comments This section described four evaluations of the WMA mix- ture at the design binder content: • Coating, • Compactability, y = -0.0019x - 0.024 R2 = 0.52 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 -34.0 -28.0 -22.0 -16.0 Low Temperature Grade, oC R at e of C ha ng e of L ow T em pe ra tu re G ra de W ith R TF O T Te m pe ra tu re , o C/ o C Figure 2. Effect of low-temperature binder grade on the rate of change of low-temperature grade with RTFOT temperature.

89 • Moisture sensitivity, and • Rutting resistance. The coating evaluation is used in lieu of the viscosity-based mixing temperature used for HMA. Coating is evaluated at the design binder content using AASHTO T 195, which measures the percentage of fully coated coarse aggregate particles. The compactability evaluation is used in lieu of the viscosity- based compaction temperature used for HMA. Compactability is evaluated by compacting specimens to Ndesign at the planned field compaction temperature and again at 54°F (30°C) below the planned field temperature. The number of gyrations to reach 92-percent relative density is then calculated from the height data. The ratio of the gyrations to 92-percent rela- tive density at the lower temperature to the higher temper- ature should be less than 1.25. Moisture sensitivity is evaluated using AASHTO T 283, the same as HMA. The criteria for AASHTO T 283 are the same as that for HMA. Finally, rutting resistance is evaluated using the flow number test in AASHTO TP 79. The test is conducted at the 50-percent reliability high pavement temperature from LTPPBind 3.1 for the project location. An unconfined flow number test with a repeated deviatoric stress of 87 psi (600 kPa) and a contact deviatoric stress of 4.4 psi (30 kPa) is used. Minimum flow numbers as a function of traffic level are provided. Basis for Critical Content Coating is one way to evaluate planned WMA produc- tion temperatures that is relevant to all WMA processes. In NCHRP Project 09-43, coating was evaluated on a number of HMA and WMA mixtures using AASHTO T 195. When a planetary mixer was used, coating was always found to be nearly 100 percent for both WMA and HMA. When a bucket mixer was used with a smaller number of WMA mixes, the coating was much lower. The mixing times and the recom- mended criterion of 95 percent were based on the planetary mixer data. The methodology for the compactability evaluation resulted from a workability study conducted in NCHRP Project 09-43. The workability study evaluated the feasibility of using various workability devices and the gyratory compactor to measure WMA workability during the mixture design process. The workability study demonstrated that it is possible to measure differences in the workability and compactability of WMA compared to HMA. The differences, however, were only sig- nificant at temperatures that are below typical WMA discharge Virgin Binder PG Grade 58–28 58–22 64–22 64–16 67–22 Average HMA Production Temperature, oF 285 285 292 292 300 Rate of Improvement of Virgin Binder Low- Temperature Grade per oC Reduction in Plant Temperature 0.035 0.025 0.025 0.012 0.025 WMA Production Temperature, oF Recommended Improvement in Virgin Binder Low- Temperature Continuous Grade for RAP Blending Chart Analysis, oC 300 NA NA NA NA 0.0 295 NA NA NA NA 0.1 290 NA NA 0.0 0.0 0.1 285 0.0 0.0 0.1 0.0 0.2 280 0.1 0.1 0.2 0.1 0.3 275 0.2 0.1 0.2 0.1 0.3 270 0.3 0.2 0.3 0.1 0.4 265 0.4 0.3 0.4 0.2 0.5 260 0.5 0.3 0.4 0.2 0.6 255 0.6 0.4 0.5 0.2 0.6 250 0.7 0.5 0.6 0.3 0.7 245 0.8 0.6 0.7 0.3 0.8 240 0.9 0.6 0.7 0.3 0.8 235 1.0 0.7 0.8 0.4 0.9 230 1.1 0.8 0.9 0.4 1.0 225 1.2 0.8 0.9 0.4 1.0 220 1.3 0.9 1.0 0.5 1.1 215 1.4 1.0 1.1 0.5 1.2 210 1.5 1.0 1.1 0.5 1.3 Table 4. Recommended improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures.

90 0 5 10 15 20 25 30 35 40 45 300 250 190 Temperature, F G yr at io ns Control Advera Sasobit Figure 4. Effect of temperature and WMA additive on gyrations to 92-percent relative density. temperatures. Figures 3 and 4 show the effect of WMA process and temperature on workability and compactability. Since the workability devices were not able to discriminate more precisely than compaction data obtained from a stan- dard Superpave gyratory compactor, the method for evalu- ating the temperature sensitivity of the compactability of WMA was developed for assessing WMA workability and com- pactability. It involves determining the number of gyrations to 8-percent air voids at the planned field compaction temper- ature and a second temperature that is approximately 54°F (30°C) lower than the planned field compaction temperature. A tentative limit allowing a 25-percent increase in the number of gyrations when the temperature is decreased was developed. This limit was investigated using data from nine WMA field 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 3. Effect of temperature and WMA additive on torque measured in the UMass workability device.

91 mixture projects sampled in NCHRP 09-43. The increase in gyrations for the WMA processes ranged from 0 to 20 percent. Workability and compactability were not reported to be a problem on any of the projects. Moisture sensitivity is evaluated using AASHTO T 283. Tests conducted during NCHRP Project 09-43 showed that the moisture sensitivity will likely be different for WMA and HMA mixtures designed using the same aggregates and binder. WMA processes that included anti-strip additives improved the tensile strength ratio of some of the mixtures included in the NCHRP Project 09-43 testing and analysis. Of the nine WMA mixtures that used a WMA process that included an anti-strip additive, the tensile strength ratio remained the same or improved in 67 percent of the mixtures. For WMA mixtures produced using processes that do not include anti-strip addi- tives, the tensile strength ratio never improved and decreased in 79 percent of the mixtures. Rutting resistance is evaluated using the flow number test. This test has also been recommended to evaluate rutting resistance for HMA mixtures in NCHRP Project 09-33. The test is conducted on specimens that have been short-term conditioned for 2 h at the compaction temperature to simu- late the binder absorption and stiffening that occurs during construction. Because lower short-term conditioning tem- peratures are used for WMA compared to HMA mixtures, binder aging in WMA mixtures is less, resulting in lower flow numbers for WMA mixtures produced with the same aggre- gates and binder. Table 5 summarizes the difference in flow numbers obtained for the field validation mixtures. The Saso- bit process increases the rutting resistance because it increases the high-temperature grade of the binder. Current criteria for the flow number and other rutting tests for HMA are based on 4 h of short-term conditioning at 275°F (135°C). The short-term conditioning study completed in NCHRP Project 09-43 shows that this level of conditioning represents the stiffening that occurs during construction as well as some time in service. Since it is inappropriate to con- dition WMA mixtures at temperatures exceeding their pro- duction temperature, the criteria for evaluating the rutting resistance of WMA mixtures were reduced compared to those currently recommended for HMA conditioned for 4 h at 275°F (135°C). Need for Further Research Bucket mixers are significantly less expensive and likely more readily available in mix design laboratories. Additional research should be conducted to develop appropriate mixing times for bucket mixers. As the draft appendix to AASHTO R 35 is used on a trial basis, data on coating and compactability should be compiled to aid in future revision of the criteria for these two evaluations. Additional research concerning the moisture sensitivity of WMA is needed and has been initiated by NCHRP in NCHRP Project 09-49, “Performance of WMA Technologies: Stage I— Moisture Susceptibility.” Additional research is needed on the development of a short-term conditioning procedure for specimens used for the evaluation of moisture sensitivity and rutting resistance that is equally applicable to both WMA and HMA. Research completed in NCHRP Project 09-43 concluded that 2 h of oven conditioning at the compaction temperature reasonably reproduces the binder absorption and stiffening that occurs during construction for both WMA and HMA mixtures. WMA mixtures that are conditioned 2 h at the compaction temperature have binder that is less stiff than similarly con- ditioned HMA mixtures because of the lower conditioning temperature. Current criteria for evaluating moisture sensi- tivity and rutting resistance are based on mixtures that have been aged to a greater degree. The conditioning originally specified in AASHTO T 283 for moisture sensitivity testing was 16 h at 140°F (60°C). Additionally, most rutting criteria are based on 4 h of conditioning at 275°F (135°C). In NCHRP Project 09-13, mixtures were conditioned for 2 h at 275°F (135°C), 4 h at 275°F (135°C), and 16 h at 140°F (60°C). Analysis of this data in NCHRP Project 09-43 concluded that 16 h at 140°F (60°C) resulted in somewhat more aging than 4 h at 275°F (135°C). The difference in aging between 2 and 4 h at 275°F (135°C) was not statistically significant. To simulate both WMA and HMA, a two-step conditioning process should be considered for specimens used for evaluation of moisture sensitivity and rutting resistance. In the first step, the mix- ture would be conditioned for 2 h at the compaction temper- ature to simulate the binder absorption and stiffening that occurs during construction. In the second step, the mixture would be further conditioned for an extended time at a repre- sentative high in-service pavement temperature to simulate a short period of time in service. Only specimens used to evalu- ate moisture sensitivity and rutting resistance would receive the second conditioning step. Volumetric design would be based on only the first step. The temperature and duration of the extended conditioning would be selected based on temper- atures from LTPPBind and typical laboratory working hours. Process Number Average Difference in Compaction Temperature (°F) Average Difference in Flow Number (%) Advera 3 46.7 39 Evotherm 2 50.0 38 LEA 1 80.0 50 Sasobit 3 48.3 +38 Table 5. Summary of average difference in flow number of WMA compared to HMA.

Most likely, the second step would require conditioning spec- imens overnight. The extended conditioning temperature and time would be selected such that HMA mixtures conditioned using the two-step process would have similar stiffness as mix- tures conditioned for 4 h at 275°F (135°C). B10. Section 9. Adjusting the Mixture to Meet Specification Properties General Comments This section provides information that can be used to adjust WMA mixtures to meet the evaluation criteria contained in the draft appendix to AASHTO R 35. For coating, compactability, and moisture sensitivity, the user is directed to consult the WMA process supplier. The effects of changing binder grade, volumetric properties, and compaction level on rutting resis- tance are provided. Basis for Critical Content Because WMA processes differ greatly, it was not possible to develop recommendations for adjusting the mixture to meet coating, compactability, and moisture sensitivity requirements. The recommendations for rutting resistance are based on the effects published in NCHRP Report 567: Volumetric Require- ments for Superpave Mix Design. Need for Further Research Additional research is needed to provide insight on how to change WMA mixtures to improve coating, compactability, and moisture sensitivity. The changes will most likely be process specific. B11. Section 10. Additional Reporting Requirements for WMA General Comments This section describes additional data that should be reported for WMA mixtures. Basis for Critical Content There is no critical content in this section. Need for Further Research There is no need for additional research. 92