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83 APPENDIX B Commentary to the Draft Appendix to AASHTO R 35

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

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85 Manufacturers of plant foaming equipment should be en- B6. Section 5. Binder couraged to develop laboratory foaming equipment that can Grade Selection be used to design foamed asphalt WMA mixtures in the lab- oratory. The laboratory foaming equipment that was used in General Comments NCHRP Project 09-43 was designed for preparing laboratory The same grade of binder should be used with WMA and samples of foamed stabilized bases, not WMA. Although it is HMA. For WMA processes with very low production temper- feasible to design WMA mixtures for plant foaming processes atures it may be necessary to increase the high-temperature using this equipment, devices specifically designed to repli- performance grade of the binder to meet rutting resistance cate the WMA foaming process and produce the smaller requirements. quantities of foamed asphalt used in mix design batches with- out extensive cleaning are needed to make the design process Basis for Critical Content efficient. Performance grading data for binders recovered from sev- eral WMA projects sampled during NCHRP Project 09-43 B5. Section 4. WMA showed only small differences in the grade of the binder for Process Selection WMA and HMA sections. Table 1 summarizes the recovered General Comments binder data from NCHRP Project 09-43. Table 2 presents average differences in the continuous grade between HMA This section lists factors to be considered when selecting a and WMA. Excluding Sasobit, which increases the high- WMA process. temperature grade of the binder, an approximately 50F (28C) reduction in production temperature resulted in less than a 1C decrease in the high-temperature grade, while an Basis for Critical Content approximately 100F (56C) reduction in production temper- There is no critical content in this section. 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 50F (28C) Need for Further Research reduction in production temperature resulted in an average There is no need for additional research. improvement in the low-temperature grade of binder of Table 1. Summary of continuous grading of recovered binders. Production Continuous Grade Temperature Project Process Temperature (C) (F) High Intermediate Low Specified NA 58.0 19.0 28.0 Control 280 59.3 14.2 30.6 Colorado I-70 Advera 250 60.0 13.7 31.6 Evotherm 250 61.3 14.1 31.1 Sasobit 250 63.9 15.1 29.9 Specified NA 58.0 16.0 34.0 Yellowstone Control 325 60.0 11.1 34.1 National Park Advera 275 56.3 8.9 36.2 Sasobit 275 60.7 10.1 35.6 New York Specified NA 64.0 22.0 28.0 Route 11 LEA 210 60.5 14.0 31.1 Specified NA 64.0 25.0 22.0 Pennsylvania Control 320 67.7 22.0 24.6 SR2007 Evotherm 250 67.2 22.0 24.9 Specified NA 64.0 25.0 22.0 Control 310 66.6 24.1 22.5 Pennsylvania Advera 250 67.0 22.9 24.1 SR2006 Gencor 250 67.5 21.7 25.7 LEA 210 63.2 21.6 25.4 Sasobit 250 72.9 23.3 22.5 Monroe, North Specified NA 70.0 28.0 22.0 Carolina Astec 275 71.5 23.7 23.9

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86 Table 2. Summary of average difference in continuous grade temperatures for WMA compared to HMA. Average Average Difference in Continuous Grade Difference in Temperature Process Number Production (C) Temperature High Intermediate Low (F) Advera 3 46.7 0.9 1.3 1.6 Evotherm 2 50.0 0.8 0.0 0.4 LEA 1 100.0 3.4 2.5 2.9 Plant Foaming 1 60.0 0.9 2.4 3.2 Sasobit 3 46.7 3.9 0.3 0.3 1.5C, while an approximately 100F (56C) reduction in Basis for Critical Content production temperature resulted in 2.9C improvement for one LEA project. 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 Need for Further Research lengths of time at the compaction temperature. The degree Additional recovered binder grade data should be collected of mixing of the RAP and new binders was evaluated by com- and analyzed to verify the conclusion from NCHRP Project paring dynamic moduli measured on mixture samples with 09-43 that binder grade changes are not necessary for WMA. 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 B7. Section 6. RAP in WMA the mixture, and adding RAP will increase the dynamic General Comments modulus significantly when the RAP is properly mixed with the new materials. The measured dynamic modulus values Research completed in NCHRP Project 09-43 found that represent the as-mixed condition. The dynamic modulus for recycled asphalt pavement (RAP) binders and new binders do the fully blended condition was estimated using the Hirsch mix at WMA process temperatures. Therefore, it is appropriate model from the shear modulus of binder recovered from the to design WMA mixtures containing RAP in the same man- dynamic modulus specimens. If the measured and estimated ner as HMA, accounting for the contribution of the RAP dynamic moduli are the same, there is good mixing of the binder to the total binder content of the mixture. From the RAP and new binders. research completed in NCHRP Project 09-43, the RAP and The findings of the laboratory mixing experiment are new binders continue to mix while the mix is held at elevated shown in Figure 1. At conditioning times of 0.5 and 1.0 h, temperature. To ensure that adequate mixing of RAP and new there is little blending of the new and recycled binders. For binders does occur, a limit is placed on the maximum stiffness all processes and temperatures, the ratio of the measured to of RAP binders for WMA. That limit is based on the planned field compaction temperature of the mixture since this tem- estimated fully blended moduli range from about 0.35 to perature will govern the temperature of the mix during stor- 0.55. At the 2-h conditioning time, the ratio of the measured age and transport. The limit is the RAP binder should have a to estimated fully blended moduli reach values approaching high-temperature grade that is less than the planned field 1.0 for the Control HMA, Advera WMA, and Sasobit WMA. compaction temperature for the WMA. RAP binders typi- The effect of temperature is also evident for these processes, cally range from PG 82 to PG 94 resulting in corresponding with the higher conditioning temperature resulting in minimum field compaction temperatures ranging from somewhat improved blending. The ratio of the measured to 180F to 200F (82C to 94C). estimated fully blended moduli for the Evotherm WMA Binders from WMA mixtures have improved low- remained low even at the 2-h conditioning time. This sug- temperature properties due to the lower amount of aging that gests that either the particular form of Evotherm used in this occurs during production. Although the improvement in low- study retards the mixing of the new and recycled binders or temperature properties is not large enough to warrant changing that the extraction and recovery process stiffens the Evotherm the low-temperature grade, it is large enough to affect the modified binder. amount of RAP that can be added to a mixture when blending Further evidence of the mixing of new and RAP binders at chart analyses are used. WMA process temperatures was obtained from a mixture

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

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88 0.045 Rate of Change of Low Temperature Grade 0.040 With RTFOT Temperature, oC/oC 0.035 y = -0.0019x - 0.024 0.030 2 R = 0.52 0.025 0.020 0.015 0.010 0.005 0.000 -34.0 -28.0 -22.0 -16.0 o Low Temperature Grade, C Figure 2. Effect of low-temperature binder grade on the rate of change of low-temperature grade with RTFOT temperature. incorporating PG XX-16, PG XX-22, and PG XX-28. These Basis for Critical Content recommended improvements are summarized in Table 4 for The specimen fabrication procedures were designed to rea- some common binder grades. For a mixture using PG 64-22 sonably reproduce the WMA process. Procedures are pro- virgin binder and a WMA production temperature of 250F, vided for: the virgin binder low-temperature continuous grade would be improved 0.6C to account for the lower WMA production WMA additives that are added to the binder. temperature. WMA additives that are added to the mixture. WMA processes incorporating wet fine aggregate and se- Need for Further Research quential mixing. Plant foaming processes. Plant mixing studies similar to the laboratory mixing study are needed to confirm that RAP and new binders mix at WMA These procedures were developed from guidance provided process temperatures for field conditions. NCHRP Project by WMA process developers and verified through laboratory 09-43 included one field project that used 30-percent RAP, the testing in NCHRP Project 09-33. Astec Double Barrel Green WMA process, and field mixing and compaction temperatures of 275F and 260F (135C and Need for Further Research 127C). For this project, the mixing analysis showed good mixing of the RAP and new binders. Additional studies of this Developers of new WMA processes should be encouraged type are needed. to prepare specimen fabrication procedures in a similar for- Recovered binder tests on WMA with RAP should be mat so that they can be added in the future to the appendix to conducted to verify the suggested improvements in low- AASHTO R 35. temperature properties for blending chart analyses. B9. Section 8. WMA Mixture Evaluations B8. Section 7. Process-Specific Specimen Fabrication General Comments Procedures This section described four evaluations of the WMA mix- General Comments ture at the design binder content: This section describes specimen fabrication procedures for Coating, several common types of WMA processes. Compactability,

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

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90 Control Advera Sasobit 450 400 350 300 Torque, in-lb 250 200 150 100 50 0 300 250 190 150 Temperature, F Figure 3. Effect of temperature and WMA additive on torque measured in the UMass workability device. temperatures. Figures 3 and 4 show the effect of WMA pactability. It involves determining the number of gyrations process and temperature on workability and compactability. to 8-percent air voids at the planned field compaction temper- Since the workability devices were not able to discriminate ature and a second temperature that is approximately 54F more precisely than compaction data obtained from a stan- (30C) lower than the planned field compaction temperature. dard Superpave gyratory compactor, the method for evalu- A tentative limit allowing a 25-percent increase in the number ating the temperature sensitivity of the compactability of of gyrations when the temperature is decreased was developed. WMA was developed for assessing WMA workability and com- This limit was investigated using data from nine WMA field Control Advera Sasobit 45 40 35 30 Gyrations 25 20 15 10 5 0 300 250 190 Temperature, F Figure 4. Effect of temperature and WMA additive on gyrations to 92-percent relative density.

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

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