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

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55 Low Strain 20 C High Strain 20 C Low Strain 4 C High Strain 4 C Fit 1.00 0.95 0.90 0.85 Damage Ratio 0.80 0.75 0.70 0.65 0.60 0.55 0.50 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 Reduced Cycles Low Strain 20 C High Strain 20 C Low Strain 4 C High Strain 4 C Equality 1.00 0.95 0.90 0.85 Predicted C 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Measured C Figure 41. Continuum damage analysis for 100-gyration Advera WMA. Table 34. Summary of continuum damage fatigue parameters. Fatigue Mixture Reference Continuum Reference Half-Life Process Gyration Modulus Damage K2 Temperature K1 Level (ksi) Shift Factor (107) 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

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56 Control Advera Evotherm Sasobit 1.00 0.90 0.80 Damage Ratio 0.70 0.60 0.50 0.40 0.30 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 Reduced Cycles Figure 42. Comparison of continuum damage fatigue curves for the 100-gyration mix. Control Advera Evotherm Sasobit 1.00 0.90 0.80 Damage Ratio 0.70 0.60 0.50 0.40 0.30 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 Reduced Cycles Figure 43. Comparison of continuum damage fatigue curves for the 75-gyration mix.

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57 Table 35. Major findings of NCHRP Project 09-43 studies. Topic Major Finding Disposition Sample reheating has a similar effect Not included directly in the products of NCHRP 09- Sample on WMA and HMA. It further 43. Considered during analysis of comparisons of Reheating stiffens the binder of the mixture. field and laboratory data. The RTFOT study showed a major Draft Appendix to AASHTO R 35 does not include a effect of temperature on high- recommendation for high-temperature binder grade temperature binder grade. This bumping. A note is included that high-temperature High- finding was not confirmed by the grade bumping may be needed if the mixture rutting Temperature recovered binder testing from the field resistance is inadequate and cannot be improved Binder Grade validation mixtures except for the through reductions in VMA or increase in the filler Selection LEA process where the mixture content of the mixture. temperature is approximately 100F (56C) lower than typical HMA production temperatures. Both the RTFOT study and the Draft Appendix to AASHTO R 35 does not include recovered binder testing from the field recommendations for changes in low-temperature Low- validation mixtures showed a minor binder grade selection. Low- and intermediate- Temperature improvement in low-temperature temperature binder grade improvements may be Binder Grade grade for WMA compared to HMA. considered for RAP blending chart analysis. A table Selection of recommended improvements as a function of production temperature was included. Short-term conditioning of 2 h at the The Draft Appendix to AASHTO R 35 recommends planned field compaction temperature 2 h of conditioning at the planned field compaction reasonably reproduces the binder temperature for volumetric design, moisture absorption and stiffening that occurs sensitivity testing, and flow number testing. Short-Term during WMA production. The draft standard practice for measuring properties Conditioning of WMA for performance analysis using the MEPDG recommends 2 h of conditioning at the planned field compaction temperature for dynamic modulus testing for structural design. RAP and new binders do mix at The Draft Appendix to AASHTO R 35 recommends WMA process temperatures when limiting the high-temperature grade of the recovered conditioned 2 h at the compaction RAP binder to the planned field compaction temperature. temperature of the WMA to ensure adequate mixing RAP of the RAP and new binders. The optimum binder content of WMA mixtures incorporating RAP should be determined 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. All of the WMA process, including The Draft Appendix to AASHTO R 35 includes Specimen- plant foaming processes, could be process-specific specimen-fabrication procedures for Fabrication reasonably reproduced in the the major categories of WMA processes. Procedures laboratory for mixture design and performance evaluation. The type of mixer used to prepare The Draft Appendix to AASHTO R 35 includes a laboratory mixtures of WMA note that the mixing ti mes included in the appendix significantly affects the coating of were developed using a mechanical planetary mixer Coating coarse aggregate particles. with a wire whip. Mixing time for bucket mixers should be determined by preparing HMA mixtures using the viscosity-based mixing temperature from AASHTO T 312, and evaluating coating. Devices that measure the torque The Draft Appendix to AASHTO R 35 does not during mixing or the force to move a include an evaluation of workability. blade though loose mix could not detect differences between HMA and Workability WMA mixtures at normal WMA production temperatures. Differences could be detected at lower temperatures associated with compaction. A primary benefit of WMA is The Draft Appendix to AASHTO R 35 includes improved compactability at lower evaluating the compactability of WMA mixtures by temperatures. The change in the determining the number of gyrations to 92-percent gyrations to reach 92-percent relative relative density at the planned field compaction Compactability density when the compaction temperature and 54F (30C) below the planned temperature was reduced 54F (30C) field compaction temperature. A maximum increase provides a simple procedure to in gyrations of 25 percent when the compaction evaluate the compactability of WMA. temperature is reduced is recommended. (continued on next page)

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

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