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C-10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Step 9. Calculate Trial Mixture Proportions by Weight and Check Dust-to-Binder Ratio These calculations are identical to those for HMA. Step 10. Evaluate and Refine Trial Mixtures This step involves the preparation and evaluation of laboratory specimens of WMA. The pro- cedure follows that for HMA with slight modification. Table 7 summarizes the steps for WMA and HMA design. The modifications required for WMA design are 1. For some processes, the WMA additive must be calculated. 2. Viscosity-based mixing temperatures are not used with WMA. Laboratory mixing is done at the planned production temperature. 3. Process-specific specimen fabrication procedures are used to prepare laboratory mixtures. 4. The short-term conditioning temperature for WMA is the planned compaction temperature. 5. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction is done at the planned compaction temperature. 6. WMA design includes an evaluation of coating and compactability using the planned pro- duction and compaction temperatures. Supporting data from NCHRP Project 9-43 for these modifications are discussed in the sections that follow. Additive Dosage The computation of WMA additive dosage rates is straightforward. The amount of additive needed may be specified by the WMA process supplier as percent by weight of binder or total Table 7. Comparison of trial specimen fabrication procedures for WMA and HMA design. Step Description HMA WMA Comment 1 Calculate batch weights X X Must calculate WMA additive content for some processes 2 Batch aggregates X X Must batch WMA additive for some processes 3 Heat aggregates and X X Use planned production temperature for asphalt binder WMA 4 Mix aggregates and X X Procedure is WMA process specific binder 5 Short-term oven X X WMA uses lower temperature. conditioning 6 Compact laboratory X X WMA uses lower temperature specimens 7 Calculate volumetric X X composition of laboratory specimens 8 Adjust aggregate X X proportions to meet volumetric requirements 9 Evaluate coating and NA X Used in WMA design in place of viscosity- compactability based mixing and compaction temperatures 10 Conduct performance X X Moisture sensitivity for all mixtures, rutting testing resistance for design traffic levels of 3 m ESALs or greater
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-11 mixture. For wet aggregate processes, water is added to a portion of the fine aggregate, and then this wet, fine aggregate is added cold to the mixture during the mixing process. The proportion of the aggregate that is added wet and the moisture content are provided by the WMA technol- ogy provider. Mixing Temperatures Viscosity-based mixing temperatures cannot be used with the wide range of WMA processes currently available. Laboratory specimens are mixed at the planned production temperature, and coating is evaluated to determine the acceptability of the WMA process. Process-Specific Specimen Fabrication Procedures For mixture design, the various WMA processes were grouped into four generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. The procedures in the report address laboratory mixing. These were developed based on recommendations from various WMA technology providers and verified during the mix design experiment completed in NCHRP Project 9-43. Once mixing is complete, specimen fabrication for all processes continues with short-term conditioning and specimen compaction. These steps are the same for all processes and the same as done with HMA. WMA mixture designs will require additional equipment. Since coating is used in lieu of viscosity-based mixing and compaction temperatures, a mechanical mixer is required. During NCHRP Project 9-43, it was observed that planetary mixers and bucket mixers do not have the same mixing efficiency. Planetary mixers are more efficient. The specimen fabrication proce- dures were developed in NCHRP Project 9-43 using a planetary mixer. 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. NCHRP Project 9-43 demon- strated that it is feasible to perform foamed asphalt WMA mixture designs in the laboratory. In NCHRP Project 9-43, a modified Wirtgen WLB-10 laboratory foaming plant was used to simu- late the Gencore Ultrafoam GX process using 1.25% water by weight of binder and the Astec Double Barrel Green process using 2.0% water by weight of binder. The modification that was required was to the replace the flow controller with a smaller, more precise flow controller to accommodate the water contents used in WMA mixtures. Short-Term Conditioning Short-term conditioning for WMA was set at 2 hours at the planned compaction temper- ature to represent the absorption and binder stiffening that occurs during construction. This level of conditioning is used for the volumetric design and for the moisture sensitivity and rutting evaluation. These conditions were selected based on comparisons of properties of laboratory-mixed, laboratory-compacted specimens with those from field-mixed, laboratory- compacted specimens.
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C-12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Figures 5 and 6 summarize the results of comparisons of maximum specific gravity and indirect tensile strength for the field sections tested in NCHRP Project 9-43. The error bars shown in Figure 5 are the single operator D2s precision from AASHTO T 209. These data show that the maximum specific gravity of the lab and field mixtures are the same, indicating that the binder absorption is the same for the lab and field mixtures. The aggregate water absorption ranged from 0.5% for the Pennsylvania SR2007 mixtures to 2.5% of the Yellowstone National Park mixtures. Figure 6 shows average differences in indirect tensile strength for the field mixtures minus the laboratory mixtures. The error bars in this figure are 95% confidence intervals for a paired t-test comparison. If the error bars do not capture zero, then the difference in the tensile strength of the field- and laboratory-mixed specimens is different from zero. Figure 6 shows that several mixtures have significantly different tensile strengths. The differences are not consistently in one direction except for the Pennsylvania SR2006 project, where the field-mixed specimens always have significantly higher tensile strengths compared to the laboratory-mixed specimens. Given that one-third of the mixtures were from this project, this difference biased the results. The average difference for all projects was 7 psi (48 kPa); not considering the Pennsylvania SR2006 project, the average difference was essentially zero. Short-term conditioning for performance evaluations, moisture sensitivity, and rutting was one of the areas where additional research was recommended in NCHRP Project 9-43. This addi- tional research was recommended because it appears that the current HMA short-term condition- ing procedure for performance evaluation, 4 hours at 275°F (135°C), represents the stiffening that occurs during construction and some short time in service. Compaction Temp 2 Hours Field Mix Monroe NC Astec PA SR2006 Sasobit PA SR2006 LEA PA SR2006 Gencor PA SR2006 Advera PA SR2006 Control Mixture/Process PA SR2007 Evotherm PA SR2007 Control YNP Sasobit YNP Advera YNP Control CO I-70 Sasobit CO I-70 Evotherm CO I-70 Advera CO I-70 Control 2.300 2.400 2.500 2.600 Maximum Specific Gravity Figure 5. Comparison of maximum specific gravity between field mixes and laboratory mixes short-term conditioned 2 hours at the compaction temperature.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-13 55 45 IDT Strength Differences, psi 35 25 15 5 YNP Sasobit PA SR2006 Sasobit CO I-70 Sasobit YNP Advera YNP Control Average PA SR2007 Evotherm PA SR2006 Gencor PA SR2006 Advera CO I-70 Evotherm CO I-70 Advera PA SR2007 Control PA SR2006 Control CO I-70 Control PA SR2006 LEA Monroe NC Astec -5 -15 -25 Mixture/Process Figure 6. Differences in indirect tensile strength between field mixes and laboratory mixes short-term conditioned 2 hours at the compaction temperature. Compaction Temperatures Viscosity-based compaction temperatures cannot be used with the wide range of WMA processes currently available. Laboratory specimens are compacted at the planned compaction temperature. Additionally a compactability evaluation is conducted to ensure that the mixture is compactable at the planned compaction temperature. WMA Evaluations Four evaluations are conducted on WMA mixtures at the design binder content: (1) coating, (2) compactability, (3) moisture sensitivity, and (4) rutting resistance. The sections below describe the supporting information from NCHRP Project 9-43 for these evaluations. Coating Coating is one way to evaluate proposed WMA production temperatures that is relevant to all WMA processes. In NCHRP Project 9-43, coating was evaluated on a number of HMA and WMA mixtures using AASHTO T 195. AASHTO T 195 counts the percentage of the coarse aggregates in the mixture that are fully coated. This is a strict criterion. When a planetary mixer was used, coat- ing 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. This indicates that the bucket mixer is less efficient than the planetary mixer. The criterion of 95% was based on the plan- etary mixer data. Though bucket mixers are less efficient than planetary mixers, they are signifi- cantly less expensive and likely more readily available in mix design laboratories. Until additional research is conducted to develop appropriate mixing times for bucket mixers, technicians and engi- neers will have to develop mixing times for their WMA mixtures based on coating evaluations for HMA mixtures produced using the traditional viscosity-based mixing temperatures.
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C-14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Compactability The compactability evaluation is used in lieu of the viscosity-based mixing 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 compaction temper- ature. The number of gyrations to reach 92% relative density is then calculated from the height data. The ratio of the gyrations to 92% relative density at the lower temperature to the higher temperature should be less than 1.25. The methodology for the compactability evaluation resulted from a workability study conducted in NCHRP Project 9-43. The workability study evaluated the feasibility of using various workabil- ity 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 significant at temperatures that are below typical WMA discharge temperatures. Figures 7 and 8 show the effect of WMA process and temperature on workability and compactability. Given that the workability devices were not able to discriminate more precisely than compaction data obtained from a standard Superpave gyratory compactor, the method for evaluating the tem- perature sensitivity of the compactability of WMA was developed for assessing WMA workability and compactability. The method involves determining the number of gyrations to 8% air voids at the proposed compaction temperature and a second temperature that is approximately 54°F (30°C) lower than the proposed compaction temperature. A tentative limit allowing a 25% increase in the number of gyrations when the temperature is decreased was developed. This limit was inves- tigated using data from nine WMA field mixture projects sampled in NCHRP 9-43. The increase in gyrations for the WMA processes ranged from 0 to 20%. Workability and compactability was not reported to be a problem on any of the projects. Moisture Sensitivity Moisture sensitivity is evaluated using AASHTO T 283, the same as HMA. The criterion for AASHTO T 283 is the same as that for HMA. Control Advera Sasobit 450 400 350 300 Torque, in-lb 250 200 150 100 50 0 300 250 190 150 Temperature, F Figure 7. Effect of temperature and WMA additive on torque measured in the UMass workability device.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-15 Control Advera Sasobit 45 40 35 30 Gyrations 25 20 15 10 5 0 300 250 190 Temperature, F Figure 8. Effect of temperature and WMA additive on gyrations to 92% relative density. Tests conducted during NCHRP Project 9-43 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 antistrip additives improved the tensile strength ratio of some of the mix- tures included in the NCHRP Project 9-43 testing and analysis. Of the nine WMA mixtures that used a WMA process that included an antistrip additive, the tensile strength ratio remained the same or improved in 67% of the mixtures. For WMA mixtures produced using processes that do not include antistrip additives, the tensile strength ratio never improved and decreased in 79% of the mixtures. Rutting Resistance Rutting resistance is evaluated using the flow number test, AASHTO TP 79. The same testing conditions that are used for HMA flow number testing are used with WMA: · Air voids of 7.0 ± 0.5% · 50% reliability high pavement temperature from LTPPBind 3.1 for the project location, 20 mm below the pavement surface, or 20 mm below the top of the sub-surface pavement layer of interest · Unconfined · Repeated deviator stress of 87 psi (600 kPa), contact deviator stress of 4.4 psi (30 kPa), Minimum flow numbers as a function of traffic level are provided and these are lower than those for HMA. Table 8 compares the recommended flow numbers for WMA and HMA. The Table 8. Flow number criteria for WMA and HMA mixtures. Traffic Level, Minimum Flow Number Million ESALs WMA HMA <3 NA NA 3 to < 10 30 50 10 to < 30 105 190 30 415 740