Mix Design Practices for Warm-Mix Asphalt (2011) / Chapter Skim
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Appendix B - Commentary to the Draft Appendix to AASHTO R 35
Appendix B - Commentary to the Draft Appendix to AASHTO R 35
Pages 83-92
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From page 83... ...
83 A P P E N D I X B Commentary to the Draft Appendix to AASHTO R 35
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From page 84... ...
. 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 Sensitivity, and Rutting Resistance; • Adjusting the Mixture to Meet Specification Requirements; and • Additional Reporting Requirements for WMA.
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From page 85... ...
Although it is feasible to design WMA mixtures for plant foaming processes using this equipment, devices specifically designed to replicate the WMA foaming process and produce the smaller quantities of foamed asphalt used in mix design batches without extensive cleaning are needed to make the design process efficient.
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From page 86... ...
Although the improvement in lowtemperature 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.
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From page 87... ...
Thus, improving the low-temperature properties of the virgin binder in the mixture 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, recommended improvements in virgin binder low-temperature continuous 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.
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From page 88... ...
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.
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From page 89... ...
Basis for Critical Content Coating is one way to evaluate planned WMA production 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.
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From page 90... ...
Since 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 temperature sensitivity of the compactability of WMA was developed for assessing WMA workability and compactability. It involves determining the number of gyrations to 8-percent air voids at the planned field compaction temperature and a second temperature that is approximately 54°F (30°C)
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From page 91... ...
WMA mixtures that are conditioned 2 h at the compaction temperature have binder that is less stiff than similarly conditioned HMA mixtures because of the lower conditioning temperature. Current criteria for evaluating moisture sensitivity and rutting resistance are based on mixtures that have been aged to a greater degree.
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From page 92... ...
The effects of changing binder grade, volumetric properties, and compaction level on rutting resistance 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.
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