NCHRP 9-43 Report 691
Before or after you settle down with the full 400-plus page report, here is my take on the Transportation Research Board’s four-year study, NCHRP 9-43, that has culminated in issuance of Report 691: “Special Mixture Design Considerations and Methods for Warm Mix Asphalt.” Principal author Ray Bonaquist, Ph.D., P.E., CEO of Advanced Asphalt Technologies, LLC, has identified several significant areas affecting lab preparation and testing of WMA mixes. Report 691 explores the WMA issues of sample handling and reheating, workability measurement, distress development, and field validation. AASHTO’s WMA and RAP Technical Working Groups continue to discuss many of the issues explored in Report 691, including binder selection and determination of optimum binder content when using WMA technologies and binder grade selection when using high RAP mixes.
AASHTO Specification R35, “Standard Practice for Superpave Volumetric Design for HMA,” is likely to be the key specification affected by the results of the Report 691. A draft Appendix, “Special Mix Design,” has been submitted for review and possible inclusion in the R35 specification. Among the topics receiving further consideration are minimum compaction temperatures with RAP mixtures and compactability using the Superpave gyratory compactor. The fact that “Special Mix Design” is being submitted as an Appendix and not an Annex is significant. Appendices in AASHTO specifications are optional. Annexes are requirements.
Regarding RAP-containing WMA mixtures, Report 691 suggests that minimum compaction temperatures should be equal to or greater than the critical temperature of the recovered RAP binder. Our experience is that Evotherm mixes with RAP have met this proposed criterion.
Regarding compaction of WMA mixtures, Report 691 suggests that the number of gyrations to 92% of Gmm for WMA mixtures compacted at 90°C (194°F)should not exceed 1.25 times the number of gyrations to 92% of Gmm for an HMA mix containing a WMA additive technology compacted at 120°C (248°F). The practicality of this proposal is debatable since lab foam mixes are not required and since a more practical comparison should be with WMA at typical temperatures and an additive-free control mix at typical HMA temperatures instead of the arbitrary 90°C and 120°C. The controversy notwithstanding, lab-made Evotherm mixes made with modified and unmodified asphalts and coarse and fine aggregate have met this proposed criterion.
The results of Report 691 have already led to funding for other sponsored WMA research. NCHRP 9-47 “Properties and Performance of WMA Technologies,” and NCHRP 9-49 “Performance of WMA Technologies: Stage I – Moisture Susceptibility” are noteworthy in this regard.
Author: Everett Crews