△ continued from page 11 Cracking Experiment Measurements Passes to Initial Cracking Figure 4a shows the number of loading passes at which the first cracks were observed for each of the tested mixtures. Overall, HRAP mixtures (45% RAP) featuring the use of either RA, or a softer binder grade withstood the highest number of passes before developing the first visible surface crack. This was followed by mixtures 45_HR, 60_HR_RA_L (based on average values from cells C1 and C2), 30_C, and finally 30_O. Cracking Quantification Figure 5 shows the cracking cells of the six experimental lanes after full-scale testing at the end of the experiment. The final cracking length, along with its severity determined by its width, was measured for each test section. During the field survey, it was observed that the majority of identified cracks in the testing sections exhibited low severity, with a crack width of 0.1 mm or less. Some cracks were of medium severity, with a crack width of 0.1–0.4 mm, while few exhibited high severity, with a crack width of greater than 0.4 mm (mainly greater than 0.75 mm). Figure 4b shows the total surface crack length for each cracking cell. Overall, HRAP mixtures (45% RAP) that utilized either RA or a softer binder grade exhibited the least amount of cracking when the experiment was concluded. Closing Remarks The OBC of all BMD mixtures was higher than the control mixture, suggesting that the BMD process may result in additional binder being used for some mixtures. The mixtures that included an RA and/or softer binder (45_HR_RA, 45_HR_L, and 60_HR_RA_L) had a lower OBC when compared to the mixture that did not include an RA and/or softer binder (45_HR). The APT rutting experiment found that BMD mixtures (30_O, 45_HR, 45_HR_RA, 45_HR_L and 60_HR_RA_L) showed higher rut depths compared to the control mixture 30_C. Meanwhile, the APT cracking experiment found that BMD mixtures (30_O, 45_HR, 45_HR_RA, and 45_HR_L) exhibited less total cracking compared to the control mixture (30_C). It was concluded that the dense-graded unmodified surface mixtures with high RAP contents exceeding 30%, as set forth by the current VDOT specifications, can be designed using the current VDOT BMD special provision. Furthermore, the effective use of BMD should include the ability to optimize mixtures using a variety of tools including gradation adjustments and the use of RAs and/ or softer binder, instead of solely relying on increasing the asphalt binder content. Finally, ensuring a sustained progress in the implementation of BMD will contribute to the production of longer-lasting, cost- effective, and environmentally sustainable asphalt mixtures. The BMD framework will continue to support innovative practices and technologies with a promising performance outlook. Acknowledgement The authors express their gratitude to Gerardo Flintsch, Bilin Tong, and Ernesto Urbaez Perez of VTTI, as well as Donald (Clyde) Landreth and Travis Higgs of VDOT Salem District for their valuable assistance with this study. The authors thank Boxley Asphalt Paving and Ingevity for their work in producing the mixtures studied herein. EVALUATING BMD CONVENTIONAL AND HIGH RAP SURFACE MIXTURES VAASPHALT.ORG 13
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