Impact of Low Asphalt Binder for Coarse HMA Mixes

Asphalt mixtures are commonly specified using volumetric controls in combination with aggregate gradation limits, like most transportation agencies, MnDOT also uses this approach. Since 2010 onward, several asphalt paving projects for MnDOT have been constructed using coarser asphalt mixtures that are manufactured with lower total asphalt binder contents. Due to the severe cold climate conditions in Minnesota, there are concerns of premature cracking and inferior durability in asphalt mixtures with lower asphalt binder contents. This research project evaluated 13 low asphalt binder content mixes from 10 actual field projects to determine whether there is potential for poor cracking performance and high permeability. Assessment of field performance indicated an average of 7.75 years of life until 100% transverse cracking level is reached. The pavement structure played a significant factor in controlling the cracking rates. Thin overlays showed almost ten times inferior transverse cracking performance as compared to asphalt wearing courses on full-depth reclamation. Asphalt mixture volumetric factors did not show a statistically significant effect on cracking rates; however, the asphalt binder grade did show a strong effect. Eight out of the 13 coarse asphalt mixtures evaluated in this study have higher permeability than the typical dense graded asphalt mixtures. Performance evaluations using lab measured properties predicted poor thermal cracking performances. No discernable trends were observed between measured or predicted cracking performance and mix volumetric measures. Use of performance tests based on specifications for design and acceptance purposes is reinforced through this study.