Development of mix desk criteria for 4.75mm Superpave® mixes

The Superpave(R) mix design system includes design criteria for a range of mixes with nominal maximum aggregate sizes (NMASs) between 9.5 and 37.5 mm. Many agencies in the United States have expressed an interest in using a 4.75-mm NMAS mix. Such a mix could provide benefits; that is, it should provide a very smooth riding surface, could be used for thin lift applications, could correct surface defects (leveling), could decrease construction time, could provide a use for manufactured screening stockpiles, and could provide a very economical surface mix for facilities with low traffic volumes. A study was done to establish the standard mix design criteria needed for 4.75-mm mixes. On the basis of the findings of the study, the recommended Superpave mix design criteria include a specified gradation control of 30% to 54% passing on the 1.18-mm (No. 16) sieve and 6% to 12% passing the 0.075-mm (No. 200) sieve. During design, the following were recommended: a design air void content of 4 %, minimum voids in mineral aggregate (VMA) of 16 % for all traffic levels, maximum VMA of 18 % for mix designs with more than 75 gyrations, voids filled with asphalt (VFA) of 75% to 78% for mix designs with 75 gyrations and above, and VFA of 75 % to 80 % for mix designs with 50 gyrations. The results of the study showed that 4.75-mm NMAS mixes can be successfully designed. These types of mixes should provide economical mixes for low-volume roadways.