Laboratory Refinement of 4.75 mm Superpave Designed Asphalt Mixtures

Abstract

In 2004 a pooled fund study was initiated to refine mix design criteria for 4.75mm NMAS Superpave designed mixes and field validate design criteria. Nine states were participants in this study, Alabama, Connecticut, Florida, Minnesota, Missouri, New Hampshire, Tennessee, Virginia, and Wisconsin. Twenty nine 4.75 mm NMAS mix designs have been performed on material from the nine participating states. Each designed mix was tested for permanent deformation, permeability, tensile strength ratio and durability. Also, four plant produced mixtures were evaluated and served as a baseline for performance. The objective of this research was to refine the current procedures and criteria for 4.75mm NMAS Superpave designed mixtures. Based on the results of this study it has been found that 4.75 mm NMAS mixtures can be designed in the laboratory to meet current AASHTO specifications. However based on performance testing, special care should be taken when using these mixtures for higher traffic volume applications. Generally, it was determined that lowering the volume of effective asphalt (Vbe) was the most effective way to control permanent deformation. Since, all the blend gradations were fine graded, the best way to control VMA was to use coarser blends or increase the dust content. The 4.75 mm NMAS mixtures were found to be durable based on fracture energy testing. Permeability is low for these mixtures even at relatively high air void contents. It is recommended that the limit on the percent passing the 0.075 mm sieve be increased to allow for higher dust content, but maintain the current maximum dust to binder ratio of 2.0 to ensure durability. Also, it is recommended that a range of 4.0 to 6.0 design air voids be permitted and VMA and VFA criteria be replaced with maximum and minimum Vbe requirements.