Evaluation of North Dakota's 4.75-mm Superpave Mixes for Thin Overlay Applications

Abstract

Because of the decline in aggregate supplies and the need for durable and rut-resistant hot-mix asphalt (HMA), the use of surplus fine aggregates in 4.75-mm mixes could be beneficial for HMA producers, aggregate suppliers, and highway agencies. The primary focus of this paper is to describe the development of and evaluate 4.75-mm Superpave® mixes for use in thin overlays on non-Interstate highway pavements and maintenance applications. The asphalt pavement analyzer (APA) is used to examine the rut resistance of the 4.75-mm mixes. The acceptable APA rut depth was specified to be 9.5 mm. Because of the nature of the 4.75-mm mixes, the Superpave mix design was adjusted to avoid the production of mixes with excessive asphalt contents. A 6% air void design was crucial for the success of North Dakota's 4.75-mm mixes. The research study entailed four mix design cases involving two binders and two aggregate blends with different proportions of natural and crushed fines. Laboratory specimens were produced at 7.0% air voids, heated to either 64°C or 58°C for 6 h, and made to endure 8,000 APA loading cycles. The results indicate that all mix design cases were rut resistant and that mixes with higher percentages of crushed fines performed the best. For the design cases in which only the asphalt binder grade (PG) is the variable, the specimens were tested at different temperatures, so the results could not be compared, but a trend for an increase in rut depth with higher asphalt concrete contents was observed. Trial mixes with high percentages of natural fines failed the APA rut depth tests.