Feasibility of 9.5-mm Stone Matrix Asphalt for Thin Lift Overlays in Washington State

Abstract

Due to budget constraints, highway agencies tend to use thin lifts as a primary rehabilitation strategy. The construction of a thin lift requires a small nominal maximum aggregate size (NMAS), e.g., 9.5 mm or smaller. However, hot-mix asphalt (HMA) with a small NMAS is prone to rutting and/or studded tire wear. Stone matrix asphalt (SMA) is a gap-graded asphalt mixture with interlocking aggregate that enhances the mixture’s mechanical properties compared with HMA, especially with regard to resistance to both rutting and studded tire wear. Therefore, SMA mixes with a small NMAS may provide a viable solution to these distress types. Few studies have evaluated the performance of small NMAS SMA for thin overlays, especially for high levels of studded tire traffic. To address this gap in the literature, this study assessed the laboratory performance of 9.5-mm NMAS SMA compared with that of commonly used 12.5-mm NMAS SMA and 9.5-mm NMAS HMA mixtures to determine the feasibility of using 9.5-mm NMAS SMA in thin lift overlays subjected to heavy traffic in the State of Washington, where the use of studded tires is widespread in winter. Three types of mixtures were tested for their resistance to bottom-up fatigue cracking, top-down fatigue cracking, thermal cracking, rutting and moisture susceptibility, and studded tire wear. The resistance to top-down fatigue cracking of the 9.5-mm SMA was comparable to that of the 9.5-mm HMA and the 12.5-mm SMA. The 9.5-mm HMA and 9.5-mm SMA had comparable resistance to bottom-up fatigue cracking and performed better than the 12.5-mm SMA. Both the 9.5-mm HMA and the 9.5-mm SMA had better thermal cracking resistance than the 12.5-mm SMA. The 12.5-mm SMA had better rutting and moisture susceptibility resistance than the 9.5-mm SMA and 9.5-mm HMA. Both the 9.5-mm SMA and 12.5-mm SMA had comparable resistance to studded tire wear and performed better than the 9.5-mm HMA. Overall, the results showed that 9.5-mm NMAS SMA is a viable material for thin-lift overlays, outperforming 9.5-mm NMAS HMA in terms of resistance to rutting, moisture, and studded tire wear. Further studies are needed to assess the field performance, constructability, and life-cycle costs of 9.5-mm SMA.