Potential of Using Stone Matrix Asphalt for Thin Overlays

Stone matrix asphalt (SMA) has been used in the United States since 1991. To date, almost all of the SMA mixes have had a nominal maximum aggregate size (NMAS) of either 12.5 or 19.0 mm. These two NMASs have been predominant because they conform to information obtained from European experiences with SMA. However, a “fine” SMA mix (for the purpose of this study defined as having an NMAS of 4.75 or 9.5 mm) could be beneficial because it could be placed in thinner lifts and should be more workable. A study was conducted to evaluate the potential of designing fine SMAs and to compare these fine SMAs with more conventional SMA mixes (larger NMASs). Study data indicate that these fine SMAs could be successfully designed to have stone-on-stone contact. Rut testing with the Asphalt Pavement Analyzer confirmed that the designed fine SMA mixes were rut resistant. Permeability testing indicated that these fine SMA mixes are less permeable than conventional SMA mixes at similar void levels and thus should be more durable. Based on information from this study, fine SMAs are a viable option for thin overlays.