Rutting and volumetric properties of SMA mixtures

Abstract

Stone mastic asphalt (SMA) surfacing mixture was developed in Germany in the mid-1960s, originally to improve the resistance to wear from studded tyres and to permanent deformation. Its very good performance and durability, as well as its good surface characteristics, became well-known and as a result SMA mixtures started to be used by a number of other countries. Today, SMA is considered as a heavy-duty surfacing mixture which is used primarily in heavy-trafficked areas. The high bitumen binder content and the resulting low voids content, in conjunction with high traffic volume and possible high in-service temperatures, require, among other things, optimum mix design if the intrinsic resistance to permanent deformation is to be sustained. This study examines the effect of voids and of synthetic fibre content on the permanent deformation of a typical 0/11 mm SMA mixture, and is quantified by the wheel-tracking test. The results show that the rutting of the SMA mixture is very sensitive to changes in voids content, particularly when the test is carried out at 60°C. An increase in fibre content has a positive effect on rutting behaviour, especially when the mixture possesses insufficient voids. Based on the findings of the study, recommendations have been made regarding the SMA mixture design in order to avoid excessive permanent deformation, particularly in locations with high in-service temperatures and high traffic volumes.