DEVELOPMENT OF A STONE MASTIC ASPHALT DESIGN METHOD FOR SOUTH AFRICAN CONDITIONS

Thin asphalt layers (of less than 40 mm thickness) are often used for surfacings in South Africa. As these layers are placed on eiher stiff (cemented) or flexible structures it is important that they be rut resistant, durable and have good fatigue properties. Stone Mastic Asphalt (SMA) has been gaining worldwide acceptance as a stable, durable and fatigue-resistant surfacing mix. The objective of this project was to develop a SMA design method suited for southern African conditions. This paper discusses: the recipe mix design methods prescribed by the various road authorities; basic principles of volumetric design; a theoretical model to determine the volumetric properties of SMA mixes; and various compaction methods, including the gyratory compactor and the Marshall impact hammers. In the design of SMAs the applicability of grading envelopes is compared with volumetric properties. The laboratory results are compared with those obtained from the model. The K-mould apparatus, a dynamic triaxial loading and data acquisition system, was used to determine the rut resistance, i.e., dynamic creep and effective stiffness, of the material.