Three types of aggregate--a dolomitic limestone, a granodiorite, and a gravel--were studied to investigate the relationship between the particle properties and their performance in terms of strength, elastic behavior, and resistance to permanent deformation. Particles of each material were assessed for their geometry at both coarse and fine scales. This was quantified using several different measures including particle shape, roughness factor, roundness, sphericity, surface friction characteristics, and angularity. Laboratory tests, using a large shear box (300 x 300 mm) and a large (280-cm-diameter) repeated-load triaxial apparatus, were carried out to understand the significance of the geometry of the particles as well as of their surface friction characteristics and the effect of grading and of compaction. Roundness and angularity are considered to be the major factors affecting ultimate shear strength and permanent deformation. The degree of compaction and consequent density also has a great effect on the shear strength and resistance to permanent deformation. However, an increase in surface friction and surface roughness tends to improve the resilient shear stiffness. A pavement trial on the three aggregates, 300 mm thick over a subgrade, was constructed. Subsequent trafficking indicated that the limestone and the granodiorite performed significantly better than the gravel. It confirms that high roundness and low angularity have a much greater impact on performance than does surface friction when rutting is the chief distress mechanism.
Abstract