Influence of creep on the small-strain stiffness of sand–rubber mixtures

A series of bender element tests was carried out to investigate the variation of small-strain stiffness (G_max) of sand–granulated rubber mixtures under long-term loading. Comparisons were made using two different host sands: a non-crushable material called Sydney sand and a crushable material composed of completely decomposed volcanic rock (CDV). The test results suggested that, at the selected stress states in the study (mean effective confining stress ranged from 250 to 333 kPa), the influence of creep on the stiffness of the pure sand specimens was more pronounced for CDV compared with Sydney sand and the presence of rubber amplified creep effects on both types of mixtures. The influence of creep on G_max was therefore associated with two major mechanisms: the increased compressibility of specimens predominantly because of grain crushing (contribution of sand fraction), and the increased compressibility because of the presence of soft rubber grains (contribution of the rubber fraction). The significant influence of grain breakage on the relationship between stiffness and creep was confirmed by post-test examination of the grain-size distribution curves of pure CDV samples. For rubber–CDV mixtures, the inclusion of soft and viscoelastic rubber grains prevented breakage, in this way reducing the creep mechanism due to grain crushing, but increased the overall influence of creep on stiffness. These conclusions were drawn based on 20% of rubber by mixture weight, and very similar behaviour was observed for 10% rubber. It was therefore understood that the aggravation of creep induced by the increased deformations due to the presence of rubber, for the given contents of rubber included in the study, was more dominant compared with the adverse influence of rubber on samples’ creep by preventing sand grain breakage.