Specimen size effect on strength behavior of cemented paste backfills subjected to different placement conditions

This paper investigates the specimen size effect on the strength properties of cemented paste backfill through unconfined compressive strength tests. Paste backfill samples were cast in different size molds (D × H: 10 × 20 cm, 7.5 × 15 cm, and 5 × 10 cm) and subjected to the four placement conditions: capped–drained C–D; uncapped–drained U–D; capped–undrained C–U; and uncapped–undrained U–U. The unconsolidated samples were also compared with consolidated backfills. Results show that the highest strengths were obtained from the C–D samples cast in the mold 5 × 10 cm, followed by the U–D, C–U and U–U samples. This could be explained by the removal of excess water within paste backfill, the decreased number of micro-cracks in grains, and the wall effects. With the drainage of excess water, particles are pulled together under capillary forces, resulting in an increase of stiffness, accelerating binder hydration and thus, producing higher strengths. Further, specimen size effect on water content, void ratio, mercury intrusion porosity, and degree of saturation of cemented paste backfill were presented. Some scale effect relationships were finally expressed and discussed.