The bond strength at the aggregate-cement paste interface is affected by the microstructure of the interfacial transition zone (ITZ). However, the influence of substrate crystal structure on the rock-shotcrete interface behaviour has received limited attention. This study investigates the impact of substrate crystal structure on the bonding strength of the rock-shotcrete interface. Gabbros and basalts used in this study have similar mineral compositions but have different grain sizes due to different cooling rates. Applying plain shotcrete to substrate samples to study the interface behaviours. Direct pull-off tests are conducted to investigate the bond strength at the interface after 28-day curing. The failed surfaces are examined using scanning electron microscopy imaging technique to further analyse the failure interface. The results indicate that hydration products can cover and embed into the rock surface. Basalt with fine-grained texture achieves denser ITZ than coarse-grained gabbro and results in stronger bond between rock and shotcrete.
Achieving strong bonding at the rock-shotcrete interface is a crucial prerequisite for the effective performance of shotcrete in its supporting functions. The bond strength of shotcrete is commonly defined as the capacity to adhere to a given surface, generally composed of rock or concrete (Bryne et al., 2014). Bonding failure consists of adhesion loss and cohesion loss. The former term refers to the separation of two materials, such as rock and cement mortar, along their interface at the contact area. The cohesion loss means the disappearance of the cohesive forces between particles within a single material, such as cement mortar (Luo et al., 2017). The predominant cause of shotcrete failure is the loss of adhesion between the shotcrete and the substrate (Chang & Stille, 1995). Barrett and McCreath (1995) provided an explanation of shotcrete failure modes and found that the performance of shotcrete is highly dependent on the bond between shotcrete and rock.
