Influence of Substrate Surface Morphology on Bonding Behaviours at Shotcrete–Substrate Interface

Abstract This study investigates the influence of substrate morphology on bond strength at the interface. Circular surfaces with varying morphologies were created on plaster substrates and quantified by three three-dimensional (3D) roughness parameters: joint roughness coefficient (JRC 3D ), area ratio ( R S ), and statistical parameter Z 2S . Plaster substrates were designed in a LEGO-style configuration, with various morphology surfaces placed on the upper surface of each cylindrical plaster base. Shotcrete was applied to these surfaces, cured for 28 days, and then subjected to direct pull-off tests to assess bond strength variations across surfaces with varying morphologies. Findings reveal that shotcrete’s shrinkage characteristics interact with surface morphologies to influence bond strength. A novel 3D surface morphological parameter, R Z × V P / V V , Adjusted Volume Ratio (AVR), was introduced. AVR showed a stronger correlation between surface morphology and bond strength than traditional 3D parameters, JRC 3D , R S , and Z 2S . Results indicate that bond strength generally increases with the AVR of the surface; however, excessively rough surfaces may reduce bond strength due to insufficient contact at the interface. Whilst 2D parameters do not represent the 3D nature of joint surfaces, the new 3D AVR, which can be incorporated into 3D surface imaging techniques such as photogrammetry, provides a reliable assessment of joint surface morphology for engineering applications.