Influence of substrate surface roughness and moisture content on tensile adhesion performance of 3D printable concrete

3D concrete printing is of great potential to replace shotcrete for hard rock tunnel linings with the advantages of high surface quality and the avoidance of rebound. However, without spraying fresh concrete onto the excavated surface at a high speed, 3D printable concrete still faces challenges of achieving strong adhesion, which closely depends on the substrate surface properties. In this work, the influence of the substrate surface roughness and moisture content on the tensile adhesion of 3D printed concrete in the fresh and hardened state was studied. A tack test and a 3D concrete printing test were used to evaluate the tensile adhesion in the fresh state. Further, the tensile adhesion of 3D printed concrete in the hardened state was measured by a pull-off test and compared with that of mold cast samples. Results showed that the printed material could not completely fill macro rough textures due to the limited pressure at the nozzle. As a result, less contact at the interface was formed when the substrate surface roughness level increased, which subsequently compromised the tensile adhesion in the fresh and hardened state. Fortunately, the tensile adhesion in the fresh and hardened state can be improved by applying a coating onto the substrate beforehand. On the other hand, an increased moisture content enhanced the tensile adhesion in the fresh state due to surface tension, while it reduced the tensile adhesion in the hardened state in the context of 3D concrete printing which can be attributed to a higher water-to-cement ratio at the contact zone