3D printing of ultra-high-performance concrete: Shape stability for various printing systems

Recently, global attention has turned to concrete 3D printing for its potential in structure optimization, life- cycle extension, emission reduction, and cost savings. However, uncertainties persist regarding the printability of ultra-high-performance concrete (UHPC). Previous studies often tailor a mix to a specific printing system and evaluate its printability based on measurements of pumpability, extrudability, and buildability, assuming consistent shape stability regardless of the chosen printing system. To further investigate this assumption, an experimental program was conducted using various printing systems on a nano-modified UHPC mix. Nano clay was incorporated into the self-leveling UHPC mix to increase its viscosity and yield stress, thereby enhancing layer stability post-extrusion. The experimental parameters included the type of robotic system, extrusion system, nozzle design, and material pumping method. Two robotic systems were utilized: a medium-scale gantry and a large-scale ABB robotic arm. Two distinct extrusion systems, a piston-type extruder and an auger system, were employed. Various nozzles, including circular and rectangular designs, were tested. Two pumps were used: a cavity pump and a Thom-Katt pump (piston-type pump). The results indicated that the shape stability of the UHPC mix is significantly influenced by the printing system, suggesting that concrete printability is dependent on the printing system rather than being an inherent material property, as suggested by other researchers. Furthermore, the use of a circular nozzle demonstrated different shape stabilities when the extrusion system was changed from a piston-type extruder to an auger system. Shape stability improved with the auger system due to the lower accumulated pressure within the system. Additionally, the method of material pumping to the extrusion system was found to be critical for the shape stability of the printed layers. The mix failed to maintain its shape post-extrusion when the cavity pump was employed, which was attributed to the higher strain rates imposed on the material during the pumping process. In contrast, this issue was not observed when the piston-type pump was used.