Optimization of 3D printing concrete with coarse aggregate via proper mix design and printing process

Considering the significant advantages of adding coarse aggregate to 3D printing concrete (3DPC) that include low cementitious material consumption, low carbon footprint, cost saving and shrinkage reduction etc., this investigation focuses on 3DPC with coarse aggregates. The effect of paste-aggregate ratio (P/A) on the printability of 3DPC was firstly investigated to ensure that a satisfactory printing quality can be obtained. Based on a suitable P/A, the rheology, mechanical properties and microstructure of 3DPC with different coarse aggregate contents are studied. The results demonstrate that the suitable content of coarse aggregate enhances the interlayer bonding, increases the mechanical strength and reduces the mechanical anisotropy of 3DPC. With suitable printing parameters, the vibration and extrusion process during printing can help eliminate the air bubbles trapped in the fresh mixture of 3DPC and densify the microstructure of hardened 3DPC. Mechanical properties of 3DPC are found to be more sensitive to specimen size as compared to cast concrete. Additionally, the rotation of coarse aggregates caused by inconsistent movement between aggregates and paste during extrusion process will cause the formation of irregular pores, with lower roundness and higher aspect ratio, nearby coarse aggregates in 3DPC.