Flexural properties of 3D printed fibre-reinforced concrete with recycled sand

The flexural properties of 3D printed concrete specimens with recycled sand reinforced by polyethylene (PE) fibres were investigated through four-point bending tests in this research. The use of recycled sand reduced the flexural stiffness and strength of 3D printed specimens loaded in various directions. From curing ages of 7 d to 14 d, the flexural strength and fracture energy of the specimens increased significantly, while the improvements from 14 d to 28 d were relatively limited. By adding PE fibres into the mix, the negative impact of adding recycled sand can be compensated, and the flexural strength of the 3D printed specimen can be obviously improved. Scanning electron microscopy results showed that the loose old cement paste attached to the recycled sand resulted in the higher porosity of the 3D printed specimens with recycled sand compared to that with only natural sand. The mechanical properties of 3D printed concrete specimens can be effectively improved by the strong interfacial bonds between the fibres and the matrix and the high tensile strength of the fibres. In practical applications, if the deformation and stiffness requirements are not strict, the replacement ratio of recycled sand can be increased if an appropriate fibre content is included.