Compressive behavior of FRP-confined 3D printed ultra-high performance concrete cylinders

Three-dimensional (3D) concrete printing technology has attracted increasing applications due to its merits such as labor-saving. Due to difficulties in implementing reinforcements in 3D printed concrete (3DPC), 3DPC structures are commonly designed to predominantly resist compressive loadings. This paper proposes to further enhance the compressive performance of 3D printed ultra-high performance concrete (3DPU) elements by fiber-reinforced polymer (FRP) wrapping as FRP confinement can enhance both the shear strength and axial compressive strength of concrete. Axial compression tests on FRP-confined 3D printed UHPC (FC3DPU) and unconfined 3D printed UHPC (UC3DPU) cylinders were conducted. The key variables include the loading directions (i.e., X, Y, Z directions) of the 3DPU cylinders and the FRP confinement thickness (i.e., one and two layers). Test results show that FRP wrapping can substantially enhance the strength and deformation capacity of 3DPU. Furthermore, the compressive strengths of the UC3DPU and FC3DPU in the X-direction are the highest, while they are the lowest in the Z-direction. The actual confinement ratio threshold for sufficient confinement of FC3DPU is 0.1. Two existing models of FRP-confined concrete were assessed, and results show that Liao et al.'s model has good performance in predicting the ultimate axial stresses of FC3DPU, whereas Teng et al.'s model is more accurate in estimating the axial strains corresponding to the ultimate axial stresses. Microscopic analysis reveals 3DPU has more large defects (i.e., equivalent diameter (Eq) > 2 mm) in interlayers but less small defects (Eq < 2 mm) than cast counterparts.