3D printing towards cost-effective design of composite UHPFRC beams: Effects of fiber distribution and orientation on flexural performances and failure mode transition

To reduce the high cost of ultra-high-performance fiber-reinforced concrete (UHPFRC) structures, this study proposes a novel composite UHPFRC beam design and a 3D printing–casting mixed fabrication method with optimal fiber orientation and distribution. Specifically, the UHPFRC is 3D-printed (3DP-UHPFRC) only in tensile stress-critical regions such as lower bending and diagonal shear, with the fibers aligned along the principal tensile stress direction to provide optimal crack constraining and bridging effects, while the fiber-free UHPC is cast elsewhere. Four-point bending tests of 12 beams considering 3 parameters, namely fabrication process, 3DP-UHPFRC layer thickness, and shear reinforcement were carried out, assisted by digital image correlation (DIC) to observe surface crack propagation and micro X-ray computed tomography (μXCT) to characterize the internal 3D microstructures around the printed interlayer interfaces and print-cast interfaces. Compared with the cast beam with a fiber volume fraction of 1.5 %, the composite beams achieved a maximum fiber reduction of 53 % for the same ultimate load and 72 % for the same nominal cracking load, with corresponding material cost reductions of 24 % and 33 %. The DIC analyses showed that 3D-printing fibers in the diagonal shear regions shifted the beam failure mode from brittle shear to ductile flexure, though the fiber saving was reduced to 35 %. The 3D μXCT images indicated that 3D printing was able to force most of the fibers to align along the printing direction and did not evidently result in more pores near the interfaces, thus hardly affecting the beam structural behavior.