Quantitative Evaluation of Orientation of Steel Fibers in 3D-Printed Ultra-High Performance Concrete

Despite the growing interest in 3D concrete printing, the inset of tensile reinforcement poses severe limitation to the advancement of the technology. Inclusion of short steel fibers is a potential alternative to improve the tensile properties of 3D-printed concrete. In the extrusion-based printing process, steel fibers tend to align predominantly in the printing direction. However, currently there is no quantitative evaluation of the orientation of fibers in 3D-printed fiber-reinforced concrete. An experimental program was designed in this study to quantitatively investigate the fiber alignment in a non-proprietary 3D-printable ultra-high performance fiber-reinforced concrete (UHPFRC). Digital image analysis was performed on thin UHPFRC specimens to quantify the fiber orientation distribution. In addition, the effect of the fiber orientation on the mechanical response of the 3D-printed UHPFRC with 2% by volume of micro steel fibers was determined by means of three-point bending tests. Conventionally mold-cast UHPFRC specimens were also prepared and tested for comparison purposes. The results of the digital image analysis revealed an enhanced fiber alignment parallel to the printing direction in the 3D-printed specimens, which in turn significantly enhanced the flexural performance of the printed UHPFRC as compared to the mold-cast counterpart.