Fiber alignment mechanism in 3D-printed ultra-high performance concrete based on fluid dynamics theory

Abstract

This paper aims to clarify fiber orientation mechanism in 3D printed ultra-high performance concrete (3DP-UHPC) within a shear flow field. Firstly, the internal flow field characteristics of 3DP-UHPC were examined under various rheological parameters and extrusion speeds. Furthermore, velocity distribution patterns of 3DP-UHPC in nozzle were analyzed by fluid dynamic theory. The fiber orientation in 3DP-UHPC was investigated by X-ray computed tomography technology (X-CT) to validate the prediction model. Results indicate that the flow field in 3DP-UHPC comprises two distinct zones: a low-velocity gradient plug flow region, which minimally impacts fiber rotation, and a high-velocity gradient shear flow region which significantly affects fiber alignment. By reducing the viscosity of UHPC and increasing the extrusion speed, the velocity gradients in both zones are enhanced, optimizing fiber alignment in the printing direction. Based on the Jeffery equation, the velocity gradient coefficient is introduced to establish the relationship between velocity gradient and fiber rotation angle in different nozzle areas, thereby refining the previously proposed fiber orientation prediction model, and effectively controlling the relative error of fiber orientation coefficient to within 6 %.