Effect of rheology modification on fiber orientation and dispersion in 3D-printed ultra-high-performance concrete

Abstract

The extrusion process in 3D concrete printing (3DCP) facilitates horizontal alignment of steel fibers along the printing direction, thus enhancing the load-bearing capacity of printed elements. However, such shear-induced fiber alignment is highly sensitive to rheological properties. This study investigates how variations in dynamic yield stress (τd) and plastic viscosity (μ), influence the extrudability, fiber orientation and dispersion, and anisotropic mechanical behavior of ultra-high-performance concrete (UHPC). Five viscosity-modifying agent (VMA) types were incorporated at two content levels to modify the rheology of printable UHPC. Results showed that moderate VMA additions improved printability, whereas excessive contents markedly increased τd and μ; in the presence of fibers, these elevated rheological parameters collectively triggered extrusion issues, such as filament nonconformity and surface voids. Higher τd and μ also reduced the proportion of fibers aligned within the 0–5° range, disrupting preferential alignment and causing heterogeneous fiber dispersion. As a result, printed elements with compromised filament quality exhibited reduced flexural strength and more pronounced anisotropy.