Orientation and spatial distribution of pores and fibers in concrete elements produced by extrusion-based 3D printing

Use of short dispersed fibers has been considered a promising solution to address the challenge of integrating reinforcement into 3D concrete printing. This study used micro-CT scanning technology to examine pore and fiber orientations and their spatial distribution of single-filament and multi-filament specimens extruded using various nozzle shapes and standoff distances. Results show that nozzle pressing and gravity-induced lateral flow progressively collapsed and filled voids in lower layers, leading to a pronounced porosity gradient along the build height. A smaller standoff distance enhanced the fiber alignment along the printing direction as the nozzle exhibited higher pressure upon deposited material. Circular nozzles promoted stronger fiber alignment compared to rectangular counterparts, primarily due to their taller outlet height, which enhanced nozzle pressing effects. Additionally, the rotating auger in the nozzle contributed to shaping the final fiber orientation profile. As a result, fibers within the inner unsheared core largely retained their orientation from the extrusion process, whereas those in the sheared outer ring aligned with the surrounding flow. Although extrusion-induced deformation reduced the difference between these two regions, a distinct variation in orientation persisted. In addition, the directional distribution of voids and fibers strongly affected compressive and flexural strengths, influenced failure modes, and contributed to anisotropy. These observations provide useful input for modelling and simulation of fiber-reinforced concrete in 3D printing. • Reduced standoff distance markedly increases longitudinal fiber alignment, driven by higher nozzle-induced confinement/pressure; nozzle shape has a minor effect on overall fiber orientation. • The final fiber-orientation profile results from coupled mechanisms: auger rotation, shear-induced material rotation in the nozzle, and post-extrusion filament deformation (nozzle pressing, gravity, creep). • Extrusion/deposition deformation reorients initially misaligned fibers toward the printing direction and homogenizes orientation, with stronger effects at low standoff distance. • μCT reveals spatially organized (non-random) pore architectures; nozzle geometry governs intersection-void morphology (continuous rod-like voids for circular vs localized patch-like voids for rectangular nozzles). • Directional distributions of fibers and pores jointly control compressive/flexural response, failure modes, and mechanical anisotropy, providing evidence to inform parameter optimization and model refinement.