The influence of pore structure and fiber orientation on anisotropic mechanical property of 3D printed ultra-high-performance concrete

This paper investigates the anisotropic mechanical properties of 3D-printed ultra-high-performance concrete (3DP-UHPC) with varying fiber contents (0 %, 1 %, 1.5 %, and 2 %) and lengths (6 mm, 10 mm, and 13 mm). X-CT analysis was used to examine the geometric characteristics and distribution of pore structure and fiber orientation. The results indicate that the anisotropy coefficients are 0.227, 0.751, and 0.667 for compressive, splitting tensile, and flexural strengths, respectively. Pore structure and fiber orientation are important factors contributing to the anisotropy of the mechanical properties, the large number of ellipsoidal pores generated along the printing direction resulted in the lowest compressive strength in the X direction, and the alignment of most steel fibers along the printing direction resulted in significantly higher split tensile and flexural strengths in the Z direction compared to the other two directions. Increasing the fiber content from 1 % to 2 % improves the flexural toughness ratios of 3DP-UHPC specimens with13 mm fibers from 0.29 to 0.54 in the X direction and 0.29–0.59 in the Y direction, shifting the failure mode from brittle to partially ductile when loaded along unfavorable directions. The higher the fiber content, the more pronounced is the influence of the printing process on the fiber orientation distribution. The average percentage of fiber distribution is about 60 % at small angles (0°–30°) and about 17 % at large angles (60°–90°) in 3DP-UHPC. A model for the prediction of anisotropic mechanical property based on pore structure and fiber orientation was developed, providing a foundation for nonlinear analysis of 3DP-UHPC structures.