Pull-Out Behavior of Novel Spherical-End Steel Fibers in Zirconium Oxide Nanoparticle-Enhanced Ultra-High-Performance Concrete

Abstract

This study introduces a novel steel fiber with spherical ends, designed to enhance mechanical interlocking in ultra-high-performance concrete (UHPC) matrices. The UHPC was further reinforced with zirconium oxide (ZrO₂) nanoparticles to improve its mechanical properties. A nonlinear finite element method (FEM), along with a multiscale modeling approach, was used to simulate fiber pull-out and flexural behavior. Experimental tests, including compressive and tensile strength measurements, were conducted to calibrate and validate the numerical models. Results show that the optimal nanoparticle content is 1.0 wt%, which increased compressive and tensile strengths by 37 % and 38 %, respectively. Compared to straight fibers, the spherical-end fibers exhibited significantly improved performance, with a 124 % increase in pull-out force and a 188 % increase in pull-out energy. In three-point bending tests, UHPC specimens with 2 vol% spherical-end fibers showed a 214 % increase in flexural strength over plain UHPC and about 102 % over those with straight fibers. These results underscore the effectiveness of spherical-end fibers in improving the toughness and load-bearing capacity of UHPC, making them highly suitable for high-performance structural applications.