Effectiveness of various fiber types and hybrid fibers in improving the compressive behavior of UHPC

The incorporation of fibers into ultra-high performance concrete (UHPC) significantly modifies its mechanical behavior. This research quantifies these effects by systematically testing five individual fiber types— straight steel (SF), hooked-end steel (HF), polypropylene (PP), polyvinyl alcohol (PVA), and basalt (BF)—alongside a multi-scale hybrid fiber (HYF) combination at 1% and 2% volume fractions. Regarding compressive strength, all fiber reinforcements provided measurable enhancements compared to the unreinforced control matrix (124.24 MPa). The SF system proved most effective, delivering a maximum strength increase of 24.97% (reaching 159.84 MPa) at a 2% volume fraction. Notably, the HYF system—comprising a synergistic blend of SF, HF, PP, and PVA— achieved a 20.86% strength gain, performing comparably to the optimal mono-steel fiber mixes while offering enhanced micro- and macro-crack control. A key finding was the universal transformation of the failure mechanism; the inclusion of any fiber type mitigated the inherent brittleness of UHPC, shifting the failure mode from explosive fragmentation to a controlled, ductile response. In contrast to compressive strength and ductility, statistical analysis (ANOVA, p>0.05) revealed that the elastic modulus remained insensitive to both fiber type and content, indicating that matrix properties predominantly govern the stiffness of this composite. This study confirms that strategic fiber hybridization is paramount for optimizing UHPC ductility and compressive strength without materially altering its elastic stiffness, providing a robust baseline for future structural applications.