Triaxial compressive behavior and failure criteria of strain-hardening UHPC with variable steel fiber geometry

This study investigates the mechanical behavior of Strain-Hardening (SH-UHPC) reinforced with steel fibers of varying geometries (straight, wave, hooked, and long-hooked) under varying levels of confinement. The effects of fiber shape and confinement on strength, ductility, and failure mechanisms. The results reveal a progressive transition in failure modes from brittle shear to ductile shear-compression with increasing confinement. Notably, hooked and long-hooked fibers significantly enhance stress redistribution, crack control, and post-peak resistance, with Long-hooked fibers providing the most pronounced confinement-activated plasticity. Several strength criteria, including Mohr-Coulomb, power-law, and Willam-Warnke models are evaluated for multiaxial stress states. A fiber geometry-dependent correction factor into the Willam-Warnke model is introduced, which significantly improves its ability to capture the distinct multiaxial responses of SH-UHPC with different fiber types. Microstructural observations further clarify the role of fiber morphology in interfacial damage and matrix adhesion. The findings provide theoretical support for the application of UHPC in protective and blast-resisting structures, offering material parameters for finite-element simulations and enhanced practical understanding of its performance under extreme loading conditions. • Investigated triaxial compressive of SH-UHPC with steel fibers of four geometries: straight, wave, hooked, and long-hooked. • Quantified fiber geometry effects on stress-strain, failure, and post-peak response under 0 to 90 MPa confinement. • Proposed a fiber-dependent correction to the Willam-Warnke failure envelope, improving SH-UHPC multiaxial predictions. • Microstructural analysis revealed fiber morphology governs confinement-activated mechanical enhancement in SH-UHPC.