Recent development of innovative steel fibers for ultra-high-performance concrete (UHPC): A critical review

This paper presents a comprehensive review of recent advancements in innovative high-strength steel fibers for ultra-high-performance concrete (UHPC). Firstly, commercially available ordinary steel fibers for UHPC products are introduced, and the analysis of fiber aspect ratio on the mechanical properties of UHPC follows. Subsequently, the historical development of deformed steel fibers, including hooked, corrugated, twisted, half-hooked, and curvilinear steel fibers for UHPC, is reviewed, accompanied by a discussion of their impact on various properties of UHPC. In response to limitations associated with specific deformed steel fibers in UHPC due to stress concentration, recent development of surface-refined straight steel fibers is presented. The paper further highlights the development of surface-roughened steel fibers through physical and chemical methods aimed at increasing surface roughness and interfacial friction in UHPC, along with an explanation of the benefits of these surface-roughened steel fibers in enhancing both fiber-matrix interfacial bond and tensile performance. Addi-tionally, the adoption of active and inert nanomaterial-based surface coatings on steel fibers for UHPC is explored to intensively reinforce the interfacial transition zone between the fiber and matrix. Hybrid surface refining methods and various chemical treatment techniques for modifying the surface properties of steel fibers in UHPC are reviewed and discussed. Finally, based on the available current database, optimal fiber types as re-inforcements for UHPC, in terms of tensile and flexural performances, are suggested.