Mechanical Performance of Ultra-High Performance Concrete Reinforced with Micro Steel Fibers: Experimental and Numerical Investigation under Uniaxial Compression and Flexural Loading

This study investigates the optimal steel fiber volume ratio (0% - 5%) in Ultra-High-Performance Concrete (UHPC) to balance mechanical performance and cost efficiency. Combining experimental testing (up to 3% fibers) and Finite Element Modeling (FEM) in ABAQUS (extended to 5%), the research evaluates two reinforcement systems: 1) monolithic 13 mm micro steel fibers (0.5% - 5%) and 2) a hybrid mix of 13 mm micro and 20 mm recycled steel fibers (2.5% - 3.5%). Results identify 3% - 3.5% as the optimal range, with hybrid fibers improving flexural strength but reducing ductility and stiffness compared to mono-fiber UHPC. FEM simulations closely matched experiments, validating computational modelling for UHPFRC analysis. The findings provide practical guidance for mix design, address gaps in standards (e.g., ACI 544.1R), and highlight the need for further research on hybrid fiber optimization. This work supports UHPFRC adoption in high-demand applications like seismic-resistant infrastructure and long-span bridges, advancing sustainable, high-performance construction.