Optimizing hybrid fiber concrete: an experimental analysis of steel and polypropylene fiber composites using RSM

Abstract Concrete inherent brittleness, limits its performance under various loading conditions, posing challenges in high-performance applications. Hybrid fiber-reinforced concrete (HYFRC), incorporating both steel and polypropylene fibers, has emerged as a promising solution to enhance crack resistance of concrete, its durability, and overall mechanical properties. This study investigates the mechanical and microstructural properties of HYFRC through a series of comprehensive experiments. Specifically, the study evaluates workability, compressive strength, split tensile strength, and microstructural characteristics using x-ray radiography and Scanning Electron Microscopy (SEM). The experimental setup involved 101 trial mixes, producing 1,212 standard cylinders, each incorporating varying combinations of steel fibers (SF) and polypropylene fibers (PP) across four hybridization schemes. The SF volume ratios were varied from 0.25% to 1.5%, and PP fiber ratios ranged from 0.2% to 1.0%. Key findings include significant improvements in compressive strength (up to 4290 psi) and split tensile strength (up to 554 psi) compared to non-fibrous control samples. Optimization through Response Surface Methodology (RSM) and ANOVA revealed an optimal mix achieving a slump of 3.32 inches and notable improvements in strength (61.19% for split tensile and 40.39% for compressive strength). Microstructural analysis confirmed that enhanced strength was primarily due to the development of calcium silicate hydrate (C-S-H) gel structures within the hybrid matrix. This study contributes valuable insights into the design and performance of HYFRC, laying the foundation for future investigations into its application in dynamic loading scenarios, such as bridge piers subjected to seismic forces. The findings support the potential of HYFRC as a durable, high-performance material for critical infrastructure, and contributing in a research gap in fiber-reinforced concrete.