Effect of Hybrid Fiber Compositions on Mechanical Properties and Durability of Ultra-High-Performance Concrete: A Comprehensive Review.

Ultra-high-performance concrete (UHPC) has emerged as a revolutionary material in structural engineering due to its exceptional mechanical properties and durability. This review comprehensively examines the influence of hybrid fiber compositions on UHPC, focusing on mechanical performance and resistance to environmental degradation. Hybrid fibers, which combine steel and synthetic and basalt fibers, improve compressive, tensile, and flexural strengths by bridging microcracks and limiting macrocrack propagation. Studies reveal that steel fiber combinations, particularly those with varying lengths and shapes, significantly improve ductility and load-bearing capacity, while steel-synthetic hybrids balance strength and flexibility. However, excessive synthetic fibers can reduce compressive strength. Basalt-synthetic hybrids, though less effective in compression, excel in tensile strength and crack resistance. Durability assessments highlight the superior resistance of UHPCs to chloride penetration, carbonation, freeze-thaw cycles, and high temperatures, and hybrid fibers further mitigate spalling and permeability. Polypropylene fibers, for instance, enhance fire resistance by creating vapor release channels. The challenge of optimizing fiber proportions and mix designs remains to minimize trade-offs between strength and workability. Future research should explore advanced fiber combinations, long-term environmental performance, and eco-friendly additives to expand the applicability of UHPC in sustainable infrastructure. This review underscores the potential of hybrid fibers to tailor UHPCs for diverse engineering demands while addressing current limitations.