Ultra-High-Performance Concrete (UHPC) Containing Non-Metallic Fibers

Ultra-high-performance concrete (UHPC) is a fiber-reinforced cementitious composite characterized by a densely packed matrix, very low permeability, high compressive strength, and sustained post-cracking tensile resistance. While steel fibers remain the reference reinforcement in most UHPC systems, non-metallic fibers are increasingly being investigated to address corrosion, density, thermal stability, multifunctionality, and sustainability. This paper presents a critical review of UHPC incorporating non-metallic fibers like polypropylene, polyvinyl alcohol, polyethylene, polyoxymethylene, basalt, glass, carbon-based fibers, and natural fibers. The review covers UHPC fundamentals and conventional mix formulations, fiber classification, and the parameters governing fiber selection including stiffness, aspect ratio, wettability, dispersion, bond characteristics, and thermal response. The influence of non-metallic fibers on the fresh-state behavior, rheology, density and air content, compressive and tensile properties, flexural performance, fracture energy, impact resistance, abrasion resistance, permeability, dimensional stability, sulfate resistance, and fire-spalling behavior is synthesized. The literature highlights that non-metallic fibers exert their strongest influence on crack control, tensile deformation capacity, fracture-related response, shrinkage mitigation, and thermal damage resistance, whereas their contribution to compressive strength is generally limited. The literature nevertheless remains fragmented, with substantial variability in matrix composition, curing regimes, fiber dosage, and testing methodology. Important knowledge gaps remain in direct tensile characterization, long-term durability validation, micromechanical bond description, and structural scale assessment, especially for carbon-based and natural fiber systems. The review highlights that non-metallic fibers are best interpreted as application specific reinforcements for next-generation UHPC and identifies the need for standardized test methods and performance-based design approaches to support broader implementation.