Effect of steel and PVA microfibers on UHPFRC: Mechanical performance and microstructural analysis

This study provides a direct comparison between steel and polyvinyl alcohol (PVA) microfibers in Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC), combining mechanical testing and microstructural analysis. The research examines how fiber type and dosage together impact strength, crack resistance, and fiber-matrix interactions at the interfacial transition zone (ITZ). An experimental program was conducted on UHPFRC mixtures with fiber volume fractions ranging from 0% to 2%. Mechanical properties were assessed through compressive strength, splitting tensile strength, Double Edge Wedge Splitting (DEWS), and pullout tests. Microstructural characterization employed Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to evaluate fiber-matrix bonding and ITZ features. The results demonstrated that steel fibers significantly enhanced the compressive and tensile strengths, with the highest compressive strength of 133.78 MPa achieved at a 2% dosage after 90 days. PVA fibers, although less effective in compression, improve crack control and ITZ adhesion due to their hydrophilic surface chemistry. DEWS confirmed the superior tensile performance of steel fibers, while pullout tests revealed reduced efficiency per fiber at higher fiber dosages, reflecting complex fiber-matrix interactions. SEM–EDS analyses showed the formation of voids around steel fibers, accompanied by denser ITZ bonding with PVA, highlighting distinct reinforcement mechanisms. These findings contribute to current knowledge by clarifying the complementary roles of steel and PVA fibers: steel fibers optimize load-bearing capacity, while PVA fibers enhance serviceability through fine crack management. The study not only establishes critical mechanical-microstructural relationships but also provides practical guidance for UHPFRC applications in various areas, including bridge decks, high-rise buildings, repair works, and protective surface layers on hydraulic structures such as spillways, where resistance to cavitation erosion and mechanical damage is crucial.