Fracture behavior and toughening mechanism of polyethylene fiber-reinforced ultra-high performance concrete (UHPC)

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Polyethylene (PE) fibers are gaining interest as a corrosion-resistant alternative to steel fibers in ultra-high performance concrete (UHPC), yet their fracture behavior remains poorly understood. This study investigated the fracture performance of PE fiber-reinforced UHPC using 27 notched beams with varying fiber type (PE vs. steel), content (0.5% and 1.0%), and PE fiber length (6, 12, 18 mm). Fracture process zone (FPZ) development was tracked by digital image correlation (DIC), while microstructural analyses were conducted to examine fiber–matrix interfaces and void distribution. Three fracture responses emerged: single-peak softening, double-peak softening, and double-peak hardening. Initial fracture toughness was dependent on matrix and fiber modulus, thus steel fibers at 1.0% gave the highest value (3.89 MPa·m 1/2 ), whereas unstable toughness was solely fiber-bridging-controlled, with 12-mm PE fibers at 1.0% showing the highest value (68.56 MPa·m 1/2 ). Fracture energy increased with fiber content and length with 18-mm PE fibers at 1.0% giving the highest value (12422 N/m) due to multiple cracking, though steel fibers offered superior ductility. DIC revealed slower FPZ growth in steel fibers due to higher modulus and distinct energy dissipation. Microstructural analyses showed longer 12- and 18-mm PE fibers increased porosity, reducing compressive strength by up to 13% relative to 6-mm PE fibers, but fiber bridging limited initial toughness reduction to 4–9%. Shorter 6-mm PE fibers showed shallower surface splits and mixed pull-out and rupture failure, confirming that fiber characteristics govern fracture performance. These findings provide a basis for designing PE fiber-reinforced UHPC with enhanced fracture resistance through optimized fiber characteristics.