Cross-Scale Synergistic Reinforcement of Ultra-High-Performance Concrete by Cellulose Nanofibers and Polyoxymethylene Fibers: Mechanical Behavior and Microstructural Mechanisms.

This study proposes a cross-scale reinforcement strategy for ultra-high-performance concrete (UHPC) using polyoxymethylene fibers (POMFs) and cellulose nanofibers (CNFs). The mechanical behavior and cross-scale synergistic reinforcement mechanisms of POMF/CNF-reinforced UHPC are systematically investigated through compressive, three-point bending, and split Hopkinson pressure bar (SHPB) tests, together with thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and pore structure characterization. The results show that POMFs improve the post-cracking load-bearing capacity of UHPC through macroscopic crack bridging and pullout-induced energy dissipation, thereby transforming the failure mode from brittle to quasi-ductile. The incorporation of CNFs further enhances the matrix strength and flexural performance. At a CNF content of 0.2 wt.%, the compressive and flexural strengths of POMF-reinforced UHPC increase by 9.7% and 21.9%, respectively. Under high strain rates, the POMF-CNF composite system exhibits a pronounced strain-rate strengthening effect. In particular, UHPC containing 2 vol.% POMFs and 0.2 wt.% CNFs achieves both high dynamic load-bearing capacity and energy dissipation capacity, with an impact toughness of 5.2 MJ/m3 at a high strain rate. Microstructural results indicate that CNFs improve matrix compactness and resistance to microcracking by promoting hydration, refining the pore structure, and providing nanoscale bridging. They also enhance stress transfer across the POM fiber-matrix interface, which acts synergistically with the macroscopic crack-bridging effect of POMFs to produce cross-scale toughening. This study provides a reference for the cross-scale design of UHPC with high strength, high toughness, and superior resistance to dynamic damage.