Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber

Ultra-High Performance Concrete (UHPC) demonstrates significant potential for extreme-load structures due to its ultra-high compressive strength (> 120 MPa) and exceptional durability, yet its inherent brittleness critically limits seismic and blast resistance. This study investigates the influence mechanism of steel fiber content on the dynamic mechanical behavior of steam-cured UHPC. Through Split Hopkinson pressure bar experiments and microscopic pore structure analysis, we reveal the strain rate strengthening effect, the synergistic regulation of fiber content and pore structure, and the impact damage mechanism. Results indicate that the dynamic compressive strength of UHPC increases exponentially with rising strain rates, accompanied by significantly enhanced dynamic increase factor and strain rate sensitivity. When steel fiber content increases from 1% to 2.5%, the dynamic compressive strength improvement rate rises from 12.3% to 13.8%, attributed to fiber bridging that inhibits microcrack propagation and enhances energy dissipation. The fiber content effect exhibits a nonlinear threshold characteristic: At 1% content, the fiber network remains incomplete, resulting in matrix-dominated behavior. At 2% content, an initial crack-blocking network forms, but weak interfacial zones induce large-pore defects (porosity for 10–100 µm pores exceeds that of the low-content group). At 2.5% content, the fiber network density surpasses the percolation threshold. Through combined physical blocking and chemical optimization (gel deposition), total porosity reduces to 2.98%, achieving peak dynamic compressive strength (215.1 MPa) and substantially improved toughness. Steam curing accelerates hydration and refines matrix pores (> 100 µm pores eliminated completely), yet high temperatures embrittle the fiber-matrix interfacial transition zone. The 2.5% fiber content mitigates interfacial weakening by optimizing pore distribution (26.5% reduction in 1–10 µm mesopores), ultimately achieving optimal strength-toughness balance. This study confirms that 2.5% steel fiber content under steam curing represents the optimal mix design, providing critical parameters for impact-resistant structural applications and establishing a theoretical foundation for high-toughness UHPC development.