Abstract: In this study, we aimed to investigate the pore characteristics and safety of high-strength concrete reinforced with polyoxymethylene (POM) fibers under impact loads. We prepared high-strength concrete samples with a design strength grade of C60, incorporating POM fibers with a length of 6 mm. The volume fractions of POM fibers used were 0%, 0.15%, 0.3%, 0.45%, and 0.6%. We further analyzed the distribution patterns of the T₂ relaxation time spectrum for specimens with different fiber contents by using nuclear magnetic resonance (NMR) technology. Additionally, we conducted quasi-static compression tests on the specimens with various fiber contents using a rock mechanics testing machine, and carried out dynamic uniaxial compression tests on the specimens at different strain rates (64.8 s^-1, 87.0 s^-1, 116.4 s^-1, and 149.1 s^-1) using a split Hopkinson pressure bar (SHPB) apparatus. Through these experiments, we analyzed the internal pore structure of POM fiber-reinforced high-strength concrete, as well as the changes in stress-strain curves, peak stress, toughness, and energy dissipation under different strain rates and fiber contents.The results reveal several key findings. The inclusion of POM fibers leads to a reduction in the peak value of the T₂ relaxation time spectrum, accompanied by a leftward shift in the spectrum curve. This indicates a decrease in the number of internal pores, a reduction in pore diameter, and a lower porosity within the specimens. The results also unveil a significant strain rate effect in the specimens, with the addition of POM fibers enhancing the overall performance.The peak stress of the specimens exhibited an initial increase followed by a decrease as the fiber content increased. Notably, when the fiber content reached 0.45%, the dynamic compressive strength of the specimens reached its peak, while excessive fiber content resulted in a decrease in strength. Moreover, the inclusion of fibers improved the toughness of the specimens in resisting external loads. The dissipated energy of the specimens increased proportionally with the incident energy, displaying a positive linear relationship. The fracture energy density of the specimens initially increased and then decreased with the increase of fiber content, with the optimal effect observed at a content of 0.45%. Furthermore, increasing strain rate amplified the variation amplitude of the energy dissipation density in the specimens.

Key words: polyoxymethylene fiber, high-strength concrete, nuclear magnetic resonance, stress-strain curve, peak stress, toughness, crushing energy consumption density