为探究聚甲醛(POM)纤维高强混凝土内部孔隙特征及在冲击荷载作用下的安全性,制备混凝土设计强度等级为C60、POM纤维长度为6 mm、体积掺量分别为0%、0.15%、0.3%、0.45%、0.6%的聚甲醛纤维高强混凝土,采用核磁共振(NMR)技术分析不同纤维掺量下试件T₂图谱分布规律,利用岩石力学试验机对不同纤维掺量下试件进行准静态压缩试验,利用霍普金森压杆(SHPB)装置对试件开展不同应变率下(64.8 s^-1、87.0 s^-1、116.4 s^-1、149.1 s^-1)的动态单轴压缩力学试验,分析不同纤维掺量下的POM纤维高强混凝土内部孔隙结构及在不同应变率下应力-应变曲线、峰值应力、韧性及能量耗散的变化规律。结果表明:POM纤维的掺入使T₂图谱峰值降低,图谱曲线向左偏移,试件内部孔隙数目减少,孔隙直径减小,孔隙率降低;试件存在明显的应变率效应,POM纤维的掺入增强了试件整体性能;不同应变率下试件峰值应力随纤维掺量的增加先增大后降低,当纤维掺量达到0.45%时试件动态压缩强度达到峰值,掺入过量纤维试件强度有所降低;纤维的掺入增强试件在抵抗外荷载时的韧性;试件耗散能随入射能的增大而增大,两者呈良好的线性关系;试件破碎耗能密度随纤维掺量的增加先增大后减小,掺量为0.45%时效果最佳,应变率的增大会使试件耗能密度变化幅值增大。

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.