Mechanical behavior of SFRC beams subjected to both impact and fire loadings
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摘要: 为了探究冲击荷载与火灾联合作用下钢纤维混凝土(steel fiber reinforced concrete, SFRC)梁的力学性能，联合应用高性能落锤试验系统、四点弯曲实验装置与装配式电炉开展了4根SFRC梁的冲击实验与高温恒载实验，观察了其破坏模式并记录了跨中位移和钢筋应变的时程曲线，探讨了冲击损伤SFRC梁的抗火性能。此外，在实验研究的基础上，考虑材料的应变率强化效应及温度软化效应，建立数值模型，首先对梁进行冲击加载模拟，并以冲击模拟结果为初始状态，采用热-力“顺序”耦合方法，对冲击加载与高温恒载联合作用下SFRC梁的力学行为进行了三维宏观有限元数值模拟。同时，考虑混凝土内部结构非均质性的影响，采用类似步骤，开展了细观模拟。宏/细观模拟结果与实验结果的良好吻合验证了本文数值方法的合理性与有效性，并体现了细观方法的优越性。研究发现，冲击能量较小时，SFRC梁在冲击荷载作用下，尽管局部混凝土开裂，梁整体残余变形较小，抗火性能有一定程度的下降；随着钢纤维掺量增大，混凝土基体抗剪强度增大，SFRC梁在冲击荷载作用下的开裂形态由弯剪裂缝并存向以弯曲裂缝为主转变；冲击损伤SFRC梁在高温恒载作用下裂缝分布较为集中，且发生脆性破坏。Abstract: To explore the mechanical behavior of SFRC beams subjected to both impact and fire loadings, 4 beams were tested with high-performance drop-weight test system, four point bending test machine and assembled electric furnace. The beams were firstly subjected to impact loadings and then exposed to fire with a constant load. During the test process, the crack patterns of beams were observed while the time histories of mid-span deflections and rebar strain were recorded. Then, the fire resistance of these beams was discussed. Based on the experiment, three-dimensional macroscopic finite element numerical model considering the effects of strain rate and high temperature was established. The impact loading process was simulated firstly; and then taking simulation results as the initial state, SFRC beams subjected to both fire and constant loading were simulated with a sequentially coupled thermal-stress analysis method. Moreover, considering the heterogeneity of concrete’s internal structure, a meso-scale simulation was also conducted with the procedures similar to that in macroscopic simulation. Good agreement between both the macro-/meso-scale simulation results and the test results illustrates the rationality and effectiveness of the present numerical analysis methods. The advantages of mesoscopic model were indicated through the comparison of macro-/meso-scopic results. It has been found that when the impact energy is low, the local concrete is cracked but a small overall deformation is remained. Nevertheless, this degrades the fire resistance of SFRC beams to some ex-tent. When the steel fiber dosage increases, resulting in an increasing shear strength of concrete matrix, the coexistence phenomenon of bending and shear cracks of beams under the impact load is changed to bending cracks as a dominant. Moreover, when subjected to elevated temperatures with a constant load, the distribution of cracks on the impact-damaged SFRC beams is relatively concentrated and a brittle failure occurs.
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图 1 SFRC梁尺寸及配筋情况（单位：mm）
Figure 1. Dimensions and reinforcement layout of SFRC beams (unit: mm)
图 4 冲击与火荷载作用后SFRC梁的开裂形态
Figure 4. Crack patterns of the SFRC beams after impact loading and fire exposure
图 5 冲击荷载作用下SFRC梁跨中挠度时程曲线
Figure 5. Mid-span deflection-time curves of the SFRC beams under impact loading
图 6 跨中挠度最大时SFRC梁的挠度分布
Figure 6. Deflection distribution along the SFRC beams when the maximum mid-span displacement occurs
图 9 冲击荷载作用下纵筋应变时程
Figure 9. Time histories of strains within the longitudinal rebars during the impact loading process
图 11 静态预加载下SFRC梁荷载-位移曲线
Figure 11. Load-displacement curves of SFRC beams under static pre-loading
图 12 火灾作用下冲击损伤SFRC梁的位移时程
Figure 12. Deflections of impact-damaged SFRC beams subjected to fire
图 14 高温下材料力学性能软化曲线
Figure 14. Degradation of material mechanical properties at elevated temperatures
图 15 数值模拟B-2梁破坏模式与实验结果的对比
Figure 15. Comparison of the simulated failure patterns with experiment observations for B-2
图 18 高温下B-2梁跨中位移时程曲线模拟值与实验值对比
Figure 18. Comparison of the simulated mid-span deflections with the experimental values for B-2 at high temperature
图 16 冲击作用下B-2梁跨中位移时程模拟值与实验值对比
Figure 16. Comparison of the simulated mid-span deflections with the experimental ones for B-2 under impact
表 1 钢纤维混凝土配合比
Table 1. Mix proportions of the steel fiber concrete
编号 钢纤维体积分数/% 体积质量/(kg·m−3) 抗压强度/ MPa 钢纤维 水 水泥 砂 粗骨料 减水剂 B-0 0 0 154 425 672 1 096 3.6 41.90 B-1 1 78 164 547 696 1 044 8.2 42.47 B-2 2 156 180 600 668 1 002 9.0 52.18 B-3 3 234 196 653 640 960 9.8 60.34 
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表 2 钢纤维物理性质
Table 2. Physical properties of the steel fibers
长度/mm 直径/mm 长径比 抗拉强度/MPa 密度/(kg·m−3) 30 0.6 50 1 100 7 800
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