循环冲击作用下冻融混凝土–砂岩组合体力学特性试验与分析

Test and analysis of mechanical properties of freeze–thaw damaged concrete–sandstone composite under cyclic impact

摘要

以寒区隧道开挖过程中衬砌结构依次承受冻融和循环动力扰动为工程背景，开展了冻融后混凝土–砂岩组合体的循环冲击压缩试验，结合 NMR 和 SEM 测试手段，研究了砂岩层厚度比和冻融次数对混凝土–砂岩组合体孔径分布、应力波波形、峰值应力、抗循环冲击次数、破坏形态和微观形貌的影响，基于纵波波速的变化定义了考虑冻融和循环冲击影响的复合损伤变量. 结果表明，组合体和砂岩试样的抗循环冲击次数随冻融次数的增加呈不同程度衰减. 随着砂岩层厚度比的增加，组合体试样微孔数量减少、中孔数量显著上升、峰值应力增大；随着冻融次数的增加，试样峰值应力呈降低趋势，对于 C–R–25% 和 C–R–75% 试样，未冻融时的峰值应力分别为 25.82 MPa 和 27.87 MPa，而经历 40 次冻融后降幅分别为 26.26% 和 28.99%. 循环冲击次数的增加会导致组合体试样峰值应力和割线模量降低、峰值应变增大. 随着冻融次数增加，组合体试样破坏形态由单一主裂纹逐渐演化为多裂纹，砂岩层和混凝土层内部损伤加剧，砂岩层内出现明显的沿晶裂纹和穿晶裂纹，但两者的界面未出现明显破坏. 混凝土、砂岩、混凝土–砂岩组合体试样的“损伤累积阈值”均随冻融次数的增加逐渐增大.

Abstract

Considering the engineering context in which lining structures in cold regions are successively exposed to freeze–thaw (F–T) cycles and cyclic dynamic disturbances during tunnel excavation, cylindrical specimens with a diameter of 50 mm were fabricated. These included pure sandstone, pure concrete, and composite specimens with sandstone layer thickness ratios of 25%, 50%, and 75%, respectively. Thereafter, constant-amplitude cyclic impact compression tests of the F–T-damaged concrete-sandstone combination were conducted using a splitting Hopkinson pressure bar system with a diameter of 50 mm. The effects of the sandstone layer thickness ratio and number of F–T cycles (0, 5, 10, 20, and 40) on the pore size distribution, stress wave characteristics (i.e., incident, reflected, and transmitted stress waves), dynamic peak stress, anti-cyclic impact times, failure mode, and micromorphology of concrete–sandstone composite specimens were investigated by integrating nuclear magnetic resonance (NMR) and scanning electron microscopy. A composite damage variable, which could consider the effects of the F–T cycle and cyclic impact, was defined based on the variation of the longitudinal wave velocity in the present study. Experimental results showed that with increasing number of F–T cycles, a notable divergence emerged in the cyclic impact resistance among the different specimen types. Specifically, the C–0% specimens maintained their original impact resistance even after 40 cycles, while the anti-cyclic impact times of both the composite and sandstone specimens exhibited varying degrees of degradation with increasing number of F–T cycles. As the sandstone layer thickness ratio increased, the number of micropores in the composite specimens decreased, while the number of mesopores increased significantly, accompanied by an increase in the dynamic peak stress. Conversely, the dynamic peak stress of specimens decreased with increasing number of F–T cycles. For the C–R–25% and C–R–75% specimens, the dynamic peak stress values without F–T treatment were 25.82 MPa and 27.87 MPa, respectively. After 40 F–T cycles, these values decreased to 19.04 MPa and 19.79 MPa, representing reductions of 26.26% and 28.99%, respectively. With the increase in the cyclic impact times, both the dynamic peak stress and dynamic secant modulus of the composite specimens decreased, while the dynamic peak strain increased. The amplitude of the reflected wave displayed an upward trend in concrete, sandstone, and concrete-sandstone composite specimens with increasing cyclic impact times, while the amplitude of the transmitted wave decreased and the occurrence of the peak value was delayed. Following cyclic impact loading, the concrete, sandstone, and composite specimens primarily exhibited two failure modes: tensile splitting and edge shear failure. The quantity and distribution of primary and secondary cracks were closely related to the sandstone layer thickness ratio within the composite specimens and number of F–T cycles. As the number of F–T cycles increased, the failure mode of the composite specimens evolved from a single main crack to multiple cracks, and the internal damage intensified within both the sandstone and concrete layers. Furthermore, distinct intergranular and transgranular cracks were observed within the sandstone layer, although no obvious damage was detected at the interface between the two layers. The “damage accumulation threshold” of concrete, sandstone, and concrete-sandstone composite specimens gradually increased with the number of F–T cycles. The findings of this study provide experimental evidence and references for analyzing the stability and durability of tunnel lining structures in cold regions.

关键词

循环冲击 / 冻融循环 / 混凝土–砂岩组合体 / 破坏形态 / 微观形貌

Key words

cyclic impact / freeze-thaw cycle / concrete-sandstone composite / failure mode / micromorphology

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基金资助

安徽理工大学青年科技基金资助项目(2024JBQN0004)