A Fundamental Study of the Evaluation of Toughness for Fiber Reinforced Concretes

研究了纤维与水泥基体的粘结性能对纤维增强混凝土韧性的影响。纤维与基体的结合由界面区的断裂韧性（即临界应变能释放率）和沿剥离界面的摩擦剪切应力表征。讨论了这些性能与纤维周围形成的微观结构的关系。通过显微硬度测定、BEI分析和荧光显微镜对纤维附近形成的微观结构进行了分析。本研究的主要结果总结如下：(1)基于柔度法的单纤维拉拔试验可以通过实验评估界面区临界应变能释放率。(2)纤维与胶凝基质界面区的临界应变能释放率较灵敏地反映了界面区的特征。(3)硅灰的加入和基体水灰比的降低导致界面区微观结构致密。然而，硅灰的加入并没有导致界面区断裂韧性的增加。(4)界面区断裂韧性受砂粒掺入的影响。(5)在荧光显微镜下对界面区的观察表明，粘结破坏模式并非单根纤维拉拔过程理论研究中假设的简单剪切破坏模式。(6)利用荧光显微镜对复杂的脱粘现象进行理论探讨还需要进一步的工作。

The significance of the bond properties between fibers and cement matrix in the toughness of fiber reinforced concretes was investigated. The bond of fiber with the matrix was characterized by the fracture toughness (i. e. the critical strain energy release rate) for the interfacial zone and the frictional shear stress along the debonded interface. Those properties were discussed relating them to the microstructure formed around fibers. The microstructure formed in the vicinity of fibers are elucidated by the microhardness measurements, BEI analysis and fluorescent microscopy.The major results obtained in this study are summarized as follows :(1) It was possible to evaluate experimentally the critical strain energy release rate for the interfacial zone from the single fiber pull-out test based on the compliance method.(2) The critical strain energy release rate for the interfacial zone between fibers and cementitious matrix appears to sensitively reflect the characteristics of the interfacial zone.(3) The addition of silica fume and the reduction of water/cement ratio of the matrix resulted in the dense microstructure of the interfacial zone. However, the dense interfacial zone formed by the addition of silica fume did not lead to the increase in fracture toughness for the interfacial zone.(4) Fracture toughness for the interfacial zone is affected by the incorporation of sand grains.(5) Observations for the interfacial zone under the fluorescent microscope revealed that the mode of bond failure was not a simple shear failure which has been assumed in the theoretical studies for the single fiber pull-out process.(6) Further work is needed to theoretically discuss the complex phenomena of debonding with the fluorescent microscope.