| This dissertation presents the investigation results on fiber/cement bond property modification in relation to interfacial microstructure. The fiber/matrix interfacial bond properties generally refer to the elastic bond between fiber and matrix, fracture energy of the fiber/matrix interface, interfacial friction, or the characteristics of the fiber/cement interfacial microstructure. Improving fiber/matrix bond properties is a key to optimize the composite performance. The focus of this study is on the evaluation of the effects of transition zone densification and fiber surface modification on bond enhancement, the age effect on bond property, and the mechanism of interfacial debonding. The results provide insights into various bond enhancing mechanisms and guidelines for bond property tailoring.; The issue of different debond modes, strength-governed or fracture-governed, is first examined. The debond mode is a material property and has to be identified by material testing. A concept of effective bond strength able to replace the fracture-based bond property, the interfacial fracture toughness, conditionally, is proposed. This theoretical concept allows the evaluation of the debond mode for any given fiber/matrix system. The experimental study carried out for steel and brass fibers with different diameters indicate that these systems exhibit a strength based debond mode.; The nature of the transition zone between fiber and cement matrix suggests various bond tailoring mechanisms, including transition zone densification and fiber surface modification. The investigation of transition zone densification in this study suggests that this mechanism would be effective in bond improvement only for the material systems with high fiber/matrix adhesion. One example has been brass fiber/cement matrix, as reported in this dissertation. For material systems with low fiber/matrix adhesion, such as most synthetic fibers, fiber surface modification is necessary. The bond properties of plasma treated polymeric fibers are examined in this study. It is found that significant improvement on the bond property using plasma treatment to modify the surface condition of synthetic fibers can be achieved. Different bond failure types, adhesive and cohesive, are identified and both types of bond failure have important implication on fiber debonding and interfacial bond property.; One of the important characteristics of cement-based materials--aging effect, is also studied in this work. The time-dependency on the interfacial bond and the formation of transition zone microstructure of two polymeric fiber systems are examined. It is found that the formation of fiber contact layer in the transition zone is mostly responsible for the bond strength development. The characteristics of time-dependency of interfacial bond, however, is expected to be different in the material systems with cohesive type of bond failure. The early development of bond strength is in favor of the strain-hardening behavior of a composite in early age. |
