| Engineered Cementitious Composites (ECC) are a unique class of high performance fiber reinforced cementitious composites featuring high tensile ductility and low fiber content. Material engineering of ECC is constructed on the paradigm of the relationships between material microstructures, processing, material properties, and performance, where micromechanics is highlighted as the unifying link between composite mechanical performance and material microstructure properties.; The thesis focuses on the development of a comprehensive understanding of matrix micromechanics associated with multiple cracking and strain-hardening behavior of ECC, and the application of this understanding in the design of ECC materials with multiple performance targets. In the aspects of micromechanics modeling, the cracking mechanism of ECC was investigated and a strength model was proposed. The revealed connection between pre-existing flaw size and matrix cracking provided insight for achieving saturated multiple cracking through matrix flaw size distribution control. In addition, the fiber bridging model was improved in terms of crack opening prediction by taking two physical fiber relaxation mechanisms into account.; The micromechanics models were then utilized to guide the microstructure tailoring in the development of several application oriented ECC materials, e.g. high early strength ECC, green ECC, and lightweight ECC. In these materials, in addition to high tensile ductility, one or more additional matrix-governed performance targets were selected for the prospective applications. Design for these performance targets may have a negative impact on composite ductility. Micromechanics tools facilitate an understanding of the influence of matrix composition changes on the composite behavior and identify the most critical microstructure property relevant to an individual performance target. Performance optimization was made through dedicated control of interface bond properties, flaw size distribution and matrix toughness. |
