Stain-Hardening Cementitious Composites (SHCC) is a group of cement-based material which exhibits a strain hardening stage in the direct tension test. In this stage, multiple cracking behavior can be observed which makes SHCC a pseudo-ductile material. This thesis investigates the micromechanical modeling process of SHCC under direct tension at multi-scale levels, i.e. from single fiber to single crack, and then further to the composite behavior.

At the single fiber scale, a fiber crossing a crack can be either perpendicular or inclined to the crack surface. For the inclined situation, the peak pullout force is experimentally observed to increase with the inclination angle. Usually the fiber is modeled as a flexible string passing over a frictional pulley, and the observed phenomenon i...[ Read more ]

Stain-Hardening Cementitious Composites (SHCC) is a group of cement-based material which exhibits a strain hardening stage in the direct tension test. In this stage, multiple cracking behavior can be observed which makes SHCC a pseudo-ductile material. This thesis investigates the micromechanical modeling process of SHCC under direct tension at multi-scale levels, i.e. from single fiber to single crack, and then further to the composite behavior.

At the single fiber scale, a fiber crossing a crack can be either perpendicular or inclined to the crack surface. For the inclined situation, the peak pullout force is experimentally observed to increase with the inclination angle. Usually the fiber is modeled as a flexible string passing over a frictional pulley, and the observed phenomenon is explained by the high local friction (or snubbing effect) near the fiber exit matrix point. However, such an explanation is not perfectly satisfactory because the bending stiffness of the fiber is not necessarily negligible and the pulley does not physically exist. Therefore, two new models which treat the fiber as a beam are developed. One emphasizes on the large deflection effect of the fiber and is physically more accurate, while the other one is based on the small deflection theory and is computationally more efficient (hence has good potential for practical application in the micromechanics-based design of SHCC). Through these two models, the entire fiber bridging stress vs crack opening relationship can be derived and the empirical snubbing effect is also revealed, even without assuming the presence of a frictional pulley. Moreover, the accuracy of the new modeling results under small crack opening, which is the operating condition for SHCC, are better than the existing frictional pulley model. Single fiber pullout tests have been conducted to verify the model directly, and good agreement was found between the model prediction and experimental results. The effects, such as fiber/matrix stiffness ratio, shear deflection effect, matrix spalling effect, are then studied based on the new model.

The new model at the single fiber scale is then used for scaling up modeling of SHCC. The single crack bridging stress vs crack opening relation can be derived through integrating the single fiber bridging behavior, and the single crack bridging behavior can be used to model the composite behavior. Different from existing scaling up modeling process that neglects the bending stiffness of fibers, the fundamental of the new scaling up modeling is to treat the fibers as beams so their bending stiffness is taken into account. Through the comparison with the test results at different scale levels, it was found the new modeling approach agrees better with the test data, which provides another direction for the scaling up modeling of SHCC.