Strain-hardening cementitious composites (SHCC) are materials reinforced with short random fibers, which exhibit tensile hardening behavior up to several percent strain accompanied by the formation of fine multiple cracks. This thesis investigates the mechanism of multiple cracking behavior from different aspects and will carry out a systematic study of the strain-hardening mechanisms in SHCC to better understand its design.

To achieve multiple cracking, steady-state cracking criterion should be satisfied so it is possible for the fibers bridging the crack to sustain a higher stress than that corresponding to first cracking of the composite. After first cracking occurs, the fiber crossing the cracked plane will continue to debond while carrying increasing loading. After the debonding...[ Read more ]

Strain-hardening cementitious composites (SHCC) are materials reinforced with short random fibers, which exhibit tensile hardening behavior up to several percent strain accompanied by the formation of fine multiple cracks. This thesis investigates the mechanism of multiple cracking behavior from different aspects and will carry out a systematic study of the strain-hardening mechanisms in SHCC to better understand its design.

To achieve multiple cracking, steady-state cracking criterion should be satisfied so it is possible for the fibers bridging the crack to sustain a higher stress than that corresponding to first cracking of the composite. After first cracking occurs, the fiber crossing the cracked plane will continue to debond while carrying increasing loading. After the debonding process is completed, the fibers will be pulled out from the matrix if they are strong enough to remain intact. In this study, experimental investigation will first be conducted on the behavior of single fiber pull-out, which is fundamental and crucial for SHCC research. The distribution of matrix strength is another key factor that influences the formation of multiple cracks and it is strongly dependent on the distribution of flaws. X-ray computed tomography technology is therefore adopted to correlate matrix strength with flaw size and shape. Experimental research is then performed at a larger scale, to study the bridging stress-crack opening relation of multiple cracks in a tensile test. A new approach to detect cracks and monitor their openings using image processing has been developed. The approach can automatically eliminate noises using a double-threshold algorithm and separate individual cracks from each other. The development of crack patterns during loading can then be tracked. More importantly, from a single tensile test, the bridging stress-crack opening relation for a large number of cracks can be obtained to provide statistical information relevant to both modeling and practical durability design.

In addition to the experimental studies, based on the mechanism of stress transfer, an analytic model and a numerical model to predict the cracking process and stress-strain relation of SHCC are developed separately. The analytic model takes into consideration the strength variation along the member, the increase of crack bridging stress in the hardening regime as well as the possibility of fiber rupture. Furthermore, the numerical model which is based on the stress field in matrix accounts for the slip hardening behavior and adopts a more reasonable description of matrix strength distribution. With material parameters from literature, the simulated stress-strain relation can be well simulated for different fiber fractions.

The above works can help the design and optimization of SHCC material regarding its mechanical properties. Based on this research, constitutive micromechanics parameters can be tailored in order to maximize structural performance of elements.