| Summary: | The role of fibers in the enhancement of the inherently low tensile stress and strain
capacities of fiber reinforced cementitious composites (FRC) has been addressed through
both the phenomenological, using concepts of continuum damage mechanics, and micromechanical
approaches leading to the development of a closing pressure that could be
used in a cohesive crack analysis. The observed enhancements in the matrix behavior is
assumed to be related to the ability of the material to transfer stress across cracks.
In the micromechanics approach, this is modeled by the introduction of a nonlinear
closing pressure at the crack lips. Due to the different nature of cracking in the pre-peak
and post peak regimes, two different micro-mechanical models of the cohesive pressure
have been proposed, one for the strain hardening stage and another for the strain
softening regime. This cohesive pressure is subsequently incorporated into a finite
element code so that a nonlinear fracture analysis can be carried out. On top of the fact
that a direct fracture analysis has been performed to predict the response of some FRC
structural elements, a numerical procedure for the homogenization of FRC materials has
been proposed. In this latter approach, a link is established between the cracking taking
place at the meso-scale and its mechanical characteristics as represented by the Young's
modulus. A parametric study has been carried out to investigate the effect of crack
patterning and fiber volume fractions on the overall Young's modulus and the
thermodynamic force associated with the tensorial damage variable.
After showing the usefulness and power of phenomenological continuum damage
mechanics (PCDM) in the prediction of FRC materials' response to a stimuli (loading), a
combined PCDM – NLFM¹ approach is proposed to model (predict, forecast) the complete
response of the composite up to failure. Based on experimental observations, this
approach assumes that damage mechanics which predicts a diffused damage is more
appropriate in the pre-peak regime whereas, NLFM is more suitable in the post-peak stage where the opening and propagation of a major crack will control the response of the
material and not a deformation in a continuum sense as opposed to the pre-cracking zone.
Tensile and compressive tests have been carried out for the sole purpose of calibrating the
constitutive models proposed and/or developed in this thesis for FRC materials. The
suitability of the models in predicting the response of different structural members has
been performed by comparing the models' forecasts with experimental results carried out
by the author, as well as experimental results from the literature. The different models
proposed in this thesis have the possibility to account for the presence of fibers in the
matrix, and give fairly good results for both high fiber volume fractions (v[sub f] > 2%) and
low fiber volume fractions (v[sub f] < 2%). Use of interface elements in a finite element code
has been shown to be a powerful tool in analyzing the behavior of concrete substrate-
FRC repair materials by the introduction of a zero thickness layer of interface elements
to account for the interface properties which usually control the effectiveness of the
repair material. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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