Micromechanical modeling of fracture and fatigue behavior of cementitious composites
Abstract
The ubiquitous use of concrete as a construction material in the civil engineering industry
demands a thorough understanding of its failure process under different types of loading.
The composite nature of cementitious materials such as concrete, consisting of multiple
phases and defects existing at different length scales, gives rise to complex mechanisms which
are responsible for the nonlinear behavior observed at the macroscopic scale. The interaction
between phases and the properties of the micro constituents are important aspects that
deserve attention while developing models to describe the mechanical response of concrete.
Concepts of continuum micromechanics have been effectively used by researchers to predict
the overall behavior of composite materials. Compared to the conventional continuum models
based on fracture mechanics or damage mechanics, micromechanical approaches have the
advantage of being physically more relevant. The models include necessary information about
the microstructural attributes of a material and the actual damage mechanisms causing the
material to fail. In this thesis, the macroscopic behavior of plain concrete has been modeled
under monotonic and fatigue loads by adopting the principles of continuum micromechanics.
Damage in concrete has primarily been ascribed to the growth of microcracks. The
internal structure of concrete is characterized by the presence of numerous microcracks, even
before it is subjected to any external load. Microcracks may also develop in the material due
to separation of the coarse aggregate from the surrounding mortar matrix. The aggregatemortar
interface is often termed as the ‘weakest link’ in concrete from where damage begins
to propagate. The distributed damage caused by aggregate debonding and the various stages
of damage incurred in concrete due to propagation of interface cracks are explicitly simulated
by meso scale models. Employing elastic solutions based on fracture mechanics at the lower scale, the resultant nonlinear macroscopic behavior is obtained through homogenization.
The interface crack model is further extended to analyze the response of concrete under
fatigue loading. An energetic criterion is used to measure the extent of fatigue damage. The
different stages of damage experienced by the material under the influence of repeated cycles
of load are modeled.
In addition to the bond cracks, the cement mortar matrix also contains a number of
microcracks, whose effects on the homogenized behavior of plain concrete are studied through
a micromechanics based damage model. The crack density parameter, representing the
number of microcracks present per unit volume of the material, is used as the damage
variable. Considering an isotropic distribution of microcracks, the additional compliance due
to the presence of cracks is evaluated. The damage criterion is given by the strain energy
release rate at the meso scale. The macroscopic response is derived systematically from the
evolution of damage, i.e., growth of the matrix microcracks. The numerical model is used to
analyze the behavior of concrete for both monotonic and fatigue loads.
A specific aim of the present study is to offer a better comprehension of the effect of the
various mesoscale properties on the homogenized behavior of concrete. This is achieved by
conducting detailed parametric studies for each of the proposed numerical models under
monotonic and fatigue loads. For monotonic loading, the variation in the macroscopic
stress-strain response of concrete reflects the influence of the different mesoscopic parameters.
Numerical S-N curves are plotted to determine the effect of the different parameters involved
on the fatigue life of plain concrete. The results of this study thus improve the understanding
of the behavior of cementitious composites under monotonic and fatigue loads
Collections
- Civil Engineering (CiE) [352]