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