Long-Term Deformation and Failure of Concrete and Shale Structures

RAHIMI-AGHDAM, SAEED

doi:https://doi.org/10.21985/n2-h60y-nn40

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Long-Term Deformation and Failure of Concrete and Shale Structures

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This thesis deals with modeling long-term deformation and failure of concrete and shale structures. First concrete structures are studied. For concrete structures, design lifetime of over a hundred years is required. Shrinkage, swelling, creep and Alkali-silica reaction are significant parts of the long-term deformation of concrete and are studied here. The aim is developing a comprehensive, physically based and computationally inexpensive model that be able to predict the deformation of concrete structures in all environmental conditions from hours after casting till several hundred years after that. This comprehensive model is lacking in literature and is necessary for designing sustainable structures. The long-term deformations of concrete structures highly depend on hydration reaction evolution and drying of concrete. Thus, predictive and computationally inexpensive models for the hydration reaction and drying process of concrete structures are required. These models are lacking in the literature. Therefore, first, a new hydration model is developed which is able to predict the hydration evolution of concrete structures for hundred-year lifespan and beyond. Next, a new method is proposed that is able to predict the humidity evolution of concrete structures during the drying process. Computationally, these models are sufficiently inexpensive to be used in finite element simulating of concrete structures. These models later are used to predict long-term deformations. To predict autogenous shrinkage and swelling, a new paradigm is proposed. The hydration process causes permanent volume expansion of cement paste as a whole due to the growth of C-S-H shells around anhydrous cement grains. In addition, a new thermodynamic formulation of unsaturated poromechanics with capillary and adsorption is proposed. Later, to predict creep, microprestress-solidification theory (MPS) is modified. the original MPS theory predicts incorrectly the diffusion size effect on drying creep and the delay of drying creep behind drying shrinkage. Presented here is an extension named XMPS that overcomes both problems and also improves a few other features of the model response. To this end, different nanoscale and macroscale viscosities are distinguished. Finally, to predict the deformation and damage induced by ASR, a new diffusion-based and creep-based chemo-mechanical model is developed. Comparisons with the existing relevant experimental evidence validate all the proposed models. In the last chapters, shale deformation is studied. shale is the main constituents of the unconventional oil reservoirs. Hydraulic Fracturing technology, aka fracking, is the technology used to extract oil from these unconventional reservoirs. This technology has become highly developed and astonishingly successful recently and made USA energy independent. However, a consistent formulation of the associated fracture mechanics that would not conflict with some observations is still unavailable. The main issue is the significantly higher permeability of reservoirs respect to intact shale. This discrepancy currently is attributed to widely open natural fractures. In this study, first, we show these natural fractures should be closed due to creep and calcification and later we show the branching of hydraulic cracks due to the presence of weak layers is the main reason for significantly higher permeability of reservoirs.

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