A Multiscale Approach to Model Thermo-Hydro-Mechanical Behaviour of non-reinforced Concrete

Cementitious materials are the main pillar of modern construction and urbanization. With their endless practical applications and diversity of utilization from small village houses to skyscrapers, power plants and even nuclear waste disposal structures, they can be seen everywhere. The main driver for this study was to investigate the thermo-hydro-mechanical (THM) behaviour of cementitious engineered barriers, in particular, the barrier for high level nuclear waste containers considered in the Belgian deep geological disposal program. The principal objective of this study is to investigate the THM behavior of concrete within a multiscale framework. More importantly, to be able to predict fundamental properties of concrete based on its composition to enable optimization of its design. This, however, cannot be achieved without an in-depth study of phenomena and parameters, which are affecting the macro-behavior of the concrete. Since such barriers are exposed to thermal loading emanating from decay of the high level waste, a range of coupled processes usually referred to as THM processes are involved in their performance. Therefore, this thesis proposes a stepwise, multi-component and multiscale framework to model THM behavior of cementitious materials starting from microstructural modelling by representing microstructure of the material based on its chemical composition and reaction condition (curing, age, temperature, etc.). This modelling tool is then coupled with an algorithmic scheme adapted to convert such microstructure to a representative pore network and simulate transport properties. Regarding the mechanical and thermal properties, including elastic modulus, coefficient of thermal expansion and heat conduction coefficient a micromechanical scheme has been implemented by means of numerical homogenization using Cast3m, where the cement microstructure is once again the main input parameter. Subsequent to the development of such hydro-micromechanical framework (pore network and micromechanical modelling), drying shrinkage was the first aspect to be addressed as an important and a potentially detrimental process for long-term performance of concrete structures. Hereby, the drying shrinkage of cementitious materials is computed using a novel and multi-mechanism drying shrinkage model. This model takes into account three individual mechanisms responsible for drying shrinkage of cementitious materials: capillary force, surface free energy and disjoining pressure. In addition, a detailed stress analysis on microstructural level is carried out, which highlights the importance of such studies. Finally, a multiscale and microstructure-informed THM simulation of an engineered barrier for high level nuclear waste container is carried out, where the material parameters are derived using the aforementioned framework (hydro-mechanical). The main objective of this application is to identify spatial regions of the engineered barrier that are prone to crack formation and propagation due to evolving thermal load and its consequences to hydro-mechanical behaviour of the barrier.