A multi-physics-based modelling approach to predict mechanical and thermo-mechanical behaviour of cementitious composite in a multi-scale framework

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PhD; 2023; Civil engineering

Concrete is a heterogeneous material whose constituents (e.g., cement paste, aggregate etc.) range from a characteristic length-scale dimension of a nanometre to a metre. Owing to the heterogeneity of concrete and the contrasting nature of its constituent’s (cement paste, aggregate) response at ambient and high temperatures, applying a homogeneous macroscopic model to predict concrete’s mechanical and thermo-mechanical performance is questionable. Hence, in this thesis, multiple physical and chemical processes that occur within the concrete constituents at different length scales are considered, and a multi-scale model is developed to study the mechanical and thermo-mechanical behaviour of concrete in a hygral-thermal-chemical-mechanical (HTCM) framework. Firstly, the governing equations of HTCM processes are described at meso-scale, a length-scale where coarse aggregate is explicitly modelled in a binding medium called mortar. After that, a hierarchical homogenization approach is employed, and the evolution of mechanical properties etc., are upscaled (from micro to meso) and used at the meso-scale. The proposed methodology is then used to predict the evolution of mechanical properties (e.g., compressive strength) and time-dependent deformation (e.g., shrinkage and creep) of cement paste, mortar and concrete for a wide variety of factors (e.g., type and content of constituents, different curing conditions, etc.). Like ambient conditions, the developed model is used to simulate thermo-mechanical responses (e.g., in terms of spalling, deformation, residual capacity, etc.) of both plain and reinforced concrete structural elements. Further, the effect of several other meso and macroscopic parameters (e.g., interfacial transition zone, aggregate shape, random configurations of aggregates etc.) on concrete’s mechanical and thermo-mechanical behaviour is studied numerically at the meso-scale. Validation of the proposed methodology with the available experimental results at both ambient and high temperatures for a wide variety of cases highlights the general applicability of the model. It has been shown that on several occasions, existing macro, meso or multi-scale models unable to reproduce the mechanical and thermos-mechanical behaviour of concrete structures. Such limitations can be overcome with the present developed approach. Further, empiricism in several calibrated parameters in the existing thermal-hygral-mechanical macroscopic models (associated with elasticity, strength, shrinkage, and creep prediction) can be avoided by using the present developed multi-scale and multi-physics-based methodology. Similarly, simulated results at high temperatures highlight several crucial aspects related to obtaining a more precise residual capacity of a concrete structure, which is impossible to reproduce with a homogenized macroscopic model. For instance, spalling out of random concrete parts at different times during high-temperature exposure cannot be simulated with a homogenized assumption. Further, unlike macroscopic models, a mesoscopic model does not require transient creep strain to be specified explicitly in the analysis. The primary influencing mechanisms behind this transient creep strain are implicitly taken into account in the present developed meso-scale model that results in such advantages.