The influences of microstructural processes on concrete exposed to high temperatures

Abstract

Concrete is the most widely used construction material across the world. It is a heterogeneous material consisting of cement paste, fine and coarse aggregates, and it is a multi-phase porous media with dry air, liquid water, vapour and other kinds of fluids filling the pores. The hygro-thermo-chemo-mechanical behaviours of concrete under elevated temperature have been investigated for decades at the macroscale level using experimental methods and numerical simulations, including material strength, material properties, variables (e.g. pore pressure) and even microstructural properties. However, the findings are generally empirically in nature, and the mechanisms underneath all these macroscopic behaviours are still not clear. The objective of this thesis is to contribute to the understanding of the influences of microstructural processes on the macro-level behaviour of concrete at elevated temperatures through a combined numerical and experimental study. The work uses finite element analysis with a fully coupled hygro-thermo-chemo-mechanical model in combination with water vapour sorption isotherms measurements using the ‘Dynamic Vapour Sorption’ method. Numerical parametric studies have been conducted for nine properties rooted in the micro-scale. These have confirmed that: permeability is key to the development of gas pressures; the description of the heat and mass boundary conditions can have a considerable effect on the predicted results; the amount of water introduced into the system as a result of dehydration of the cement paste, the influence of micro-scale gas flow behaviour and the evolution of capillary pressures are all found to have a considerable effect on the development of macro-scale behaviours. Furthermore, the transient behaviour of moisture under elevated temperatures are found to be significantly affected by the formulation of the sorption isotherm, especially where that relates to microstructural behaviour. To further investigate this and potential contradictions between theoretical or observed microstructural behaviour and macro-scale model formulations, a series of experiments were conducted to measure water vapour sorption isotherms using the ‘Dynamic Vapour Sorption (DVS)’ method. Investigations were carried out to determine the potential microstructural changes of hardened cement pastes (CEMI with water to cement ratio of 0.4) subjected to different relative humidity ranges. The results indicate that the microstructure of cement paste is not affected by elevated temperatures until 80°C, after removing confounding effects from irreversible changes upon first drying and harsh drying. The only microstructural changes consistent with the presented results, during desorption and adsorption, appear to be reversible. The temperature dependency of sorption isotherms was investigated as well. The results confirm and extend the interpretation that the adsorption isotherm is near-equilibrium, and the desorption isotherm is not. These results are qualitatively confirmed by classical Density Functional Theory (DFT) theory. It is also confirmed that the adsorption isotherm is weakly temperature-dependent and desorption is much less temperature-dependent than from desiccator tests, where a marked increase of cavitation pressure is observed in the desorption branch with increased temperature. Additionally, the mechanism of interlayer water was investigated in this project by drying the samples to nominal RH=0%, instead of drying at 5% RH. The results suggested that the interlayer water play a significant role in the desorption range below 5% RH and the hysteresis in isotherms suggested that until the interlayer water was evaporated sufficiently, the interlayer spaces were never filled even when re-wetting to full saturation. All these results and their implications indicate the need for a revision of the models linking water content with humidity at high temperature, with possibly important implications for the understanding and prediction of temperature-induced damage in concrete. All these results indicated that microstructural processes have significant influences on the behaviours of concrete at the macroscale level when exposed to high temperature. However, these microstructural processes are still vague, which are strongly affected by the pore size distribution that needs to be explored and clarified further.