Usage
  • 216 views
  • 436 downloads

The Fractal Nature of Cement-Based Systems Under Sustained Elevated Temperatures

  • Author / Creator
    Mamun, Muhammad
  • Macro-mechanical behaviour of a material in response to applied loads is governed by its microstructural characteristics. The relative volume fraction of the constituent phases, their connectivity and interface are some of the microstructural features that have significant impact on the load carrying capacity of the material. During its service life, a structure may be exposed to extreme environmental actions, such as elevated temperatures, causing hygro-thermal degradations to the material. The severity of this deterioration depends largely on the nature of the source of exposure, the rate of temperature rise, the density of the material, and its moisture content. Cement-based materials are particularly susceptible to damage upon exposure to sustained elevated temperatures. The physical and chemical aspect of the properties of hydrated cement paste are the major sources of thermal damage. Furthermore, the thermally driven coupled heat and mass transport phenomena is controlled by the prevailing pore structure that is formed during cement hydration. In order to understand the behaviour of cement-based systems under sustained elevated temperatures, it is essential to examine the evolution of the microstructural parameters with an increase in the temperature and their impact on the thermal and mechanical response of the material. Notwithstanding the extensive literature on the behaviour of cement-based composites under elevated temperature, further investigations were deemed necessary to explain the mechanisms of the deterioration. Accordingly, this thesis examines the evolving microstructure and models the thermal properties using a fractal approach for sustained temperatures up to 600 °C at a heating rate of 5 °C/minute. In turn, the fracture mechanical behaviour and mechanical response are predicted. To this end, several state-of-the-art experimental methods were employed in this study. These include the Transient Plane Source method of thermal analyses per ISO, Backscattered Electron Imaging, Computed Micro-Tomography, and fracture mechanical tests. The experimental results were analyzed and discussed using appropriate modelling approaches. The findings show that the pore volume fraction increases with an increase in the temperature of exposure across all mortar mixtures. In addition, the fibre reinforced mortar shows a marginally lower porosity than the plain ones. However, it is essential to note that the lowering of porosity does not necessarily indicate a reduction in pore connectivity, which is the dominant factor in terms of the efficiency of polymeric fibres against spalling. The two-point correlation function, a morphological image processing technique, shows that the presence of monotonic nature of the asymptotic end of the function indicates a spatially uncorrelated nature of the pore structure beyond the distance associated with characteristic pore radius. Further, the thermal exposure results in a coarsening effect in capillary pores as well as in the meso pores. Additional image processing shows that the box-count based fractal dimension of the mortar increases with temperature, indicating an increasingly rougher pore perimeter. The three-dimensional pore fractal dimension, estimated through the computed microtomography, exhibits a space filling nature of the thermally degraded microstructure of the mortar. In addition, the change in fractal dimension with temperature implies an increase in compliance. Investigations regarding the fracture mechanical behaviour reveal that the crack growth resistance of the heated mortar drops with an increase in the temperature of exposure. In addition, according to the R-curve response of cement mortar—based on both the effective crack length and the crack tip opening displacement—the crack-growth resistance of the mortar approaches a constant value at elevated temperatures, which is characteristic of brittle materials. These findings are expected to broaden the knowledge base on the behaviour of cement-based composites at elevated temperature. In other words, the information generated through this thesis will provide guidelines regarding the development of mixture proportioning process, where the selection of a composite reinforced with moderate dosage of polymeric microfibre will be capable of absorbing higher fracture energy and render a more durable structure. Further, the fractal-based model can be used to study the temperature distribution within any cement-based system, which will enable the prediction of fire resistance rating. It is also expected that the discussion presented in this thesis on the characteristics of cement mortar will facilitate the development of cement-based insulating materials for various components of a structure.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R3C24R33C
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.