Kunjie Fan


Material properties of concrete at elevated temperatures are imperative to structural fire safety assessment. However, fire performance of concrete at high temperature is extremely complex since concrete is a heterogeneous material with considerable variations. When it is subjected to thermal exposure, such as fire, concrete undergoes a series of significant physicochemical changes. Its mechanical behaviour is not only temperature dependent, but also loading history dependent, during the heating process. Therefore, the characterization of concrete properties at elevated temperatures is very challenging and the development of reliable and accurate constitutive models remains an unaccomplished task. The main aim of this thesis can be divided into two scopes: 1) to examine the effect of using fly ash (FA) as the supplemental cementitious material (SCM) on the fire performance of concrete; 2) to investigate the mechanical behaviour of concrete under different thermomechanical conditions and obtain a better understanding of the mechanism of how the pre-fire load affects the fire performance of concrete. Finally, an advanced thermo-mechanical constitutive model for concrete under uniaxial compression at elevated temperatures is expected to be established, for applications in structural fire engineering. The model has to capture the behaviour of structural concrete with different heating-loading sequences accurately and consider the effect of using FA as SCM on the variation of the properties of concrete during thermal exposure. An apparatus is specially designed for testing “hot” mechanical properties of concrete materials with different heating-loading regimes. Through the experimental research, the mechanical properties, including compressive strength, peak strain, elastic modulus, complete stress-strain relationship and transient thermal creep (TTC) of concrete under uniaxial compression at elevated temperatures have been investigated. In the experimental programme, both conventional ordinary Portland cement (Karakurt & Topçu) based concrete and FA concrete specimens were tested to examine the difference. In addition, a novel numerical method was proposed, to quantify the effect of temperature gradient on TTC of stressed concrete in transient state tests, so that an explicit TTC model could be formulated. Through the experimental research and numerical analysis presented in this thesis, the fire performance of FA concrete was examined. It was found that 25% replacement of OPC with FA in the concrete, mitigated the deterioration of the compressive strength, the development of TTC, and the nonlinearity of stress-strain response at elevated temperatures, but hardly influenced the value of the elastic modulus and the peak strain. The applicability of Eurocode EN1992-1-2 to normal strength concrete with 25% replacement of FA as SCM has been verified to be safe. In addition, the effect of loading history during thermal exposure, on the mechanical properties of concrete at the thermal steady state, has been investigated too. This thesis contributes to a better understanding on the mechanism of how loading-heating sequences influence the fire performance of concrete materials. Finally, an advanced constitutive model for concrete at high temperature has been proposed and verified. Compared with previous models, it has the following advantages of: 1) incorporating a parameter n to consider the variation of the nonlinearity of stress-strain curves with temperature, 2) formulating the elastic modulus separately from the stress-strain curve, 3) distinguishing the stress-strain response of the stressed condition from the unstressed condition and 4) calculating TTC in a completely explicit way, by quantifying the interference caused by the thermal gradient in transient state tests. The relevant parameters in the model have been particularly calibrated for FA concrete as a recommendation for practical engineering.

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