Experimental study of UHPC with high fire resistance and meso-scale modelling

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Despite the use of state-of-the-art technology and materials, modern buildings are still vulnerable to fire. Damage made to ultra-high performance concrete (UHPC) by fire or high temperature is usually severer than normal strength concrete (NSC) or high performance concrete (HPC) due to its compact internal structure. For example, strength loss of UHPCs can reach up to 80% after exposure to 800 °C and explosive spalling is a common disaster to UHPCs. To develop a UHPC with high fire resistance, a total of six UHPC mixtures were designed and tested after subjected to elevated temperatures up to 1000 °C in this study. The effects of aggregate type, fibre type and heating rate were investigated. Residual compressive strengths and stress-strain relationships were studied. Besides, attention was paid to explosive spalling. Scanning electron microscope (SEM) analysis was conducted to help understand the mechanism of variation of internal micro-structure under different temperatures. It was found the mixture containing steel slag and hybrid fibre, i.e. steel fibre and polypropylene (PP) fibre, had excellent fire resistance. After being subjected to 1000 °C, this mixture retained a residual compressive strength of 112.8 MPa or a relative value of 69%. Furthermore, to study the behaviour of the newly developed UHPC under simultaneous effect of fire and blast, both compressive and splitting tensile split Hopkinson pressure bar (SHPB) tests were carried out under combined action of high temperatures up to 800 °C and impact loading. The dynamic tests were done both under high temperatures (hot test) and after cooling down (cool test) and comparisons were made between the two scenarios. Based on the tests on this UHPC, mechanic and physical characteristics under the combined effect were studied. Besides, explosive spalling was observed in the tests and analysed in this work. It was interesting to find PP fibre could play a negative role in preventing explosive spalling between 320 and 380 °C. To investigate the effect of steel fibre on thermal conductivity of steel fibre reinforced concrete (SFRC) (including UHPC), a meso-scale model for heat analysis was developed. Delaunay triangulation was employed to generate the unstructured mesh for SFRC materials. The model was validated using existing experimental data. Then, it was used to study how model thickness affected simulation outcomes of thermal conductivity of models with different fibre lengths, by which an appropriate thickness was determined for the later analyses. The validated and optimised model was applied to study of relationships between thermal conductivity and factors such as fibre content, fibre aspect ratio and different parts of an SFRC block by conducting steady-state heat analyses with the finite element analysis (FEA) software ANSYS. The simulation results reveal that presence of steel fibres has an obvious impact on the distribution of temperature and heat flux vector of the SFRC blocks. Besides, fibre content improves thermal conductivity considerably, while fibre aspect ratio only has an insignificant effect. Based on the Delaunay triangulation meshing method applied above for thermal analysis, a 3D meso-scale model for mechanical analysis of SFRCs is also successfully developed and verified, which has the potential to more accurately simulate behaviour of SFRCs under elevated temperatures in the future. This approach modelling fibre and concrete separately and linking them with slide line contact has the capability to truly reflect the interfacial behaviour of fibre and mortar, and thus achieve high fidelity of numerical simulations. However, meso-scale modelling usually means tremendous complexity and long computational time. This study proposes a model to achieve relatively high computation efficiency, as well as accuracy. Besides, the model has the ability to deal with small specimens cut from SFRC blocks.
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