Fire or high temperature is a big challenge to ultra-high performance concrete (UHPC). Strength loss of UHPCs can reach up to 80% after exposure to 800 °C. In this study, a total of six UHPC mixtures were designed and tested after subjected to elevated temperatures up to 1000 °C. 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 since UHPCs are usually of compact structure and thus more vulnerable to explosive spalling than other concretes. Scanning electron microscope (SEM) analysis was conducted to help understand the mechanism of variation of internal structure under different temperatures. It was found the mixture containing steel slag and hybrid 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%.
Based on Delaunay triangulation, a 3D meso-scale model is successfully developed and verified. 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 paper proposes a model to achieve relatively high computation efficiency, as well as accuracy. Besides, the model has the potential to deal with small specimens cut from steel fibre reinforced concrete (SFRC) blocks.
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