Microscale failure analysis of the ultra-high-performance polypropylene fibre reinforced concrete panel subjected to high thermal loading induced by fire exposure

To date, many buildings are prone to fire from non-structure systems like high-pressure laminates and insulated cladding systems. Ultra-high-performance concrete (UHPC) has been applied to the cladding systems due to its excellent mechanical properties at room temperature and non-flammable characteristic. To improve its mechanical properties under fire condition, different fibres like PP fibre have been employed to reinforce UHPC. The tensile strengths of these ultra-high-performance PP fibre reinforced concrete (UHPPFRC) panels vary due to the different orientations of internal PP fibres. However, few studies have been carried out on the internal stress and fire resistance of these claddings. Study on understanding the fire behaviours and failure process of the UHPPFRC panel is essential for developing claddings with better fire resistance and durability. In this study, a coupled hygro-thermal-mechanical model is developed to predict the internal stress and temperature distribution within the UHPPFRC panel. The microscopic failure process of PP fibre and porosity change within the panel during heating are all embodied in the model, and its accuracy has been validated. The initial and the residual tensile strengths of the UHPPFRC panel are considered to fluctuate between two extrema due to the random fiber orientation. Based on this, Program Evaluation and Review Technique (PERT) is applied to investigate the effects of fibre dosage and heating rate on retarding crack propagation and failure of the UHPPFRC panel under high temperature. Results from the numerical investigation indicate that 0.55% dosage of PP fibre can effectively postpone crack occurrence and propagation within the UHPPFRC panel at linear elastic phase, and faster heating rate can accelerate the pore pressure generation process. Also, internal fibre orientation is a significant factor affecting the fire endurance of the UHPPFRC panel subjected to elevated temperature, which should be considered for safety and reliability evaluation of the structure.