Post-fire Mechanical Behaviour of Concrete Reinforced with Phragmites Australis Fibres

Abstract The use of natural fibres as concrete reinforcement has gained increasing attention due to their environmental benefits, renewability, and cost-effectiveness. This study investigates the post-fire mechanical behaviour of concrete reinforced with Phragmites australis fibres (PAF), a locally available and sustainable material. Concrete mixes containing 0%, 0.5%, 1%, 1.5%, and 2% fibre volume fractions were exposed to temperatures up to 600 °C for 2 h to simulate fire conditions. The effects of fibre content and temperature were assessed through measurements of slump, compressive strength, splitting tensile strength, flexural strength, and ultrasonic pulse velocity (UPV). Results showed that PAF-reinforced concrete retained considerable strength and integrity up to 400 °C, with a pronounced reduction observed at 600 °C due to fibre degradation and microcrack propagation. Regression models developed from the experimental data demonstrated strong correlations between fibre dosage, temperature, and residual mechanical properties. The analysis identified an optimal fibre content between 0.5% and 1%, which provided the best balance between thermal stability and mechanical performance. These findings highlight the potential of Phragmites australis fibres as an effective and sustainable reinforcement for improving the fire resistance of concrete structures.