Effects of hybrid fibres on freeze–thaw durability and mechanical behaviour of ultra-high performance concrete

Freeze-thaw damage is a critical durability concern for concrete structures in cold regions, often leading to early deterioration and significant maintenance expenses. This study investigates the freeze-thaw resistance of ultra-high performance concrete (UHPC) containing different types and dosages of fibres. Effects from mono steel fibre (SF), polyvinyl alcohol (PVA) fibre at varying dosages (0%, 0.5%, 1%, 1.5%, and 2% by volume) and hybrid steel and PVA fibres (SP) in different ratios are investigated. Specimens were subjected to 100 to 500 freeze-thaw cycles with temperature range between -18 ± 2 °C and 5 ± 2 °C. The residual performance was assessed through mass loss rate, relative dynamic elastic modulus and relative compressive strength. Results indicate that both steel and PVA fibres significantly enhance the freeze-thaw resistance of UHPC, with optimal performance achieved at specific dosages. Steel fibre reinforcement showed optimal performance at 1% content, exhibiting mass loss rates of 0.34%, 0.57%, 0.65%, 1.04%, and 1.81% after 100, 200, 300, 400 and 500 cycles, respectively with corresponding relative dynamic elastic moduli of 95.84%, 92.89%, 91.28%, 86.75% and 80.97% as compared to those with 0 cycle. PVA fibres demonstrated optimal performance at 2% content. The hybrid combination with equal proportions of steel and PVA fibres (1% each) (SP1) exhibited optimal freeze-thaw resistance, maintaining a relative dynamic elastic modulus of 82.60% after 500 cycles with a mass loss rate of 1.79%. The SP1 also showed the lowest compressive strength reduction, outperforming single fibre reinforced SF2 and PF2 by 12.15% and 8.01%, respectively. Scanning electron microscopy observations confirm that freeze–thaw cycling causes microcrack propagation in UHPC.