Effects of hybrid recycled polypropylene and natural kenaf fibres on spalling prevention of ultra-high performance concrete at elevated temperatures

The diverse thermal responses of polymer and natural fibres have often been exploited to mitigate explosive spalling of ultra-high performance concrete (UHPC) in the literature. The effectiveness of these fibres is largely attributed to their ability to increase permeability – an important factor that governs the spalling behaviour of UHPC at elevated temperatures. With an increasing emphasis on sustainable solutions for advanced building materials, this study investigates the combined effects of recycled polypropylene (PP) fibres and natural kenaf fibres on the workability, spalling behaviour, residual permeability, compressive and flexural strengths of UHPC exposed to elevated temperatures. In addition, thermal analyses and microscopic observations were performed to elucidate the mechanisms behind the influence of hybrid fibres on the spalling behaviour and material properties of UHPC at elevated temperatures. The test results gathered from sixteen UHPC mixes containing different recycled PP (0, 2, 4 and 6 kg/m³) and kenaf fibre (0, 4, 8 and 12 kg/m³) contents showed that explosive spalling of UHPC could be completely mitigated with low contents of hybridised fibres. This effective approach was due to the complementary actions of these two fibres: shrinkage of kenaf fibres forming cavities and fissures, and expansion and melting of PP fibres creating microcracks and empty channels. These synergistic effects significantly enhanced permeability, facilitating vapour migration at high temperatures, while the hybrid fibres also improved spalling resistance, preserving residual compressive and flexural strengths of UHPC. Overall, an optimal hybrid fibre combination of 6 kg/m³ of recycled PP fibres and 4 kg/m³ of kenaf fibres was recommended, striking a balance between workability in the fresh state and spalling resistance at elevated temperatures.