Synergistic integration of waste fibres and supplementary cementitious materials to enhance sustainability of ultra-high-performance concrete (UHPC)

Efforts to improve the environmental sustainability of the construction industry involve exploring waste materials, environmentally friendly production processes and high performance materials like ultra-high-performance concrete (UHPC). Despite UHPC's benefits, its widespread use is hampered by its high cost and environmental concerns. To address this, limestone filler and metakaolin were used to replace 30% of the cement, while recycled tyre steel fibres (RTSF) and production waste carbon fibres (CF) replaced the conventional factory-made fibres. Eight mixes underwent various tests in fresh state and hardened state, revealing initial negative effects of supplementary cementitious materials (SCMs) on strength and capillary absorption, which improved considerably over time. Fibres addition, especially hybrid fibres, increased strength and toughness without significantly affecting modulus of elasticity. RTSF at 1 vol% and 1.5 vol% positively impacted compressive strength and toughness, while 1 vol% CF improved flexural strength and capillary water absorption. Optimal contents varied for the hybrid fibre mixes depending on the tested property, with overall best results in compressive strength, toughness and specific energy absorption. The hybrid mixes outperformed the RTSF mixes but lagged behind the CF mixes in terms of flexural strength and capillary water absorption. A multi-criteria analysis was conducted to determine the optimum mix design, taking into account factors such as performance, cost, and environmental impact. This analysis highlighted the potential of the supplementary materials and fibres to improve the environmental and economic efficiency of UHPC while maintaining or even improving the tested properties.