Upcycling recycled carbon fibres into hybrid limestone calcined clay cement (LC3) composites for strengthening applications

A large share of structures worldwide cannot meet current design requirements due to updated anti-seismic guidelines, traffic growth, ageing, and severe climate change. High-Performance Fibre-Reinforced Cementitious Composites (HP-FRCC) represent an effective solution for structural strengthening; however, their widespread adoption is made less attractive by their high clinker content and reliance on virgin reinforcing fibres. This study investigates the development of sustainable HP-FRCCs based on limestone calcined clay cement (LC3) incorporating recycled carbon fibres (rCFs) recovered from end-of-life sources and brass-coated steel fibres (BSFs). Two commercially available rCFs were first characterised to identify the most suitable reinforcement before developing hybrid LC3-based composites. The incorporation of rCFs increased compressive strength by up to 25% and flexural strength by more than 30% through microstructural refinement, while BSFs governed post-cracking behaviour and energy absorption. Hybridisation enabled the replacement of one third of the steel fibre content while maintaining confinement performance comparable to that of reference steel-fibre systems. Concrete cylinders strengthened with LC3-HP-FRCC jackets exhibited compressive strength increases of up to 107% compared with unconfined specimens, together with significantly improved damage tolerance. The results demonstrate that combining rCFs with BSFs and LC3 binders provides sustainable high-performance cementitious composites for structural retrofitting.