Influence of fibre surface treatment on the performance of ultra high-performance concrete with metallic and non-metallic fibres

In ultra-high-performance concrete (UHPC), there is a need to address the challenges due to the presence of fibres on workability, dispersion and bonding with the matrix. This study addresses these concerns by applying treatments on metallic (steel) and non-metallic (glass, basalt, and carbon) fibres using Sodium Hydroxide, Potassium Dichromate, Ethylene Diamine Tetraacetic Acid, and Nitric Acid to enhance UHPC performance. Characterisation techniques such as Scanning Electron Microscopy, X-ray Diffraction, and Atomic Force Microscopy were used to evaluate the impact of these treatments. For UHPC with steel fibres, sodium hydroxide treatment improved workability by 20%, with marginal increase in compressive and flexural strength. UHPC with glass fibres exhibited over a 60% enhancement in workability at 1% fibre content, with flexural strength gaining up to 47%. UHPC with Basalt fibres showed enhanced workability and increased flexural strength up to 43% after treatment. UHPC with Carbon fibres demonstrated a 35% improvement in workability and a 43% increase in flexural strength. Sodium hydroxide treatment was preferred for steel, glass, and carbon fibres, while nitric acid treatment was most effective for basalt fibres. Post-treatment analyses revealed improved surface energy, contact angle, and oxygen/carbon ratios, enhancing the wettability of non-metallic fibres. UHPC with hybrid combinations of metallic and non-metallic fibres were also explored, achieving compressive strengths around 148 MPa and flexural strengths around 22 MPa. The flexural toughness reached 90.4 J and treated hybrid combinations could absorb up to 1353 J of impact energy without scabbing under the ballistic impact in specific test conditions, indicating significant improvement in UHPC's performance. This study provides insights on appropriate treatment for metallic and non-metallic fibres in enhancing the performance of UHPC.