Performance Enhancement of Concrete Using Carbon Nanotubes, Steel Fibres, and 100% Artificial Sand

The depletion of natural river sand and the environmental impact of excessive extraction have created an urgent need for sustainable alternatives in concrete production. Artificial sand (AS), produced from crushed rock, has emerged as a viable and eco-friendly substitute. However, AS can influence workability and mechanical performance, necessitating the use of advanced reinforcement strategies. This study aims to enhance the mechanical properties of high-performance concrete (HPC) incorporating 100% AS by utilizing hybrid reinforcement with carbon nanotubes (CNTs) and steel fibers (SFs). CNTs were added at 1%, 2%, and 3% by weight of cement, while SFs were maintained at a constant 5% by volume of concrete. The concrete mixes were subjected to standard mechanical tests, including compressive strength, flexural strength, and splitting tensile strength, at specified curing ages. The experimental results demonstrated that the synergistic use of CNTs and SFs significantly improved strength characteristics and post-cracking performance compared to control mixes with AS alone. The mix with 2% CNTs showed the most balanced improvement across all parameters, with notable increases in compressive strength, flexural toughness, and tensile resistance. This enhancement is attributed to the high tensile strength and Nano-scale bridging effect of CNTs, coupled with the crack-arresting ability of SFs. Conversely, at 3% CNTs, reduced workability and slight declines in strength gains were observed, primarily due to nanoparticle agglomeration affecting dispersion. Overall, the hybrid CNT–SF approach presents a promising pathway for producing sustainable, high-performance concrete using artificial sand, thereby reducing dependence on natural resources while achieving superior structural performance. These findings contribute to the growing body of research on eco-friendly, fibre-reinforced, Nano-modified concretes, supporting their adoption in structural applications where both strength and durability are critical.