Performance of CNT-enhanced GGBS/fly ash concrete for precast U-drain applications

This study evaluates the mechanical and durability properties of M40-grade concrete modified with ground granulated blast furnace slag (GGBS), fly ash, and carbon nanotubes (CNTs). The objective is to address the environmental and performance limitations of conventional concrete, which is heavily reliant on cement. Four concrete mixes were examined: M40 (control), M40 incorporating 40% GGBS, M40 containing 30% fly ash, and M40 with CNTs (0.01-0.15%). The mechanical properties compressive, split tensile, and flexural strength were tested at 3, 7, and 28 days, while durability tests such as rapid chloride penetration and water permeability were also performed. The results showed that CNTs, particularly at 0.03-0.05% concentrations, significantly enhanced the concrete's performance, with compressive strength increasing by 56%, tensile strength by 20%, and flexural strength by 39%. Durability improved, with a 100% reduction in water permeability and a 34.56% reduction in chloride penetration. Microstructural analysis via SEM indicated matrix densification and better hydration product distribution at effective CNT dosages. XRD confirmed enhanced C-S-H formation, but higher CNT dosages led to clustering, which reduced efficiency. GGBS-modified concrete outperformed fly ash-modified concrete, and CNT-modified concrete showed the greatest improvements. These findings suggest that CNT-modified concrete is particularly suitable for precast U-drain systems, where enhanced early-age strength, reduced cracking, and improved durability against chloride ingress can improve the service life and performance of drainage infrastructure. This concrete mix shows strong potential for high-performance, sustainable construction.