Upcycled C&D waste as recycled concrete aggregate for green cementitious composites: Mechanical, NDT and post-fire performance

Although recycled aggregate concrete (RAC) has been widely investigated, its performance remains strongly dependent on the origin, quality, and processing history of the recycled aggregate. This study addresses this issue through a context-specific experimental assessment of recycled coarse aggregates (RCA) produced from locally sourced construction and demolition (C&D) concrete waste in Pakistan, where natural aggregate extraction is increasingly constrained and systematic RCA data remain limited. Unlike many previous studies based on generic or laboratory-produced recycled aggregates, the RCA used here was obtained from demolished rigid pavement concrete along the Old Grand Trunk (GT) Road corridor, providing a traceable and representative waste stream for local infrastructure applications. Natural coarse aggregates were replaced by RCA at 0%, 25%, 50%, 75%, and 100% by mass. The experimental programme combined aggregate characterization, destructive mechanical testing, non-destructive evaluation, post-fire residual strength assessment after exposure to 500° C, and microstructural/mineralogical analyses using SEM, EDS, and XRD. RCA exhibited higher water absorption and lower specific gravity than natural aggregates because of adhered mortar, microcracking, and increased porosity. Mechanical results showed a gradual reduction in compressive, flexural, and tensile strength with increasing RCA content; however, mixtures containing up to 50% RCA achieved 28-day compressive strengths suitable for structural-grade concrete. One-way ANOVA indicated that the differences among replacement levels were not statistically significant at the 95% confidence level under the investigated conditions. The microstructural observations confirmed that RCA incorporation mainly affects concrete through increased porosity and thickening of the interfacial transition zone, while the fundamental hydration products remain comparable to those of conventional concrete. The results demonstrate that locally sourced Pakistani RCA can be incorporated in structural concrete, particularly up to 50% replacement, with acceptable mechanical performance. The study therefore contributes region-specific evidence for sustainable aggregate substitution, C&D waste valorisation, and circular construction practices in Pakistan.