Auto Shredder Residue for Sustainable Concrete: Performance and Potential Economic Benefits

The increasing demand for civil infrastructure has contributed significantly to CO 2 emissions and global warming potential (GWP), largely due to concrete production, cement manufacturing, and natural aggregate extraction. Automotive shredder residue (AutoSR) offers a sustainable alternative; however, its effects on concrete performance remain poorly understood. This study evaluates AutoSR fines, termed eco-friendly aggregates (EFAs), used at 10% volumetric replacement for natural fine aggregate in eight concrete mixtures. Fresh and hardened properties were assessed for EFAs with varying particle characteristics. Microstructural features, including the interfacial transition zone (ITZ), and maturity indicators based on the temperature–time factor (TTF) and ultrasonic pulse velocity (UPV) were examined. All EFA mixtures showed reduced workability compared to the control mix, while hydration behavior and UPV responses remained comparable, allowing the development of maturity master curves. Mechanical performance was strongly influenced by EFAs’ characteristics, with an increased ITZ thickness identified as the primary driver of strength reduction. The study establishes a clear mechanistic link between EFA absorption, ITZ development, and strength loss, supporting a practical 10% replacement level for structural applications and providing guidance for future optimization. The potential economic benefits are also briefly discussed.