Drained shear behaviour of recycled construction and demolition waste mixed with natural aggregates and their optimal mix proportion

This study investigates the performance of mixed recycled aggregates (MRA) derived from construction and demolition waste (CDW), which incorporates approximately 81% recycled concrete aggregates and 16% crushed brick, when blended with natural gravel aggregates through a comprehensive experimental program. Shear behaviour and dilatancy characteristics of these materials are evaluated within the critical state soil mechanics framework using consolidated drained monotonic triaxial tests. The results reveal that while pure MRA (MRA100) attains higher peak shear strength than pure NGA (NGA100), it also exhibits potential limitations, including the most pronounced strain softening (SS) behaviour, the highest maximum dilation angle ( ψ max ), and a substantial difference between soaked and unsoaked CBR values (ΔCBR). Incorporating NGA into MRA substantially mitigates these issues, resulting in blends with reduced SS, lower ψ max and smaller ΔCBR compared to MRA100, while achieving higher peak shear strength than NGA100. NGA/MRA mixtures generally exhibit greater effective friction angles than their parent materials (NGA100 and MRA100) under both peak ( ϕ' peak ) and critical ( ϕ' cs ) states. With increasing MRA percentage in the mixtures, ϕ' peak increases and ϕ' cs remains relatively constant, but the peak apparent cohesion ( c' peak ) decreases. Among all mixtures, NGA25/MRA75 emerges as the optimal mix, demonstrating a balance of strength, moisture resilience and ductility. This is evidenced by NGA25/MRA75 exhibiting the highest ϕ' peak and CBR values (both soaked and unsoaked), alongside the lowest ΔCBR, and minimal SS and ψ max . These findings highlight the synergistic benefits of blending recycled CDW with NGA, producing a composite with superior mechanical properties and improved resilience under saturated conditions. By providing critical insights into the engineering behaviour of recycled waste materials, this study supports their broader adoption in transportation infrastructure projects and promotes sustainable construction practices. • Drained shear behaviour and stress-dilatancy of CDW-derived MRA and NGA mixtures are investigated through triaxial tests. • The influence of MRA content on dilatancy, peak shear strength, and critical state behaviour of the mixtures is evaluated. • NGA25/MRA75 emerges as the optimal blend, balancing peak shear strength, moisture resilience, and ductility. • A rationale for integrating NGA into recycled CDW-derived aggregates is presented within the CSSM framework.