A state-of-the-art of bond and lap-splice of steel/FRP bars in recycled aggregate concrete

Recycled aggregate concrete (RAC) is rapidly emerging as a viable structural material; however, its comparatively weaker microstructure poses challenges for bond and splice behaviour, which are critical to structural safety. This review systematically analyses 38 experimental and numerical studies concerning the effects of RAC replacement ratio, reinforcement type, and confinement systems on bond and splice behaviour. When fully replacing natural aggregate, RAC demonstrates a bonding capacity approximately 25% lower, primarily due to the porous nature of the interfacial transition zone. Consequently, the required splice lengths under unconfined conditions are 35–40 bar diameters for steel reinforcement and 60–75 bar diameters for fibre-reinforced polymer (FRP) reinforcement. Confinement strategies, particularly transverse reinforcement and external FRP wrapping, effectively mitigate brittle splitting failures and promote ductile behaviour. Current design codes (ACI 318; Eurocode 2), calibrated for natural-aggregate concrete, inadequately address the distinct properties of RAC. To bridge this gap, this review proposes a recycled-aggregate reduction factor (ψ_r), analogous to the lightweight-concrete factor in existing codes. This factor provides engineers with a code-compliant methodology for the safe design of RACs, supporting the broader adoption of sustainable construction materials.