Aggregate Interlock Degradation to Shear Behaviour of FRP-Reinforced Recycled Aggregate Concrete Members: A Critical Review

This review critically examines the shear behaviour of fibre-reinforced polymer (FRP)–reinforced recycled aggregate concrete (RAC) members, with particular emphasis on the role of aggregate interlock degradation in governing shear transfer mechanisms and structural performance. The increasing adoption of recycled concrete aggregates and FRP reinforcement offers significant environmental and durability benefits. However, their combined use fundamentally modifies the mechanisms responsible for resisting shear. Based on a systematic literature review following a PRISMA methodology, seventy peer-reviewed studies were synthesised to evaluate the influence of recycled aggregate characteristics, FRP reinforcement type, interfacial bond behaviour, aggregate interlock degradation, dowel action, diagonal compression strut behaviour, crack propagation, failure modes, strengthening strategies, and analytical modelling on the shear response of FRP-RAC members. The reviewed studies consistently demonstrate an observed structural trend: increasing recycled aggregate replacement ratios results in reduced shear capacity, wider diagonal cracks, and more brittle failure behaviour. Mechanistically, this trend is widely attributed to a weakened interfacial transition zone and the degradation of aggregate interlock. Furthermore, the lower transverse stiffness of FRP reinforcement limits dowel action, which compoundingly intensifies the observed reductions in member shear resistance. Conversely, the incorporation of steel or hybrid fibres and the application of FRP strengthening systems effectively enhance crack bridging, compensate for reduced aggregate interlock, delay crack propagation, and restore member shear capacity and ductility. By integrating material characteristics with the underlying shear transfer mechanisms, this review provides a comprehensive understanding of the factors governing the shear behaviour of FRP-RAC members, identifies existing research gaps, and highlights future research directions for advancing sustainable and durable structural concrete systems.