Bond of steel and FRP reinforcement in recycled aggregate concrete: a critical review

The use of recycled concrete aggregates (RCA) in recycled aggregate concrete (RAC) presents a solution to reduce dependence on raw natural resources. However, the bond behaviour of reinforcing bars embedded in RAC is challenging, mainly due to contradictory results in literature. This critical review examines the mechanisms influencing bond strength of steel and fibre reinforced polymer (FRP) reinforcement in RAC, emphasising key factors such as surface roughness, interaction at the interfacial transition zone (ITZ), and the concrete’s properties. Additionally, the review discusses the main differences in bond performance between FRP reinforcement and normal steel bars, highlighting the FRP bars’ reliance on adhesion and friction, rather than mechanical interlocking. Various testing methods in line with current standards are discussed, alongside relevant design equations from European and North American guidelines for anchorages and lap splices. It is found that moderate RCA replacement levels (50%-75%) can improve the bond strength of bars due to a rougher interfacial transition zone. However, high levels of RCA replacement (~100%) can reduce bond strength by up to 38%. Current design codes primarily focus on steel and FRP reinforcement embedded in normal concrete, and therefore these should be revised to extend their applicability to RAC elements to promote the faster adoption of RAC in engineering practice. Future research needs are also provided. This study contributes towards a better understanding of bond behaviour of reinforcement in RAC, which in turn is expected to facilitate the broader adoption of circular economy practices in construction.