Can a local bond test truly reflect impact of recycled aggregate on the bond between deformed steel bars and recycled aggregate concrete?—A critical assessment and development of a generic model

It is essential, for promoting the structural use of recycled aggregate concrete (RAC), to understand the impact of the incorporation of recycled aggregate (RA) on the bond between steel reinforcement and RAC. However, conflicting findings in existing literature indicate a need for re-evaluating the RAC-steel bond, as well as establishing a unified bond characterization and modelling method. This study provides a critical assessment to address this pivotal issue, in light of a large experimental database built from an extensive survey of the up-to-date literature. Following this, a partial-interaction mechanics-based model is developed to simulate the bond-slip response between steel bars and RAC, covering scenarios from a steel bar pulled out from RAC to the tension stiffening in reinforced RAC members. A parametric study is then undertaken to look into the actual impacts of RA on the global bond behaviour. It turns out that the effects of the attributes of RAC and steel on the bond vary markedly at different scales and these effects further alter with the specimen failure mode. For a RAC specimen with a short anchorage experiencing bar pull-out failure, there appears no clear correlation between the local bond characteristic and the RA replacement level; nonetheless, for a similar specimen yet suffering splitting failure, a higher content of RA or lower strength of RAC tends to impair the local bond capacity. Furthermore, the RA incorporation has a real impact on the global bond behaviour when the bar embedment length is well over five times the bar diameter. The model developed is capable of producing results in good agreement with the experimental measurements, approving that the RA substitution does weaken the global bond characteristic of RAC, due primarily to its lower modulus compared to that of natural aggregate concrete.