## 1 Introduction
Therefore, more full-scale research was recommended to be conducted for an in-depth understanding of the structural performance of such composites. The usefulness of structural concrete (RC) is derived from the combined action of reinforcing rebars and concrete. Load transfer must occur between these two materials to ensure composite action. In this regard, force transfer from rebars to concrete or from concrete to steel rebars will only be effective if a good bond exists between both materials [36]. Therefore, one of the important factors in the design of RC structures is the bond behavior between steel reinforcing bars and concrete. Extensive research work has been conducted over the past many decades on the bond stress-slip response of natural aggregate concrete (NAC) and steel reinforcing deformed bars, and bond strength for such composites has already been formulated and quantified [37; 38; 39; 40; 41]. However, very limited literature is found dealing with the bond stress-slip response of steel reinforcing bars and RAC containing RCAs and/or RBAs. [PERSON] and [PERSON] [42] conducted pull-out tests to examine the bond behavior between steel rebar and RAC. They prepared RAC by replacing the coarse NAs with coarse RCAs at replacement levels of 0, 25, 50, 75, and 100%. Fromthe findings of this study, they concluded that the relative bond strength of RAC prepared with coarse RCAs was higher. Compared to concrete prepared with full NA, the maximum bond strength was obtained at a replacement level of 100%. Likewise, [PERSON] and [PERSON] [43] investigated the bond-slip performance of ordinary recycled brick-aggregate (RBA) concrete, considering the replacement ratio from 0% to 100% with an increment of 25% as the primary varying parameter for 40 pull-out specimens. They found that as the replacement ratio of RBAs varied from 0% up to 50% and then up to 100%, the decrease in the bond strength was 19.91-28.11% and 46.32-49.63%, respectively, corresponding to different embedment lengths. Another study was conducted by [PERSON] et al. [44] to analyze the bond behavior between steel and RAC in slabs. Test specimens were prepared by replacing the coarse NAs with coarse RCAs at 0, 50, and 100% replacement levels. They observed a 4-20% decrease in the ultimate bond strength and an 8-14% decrease in the stiffness in the presence of RCAs in the concrete. [PERSON] et al. [45] analyzed the bond response between concrete incorporating RBAs as coarse aggregates and steel rebar. Based on the experimentation, they developed an analytical model to calculate the bond strength of brick aggregate concrete (BAC). Moreover, they found that BAC exhibited a higher bond strength compared to that predicted using the model proposed by ACI 318 [46] for stone aggregate concrete. The factors that affect the bond strength include reinforcement size and geometry, rebar position, bonded length, amount of transverse reinforcement, concrete strength, and thickness of concrete cover [47; 48; 49]. Since the concrete strength is altered by replacing NAs with RCAs or RBAs [50; 51; 52], experimental investigation is required to assess the impact of fine and coarse RCAs and/or RBAs on the bond properties to avoid underestimation of bond strength, which may lead to financial losses, or overestimation, which may lead to failure. Keeping in view the structural performance of reinforced RAC and its real application in concrete structures, the investigation of its flexural behavior is of great significance. [PERSON] et al. [26] studied the flexure performance of multi-generation recycled concrete beams. Three different generations (RA-1, RA-2, and RA-3) were used to prepare RAC. In the RA-1 generation, RCAs were produced by crushing virgin aggregate concrete, while in the RA-2 generation, RCAs were produced by crushing RA-1 concrete. Similarly, in the RA-3 generation, RCAs were produced by crushing RA-2 concrete. RC beams were cast and tested under four-point bending loading.