## 4 Results and Discussion
### Bond Strength  
The impact of different RAs on the bond performance of steel rebar embedded in RAC was evaluated by pullout tests. The average bond strength was determined using the following equation, where \(F\) is the maximum experimental force obtained in pull out tests, \(d_{b}\) is the diameter of the embedded steel bar, and \(l_{b}\) is the bonded length (i.e., taken as equal to 5 \(d_{b}\) in this study):  
\[\tau=\frac{F}{\pi d_{b}l_{b}} \tag{2}\]  
Figure 8: Compressive strength results (Minimum compressive strength requirement of structural concrete is 17 MPa [46]). The data obtained from pull-out tests for each concrete mix were analyzed in terms of bond stress-slip response, as presented in Figures 9 and 10 (average of two samples with a maximum variation of 9% in values). Since the compressive strength of concrete mix significantly affects the maximum bond strength, the peak bond strength exhibited by each concrete mix was normalized by its compressive strength, as indicated in Equation (3):  
\[\tau_{n}=\frac{\tau_{max}}{\sqrt{f_{c}^{\prime}}} \tag{3}\]  
where \(\tau_{n}\) is the normalized bond strength, \(\tau_{max}\) is the maximum bond stress, and \(f_{c}^{\prime}\) is the compressive strength of the concrete. The maximum and normalized bond strength values of all concrete mixes are presented in Figures 11 and 12, respectively. The bond stress vs. slip response of concrete mixes containing 100% fine RBAs is presented in Figure 9, along with the same response of the control concrete mix. Among these mixes, the lowest response was exhibited by the mix containing 100% coarse RBAs (i.e., F(RBA) + C(RBA)). However, an improvement in the overall response was observed when coarse RBAs were partially or fully replaced with coarse NAs. It was noticed that the post-peak bond behavior of concrete mix containing 100% fine RBAs and 100% coarse NAs was even better than the control concrete mix (i.e., F(NA) + C(NA)). Due to the presence  
Figure 10: Bond stress–slip response (mixes containing fine RCAs). Figure 9: Bond stress–slip response (mixes containing fine RBAs). of weak RBAs, the bearing strength of RAC in contact with the ribs of a deformed steel bar is significantly reduced, causing early development and quick propagation of splitting cracks in the relatively weak concrete matrix surrounding the steel bar. Consequently, a detrimental effect on bond strength and a sharp degradation of the post-beak-peak bond stress-slip response in RAC were observed. However, the replacement of weak coarse RBAs with relatively stronger coarse NAs in the concrete mix gradually improved its overall bond stress-slip response. The results presented in Figure 9 revealed that the replacement of 100% coarse RBAs with RCAs improved the overall response of concrete when compared to a concrete mix containing 100% coarse RBAs. The positive impact of coarse RCAs on the residual bond strength at a larger slip value was obvious from the results of this study. The slip of deformed steel rebar corresponding to peak bond strength was almost similar for all concrete mixes except for the mix containing 100% fine and coarse RBAs [F(RBA) + C(RBA)], where it was significantly less. The bond stress-slip response of concrete mixes containing 100% fine RCAs is presented in Figure 10 along with the response of the control concrete mix. The peak bond strength exhibited by all RAC mixes was less than the bond strength of the control mix; however, the slip value at peak bond strength for RAC mixes containing fine and coarse RCAs was higher than that of the control mix. The results clearly indicated that the replacement of coarse RCAs with coarse NAs improved the overall bond performance of steel rebar in concrete. At slip greater than 2 mm, the residual bond strength of RAC mixes containing 100% coarse RCAs or coarse RCAs and NAs was observed to be higher than  
Figure 11: Maximum bond strength. Figure 12: Normalized bond strength. the control mix.