## 3 Test Results and Discussion
### Effect of Bar Diameter and Embedment Length  
It has been widely reported in the literature [18; 19; 36; 40; 41; 45; 46; 47; 48] that the bond strength decreases when there is an increase in the bar diameter, due to the non-linear distribution of stresses along the loaded end of the bar to the free end of the bar at the time of pull-out testing. Hence, in this study, two diameters of the basalt FRP bars--one belonging to the smaller diameter range (6 mm), and one belonging to the larger diameter range (10 mm)--have been selected.  
Figure 14: Wear and tear on rib fibres on the pulled-out bar surface.  
Figure 12: Pull-out failure of 10-mm basalt FRP bar specimens.  
Figure 13: Pull-out failure of steel bar specimens.  
From the experimental observations, it is understood that the smaller diameter bars (6 mm) exhibited lower bond strength, which can be attributed to the early pull-out of the bars from the concrete matrices due to bonding failure. For the smaller diameter bars, even before reaching the peak stress values, the bar grip was loosened from the surrounding concrete, leading to early pull-out failure. The surface characteristics of the bar are another reason for this, as they are smooth when compared to the ribbed surfaces of the steel bars. The surface characteristics of the reinforced bars are of significance in direct pull-out studies, and it has been reported in many studies that a rougher surface--such as those in ribbed, sand-coated surfaces--offers better bonding due to friction in the bar-concrete interface [37; 49; 50].  
Unlike the 6-mm bars, the bond stress of the 10-mm bars decreased with the increase in the bonded length, and was similar to that of steel rebars. The 10-mm BFRP and 10-mm steel rebars reported similar values of bond strength at an embedment length of 5 times the bar diameter. The cause of the decrease in bond stress with an increase of embedment length is attributed to the non-linear distribution of stresses along longer lengths [9; 13; 49; 51].  
The other variable used in this study is the embedment length (or development length), which is considered to be the length of the reinforcement bar required in any reinforced concrete structural element in order to provide sufficient bond strength between the reinforcement and the concrete [11; 16; 48; 52]. The current study aims to identify the best suited embedment length when using basalt FRP reinforcement.  
The bond stress values with respect to the different embedment length to bar diameter ratios (L/d\({}_{\text{b}}\)) obtained from the tests are depicted in Figure 16 below:  
Figure 16: Bond stress vs. embedment length to bar diameter (L/d\({}_{\text{b}}\)).  
Figure 15: Cross-sectional view of cut Pull-out specimens.  
The basalt FRP bars of 5 d\({}_{\rm b}\) bonded length corresponding to an L/d\({}_{\rm b}\) ratio of 5 report higher bond strength when compared to L/d\({}_{\rm b}\) ratios of 10 and 15. There is a decrease in bond stress values for 10-mm bars when there is an increase in bond length, due to the non-linear distribution of stresses in the bars [38; 53]. For the smaller 6-mm BFRP bars, the bond capacity of the concrete was very low even for longer embedment lengths, and the bars pulled out under peak loads as reported in a study by [PERSON] and [PERSON] [16]. In the case of 6-mm bars, an embedment length of 15 d\({}_{\rm b}\) showed a slight increase in the bond strength; however, in all three embedment lengths, the peak load capacity of the 6-mm BFRP was not achieved.  
Even though the only mode of failure reported in this study was the pull-out type, we observed that in specimens with 15 d\({}_{\rm b}\) embedment length, while the bar firstly pulled out of the SCGC, a crack of 1-2 mm width appeared on the top surface of the specimen immediately afterwards, as seen in Figure 17: