# Study of Basalt Fibers and Graphene Enriched Polymers on Bond Behavior of FRP Bars in Concrete
## 4 Results and Discussions  
### Room Temperature Results  
Table 5 presents the values of the maximum bond stress \(\tau_{1}\) (bond strength), corresponding slip \(s_{1}\) and the rupture mode developed for each pull-out test. It is also presents the average maximum bond stress \(\tau_{1}\) of the specimens and the corresponding standard deviation. In the Appendix A model parameters used in the numerical simulations are given for the average of each case considered. In all specimens a rupture by pull-out occurred, with the outer surface of the bars being damaged, as can be seen in Figure 6. No signs of cracking were observed in the concrete, indicating an adequate confinement of the bars. For smooth surfaces, partial delamination occurred (Figure 6a). For helically-wounded surfaces, it was noted an abrasion of the outer surface (light gray areas in Figure 6b). This type of rupture have been also reported by other authors, such as [PERSON] and [PERSON] [PERSON] and [PERSON] (2016) and [PERSON] et al. [PERSON] et al. (2017). In helically-wrapped surfaces the strings wrapping the bars tended to delaminate, as seen Figure 6c. Figure 7a, shows all average bond stress vs slip plots for GRFP cases. Experimental and numerical results are shown. An excellent fit between them is obtained. Figure 7b shows the average of the three specimens used per case. Based on these averages it is possible to access the influence of concrete strength increase on bond strength. For 12.5-mm bars there was an increase in bond and for 8-mm bars the bond actually decreased. A new set of average values is presented in Figure 7c, for both diameters considered. It can be seen a clear increase in bond with the increase in diameter. Finally Figure 7d shows a comparison of the average 12.5 mm GFRP bars with some results available in the literature, considering GFRP bars in similar conditions. It is observed that the present experiments do not depart significantly from what is expected by the literature, except the case tested by [PERSON] et al. [PERSON] et al. (2016), where a gro  
Figure 6: Typical failure mode: pull-out by damage of the rebar surface. (**a**) Smooth surface, (**b**) helically-wounded surface and (**c**) helically-wrapped surface. Figure 8a, shows all average bond stress vs slip plots for GRFPg cases. Experimental and numerical results are shown. An excellent fit is obtained, except for SP2 and SP3 C30-12 Cg cases where an oscillatory answer was obtained experimentally during post-peak. The model used was not able to capture such oscillations but it was able to capture the behavior on average. Figure 8b shows the relations for the average of the three specimens used per case. The observed effect of the increase in concrete strength on bond strength is the opposite of the effect seen in GFRP bars, i.e., For 8-mm bars there was an increase in bond and for 12.5-mm bars the bond decreased. The average values for both diameters considered is presented in Figure 8c. It can be seen again an increase of the bond with the increase of diameter. As no other test of bars with GNP addition was found in the literature, the effect of the increase of the bond strength on bond strength is also observed. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Combinations** & **Specimens** & \(\tau_{1}\) **(MPa)** & \(s_{1}\) **(mm)** & **Rupture** & \(\overline{\tau}_{1}\) **(MPa)** & **Std Dev.