# Experimental Research on Bond Behavior between GFR Bars and Stirrups-Confined Concrete
## 4 Parameterization Analysis
### Influence of Concrete Strength  
Research on the bond behavior of steel bars has indicated that, when pullout failure occurs, there is an increase in bond strength of steel bars, along with an enlargement of concrete strength. However, this conclusion is not always applicable in the case of GFRP bars. Figure 11 shows a comparison between the average bond strength of R1 and R3 bars in C1 and C2 concrete without stirrups. With the occurrence of pullout failure, the bond failure interface only transpires at the surface of the R1 bar. When the concrete strength increases from C1 to C2, the average bond strength of specimens R1-C1-8-N, R1-C1-12-N, and R1-C1-16-N becomes essentially akin to that of specimens R1-C2-8-N, R1-C2-12-N, and R1-C2-16-N (Figure 11); the shape of the \(\tau\)-s curves also appear to be similar (Figures 5a and 9a), which is concordant with the research that was noted in the references [7; 9; 42; 43]. However, the bond failure interface occurs at the surfaces of both the R3 bar and concrete. When the concrete strength increases from C1 to C2, the \(\tau\)-s curves of specimens R3-C1-8-N and R3-C1-12-N (Figure 7a) appear similar in shape to those that are evident in specimens R3-C2-8-N and R3-C2-12-N (Figure 9b). However, the average bond strength (\(\tau^{*}\)) increases by 37.3% and 23.6%, respectively (Figure 11). It can be concluded that the bond failure interface takes place in the concrete, and the bond strength of the GFRP bar is directly related to concrete strength. The conclusion is unaffected by stirrups (Figures 5, 7 and 9). Furthermore, improving the concrete strength can alter the bond failure mode of the GFRP bars. Concrete splitting failure occurred in specimens R1-C1-20-N, R3-C2-16-N, and R3-C2-20-N (Figures 5a and 7a). However, when the concrete strength increased to C2, their failure mode changed into that of a pullout failure (Table 3 and Figure 9), and there were average bond strength increases of 17.4%, 48%, and 93.8%, respectively (Figure 11). diameter, as shown in Table 3. However, the failure modes of their stirrup-containing counterparts changed into pullout failure with many micro-cracks appears on the concrete surface at the loading end e.g., R3-C1-20-Y-2 (Figure 12b). This is with the exception of specimen R4-C1-20-Y. Despite the concrete splitting failure on specimen R4-C1-20-Y, unlike specimen R4-C1-20-N, many penetrating and non-penetrating cracks appeared on the concrete surface at its loaded end, e.g., R4-C1-20-Y-2 (Figure 12c). This phenomenon is also reflected in the research by [PERSON] and [PERSON] [32] on the bond behavior of steel bars. Largely due to the existence of stirrups, the circumferential pressure of the steel bar to concrete becomes more uniform, resulting in the appearance of more cracks on the concrete surface. Figure 13 shows the ratio of the average bond strength (\(\tau\)* (Y) / \(\tau\)* (N)) and the corresponding average slip (\(s\)* (Y) /\(s\)* (N)) of the specimens with and without stirrups. It is evident that the \(\tau\)* and \(s\)* of specimens that contain stirrups will be increased if their stirrup-lacking counterparts undergo concrete splitting failure. The larger the diameter of the similar bars, the more evident was the improvement in its performance. Different types of bars ranged in the amount of ostensible improvement, from R1, R2, R3, and R4 bars with same diameter. The \(\tau\)* (Y) / \(\tau\)*(N) and \(s\)* (Y) / \(s\)* (N) values of R2, R3, and R4 bar of the 16 mm diameter were 1.25, 1.32, 1.44 and 1.38, 1.57, 1.79, respectively; while the values of R1, R2, R3, and R4 bar of 20 mm diameter were 1.12, 1.41, 1.58, 1.69 and 1.31, 1.68, 2.03, 2.18, respectively. This may be because the larger the diameter and bond stress between the bar and concrete, the higher the likelihood of the occurrence of concrete splitting failure, which in turn renders an increase in the confinement of stirrups on concrete. In addition, no splitting cracks were apparent in specimens that underwent pullout failure.