# Experimental Research on Bond Behavior between GFR Bars and Stirrups-Confined Concrete
## 4 Parameterization Analysis  
### Influence of Bar Surface Characteristics  
In this paper, the tests compare the bond behavior of three types of surface characteristics that are observable in GFRP bars, R1/helical wrapping, R2/helical wrapping and sand coating, and R3/Ribbed.  
Figure 10 shows the typical failure modes of GFRP bars that are exhibited by specimens with pullout failure. Close examination of the surface of GFRP bars indicated that the bond stress collapse of the GFRP bar was primarily due to the detachment of the fiber spirals and resin layer, for R1 bars (Figure 10a), and sand grains for R2 (Figure 10b) bars, while the concrete surface remained damaged. It was evident that a friction-resistant type mechanism had been activated via this failure mode. The bond stress was predominantly dependent upon chemical adhesion and after slip upon friction. Furthermore, sanding provides many small anchoring points that are distributed over the surface, leading to an increase of friction between GFRP bar and concrete, thereby increasing bond strength. Table 3 shows that the average bond strength of R2 bars is 1.2-1.35 times that of R1 bars with the same diameter. This is also reflected in the experiments by [PERSON] et al. [42]. Subsequent to pullout failure, the R1 bars exhibited a constant residual bond stress that is caused by the friction between the bar and concrete, whereas the R2 bars exhibited a small amount of oscillating residual bond stress due to the existence of mechanical interlocks between the shallow sand coating and the concrete (Figures 5 and 6). Close examination the interface between the R3 and R4 bars and the concrete revealed that the  
Figure 8: Representative \(\tau\)-\(s\) curves for R4 bars for the (**a**) without and (**b**) with stirrups.  
Figure 9: Representative \(\tau\)-\(s\) curves for C2 concrete for the (**a**) R1 bar (**b**) R3 bars.  
bond stress collapse of R3 bar was a consequence of the crushing of concrete and the shearing off of ribs (Figure 10c), whilst the failure of R4 bars was solely due to the crushing of concrete (Figure 10d). This failure mode verified that a bearing type mechanism was activated. The bond stress was primarily dependent on the interlocking interaction between ribs and concrete. Therefore, R3 and R4 bars exhibited a higher bond strength than friction-resistant type bars (R1, R2). The average bond strength of R3 bar manifested as 1.3-1.5 times that of R1 bar; that of R4 bar manifested as 1.75 times that of R1 bar with the same diameter (Table 3). After pullout failure, R3 exhibited a large oscillating residual bond stress due to the existence of interlocking between the residual ribs and the concrete (Figure 7).  
When concrete splitting failure became apparent in the specimen, the bond stress-slip curves (Figures 5-8) and the bond strength of the GFRP bars or steel bars with the same diameter were similar. The average bond strength of specimens R2-C1-16-N, R3-C1-16-N, and R4-C1-16-N is 10.32 MPa, 10.44 MPa, and 11.3 MPa, and that of specimens R1-C1-20-N, R2-C1-20-N, R3-C1-20-N, R4-C1-20-N is 7.42 MPa, 7.11 MPa, 6.9 MPa, and 7.14 MPa, which is in line with the results by [PERSON] and [PERSON] [32] on bond behavior of steel bar. When the specimen exhibited splitting failure, the bond strength of the steel bar was not reliant on surface characteristics. The principal reason behind the failure of specimens was that the strain of concrete loop tensile stress direction around the bar exceeded the tensile strain limit of the concrete, causing the internal creation of cracks that were rapidly transmitted to the surface of the concrete, culminating in splitting failure.