# Experimental Research on Bond Behavior between GFR Bars and Stirrups-Confined Concrete
## 6 Conclusions  
A total of 100 pullout tests were carried out in this paper in order to examine the bond behavior of the GFRP bars in concrete. Based on the results of this experimental and analytical study, the following conclusions may be drawn:  
(1) The surface characteristics of the bar influence the bond behavior of GFRP bars. In the tests, the bond stress collapse of GFRP bars with surface of helical wrapping (R1) and helical wrapping and sand coating (R2) was due to the detachment of fiber spirals, the resin layer, and sand grains; the bond strength of these GFRP bars was governed by the fiction between the GFRP bar and the concrete. However, the bond stress collapse that is evident in the GFRP bars with a ribbed surface (R3) was due to the crushing of concrete and the shearing off of ribs. The bond strength of these GFRP bars was governed by the interlocking interaction between the GFRP bar and the concrete. Therefore, the R3 bars exhibit higher bond strength than that of both the R1 and R3 bars. The average bond strength of R3 bars is 1.3-1.5 times that of R1 bars and 1.2-1.36 times that of R2 bars. However, it is notable that the bond strength of the specimens that displayed concrete splitting failure is not affected by the surface characteristics of the GFRP bar. (2) The test results confirmed the notion that larger bar diameters develop lower bond strength. This tendency is not influenced by the surface characteristics of the GFRP bar, concrete strength, or the confined effect of stirrups. (3) The improvement of concrete strength does not necessarily render an increase in the bond strength of the GFRP bars. When bond failure interface only occurs on the surface of a GFRP bar, the bond strength of a GFRP bar is not dependent upon the concrete strength. However, when a bond failure interface coexists on the surface of a GFRP bar and the concrete, an enhancement of concrete strength is able to effectively increase the bond strength of a GFRP bar. As a result, in the design of a GFRP-RC structure, an appropriate concrete strength should be selected according to the surface of the GFRP bar. (4) Stirrups provide confinement to concrete, which in turn changes the failure modes and the \(\tau\)-\(s\) relationship of the pullout specimens. If concrete splitting failure occurs in specimens without stirrups, their stirrup-containing counterparts have a tendency to exhibit pullout failure, leading to an increase in the bond strength and the corresponding slip observable in the specimens. The level of improvement ranges depending on the amount of increase of the diameter of the GFRP bar, and the bond stress between the GFRP bar and the concrete. As opposed to the beam test, the concrete was not subjected to bending stress during the pullout testing. No sign of splitting cracks was evident on these specimens due to the relatively small diameter of the bars in comparison to the dimensions of the specimens or the relatively high concrete strength. The bond behavior of these specimens was almost completely unaffected by the presence of stirrups. As a result, further research into the subject of the beam testing of GFRP bars should be taken into consideration in the presence of stirrups. Figure 16: Comparisons between experimental and analytical \(\tau\)-\(s\) curves: (**a**) R1-C1-20-Y-2; (**b**) R2-C1-16-Y-3; (**c**) R2-C1-20-Y-1; (**d**) R3-C1-16-Y-1; and, (**e**) R3-C1-20-Y-3. (5) The BPE model and the CMR model have relatively simple form and reliable results that can be applied to investigate \(\tau\)-\(s\) relationship of GFRP bars to stirrups-confined concrete. The fitting parameters, \(\alpha\)\(\beta\)\(s_{\tau}\), specified in these two models, were generated based on the test results of this paper, and the suggested parameter values that were classified based on surface characteristics were proposed. However, there are no universal analytical models that can be applicable to the general \(\tau\)-\(s\) relationship of GFRP bar to stirrups-confined concrete.