## 3 Test Results and Discussions
### Cyclic Load-Displacement Relationship  
Figure 8 shows the applied cyclic load-displacement relationships measured in the beam element. For F-series specimens, the initial stiffness was significantly affected by the type of longitudinal reinforcement rather than \(f_{c}^{\prime}\). The specimens F-N-G and F-H-G exhibited approximately 70% lower initial stiffness before cracking when compared with the companion specimens F-N-C and F-H-C. In addition, the former specimens possessed a lower initial flexural cracking strength than the latter specimens. The beam displacement increased sharply with the occurrence of the initial flexural cracks, indicating a greater increasing rate in the displacement in the specimens F-N-G and F-H-G than the companion specimens with conventional steel reinforcements. The longitudinal steel reinforcement in the specimens F-N-C and F-H-C reached its yield strain at approximately 75 to 90% of the peak load. The specimen F-N-C exhibited near plastic flow performance after the yielding of the longitudinal steel bars and reached failure with the buckling of such bars and the opening of the 90-deg hook of the shear reinforcement, as shown in Figure 8a. Thus, the flexural behavior of the specimen F-N-C was almost similar to the typical trend observed in the conventional RC beams [14; 15; 16]. The specimen F-N-G exhibited a similar pre-peak behavior to the companion specimen F-N-C, as shown in Figure 8b; however, the former possessed a 21% lower flexural capacity of the beam and displayed a rapid decrease of the applied load after the peak load because of the pull-out failure of the GFRP bars, when compared with those observed in the latter. The specimen F-H-C exhibited a very similar trend to the load-displacement curve recorded in the specimen F-N-C, as shown in Figure 8c. Thus, a rapid decrease in the applied load at the post-peak state was not observed in the specimen F-H-C despite the use of high-strength materials. Meanwhile, the initial flexural cracking strength and flexural capacity of the beam element of the specimen F-H-C were, respectively, 1.34 and 2.68 times higher than those measured in the specimen F-N-C because of the use of high-strength materials in the former. The specimen F-H-G exhibited a greater increasing rate in displacement at the pre-peak state when compared with the companion specimen F-H-C, as shown in Figure 8d. In addition, the specimen F-H-G did not display the plastic flow characteristic after the peak load, resulting in a rapid decrease in the applied load due to the tensile rupture of GFRP bars. Overall, the low elastic modulus and elongation capacity of GFRP bars resulted in large displacement and brittle post-peak performance of beams.  
S-series specimens exhibited a great increasing rate in the beam displacement with the occurrence of the diagonal cracks and no plastic flow behavior after the peak load due to the severe diagonal cracks in the beam web, as shown in Figure 8e,f. The diagonal shear crack stresses were \(0.18\sqrt{f_{c}^{\prime}}\) and \(0.14\sqrt{f_{c}^{\prime}}\) for the specimen S-H-C and S-H-G, respectively, indicating a lower diagonal shear crack resistance in the latter beam, as given in Table 4. This may be attributed to the fact that the lower flexural crack strength and faster propagation of flexural cracks in the beam web due to the low elastic modulus of GFRP bars reduce the diagonal tensile transfer capacity of concrete [6]. In addition,the specimen S-H-G exhibited as much as 41% lower shear capacity and a smaller area of the envelop load-displacement curve when compared with the companion specimen S-H-C. Overall, the reduced diagonal tensile transfer capacity of concrete in the beam reinforced with longitudinal GFRP bars decreases the ultimate shear capacity of the beam.  
Figure 8: _Cont._