## 4 Test Results and Discussions  
### Quasi-Static Compression Tests  
For unconfined specimens, the failure was caused by concrete longitudinal cracks. The axial stress-strain curves are shown in Figure 3a. For the FCHC specimens, although the number of FRP layers and the types of FRP jackets were different, the failure patterns (mid-height FRP rupture failure) of the all the FCHC specimens were similar (Figure 3b-d). The FRP strain development and the column fracture phenomena are described as follows below: The lateral deformations of concrete were very small before the load reached the peak load of the unconfined specimens, indicating that the confinement provided by the FRP jacket was very small at this stage. The FRP hoop strains then increased rapidly until the FRP jacket experienced fracture failure. Due to the existence of confinement from the strips applied at the end regions, the FRP fracture generally occurred at or near the mid-height of the specimens, as seen in Figure 3b-d. Figure 4 shows the stress-strain curves of all the unconfined high-strength concrete specimens under quasi-static compression, with these curves terminating at the point when the concrete crushing failure occurred. The stress-strain curves of the FCHC specimens under quasi-static compression are reported in Figures 5-7. For the FCHC specimens, the stress-strain curves terminate at the point related to the FRP rupture. The three curves of the three nominal identical specimens in each group are generally close to one another, except that of Specim S-60-B2-3 and S-60-A1-1. The above discrepancy was probably caused by the inherent scatter of the concrete strength, which is due to the inhomogeneity of the concrete made in the laboratory, although all these specimens were cast with the same batch of concrete. The agreement in the stress-strain curves between the three nominally identical specimens indicates the good repeatability of the tests and the high reliability of the test results. As seen from Figures 5-7, compared with the unconfined high-strength concrete specimens, the AFRP, BFRP, and CFRP wraps increased the strength and ultimate axial strain of the specimens and the strength and ultimate axial strain increased with the FRP thickness (i.e., confinement stiffness) for all the FCHC, concurring well with the knowledge documented in the literature [55; 56]. With the confinement  
Figure 3: Typical failure modes of the unconfined concrete and FCHC specimens: (**a**) S-0; (**b**) S-C1; (**c**) S-B1; and (**d**) S-A1. provided by the different types of FRPs but with the same value of the FRP layer, the strength of the CFRP-confined concrete is the largest, while that of the BFRP-confined concrete is the smallest. This is due to the strength difference among the three types of FRP. Note that for specimens wrapped with a three-layer CFRP jacket, only results of two specimens out of the three specimens were presented due to the unexpected stop of the loading machine during the test on the other specimen. Figure 4: Stress–strain curves of the unconfined high-strength concrete specimens under quasi-static compression. Figure 5: Stress–strain curves of the BFRP-confined concrete specimens under quasi-static compression. **(a)** One-layer BFRP-confined specimens; **(b)** Two-layer BFRP-confined specimens; **(c)** Three-layer BFRP-confined specimens; and **(d)** BFRP-confined and unconfined specimens. Figure 6: Stress–strain curves of the AFRP-confined concrete specimens under quasi-static compression. (**a**) One-layer AFRP-confined specimens; (**b**) Two-layer AFRP-confined specimens; (**c**) Three-layer AFRP-confined specimens; and (**d**) AFRP-confined and unconfined specimens. Figure 7: _Cont._  
The key test results of the FCHC specimens under quasi-static compression are given in Table 3. Obviously, the compressive strength and ultimate axial strain increase with the FRP thickness.