*3.4. Load–Displacement Curves*

Figure 10 shows the load–displacement curves of specimens. The curves of all the specimens consisted of three stages: elasticity, elastoplasticity, and failure. In these curves, the load–displacement relationship of the unreinforced column was consistent with the axial compression test of the plain concrete column [25]; that is, from the beginning to 80% of the peak load was the elastic stage, then the crack occurred at the beginning of the elastoplastic stage and the slope of the curve gradually decreased until the bearing capacity reached the peak, and finally the bearing capacity decreased rapidly until failure. It should be noted that the failure characteristic for specimen P1 was brittle failure. For the strengthened specimens, the load–displacement curves were similar. From the beginning to 80% of the peak load was the elastic stage. After this, the stiffness of the specimens gradually decreased with the development of the crack and the bearing capacity reached the peak when several segments broke. This process was referred to as the elastoplastic stage. Finally, the failure stage was reached. The bearing capacity of the specimen decreased until it failed and the descending speed was slower than that of the unreinforced column, showing the ductile failure characteristics of the strengthened specimens. The slopes of the load–displacement curves for the strengthened specimens were larger than those of the corresponding unreinforced columns, indicating that the stiffness increased after strengthening. In the same way, the slopes of the curves for the self-stressed strengthened specimens were larger than that of the ordinary strengthened specimen, indicating that self-stress enhances the stiffness of the strengthened specimens.

**Figure 10.** Load–displacement curves of the specimens.

#### *3.5. Load–Strain Curves*

Figure 11 shows the load–strain curves of the specimens. For unreinforced columns, only the data before the peak load were analyzed, because the concrete strain was unstable and inaccurate after the peak load. Before the loads of P1 and P2 reached their peaks, the longitudinal and circumferential strains increased linearly. When it was close to the peak load, the longitudinal strain curve showed a downward bending trend with faster strain growth, which was similar to the axial compression in concrete column [9,25]. For the strengthened specimens, the load–strain curve was similar, whereby the longitudinal strains generally increased linearly and the downward bending tends appeared when it was close to the peak. The ultimate strains were much larger than for the unreinforced columns, which was in agreement with the conclusion found in [9,20]. The circumferential strains were small before the peak load and the strain increased obviously when the load approached the peak. This was due to the segments limiting the circumferential deformation of the specimens before the peak load, while the circumferential strains increased significantly after the failure of the segments.

**Figure 11.** Load–strain curves of the specimens.

#### *3.6. Results and Discussion*

Table 7 indicates the axial compression test data for the specimens. The results of the test were reasonable and were consistent with the related studies [9–12,20,25]. Hence, the correctness of the axial compression test on the specimens was verified. According to the objective of this test, the test results were compared to verify the reinforcement effect

of the IPCSAM and to analyze the impacts of the reinforcement parameters on the axial compression performance of the strengthened specimens.


**Table 7.** Results of the axial compression test.


to the loss of the restriction provided by the sleeve, finally leading to the decreased bearing capacity.

(5) The influence of the thickness of the filled concrete on the strengthening effect: By comparing S1-S with S2-S, it was clear that the cross-sectional areas of these two strengthened specimens were the same, although the larger the thickness of filled concrete, the lower the bearing capacity of the specimen. The bearing capacity was borne by the filled concrete of the strengthened specimens, which complied with the code in [24], while the utilization factor of the filled concrete was 0.8 compared with the column; hence, for S2-S, the thickness of the filled concrete was larger than S1-S, so the bearing capacity was lower than S1-S.

### **4. Axial Compression Bearing Capacity**

The axial compression bearing capacity of the strengthened pier was related to parameters such as the cross-sectional area and strength of the SSAWC, the cross-sectional area and concrete strength of the LCSS, and the self-stress value product produced by the SSAWC. Due to the complexity, long production time, and high cost of the specimens, only one strengthened specimen was prepared for each parameter. Referring to relevant research [9], the test models were supplemented by the finite element models while considering the influence of different parameters. On the basis of the results of the tests and extended analysis, the calculation formula for the bearing capacity was established.
