## 3 Experimental Procedures
### Experimental Design
#### 3.1.1 Specimens Design  
1. Dimensions of the specimens: The strengthened specimens were composed of three parts: the lining concrete segment sleeve (LCSS), the unreinforced column, and the filled concrete (SSAWC). Referring to the specimen designs found in relevant studies [17; 23], 1/5 model proportions were adopted to avoid difficulties in preparation and deviation of the test results due to the small size. The dimensions of the unreinforced columns were as follows: diameters of 250 mm and 200 mm, height of 750 mm.
2. Representativeness and number of test specimens: The control variables of the test specimens selected in this paper were the diameter of the unreinforced column, axial compressive strength, thickness, self-stress, and axial compressive strength of the LCSS. In order to investigate the influence of the above variables and ensure the specimens were representative, the initial values of the control variables were determined by the code in [24], involving existing pier-strengthening technologies and the design parameters of ordinary piers. The diameter of the unreinforced column was 250 mm as a result of the 1/5 scale, the axial compressive strength of the filled concrete was 21.7 MPa, the thickness was 40 mm as a result of the 1/5 scale, and the axial compressive strength of the segment concrete was 20.9 MPa. Because it took a lot of time to prepare the reinforced test specimens, time and funds were limited when three specimens were prepared under the same parameter conditions. For the sake of speeding up the test process, the following strategy was adopted. According to reference [9], it was known that the errors for three specimens with the same parameters are generally within 10%. Therefore, only one specimen with the same parameters was prepared in this paper. In order to ensure the rationality and accuracy of the test data, the finite element model with the same parameters was established and the parametric analysis was performed. Based on the above principles, as well as the premise of each variable with a control group, a total of 6 test specimens were prepared in this paper, as shown in Table 3.
3. Specim grouping: The specimens were divided into three groups according to the objectives of this test. Table 3 shows the details. (1) The unreinforced group (UG) had two specimens with diameters of 200 mm and 250 mm respectively, which were used for comparison with the corresponding strengthened specimens to verify the strengthening effect. (2) The ordinary reinforced group (ORG) was a specimen strengthened with AWC, which was used for comparison with specimens in the other group to verify the influence of the SSAWC and the LCSS on the strengthening effect. (3) The self-stressed reinforced group (SRG) was composed of specimens filled with SSAWC, among which the concrete strength of the LCSS of specimen S1-SS was reduced to verify the influence of the strength of the sleeve concrete on the bearing capacity of the strengthened specimens, while the thickness of the filled concrete of specimen S2-S was increased to verify the effect of the thickness of the filled concrete on the bearing capacity of the strengthened specimens.