## 2 Ospec Performance Verification
### Long-Term Performance Verification-Relaxation Test  
In order to verify the long-term performance of the OSPC strand, a relaxation test was performed in addition to the creep test. The relaxation test, which examined the change in the load under the condition with the displacement kept constant, was performed according to the test method of KS D 7002 [17]. The loading rate was (200 \(\pm\) 50) N/mm\({}^{2}\) per min, and the load corresponding to 70% of the minimum value of the tensile load was applied and maintained for (120 \(\pm\) 2) s. The decrease in load was measured while maintaining the distance between the anchorages for 1000 h. The final relaxation value was expressed as a percentage value for the reduced load with respect to the original load. Figure 5 shows a schematic diagram of the test apparatus. The specimen was designed to be mounted on a rigid steel frame to prevent deformation, and the amount of change in the load was measured through a load cell. Both ends of the specimen were fixed using a mono anchorage each, and the specimen was prestressed with a hydraulic jack. When a  
Figure 3: Creep test results—relative displacement (**a**) Normal PC Strand, (**b**) OSPC Strand. short strand was prestressed using a hydraulic jack, a significant amount of instantaneous loss occurred depending on the wedge slip amount and the performance of the hydraulic jack. Therefore, a hydraulic nut was installed between the frame and the load cell to recover the loss immediately after prestressing. The prestress force was first introduced by the hydraulic jack, and the hydraulic nut was used to meet the target load in order to compensate for the immediate loss caused by the removal of the hydraulic jack. The normal PC strand to be compared was SWPC7 BL, with a tensile load of 261 kN, and a 70% load at 182.7 kN was applied to the specimen in this study. After prestressing to 182.7 kN and maintaining the load for 120 s, the amount of load reduction and change in strain over 1000 h were examined. Furthermore, strain sensors were attached to both sides of the steel frame in preparation for a case in which the load was reduced in the steel frame. Figure 6 shows the introduction of a load through a mono prestressing jack and a picture of the experiment. Figure 4: Creep test results—change in strain. Figure 5: Relaxation test simulation. Figure 7a shows the change curve of the load decrease with time for the normal PC and OSPC strands. In the figure, the degrees of load reduction, compared to the initial loads, were about 5.75 kN for the normal PC strand and about 5.25 kN for the OSPC strand. The initial loads were 180.50 kN for the general strand and 182.70 kN for the OSPC strand, which were about 3.19% and 2.87%, respectively, when calculated as a ratio. These results indicated that the relaxation performance of the OSPC strand was higher than that of the normal PC strand. However, such a load reduction did not satisfy the standard as it exceeded the standard value of 2.5% for normal low relaxation strands. Nonetheless, this value did not consider the load reduction in the steel frame. With respect to the strain reduction in the steel frame shown in Figure 7b, there was a difference in strain owing to temperature change over time, with a reduction of about 5 to 10 \(\mu\epsilon\) being observed. Considering the dimension of the steel frame of 150 mm \(\times\) 75 mm \(\times\) 6.5 mm \(\times\) 10 mm with a cross-sectional area of 2371 mm\({}^{2}\), the load was about 1.2 to 2.4 kN. Because the frame was installed on both sides, the load reduction could be seen as about 2.4 to 4.8 kN. Considering the load reduction in the frame calculated in this way, the load reduction for the normal PC strand ranged from 0.95 to 3.35 kN, and the load reduction for the OSPC strand was 0.45 to 2.85 kN. Calculated as a ratio, it was about 0.53% to 1.86% for the normal PC strand and about 0.25% to 1.56% for the OSPC strand, with both strands satisfying the standard of 2.5% for low relaxation strands.