## 2 Ospec Performance Verification
### Long-Term Performance Verification--Creep Test  
A creep test was performed to verify the long-term performance of the OSPC strand alone. For comparison, the test was performed on a normal PC strand as well as the OSPC strand. The creep test examined the change in strain under constant stress, and it was crucial to apply a constant load to the OSPC strand throughout the test. To enable this, this study devised a load amplifier, as shown in Figure 2, and applied it to the experiment. The load amplifier used the principle of the lever and was designed to use a steel plate as a weight and to apply the amplified load to the specimen according to the distance ratio of the lever. Depending on the length ratios of a to b and c to d in Figure 2a, a load equivalent to about 80 times the actual weight was applied to the specimen. For accurate testing, monoheads were used at both ends of the specimen to create anchorages and were installed in the load amplifier. The strain was measured with an optical fiber sensor in the OSPC strand, and an electrical resistance strain sensor attached to the surface of the outer wire was used. In addition, to measure the relative slip between the core wire and the outer wire, a 10-mm displacement gauge was installed at the upper and lower parts of the specimen, and a load cell was installed to measure the change in the load. The load introduced by the load amplifier was about 175 kN, and data were obtained by performing measurements with each sensor while carrying out the test for more than 1000 h.  
Figure 2: Creep test (**a**) Load amplifier, (**b**) Creep test, (**c**) Installation of sensor, (**d**) Upper and lower displacement meters.  
Figure 3 shows the relative displacement-time curve, which is one of the creep test results of the normal PC and OSPC strands. The relative displacement was obtained by subtracting the displacement of the outer wire, located outside, from the displacement of the core wire, located in the center of the strand. The relative displacement of the OSPC strands ranged from about 0.3 mm to 1.6 mm at the beginning of loading in the upper and lower parts. The length of the specimen used in the creep test was very short, and the relative displacement varied for the normal PC strand and the OSPC strand, depending on the installation condition of the anchorage before loading. However, both strands showed a tendency to decrease the amount of change in relative displacement as the load was continuously applied over time. After a certain period of time, there was only a difference in the amount of change in the initial relative slip amount, with almost no subsequent change being observed after a certain period of time.  
Figure 4 shows a comparison of the strain-time curves of the normal PC and OSPC strands. While the change in strain was observed somewhat in the initial stage of loading, there was a section where it gradually stabilized over time. The abrupt change seen in the middle of the curve indicated the difference in the condition from the initial condition in the constant-temperature and constant-humidity (CTCH) chamber, owing to a power outage that occurred during the test. The CTCH chamber was repaired to continue with the test. This changed the temperature inside the laboratory for a certain period of time, and such a change in temperature resulted in a change in the strain of the specimen. After the laboratory temperature was normalized, the strain returned to the original pattern for both the normal PC and OSPC strands. After examining the change in strain of the OSPC strand under the load condition for more than 1000 h after the load was introduced, almost the same pattern was observed as in the normal PC strand. Therefore, the OSPC strand developed in this study showed high reliability measurement performance.