# Smart Sensing of PSC Girders Using a PC Strand with a Built-in Optical Fiber Sensor
## 2 Fabrication and Test
### Prestressing of Full-Scale BH Girder Bridge Prototype  
The prestressing of the BH girder bridge was executed by tensioning the tendons sequentially with respect to their number, as given in Figure 5 and Table 1. The process was performed in six steps by introducing prestress in tendons one to three of up to 3200 kN and, then, tensioning tendon four up to 2100 kN. The calculation provided an average prestressing force of 177.8 kN in the strands of tendons one to three and of 175.0 kN in the strands of tendon four. The prestressing force measured by the OSPC strand installed in each tendon was 173.1 kN for tendon one, 164.0 kN for tendon two, 165.7 kN for tendon three, and 168.5 kN for tendon four. These values were slightly lower than the target prestressing force and about 92.3% to 97.4% to the target values. However, if prestressing is conducted simultaneously using multiple hydraulic lacks for several strands, the resulting prestressing force for each individual strand would naturally exhibit a slight difference. This situation was reported in 2005 by [PERSON] and [PERSON] [15] through actual measurements. Moreover, [PERSON] et al. [16] analyzed the distribution of the prestressing force in the strands of the tendon and revealed that this distribution was normal with a variation of about \(\pm\)4% around the mean value. Accordingly, even if the prestressing force measured by the OSPC strand is slightly lower than the target value, the measurement can be assumed to be correct. Figure 8 plots the change of the prestressing force measured in tendon one by the OPSC strand at each tensioning step. It could be observed that the prestressing force presents different distributions along the longitudinal direction. It is well known that the prestressing force generally exhibits its largest value at the end that is tensioned (live end), but Figure 8 revealed that one sensor measured a slightly lower value near the hydraulic jack when the maximum tensioning force was attained. This discrepancy can be credited to the helical arrangement of the strands inside the tendon. Figure 8: Variation of prestress force in tendon one of full-scale BH girder bridge prototype per tensioning step measured by OSPC strand. Besides, the loss of prestressing occurring during the removal of the hydraulic jack after the tensioning process could also be measured. This loss is called the immediate loss and is arranged per anchor in Table 2. Assuming a loss of 5 mm due to the slip of the wedge in the anchor, the immediate loss that may occur in the anchors runs between 2.3% and 3.9%. However, the values of the immediate loss in Table 2 fell slightly beyond this range. This greater immediate loss can be explained by some imprecision in the relative arrangement of the wedge and strands and in the fixation of the wedge of the hydraulic jack. Nevertheless, these higher values for the loss are acceptable considering that the loss of the prestress force generally ranged between 15% and 30% by accounting for the immediate loss as well as the loss according to time. In addition, the anchor set length that is the distance over which the difference in the prestressing force due to the slip of the wedge at the removal of the hydraulic jack (step-7) and the maximum tensioning force (step-6) had an effect that was seen to be longer than 20 m in Figure 8. This length was significantly longer than the value of 13.9 m obtained by calculation for the anchor set length. This calculated value is usually adopted in studies related to the anchor set length because of the difficulty to acquire actual data all along the tendon in the PSC structure. Note that KICT is currently developing an OSPC strand with distributed sensing to improve the present OSPC strand with a finite number of sensing points. The future application of this new OSPC strand to PSC structures will make it possible to compute the anchor set length more accurately. The so-prestressed BH girder bridge prototype was finally completed by injecting grout and placing the top concrete.