## 4 Conclusions  
In this study, the material properties of 45-year-old PSC girder bridges were investigated and the strengthening effect of applying the external prestressing method was experimentally verified. For the purpose of this study, concrete and tendon material properties and effective tension evaluation were performed on the deteriorated bridge, and the load carry capacity of the bridge was compared through four-point loading tests before and after strengthening.  
The results of this experiment are expected to contribute to verifying the performance of the existing deteriorated prestressed concrete bridge and determining the required amount of strengthening.  
The results derived from this study are as follows.  
1. As a result of conducting a concrete material test by collecting approximately 50 cores from the girders of an old bridge, the compressive strength of concrete was found to be 33 MPa, which was 22% higher than that used in the design (27 MPa). However, it was not possible to confirm the change in compressive strength due to the absence of the experimental values during the bridge's construction. In addition, as a result of performing the tensile strength test by cutting five tendons in the deteriorated bridge, it can be concluded that the decrease in tensile performance was not significant as the tendons used in the bridge exposed for 45 years were also sealed with grout.
2. After exposing the internal tendon of the deteriorated bridge, the tendon was pulled in the transverse direction to evaluate the effective tension of the existing tendon. The effective tension is 25.1-32.7 kN and assuming the maximum introduced tension, the effective tension of 40-53% is measured; it is conceivable that about 50% of the prestressing force is lost during its service period.
3. When designing the anchorage for strengthening using the external prestressing method, in some cases, early failure of the anchorage was confirmed by considering the design strength up to the applied tension and not the tensile strength of the tendon. However, in this study, the anchorage did not fail until the member was destroyed because of the design reflecting the tensile strength of the tendon, the regulation of the anchor bolt spacing, and the strict consideration of the side anchors not considered in the design. Thus, it was confirmed that sufficient attention is required when managing strengthening design.
4. The four-point loading test before and after strengthening was performed, and the strengthening effect was determined through the increase in the measured crack load. Moreover, the behavior of the bridge was relatively accurately predicted by applying the effective moment of inertia. In addition, it was difficult to determine the strengthening effect before cracking because the external tensioning method had a minimal contributive effect on the stiffness before cracking.  
Conceptualization, S.-H.K. and J.-S.P.; methodology, S.-H.K. and J.-S.P.; software, S.-H.K. and T.-K.K.; validation, S.-H.K. and H.-B.P.; formal analysis, S.-H.K.; investigation, W.-T.J.; resources, S.-H.K. and W.-T.J.; data curation, S.-H.K.; writing--original draft preparation, S.-H.K.; writing--review and editing, S.-H.K. and J.-S.P.; visualization, S.-H.K.; supervision, J.-S.P.; project administration, J.-S.P.; funding acquisition, J.-S.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Smart Civil Infrastructure Research Program for the Ministry of Land, Infrastructure and Transport of the Korean Government, grant number 17 SCIP-B128496-01.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** This research was supported by a grant (17 SCIP-B128496-01) from the Smart Civil Infrastructure Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government.  
**Conflicts of Interest:** The author declares no conflict of interest.