# Assessment of Mechanical Properties of Corroded Prestressing Strands
## 5 Conclusions  
In this study, a total of 86 seven-wire strands taken from existing post-tensioned segmental box girder bridges were investigated for section loss due to corrosion. The mechanical properties of the corroded strands derived from a 95% lower reliability curve were estimated by tensile tests. The flexural behaviors of four corroded PSC beams with different levels and locations of corrosion were also introduced. A method to evaluate flexural strength using an approximated \(f_{c.ps}\) calculation was suggested. The conclusions of this study are as follows. 1. A higher number of corroded wires in one strand tends to be associated with a higher level of corrosion (section loss) in the wire. This indicates that the corrosion reactions continue to occur in some wires, and once a certain level of corrosion occurs, other wires also begin to corrode. Because a plurality of strands is bundled together in one tendon due to tension, the inner wires are protected from corrosive conditions as compared to the outermost wires; therefore, inner wires corrode sequentially after the outer wires are sufficiently corroded. 2. The maximum wire-unit corrosion in a strand can be very different between corroded strands, even though they have the same level of strand-unit corrosion. If the strand fracture is defined as the moment at which a single wire fractures, it is necessary to consider the cross-sectional loss in the wire, after at least a 2% section loss of a strand. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & \multicolumn{2}{c}{**Ultimate Strength**} & \multirow{2}{*}{\(f_{c.ps}\)} & \multirow{2}{*}{**Maximum Load (Test)**} & \multirow{2}{*}{**Maximum Load**} & \multirow{2}{*}{**Difference**} \\  & **of Corroded Strand** & & & & \\  & **Based on 95\% Lower** & & & & \\  & **Reliability Limit** & & & & \\ \hline RB & 1860 MPa & 1614 MPa & 163.9 kN & 154.47 kN & \(-\)5.75\% \\ QB & 1432 MPa & 1293 MPa & 161.0 kN & 154.47 & \(-\)4.06\% \\ MB1 & 1423 MPa & 1289 MPa & 146.4 kN & 138.53 kN & \(-\)5.38\% \\ MB2 & 1469 MPa & 1327 MPa & 135.6 kN & 140.6 kN & 3.69\% \\ \hline \hline \end{tabular}  
* Ultimate load for the section at point of corrosion (1/4 of span length).  
\end{table} 
Table 4: Flexural strength evaluation using approximated determination of \(f_{ps}\) with Equation (7). Figure 17: Determination of \(f_{c.ps}\) from 95% lower reliability limit of ultimate strength. Within the wire-unit section loss of 45.79% and the strand-unit section loss of 22.52% (the most corroded specimen among the 86 specimens), the yield strength and strain showed no significant gap even under the increase of section loss, because even if one wire reaches a yield first due to cross-sectional loss, the remaining wires can carry the load until they yield. However, the yield strength is likely to decrease if the number of corroded wires and the corrosion level are high. 4. The ultimate strength and strain for the corroded strand were introduced by the lower limit curve of 95% reliability from the tensile test result. The ultimate strength proportionally decreased; the ultimate strength was reduced by 39.11% if the wire-unit section loss was 45.79% based on the lower limit curve of 95% reliability. The ultimate strain of the corroded strand could not meet the requirement of minimum tensile strain (3.5%) from KS D 7002 [21] if the wire-unit section loss exceeded approximately 4%. Formulas for the lower limit curve of 95% reliability were also introduced. 5. Based on the test results of the PSC beam specimens with a single corroded strand, a method to evaluate the flexural strength using \(f_{c,ps}\) was suggested. The proposed method showed an approximate estimate of flexural strength with a difference of 4.72% on average; however, it is limited to considering the rapid reduction of the ultimate strain of corroded strands. A further study is required for the following issues. This study considered the single-strand PSC beam.