# Assessment of Mechanical Properties of Corroded Prestressing Strands
## 3 Tensile Test of Corroded Strands
### Residual Mechanical Properties from Test Result  
In this study, the residual mechanical properties due to corrosion were defined as the relation between the section loss and yield (or ultimate) strength. The section loss is explained in terms of the strand-unit and wire-unit section losses. The reason the wire-unit section loss was considered is because of the assumption that the corrosion of a strand can be localized in a few of the seven wires. A premature wire fracture due to corrosion would be the determining factor in the performance of the corresponding strand. This was validated by the decrease in strength owing to wire-unit fracture shown in the experiment (Figure 10). ASTM A416 [19] defines the yield load as a 1% extension or 10,000 \(\mu\epsilon\), and EC2 [20] defines it as a 0.1% proof stress. However, as the extracted strand used in this research was manufactured under the KS D 7002 [21] standard, the definition of yield was a 0.2% proof stress, and the yield strength of a corroded strand was defined as a 0.2% proof stress. Specimens with severe corrosion that did not show a ductility up to a 0.2% strain rate were excluded from the yield behavior graph. Figure 12a,b shows the relation of a 15.2 type strand's section loss with its yield strength and yield strain, respectively. Figure 12c,d shows the relation of a 12.7 type strand's section loss with its yield strength and yield strain, respectively. The coefficient of determination, R\({}^{2}\), had a maximum value of 0.0409, which indicated that the regression analysis was ineffective and the correlation of two factors was very low. This means that with the section loss of 22.52% (the most corroded specimen), the yield strength and strain showed no significant gap even under the increase in section loss, as explained in Figure 10b. However, Figure 12b, which includes numerous highly corroded strands, indicates that the yield strength was likely to decrease if the number of corroded wires and corrosion level are high. Figure 13 shows the corroded strands' yield strength and strain according to the section loss of the most corroded wire within each strand. As shown in Figure 12, the yield properties and level of corrosion from the most corroded wire also showed no significant correlation. Before evaluating the ultimate behavior of the corroded strands, as the two strands had different ultimate strengths, to compare them, the ultimate strength was normalized through the calculation of \(P_{u}/P_{0}\). \(P_{u}\) refers to each corroded strand specimen's ultimate strength, and \(P_{0}\) refers to the ultimate strength of a non-corroded strand. \(P_{0}\), the ultimate loads from \(f_{pu}\) in Table 1, were 261.20 kN and 184.28 kN for the 15.2 type and 12.7 type, respectively. If \(P_{u}/P_{0}\) is equal to 1.0, it means the strand's strength did not decrease. Figure 14a,b shows the relations between the strand-unit section loss and the ultimate strength and strain. As the graph indicates, unlike for the yield strength, the two factors seemed to be correlated. Furthermore, the decrease in ultimate strain could be a more serious problem than the decrease in strength. For example, for a 5% section loss, a 20% maximum strength loss can occur; however, in the case of ultimate strain, for a 5% section loss, an 85% maximum decrease in deformation capacity can occur. Figure 12: Yield properties of corroded strands, evaluated via section loss of strands. Figure 13: _Cont._  
Figure 14: Ultimate properties of corroded strands. Figure 13: Yield properties of corroded strands, evaluated via the section loss of the most corroded wire in each strand. Regarding the test results, the strength of the non-corroded strands (\(P_{u}/P_{0}=1\)) and ultimate strain (7.89%, average of the ultimate strain of the two types listed in Table 1) were set as the axis and intercept, and a regression analysis was conducted. The ultimate strain showed a high deviation, implying a low credibility of the index regression analysis result (R\({}^{2}=-0.015\)).