# Strengthening Effect of the External Prestressing Method That Simulated a Deterioration Bridge
## 3 Test Result
The crack load of the 20-MPa specimen with low concrete compressive strength was lower than that of the 40-MPa specimen, and the 3-D19 specimen with small reinforcement quantity had a lower yield strength than that of the 3-D22 specimen. The behavior was similar depending on the reinforcement quantity after yielding, and it was found that the strength and stiffness of the non-strengthened RC beam was controlled by the reinforcement quantity rather than the concrete strength. Figure 7: Failure shape: (**a**) non-strengthened specimen and (**b**) strengthened specimen. In the case of the strengthened specimen, the crack load of the 20-MPa specimen with low concrete compressive strength was lower than that of the 40-MPa specimen, as in the non-strengthened specimen shown in Figure 9. Moreover, the 3-D19 specimen with a small reinforcement had a lower yield strength than that of the 3-D22 specimen. Only the crack and yield loads were analyzed as it was difficult to examine the failure load, which exhibits the maximum strengthening performance, owing to the failure of the anchorage in the strengthened specimen. Nevertheless, the increase in the crack and yield loads was examined before and after strengthening through the relationship between the load and strain rate of the tension reinforcement in the middle of the span, as shown in Figure 10. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Name of Specim**} & \multicolumn{2}{c}{**Crack (P\({}_{\text{cr}}\), kN)**} & \multicolumn{2}{c}{**Yield (P\({}_{\text{y}}\), kN)**} & \multicolumn{2}{c}{**Failure (P\({}_{\text{u}}\), kN)**} \\ \cline{3-8}  & & **Design** & **Measured** & **Design** & **Measured** & **Design** & **Measured** \\ \hline
1 & C40-D22-NS & 52.7 & 53.9 (102\%) & 162.7 & 188.9 (116\%) & 182.1 & 217.1 (119\%) \\
2 & C40-D19-NS & 52.7 & 57.7 (109\%) & 116.4 & 143.1 (123\%) & 127.3 & 163.5 (128\%) \\
3 & C20-D22-NS & 33.9 & 30.9 (91\%) & 161.0 & 184.9 (115\%) & 165.6 & 200.6 (121\%) \\
4 & C20-D19-NS & 33.9 & 31.2 (92\%) & 115.7 & 134.4 (116\%) & 118.9 & 155.4 (131\%) \\
5 & C40-D22-EP & 97.1 & 135.3 (139\%) & 272.4 & 303.1 (111\%) & 250.9 & 350.2 (140\%) \\
6 & C40-D19-EP & 103.7 & 128.9 (124\%) & 235.2 & 270 (115\%) & 205.7 & 291.4 (142\%) \\
7 & C20-D22-EP & 55.8 & 60.7 (109\%) & 241.3 & 253.6 (105\%) & 247.1 & 322.3 (130\%) \\
8 & C20-D19-EP & 58.9 & 58.9 (100\%) & 200.0 & 214.3(107\%) & 200.2 & 244.9 (122\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental results. Figure 8: Detachment of anchorage and anchor bolt installation shape (unit: mm): (**a**) detachment of anchorage and (**b**) anchor bolt installation in section. Figure 9: Load–displacement relationship: (**a**) non-strengthened specimen and (**b**) strengthened specimen. In addition, the effect of increasing the stiffness of external prestressing was unexpected as the difference in stiffness before and after strengthening prior to cracking was insignificant. It will also be difficult to examine the strengthening effect prior to cracking in the case of strengthening of a real bridge as estimating the crack load is challenging. The strengthening effect examined through the ratio of the experimental results of the strengthening to the experimental results of the non-strengthening of the crack load, yield load, and ultimate strength is as shown in Figure 11.  
In the case of the external prestressed specimen, the crack load increased by 215%, and the yield load increased by 161% on average compared to the non-strengthened specimen. The effect of increasing the crack load of the 40-MPa specimen with a large reinforcement  
Figure 11: Comparison of strengthening effects. Figure 10: Load–strain relationship: (**a**) C40-D22 specimen; (**b**) C40-D19 specimen; (**c**) C20-D22 specimen; and (**d**) C20-D19 specimen. quantity was greater than that of the 20-MPa specimen.