## 6 Discussion on the Effect of Cover Damage and Mortar Repair  
As shown in Figure 7, the removal of the concrete cover resulted in decreases of 14.4% and 22.2% in both initial stiffness and secant stiffness, respectively, as compared to the control case. However, the peak force only showed 1.2% reduction. This is mainly because the failure modes of the control and damaged cases (see Figure 8) were identical; both attributed to the pullout of the prestressing strands. Moreover, in the damaged specimen, the remaining core concrete was able to resist the compression diagonal strut force, despite the cover removal, without showing any signs of crushing. This was primarily attributed to the high concrete compressive strength. Nevertheless, the removal of the concrete cover showed a more detrimental effect on the stiffness and ductility than on peak load. The ductility of the damaged girder was reduced by 39.3% as compared to the control case.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multirow{2}{*}{**Peak**} & \% of & \% of & \% of Control & \% of & **Strand** & **Max. FRP** \\  & & **Control** & **Initial** & **Stiffness at** & **Control** & **Slip at** & **Strain,** \\  & & **Peak Load** & **Stiffness** & **Tielding** & **Ductility** & **Load, mm** & **mm/mm** \\ \hline Control & 543.1 & 100.0 & 100.0 & 100.0 & 100.0 & n.d. & n.d. \\ Damage & 536.4 & 98.8 & 85.6 & 77.8 & 60.7 & 0.34 & n.d. \\ Mortar & 496.4 & 91.4 & 84.7 & 80.0 & 46.5 & 0.40 & n.d. \\ CFRP & 557.3 & 102.6 & 97.7 & 81.2 & 109.6 & 0.25 & 0.0011 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test results of full-scale PC girders.  
Figure 8: Failure mode of control and damaged specimens (1:27): (**a**) control case after failure and (**b**) damage case after failure.  
Figure 7: Load, end slip vs. deflection curves.  
From the load vs. deflection curves, it can be seen that the mortar repair case nearly coincided with the damaged case. Compared to the control case, the mortar repaired case showed a reduction of 8.6%, 15.3% and 20.0% in the peak load, initial stiffness and secant stiffness at the yielding point, respectively. The ductility of mortar repair case was even worse than the damaged case with 53.5% reduction compared to the control case. Based on the test results, it seemed that the mortar repair alone failed to restore the shear capacity and ductility of the girder. There are several reasons that might have contributed to such unexpected behavior. First, the bond between the mortar mix and base concrete was not strong enough to allow stress transferred to mortar. Therefore, unlike the control and damaged cases, the mortar repair case only showed one main shear crack as shown in Figure 9a. Second, the strength and stiffness of the mortar layer was low compared to the base concrete and the mortar was not prestressed as the base concrete. Third, cracks within the mortar above the bearing plate were observed on both sides of the girder as shown in Figure 9b. This was due to the inability of the mortar to resist the vertical reaction as shown in Figure 9c. Such cracks impacted the integrity of the mortar cover and limited its participation in resisting shear stress. To prevent such cracks from developing, it is recommended that mortar repair should be terminated slightly above the bearing plate to prevent any contact with the bearing plate.