## 3 Test Results and Analysis  
### Surface Damage  
A total of 682 records of the surface damage were found on the superstructures of thirteen bridges. According to the different components on the bridges (single hollow slab girder and hinge joint), typical defects were classified as shown in Table 5. Two defects were mainly observed during the inspection, i.e., transverse cracks on the bottom plates of the single girders and longitudinal cracks on the hinge joints. They account for more than 80% of the surface damage of the single girder or the joint, and typical images of the main defects are shown in Figure 7.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{5}{c}{**Grade**} \\ \cline{2-6}  & **A** & **B** & **C** & **D** & **E** \\ \hline \(\rm D_{b}\) & \(\geq\)0.95 & 0.90 to 0.95 & 0.80 to 0.90 & 0.70 to 0.80 & \(<\)0.70 \\ \(\rm D_{c}\) & \(\geq\)0.98 & 0.95 to 0.98 & 0.90 to 0.95 & 0.80 to 0.90 & \(<\)0.80 \\ \(\rm D_{d}\) & \(\geq\)0.95 & 0.85 to 0.95 & 0.70 to 0.85 & 0.55 to 0.70 & \(<\)0.55 \\ \(\rm D_{e}\) & \(\geq\)0.80 & 0.60 to 0.80 & 0.40 to 0.60 & 0.20 to 0.40 & \(<\)0.20 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Evaluation criteria based on the different parameters.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Component** & **Defect** & **Number** & **Total** & **Proportion (\%)** \\ \hline \multirow{4}{*}{Single girder} & Transverse crack on the & 313 & & 85.1 \\  & bottom plates & 27 & 368 & 7.3 \\  & Rebar corrosion & 18 & 4.9 \\  & Longitudinal crack on & 10 & & 2.7 \\  & the bottom plates & & & \\ \hline \multirow{4}{*}{Hinge joint} & Longitudinal crack & 256 & & 81.5 \\  & Rebar corrosion & 30 & 9.6 \\ \cline{1-1}  & Transverse crack & 21 & 6.7 \\ \cline{1-1}  & Seepage & 7 & & 2.2 \\ \hline Total & & 682 & 682 & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Surface damage at different components.  
In order to quantitatively evaluate the surface damage, the characteristics of the main defects were obtained including the location and length and width of the cracks. Table 6 shows the location of the cracks in the longitudinal direction. It could be found that the transverse cracking on the bottom plates of the girders mainly occurred in the mid-span of the bridges, while the distribution of the longitudinal cracks on the joints was quite uniform along the longitudinal location except the through cracks. The through cracks along the span length account for 37.1% of all the cracks, indicating the severe damage of the joints after 25 years' service life.  
The dimension distribution of two kinds of cracks is shown in Table 7. Most of the crack length and width of the transverse cracks on the bottom plates of the girders is within 0.5 m and 0.5 mm, respectively. The number of cracks decreases when the crack length increases from 0.2 to 0.5 m or when the crack width increases from 0.2 to 0.5 mm. For the longitudinal cracks on the joints, the crack length has a wide range, and the crack width is mainly less than 6 mm. Most of the cracks which extend partly on the joints were observed near the bearings.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Transverse Cracks on the**} & \multicolumn{2}{c}{**Longitudinal Cracks on the**} \\  & \multicolumn{2}{c}{**Bottom Plates**} & \multicolumn{2}{c}{**Hinge Joints**} \\ \hline
**Longitudinal** & \multirow{2}{*}{**Number**} & \multirow{2}{*}{**Proportion (\%)**} & \multirow{2}{*}{**Number**} & \multirow{2}{*}{**Proportion (\%)**} \\
**Location** & & & & \\ \hline
0–1/8 span & 17 & 5.4 & 31 & 12.1 \\
1/8 span–3/8 span & 61 & 19.5 & 26 & 10.2 \\
3/8 span–5/8 span & 220 & 70.3 & 47 & 18.4 \\
5/8 span–7/8 span & 10 & 3.2 & 36 & 14.1 \\
7/8 span–1 span & 5 & 1.6 & 21 & 8.2 \\ Through crack & 0 & 0.0 & 95 & 37.1 \\ \hline Total & 313 & & 256 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Location distribution of the main defects.  
Figure 7: Main defects in the visual inspection.