## 4 Experimental Results
### Model Samples of 750 mm Thickness for Box-Shape PC Grider Bridge  
Now, we explain the similar filled/unfilled grout analysis for the side WEB of a box-shaped PC girder bridge using the 3.95 MeV X-ray source. Because the PC concrete thickness is far greater than 400 mm, we needed to use the 3.95 MeV X-ray source here. Real bridge inspection continued, and the obtained results are currently under evaluation. Hence, the results of the preparatory experiments modeling the real situation are introduced in this paper. The target was a 750 mm thick PC WEB wall. We constructed model samples using pieces cut from real old bridges. The 750 mm thick assemblies are located between the 3.95 MeV X-ray source and the X-ray FPD detector, as shown in Figure 29a. The photograph is shown in (b). The PC wire used for insertion is shown in (c). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Location** & \(\mathbf{G_{w}}\) & \(\mathbf{G_{w\_few}}\) & \(\mathbf{G_{grout}}\) & \(\mathbf{K_{fw}}\) (\(\mathbf{G_{no\_grout}}\)/\(\mathbf{G_{w}}\)) & \(\mathbf{K_{fwf}}\) (\(\mathbf{G_{no\_grout}}\)/\(\mathbf{G_{w\_few}}\)) \\ \hline Left & 115 & 141 & 150 & 1.30 & 1.06 \\ Center & 135 & 152 & 175 & 1.30 & 1.15 \\ Right & 110 & 134 & 147 & 1.34 & 1.10 \\ Average & 120 & 142 & 157 & 1.31 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Gray values and \(K\) ratios of Figure 28 for 200 mm thick T-shape PC girder bridge by 950 keV. The grout is filled. Figure 28: Gray value profiles of (**a**) left; (**b**) center; (**c**) right in Figure 27 for 200 mm thick T-shape PC girder bridge by 950 keV. Figure 29: _Cont._For better image quality, multiple X-ray shots should be averaged into a single image. As shown in Figure 29, the 750 mm-thick assembly is irradiated by the 3.95 MeV X-ray source. Figure 30 shows the X-ray images for 1, 25, 50, and 100 shots, where one shot means 10 s exposure, and their averages are taken for each image to smooth the image [19]. When the exposure time is short and the transmitted X-ray intensity is weak, the image appears spotty and the gray value profile becomes noisy. If the exposure time is increased, the profile is expected to be smoother. Such a denoising effect by stacking 1, 25, 50, and 100 X-ray shots is clear, as shown in Figure 31. The noise attributed to the lack of X-ray intensity is remarkably reduced for many stacking shots. Because the concrete is significantly thick, the intensity of the transmitted X-ray becomes attenuated. Therefore, stacking using appropriate shots is necessary for an accurate quantitative analysis of filled and unfilled groups based on their gray value difference. Now, X-ray images taken by the 3.95 MeV source for the model samples of a box-shaped PC girder bridge were analyzed. The inner contents of the samples are shown in Figure 32. Two declined sheaths can be observed. In this case, only a few wires are inserted into the remaining grout in the upper sheath 1, and many wires are located in the lower sheath 2. The gray value profiles were evaluated on the indicated line segments. Gray value profiles and analysis were performed, as shown in Figure 33. A few wires are clearly recognized by the small negative peak in the figure. The calculated values are listed in Table 5. Because there were only a few wires in sheath 2, only \(K_{uw\_few}\) and \(K_{fw\_few}\) were evaluated. Additionally, \(K_{twd}\) was calculated. Figure 29: Experimental configuration (**a**) and photograph (**b**) for modeling the 750 mm thick side WEB wall of a box-shape PC girder bridge with the 3.95 MeV X-ray source and FPD. (**c**) PC wire for insertion. Figure 31: Denoising effects by stacking 1, 25, 50, and 100 X-ray shots. Figure 30: Measured X-ray images for 1 (**a**), 25 (**b**), 50 (**c**) and 100 shots (**d**). Gray value analysis lines are also indicated.