## 4 Experimental Results
### WEB Part of T-Shape PC Girder Bridge
The gray value of the line segment, where only concrete exists (see Figure 14), is plotted in Figure 16. An 8 th-degree polynomial fitting curve was added to the original profile. At that stage, the relative background X-ray intensity distribution was obtained. These data were utilized to compensate for the gray values of unfilled grout and PC wires to obtain their ratio, which is the proposed index for the stage of unfilled grout. Schematic gray value profiles of the filled and unfilled grout cases are shown in Figure 17a. In fact, these lie on the background of the X-ray point source, as shown in (b). In this measurement by the X-ray FPD, heavy and light materials appear dark and gray, respectively. The gray values of the dark and gray parts correspond to the low and high gray values, respectively. We needed to evaluate the background X-ray intensity distribution first and subtract it from the measured data. We tried to represent the gray value at the plateau, \(G_{w}\), for PC wires, as shown in (a). The boundary to the grout area appeared to be less dark and its gray value, \(G_{w\_faw}\), became lower because only a few wires were located. There was gradation in the region of the filled grout as the effective thickness changes (see (a)). Thus, the medium gray value was selected for the filled grout as \(G_{grout}\). The gray value at the high/bright peak was regarded as \(G_{no\_grout}\). The ratio, \(K\), of the gray values at filled/unfilled grout and PC wires was calculated for several situations as  
Figure 12:0  
Then,  
Figure 12:1  
Figure 12:2  
Figure 12:3  
Figure 12:4  
Figure 12:5 Gray value profile and the polynomial fitting along the line segment in Figure 14 as the background X-ray intensity distribution. Because \(G_{w}\) is lower than \(G_{grout}\) and \(G_{no\_gravit}\), both \(K_{uw}\) and \(K_{fu}\) must be higher than 1. \(K_{uw}\) at the unfilled grout must also be higher than \(K_{fu}\) at the filled grout, as explained above. The inner contents of the image in Figure 14 are explained in Figure 12:6 The grout appears wholly unfilled in the broken line frame. The bright blank indicates the vacancy at this location, leading to a high dose at the FPD, which reveals a grout flaw. In comparison, the dark straight region above the broken line frame indicates the existence of PC wires, which remarkably attenuate X-rays. To make a quantitative evaluation, it was necessary to establish a method based on sufficient numerical tools. Gray value evaluation of the raw images was expected to be effective on this occasion. Figure 19 shows the gray value profile plotted along the line segment that crosses the PC sheath transversely. The diameter and thickness of the PC sheath tube were \(\sim\)38 mm and \(\sim\)1 mm, respectively. Approximately 15 PC wires of \(\sim\)7 mm in diameter were installed in a sheath. Each wire can be recognized almost entirely. The magnified profile of the gray value across the PC sheath is shown in Figure 12:7 We can clearly observe the locations of the PC wires and grout in the concrete. The dark and bright/white parts of the PC  
Figure 12:8 Schematic drawing of the correlation between filled/unfilled grouts and gray value profiles. (**a**) Typical gray value profiles for filled and unfilled grout cases; (**b**) The profiles lie on the intensity background due to point source X-rays. Figure 12:9 X-ray Image acquired from 200 mm thick WEB of the T-shape PC girder bridge and explanation of the inner structure. wires and grout correspond to the lower and upper gray value peaks, respectively. This quantitative result is consistent with our visual recognition of the original X-ray image. That is, the bright/white part beneath the PC wires appears as a fully unfilled grout. This is supported by gray value analysis. Thus, this case can be evaluated as a fully unfilled grout. Next, we proposed a quantitative index of the stage of the unfilled grout.