## 4 Experimental Results  
### WEB Part of T-Shape PC Girder Bridge  
X-ray transmission images for a typical T-shaped PC girder bridge were acquired onsite, as shown in Figure 12a. The portable X-ray source box and magnetron box were lifted and finely positioned to the WEB part of the T-shaped PC girder bridge. The electric power source and water chiller are installed in a special vehicle with a 20 kVA electric diesel generator. The X-ray FPD was attached to the opposite side of the WEB part with respect to the X-ray source. Based on visual inspection, the WEB part indicated by the lines became the target ((b)). We found large surface concrete cracks and an exuded white Ca water stain. The inspection region of the PC wires, shown in (b), was examined using an X-ray source. Figure 12c shows the surface cracks and the stain and X-ray transmitted images at the two flaws. In the two X-ray images, we observed declined PC sheaths, where PC wires and grout were inserted, and vertical/horizontal reinforcing rods. In the upper and lower cases, the lower halves beneath the PC wires appear dark/black and bright/white, respectively. In particular, it was speculated that the grout was unfilled and as a result, rainwater was retained in (c). Then, the PC sheath was corroded, and water exuded from the corroded  
Figure 11: Procedure of X-ray inspection, flaw evaluation, and structural analysis. cracks of the sheath to the surface via cracks in the concrete. The obtained X-ray images were attached to the design drawing, as shown in Figure 12d. The inner situation of the PC sheaths in a rather wide region was then visualized. We then evaluated the stage of unfilled grout in the PC sheath by gray value plotting, using the ratio of the gray values of unfilled grout and PC wires from the measured X-ray images. Here, we needed to consider and calibrate the uniformity of the background X-ray intensity distribution. The X-rays used were emitted from a point with diameter of 2 mm at the W target and collimated by 17\({}^{\circ}\) by the W collimator. Therefore, the X-ray intensity at the FPD had an axisymmetric distribution. As the radius increased, the intensity decreased. As shown in Figure 13, if the source releases the X-ray beam in parallel, the dose is equally received by  
Figure 12: Photographs of onsite inspection and measured X-ray images by 950 keV X-ray source for a T-shape PC girder bridge. (**a**) Way of inspection; (**b**) inspection region; (**c**) filled and unfilled grouts in the X-ray images; (**d**) overall situation composed by several images. the detector at every location. The beam distribution is uniform. However, the X-rays from our point-source have a cone shape and radial intensity distribution at the FPD, as shown on the right-hand side of the figure. A typical X-ray image is shown in Figure 14. This was acquired from a 200 mm-thick WEB part of the T-shaped PC girder bridge using the 950 keV X-ray source. The PC sheath, wires, and other rods are observed. The definition of the gray value is shown in Figure 15. In this measurement, the black and white parts correspond to highly and slightly X-ray-attenuated parts, respectively. According to the X-ray attenuation coefficient, PC wires appear very dark while grout in the PC sheath and concrete appears bright. The locations of typical materials such as PC wires, rods, concrete, and filled/unfilled grouts are shown in Figure 14.  
Figure 14: Typical X-ray image of circular shape at the FPD screen and intensity profile evaluating line. Figure 13: Parallel beam vs. cone beam in the dose regarding location. Figure 15: Gray value from low to high and typical materials. Then, we checked the background X-ray intensity distribution on the line segment, as indicated in Figure 14. Here, we used an 8-bit system so that the full scale corresponds to 256. Darkness, or a lower gray value near the corners, is a result of an ununiformly distributed X-ray dose. The effect must be considered when the evaluation is based on gray values.