## 3 Experiment  
Recalling that the tendon is made up of a bundle of strands and each strand is made up of wires, the tendon considered in this study is made of fifteen 7-wire strands. The sectional loss of the external prestressing tendon is simulated as shown in Figure 4 by the number of snapped strands and the length of the damage in the tendon. For the convenience of the experiment, the HDPE duct of the external tendon is replaced with a transparent acrylic duct having similar relative permeability to HDPE. The tendon is placed inside the acrylic duct, and the length of the damage near the center of the tendon is changed from 0 to 10 cm. The number of snapped strands is increased from one to three to observe the secondary coil output according to the amount of damage. Table 1 describes the cases setup for the 3-strand damage case. The device delivering the magnetic field to the external tendon and detecting the variation of the magnetic flux is made by the assemblage of two 37-cm long plastic half-cylinders as shown in Figure 5a. The secondary coil with 18 turns is installed at the center of the frame for detecting the magnetic flux signal and the primary coil with 400 turns over a length of 30 cm is placed over the secondary coil to generate the magnetic field. The stability of the magnetic field along the longitudinal axis measured at the center of the cross section is verified in Figure 5b by comparison with the theoretical values. However, the device in Figure 5 is unsuitable for in situ inspection that needs frequent assembly and disassembly due to the large number of numerous turns. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{Length of Section Loss (cm)} \\ \hline
0 & 2 & 4 & 6 & 8 & 10 \\ \hline  
Accordingly, our research team developed a mountable frame where the primary and secondary can be easily connected by plugging as shown in Figure 6a to avoid the repeated winding of the coils in situ. This so-called wrapping solenoid-shaped device also exhibited stable magnetic field through the comparison of the measured and theoretical values (Figure 6b). Nevertheless, the original setup of Figure 5 is employed here for evaluating the feasibility of the TFL method in detecting the sectional loss in the external tendon. Research about the improved device shown in Figure 6 will be implemented in the future to investigate its accuracy and performance. In addition, the operation of the solenoid-shape device with a battery will be tested in the future to verify the stableness of the device's results with a less stable power supply for in situ inspection. Although an automatic movement of the device can reduce the uncertainty from the inconstant moving speed of the device, the automation of the device movement is not considered in this study. In general, near the anchorage where is damages are often found, tendons are closely spaced with each other, and an available space that the device can use is limited. The automation equipment also increases the weight of the device that is another limitation for field application. Therefore, the device was designed to remove inessential function in order to fit into the limited space and weight. But the automation equipment can be installed without difficulty if necessary. Figure 7 shows the external tendon specimen with the installed device. During the inspection, the distance travelled by the device along the axis of the tendon was measured in real time with respect to the reference point using a laser distance meter. An AC with frequency of 10 Hz and amplitude of \(\pm\)0.6 A was delivered continuously to the primary coil of the device using the NI (National Instrument) equipment as shown in Figure 8. The signal was generated by the LabVIEW program on a laptop and  
Figure 5: Fabrication and installation of solenoid-shape device: (**a**) installed device; (**b**) distribution of theoretical and measured longitudinal magnetic fields at center of device.