## 5 Conclusions  
A new TFL method is proposed to estimate the location and amount of damage in the external tendon of PSC structures. This TFL method uses AC-delivering solenoid with a secondary coil inserted in an external primary coil enclosing the tendon. The primary coil is fed with a current in the device to cause the magnetic field, and the change in the magnetic flux at the damaged area of the external tendon is detected and quantified by the variation of the voltage in the secondary coil. Unlike the MMF (Main Magnetic Flux) method, which usually uses a heavy magnetic yoke, the proposed TFL method uses a light solenoid-shaped device. In addition, the proposed TFL method can provide a stable output due to the AC input signal and can be applied to any cable system with minor modification.  
The following conclusions can be derived from a series of experiments considering various degrees and lengths of damage in the tendon.  
1. Sufficient output signal could be acquired from the secondary coil for identifying the damage on the external tendon even when using the device with light and simple structure and fed with low-level  
Figure 13: Rate of decrement in induced voltage of secondary coil.  
AC. The proposed method was proved to satisfy the simplicity, portability, and low-energy power requirements for detecting damage in the external tendon of the PSC structure.
2. There is concern about the occurrence of undesired disturbance in the induced voltage from the secondary coil if the moving speed is not constant in the process of detecting the damage location by moving the device throughout the length of the external tendon. In this research, a sufficiently stable magnetic flux was generated by means of AC that inverted the direction of the power source as much as its frequency even though the operator was moving the device without specific speed control.
3. The signal induced in the secondary coil by the AC delivered to the primary coil of the device is an AC signal with the same frequency as the input signal on the primary coil and is governed only by the tendon area in the duct. The signal measured in the secondary coil reflected the change of the total flux throughout the length of the external tendon. The location of the damage could be identified by analyzing only the peaks of the induced signal measured when moving the device throughout the length of the external tendon. Intuitive detection of the rupture of 1 or more strands or damage length of 2 cm or longer could be achieved by identifying the reduction of the amplitude of the signal measured in the secondary coil. The proposed method proved its feasibility in effectively detecting the damage by measuring the change of the total flux in the external tendon.  
Conceptualization, I.K. and C.J.; methodology, I.K. and J.-Y.C.; software, H.K. and K.-Y.P.; validation, I.K., H.K., and C.J.; resources, J.-Y.C.; data curation, K.-Y.P.; writing--original draft preparation, I.K.; writing--review and editing, C.J.; visualization, K.-Y.P.; project administration, C.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Korea Institute of Civil Engineering and Building Technology (KICT) of the Republic of Korea, Project No. 2020-0033.  
The authors declare no conflict of interest.