## 1 Introduction  
The extensive application of prestressed concrete (PSC) in structures can be credited to its structural and economic efficiency. In the PSC structure, the tendon plays a critical role to introduce prestress. The tendon is anchored to the concrete member at its ends and is inserted in a duct generally made of HDPE (High Density PolyEthylene) duct filled with grout to protect it from external ingress. However, the corrosion of the prestressing tendon accelerates in corrosive environments as the tension force becomes higher [1]. There is a long list of structures that suffered severe degradation or even collapse because of the rupture or sectional loss of the corroded tendons' strands [2, 3]. Following the risk of such corrosion-induced damages, Germany and Japan implemented large-scale investigations on their PSC bridge inventories, which made it possible to detect numerous damages, from minor damage like early corrosion to severe damage like tendon failure [4, 5]. The common causes of damage in the external tendon of PSC structures are known to be the low quality of grouting, the formation of air voids in the duct, and the chemical reaction of deicing chemical and chloride ion [6, 7]. For example, corrosion of the tendons near their anchorage and cracks in the HPDE duct occurred at the Mid-Bay Bridge, Florida, USA, in 2000. One external tendon failed completely, another one experienced partial damage, a tendon suffered severe corrosion near its anchorage, and huge quantities of air voids were found in the ducts at mid-span and around their anchorages [8]. In 2007, two tendons among the 480 tendons with grouted post-tensioned strands of Varina Enon Bridge, Virginia, USA, were replaced because of the formation of voids at their high ends which compromised the corrosion protection of the tendons [9]. In 2016, an external tendon of Jeongneungcheon Bridge, Seoul, Korea, broke due to corrosion and the investigation could attribute it to the poor quality of the grouting [10; 11; 12]. Later in 2019, corrosion literally exploded a 1000-ft cable on Wando River Bridge, South Carolina, USA, and sent scraps of grout and plastic as far as 30 m. The corrosion was caused by water seeping through the concrete deck [13]. From these examples, the poor quality of the grouting in the PSC structure appears to be one major initiator of the corrosion or damage of the tendon. Periodic non-destructive evaluation (NDE) of the PSC structure is thus necessary for proper maintenance, and repair shall be implemented when the sectional loss reaches a definite level. Accordingly, the FHWA guideline(FHWA-HRT-13-027) states that the tendon must be replaced when the sectional loss exceeds 5% [14]. This study pays attention to the non-destructive method to detect the change of the magnetic flux induced by the delivered magnetic field at the sectional loss area. Typical NDE methods delivering the magnetic field to the tendon are RMF (Remnant Magnetic Field), MFL (Magnetic Flux Leakage), and IMF (Induced Magnetic Field). These methods have shown the potential to evaluate the corrosion in the tendon section [15; 16]. The disadvantage of the MFL method is that the Hall sensor measuring the leakage of the magnetic field is sensitive to the distance to the damage in the tendon. In other words, a weak signal from the Hall sensor cannot tell if the damage exists or if the distance to the intact tendon from the Hall sensor is far. According to the research using the MFL method on the external tendon, the existence of the sectional loss was well identified, but the difference between the estimated amount of sectional loss and the actual amount of the sectional loss was bigger as the amount of sectional loss increased [17]. In another study for improving the MFL method, a method to measure the residual flux leakage induced by a permanent magnet was sensitive to the magnetization of the tendon [18].