Access the full text.
Sign up today, get DeepDyve free for 14 days.
Loading next page...
/lp/multidisciplinary-digital-publishing-institute/non-destructive-evaluation-for-sectional-loss-of-external-tendon-of-E1G0eOXaBD
- Publisher
- Multidisciplinary Digital Publishing Institute
- Copyright
- © 1996-2020 MDPI (Basel, Switzerland) unless otherwise stated Disclaimer The statements, opinions and data contained in the journals are solely those of the individual authors and contributors and not of the publisher and the editor(s). MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Terms and Conditions Privacy Policy
- ISSN
- 2076-3417
- DOI
- 10.3390/app10217398
- Publisher site
- See Article on Publisher Site
Abstract
applied sciences Article Non-Destructive Evaluation for Sectional Loss of External Tendon of Prestressed Concrete Structures Using Total Flux Leakage Imjong Kwahk, Kwang-Yeun Park, Ji-Young Choi, Hungjoo Kwon and Changbin Joh * Department of Infrastructure Safety Research, Korea Institute of Civil Engineering and Building Technology, 283, Goyangdae-Ro, Goyang-Si, Gyeonggi-Do 10223, Korea; kwakim@kict.re.kr (I.K.); kypark@kict.re.kr (K.-Y.P.); legion@kict.re.kr (J.-Y.C.); kwonhj3@kict.re.kr (H.K.) * Correspondence: cjoh@kict.re.kr; Tel.: +82-31-910-0332 Received: 22 September 2020; Accepted: 19 October 2020; Published: 22 October 2020 Abstract: A non-destructive evaluation method is proposed to identify the sectional loss of the external tendon of prestressed concrete structures by detecting the change of the magnetic flux in the external tendon exposed to a magnetic field. The method uses a solenoid-shaped device with two coils: a primary coil for producing magnetic field and secondary coil for damage detection, wrapping the external tendon. A current applied to the primary coil in the device causes the magnetic field. Then, the change in the magnetic flux by the damage in the external tendon is detected by the variation of the voltage in the secondary coil in the device as the device moves along the tendon. An alternating current is applied to the primary coil to minimize the e ect of the moving speed of the device. As a result, the damaged area can be detected with a low-level energy current. In addition, a wrapping solenoid-shaped device that is easy to disassemble and assemble was developed for in situ inspection. The measured signal from the secondary coil has a sinusoidal form with the same frequency as the applied current to the primary coil, and the peak curve of the measured signal provides enough information to detect the damage. It is shown that the proposed method can quantitatively identify one or multiple damaged-tendon locations as well as damages of at least 2 cm. Keywords: total flux leakage; non-destructive evaluation(NDE); Prestressed Concrete (PSC); corrosion of the prestressing tendon; section loss of external tendon 1. Introduction The extensive application of prestressed concrete (PSC) in structures can be credited to its structural and economic eciency. 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 su ered 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 Appl. Sci. 2020, 10, 7398; doi:10.3390/app10217398 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7398 2 of 12 failed completely, another one experienced partial damage, a tendon su ered 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 [10–12]. 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 di erence 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]. Another method proposed to measure the variation of the total magnetic field flux through the yoke rather than measuring the magnetic field leakage is MMF (Main Magnetic Flux). Since the MMF method can quantify the section loss of the tendon, it can be utilized in combination with the MFL method [19]. Although the MMF method basically estimates the relative sectional loss, a method to figure out the absolute sectional loss by measuring the maximum and minimum of the magnetic flux change after delivering a strong bi-polar magnetic field to the tendon was proposed [20]. Another study proposed to quantify the sectional loss by comparing the slope of the B-H loop with the reference slope [21]. However, there was a diculty in distinguishing the slope change resulting from the sectional loss and the tensile stress in the tendon. The IMF method that is conceptually similar to the MMF method has shown a capability of detecting the damage location but there remains some doubt about the quantification of the damage [22]. In addition, another method called spontaneous MFL was suggested to estimate the sectional loss by measuring the magnetic flux delivered by the Earth’s magnetic field instead of an artificial magnetic field to the tendon [23,24]. However, the leaked magnetic flux was so small that an extremely sensitive magnetic sensor had to be used to measure it and the method also required separating the measured signal from the noise and the external magnetic field source. For the external tendon of the PSC structure, a heavy magnetizing system using a yoke for NDE is not suitable practically. It is also dicult to move the magnetizing system along the length of the tendon with a constant speed. Accordingly, this research adopts a simple solenoid to generate the magnetic field instead of the heavy yoke. In this framework, when the DC (direct current) is delivered to the solenoid, the induced voltage of the secondary coil related to the sectional change can be influenced by the moving speed of the device. Therefore, the AC (alternating current) is delivered to the primary coil to reduce the e ect of the moving speed of the device and generate larger induced Appl. Sci. 2020, 10, 7398 3 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 voltage in the secondary coil so that the flux leakage can be measured easily. The proposed method is voltage in the secondary coil so that the flux leakage can be measured easily. The proposed method voltage in the secondary coil so that the flux leakage can be measured easily. The proposed method expected to identify the sectional damage of the external tendon e ectively by increasing the sensitivity is expected to identify the sectional damage of the external tendon effectively by increasing the is expected to identify the sectional damage of the external tendon effectively by increasing the of the secondary coil. sensitivity of the secondary coil. sensitivity of the secondary coil. This paper proposes a novel damage-detecting method for an external tendon of PSC structures This paper proposes a novel damage‐detecting method for an external tendon of PSC structures This paper proposes a novel damage‐detecting method for an external tendon of PSC structures using a solenoid-shaped device. The principle of detecting sectional loss using a magnetic field is using a solenoid‐shaped device. The principle of detecting sectional loss using a magnetic field is using a solenoid‐shaped device. The principle of detecting sectional loss using a magnetic field is explained in Section 2, and the experimental demonstration of the method is presented in Section 3. explained in Section 2, and the experimental demonstration of the method is presented in Section 3. explained in Section 2, and the experimental demonstration of the method is presented in Section 3. The discussion of the experimental results and the conclusion of the research are drawn in Sections 4 The discussion of the experimental results and the conclusion of the research are drawn in Sections 4 The discussion of the experimental results and the conclusion of the research are drawn in Sections 4 and 5. and 5. and 5. 2. TFL Method 2. TFL Method 2. TFL Method The proposed TFL (Total Flux Leakage) method employs a solenoid similar to the EM The proposed TFL (Total Flux Leakage) method employs a solenoid similar to the EM The proposed TFL (Total Flux Leakage) method employs a solenoid similar to the EM (electromagnetic) sensor with a primary and secondary coil in order to estimate the sectional loss in (electromagnetic) sensor with a primary and secondary coil in order to estimate the sectional loss in (electromagnetic) sensor with a primary and secondary coil in order to estimate the sectional loss in the external tendon. The EM sensor was introduced to measure the residual stress in the tendon for the external tendon. The EM sensor was introduced to measure the residual stress in the tendon for the external tendon. The EM sensor was introduced to measure the residual stress in the tendon for NDE. The EM sensor consists of the primary coil to generate a magnetic field on the external tendon NDE. The EM sensor consists of the primary coil to generate a magnetic field on the external tendon NDE. The EM sensor consists of the primary coil to generate a magnetic field on the external tendon and the secondary coil installed at the center to detect the generated magnetic field. The governing and the secondary coil installed at the center to detect the generated magnetic field. The governing and the secondary coil installed at the center to detect the generated magnetic field. The governing equation of the EM sensor is well established [25,26]. Once a current is passed through the primary equation of the EM sensor is well established [25,26]. Once a current is passed through the primary equation of the EM sensor is well established [25,26]. Once a current is passed through the primary coil installed at the surface of HDPE duct of the external tendon, a magnetic field is generated inside coil installed at the surface of HDPE duct of the external tendon, a magnetic field is generated inside coil installed at the surface of HDPE duct of the external tendon, a magnetic field is generated inside and outside of the duct as shown in Figure 1. and outside of the duct as shown in Figure 1. and outside of the duct as shown in Figure 1. Figure 1. Magnetic flux distribution around electromagnetic solenoid. Figure 1. Magnetic flux distribution around electromagnetic solenoid. Figure 1. Magnetic flux distribution around electromagnetic solenoid. If a ferromagnetic material like the tendon is placed inside the duct along the longitudinal direction If a ferromagnetic material like the tendon is placed inside the duct along the longitudinal If a ferromagnetic material like the tendon is placed inside the duct along the longitudinal direc of thetion solenoid of the so aslenoid shown as in shown Figur in e Figure 2, and 2, if and the if tendon the tend is on intact, is intthe act, magnetic the magne flux tic fldeveloped ux developed by direction of the solenoid as shown in Figure 2, and if the tendon is intact, the magnetic flux developed the current in the primary coil will be distributed along the longitudinal direction of the tendon. by the current in the primary coil will be distributed along the longitudinal direction of the tendon. by the current in the primary coil will be distributed along the longitudinal direction of the tendon. From From th thee perspect perspective ive of of ma magnetic gnetic ma material, terial, th thee gr grout out f fill illing ing the the ex external ternal ten tendon don ha hass the the sa same me re relative lative From the perspective of magnetic material, the grout filling the external tendon has the same relative permeability with the air ( = 1) and has thus no e ect on the magnetic flux developed by the current permeability with the air (μr = 1) and has thus no effect on the magnetic flux developed by the current permeability with the air (μr = 1) and has thus no effect on the magnetic flux developed by the current in in th thee prim primary ary coil. coil. in the primary coil. Figure 2. Magnetic flux of solenoid installed on surface of tendon of the prestressed concrete (PSC) Figure 2. Magnetic flux of solenoid installed on surface of tendon of the prestressed concrete (PSC) Figure 2. Magnetic flux of solenoid installed on surface of tendon of the prestressed concrete structure. structure. (PSC) structure. If, additionally, a secondary coil is installed at the center of the solenoid as shown in Figure 3a, If, additionally, a secondary coil is installed at the center of the solenoid as shown in Figure 3a, If, additionally, a secondary coil is installed at the center of the solenoid as shown in Figure 3a, any change in the magnetic flux can be detected by the change in the induced voltage in the secondary any any chan change ge in in th thee ma magneti gneticc flux flux ca can n be be de detected tected by by the the change change in in the the induce induced d voltage voltage in in the the seconda secondary ry coil. Unlike the undamaged tendon case (Figure 3a), when the tendon suffers partial damage like a coil. Unlike the undamaged tendon case (Figure 3a), when the tendon su ers partial damage like a coil. Unlike the undamaged tendon case (Figure 3a), when the tendon suffers partial damage like a sectional loss due to the rupture of some strands, a part of the magnetic flux around the damaged sectional sectional loss loss due dueto tothe ther uptur ruptur e e of of some some strands, strands a, part a paof rt the of the magnetic magnet flux ic fl ar ux ound around the damaged the damaged area area is absorbed by the neighboring and undamaged strands and the remaining part of the magnetic area is absorbed by the neighboring and undamaged strands and the remaining part of the magnetic Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7398 4 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13 is absorbed by the neighboring and undamaged strands and the remaining part of the magnetic flux is flux is leaked (Figure 3b). In such a case, the secondary coil around the damaged area detects the leaked (Figure 3b). In such a case, the secondary coil around the damaged area detects the remaining remaining magnetic flux that has not been leaked. magnetic flux that has not been leaked. (a) (b) Figure 3. Change in magnetic flux distribution of solenoid according to eventual sectional loss of Figure 3. Change in magnetic flux distribution of solenoid according to eventual sectional loss of external tendon: (a) undamaged tendon; (b) damaged tendon. external tendon: (a) undamaged tendon; (b) damaged tendon. The electromotive force of the secondary coil induced by the current in the primary coil of the The electromotive force of the secondary coil induced by the current in the primary coil of the solenoid is expressed in Equation (1). The induced electromotive force, V , is simply the product of ind solenoid is expressed in Equation (1). The induced electromotive force, 𝑉 , is simply the product of dF the number N of turns in the secondary coil and , the first time derivative of the magnetic flux. 2nd dt the number 𝑁 of turns in the secondary coil and , the first time derivative of the magnetic flux. Note that V acts in the direction opposite the change of the magnetic flux. The magnetic flux F is ind Note that 𝑉 acts in the direction opposite the change of the magnetic flux. The magnetic flux 𝛷 is represented as the integration of the magnetic flux density, B, over the total internal cross-sectional represented as the integration of the magnetic flux density, 𝐵 , over the total internal cross‐sectional area, A, of the solenoid. ! ! dF d area, 𝐴 , of the solenoid. V (t) = N = N BdA (1) ind 2nd 2nd dt dt 𝑑𝛷 𝑑 ⃗ ⃗ 𝑉 𝑡 𝑁 𝑁 𝐵 ∙𝑑 𝐴 (1) where the total internal cross-sectional area of the solenoid A = A + A + A + A ; A is tendon air duct concrete air 𝑑𝑡 𝑑𝑡 the cross-sectional area of air in the cross-section of the solenoid; A is the cross-sectional area of the duct where the total internal cross‐sectional area of the solenoid 𝐴𝐴 𝐴 𝐴 𝐴 ; HDPE duct in the cross-section of the solenoid; A is the cross-sectional area of concrete in the concrete 𝐴 is the cross‐sectional area of air in the cross‐section of the solenoid; 𝐴 is the cross‐sectional cross-section of the solenoid; and, A is the cross-sectional area of the tendon in the cross-section of tendon area of the HDPE duct in the cross‐section of the solenoid; 𝐴 is the cross‐sectional area of the solenoid. concrete in the cross‐section of the solenoid; and, 𝐴 is the cross‐sectional area of the tendon in When a time-variant AC flows in the primary coil of the solenoid, the magnetic field changes the cross‐section of the solenoid. together and results in the change of the magnetic flux around the solenoid. The magnitude of When a time‐variant AC flows in the primary coil of the solenoid, the magnetic field changes the induced electromagnetic field is a function of A = A + (A + A + A ) = A + A . concrete tendon air duct together and results in the change of the magnetic flux around the solenoid. The magnitude of the Since A , A and A in A can be assumed as constants A , the induced electromotive force a air duct concrete induced electromagnetic field is a function of 𝐴𝐴 𝐴 𝐴 𝐴 𝐴 𝐴 . function of A = A . Compared to the permeability of the ferromagnetic tendon, the permeability tendon Since 𝐴 , 𝐴 and 𝐴 in 𝐴 can be assumed as constants 𝐴 , the induced electromotive of the air, concr ete, and HDPE duct is very small and practically identical to the permeability in vacuum, force a function of 𝐴 𝐴 . Compared to the permeability 𝜇 of the ferromagnetic tendon, the . Considering that all the parameters at the exception of A are controllable or constant, V in 0 ind permeability of the air, concrete, and HDPE duct is very small and practically identical to the Equation (1) can be expressed as a function of the area of the tendon, A as shown in Equation (2). permeability in vacuum, 𝜇 . Considering that all the parameters at the exception of 𝐴 are 2 3 Z Z Z ! ! ! ! ! ! 6 7 controllable or constant, 𝑉 ind Equation (1) can be expressed as a function d of the area of the tendon, 6 7 6 7 V (t) = N 6 HdA + BdA 7 N BdA (2) ind 2nd 0 2nd 4 0 5 𝐴 as shown in Equation (2). dt dt A A A 𝑑 𝑑 where H is magnetic field intensity. ⃗ ⃗ ⃗ ⃗ ⃗ ⃗ 𝑉 𝑡 𝑁 𝜇 𝐻 ∙𝑑 𝐴 𝐵 ∙𝑑 𝐴 ≅𝑁 𝐵 ∙𝑑 𝐴 (2) 𝑑𝑡 𝑑𝑡 where 𝐻 is magnetic field intensity. Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7398 5 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13 Appl. Appl. Appl. Appl. Appl. Appl. Sci. Sci. Sci. Sci. Sci. Sci. 2020 2020 2020 2020 2020 2020 ,, , 10 , 10 , 10 , 10 10 ,10 , , x, x, x , x FOR x FOR x FOR FOR FOR FOR P P P EER PEER PEER PEER EER EER RE RE RE RE RE RE VVVV IV IE V IEIEIW EIW EW EW W W 555 5 5 of 5 of of of of of 13 13 13 13 13 13 Equation (2) indicates that the measured variation of the induced voltage in the second coil by Equation (2) indicates that the measured variation of the induced voltage in the second coil by moving the solenoid throughout the length of the external tendon gives a means to detect the change moving the solenoid throughout the length of the external tendon gives a means to detect the change Equation Equation Equation Equation Equation Equation (2 (2 (2 (2 (2 )(2 ) ) )indi ) indi )indi indi indi indi cat cat cat cat cat cat eeesesese s sth sth th th th th aaaatta ta tthe t the tthe the the the mea mea mea mea mea mea sssured sured sured sured ured ured va va va va va va riat riat riat riat riat riat ion ion ion ion ion ion of of of of of of the the the the the the ind ind ind ind ind ind uuuuced uced uced ced ced ced vo vo vo vo vo vo lltltlatlatltaagtg ag agegege e ein ein in in in in the the the the the the second second second second second second co co co co co co ilililil il il by by by by by by of the sectional area of the tendon. of the moving moving sectional the the sar osolenoid ea lenoid of the thro thro tendon. uugghout hout the the len lenggthth of of the the ex external ternal tendon tendon gives gives a a mea meanns sto to detect detect the the change change moving moving moving moving the the the the ss o sosolenoid olenoid lenoid lenoid thro thro thro thro uuugugghout ghout hout hout the the the the len len len len gggth gth th th of of of of the the the the ex ex ex ex ternal ternal ternal ternal tendon tendon tendon tendon gives gives gives gives aa a amea mea mea mea nnnsns s to sto to to detect detect detect detect the the the the change change change change of of of of of of th th th th th th eee e esecti esecti secti secti secti secti oooona ona ona na na na ll l lar lar lar ar ar ar ea ea ea ea ea ea of of of of of of the the the the the the tendon. tendon. tendon. tendon. tendon. tendon. 3. Experiment 3. Experiment 3.3. Ex Expe peririme mentnt 3. 3.3. 3. Ex Ex Ex Ex pe pe pe pe riririri me me me me nt nt nt nt Recalling that the tendon is made up of a bundle of strands and each strand is made up of wires, Recalling that the tendon is made up of a bundle of strands and each strand is made up of wires, Rec Rec Rec Rec Rec Rec aaaallaling laling lling lling lling ling tha tha tha tha tha tha tt t tthe t the tthe the the the tend tend tend tend tend tend on on on on on on is is is is is is ma ma ma ma ma ma de de de de de de up up up up up up of of of of of of a a a a abundle abundle bundle bundle bundle bundle of of of of of of strands strands strands strands strands strands and and and and and and ea ea ea ea ea ea ccchchchch h str h str str str str str aaaand nd and and nd nd is is is is is is ma ma ma ma ma ma de de de de de de up up up up up up of of of of of of wires, wires, wires, wires, wires, wires, the tendon considered in this study is made of fifteen 7‐wire strands. The sectional loss of the external the tendon considered in this study is made of fifteen 7-wire strands. The sectional loss of the external the the the the the the tendon tendon tendon tendon tendon tendon co co co co co co nsider nsider nsider nsider nsider nsider ed ed ed ed ed ed in in in in in in th th th th th th is isis isis is study study study study study study is is is is is is ma ma ma ma ma ma ddddedede e eof of e of of of of fi fi fi fift fift fiftftft een een fteen een een een 7 7 7 7‐ ‐7wire ‐7wire ‐‐wire wire ‐wire wire st st st st st ra st ra ra ra ra ra nds. nds. nds. nds. nds. nds. The The The The The The secti secti secti secti secti secti oooonal onal onal nal nal nal los los los los los los sss s of s of s of of of of the the the the the the external external external external external external prestressing tendon is simulated as shown in Figure 4 by the number of snapped strands and the prestressing tendon is simulated as shown in Figure 4 by the number of snapped strands and the prest prest prest prest prest prest rrressing ressing ressing ressing essing essing tendon tendon tendon tendon tendon tendon is is is is is is si si si si si mu si mu mu mu mu mu la la la la la tla tetetted etd ed ed d das as as as as as sho sho sho sho sho sho w w w w w n w nnn n n in in in in in in Fi Fi Fi Fi Fi gu Fi gu gu gu gu gu rrrererer e e 4e 4 44 4 by 4 by by by by by th th th th th th eee e e number e number number number number number of of of of of of snap snap snap snap snap snap ped ped ped ped ped ped st st st st st ra st ra ra ra ra ra nds nds nds nds nds nds and and and and and and the the the the the the length of the damage in the tendon. For the convenience of the experiment, the HDPE duct of the length of the damage in the tendon. For the convenience of the experiment, the HDPE duct of the leng lengthth of of the the damage damage in in th the etendon tendon. .For For the the convenience convenience of of th the eex experiment, periment, the the HDPE HDPE du ductc tof of the the leng leng leng leng th th th th of of of of the the the the damage damage damage damage in in in in th th th th ee e etendon tendon tendon tendon .. .For . For For For the the the the convenience convenience convenience convenience of of of of th th th th ee e eex ex ex ex periment, periment, periment, periment, the the the the HDPE HDPE HDPE HDPE du du du du cctctc t of t of of of the the the the external tendon is replaced with a transparent acrylic duct having similar relative permeability to external tendon is replaced with a transparent acrylic duct having similar relative permeability to extern extern extern extern extern extern al al al al al al tend tend tend tend tend tend on on on on on on is is is is is is re re re re re re pla pla pla pla pla pla ccced ced ced ced ed ed with with with with with with a a aa a tr a tr tr tr tr an tr an an an an an sp sp sp sp sp sp aren aren aren aren aren aren tt t tt a at aa ccacarylic crylic crylic crylic rylic rylic duc duc duc duc duc duc tt t tt ha ha t ha ha ha ha vvvviviv ng ing ing ing ing ng si si si si si mil si mil mil mil mil mil aaaarrara r r re r re re re re re lat lat lat lat lat lat iive ive ive ive ive ve perme perme perme perme perme perme aaaabbababili bili bili ili ili ili ty ty ty ty ty ty to to to to to to HDPE. The tendon is placed inside the acrylic duct, and the length of the damage near the center of HDPE. The tendon is placed inside the acrylic duct, and the length of the damage near the center of the HDPE. HDPE. HDPE. HDPE. HDPE. HDPE. The The The The The The te te te te te ndon te ndon ndon ndon ndon ndon is is is is is is pla pla pla pla pla pla ccced ced ced ced ed ed insi insi insi insi insi insi de de de de de de th th th th th th eee e eacryli eacryli acryli acryli acryli acryli ccc c cduc cduc duc duc duc duc tt,t,t t, ,and t ,and ,and and and and the the the the the the len len len len len len ggggth gth gth th th th of of of of of of the the the the the the da da da da da da mage mage mage mage mage mage near near near near near near the the the the the the cen cen cen cen cen cen tter ter tter er ter er of of of of of of the tendon is changed from 0 to 10 cm. The number of snapped strands is increased from one to three tendon is changed from 0 to 10 cm. The number of snapped strands is increased from one to three to the the the the the the tendon tendon tendon tendon tendon tendon is is is is is is changed changed changed changed changed changed from from from from from from 0 0 0 0 0to 0to to to to to 10 10 10 10 10 10 cm. cm. cm. cm. cm. cm. Th Th Th Th Th Th eee e enumber number e number number number number of of of of of of sna sna sna sna sna sna pped pped pped pped pped pped st st st st st rands st rands rands rands rands rands is is is is is is incre incre incre incre incre incre aaaassasaesesesd ed ed ed d from d from from from from from one one one one one one to to to to to to three three three three three three to observe the secondary coil output according to the amount of damage. Table 1 describes the cases observe to to to to to to observe observe observe observe observe observe the secondary th th th th th th eee e esecond esecond second second second second coil ar ar ar ar ar ar yyyy y co y co output co co co co iilili li loutput i loutput loutput output output output accor acc acc acc acc acc acc o ding ooording ording ording rding rding rding to to to to to to the to th th th th th th eee e amount eam eam am am am am oun oun oun oun oun oun tt t tof t of tof of of of of dama dama dama dama damage. dama dama ggggegege.e.e .e .Tab Tab . Tab . Tab Tab Tab Tllable elelel el e1 e1 1 1 1descr 1descr descr descr descr 1descr describes iibes ibes ibes ibes ibes bes th th th th th th eee e ecases ecases cases cases the cases cases cases setup for the 3‐strand damage case. setup setup setup setup setup setup fo fo fo fo fo fo rrr r th r th r th th th th eee e e3 e3 3 3‐ ‐3str ‐3str ‐‐str str ‐str str aaaand nd and and nd nd dam dam dam dam dam dam aaaage ge age age ge ge case. case. case. case. case. case. setup for the 3-strand damage case. 1, 1,1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3 3 3 3 3da 3da da da da da m m m m m m aaage age age age ge ge dddd d str d str str str str str aaan ananandndnddsdsdssss 1, 2, 3 damaged strands 15 15 15 15 15 15 str str str str str str aaand and and and nd nd ssssss 15 strands 0, 0,0, 0, 0, 0, 2, 2, 2, 2, 2, 2, 4, 4, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6, 8, 8, 8, 8, 8, 8, 10 10 10 10 10 10 cm cm cm cm cm cm 163 163 163 163 163 163 cm cm cm cm cm cm 0, 2, 4, 6, 8, 10 cm 163 cm 330 330 330 330 330 330 cm cm cm cm cm cm 330 cm Figure Figure Figure Figure Figure Figure 4. 4. 4. 4. 4. 4. Cons Cons Cons Cons Cons Cons idered idered idered idered idered idered sect sect sect sect sect sect io io io io io nal io nal nal nal nal nal d d d dad ad amage amage amage amage mage mage ca ca ca ca ca ses ca ses ses ses ses ses of of of of of of external external external external external external PSC PSC PSC PSC PSC PSC tendon tendon tendon tendon tendon tendon (1 (1 (1 (1 (1 5(1 55‐5‐5strand ‐5strand ‐strand ‐strand ‐strand strand tendon, tendon, tendon, tendon, tendon, tendon, 7 7 7 ‐7‐7wire ‐7wire ‐wire ‐wire ‐wire wire strand). strand). strand). strand). strand). strand). Figure 4. Considered sectional damage cases of external PSC tendon (15‐strand tendon, 7‐wire Figure 4. Considered sectional damage cases of external PSC tendon (15-strand tendon, 7-wire strand). strand). Table Table Table Table Table Table 1. 1. 1. 1. 1. 1. Damage Damage Damage Damage Damage Damage case case case case case case sss s for s for s for for for for 3 3 3 ‐3‐ 3strand ‐3strand ‐strand ‐strand ‐strand strand d d d da d ad amage amage amage amage mage mage of of of of of of 15 15 15 15 15 15‐‐strand ‐strand ‐strand ‐strand ‐strand strand te te te te te ndon. te ndon. ndon. ndon. ndon. ndon. Table 1. Damage cases for 3-strand damage of 15-strand tendon. Length Length Length Length Length Length of of of of of of S S S S eSeSeectio ctio ectio ectio ctio ctio nnnn n n Loss Loss Loss Loss Loss Loss (c (c (c (c (c m) (c m) m) m) m) m) Table 1. Damage cases for 3‐strand damage of 15‐strand tendon. Length of Section Loss (cm) 0000 0 0 2222 2 2 4444 4 4 6666 6 6 8888 8 8 10 10 10 10 10 10 0 2 4 6 8 10 Length of Section Loss (cm) 0 2 4 6 8 10 The The The The The The dev dev dev dev dev dev iice ice ice ice ice ce d d dd ededeliver eliver eliver eliver liver liver ing ing ing ing ing ing the the the the the the ma ma ma ma ma ma gnet gnet gnet gnet gnet gnet ic icic ic ic ic fi fi fifi eld field field eld eld eld to to to to to to the the the the the the ex ex ex ex ex ex ter ter ter ter ter ter nnnnal nal nal al al al tendon tendon tendon tendon tendon tendon and and and and and and de de de de de de tec tec tec tec tec tec ttin tin ttin in tin in gggg g gthe the the the the the va va va va va va riation riation riation riation riation riation of of of of of of The device delivering the magnetic field to the external tendon and detecting the variation of the the the the the the the magnetic magnetic magnetic magnetic magnetic magnetic flux flux flux flux flux flux is is is is is is ma ma ma ma ma ma ddddedede e eby eby by by by by the the the the the the assem assem assem assem assem assem bbbblage blage blage lage lage lage of of of of of of two two two two two two 37 37 37 37 37 37‐‐cm ‐cm ‐‐cm cm ‐cm cm lon lon lon lon lon lon gggg g gplastic plastic plastic plastic plastic plastic half half half half half half‐‐cy‐cy‐‐cy cy‐cy cy linders linders linders linders linders linders as as as as as as sho sho sho sho sho sho w w w w w n w nnn n nin in in in in in Fig Fig Fig Fig Fig Fig uuuure ure ure re re re 5a 5a 5a 5a 5a 5a .. . .Th .Th . Th Th Th Th eee e esecond esecond second second second second ary ary ary ary ary ary co co co co co co iilili li liwi lwi l wi wi wi wi tth th tthh t h h18 18 18 18 18 18 turns turns turns turns turns turns is is is is is is ins ins ins ins ins ins ttatattaalle tlle alle alle lle lle dddd d dat at at at at at the the the the the the ce ce ce ce ce ce nnnntntner ter tter er ter er of of of of of of the the the the the the fr fr fr fr fr am fr am am am am am eee e efor efor for for for for de de de de de de tectin tectin tectin tectin tectin tectin gggg g gthe the the the the the magnetic flux is made by the assemblage of two 37-cm long plastic half-cylinders as shown in Figure 5a. The device delivering the magnetic field to the external tendon and detecting the variation of ma ma ma ma ma ma gnet gnet gnet gnet gnet gnet ic icic ic ic ic fl fl flfl u flufluuxuxuxx xsignal xsignal signal signal signal signal and and and and and and the the the the the the prim prim prim prim prim prim aaaarrarayryryry y yco co co co co co iilili li liwit lwit l wit wit wit wit hhhh h h40 40 40 40 40 40 0000 0turns 0turns turns turns turns turns over over over over over over a a a a alengt alengt lengt lengt lengt lengt hhhh h hof of of of of of 30 30 30 30 30 30 cm cm cm cm cm cm is is is is is is pl pl pl pl pl pl aaaaccacaecececd ed ed ed d dove ove ove ove ove ove rrr r rthe rthe the the the the The secondary coil with 18 turns is installed at the center of the frame for detecting the magnetic flux the magnetic secondary secondary secondary secondary secondary secondary flux co co co co co co ilililil il to ilto to is to to to gen gen gen ma gen gen gen eeed rate erate erate erate erate rate by the the the the the the the ma ma ma ma ma ma gnetic assem gnetic gnetic gnetic gnetic gnetic fifi b fi field. fi eld. filage eld. eld. eld. eld. The The The The The of The sta sta two sta sta sta sta bbbbili bili bili ili ili 37 ili ttytyttyyt y‐ of y of cm of of of of th th th th th th eelon e e ema ma e ma ma ma ma ggn gn gn gn gn plastic gn eeeteteiteitcticitci ci field c field c field field field field half alo alo alo alo alo alo ‐ncy nnngngngg linders g th g th th th th th eee e elon lon e lon lon lon lon gg ggigas itgitiu tiu tituu t dudusho ddina dina dina ina ina ina lw l l l l l n in signal and the primary coil with 400 turns over a length of 30 cm is placed over the secondary coil to axi axi axi axi axi axi sss s s mea s mea mea mea mea mea sssured sured sured sured ured ured at at at at at at the the the the the the ce ce ce ce ce ce nnnntntnetetteretrere r r of r of of of of of the the the the the the cro cro cro cro cro cro sssssssss s s secti s secti secti secti secti secti oooonononn n n is is is isis is verified verified verified verified verified verified in in in in in in Figure Figure Figure Figure Figure Figure 5b 5b 5b 5b 5b 5b by by by by by by co co co co co co mpa mpa mpa mpa mpa mpa rrririrsisrisiosiososonononn n n wi wi wi wi wi wi tth th tthh t h h the the the the the the Figure 5a. The secondary coil with 18 turns is installed at the center of the frame for detecting the generate the magnetic field. The stability of the magnetic field along the longitudinal axis measured theore theore theore theore theore theore tic tic tic tic tic tic aaaalla la l lva va l va va va va llu lu lu lules ues ues es es es .. . . Howev . Howev . Howev Howev Howev Howev eeer, er, er, er, r, r, the the the the the the devic devic devic devic devic devic eee e e in e in in in in in Figur Figur Figur Figur Figur Figur eee e e 5e 5 5 5 5 5is is is isis is un un un un un un su su su su su su itabl itabl itabl itabl itabl itabl eee e e for e for for for for for in in in in in in situ situ situ situ situ situ in in in in in in spect spect spect spect spect spect iion ion ion ion ion on tha tha tha tha tha tha tt t tt needs needs t needs needs needs needs magnetic flux signal and the primary coil with 400 turns over a length of 30 cm is placed over the at the center of the cross section is verified in Figure 5b by comparison with the theoretical values. freq freq freq freq freq freq uen uen uen uen uen uen tt t tas t as tas as as as se se se se se se m m m m m m bbbbly bly bly ly ly ly an an an an an an dddd d d d d d d idid sassembly isassembly isassembly isassembly isassembly sassembly due due due due due due to to to to to to the the the the the the la la la la la rge la rge rge rge rge rge nu nu nu nu nu nu mbe mbe mbe mbe mbe mbe rrr r of r of r of of of of numerous numerous numerous numerous numerous numerous turns. turns. turns. turns. turns. turns. secondary coil to generate the magnetic field. The stability of the magnetic field along the longitudinal However, the device in Figure 5 is unsuitable for in situ inspection that needs frequent assembly and axis measured at the center of the cross section is verified in Figure 5b by comparison with the disassembly due to the large number of numerous turns. 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. Appl. Appl. Appl. Appl. Appl. Appl. Sci. Sci. Sci. Sci. Sci. Sci. 2020 2020 2020 2020 2020 2020 ,, , 10 , 10 , 10 , 10 10 ,10 , , x; , x; , x; , x; x; doi: x; doi: doi: doi: doi: doi: FOR FOR FOR FOR FOR FOR PEE PEE PEE PEE PEE PEE RRRR R REVIEW R REVIEW REVIEW REVIEW REVIEW REVIEW www.mdpi.com/ www.mdpi.com/ www.mdpi.com/ www.mdpi.com/ www.mdpi.com/ www.mdpi.com/ jjournal/applsci ournal/applsci jjournal/applsci journal/applsci ournal/applsci journal/applsci Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13 Appl. Sci. 2020, 10, 7398 6 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13 (a) (b) 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. 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 (a) (b) of Figure 5 is employed here for evaluating the feasibility of the TFL method in detecting the sectional loss Figure in the 5. ext Fabrication ernal ten and don. installation Research of solenoid-shape about the improved device: (a ) de installed vice shown device; (in b) distribution Figure 6 will of be Figure 5. Fabrication and installation of solenoid‐shape device: (a) installed device; (b) distribution of implemen theoretical ted in and the measur future ed to longitudinal investigat magnetic e its accu fields racy at and center perform of device. ance. In addition, the operation of theoretical and measured longitudinal magnetic fields at center of device. the solenoid‐shape device with a battery will be tested in the future to verify the stableness of the Accordingly, our research team developed a mountable frame where the primary and secondary device’s results with a less stable power supply for in situ inspection. 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 can be Although easily connected an autom aby tic plugging movement as shown of the devic in Figur e caen 6a re duce to avoid the the unce rertain peatteyd from windin theg iof nc th ons e ta coils nt in situ. This so-called wrapping solenoid-shaped device also exhibited stable magnetic field through moving speed of the device, the automation of the device movement is not considered in this study. 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 In the gener comp al,arison near th ofe the ancho measured rage where and th is eoret damaica gel sva are lue often s (Fig fo urund, e 6b) .tendons Nevert he are les close s, thely or space iginadl setu withp Figure 5 is employed here for evaluating the feasibility of the TFL method in detecting the sectional loss each other, and an available space that the device can use is limited. The automation equipment also of Figure 5 is employed here for evaluating the feasibility of the TFL method in detecting the sectional in the external tendon. Research about the improved device shown in Figure 6 will be implemented in incre loss ain ses the the weigh externta of l ten thed device on. Resea thatr is ch anothe aboutr limi the tat improved ion for fie de ld vice appl ica shown tion. Therefore in Figure, th6e will device be the future to investigate its accuracy and performance. In addition, the operation of the solenoid-shape was designed to remove inessential function in order to fit into the limited space and weight. But the implemented in the future to investigate its accuracy and performance. In addition, the operation of device with a battery will be tested in the future to verify the stableness of the device’s results with a au the tom soaleno tionid eq‐shape uipme de ntvi can ce be with in staal ba led tte witho ry willu tbe di fftes ictul edty in if necessa the futury re. to verify the stableness of the less stable power supply for in situ inspection. 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. (a) (b) Figure 6. Mountable wrapping solenoid-shaped device for in situ inspection: (a) mounted wrapping Figure 6. Mountable wrapping solenoid‐shaped device for in situ inspection: (a) mounted wrapping device; (b) distribution of theoretical and measured longitudinal magnetic fields at the center of device; (b) distribution of theoretical and measured longitudinal magnetic fields at the center of the the device. device. Although an automatic movement of the device can reduce the uncertainty from the inconstant Figure 7 shows the external tendon specimen with the installed device. During the inspection, moving speed of the device, the automation of the device movement is not considered in this study. the distance travelled by the device along the axis of the tendon was measured in real time with In general, near the anchorage where is damages are often found, tendons are closely spaced with respect to the reference point using a laser distance meter. An AC with frequency of 10 Hz and (a) each other, and an available space that the device can use is limited. The automation equipment also (b) amplitude of ±0.6 A was delivered continuously to the primary coil of the device using the NI increases the weight of the device that is another limitation for field application. Therefore, the device Figure 6. Mountable wrapping solenoid‐shaped device for in situ inspection: (a) mounted wrapping was designed to remove inessential function in order to fit into the limited space and weight. But the Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci device; (b) distribution of theoretical and measured longitudinal magnetic fields at the center of the automation equipment can be installed without diculty if necessary. device. 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 Figure 7 shows the external tendon specimen with the installed device. During the inspection, to the reference point using a laser distance meter. An AC with frequency of 10 Hz and amplitude of the distance travelled by the device along the axis of the tendon was measured in real time with 0.6 A was delivered continuously to the primary coil of the device using the NI (National Instrument) respect to the reference point using a laser distance meter. An AC with frequency of 10 Hz and equipment as shown in Figure 8. The signal was generated by the LabVIEW program on a laptop and amplitude of ±0.6 A was delivered continuously to the primary coil of the device using the NI Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13 Appl. Sci. 2020, 10, 7398 7 of 12 (National Instrument) equipment as shown in Figure 8. The signal was generated by the LabVIEW (National Instrument) equipment as shown in Figure 8. The signal was generated by the LabVIEW program on a laptop and the NI PXI‐5412 AWG (arbitrary waveform generator). The generated signal program on a laptop and the NI PXI‐5412 AWG (arbitrary waveform generator). The generated signal the NI PXI-5412 AWG (arbitrary waveform generator). The generated signal was magnified by the NF was magnified by the NF BP bipolar DC power supply. The output voltage from the secondary coil was magnified by the NF BP bipolar DC power supply. The output voltage from the secondary coil BP bipolar DC power supply. The output voltage from the secondary coil of the device was measured of the device was measured by the NI PXI‐4462 DAQ (data acquisition) system. The device was of the device was measured by the NI PXI‐4462 DAQ (data acquisition) system. The device was by the NI PXI-4462 DAQ (data acquisition) system. The device was moved manually and slowly along moved manually and slowly along the length of the tendon by the operator without a specific power moved manually and slowly along the length of the tendon by the operator without a specific power the length of the tendon by the operator without a specific power unit. unit. unit. Figure 7. View of external PSC tendon installed for measurement test of sectional loss by the Total Figure 7. View of external PSC tendon installed for measurement test of sectional loss by the Total Figure 7. View of external PSC tendon installed for measurement test of sectional loss by the Total Flux Flux Leakage (TFL) method. Flux Leakage (TFL) method. Leakage (TFL) method. Figure 8. Equipment used to deliver AC to the primary coil of the device and measure the output of Figure 8. Equipment used to deliver AC to the primary coil of the device and measure the output of Figure 8. Equipment used to deliver AC to the primary coil of the device and measure the output of the secondary coil. the secondary coil. the secondary coil. 4. Results 4. Results 4. Results Figur Figure e 99 shows shows aa typical typicalpattern patternof ofthe theoutput output signal signal measur measured ed in in the the secondary secondarcoil y coin il in the th case e cas ofe Figure 9 shows a typical pattern of the output signal measured in the secondary coil in the case damaged external tendon. As the AC is delivered to the primary coil, the output signal of the secondary of damaged external tendon. As the AC is delivered to the primary coil, the output signal of the of damaged external tendon. As the AC is delivered to the primary coil, the output signal of the coil secondary has a form coil similar has a form to the siinput milar signal to thedeliver input ed sigto nal the delivere primary d to coil the with prima the same ry coil fr equency with the and same is secondary coil has a form similar to the input signal delivered to the primary coil with the same symmetric with reference to the horizontal axis. In the case where the DC is delivered to the primary frequency and is symmetric with reference to the horizontal axis. In the case where the DC is frequency and is symmetric with reference to the horizontal axis. In the case where the DC is coil delive instead red toof the an prima AC and ry the coilmoving insteadspeed of an of AC the and device the is mov irregular ing speed , undesir of th ed e device disturbance is irre would gular, delivered to the primary coil instead of an AC and the moving speed of the device is irregular, occur in the induced voltage on the secondary coil. In this research, a suciently stable magnetic flux undesired disturbance would occur in the induced voltage on the secondary coil. In this research, a undesired disturbance would occur in the induced voltage on the secondary coil. In this research, a was suffic generated iently staby blemeans magnet ofic AC flux that was inverted generated the dir by ection mean ofs the of AC power thasour t inver ce as ted much the di asre its ctifroequency n of the sufficiently stable magnetic flux was generated by means of AC that inverted the direction of the even though the operator was moving the device without specific speed control. Since the device power source as much as its frequency even though the operator was moving the device without power source as much as its frequency even though the operator was moving the device without was specif moved ic speeslowly d contro from l. Sin the ce lef the t end device to the was right moved end sl of owl the y external from thetendon, left enda to very the lar righ get number end of the of specific speed control. Since the device was moved slowly from the left end to the right end of the positive and negative peaks of the secondary coil output signal was recorded according to the distance external tendon, a very large number of positive and negative peaks of the secondary coil output external tendon, a very large number of positive and negative peaks of the secondary coil output travelled signal waduring s recorded the acco totalrding time of to the motion. distance The tr measur avelled ed durin secondary g the tot coil al tim output e of motion. takes thus The the measured form of signal was recorded according to the distance travelled during the total time of motion. The measured the compressed sinusoidal wave along the horizontal axis shown in Figure 9. Only the positive peaks secondary coil output takes thus the form of the compressed sinusoidal wave along the horizontal secondary coil output takes thus the form of the compressed sinusoidal wave along the horizontal wer axise shown picked fr inom Fig the uremeasur 9. Only ed the signal positive for the pea convenience ks were pi ofck the ed analysis. from the measured signal for the axis shown in Figure 9. Only the positive peaks were picked from the measured signal for the convenience of the analysis. convenience of the analysis. Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 Appl. Sci. 2020, 10, 7398 8 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 Figure 9. Induced voltage and its peaks in the secondary coil. The peak curves of the voltage induced in the secondary coil in accordance with the change in length of one‐strand damage of the external tendon are presented in Figure 10. The lowest point in the curves corresponds the center of the damage length. The centerline of the damage length is shifted slightly to the right due to the fixed left end of the damage as shown in Figure 4. It is observed that this phenomenon is reflected on the measured peak curves. In the same manner, the peak curves of the voltage induced in the secondary coil in accordance with the change in length of two‐ and three‐strand‐damage of the external tendon are presented in Figure 11 and Figure 12 Figure , respectively 9. Induced. In vo ad ltage di tion, and its the peaks second in the ary se vo condary ltage red coil.u ction ratios of the one Figure 9. Induced voltage and its peaks in the secondary coil. to three‐strands damage with respect to the damage length are presented in Figure 13. The damage The peak curves of the voltage induced in the secondary coil in accordance with the change in The peak curves of the voltage induced in the secondary coil in accordance with the change in detectable level in Figure 13 was visually suggested as the peak curve for the 1‐strand damage with length of one-strand damage of the external tendon are presented in Figure 10. The lowest point in the length of one‐strand damage of the external tendon are presented in Figure 10. The lowest point in 0 cm damage length in Figure 10 is difficult to be distinguished and can be hidden if the signal to curves corresponds the center of the damage length. The centerline of the damage length is shifted the curves corresponds the center of the damage length. The centerline of the damage length is shifted noise ratio decreases. The decrement ratio in Figure 13 is the voltage change due to the damage to the slightly to the right due to the fixed left end of the damage as shown in Figure 4. It is observed that this slightly to the right due to the fixed left end of the damage as shown in Figure 4. It is observed that voltage value just before the decrement. phenomenon is reflected on the measured peak curves. this phenomenon is reflected on the measured peak curves. In the same manner, the peak curves of the voltage induced in the secondary coil in accordance with the change in length of two‐ and three‐strand‐damage of the external tendon are presented in Figure 11 and Figure 12, respectively. In addition, the secondary voltage reduction ratios of the one to three‐strands damage with respect to the damage length are presented in Figure 13. The damage detectable level in Figure 13 was visually suggested as the peak curve for the 1‐strand damage with 0 cm damage length in Figure 10 is difficult to be distinguished and can be hidden if the signal to noise ratio decreases. The decrement ratio in Figure 13 is the voltage change due to the damage to the voltage value just before the decrement. Figure 10. Secondary coil output peak induced by 1‐strand damage according to damage length. Figure 10. Secondary coil output peak induced by 1-strand damage according to damage length. In the same manner, the peak curves of the voltage induced in the secondary coil in accordance with the change in length of two- and three-strand-damage of the external tendon are presented in Figures 11 and 12, respectively. In addition, the secondary voltage reduction ratios of the one to three-strands damage with respect to the damage length are presented in Figure 13. The damage Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci detectable level in Figure 13 was visually suggested as the peak curve for the 1-strand damage with 0 cm damage length in Figure 10 is dicult to be distinguished and can be hidden if the signal to noise ratio decreases. The decrement ratio in Figure 13 is the voltage change due to the damage to the voltage value just before the decrement. Figure 10. Secondary coil output peak induced by 1‐strand damage according to damage length. Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13 Appl. Sci. 2020, 10, 7398 9 of 12 Figure 11. Secondary coil output peak induced by 2‐strand damage according to damage length. Figure Figure 11. 11. Secondary Secondary coil coil output output peak peak induced induceby d by 2-strand 2‐strand damage damage accor accordi dingnto g to damage damage length. length. Figure 12. Secondary coil output peak induced by 3‐strand damage according to damage length. Figure 12. Secondary coil output peak induced by 3-strand damage according to damage length. Figure 12. Secondary coil output peak induced by 3‐strand damage according to damage length. The location of the sectional loss can be estimated simply and intuitively. Moreover, the change in the damage length is also well reflected in Figures 10–12. The reduction of the magnitude of the voltage induced in the secondary coil caused by the magnetic flux leaked from the damaged section that is not permeating the secondary coil shown in Figure 3 can be clearly distinguished in the graphs. In addition, it appears that deeper and longer damage introduces stronger magnetic flux leakage as well as increases the magnitude of the decrement of the voltage induced in the secondary coil as shown in Figure 13. The ratio of decrement in induced voltage of the secondary coil for all cases except for the 1-strand damage with a length of 0 cm case were above the designated damage detectable level. However, as such the case is neglectable as the external tendon is always in tension and the damage length would not be zero when cut. Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7398 10 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13 Figure 13. Rate of decrement in induced voltage of secondary coil. Figure 13. Rate of decrement in induced voltage of secondary coil. The The maximum location of peak the sec curve tiona of l lo the ss voltage can be estimated induced in simpl the secondary y and intuicoil tivemeasur ly. More ed over, by moving the change the device throughout the length of the external tendon can provide information about the damage location in the damage length is also well reflected in Figures 10–12. The reduction of the magnitude of the by voltage identif ind ying ucethe d inv the ariation secondary of the coil curve. cauThe sed by reduction the magnet of the ic fl voltage ux leake induced d from in the the dam secondary aged section coil by the rupture of 1 or more strands or damage length of 2 cm or longer can be intuitively identified. that is not permeating the secondary coil shown in Figure 3 can be clearly distinguished in the graphs. Hence, In additthis ion, validates it appearsthat thatthe deeper method and measuring longer damag theechange introduce of the s strong totaler flux macan gnetbe ic fl used ux leto akdetect age as damage in the external tendon. This result satisfies the FHWA guideline of tendon replacement at 5% well as increases the magnitude of the decrement of the voltage induced in the secondary coil as sectional shown inloss Figu asre well 13. as The the ra pr tio oject’s of decre detection ment in accuracy induced of volt figuring age of out the 1 out second of 15 ary tendons. coil for all cases The external tendon of the PSC structure has a limited profile and size inside the structure section except for the 1‐strand damage with a length of 0 cm case were above the designated damage unlike detectable other lev cases el. However with a complicated , as such the pr case ofile is and neglectable size. Since asthe thedevice external wraps tendon the is entir always e section in tensi of the on external tendon, the secondary coil can measure almost any change of the magnetic flux induced by and the damage length would not be zero when cut. the change The ma ofximum the sectional peak curve area of inside the voltage the tendon. induce The d in pr the oposed second method ary coil has mea shown suredthe by potent moving ial the to identify and quantify the sectional loss in the external tendon by using a solenoid-shaped device fed device throughout the length of the external tendon can provide information about the damage with location AC and by ipr dent esenting ifying ath simple e variat shape ion of and the light cur weight ve. The even reduc though tion the of the uncertainty voltage of induce the pr doposed in the method increases if a strong exterior magnetic source is close to the device or the power supply is secondary coil by the rupture of 1 or more strands or damage length of 2 cm or longer can be unstable. intuitivelyThe identi prfie oposed d. Hence TFL, th method is valida o te ers s tha thet the advantage method of mea a low-power suring the change source of which the to makes tal flux it convenient for actual NDE conditions in terms of portability and long-term usage. can be used to detect damage in the external tendon. This result satisfies the FHWA guideline of tendon replacement at 5% sectional loss as well as the project’s detection accuracy of figuring out 1 5. Conclusions out of 15 tendons. The external tendon of the PSC structure has a limited profile and size inside the structure A new TFL method is proposed to estimate the location and amount of damage in the external section unlike other cases with a complicated profile and size. Since the device wraps the entire tendon of PSC structures. This TFL method uses AC-delivering solenoid with a secondary coil inserted section of the external tendon, the secondary coil can measure almost any change of the magnetic flux in an external primary coil enclosing the tendon. The primary coil is fed with a current in the device induced by the change of the sectional area inside the tendon. The proposed method has shown the to cause the magnetic field, and the change in the magnetic flux at the damaged area of the external potential to identify and quantify the sectional loss in the external tendon by using a solenoid‐shaped tendon is detected and quantified by the variation of the voltage in the secondary coil. Unlike the MMF device fed with AC and presenting a simple shape and light weight even though the uncertainty of (Main Magnetic Flux) method, which usually uses a heavy magnetic yoke, the proposed TFL method the proposed method increases if a strong exterior magnetic source is close to the device or the power uses a light solenoid-shaped device. In addition, the proposed TFL method can provide a stable output supply is unstable. The proposed TFL method offers the advantage of a low‐power source which due to the AC input signal and can be applied to any cable system with minor modification. makes it convenient for actual NDE conditions in terms of portability and long‐term usage. The following conclusions can be derived from a series of experiments considering various degrees and lengths of damage in the tendon. 5. Conclusions (1) Sucient output signal could be acquired from the secondary coil for identifying the damage on the A new TFL method is proposed to estimate the location and amount of damage in the external external tendon even when using the device with light and simple structure and fed with low-level 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 Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7398 11 of 12 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 suciently 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 e ectively detecting the damage by measuring the change of the total flux in the external tendon. Author Contributions: 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. Funding: This research was funded by the Korea Institute of Civil Engineering and Building Technology (KICT) of the Republic of Korea, Project No. 2020-0033. Conflicts of Interest: The authors declare no conflict of interest. References 1. Lee, B.Y.; Koh, K.T.; Ismail, M.A.; Ryu, H.-S.; Kwon, S.J. Corrosion and strength behaviors in prestressed tendon under various tensile stress and impressed current conditions. Adv. Mater. Sci. Eng. 2017, 2017, 1–7. [CrossRef] 2. Mahmoodian, M.; Li, C.Q. Failure assessment of a pre-stressed concrete bridge using time dependent system reliability method. In Proceedings of the 29th Annual International Bridge Conference, Pittsburgh, PA, USA, 10–13 June 2012. 3. Naito, C.; Sause, R.; Hodgson, I.; Pessiki, S.; Macioce, T. Forensic Examination of a Noncomposite Adjacent Precast Prestressed Concrete Box Beam Bridge. J. Bridg. Eng. 2010, 15, 408–418. [CrossRef] 4. Weiher, H.; Zilch, K. Condition of post-tensioned concrete bridges-assessment of the German stock by a spot survey of damages. In Proceedings of the First International Conference on Advances in Bridge Engineering, Brunel University, Uxbridge, UK, 26–28 June 2006. 5. Mutsuyoshi, H. Present situation of durability of post-tensioned PC bridges in Japan. Bulletin-Fib 2001, 15, 75–88. 6. Fricker, S.; Vogel, T. Detecting wire breaks in a prestressed concrete road bridge with continuous acoustic monitoring. Bridge Maint. Saf. Manag. Life-Cycle Perform. Cost 2006, 847–850. [CrossRef] 7. Hurlebaus, S.; Hueste, M.; Karthik, M.; Terzioglu, T. Condition Assessment of Bridge Post-Tensioning and Stay Cable Systems Using NDE Methods; NCHRP Report 14-28; NCHRP, Transportation Research Board of the National Academies: Washington, DC, USA, 2016; pp. 7–19. 8. Trejo, D.; Hueste, M.B.D.; Gardoni, P. E ect of Voids in Grouted, Post-tensioned Concrete Bridge Construction: Volume 1—Electrochemical Testing and Reliability Assessment; (No. FHWA/TX-09/0-4588-1 Vol. 1); Texas Transportation Institute, Texas A&M University: College Station, TX, USA, 2009. 9. Sprinkel, M.M. VDOT Experience with Grouts and Grouted Post-Tensioned Tendons. PTI J. 2015, 11, 51–61. Appl. Sci. 2020, 10, 7398 12 of 12 10. Yu, C.; Kim, K.; Seol, D.; Youn, S. Endoscope survey of internal, external tendon in PSC Box Girder. In Proceedings of the Annual Conference of Korea Concrete Institute, Andong-Si, Korea, 2 November 2017. 11. Yoon, I.-S.; Kang, T.H.K.; Shin, H.-Y. Evaluation of corrosion prevention systems of strands for PSC structures. J. Korea Concr. Inst. 2019, 31, 557–565. [CrossRef] 12. Kim, K.; Youn, S.; Youn, S. Corrosion of Post-Tensioned Tendons and Technology of Grouting; Magazine of Korea Concrete Institute: Seoul, Korea, 2019; Volume 31, pp. 18–23. 13. Adcox, S. DOT: Corrosion ‘Exploded’ Cable in Wando Bridge, Sending Grout Flying 100 Feet. The Post and Courier. Available online: https://www.postandcourier.com/politics/dot-corrosion-exploded-cable-in- wando-bridge-sending-grout-flying-100-feet/article_b f0fa0-7568-11e8-a1a7-0bfbef6a71fe.html (accessed on 7 September 2020). 14. FHWA. Guidelines for Sampling, Assessing, and Restoring Defective Grout in Prestressed Concrete Bridge Post-Tensioning Ducts; FHWA-HRT-13-027; FHWA: Washington, DC, USA, 2013. 15. Fernandes, B.; Titus, M.; Nims, D.; Ghorbanpoor, A.; Devabhaktuni, V.K. Practical assessment of magnetic methods for corrosion detection in an adjacent precast, prestressed concrete box-beam bridge. Nondestruct. Test. Evaluation 2013, 28, 99–118. [CrossRef] 16. Azizinamini, A.; Gull, J. Improved inspection techniques for steel prestressing/post-tensioning strand. In FDOT Contract No. BDK80 977-13 Final Report-Volume I; Florida Department of Transportation, Florida International University: Miami, FL, USA, 2012. 17. Karthik, M.M.; Terzioglu, T.; Hurlebaus, S.; Hueste, M.B.; Weischedel, H.; Stamm, R. Magnetic flux leakage technique to detect loss in metallic area in external post-tensioning systems. Eng. Struct. 2019, 201, 109765. [CrossRef] 18. Makar, J.; Desnoyers, R. Magnetic field techniques for the inspection of steel under concrete cover. NDT E Int. 2001, 34, 445–456. [CrossRef] 19. Fernandes, B.; Nims, D.; Devabhaktuni, V.K. Comprehensive MMF–MFL inspection for corrosion detection and estimation in embedded prestressing strands. J. Civ. Struct. Health Monit. 2013, 4, 43–55. [CrossRef] 20. Moriya, T.; Sugawara, M.; Tsukada, K. Magnetic nondestructive evaluation of corrosion in wire ropes. 0 0 In Proceedings of the Oceans 04 MTS/IEEE Techno-Ocean 04 (IEEE Cat. No.04CH37600), Kobe, Japan, 9–12 November 2004; Institute of Electrical and Electronics Engineers (IEEE): New York, NY, USA, 2004; pp. 1910–1917. 21. Kim, J.-W.; Kim, J.; Park, S. Cross-Sectional Loss Quantification for Main Cable NDE Based on the B-H Loop Measurement Using a Total Flux Sensor. J. Sensors 2019, 2019, 8014102. [CrossRef] 22. Nims, D.K.; Devabhaktuni, V. Magnetic Sensor for Nondestructive Evaluation of Deteriorated Prestressing Strand-Phase II; The University of Toledo University Transportation Center: Toledo, OH, USA, 2011. 23. Qu, Y.; Zhang, H.; Zhao, R.; Liao, L.; Zhou, Y. Research on the Method of Predicting Corrosion width of Cables Based on the Spontaneous Magnetic Flux Leakage. Materials 2019, 12, 2154. [CrossRef] [PubMed] 24. Zhao, Q.; Zhou, J.; Xia, Q.; Zhang, S.; Zhang, H. Non-Destructive Testing of Steel Corrosion Fluctuation Parameters Based on Spontaneous Magnetic Flux Leakage and Its Relationship with Steel Bar Diameter. Materials 2019, 12, 4116. [CrossRef] 25. Sun, Z. Cable tension sensor based on magneto-elastic e ect. In Proceedings of the 14th World Conference on Earthquake Engineering, Beijing, China, 12–17 October 2008. 26. Cho, S.; Yim, J.; Shin, S.W.; Jung, H.-J.; Yun, C.-B.; Wang, M.L. Comparative Field Study of Cable Tension Measurement for a Cable-Stayed Bridge. J. Bridg. Eng. 2013, 18, 748–757. [CrossRef] Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Journal
Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Oct 22, 2020