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Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive Testing

Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive... applied sciences Article Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive Testing 1 1 Dmitry Valeryevich Sannikov , Alexander Sergeevich Kolevatov , 1 , 2 , 1 Vladimir Platonovich Vavilov * and Marina Valeryevna Kuimova School of Nondestructive Testing and Safety, Tomsk Polytechnic University, Lenin Av., 30, 634050 Tomsk, Russia; sanndv72@gmail.com (D.V.S.); shiryaev@tpu.ru (A.S.K.); kuimova@tpu.ru (M.V.K.) Department of Theoretical and Computational Mechanics, Tomsk State University, Lenin Av., 36, 634050 Tomsk, Russia * Correspondence: vavilov@tpu.ru Received: 11 December 2017; Accepted: 24 January 2018; Published: 1 February 2018 Abstract: Thermal nondestructive testing can be used to inspect reinforced concrete supports that are widely used in various industries. Corrosion damage is a typical defect found in these supports. Corrosion usually starts as a separation between the concrete and the steel rebar. Damage is exacerbated by pressure that is caused by the formation of corrosion products. The most logical method for using IR to detect corrosion or a disbond would be to heat up the rebar by resistive or inductive heating. In both cases, monitoring the dynamic temperature distributions on the pole allows for the evaluation of reinforcement quality. The thermal properties of steel, concrete, air, and corrosion products differ greatly. The magnitude of temperature anomalies and their behavior over time depend on the presence of corrosion products, air gaps, and the quality of contact between rebar and concrete. Keywords: thermal testing; electric railway pole; inductive heating; numerical modeling 1. Introduction In the inspection of reinforced concrete, a combination of induction heating and IR thermography was first suggested by Hillemeier et al. in 1982 [1]. General applications of this inspection technique were summarized by Maierhofer et al. [2]. Kobayashi and Banthia focused on the detection of corrosion by applying induction heating [3]. Milovanovic ´ et al. demonstrated that external optical stimulation of reinforcement in concrete enables detection of 10-cm structural elements in concrete at depths up to 7 cm [4]. Smaller thermal anomalies, such as 2  2 cm, can be detected up to a depth of about 4 cm [5]. Szymanic et al. described the use of microwave heating in the detection of rebars in concrete with a depth limit of about 2 cm [6]. This study covers the development of a thermal nondestructive testing (TNDT) technique for inspecting reinforced concrete supports of several types, such as overhead line poles, pipes, beams, pilings, etc., which are widely used in various industries, and in both the manufacturing and operation stages [7–9]. Historically, Russian railways represent one of the most important industrial areas in the country. It is worth mentioning that over 80% of cargo in Russia is being transported by means of railways (not counting the pipeline transport) [10]. Uncontrolled damage and full destruction of electrical supports may cause financial losses and even casualties, particularly in densely populated areas. Therefore, research intended to enhance the reliability of railway overhead lines is of a high priority in Russia, being supported by the corresponding federal directive documents. The mass electrification of Russian railways started in the 1980s. The lifetime of reinforced concrete poles is Appl. Sci. 2018, 8, 222; doi:10.3390/app8020222 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 222 2 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 2 of 13 reinforced concrete poles is assumed to be about 50 years, hence, in the near future, efficient assumed to be about 50 years, hence, in the near future, efficient inspection techniques to evaluate the inspection techniques to evaluate the quality of supports will become increasingly important. quality of supports will become increasingly important. Some poles may reveal an essential loss of their mechanical strength much earlier than within Some poles may reveal an essential loss of their mechanical strength much earlier than within their normative lifetime. This may be conditioned by either poor workmanship or some ambient their normative lifetime. This may be conditioned by either poor workmanship or some ambient factors. Pole damage often starts in underground sections of supports, where corrosion appears factors. Pole damage often starts in underground sections of supports, where corrosion appears because of varying soil moisture and high mechanical stresses. Corrosion damage is related to cracks because of varying soil moisture and high mechanical stresses. Corrosion damage is related to cracks that appear in concrete boundary layers adjacent to steel rebars because of enhanced pressure that appear in concrete boundary layers adjacent to steel rebars because of enhanced pressure rendered rendered by corrosion products. It is important to note that corrosion defects are located under the by corrosion products. It is important to note that corrosion defects are located under the concrete concrete surface layer and often hidden in the soil, thus are visually undetectable. surface layer and often hidden in the soil, thus are visually undetectable. Figure 1 shows a scheme of an electric railway pole and typical corrosion defects that appear at Figure 1 shows a scheme of an electric railway pole and typical corrosion defects that appear a pre-failure period of operation. The most dangerous is corrosion damage, which appears in at a pre-failure period of operation. The most dangerous is corrosion damage, which appears in underground pole sections (Figure 1b). underground pole sections (Figure 1b). (a) (b)(c) Figure 1. (a) Electric railway pole scheme; (b,c) and examples of corrosion damage. Figure 1. (a) Electric railway pole scheme; (b,c) and examples of corrosion damage. A number of nondestructive testing (NDT) techniques have been applied to detect early A number of nondestructive testing (NDT) techniques have been applied to detect early corrosion corrosion in electric railway poles. They differ by sensitivity and test productivity but can hardly be in electric railway poles. They differ by sensitivity and test productivity but can hardly be applied to applied to the evaluation of pole wear. The most usable ultrasonic inspection technique evaluates the evaluation of pole wear. The most usable ultrasonic inspection technique evaluates the strength the strength of concrete by measuring ultrasound velocity in different directions or recording of concrete by measuring ultrasound velocity in different directions or recording acoustic emission acoustic emission signals. Another ultrasonic test method involves the analysis of resonance features signals. Another ultrasonic test method involves the analysis of resonance features of the reinforcement. of the reinforcement. Both techniques are predominantly applicable for inspecting concrete surface Both techniques are predominantly applicable for inspecting concrete surface layers and often require layers and often require the use of special references. the use of special references. One more inspection technique involves electromagnetic (induction) heating of a pole to locate One more inspection technique involves electromagnetic (induction) heating of a pole to locate reinforcement in concrete, but this does not help with the evaluation of residual reinforcement reinforcement in concrete, but this does not help with the evaluation of residual reinforcement thickness. thickness. Classical TNDT based on surface heating of test objects seems to be inappropriate in the Classical TNDT based on surface heating of test objects seems to be inappropriate in the inspection of inspection of concrete poles because of the considerable pole thickness and low thermal conductivity of concrete with regard to highly conductive steel. Alternatively, performing heating within internal Appl. Sci. 2018, 8, 222 3 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 3 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 3 of 13 cavities that are present in some types of poles is practically difficult. The use of solar heating for concrete poles because of the considerable pole thickness and low thermal conductivity of concrete cavities that are present in some types of poles is practically difficult. The use of solar heating for detecting damaged concrete in electric railway poles was reported elsewhere [11], but such a with regard to highly conductive steel. Alternatively, performing heating within internal cavities detecting damaged concrete in electric railway poles was reported elsewhere [11], but such a technique obviously cannot ensure high test reliability because of variable and low-power heating that are present in some types of poles is practically difficult. The use of solar heating for detecting technique obviously cannot ensure high test reliability because of variable and low-power heating and the presence of multiple sources of thermal noise. damaged concrete in electric railway poles was reported elsewhere [11], but such a technique obviously and the presence of multiple sources of thermal noise. cannot ensure high test reliability because of variable and low-power heating and the presence of 2. Inspection Schemes multiple sources of thermal noise. 2. Inspection Schemes The authors have suggested the stimulation of hidden metallic rebars by applying electric 2. Inspection Schemes The authors have suggested the stimulation of hidden metallic rebars by applying electric current or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic current or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic tempera Thetauthors ure distri have butisuggested ons on the externa the stimulation l bottom sect of hidden ion of metallic a poler a ebars llows by for applying evaluatelectric ing the curr qual ent ity temperature distributions on the external bottom section of a pole allows for evaluating the quality or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic temperature of reinforcement. Since steel, concrete, air, and corrosion products differ by their thermal properties, of reinforcement. Since steel, concrete, air, and corrosion products differ by their thermal properties, distributions significant teon st p the ara external meters, s bottom uch as section magnit ofude a pole of t allows emperat forure evaluating anomalies the and quality their b of reinfor ehavicement. or over significant test parameters, such as magnitude of temperature anomalies and their behavior over Since time, depen steel, concr d on the presence of corro ete, air, and corrosionsion produc products dif ts fer and the by their qua thermal lity of conta properties, ct between rebars significant test and time, depend on the presence of corrosion products and the quality of contact between rebars and parameters, concrete. Both power such as magnitude and dura oftion of temperatur electri e anomalies c pulses and shoul their d be behavior optimizover ed to ensure detecta time, depend on the ble concrete. Both power and duration of electric pulses should be optimized to ensure detectable pr sur esence face teof mp corr erat osion ure sipr gn oducts als. and the quality of contact between rebars and concrete. Both power surface temperature signals. and duration There are of electric four test configur pulses should ations en be optimized abling the to ensur inteernal st detectable imulat surface ion of reb temperatur ars. In t e h signals. e first There are four test configurations enabling the internal stimulation of rebars. In the first scheme, a so There arurce o e fourftest electric configurations current 5 is d enabling irectly the connect internal ed to stimulation rebars 2 via cont of rebars. act cIn lam the psfirst 6 (Fig scheme, ure 2). scheme, a source of electric current 5 is directly connected to rebars 2 via contact clamps 6 (Figure 2). aFisour gure ce 3 s ofh elect owsric the us current e of5 el isedir ctrectly odes connected 7 placed on to the surface rebars 2 via o contact f concrete wi clampsthout di 6 (Figurrect e 2).a Figur ccess to e 3 Figure 3 shows the use of electrodes 7 placed on the surface of concrete without direct access to shows rebars. The t the useh of ird t electr estodes conf7igur placed ation on us the es t surface he natof ura concr l groun ete without ding of concret direct access e poles t to rebars. hat arThe e deeply third rebars. The third test configuration uses the natural grounding of concrete poles that are deeply test buriconfiguration ed in soil. In thi uses s ca the se, one condu natural grounding ctor is of conne concr cted to ete poles a na that tura arl or arti e deeply ficial buried earth el in soil. ectrode In this8 buried in soil. In this case, one conductor is connected to a natural or artificial earth electrode 8 case, (Figur one e 4). However, conductor the au is connected thors’ pr to actical exper a natural or ienc artificial e has shown t earth electr hat the most ode 8 (Figur effecte ive tech 4). However nique is , (Figure 4). However, the authors’ practical experience has shown that the most effective technique is the induct authors’ ive stpractical imulation experience of reinfohas rcemen shown t (Fthat igurthe e 5) most usin efg fective high-fr technique equency ind is inductive uctors 1stimulation 0 allowing inductive stimulation of reinforcement (Figure 5) using high-frequency inductors 10 allowing of cont reinfor actless cement heating o (Figur f met e 5)ausing llic rebar high-fr s insi equency de concret inductors e poles.10 allowing contactless heating of metallic contactless heating of metallic rebars inside concrete poles. rebars inside concrete poles. Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR imager, Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 6—contact clamps). 4—cables, 5—current source, 6—contact clamps). imager, 4—cables, 5—current source, 6—contact clamps). Figure 3. Using surface electrodes for heating rebars: (1—test object, 2—rebars, 3—IR imager, 4—cables, Figure 3. Using surface electrodes for heating rebars: (1—test object, 2—rebars, 3—IR imager, 5—curr Figure 3. ent Us sour ing s ce, 7—surface urface elecelectr trodeodes). s for heating rebars: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 7—surface electrodes). 4—cables, 5—current source, 7—surface electrodes). Appl. Sci. 2018, 7, x FOR PEER REVIEW 4 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 4 of 13 Below we describe general principles and a step-by-step procedure of TNDT of railway electric poles by using inductive heating. Below we describe general principles and a step-by-step procedure of TNDT of railway electric The inspection is performed by using at least one IR camera 1 (Figure 5). The inspection area poles by using inductive heating. can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high The inspection is performed by using at least one IR camera 1 (Figure 5). The inspection area reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high energy supply intended for rebar stimulation should be chosen taking into account the configuration reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric and mass of test objects. Stimulation time and power are to be optimized by modeling particular test energy supply intended for rebar stimulation should be chosen taking into account the configuration cases. Note that, by delivering a maximal heating power, one must avoid material damage and mass of test objects. Stimulation time and power are to be optimized by modeling particular test accompanied by the appearance of burns and slags. cases. Note that, by delivering a maximal heating power, one must avoid material damage Appl. Sci. 2018, 8, 222 4 of 13 accompanied by the appearance of burns and slags. Figure 4. TNDT by using pole natural grounding and earth electrode: (1—test object, 2—rebars, 3—IR imager, Figure 4. TND4—cables, 5— T by using polcurrent source, 6—contact e natural grounding and ea cla rthm elps, 8—natu ectrode: (1— ral/artific test objeial eart ct, 2—re h electrod bars, 3—Ie, R Figure 4. TNDT by using pole natural grounding and earth electrode: (1—test object, 2—rebars, imager, 4—cables, 5—current source, 6—contact clamps, 8—natural/artificial earth electrode, 11—IR mirror). 11—IR mirror). 3—IR imager, 4—cables, 5—current source, 6—contact clamps, 8—natural/artificial earth electrode, 11—IR mirror). Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, 3—IR 3—IR imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, 3—IR inductor, 11—IR mirror). inductor, 11—IR mirror). imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency inductor, 11—IR mirror). Synchronously with turning on a stimulation source, one starts recording a sequence of IR Below we describe general principles and a step-by-step procedure of TNDT of railway electric thermograms that reflects the evolution of pole surface temperature over time. Note that most of the polesSynchrono by using inductive usly with turn heating. ing on a stimulation source, one starts recording a sequence of IR thermogra The inspection ms that refl is ects the evoluti performed by using on of pol at least e surf one ace temperat IR camera ur 1 (Figur e overe time. 5). The Note tha inspecti t most of the on area can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric energy supply intended for rebar stimulation should be chosen taking into account the configuration and mass of test objects. Stimulation time and power are to be optimized by modeling particular test cases. Note that, by delivering a maximal heating power, one must avoid material damage accompanied by the appearance of burns and slags. Appl. Sci. 2018, 8, 222 5 of 13 Synchronously with turning on a stimulation source, one starts recording a sequence of IR thermograms that reflects the evolution of pole surface temperature over time. Note that most of the thermal signals appropriate for further evaluation take place at a cooling stage when hidden defects can be detected by their specific temperature patterns on the inspected surface. By analyzing IR images, a thermographer is to outline areas with rebars and detect possible hidden defects, whose identification strongly depends on thermographer ’s experience. It has been found that an additional detection criterion, along with defect pattern amplitude and shape, is a rate of temperature variations in suspicious areas, i.e., the corresponding temperature derivatives. The efficiency of the above test procedure depends on how uniform the heating of rebars is. This can be achieved by: (1) using transverse reinforcement (if available); (2) binding reinforcement with additional wires; (3) using grounding elements or special surface electrodes. 3. Modeling Test Cases 3.1. Inductive Heating We are dealing with a 3D heat conduction problem for a non-adiabatic solid body with internal heat sources. The cylindrical geometry of a pole (Figure 6a) was replaced with a 3D Cartesian model shown in Figure 6b and numerically solved by using the ThermoSource software from Tomsk Polytechnic University (Figure 6c). The model simulates a 55 mm thick pole shell made of reinforced concrete (pole type SK8 by the Russian nomenclature). It is worth noting that in real poles, plenty of rebar bunches each including four 5 mm diameter steel rebars are regularly placed within a pole, as shown in Figure 6a. Each rebar bunch was simulated as a parallelepiped-like defect (cross section: 20  5 mm) and placed at depths of 10 and 20 mm from the external pole surface, and high-frequency powerful induction heating was simulated as energy discharge in a rebar (released power 10 MW/m , heating duration 20 s). The mathematical formulation of such TNDT problems was thoroughly discussed elsewhere [12,13]. Three characteristic areas are specified on the pole surface: a defect-free area (area 1 in Figure 6c), diminution of the rebar cross section by 25% (area 2), and the same rebar thinning but additionally accompanied with a 1 mm thick air-filled delamination (area 3). The latter defect is to simulate the practical situations, where the process of corrosion wear is accompanied by corrosion products that appear between concrete and reinforcement and provide pressure on the adjacent concrete, thus causing voids and cavities. Two IR thermograms taken at 60 and 120 s are shown in Figure 7 to demonstrate that, over defect-free areas, the surface excess temperature DT reaches 8 C (at 120 s if the defect depth is 10 mm); the rebar thinning results in a weak decrease of surface temperature because of the lower released energy while a significant temperature decrease occurs over the air-filled delamination because of its high thermal resistance. Temperature evolutions at three characteristic points (see Figure 7) in time () are presented in Figure 8. Similarly to diffusivity measurement by using the known Parker technique, the fronts of temperature signals in Figure 8 depend on material thickness and thermal properties. This is also illustrated by Figure 9, where steel rebars are located at a depth of 20 mm. The thicker concrete layer diminishes both the excess surface temperature (down to 3 C) and the rate of the temperature change. The same dependencies are shown in Figure 9 for the case where steel rebars are located at a depth of 20 mm. The thicker concrete layer diminishes the surface temperature up to 3 C and makes the rate of change in temperature slower. Appl. Sci. 2018, 8, 222 6 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 6 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 6 of 13 (a) (b) (a) (b) (c) Figure 6. Railway pole 3D numerical model (wall thick (c) ness 55 mm, 5 mm diameter steel rebars at 10 Figure 6. Railway pole 3D numerical model (wall thickness 55 mm, 5 mm diameter steel rebars at (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, 10 (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, Figure 6. Railway pole 3D numerical model (wall thickness 55 mm, 5 mm diameter steel rebars at 10 3—same as (2), additional 1 mm–thick air-filled delamination): (a) pole scheme; (b) Cartesian model; 3—same as (2), additional 1 mm–thick air-filled delamination): (a) pole scheme; (b) Cartesian model; (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, (c) ThermoSource software model, two rebars shown). (c) 3—same as (2), additional 1 ThermoSource software model, mm–th two ick air-filled rebars shown). delamination): (a) pole scheme; (b) Cartesian model; (c) ThermoSource software model, two rebars shown). (a) (b) Figure 7. Synthetic IR thermograms of railway pole (a) ( at 60 s (a) and 120 bs ( ) b) (model in Figure 6, reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). Figure 7. Synthetic IR thermograms of railway pole at 60 s (a) and 120 s (b) (model in Figure 6, Figure 7. Synthetic IR thermograms of railway pole at 60 s (a) and 120 s (b) (model in Figure 6, reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). Appl. Sci. 2018, 8, 222 7 of 13 Appl. Appl. Sci. Sci. 2018 2018, , 7 7, , x FO x FOR P R PEER EER RE REVIEW VIEW 7 of 7 of 13 13 Figure Figure 8. 8. Temperature signal ( Temperature signal (D ΔT T)) v vs. s. ti time me ( (τ)) in in the the ca case se o off indu inductive ctive hea heating ting at som at somee ch characteri aracteristi sticc Figure 8. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic surface surface point pointss (model (model in in F Figur igure 6, e 6, ste steel el re reinfor inforcem cement ent at 10 at 10 m mm m depth, depth, po points ints of of intere interest st shown shown in in surface points (model in Figure 6, steel reinforcement at 10 mm depth, points of interest shown in Figur Figure 7, 1—d e 7, 1—defeefect-free ct-free case, case 2—defective , 2—defective case, case reinfor , reinforce cement m crent cross oss sectionsec diminished tion dimi by nishe 25%, d by 3—same 25%, Figure 7, 1—defect-free case, 2—defective case, reinforcement cross section diminished by 25%, as 3—sam (2) but e as additional (2) but addit 1 mm ional thick 1 m air m -filled thick air- delamination). filled delamination). 3—same as (2) but additional 1 mm thick air-filled delamination). Figure 9. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic Figure 9. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic Figure 9. Temperature signal (DT) vs. time () in the case of inductive heating at some characteristic surface points (model in Figure 6, steel reinforcement at 20 mm depth, points of interest shown in surface surface point pointss (model (model in in F Figur igure 6, e 6, ste steel el re reinfor inforcem cement ent at 20 at 20 m mm m depth, depth, po points ints of of intere interest st shown shown in in Figure 7, 1—defect-free case, 2—defective case, reinforcement cross section diminished by 25%, Figur Figure 7, 1—d e 7, 1—defeefect-free ct-free case, case 2—defective , 2—defective case, case reinfor , reinforce cement m crent cross oss sectionsec diminished tion dimi by nishe 25%, d by 3—same 25%, 3—same as (2) but additional 1 mm thick air-filled delamination). as 3—sam (2) but e as additional (2) but addit 1 mm ional thick 1 m air m -filled thick air- delamination). filled delamination). 3.2. Heating with Electric Current 3.2. Heating with Electric Current 3.2. Heating with Electric Current Mathematically, heating with an electric current can be reduced to the same 3D heat conduction Mathematically, heating with an electric current can be reduced to the same 3D heat conduction Mathematically, heating with an electric current can be reduced to the same 3D heat conduction problem of heating a body by internal heat sources, as described above. In practice, electric current problem of heating a body by internal heat sources, as described above. In practice, electric current problem of heating a body by internal heat sources, as described above. In practice, electric current stimulation provides lower power release (400 W per linear meter of pole in our case) and thus stimulation provides lower power release (400 W per linear meter of pole in our case) and thus stimulation provides lower power release (400 W per linear meter of pole in our case) and thus should should operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows should operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows synthetic synthetic IR images of dynamic temperature distributions. The images consist of two sections: the synthetic IR images of dynamic temperature distributions. The images consist of two sections: the IR images of dynamic temperature distributions. The images consist of two sections: the upper upper half-image is related to the above-ground section of the pole, while the bottom half-image upper half-image is related to the above-ground section of the pole, while the bottom half-image half-image is related to the above-ground section of the pole, while the bottom half-image exhibits exhibits the temperature distributions on the pole section buried in moistened soil. Rebar damage exhibits the temperature distributions on the pole section buried in moistened soil. Rebar damage the temperature distributions on the pole section buried in moistened soil. Rebar damage has been has been modeled by 20% rebar cross section thinning separated from the concrete surface layer by a has been modeled by 20% rebar cross section thinning separated from the concrete surface layer by a modeled by 20% rebar cross section thinning separated from the concrete surface layer by a 1 mm thick 1 mm thick air gap. Defect-free reinforcement is characterized by excess temperature of about 10 °C 1 mm thick air gap. Defect-free reinforcement is characterized by excess temperature of about 10 °C air gap. Defect-free reinforcement is characterized by excess temperature of about 10 C (Figure 10a), (Figure 10a), while the presence of an air gap diminishes surface temperature signals in the (Figure 10a), while the presence of an air gap diminishes surface temperature signals in the above-ground bottom section of the pole by about 3–4 °C (Figure 10b). above-ground bottom section of the pole by about 3–4 °C (Figure 10b). Appl. Sci. 2018, 8, 222 8 of 13 while the presence of an air gap diminishes surface temperature signals in the above-ground bottom section Appl. Sci. of the 2018, pole 7, x FO by R P about EER RE3–4 VIEW C (Figure 10b). 8 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 8 of 13 Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions look similar to those in Figures 8 and 9, and the temperature values conditioned by total released look similar to those in Figures 8 and 9, and the temperature values conditioned by total released look similar to those in Figures 8 and 9, and the temperature values conditioned by total released energy are close to the case of inductive heating. The modeling shows that the temperature maximums energy are close to the case of inductive heating. The modeling shows that the temperature energy are close to the case of inductive heating. The modeling shows that the temperature appear at about the same times over rebars of different thickness but a 1 mm thick air gap in a maximums appear at about the same times over rebars of different thickness but a 1 mm thick air maximums appear at about the same times over rebars of different thickness but a 1 mm thick air pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with regard gap in a pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with gap in a pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate (temperature regard to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate regard to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate derivative) can be used for identifying air gaps. However, the use of this phenomenon in practice may (temperature derivative) can be used for identifying air gaps. However, the use of this phenomenon (temperature derivative) can be used for identifying air gaps. However, the use of this phenomenon require smoothing experimental temperature evolutions because taking derivatives tends to enhance in practice may require smoothing experimental temperature evolutions because taking derivatives in practice may require smoothing experimental temperature evolutions because taking derivatives high-frequency noise. tends to enhance high-frequency noise. tends to enhance high-frequency noise. (a) (b) (a) (b) Figure 10. Synthetic temperature distributions in case of non-defective (a) and defective (b) Figure Figure 10. 10. Syn tSy henthetic tem tic temperatp ueratu re disrte distribu ributions t ii nons in case ofcase non -dof non-defect efective (a) aniv de ( dea fe) and de ctive (b) rfective einfor ( ceb m ) ent. reinforcement. reinforcement. Figure 11. Temperature signal (ΔT) vs. time (τ) in the case of heating with electric current: Figure 11. Temperature signal (ΔT) vs. time (τ) in the case of heating with electric current: Figure 11. Temperature signal (DT) vs. time () in the case of heating with electric current: 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section diminished by 20% with additional 1 mm thick air gap. diminished by 20% with additional 1 mm thick air gap. diminished by 20% with additional 1 mm thick air gap. Appl. Sci. 2018, 8, 222 9 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 9 of 13 Figure 12. Temperature derivative (ΔT/Δτ) vs. time (τ) (see caption for Figure 11). Figure 12. Temperature derivative (DT/D) vs. time () (see caption for Figure 11). 4. Experimentation and Discussion 4. Experimentation and Discussion The authors’ practical experience suggested that these testing techniques can be applied to the The authors’ practical experience suggested that these testing techniques can be applied to the inspection of in-field railway electric poles. If inspection is performed by the scheme of Figure 4, i.e., inspection of in-field railway electric poles. If inspection is performed by the scheme of Figure 4, by using natural grounding, one can avoid digging out an underground pole section due to the i.e., by using natural grounding, one can avoid digging out an underground pole section due to temperature distribution at the ground level allowing for the evaluation of the electric current the temperature distribution at the ground level allowing for the evaluation of the electric current flowing down through rebars. The corresponding surface temperature pattern reflects the severity flowing down through rebars. The corresponding surface temperature pattern reflects the severity and and localization of damage in the pole underground section. In its turn, the monitoring of a 1–2 m localization of damage in the pole underground section. In its turn, the monitoring of a 1–2 m long long above-ground pole section allows for the detection of such defects as thinning of rebar above-ground pole section allows for the detection of such defects as thinning of rebar cross-section, cross-section, the presence of corrosion products, concrete delaminations, etc. To simultaneously the presence of corrosion products, concrete delaminations, etc. To simultaneously view the rear side view the rear side of the poles during a single heating cycle, one can use, as mentioned above, IR of the poles during a single heating cycle, one can use, as mentioned above, IR mirrors or additional mirrors or additional IR cameras. IR cameras. In TNDT, test results can often be improved by increasing heating power. In order to avoid In TNDT, test results can often be improved by increasing heating power. In order to avoid damaging a pole protective layer by using additional electrodes, it is convenient to apply damaging a pole protective layer by using additional electrodes, it is convenient to apply non-contact non-contact inductive heating by the scheme of Figure 5 in a narrow circular area on the pole inductive heating by the scheme of Figure 5 in a narrow circular area on the pole surface. By moving surface. By moving both the heater and the IR camera, one can observe pole temperature response both the heater and the IR camera, one can observe pole temperature response under constant heating under constant heating conditions. Since the thermal conductivity of concrete is relatively low, an conditions. Since the thermal conductivity of concrete is relatively low, an optimal inspection area is optimal inspection area is located several centimeters from the heated zone, thus being unaffected located several centimeters from the heated zone, thus being unaffected by powerful thermal radiation by powerful thermal radiation from the heating zone. A similar procedure was used by Green in the from the heating zone. A similar procedure was used by Green in the 1970s to test quality of nuclear 1970s to test quality of nuclear fuel elements [14]. Theoretical aspects of line heating were discussed fuel elements [14]. Theoretical aspects of line heating were discussed elsewhere [12], and its practical elsewhere [12], and its practical implementation was done in the inspection of boiler sections [15]. implementation was done in the inspection of boiler sections [15]. Experimental section of this study was performed on the Transsiberian railway (JSCo “Russian Experimental section of this study was performed on the Transsiberian railway (JSCo “Russian Railways”) by using a pilot experimental setup. We have checked 14 operating poles with a Railways”) by using a pilot experimental setup. We have checked 14 operating poles with a pre-stressed pre-stressed steel reinforcement of the SK type (same as simulated) by using three NDT methods: IR steel reinforcement of the SK type (same as simulated) by using three NDT methods: IR thermographic, thermographic, ultrasonic, and vibroacoustic. ultrasonic, and vibroacoustic. Thermal stimulation of poles was accomplished by means of an inductive heater powered from Thermal stimulation of poles was accomplished by means of an inductive heater powered from an autonomous gasoline generator that was mounted on a truck. The inductor coil was moving up an autonomous gasoline generator that was mounted on a truck. The inductor coil was moving up along a test pole with velocity from 5 to 10 mm/s by using a special lifting device (see Figure 13a). along a test pole with velocity from 5 to 10 mm/s by using a special lifting device (see Figure 13a). Bottom sections of the poles under test were dug out by about 0.5 m to allow their direct IR Bottom sections of the poles under test were dug out by about 0.5 m to allow their direct IR surveying. surveying. Each pole area was inductively heated for about 20 s, the same as in the modeling. Each pole area was inductively heated for about 20 s, the same as in the modeling. Thermal NDT was accomplished on a 1 m long pole area including a 0.5 m long underground section. The inspected area was outlined with aluminum markers that were visible in both IR and visual images. Pole surface temperature was monitored on a dry sunny day (ambient temperature 25–30 °C) by using a FLIR P60 IR camera mounted on a tripod (Figure 13a). A typical IR thermogram Appl. Sci. 2018, 7, x FOR PEER REVIEW 10 of 13 of a defect-free pole is presented in Figure 13b to clearly show the warmed-up subsurface Appl. Sci. 2018, 8, 222 10 of 13 reinforcement. (a) (b) Figure 13. (a) Inspecting quality of electric railway poles: test site view; (b) typical IR thermogram. Figure 13. (a) Inspecting quality of electric railway poles: test site view; (b) typical IR thermogram. Recorded images were processed to produce synthetic images containing extreme values of Thermal NDT was accomplished on a 1 m long pole area including a 0.5 m long underground parameters chosen as significant—for example, images of maximal temperatures achieved during section. The inspected area was outlined with aluminum markers that were visible in both IR and the whole thermal process (the so-called “maxigrams”). Figure 14a shows a defect-free pole section visual images. Pole surface temperature was monitored on a dry sunny day (ambient temperature located above the ground level. It is clearly seen that the surface ‘footprints’ of the reinforcement are 25–30 C) by using a FLIR P60 IR camera mounted on a tripod (Figure 13a). A typical IR thermogram of regularly located and characterized by the same excess temperature in the range from 7 to 8 °C. In a defect-free pole is presented in Figure 13b to clearly show the warmed-up subsurface reinforcement. the case of a damaged pole (Figure 14b), the corresponding maxigram shows a distinct area of Recorded images were processed to produce synthetic images containing extreme values of decreased temperature that apparently corresponds to debonding between the concrete and the parameters chosen as significant—for example, images of maximal temperatures achieved during reinforcement. In defect areas, the excess temperature dropped by about 4–5 °C, being in the same the whole thermal process (the so-called “maxigrams”). Figure 14a shows a defect-free pole section range of magnitude as predicted by the simulation. located above the ground level. It is clearly seen that the surface ‘footprints’ of the reinforcement are regularly located and characterized by the same excess temperature in the range from 7 to 8 C. In the case of a damaged pole (Figure 14b), the corresponding maxigram shows a distinct area of decreased temperature that apparently corresponds to debonding between the concrete and the reinforcement. Appl. Sci. 2018, 8, 222 11 of 13 In defect areas, the excess temperature dropped by about 4–5 C, being in the same range of magnitude as predicted by the simulation. Appl. Sci. 2018, 7, x FOR PEER REVIEW 11 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 11 of 13 (a) (b) (a) (b) Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). In general, it has been found that both types of reinforcement, namely, circumferential and In general, it has been found that both types of reinforcement, namely, circumferential and In general, it has been found that both types of reinforcement, namely, circumferential and longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has been good enough to reliably identify the localization of reinforcement. been good enough to reliably identify the localization of reinforcement. been good enough to reliably identify the localization of reinforcement. Similar results appeared when we inspected the below-ground pole sections (see the example of Similar results appeared when we inspected the below-ground pole sections (see the example Similar results appeared when we inspected the below-ground pole sections (see the example of a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. In of a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. In particular, no indications of circumferential reinforcement were found in the pole underground In particular, no indications of circumferential reinforcement were found in the pole underground particular, no indications of circumferential reinforcement were found in the pole underground sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, the sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, the temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times with the temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times with regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum with regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of hidden appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of hidden delaminations between the concrete and the reinforcement. delaminations between the concrete and the reinforcement. hidden delaminations between the concrete and the reinforcement. Visual inspection of the underground pole section revealed some cracks and weak traces of Visual inspection of the underground pole section revealed some cracks and weak traces Visual inspection of the underground pole section revealed some cracks and weak traces of corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the of corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the corresponding norms accepted in Russia, thus confirming the IR thermographic findings. corresponding norms accepted in Russia, thus confirming the IR thermographic findings. corresponding norms accepted in Russia, thus confirming the IR thermographic findings. (a) (b) (a) (b) Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Appl. Sci. 2018, 8, 222 12 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 12 of 13 Figure 16. Experimental temperature profiles at the ground level: pole excess temperature vs. time Figure 16. Experimental temperature profiles at the ground level: pole excess temperature vs. time (1—+50 cm, 2—0 cm, 3—−30 cm). (1—+50 cm, 2—0 cm, 3—30 cm). Following the results of IR thermographic surveys, the two inspected poles were replaced. The Following the results of IR thermographic surveys, the two inspected poles were replaced. presence of electric corrosion damage was confirmed by a mechanical destruction test. In parallel, The presence of electric corrosion damage was confirmed by a mechanical destruction test. In parallel, the acceptable quality of other inspected poles was confirmed by comparing the results of ultrasonic the acceptable quality of other inspected poles was confirmed by comparing the results of ultrasonic and vibroacoustic testing (these results will be discussed in a forthcoming paper). and vibroacoustic testing (these results will be discussed in a forthcoming paper). 5. Conclusions 5. Conclusions In this study, we have used the known techniques of IR thermographic nondestructive testing In this study, we have used the known techniques of IR thermographic nondestructive testing and the scanning heating of cylindrical objects. However, these techniques are innovatively applied and the scanning heating of cylindrical objects. However, these techniques are innovatively applied to to a new class of test objects. Also, the implementation of heating devices and data processing using a new class of test objects. Also, the implementation of heating devices and data processing using a a concept of maxigrams is novel. concept of maxigrams is novel. TNDT has been used for performing evaluations of reinforced concrete poles on operating TNDT has been used for performing evaluations of reinforced concrete poles on operating railways. This technique is safe for personnel and fully nondestructive since the excess pole railways. This technique is safe for personnel and fully nondestructive since the excess pole temperature does not exceed 15 °C. temperature does not exceed 15 C. IR imaging allows the visualization of both longitudinal and circumferential rebar IR imaging allows the visualization of both longitudinal and circumferential rebar reinforcement, reinforcement, and whether corrosion has caused a separation between the rebar and the concrete. and whether corrosion has caused a separation between the rebar and the concrete. By heating By heating poles inductively (released power: about 10 MW/m ) or applying electric current (up to poles inductively (released power: about 10 MW/m ) or applying electric current (up to 400 W per 400 W per linear meter of a pole), surface temperature signals in defect areas may reach 3–10 °C linear meter of a pole), surface temperature signals in defect areas may reach 3–10 C depending depending on reinforcement depth, as well as a grade of rebar thinning and presence of corrosion on reinforcement depth, as well as a grade of rebar thinning and presence of corrosion products products characterized by high thermal resistance. The same defects diminish the rate of characterized by high thermal resistance. The same defects diminish the rate of temperature changes temperature changes (temperature derivative) by 2–4-fold, which can be used for defect (temperature derivative) by 2–4-fold, which can be used for defect identification. identification. It has been experimentally determined that galvanic corrosion in concrete pole reinforcement can It has been experimentally determined that galvanic corrosion in concrete pole reinforcement be detected in pole sections located higher than 20 cm above the ground level. To evaluate underground can be detected in pole sections located higher than 20 cm above the ground level. To evaluate sections, a pole should be unearthed for about 20–30 cm below the ground level, although in some cases underground sections, a pole should be unearthed for about 20–30 cm below the ground level, it is not necessary. It is worth mentioning that, until now, corrosion damage could only be detected although in some cases it is not necessary. It is worth mentioning that, until now, corrosion damage just prior to the failure of a pole. However, this technique can detect damage well before failure. could only be detected just prior to the failure of a pole. However, this technique can detect damage TNDT surveys can detect reinforcement displacement and predict the residual strength of poles well before failure. by analyzing concrete cracking in the near-ground pole section. This is a goal of the ongoing research. TNDT surveys can detect reinforcement displacement and predict the residual strength of poles Acknowledgments: by analyzing concret This e cr work acking was in supported the near by -gro a und Russian pole s Scientific ection. F This oundation is a grant goal of #17-19-01047 the ongoing (3D numerical modeling) and in part by a State Order of the Russian Ministry of Higher Education for 2017–2019, NIR research. #9.5966.2017/BY (experimental implementation). Author Contributions: Dmitry Valeryevich Sannikov conceived and designed the experiments. Alexander Acknowledgments: This work was supported by a Russian Scientific Foundation grant #17-19-01047 (3D Sergeevich Kolevatov performed the experiments. Vladimir Platonovich Vavilov did the numerical modeling of numerical modeling) and in part by a State Order of the Russian Ministry of Higher Education for 2017–2019, defect situations. Marina Valeryevna Kuimova wrote the paper. NIR #9.5966.2017/BY (experimental implementation). Conflicts of Interest: The authors declare no conflict of interest. Author Contributions: Dmitry Valeryevich Sannikov conceived and designed the experiments. Alexander Sergeevich Kolevatov performed the experiments. Vladimir Platonovich Vavilov did the numerical modeling of defect situations. Marina Valeryevna Kuimova wrote the paper. Appl. Sci. 2018, 8, 222 13 of 13 References 1. Hillemeier, B. Method of Determining the Location, Orientation and Pattern of Reinforcing Members in Reinforced Concrete. U.S. Patent 4309610, 5 January 1982. 2. Maierhofer, C.; Reinhardt, H.W.; Dobmann, G. (Eds.) Non-Destructive Evaluation of Reinforced Concrete Structures; Woodhead Publishing CRC Press: Cambridge, UK, 2010; 624p, ISBN 978184569950. 3. Kobayashi, K.; Banthia, N. Corrosion detection in reinforced concrete by using induction heating and infrared thermography. J. Civ. Struct. Health Monit. 2011, 1, 25–35. [CrossRef] 4. Milovanovic, ´ B. Detecting defects in reinforced concrete using the method of infrared thermography. CrSNDT J. 2013, 3, 3–13. 5. Cannard, H.; Mahrez, M.; Perrin, T.; Muzet, V.; Prybyla, D.; Brachelet, F. The use of infrared thermography for defects detection on reinforced concrete bridges. In Proceedings of the 12th Quantitative InfraRed Thermography Conference, Bordeaux, France, 7–11 July 2014. [CrossRef] 6. Szymanik, B.; Karol-Frankowski, P.; Czady, T.; Chelliah, C. Detection and inspection of steel bars in reinforced concrete structures using active infrared thermography with microwave excitation and eddy current sensors. Sensors 2016, 16, 234. [CrossRef] [PubMed] 7. American Society of Civil Engineers, USA. Guide for the Design and Use of Concrete Poles; American Society of Civil Engineers: Reston, VA, USA, 1987; 53p, ISBN 0-87262-596-6. 8. Tsunemoto, M.; Shimizu, M.; Kondo, Y.; Kudo, T.; Ueda, H.; Ijima, T. Replacement criteria for concrete catenary poles. Q. Rep. RTRI 2017, 58, 270–276. [CrossRef] 9. Ausgrid Network Standard NS145. Pole Inspection and Treatment Procedures. Available online: www.ausgrid. com.au/-/media/Files/Network/Documents/NS-and-NUS/NS145.pdf (accessed on 3 August 2017). 10. Russian Federation Government Analytical Centre. Bulletin of the Socio-Economic Crisis in Russia: Dynamics of Cargo Transportation in Russia; Russian Federation Government Analytical Centre: Moscow, Russia, 2015; 24p. (In Russian) 11. Frumuselu, D.; Radu, C. IR thermography applied to ground-level reinforced concrete constructions belonging to electricity networks. Insight 1998, 40, 501–504. 12. Vavilov, V.; Taylor, R. Theoretical and practical aspects of the thermal NDT of bonded structures. In Research Techniques in NDT; Sharpe, R., Ed.; Academic Press: London, UK, 1982; Volume 5, pp. 239–280, ISBN 0-12-639055-X. 13. Vavilov, V.P.; Burleigh, D.D. Review of pulsed thermal NDT: Physical principles, theory and data processing. NDT E Int. 2015, 73, 28–52. [CrossRef] 14. Green, D.R. Principles and applications of emittance-independent infrared nondestructive testing. Appl. Opt. 1968, 7, 1796–1805. [CrossRef] [PubMed] 15. Woolard, D.; Cramer, K. The thermal photocopier: A new concept for thermal NDT. Proc. SPIE 2004, 5405, 366–373. [CrossRef] © 2018 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/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive Testing

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applied sciences Article Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive Testing 1 1 Dmitry Valeryevich Sannikov , Alexander Sergeevich Kolevatov , 1 , 2 , 1 Vladimir Platonovich Vavilov * and Marina Valeryevna Kuimova School of Nondestructive Testing and Safety, Tomsk Polytechnic University, Lenin Av., 30, 634050 Tomsk, Russia; sanndv72@gmail.com (D.V.S.); shiryaev@tpu.ru (A.S.K.); kuimova@tpu.ru (M.V.K.) Department of Theoretical and Computational Mechanics, Tomsk State University, Lenin Av., 36, 634050 Tomsk, Russia * Correspondence: vavilov@tpu.ru Received: 11 December 2017; Accepted: 24 January 2018; Published: 1 February 2018 Abstract: Thermal nondestructive testing can be used to inspect reinforced concrete supports that are widely used in various industries. Corrosion damage is a typical defect found in these supports. Corrosion usually starts as a separation between the concrete and the steel rebar. Damage is exacerbated by pressure that is caused by the formation of corrosion products. The most logical method for using IR to detect corrosion or a disbond would be to heat up the rebar by resistive or inductive heating. In both cases, monitoring the dynamic temperature distributions on the pole allows for the evaluation of reinforcement quality. The thermal properties of steel, concrete, air, and corrosion products differ greatly. The magnitude of temperature anomalies and their behavior over time depend on the presence of corrosion products, air gaps, and the quality of contact between rebar and concrete. Keywords: thermal testing; electric railway pole; inductive heating; numerical modeling 1. Introduction In the inspection of reinforced concrete, a combination of induction heating and IR thermography was first suggested by Hillemeier et al. in 1982 [1]. General applications of this inspection technique were summarized by Maierhofer et al. [2]. Kobayashi and Banthia focused on the detection of corrosion by applying induction heating [3]. Milovanovic ´ et al. demonstrated that external optical stimulation of reinforcement in concrete enables detection of 10-cm structural elements in concrete at depths up to 7 cm [4]. Smaller thermal anomalies, such as 2  2 cm, can be detected up to a depth of about 4 cm [5]. Szymanic et al. described the use of microwave heating in the detection of rebars in concrete with a depth limit of about 2 cm [6]. This study covers the development of a thermal nondestructive testing (TNDT) technique for inspecting reinforced concrete supports of several types, such as overhead line poles, pipes, beams, pilings, etc., which are widely used in various industries, and in both the manufacturing and operation stages [7–9]. Historically, Russian railways represent one of the most important industrial areas in the country. It is worth mentioning that over 80% of cargo in Russia is being transported by means of railways (not counting the pipeline transport) [10]. Uncontrolled damage and full destruction of electrical supports may cause financial losses and even casualties, particularly in densely populated areas. Therefore, research intended to enhance the reliability of railway overhead lines is of a high priority in Russia, being supported by the corresponding federal directive documents. The mass electrification of Russian railways started in the 1980s. The lifetime of reinforced concrete poles is Appl. Sci. 2018, 8, 222; doi:10.3390/app8020222 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 222 2 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 2 of 13 reinforced concrete poles is assumed to be about 50 years, hence, in the near future, efficient assumed to be about 50 years, hence, in the near future, efficient inspection techniques to evaluate the inspection techniques to evaluate the quality of supports will become increasingly important. quality of supports will become increasingly important. Some poles may reveal an essential loss of their mechanical strength much earlier than within Some poles may reveal an essential loss of their mechanical strength much earlier than within their normative lifetime. This may be conditioned by either poor workmanship or some ambient their normative lifetime. This may be conditioned by either poor workmanship or some ambient factors. Pole damage often starts in underground sections of supports, where corrosion appears factors. Pole damage often starts in underground sections of supports, where corrosion appears because of varying soil moisture and high mechanical stresses. Corrosion damage is related to cracks because of varying soil moisture and high mechanical stresses. Corrosion damage is related to cracks that appear in concrete boundary layers adjacent to steel rebars because of enhanced pressure that appear in concrete boundary layers adjacent to steel rebars because of enhanced pressure rendered rendered by corrosion products. It is important to note that corrosion defects are located under the by corrosion products. It is important to note that corrosion defects are located under the concrete concrete surface layer and often hidden in the soil, thus are visually undetectable. surface layer and often hidden in the soil, thus are visually undetectable. Figure 1 shows a scheme of an electric railway pole and typical corrosion defects that appear at Figure 1 shows a scheme of an electric railway pole and typical corrosion defects that appear a pre-failure period of operation. The most dangerous is corrosion damage, which appears in at a pre-failure period of operation. The most dangerous is corrosion damage, which appears in underground pole sections (Figure 1b). underground pole sections (Figure 1b). (a) (b)(c) Figure 1. (a) Electric railway pole scheme; (b,c) and examples of corrosion damage. Figure 1. (a) Electric railway pole scheme; (b,c) and examples of corrosion damage. A number of nondestructive testing (NDT) techniques have been applied to detect early A number of nondestructive testing (NDT) techniques have been applied to detect early corrosion corrosion in electric railway poles. They differ by sensitivity and test productivity but can hardly be in electric railway poles. They differ by sensitivity and test productivity but can hardly be applied to applied to the evaluation of pole wear. The most usable ultrasonic inspection technique evaluates the evaluation of pole wear. The most usable ultrasonic inspection technique evaluates the strength the strength of concrete by measuring ultrasound velocity in different directions or recording of concrete by measuring ultrasound velocity in different directions or recording acoustic emission acoustic emission signals. Another ultrasonic test method involves the analysis of resonance features signals. Another ultrasonic test method involves the analysis of resonance features of the reinforcement. of the reinforcement. Both techniques are predominantly applicable for inspecting concrete surface Both techniques are predominantly applicable for inspecting concrete surface layers and often require layers and often require the use of special references. the use of special references. One more inspection technique involves electromagnetic (induction) heating of a pole to locate One more inspection technique involves electromagnetic (induction) heating of a pole to locate reinforcement in concrete, but this does not help with the evaluation of residual reinforcement reinforcement in concrete, but this does not help with the evaluation of residual reinforcement thickness. thickness. Classical TNDT based on surface heating of test objects seems to be inappropriate in the Classical TNDT based on surface heating of test objects seems to be inappropriate in the inspection of inspection of concrete poles because of the considerable pole thickness and low thermal conductivity of concrete with regard to highly conductive steel. Alternatively, performing heating within internal Appl. Sci. 2018, 8, 222 3 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 3 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 3 of 13 cavities that are present in some types of poles is practically difficult. The use of solar heating for concrete poles because of the considerable pole thickness and low thermal conductivity of concrete cavities that are present in some types of poles is practically difficult. The use of solar heating for detecting damaged concrete in electric railway poles was reported elsewhere [11], but such a with regard to highly conductive steel. Alternatively, performing heating within internal cavities detecting damaged concrete in electric railway poles was reported elsewhere [11], but such a technique obviously cannot ensure high test reliability because of variable and low-power heating that are present in some types of poles is practically difficult. The use of solar heating for detecting technique obviously cannot ensure high test reliability because of variable and low-power heating and the presence of multiple sources of thermal noise. damaged concrete in electric railway poles was reported elsewhere [11], but such a technique obviously and the presence of multiple sources of thermal noise. cannot ensure high test reliability because of variable and low-power heating and the presence of 2. Inspection Schemes multiple sources of thermal noise. 2. Inspection Schemes The authors have suggested the stimulation of hidden metallic rebars by applying electric 2. Inspection Schemes The authors have suggested the stimulation of hidden metallic rebars by applying electric current or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic current or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic tempera Thetauthors ure distri have butisuggested ons on the externa the stimulation l bottom sect of hidden ion of metallic a poler a ebars llows by for applying evaluatelectric ing the curr qual ent ity temperature distributions on the external bottom section of a pole allows for evaluating the quality or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic temperature of reinforcement. Since steel, concrete, air, and corrosion products differ by their thermal properties, of reinforcement. Since steel, concrete, air, and corrosion products differ by their thermal properties, distributions significant teon st p the ara external meters, s bottom uch as section magnit ofude a pole of t allows emperat forure evaluating anomalies the and quality their b of reinfor ehavicement. or over significant test parameters, such as magnitude of temperature anomalies and their behavior over Since time, depen steel, concr d on the presence of corro ete, air, and corrosionsion produc products dif ts fer and the by their qua thermal lity of conta properties, ct between rebars significant test and time, depend on the presence of corrosion products and the quality of contact between rebars and parameters, concrete. Both power such as magnitude and dura oftion of temperatur electri e anomalies c pulses and shoul their d be behavior optimizover ed to ensure detecta time, depend on the ble concrete. Both power and duration of electric pulses should be optimized to ensure detectable pr sur esence face teof mp corr erat osion ure sipr gn oducts als. and the quality of contact between rebars and concrete. Both power surface temperature signals. and duration There are of electric four test configur pulses should ations en be optimized abling the to ensur inteernal st detectable imulat surface ion of reb temperatur ars. In t e h signals. e first There are four test configurations enabling the internal stimulation of rebars. In the first scheme, a so There arurce o e fourftest electric configurations current 5 is d enabling irectly the connect internal ed to stimulation rebars 2 via cont of rebars. act cIn lam the psfirst 6 (Fig scheme, ure 2). scheme, a source of electric current 5 is directly connected to rebars 2 via contact clamps 6 (Figure 2). aFisour gure ce 3 s ofh elect owsric the us current e of5 el isedir ctrectly odes connected 7 placed on to the surface rebars 2 via o contact f concrete wi clampsthout di 6 (Figurrect e 2).a Figur ccess to e 3 Figure 3 shows the use of electrodes 7 placed on the surface of concrete without direct access to shows rebars. The t the useh of ird t electr estodes conf7igur placed ation on us the es t surface he natof ura concr l groun ete without ding of concret direct access e poles t to rebars. hat arThe e deeply third rebars. The third test configuration uses the natural grounding of concrete poles that are deeply test buriconfiguration ed in soil. In thi uses s ca the se, one condu natural grounding ctor is of conne concr cted to ete poles a na that tura arl or arti e deeply ficial buried earth el in soil. ectrode In this8 buried in soil. In this case, one conductor is connected to a natural or artificial earth electrode 8 case, (Figur one e 4). However, conductor the au is connected thors’ pr to actical exper a natural or ienc artificial e has shown t earth electr hat the most ode 8 (Figur effecte ive tech 4). However nique is , (Figure 4). However, the authors’ practical experience has shown that the most effective technique is the induct authors’ ive stpractical imulation experience of reinfohas rcemen shown t (Fthat igurthe e 5) most usin efg fective high-fr technique equency ind is inductive uctors 1stimulation 0 allowing inductive stimulation of reinforcement (Figure 5) using high-frequency inductors 10 allowing of cont reinfor actless cement heating o (Figur f met e 5)ausing llic rebar high-fr s insi equency de concret inductors e poles.10 allowing contactless heating of metallic contactless heating of metallic rebars inside concrete poles. rebars inside concrete poles. Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR imager, Figure 2. Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 6—contact clamps). 4—cables, 5—current source, 6—contact clamps). imager, 4—cables, 5—current source, 6—contact clamps). Figure 3. Using surface electrodes for heating rebars: (1—test object, 2—rebars, 3—IR imager, 4—cables, Figure 3. Using surface electrodes for heating rebars: (1—test object, 2—rebars, 3—IR imager, 5—curr Figure 3. ent Us sour ing s ce, 7—surface urface elecelectr trodeodes). s for heating rebars: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 7—surface electrodes). 4—cables, 5—current source, 7—surface electrodes). Appl. Sci. 2018, 7, x FOR PEER REVIEW 4 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 4 of 13 Below we describe general principles and a step-by-step procedure of TNDT of railway electric poles by using inductive heating. Below we describe general principles and a step-by-step procedure of TNDT of railway electric The inspection is performed by using at least one IR camera 1 (Figure 5). The inspection area poles by using inductive heating. can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high The inspection is performed by using at least one IR camera 1 (Figure 5). The inspection area reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high energy supply intended for rebar stimulation should be chosen taking into account the configuration reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric and mass of test objects. Stimulation time and power are to be optimized by modeling particular test energy supply intended for rebar stimulation should be chosen taking into account the configuration cases. Note that, by delivering a maximal heating power, one must avoid material damage and mass of test objects. Stimulation time and power are to be optimized by modeling particular test accompanied by the appearance of burns and slags. cases. Note that, by delivering a maximal heating power, one must avoid material damage Appl. Sci. 2018, 8, 222 4 of 13 accompanied by the appearance of burns and slags. Figure 4. TNDT by using pole natural grounding and earth electrode: (1—test object, 2—rebars, 3—IR imager, Figure 4. TND4—cables, 5— T by using polcurrent source, 6—contact e natural grounding and ea cla rthm elps, 8—natu ectrode: (1— ral/artific test objeial eart ct, 2—re h electrod bars, 3—Ie, R Figure 4. TNDT by using pole natural grounding and earth electrode: (1—test object, 2—rebars, imager, 4—cables, 5—current source, 6—contact clamps, 8—natural/artificial earth electrode, 11—IR mirror). 11—IR mirror). 3—IR imager, 4—cables, 5—current source, 6—contact clamps, 8—natural/artificial earth electrode, 11—IR mirror). Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, 3—IR 3—IR imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency Figure 5. TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, 3—IR inductor, 11—IR mirror). inductor, 11—IR mirror). imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency inductor, 11—IR mirror). Synchronously with turning on a stimulation source, one starts recording a sequence of IR Below we describe general principles and a step-by-step procedure of TNDT of railway electric thermograms that reflects the evolution of pole surface temperature over time. Note that most of the polesSynchrono by using inductive usly with turn heating. ing on a stimulation source, one starts recording a sequence of IR thermogra The inspection ms that refl is ects the evoluti performed by using on of pol at least e surf one ace temperat IR camera ur 1 (Figur e overe time. 5). The Note tha inspecti t most of the on area can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric energy supply intended for rebar stimulation should be chosen taking into account the configuration and mass of test objects. Stimulation time and power are to be optimized by modeling particular test cases. Note that, by delivering a maximal heating power, one must avoid material damage accompanied by the appearance of burns and slags. Appl. Sci. 2018, 8, 222 5 of 13 Synchronously with turning on a stimulation source, one starts recording a sequence of IR thermograms that reflects the evolution of pole surface temperature over time. Note that most of the thermal signals appropriate for further evaluation take place at a cooling stage when hidden defects can be detected by their specific temperature patterns on the inspected surface. By analyzing IR images, a thermographer is to outline areas with rebars and detect possible hidden defects, whose identification strongly depends on thermographer ’s experience. It has been found that an additional detection criterion, along with defect pattern amplitude and shape, is a rate of temperature variations in suspicious areas, i.e., the corresponding temperature derivatives. The efficiency of the above test procedure depends on how uniform the heating of rebars is. This can be achieved by: (1) using transverse reinforcement (if available); (2) binding reinforcement with additional wires; (3) using grounding elements or special surface electrodes. 3. Modeling Test Cases 3.1. Inductive Heating We are dealing with a 3D heat conduction problem for a non-adiabatic solid body with internal heat sources. The cylindrical geometry of a pole (Figure 6a) was replaced with a 3D Cartesian model shown in Figure 6b and numerically solved by using the ThermoSource software from Tomsk Polytechnic University (Figure 6c). The model simulates a 55 mm thick pole shell made of reinforced concrete (pole type SK8 by the Russian nomenclature). It is worth noting that in real poles, plenty of rebar bunches each including four 5 mm diameter steel rebars are regularly placed within a pole, as shown in Figure 6a. Each rebar bunch was simulated as a parallelepiped-like defect (cross section: 20  5 mm) and placed at depths of 10 and 20 mm from the external pole surface, and high-frequency powerful induction heating was simulated as energy discharge in a rebar (released power 10 MW/m , heating duration 20 s). The mathematical formulation of such TNDT problems was thoroughly discussed elsewhere [12,13]. Three characteristic areas are specified on the pole surface: a defect-free area (area 1 in Figure 6c), diminution of the rebar cross section by 25% (area 2), and the same rebar thinning but additionally accompanied with a 1 mm thick air-filled delamination (area 3). The latter defect is to simulate the practical situations, where the process of corrosion wear is accompanied by corrosion products that appear between concrete and reinforcement and provide pressure on the adjacent concrete, thus causing voids and cavities. Two IR thermograms taken at 60 and 120 s are shown in Figure 7 to demonstrate that, over defect-free areas, the surface excess temperature DT reaches 8 C (at 120 s if the defect depth is 10 mm); the rebar thinning results in a weak decrease of surface temperature because of the lower released energy while a significant temperature decrease occurs over the air-filled delamination because of its high thermal resistance. Temperature evolutions at three characteristic points (see Figure 7) in time () are presented in Figure 8. Similarly to diffusivity measurement by using the known Parker technique, the fronts of temperature signals in Figure 8 depend on material thickness and thermal properties. This is also illustrated by Figure 9, where steel rebars are located at a depth of 20 mm. The thicker concrete layer diminishes both the excess surface temperature (down to 3 C) and the rate of the temperature change. The same dependencies are shown in Figure 9 for the case where steel rebars are located at a depth of 20 mm. The thicker concrete layer diminishes the surface temperature up to 3 C and makes the rate of change in temperature slower. Appl. Sci. 2018, 8, 222 6 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 6 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 6 of 13 (a) (b) (a) (b) (c) Figure 6. Railway pole 3D numerical model (wall thick (c) ness 55 mm, 5 mm diameter steel rebars at 10 Figure 6. Railway pole 3D numerical model (wall thickness 55 mm, 5 mm diameter steel rebars at (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, 10 (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, Figure 6. Railway pole 3D numerical model (wall thickness 55 mm, 5 mm diameter steel rebars at 10 3—same as (2), additional 1 mm–thick air-filled delamination): (a) pole scheme; (b) Cartesian model; 3—same as (2), additional 1 mm–thick air-filled delamination): (a) pole scheme; (b) Cartesian model; (or 20) mm depth, 1—defect-free case, 2—defective case, rebar cross section diminished by 25%, (c) ThermoSource software model, two rebars shown). (c) 3—same as (2), additional 1 ThermoSource software model, mm–th two ick air-filled rebars shown). delamination): (a) pole scheme; (b) Cartesian model; (c) ThermoSource software model, two rebars shown). (a) (b) Figure 7. Synthetic IR thermograms of railway pole (a) ( at 60 s (a) and 120 bs ( ) b) (model in Figure 6, reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). Figure 7. Synthetic IR thermograms of railway pole at 60 s (a) and 120 s (b) (model in Figure 6, Figure 7. Synthetic IR thermograms of railway pole at 60 s (a) and 120 s (b) (model in Figure 6, reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). reinforcement depth 10 mm, released power 10 MW/m , inductive heating duration 20 s). Appl. Sci. 2018, 8, 222 7 of 13 Appl. Appl. Sci. Sci. 2018 2018, , 7 7, , x FO x FOR P R PEER EER RE REVIEW VIEW 7 of 7 of 13 13 Figure Figure 8. 8. Temperature signal ( Temperature signal (D ΔT T)) v vs. s. ti time me ( (τ)) in in the the ca case se o off indu inductive ctive hea heating ting at som at somee ch characteri aracteristi sticc Figure 8. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic surface surface point pointss (model (model in in F Figur igure 6, e 6, ste steel el re reinfor inforcem cement ent at 10 at 10 m mm m depth, depth, po points ints of of intere interest st shown shown in in surface points (model in Figure 6, steel reinforcement at 10 mm depth, points of interest shown in Figur Figure 7, 1—d e 7, 1—defeefect-free ct-free case, case 2—defective , 2—defective case, case reinfor , reinforce cement m crent cross oss sectionsec diminished tion dimi by nishe 25%, d by 3—same 25%, Figure 7, 1—defect-free case, 2—defective case, reinforcement cross section diminished by 25%, as 3—sam (2) but e as additional (2) but addit 1 mm ional thick 1 m air m -filled thick air- delamination). filled delamination). 3—same as (2) but additional 1 mm thick air-filled delamination). Figure 9. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic Figure 9. Temperature signal (ΔT) vs. time (τ) in the case of inductive heating at some characteristic Figure 9. Temperature signal (DT) vs. time () in the case of inductive heating at some characteristic surface points (model in Figure 6, steel reinforcement at 20 mm depth, points of interest shown in surface surface point pointss (model (model in in F Figur igure 6, e 6, ste steel el re reinfor inforcem cement ent at 20 at 20 m mm m depth, depth, po points ints of of intere interest st shown shown in in Figure 7, 1—defect-free case, 2—defective case, reinforcement cross section diminished by 25%, Figur Figure 7, 1—d e 7, 1—defeefect-free ct-free case, case 2—defective , 2—defective case, case reinfor , reinforce cement m crent cross oss sectionsec diminished tion dimi by nishe 25%, d by 3—same 25%, 3—same as (2) but additional 1 mm thick air-filled delamination). as 3—sam (2) but e as additional (2) but addit 1 mm ional thick 1 m air m -filled thick air- delamination). filled delamination). 3.2. Heating with Electric Current 3.2. Heating with Electric Current 3.2. Heating with Electric Current Mathematically, heating with an electric current can be reduced to the same 3D heat conduction Mathematically, heating with an electric current can be reduced to the same 3D heat conduction Mathematically, heating with an electric current can be reduced to the same 3D heat conduction problem of heating a body by internal heat sources, as described above. In practice, electric current problem of heating a body by internal heat sources, as described above. In practice, electric current problem of heating a body by internal heat sources, as described above. In practice, electric current stimulation provides lower power release (400 W per linear meter of pole in our case) and thus stimulation provides lower power release (400 W per linear meter of pole in our case) and thus stimulation provides lower power release (400 W per linear meter of pole in our case) and thus should should operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows should operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows operate for a longer time (10 min) to ensure detectable temperature signals. Figure 10 shows synthetic synthetic IR images of dynamic temperature distributions. The images consist of two sections: the synthetic IR images of dynamic temperature distributions. The images consist of two sections: the IR images of dynamic temperature distributions. The images consist of two sections: the upper upper half-image is related to the above-ground section of the pole, while the bottom half-image upper half-image is related to the above-ground section of the pole, while the bottom half-image half-image is related to the above-ground section of the pole, while the bottom half-image exhibits exhibits the temperature distributions on the pole section buried in moistened soil. Rebar damage exhibits the temperature distributions on the pole section buried in moistened soil. Rebar damage the temperature distributions on the pole section buried in moistened soil. Rebar damage has been has been modeled by 20% rebar cross section thinning separated from the concrete surface layer by a has been modeled by 20% rebar cross section thinning separated from the concrete surface layer by a modeled by 20% rebar cross section thinning separated from the concrete surface layer by a 1 mm thick 1 mm thick air gap. Defect-free reinforcement is characterized by excess temperature of about 10 °C 1 mm thick air gap. Defect-free reinforcement is characterized by excess temperature of about 10 °C air gap. Defect-free reinforcement is characterized by excess temperature of about 10 C (Figure 10a), (Figure 10a), while the presence of an air gap diminishes surface temperature signals in the (Figure 10a), while the presence of an air gap diminishes surface temperature signals in the above-ground bottom section of the pole by about 3–4 °C (Figure 10b). above-ground bottom section of the pole by about 3–4 °C (Figure 10b). Appl. Sci. 2018, 8, 222 8 of 13 while the presence of an air gap diminishes surface temperature signals in the above-ground bottom section Appl. Sci. of the 2018, pole 7, x FO by R P about EER RE3–4 VIEW C (Figure 10b). 8 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 8 of 13 Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives Figures 11 and 12 present graphs of both surface temperature and temperature time derivatives for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions for three test cases, respectively (see the figure caption). Qualitatively, the temperature evolutions look similar to those in Figures 8 and 9, and the temperature values conditioned by total released look similar to those in Figures 8 and 9, and the temperature values conditioned by total released look similar to those in Figures 8 and 9, and the temperature values conditioned by total released energy are close to the case of inductive heating. The modeling shows that the temperature maximums energy are close to the case of inductive heating. The modeling shows that the temperature energy are close to the case of inductive heating. The modeling shows that the temperature appear at about the same times over rebars of different thickness but a 1 mm thick air gap in a maximums appear at about the same times over rebars of different thickness but a 1 mm thick air maximums appear at about the same times over rebars of different thickness but a 1 mm thick air pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with regard gap in a pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with gap in a pole with a 55 mm thick wall diminishes the temperature evolution rate up to 3-fold with to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate (temperature regard to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate regard to non-defect areas (Figure 12). Hence, variations in the temperature evolution rate derivative) can be used for identifying air gaps. However, the use of this phenomenon in practice may (temperature derivative) can be used for identifying air gaps. However, the use of this phenomenon (temperature derivative) can be used for identifying air gaps. However, the use of this phenomenon require smoothing experimental temperature evolutions because taking derivatives tends to enhance in practice may require smoothing experimental temperature evolutions because taking derivatives in practice may require smoothing experimental temperature evolutions because taking derivatives high-frequency noise. tends to enhance high-frequency noise. tends to enhance high-frequency noise. (a) (b) (a) (b) Figure 10. Synthetic temperature distributions in case of non-defective (a) and defective (b) Figure Figure 10. 10. Syn tSy henthetic tem tic temperatp ueratu re disrte distribu ributions t ii nons in case ofcase non -dof non-defect efective (a) aniv de ( dea fe) and de ctive (b) rfective einfor ( ceb m ) ent. reinforcement. reinforcement. Figure 11. Temperature signal (ΔT) vs. time (τ) in the case of heating with electric current: Figure 11. Temperature signal (ΔT) vs. time (τ) in the case of heating with electric current: Figure 11. Temperature signal (DT) vs. time () in the case of heating with electric current: 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section 1—defect-free reinforcement, 2—rebar cross section diminished by 20%, 3—rebar cross section diminished by 20% with additional 1 mm thick air gap. diminished by 20% with additional 1 mm thick air gap. diminished by 20% with additional 1 mm thick air gap. Appl. Sci. 2018, 8, 222 9 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 9 of 13 Figure 12. Temperature derivative (ΔT/Δτ) vs. time (τ) (see caption for Figure 11). Figure 12. Temperature derivative (DT/D) vs. time () (see caption for Figure 11). 4. Experimentation and Discussion 4. Experimentation and Discussion The authors’ practical experience suggested that these testing techniques can be applied to the The authors’ practical experience suggested that these testing techniques can be applied to the inspection of in-field railway electric poles. If inspection is performed by the scheme of Figure 4, i.e., inspection of in-field railway electric poles. If inspection is performed by the scheme of Figure 4, by using natural grounding, one can avoid digging out an underground pole section due to the i.e., by using natural grounding, one can avoid digging out an underground pole section due to temperature distribution at the ground level allowing for the evaluation of the electric current the temperature distribution at the ground level allowing for the evaluation of the electric current flowing down through rebars. The corresponding surface temperature pattern reflects the severity flowing down through rebars. The corresponding surface temperature pattern reflects the severity and and localization of damage in the pole underground section. In its turn, the monitoring of a 1–2 m localization of damage in the pole underground section. In its turn, the monitoring of a 1–2 m long long above-ground pole section allows for the detection of such defects as thinning of rebar above-ground pole section allows for the detection of such defects as thinning of rebar cross-section, cross-section, the presence of corrosion products, concrete delaminations, etc. To simultaneously the presence of corrosion products, concrete delaminations, etc. To simultaneously view the rear side view the rear side of the poles during a single heating cycle, one can use, as mentioned above, IR of the poles during a single heating cycle, one can use, as mentioned above, IR mirrors or additional mirrors or additional IR cameras. IR cameras. In TNDT, test results can often be improved by increasing heating power. In order to avoid In TNDT, test results can often be improved by increasing heating power. In order to avoid damaging a pole protective layer by using additional electrodes, it is convenient to apply damaging a pole protective layer by using additional electrodes, it is convenient to apply non-contact non-contact inductive heating by the scheme of Figure 5 in a narrow circular area on the pole inductive heating by the scheme of Figure 5 in a narrow circular area on the pole surface. By moving surface. By moving both the heater and the IR camera, one can observe pole temperature response both the heater and the IR camera, one can observe pole temperature response under constant heating under constant heating conditions. Since the thermal conductivity of concrete is relatively low, an conditions. Since the thermal conductivity of concrete is relatively low, an optimal inspection area is optimal inspection area is located several centimeters from the heated zone, thus being unaffected located several centimeters from the heated zone, thus being unaffected by powerful thermal radiation by powerful thermal radiation from the heating zone. A similar procedure was used by Green in the from the heating zone. A similar procedure was used by Green in the 1970s to test quality of nuclear 1970s to test quality of nuclear fuel elements [14]. Theoretical aspects of line heating were discussed fuel elements [14]. Theoretical aspects of line heating were discussed elsewhere [12], and its practical elsewhere [12], and its practical implementation was done in the inspection of boiler sections [15]. implementation was done in the inspection of boiler sections [15]. Experimental section of this study was performed on the Transsiberian railway (JSCo “Russian Experimental section of this study was performed on the Transsiberian railway (JSCo “Russian Railways”) by using a pilot experimental setup. We have checked 14 operating poles with a Railways”) by using a pilot experimental setup. We have checked 14 operating poles with a pre-stressed pre-stressed steel reinforcement of the SK type (same as simulated) by using three NDT methods: IR steel reinforcement of the SK type (same as simulated) by using three NDT methods: IR thermographic, thermographic, ultrasonic, and vibroacoustic. ultrasonic, and vibroacoustic. Thermal stimulation of poles was accomplished by means of an inductive heater powered from Thermal stimulation of poles was accomplished by means of an inductive heater powered from an autonomous gasoline generator that was mounted on a truck. The inductor coil was moving up an autonomous gasoline generator that was mounted on a truck. The inductor coil was moving up along a test pole with velocity from 5 to 10 mm/s by using a special lifting device (see Figure 13a). along a test pole with velocity from 5 to 10 mm/s by using a special lifting device (see Figure 13a). Bottom sections of the poles under test were dug out by about 0.5 m to allow their direct IR Bottom sections of the poles under test were dug out by about 0.5 m to allow their direct IR surveying. surveying. Each pole area was inductively heated for about 20 s, the same as in the modeling. Each pole area was inductively heated for about 20 s, the same as in the modeling. Thermal NDT was accomplished on a 1 m long pole area including a 0.5 m long underground section. The inspected area was outlined with aluminum markers that were visible in both IR and visual images. Pole surface temperature was monitored on a dry sunny day (ambient temperature 25–30 °C) by using a FLIR P60 IR camera mounted on a tripod (Figure 13a). A typical IR thermogram Appl. Sci. 2018, 7, x FOR PEER REVIEW 10 of 13 of a defect-free pole is presented in Figure 13b to clearly show the warmed-up subsurface Appl. Sci. 2018, 8, 222 10 of 13 reinforcement. (a) (b) Figure 13. (a) Inspecting quality of electric railway poles: test site view; (b) typical IR thermogram. Figure 13. (a) Inspecting quality of electric railway poles: test site view; (b) typical IR thermogram. Recorded images were processed to produce synthetic images containing extreme values of Thermal NDT was accomplished on a 1 m long pole area including a 0.5 m long underground parameters chosen as significant—for example, images of maximal temperatures achieved during section. The inspected area was outlined with aluminum markers that were visible in both IR and the whole thermal process (the so-called “maxigrams”). Figure 14a shows a defect-free pole section visual images. Pole surface temperature was monitored on a dry sunny day (ambient temperature located above the ground level. It is clearly seen that the surface ‘footprints’ of the reinforcement are 25–30 C) by using a FLIR P60 IR camera mounted on a tripod (Figure 13a). A typical IR thermogram of regularly located and characterized by the same excess temperature in the range from 7 to 8 °C. In a defect-free pole is presented in Figure 13b to clearly show the warmed-up subsurface reinforcement. the case of a damaged pole (Figure 14b), the corresponding maxigram shows a distinct area of Recorded images were processed to produce synthetic images containing extreme values of decreased temperature that apparently corresponds to debonding between the concrete and the parameters chosen as significant—for example, images of maximal temperatures achieved during reinforcement. In defect areas, the excess temperature dropped by about 4–5 °C, being in the same the whole thermal process (the so-called “maxigrams”). Figure 14a shows a defect-free pole section range of magnitude as predicted by the simulation. located above the ground level. It is clearly seen that the surface ‘footprints’ of the reinforcement are regularly located and characterized by the same excess temperature in the range from 7 to 8 C. In the case of a damaged pole (Figure 14b), the corresponding maxigram shows a distinct area of decreased temperature that apparently corresponds to debonding between the concrete and the reinforcement. Appl. Sci. 2018, 8, 222 11 of 13 In defect areas, the excess temperature dropped by about 4–5 C, being in the same range of magnitude as predicted by the simulation. Appl. Sci. 2018, 7, x FOR PEER REVIEW 11 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 11 of 13 (a) (b) (a) (b) Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). Figure 14. Maxigrams of a defect-free (a) and damaged (b) pole (pole section above ground level). In general, it has been found that both types of reinforcement, namely, circumferential and In general, it has been found that both types of reinforcement, namely, circumferential and In general, it has been found that both types of reinforcement, namely, circumferential and longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has longitudinal, can be clearly detected 20 cm above the ground level, and the temperature contrast has been good enough to reliably identify the localization of reinforcement. been good enough to reliably identify the localization of reinforcement. been good enough to reliably identify the localization of reinforcement. Similar results appeared when we inspected the below-ground pole sections (see the example of Similar results appeared when we inspected the below-ground pole sections (see the example Similar results appeared when we inspected the below-ground pole sections (see the example of a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. In of a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. a tested pole in Figure 15a). In this case, the maxigram revealed some electric corrosion. In particular, no indications of circumferential reinforcement were found in the pole underground In particular, no indications of circumferential reinforcement were found in the pole underground particular, no indications of circumferential reinforcement were found in the pole underground sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, the sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, sections (Figure 15b). In this case, except for a general decay of temperature on the pole surface, the temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times with the temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times temperature vs. time curves revealed a decrease in temperature variation rate by 2–4 times with regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum with regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum regard to defect-free areas (Figure 16). In conjunction with a delayed temperature maximum appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of hidden appearing at about 170 s, this, according to the theoretical predictions, suggests the presence of hidden delaminations between the concrete and the reinforcement. delaminations between the concrete and the reinforcement. hidden delaminations between the concrete and the reinforcement. Visual inspection of the underground pole section revealed some cracks and weak traces of Visual inspection of the underground pole section revealed some cracks and weak traces Visual inspection of the underground pole section revealed some cracks and weak traces of corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the of corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the corrosion products. The results of ultrasonic and vibroacoustic tests were also beyond the corresponding norms accepted in Russia, thus confirming the IR thermographic findings. corresponding norms accepted in Russia, thus confirming the IR thermographic findings. corresponding norms accepted in Russia, thus confirming the IR thermographic findings. (a) (b) (a) (b) Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Figure 15. Photo (a) and maxigram (b) of a damaged pole (pole section below ground level). Appl. Sci. 2018, 8, 222 12 of 13 Appl. Sci. 2018, 7, x FOR PEER REVIEW 12 of 13 Figure 16. Experimental temperature profiles at the ground level: pole excess temperature vs. time Figure 16. Experimental temperature profiles at the ground level: pole excess temperature vs. time (1—+50 cm, 2—0 cm, 3—−30 cm). (1—+50 cm, 2—0 cm, 3—30 cm). Following the results of IR thermographic surveys, the two inspected poles were replaced. The Following the results of IR thermographic surveys, the two inspected poles were replaced. presence of electric corrosion damage was confirmed by a mechanical destruction test. In parallel, The presence of electric corrosion damage was confirmed by a mechanical destruction test. In parallel, the acceptable quality of other inspected poles was confirmed by comparing the results of ultrasonic the acceptable quality of other inspected poles was confirmed by comparing the results of ultrasonic and vibroacoustic testing (these results will be discussed in a forthcoming paper). and vibroacoustic testing (these results will be discussed in a forthcoming paper). 5. Conclusions 5. Conclusions In this study, we have used the known techniques of IR thermographic nondestructive testing In this study, we have used the known techniques of IR thermographic nondestructive testing and the scanning heating of cylindrical objects. However, these techniques are innovatively applied and the scanning heating of cylindrical objects. However, these techniques are innovatively applied to to a new class of test objects. Also, the implementation of heating devices and data processing using a new class of test objects. Also, the implementation of heating devices and data processing using a a concept of maxigrams is novel. concept of maxigrams is novel. TNDT has been used for performing evaluations of reinforced concrete poles on operating TNDT has been used for performing evaluations of reinforced concrete poles on operating railways. This technique is safe for personnel and fully nondestructive since the excess pole railways. This technique is safe for personnel and fully nondestructive since the excess pole temperature does not exceed 15 °C. temperature does not exceed 15 C. IR imaging allows the visualization of both longitudinal and circumferential rebar IR imaging allows the visualization of both longitudinal and circumferential rebar reinforcement, reinforcement, and whether corrosion has caused a separation between the rebar and the concrete. and whether corrosion has caused a separation between the rebar and the concrete. By heating By heating poles inductively (released power: about 10 MW/m ) or applying electric current (up to poles inductively (released power: about 10 MW/m ) or applying electric current (up to 400 W per 400 W per linear meter of a pole), surface temperature signals in defect areas may reach 3–10 °C linear meter of a pole), surface temperature signals in defect areas may reach 3–10 C depending depending on reinforcement depth, as well as a grade of rebar thinning and presence of corrosion on reinforcement depth, as well as a grade of rebar thinning and presence of corrosion products products characterized by high thermal resistance. The same defects diminish the rate of characterized by high thermal resistance. The same defects diminish the rate of temperature changes temperature changes (temperature derivative) by 2–4-fold, which can be used for defect (temperature derivative) by 2–4-fold, which can be used for defect identification. identification. It has been experimentally determined that galvanic corrosion in concrete pole reinforcement can It has been experimentally determined that galvanic corrosion in concrete pole reinforcement be detected in pole sections located higher than 20 cm above the ground level. To evaluate underground can be detected in pole sections located higher than 20 cm above the ground level. To evaluate sections, a pole should be unearthed for about 20–30 cm below the ground level, although in some cases underground sections, a pole should be unearthed for about 20–30 cm below the ground level, it is not necessary. It is worth mentioning that, until now, corrosion damage could only be detected although in some cases it is not necessary. It is worth mentioning that, until now, corrosion damage just prior to the failure of a pole. However, this technique can detect damage well before failure. could only be detected just prior to the failure of a pole. However, this technique can detect damage TNDT surveys can detect reinforcement displacement and predict the residual strength of poles well before failure. by analyzing concrete cracking in the near-ground pole section. This is a goal of the ongoing research. TNDT surveys can detect reinforcement displacement and predict the residual strength of poles Acknowledgments: by analyzing concret This e cr work acking was in supported the near by -gro a und Russian pole s Scientific ection. F This oundation is a grant goal of #17-19-01047 the ongoing (3D numerical modeling) and in part by a State Order of the Russian Ministry of Higher Education for 2017–2019, NIR research. #9.5966.2017/BY (experimental implementation). Author Contributions: Dmitry Valeryevich Sannikov conceived and designed the experiments. Alexander Acknowledgments: This work was supported by a Russian Scientific Foundation grant #17-19-01047 (3D Sergeevich Kolevatov performed the experiments. Vladimir Platonovich Vavilov did the numerical modeling of numerical modeling) and in part by a State Order of the Russian Ministry of Higher Education for 2017–2019, defect situations. Marina Valeryevna Kuimova wrote the paper. NIR #9.5966.2017/BY (experimental implementation). Conflicts of Interest: The authors declare no conflict of interest. Author Contributions: Dmitry Valeryevich Sannikov conceived and designed the experiments. Alexander Sergeevich Kolevatov performed the experiments. Vladimir Platonovich Vavilov did the numerical modeling of defect situations. Marina Valeryevna Kuimova wrote the paper. Appl. Sci. 2018, 8, 222 13 of 13 References 1. Hillemeier, B. Method of Determining the Location, Orientation and Pattern of Reinforcing Members in Reinforced Concrete. U.S. Patent 4309610, 5 January 1982. 2. Maierhofer, C.; Reinhardt, H.W.; Dobmann, G. (Eds.) 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Theoretical and practical aspects of the thermal NDT of bonded structures. In Research Techniques in NDT; Sharpe, R., Ed.; Academic Press: London, UK, 1982; Volume 5, pp. 239–280, ISBN 0-12-639055-X. 13. Vavilov, V.P.; Burleigh, D.D. Review of pulsed thermal NDT: Physical principles, theory and data processing. NDT E Int. 2015, 73, 28–52. [CrossRef] 14. Green, D.R. Principles and applications of emittance-independent infrared nondestructive testing. Appl. Opt. 1968, 7, 1796–1805. [CrossRef] [PubMed] 15. Woolard, D.; Cramer, K. The thermal photocopier: A new concept for thermal NDT. Proc. SPIE 2004, 5405, 366–373. [CrossRef] © 2018 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/).

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Applied SciencesMultidisciplinary Digital Publishing Institute

Published: Feb 1, 2018

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