Research Article Feasibility Study of Stress Measurement ...downloads.hindawi.com/journals/ijdsn/2013/249829.pdf · prestressing tendon of the prestressed concrete bridge by means

Hindawi Publishing CorporationInternational Journal of Distributed Sensor NetworksVolume 2013, Article ID 249829, 8 pageshttp://dx.doi.org/10.1155/2013/249829

Research ArticleFeasibility Study of Stress Measurement in Prestressing TendonsUsing Villari Effect and Induced Magnetic Field

Changbin Joh, Jung Woo Lee, and Imjong Kwahk

Korea Institute of Construction Technology, Goyang, Gyeonggi 411-712, Republic of Korea

Correspondence should be addressed to Changbin Joh; [email protected]

Received 20 August 2013; Accepted 16 October 2013

Academic Editor: Jeong-Tae Kim

Copyright © 2013 Changbin Joh et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper investigates experimentally the feasibility of estimating the stress in the prestressing tendons of prestressed concretebridges using the magnetic field induced by an electromagnet and the Villari effect in which the magnetic susceptibility orpermeability of a ferromagnetic material changes when subjected to a mechanical stress. The test results show the good linearitybetween the stress in the prestressing tendon and the inducedmagnetic flux density within the practical stress range of the tendons.In addition, the induced magnetic flux density in the tendon appears to depend on the intensity of the electromagnet and thedistance between the electromagnet and the tendon regardless of the concrete cover. Accordingly, although further studies areneeded for practical applications, the stress in the prestressing tendonof a prestressed concrete bridge can be estimated bymeasuringthe induced magnetic flux density generated in the tendon and by using the linear relationship between this induced magnetic fluxdensity and the stress in the prestressing tendon.

1. Introduction

Prestressed concrete bridges started to be erected effectivelyworldwide in the 1980s to achieve bridges with spans longerthan concrete bridges. The prestressed concrete bridge is astructure in which the deflection or the cracks are reducedby introducing prestress forces through the use of prestress-ing tendons. Owing to such structural characteristics, theprestress force introduced by means of the tendons hassignificant effect on the performance of the prestressed con-crete bridge. The prestress force of the prestressing tendonsexperiences changes immediately after its introduction dueto not only the loss of prestress but also due to other variousreasons during the lifespan of the structure like creep, dryingshrinkage, relaxation, and corrosion, which should be man-aged appropriately through evaluation all along the servicelife [1]. Failing in the management of the prestress force willresult in the increase of the deflection or cracks accordingto the loss of prestress and lead to severe degradation ofthe performance or, in worst case, to the collapse of theprestressed concrete bridge [2]. In view of this necessity,diversified nondestructive test (NDT) methods have been

studied to estimate the prestress force of the prestressingtendons during the lifespan of prestressed concrete bridgesbut without successful application on site. In particular, thereis practically no example in which economically efficientestimation of the prestress force has been realized for thebonded prestressing tendons applied in existing prestressedconcrete bridges.

Research for the estimation of the prestress force in thebonded prestressing tendons applied in existing prestressedconcrete bridges can be subdivided into those using systemidentification (SID), those using guided ultrasonic waves,and those using stress waves. The SID method monitorsthe change in modal properties in relation to the change inprestress forces. Kim et al. [3] and Ho et al. [4] developed amultiphase SID scheme to detect the occurrence of prestressloss systematically. Kim et al. [5] proposed the vibration andimpedance monitoring method to improve the detection ofoccurrence and location of prestress loss. In general, theSID method has a strong point for overall structural healthmonitoring but has still limits to estimate the prestress forceslocally and directly.

2 International Journal of Distributed Sensor Networks

The guided ultrasonic wave method is a NDT methodusing the vibrational characteristics or propagation velocitycharacteristics of the ultrasonic wave varying with the stressof the prestressing tendon [6, 7]. This method is widelyadopted for the inspection of the cracks or corrosion ofpipelines but presents limitations when applied on bondedprestressing tendons. This means that the guided ultrasonicwave experiences loss when penetrating concrete, whichmakes it difficult to measure the guided wave at distanceof merely 1.5m from the anchorage of the prestressed con-crete bridge [6]. Recently, Salamone et al. [7] found outexperimentally that the vibrational frequency ratio of theprestressing tendon is proportional to the stress rather thanthe wave propagation velocity, but this result could not beconsidered as having solved the loss of the guided wavedue to grouting since the experiences were conducted onlyon specimens shorter than 1.5m. Considering that most ofthe prestressed concrete bridges in Korea are longer than40m and were erected with bonded prestressing tendons,need is for further research to exploit the guided ultrasonicwave method for the estimation of the stress in prestressingtendons.

Kim et al. [8] estimated the stress in prestressing tendonsusing the increase of the velocity of the longitudinal stresswave along the tendon according to the prestress force.Differently to the guided wave method, this method enablesto measure the stress wave at any position even in bondedprestressing tendons longer than 40m. However, the sensi-tivity of the stress to the velocity of the stress wave showssignificant degradation at stress level larger than 40% of theyield stress of the tendon. Jang et al. [9] applied successfullythis method to estimate the residual stress of the bondedprestressing tendons in the containment structure of nuclearpower plants, but this method also necessitates additionalimprovement to measure accurately the stress in the tendonaccounting that the stress in the tendon reaches 50 to 60%of the yield stress under the service load of the prestressedconcrete bridge.

Themethod using the magnetic field can be envisaged forovercoming the disadvantages encountered in the methodsusing SID, ultrasonic wave, or stress wave. This method usesan external magnetic field to generate an induced magneticfield in the prestressing tendon adopted as magnetic bodyand applies the Villari effect. The Villari effect, also knownas the inverse magnetostriction, was discovered in 1865 byVillari and describes the phenomenon by which themagneticcharacteristics like the permeability, 𝜇, of the magneticbody changes according to its stress [10]. Conceptually,the method forms a closed magnetic circuit including theprestressing tendon of the prestressed concrete bridge bymeans of an external electromagnet and exploits the changein the magnetic flux density, 𝐵, of the induced magneticfield flowing in the prestressing tendon according to thestress.

This paper derives a method measuring the prestressforce of bonded prestressing tendons in prestressed concretebridge using the Villari effect and the induced magnetic fieldgenerated by an electromagnet. The feasibility of the methodis verified experimentally through scaled models.

−Bm

−Hm

Bm

B

HHm

𝜎 = 0

Hysteresis loop

𝜎 = 𝜎1 > 0

(a) Positive magnetostriction

−Bm

−Hm

Bm

B

HHm

𝜎 = 0

Hysteresis loop

𝜎 = 𝜎1 > 0

(b) Negative magnetostriction

Figure 1: 𝐵-𝐻 hysteresis loop and concept of Villari effect.

2. Estimation of the Stress of PrestressingTendon Using the Villari Effect and InducedMagnetic Field

The relation between the intensity, 𝐻, of the magnetic fieldand the magnetic flux density, 𝐵, can be defined by themagnetic permeability, 𝜇, as expressed in the following:

𝐵 = 𝜇𝐻. (1)

In the physics of magnetic material, the Villari effect, alsoknown as the inverse magnetostrictive effect, is the namegiven to the change of the magnetic permeability, 𝜇, of amagnetic material when subjected to a mechanical stress[11]. Figure 1 describes conceptually the Villari effect. This isdue to the rearrangement of the magnetic domains within

International Journal of Distributed Sensor Networks 3

Strand

Concrete cover

N SB1

Separation

Hall sensor

Yoke-shaped electromagnet

𝜎1

Strand

Concrete cover

N S

B2

Separation

Hall sensor


𝜎2

𝜎1 ≠ 𝜎2, B1 ≠ B2

Figure 2: Illustration of induced magnetic fields in tendons ofdifferent stresses.

the structure of the material. The Villari effect can be eitherpositive or negative depending on the specific material andthe intensity of magnetic field (𝐻).

This phenomenon is very effective in amorphous alloysand exhibits outstanding thermal characteristics and dura-bility, which boosted research dedicated to strain gages andtorsion gages using such properties [12]. As a typical example,Baudendistel and Turner [13] investigated the change in thepermeability, 𝜇, with respect to the stress and temperature ofa Fe-Ni alloy and used it to propose an innovative strain gageconcept.

As shown in Figure 2, an induced magnetic field is gen-erated in the magnetic body that is the prestressing tendonwhen a yoke-shaped electromagnet is disposed below the pre-stressed concrete structure including the prestressing tendon.This layout forms a magnetic circuit in which a magneticflux,Φ, flows through the electromagnet and the prestressingtendon. The magnetic flux density, 𝐵, representing the sizeof this induced magnetic field in the prestressing tendondepends on the permeability, 𝜇, and the cross-sectional areaof the magnetized prestressing tendon and the continuity ofthe magnetic circuit (closed magnetic circuit). The magneticflux density, 𝐵, crossing the prestressing tendon is measuredby a Hall sensor installed on the pole face. The accuratemeasurement necessitates near magnetic saturation of theprestressing tendon. The continuity of the magnetic circuitdepends on the separation between the prestressing tendonand the pole face. Since larger separation results in largerloss of the magnetic flux forming the closed magnetic circuit,a relatively strong electromagnet is required to magnetizethe prestressing tendon. In addition, according to Bozorth[14], the intensity of magnetic field (𝐻) changes the sign of

magnetostriction of ferromagnetic material and, accordingly,the sign of variation of permeability under stress. So, thiseffect should be considered for a prestressing tendon, one offerromagnetic materials. Considering that concrete is a non-magnetic material, concrete does not disperse the magneticflux flowing in the prestressing tendon and the concrete coverhas practically no effect. Fernandes et al. [15, 16] exploitedthe fact that the magnetic flux density of the magnetic fieldis influenced by the cross sectional area of the prestressingtendon to measure the degree of corrosion of the tendon in aprestressed concrete structure.

Therefore, since the permeability, 𝜇, of the prestressingtendon subjected to stress varies according to the stress whenthe tendon is magnetized by means of a permanent magnetor an electromagnet with a magnetic field intensity, 𝐻, thedensity,𝐵, of themagnetic flux flowing in the tendonwill varywith respect to the stress owing to the Villari effect. Even ifthe cross sectional area of the tendon decreases under largerstress due to the Poisson’s effect, this effect can be assumedto be relatively insignificant. Consequently, the measurementof the magnetic flux density, 𝐵, flowing in the tendon willenable to estimate the stress of the tendon by comparisonwiththe stress of the tendon that has been preliminary measuredand the magnetic hysteresis loop 𝐵-𝐻. To that goal, need is tocollect a database bymeasuring in advance the𝐵-𝐻 hysteresisloop according to the stress of the prestressing tendon used inthe prestressed concrete structure. In this process, the effectsof the intensity of an electromagnet, the concrete cover, theseparation, and temperature must be considered.

3. Model Test for the Proposed Method

3.1. Summary of Test. The feasibility of themethodmeasuringthe stress of the prestressing tendon by means of the Villarieffect and the induced magnetic field is examined experi-mentally by measuring the relation between the stress of thetendon and the induced magnetic flux density.

Figure 3 illustrates the concept underlying the test. Aloading device and a frame are prepared so as to vary thestress of the tendon during the test, and an electromagnetis disposed at a position below the tendon to secure definiteseparation. TwoHall sensors are installed on the face poles ofthe electromagnet to measure the magnetic flux density, 𝐵.

The loading frame for the prestress of the tendon wasfabricated using 100 × 100mm H-beam (Figure 4). Loadingwas applied by means of a hydraulic nut (Figure 5). Theintroduced prestress force was measured using a center holedload cell with capacity of 50 ton disposed between the loadingframe and the hydraulic nut (Figure 5). The temperature ofthe tendon during the test was measured by a thermosensorattached to the tendon and the ambient temperature by adigital thermometer.

The electromagnet used to induce the magnetic field inthe tendon was fabricated to provide a maximum magneticinduction of 4000 Gauss at the pole face (Figure 6). TwoHall sensors (model WSH135 of Winson) are attached at thecenters of N-pole and S-pole for measuring the magnetic fluxdensity (Figure 7). The voltage occurring at the Hall sensor



Hall sensorHall sensor

StrandConcrete cover

Load cell

Hydraulic pump

Hydraulic nutFixed end

Loading frame

Datalogger

Multimeter

Thermal sensor

OffOn Power on

Figure 3: Illustrative layout for the test.

Figure 4: Test setup.

Figure 5: Multimeter, load cell, and hydraulic nut.

Figure 6: Prestressing tendon and installedHall sensor, thermosen-sor, and electromagnet.

is measured by means of a multimeter (model 34410A ofAgilent) (Figure 5).

In order to examine the effect of the concrete cover on themagnetic flux density, tests were performed using a movableconcrete plate device disposed at the bottom of the tendon(Figures 8 and 9). The thickness of the concrete plate is22.7mm.

Using this experimental setup, the magnetic flux densityor magnetic induction, 𝐵, induced in the tendon by theelectromagnet was measured by varying the stress of theprestressing tendon. Referring to the KoreanHighway BridgeDesign Code (2010) [17], the stress of the prestressing tendonwas varied from 14% of the yield stress to a value of 80% of theyield stress close to the tensile allowable stress of the tendon


Figure 7: Hall sensor installed at the N-pole of the electromagnet.

Figure 8: Case with concrete cover.

Table 1: Test variables for the test.

Variable Values RemarksInput current ofelectromagnet (A) 4, 5 See Figure 10

Stress of prestressingtendon (𝑓

𝑦)

0.14, 0.27, 0.41,0.54, 0.68, 0.81

𝑓𝑦= 1600MPa

(SWPC7B 15.2mm7-strand)

Separation betweenthe tendon and theelectromagnet (mm)

25, 46 See Figure 2

immediately after the introduction of prestress. The tendonadopted for the test is SWPC7B tendon (7 strands of 15.2mm)widely used for prestressed concrete structures. The inputcurrent determining the intensity of the magnetic field andthe separation, which is the distance between the pole face ofthe electromagnet and the bottom of the prestressing tendon,was decided so as to magnetize the tendon to near saturationlevel. The variables used in the test are arranged in Table 1.

3.2. Measurement of Magnetic Saturation of Prestressing Ten-don. Prior to investigate the relation between the stress andinduced magnetic flux density in the prestressing tendon,the input current of the electromagnet was varied to findthe input current and separation enabling to magnetize

Figure 9: Case with removed concrete cover.

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5 6 7

Mag

netic

fiel

d in

duce

d (G

auss

)

Input current of electromagnet (A)

Electromagnet onlyTendon (7-wire strand), separation = 40mmSingle wire (d = 5mm), separation = 40mm

Figure 10: Magnetization of the prestressing tendon to near satura-tion.

the tendon to near saturation level. Comparison of themagnetic flux densities of the electromagnet was conductedfor the cases corresponding to the absence of tendon andcorresponding to the tendon without prestress. In addition,the induced magnetic flux density of 1 wire of 5mm diametercomposing the tendon was also measured to test the effectof the cross sectional area of the tendon on the magneticinduction. For both cases, the separation was set to 40mm.

In view of the test results in Figure 10, when the inputcurrent of the electromagnet ranges between 0 to 3.67A, thedifference between magnetic flux densities of electromagnetand the prestressing tendon, that is, the increased magneticflux density due to the presence of the prestressing tendon,tends to increase with the varying input current. However,for input currents larger than 3.67A, this increase convergesto a constant value.This means that near magnetic saturationof the tendon can be achieved for input current larger than3.67A in the case of a separation of 40mm. Besides, forthe wire with diameter of 5mm, difference could practicallynot be observed between the cases where there was only


1700

1800

1900

2000

2100

2200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

y = −97.626x + 2111.7

R2 = 0.9903

y = −120.87x + 1852.7

R2 = 0.9967

Input = 4A, separation = 25.9mmInput = 5A, separation = 24.9mm

Mag

netic

fiel

d in

duce

d,B

(Gau

ss)

Ratio of tendon stress (f/fy = 1600MPa)

Figure 11: Stress of prestressing tendon versus magnetic induction𝐵 (separation of 25mm).

the electromagnet and where the wire was present. Thisobservation can be attributed to the fact that near magneticsaturation is easily achieved for small cross sectional areas.

3.3. Measurement of the Induced Magnetic Field of the Pre-stressing Tendon. Referring to Figure 10, the input current ofthe electromagnet was set to 4A and 5A.The separation wasdecided as 25mm and 46mm (practically close to 25mm and46mm, resp.) considering the cover thickness in real concretestructures. During the tests, the variation of temperatureremained insignificant. During the tests with separation of25mm, the ambient temperature was 23.5∘C (4A) and 23.7∘C(5A), and the temperature of the tendon ranged between22.6 and 23.3∘C for input currents of 4A and between 22.9and 23.8∘C for input currents 5 A. During the tests withseparation of 46mm, the ambient temperature was 21.8∘C(4A) and 22.9∘C (5A), and the temperature of the tendonranged between 21.4 and 21.8∘C for input current of 4A andbetween 22.6 and 22.8∘C for input current of 5 A.

In view of the test results for a separation of 25mmin Figure 11, a highly linear relationship can be observedbetween the stress of the tendon and the induced magneticinduction 𝐵 for both input currents. The linear regression ofthe relation between the stress of the tendon and the inducedmagnetic induction reveals a coefficient of determination(𝑅2) of 0.9967 for the input current of 4A and of 0.9903 for5A. Besides, the size of the magnetic flux density induced inthe tendon increases when the input current rises from 4A to5A.

From the test results for a separation of 46mm inFigure 12, the highly linear relationship between the stressof the tendon and the induced magnetic induction appearsalso for the input currents of 4A and 5A with coefficients ofdetermination (𝑅2) of 0.9964 (4A) and 0.9982 (5A).

The relation between the separation and magnetic fluxdensity induced in the tendon for the input current of 5 Awas examined (Figure 13). The magnetic induction is seen to

1300

1400

1500

1600

1700

1800

1900

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

y = −47.586x + 1826.1

R2 = 0.9982

y = −55.919x + 1435.6

R2 = 0.9964


Mag

netic

fiel

d in

duce

d,B

(Gau

ss)


Figure 12: Stress of prestressing tendon versus magnetic induction𝐵 (separation of 50mm).

1700

1800

1900

2000

2100

2200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Mag

netic

fiel

d in

duce

d,B

(Gau

ss)



Figure 13: Reduction of the magnetic induction according to theseparation.

reduce by 281Gauss in average according to the increase ofthe separation from 25mm to 46mm.

Figure 14 plots the results of the tests investigating therelation between the thickness of the concrete cover andthe induced magnetic induction for the input current of 4Aand separation of 46mm. It appears that the cover madeof nonmagnetic concrete has practically no effect on themagnetic flux density induced in the tendon. Despite of slightchange in the magnetic induction 𝐵 due to the concretecover, this change remained within the measurement erroroccurring during the measurement of the voltage of the Hallsensors, which means that the concrete cover has practicallyno effect on the magnetic flux density.


1100

1200

1300

1400

1500

1600

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Input = 4A, separation = 46.8mm, no cover

Mag

netic

fiel

d in

duce

d,B

(Gau

ss)


Input = 4A, separation = 46.5mm, concrete cover = 22.7mm

Figure 14: Relation between the concrete cover and the inducedmagnetic induction 𝐵.

4. Conclusions

This paper investigated the feasibility of the method estimat-ing the stress in the bonded tendon of prestressed concretebridge using the Villari effect and the induced magneticfield. To that goal, a magnetic field was generated in theprestressing tendon by means of an electromagnet and therelation between the stress of the tendon and the so-inducedmagnetic flux density was observed. The test results revealedthat, within the stress range (from 0.14 𝑓𝑦 to 0.81 𝑓𝑦) of theprestressing tendon used in field, there is a linear relationshipbetween the stress of the tendon and the induced magneticinduction. Moreover, the magnetic flux density induced inthe tendon depended on the intensity of the electromagnetand the separation but was not affected by the concretecover.

Accordingly, this linear relation between the stress ofthe tendon and the induced magnetic flux density can beused to estimate the stress of the bonded prestressing tendonin existing prestressed concrete bridges. This means that ifHall sensors are used to measure the magnetic flux densityinduced in the prestressing tendon by an external electro-magnet attached to the prestressed concrete bridge, the stressof the tendon can be estimated by the Villari effect in whichthe magnetic permeability varies according to the stress ofthe magnetic body. However, additional studies should beconducted for further practical application of the method.Concretely, the magnetic 𝐵-𝐻 hysteresis loops should beconstructed per stress of the prestressing tendon, and studyshould be performed to examine systematically the effectof the separation and the intensity of the electromagnet onthe magnetic flux density induced and the sign of magne-tostriction in the prestressing tendon. In addition, methodsenabling to consider the effects of the diverse details in actualprestressed concrete bridges like the effect of longitudinal andtransverse reinforcements, the case of prestressing tendonsarranged in bundles, and the ordinary use of metallic sheathshall also be investigated.

Acknowledgment

This research was supported by a grant from a StrategicResearch Project (Diagnostic Techniques for the PrestressingSystem in Concrete Bridges) funded by the Korea Institute ofConstruction Technology.

References

[1] M. K. Tadros, N. A. Omaishi, S. J. Seguirant, and J. G. Gallt,“Prestress losses in pretensioned high-strength concrete bridgegirders,” Transportation Research Board, NCHRP Report 496,2001.

[2] R. Woodward and F. Williams, “Collapse of the Ynys-y-Gwasbridge, West Glamorgan,” in Proceedings of the Institution ofCivil Engineers, vol. 84, pp. 635–669, London, UK, August 1988.

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[16] B. Fernandes, M. D. Titus, D. K. Nims, A. Ghorbanpoor, and V.K. Devabhaktuni, “Practical assessment of magnetic methodsfor corrosion detection in an adjacent precast prestressed con-crete box-beam bridge,” Nondestructive Testing and Evaluation,vol. 28, no. 2, pp. 99–118, 2012.

[17] Korea Road & Transportation Association, Highway BridgeDesign Code, 2010, (Korean).

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Research Article Feasibility Study of Stress Measurement ...downloads.hindawi.com/journals/ijdsn/2013/249829.pdf · prestressing tendon of the prestressed concrete bridge by means