Reinforced concrete is used to build many modern structures such as bridges and buildings. The strength of these structures is dependent upon the integrity of the steel reinforcing embedded in the concrete. Over a period of time the rebars may begin to corrode and crack. The loss of steel due to corrosion or the loss of strength due to cracking may lead to structural failure particularly for prestressed concrete. The work described in this paper is the development of active electromagnetic induction sensors capable of detecting reinforcing bars within concrete and the identification of surface breaking defects on the reinforcing bars

Gaydecki, P. A.

Simmonds, J. L.

English

J. L. Simmonds

P. A. Gaydecki

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Defect Visualization and Location of Reinforcing Bars and Cables in Concrete Using Active Electromagnetic Induction Imaging

Reinforced concrete is used to build many modern structures such as bridges and buildings. The strength of these structures is dependent upon the integrity of the steel reinforcing embedded in the concrete. Over a period of time the rebars may begin to corrode and crack. The loss of steel due to corrosion or the loss of strength due to cracking may lead to structural failure particularly for prestressed concrete. The work described in this paper is the development of active electromagnetic induction sensors capable of detecting reinforcing bars within concrete and the identification of surface breaking defects on the reinforcing bars.</p

Simmonds, J.

Gaydecki, P.

Digital Repository @ Iowa State University (ISU)

DEFECT VISUALIZATION AND LOCATION OF REINFORCING BARS AND CABLES IN CONCRETE USING ACTIVE ELECTROMAGNETIC INDUCTION IMAGING J. L. Simmonds Department of instrumentation and analytical science University of Manchester institute of science and technology Manchester, P.O. Box 88, M60 IQD, UK. P. A. Gaydecki Department of instrumentation and analytical science University of Manchester institute of science and technology Manchester, P.O. Box 88, M60 IQD, UK. INTRODUCTION Reinforced concrete is used to build many modern structures such as bridges and buildings. The strength of these structures is dependent upon the integrity of the steel reinforcing embedded in the concrete. Over a period of time the rebars may begin to corrode and crack. The loss of steel due to corrosion or the loss of strength due to cracking may lead to structural failure particularly for prestressed concrete. The work described in this paper is the development of active electromagnetic induction sensors capable of detecting reinforcing bars within concrete and the identification of surface breaking defects on the reinforcing bars. The method is active because direct physical contact is made with the bar under examination. An ac voltage is applied across the length of bar to be examined, this generates an alternating current along the bar. Because of the frequencies used in this project (~ 1 kHz), the skin effect causes the current to flow along a thin outer layer of the bar. Any surface breaking defects on the bar will cause a portion of the current to be deviated across the bar and induce a corresponding alteration in the magnetic field around the bar. An array of coils is used to detect both field components induced by the current flow directly along the bar and field components induced by current deviations around surface defects on the bar [1-4]. 1565 THEORY The x components of the magnetic flux density aroWld a bar carrying a current 1 and lying along the z-axis of our co-ordinate system, are given by the equations: (1) If a second conductor is placed in the magnetic field produced by the bar, by mutual induction an emfwill be induced within the second conductor. The emf is governed by the equation: dJ ~=-M­dt (2) where M is the mutual inductance of the pair of conductors [5]. The mutual inductance between the bar and a coil lying in the x-direction (that is perpendicular to the bar) is given by: (3) where r is the coil radius and N is the number of turns on the coil. Hence for a coil aligned with the x-axis: q =_pyr2N1oOJ.cos(wt) x 2(x2 + i) (4) where the bar current is sinusoidal and 10 is the current amplitude, ro is the angular frequency and 11 is the permeability of the coils core and. A corresponding equation can be obtained for a coil in the y direction. Because the current on the bar flows in the z-direction, it induces a magnetic field only in the plane perpendicular to the z-axis (the xy plane), hence the emf induced in a coil aligned in the z-direction is ordinarily zero. But as mention in the introduction, surface defects cause deviations in the current path resulting in a flux density component in the z-direction. Coils above the bar orientated in the x-direction are sensitive to current flow along the bar whilst coils parallel to the bar (the z-direction) are sensitive to current across the bar. Thus a combination of coils is used to detect the bar and its surface defects [6]. FINITE ELEMENT ANALYSIS Finite element modeling software produced by 'Vector Fields' of Oxford England, was used to model the system of bars and their defects [7&8]. The modeling was mainly concerned with the magnetic flux density aroWld the bars and the current density on the bars. Models were generated of single bars, groups of parallel bars and bars within meshes, for both perfect bars and bars with surface defects; in each case only one bar carries an applied current. The current density on the bars and the magnetic flux density around the bars were analyzed. 1566 The modeling has shown that the x and y components of flux density Bx and By are constant along the length of the bar for constant x and y distances from the bar (figure 1), this is as expected from equation(l). Along the length of an undamaged bar the current density flows totally in the z-direction resulting in magnetic flux density in the x, y-plane and a Bz component which is virtually zero (figure 2). When defects on a bar are encountered the models show that the x and y components of current density increase and peak at either edge of the defect, this results in the Bz field component becoming non zero, peaking at either side of the defect (figure 3). 70Eo08 ,--"" --------------- ---" I 1 I 1 Magnetic Oux 6~ I I density I \ I 1 I I (T esla) 6.OEo08 , I 5~008 I I , I , I 50Eo08 I I 1 I I 4~008 I I , I 40Eo08 : I 3~o08L-----L---~L---~L---~----~ lOCll X toenI 0.G3 0 03 0.03 0.03 003 00 112 003 lOCll Y toenI 0,0 00 0,0 00 lOCllZtoenI 00 028 OM oao ~~(m) __ ~8Y. ~·95885<1e.oe Figure 1. Magnetic flux density component, By at constant x-y distance from the bar. ,.E-23 j ,I Magnetic 1 2E·23 I' nux 1 0E-23 h I density 8 OE·24 ~ \ (G )6.0E.24 \ " auss 'QE.24 I I • , I, , \ 1\' II " '1 2.0E·24 I,} I ~ ) I I ,_ " I I" 00 .20E.24 ': ',I,,, II I; II 'I 1\ " ,I II "0E-2' I , I ~0E-2' -a0E-2' I I ~ /I II II 1\ u: " "1/".' v ': l I - 10E-23 I' -12e-23 I' · 1 'E·23 ~ · 1 6E·23 "-____ '-____ L_ ____ L_ ____ L_ ___ .JJ I.localXcocnl 00 00 00 00 00 00 t=~= io~ :'00 :'00 ~~ '~O I~O(cm) _ _ Bl. 1nIeg-oI . , 2985£-23 Figure 2. Bz component, when there is no defect on the bar. 1567 000:1 Magnetic flux 0001! density (Gauss) 0001 0000l! ,; I, I, I, , , , I , I : I I~\ OOF-~~~~~~,-+~,~~~~~~ -00006 -0001 -00016 I I , I , ' I' , , ,I -0002 " - _--'-__ --'-_.1:.,'--1-LOQ/XCOCfd 00 00 00 00 LOQ/YCOCfd 30 30 30 30 LOQ/ZCOCfd ~O ~O -100 100 8Z. ~' · 527509E· 18 00 30 300 00 i>~ (em) Figure 3. Bz component when there is a single v-shaped cut on the center of the bar. ,..---.. ToPC Coils pel']pendJQ111lar to the bar ~z 1 to 10kHz, 0.2 to I amp. Figure 4. The positioning of the coils in the scanning sensor and their alignment with respect to the test bar. THE SCANNING SENSOR The modeling results were used to design and build a four coil scanning sensor. The scanning assembly has two stepper motors, controlled by a microprocessor linked to a PC; the motors move the sensor in the horizontal (x, z) plane. As well as controlling the scanning process, the host PC collects and stores data from the sensor via a 12 bit ADC. It also process the data, this is achieved using software written for this system. The four coils of the sensor are arranges in two pairs; when the sensor is mounted on the scanning rig the coils of a pair have the same x,z position but are separated by 27 mm in height (v-direction). The two pairs of coils are perpendicular to each other (figure 4). The scanning assembly and test bar are aligned so that the two pairs of coils in the sensor are approximately aligned with the x and z axes of the bar. 1568 Figure 5. An image of a section of bar that has no defects, the bar diameter is 16 rnm, scan height 32 rnm. Figure 6. An image of a bar with 3 v-shaped notch cut out of the right side 6,4,8 rnm deep on a 16 rnm diameter bar, scan height 38 rnm. EXPERIMENTAL RESULTS FOR THE SCANNING SENSOR The images shown (figure 5 & 6) were produced after scanning test bars at a resolution of 1.1 rnm; these results are for single bars only. The images were generated using the algorithm shown in equation (5). ~J to ~4 are the output levels from the circuitry for coils 1 to 4 and V is the resultant pixel value at point (x, z). Images can be generated in 16 -level false color or gray scale. All of the images shown were produced using a bar current of 1 A at a frequency of 1.3 kHz. Equation (6) shows how coils set at different heights allow bar depth measurements to be made, where s is the separation between the coils. (5) (6) 1569 Figure 5, shows an image created when a section of 16 mm diameter bar that has no defects, was scanned with the bar current and frequency mentioned above. The image intensity decreases evenly as we move from the center of the bar and falls to zero at ~30 mm either side of the bar. Figure 6 also shows an image of a 16 mm diameter bar. This section has three v-shaped cuts taken out of the right hand side. The cut depths are 6, 4 & 8 mm starting from the top of the bar. Each of the cuts is clearly seen on the image, with progressively deeper cuts extending further into the image of the bar. THE HAND HELD SENSOR A hand held version of the sensor was also built. This is designed to be tracked along the concrete surface above the reinforcing bars. This sensor has a different coil configuration from that of the scanning sensor: the fourth coil in the scanning sensor was aligned along the z-axis, this has be replaced with a coil aligned along the y-axis meaning that all 3 axes are know monitored. This sensor operates with a bar current of200 rnA at a frequency of 10kHz. The sensor is linked to a PC via an ADC connected via the parallel port. The control program guides the user to align the sensor with the bar for fault detection and the performance of depth measurements. Depth measurement is performed using equation (6), and the accuracy of such measurements is increased with an increased coil separation s, which for this sensor was 60 mm. Figure 7 shows a screen capture from the control program as the sensor passes over a 16 mm diameter bar that has a 5mm deep v-shaped cut taken out. Figure 7 shows how the outputs of coils 1 &3 vary as the sensor is moved along the bar. The straight line represents coil 1 output, indicating current flow directly along the bar; the second line represents coil 3 output indicating current flow across the bar. The 5 mm deep v-cut results in two peaks in coil 3 output, one either side of the cut as one moves along the bar. The separation for the two peaks increases with sensor height above the bar. Figures 8 & 9 show how coil 3 output changes with sensor height above the test bar. These figures show results for a 2.5 mm deep by 5 mm wide v-cut and a 6 mm deep by 12 mm wide v-cut respectively, both on 16 mm diameter bars. The 2.5 mm deep cut is detectable at depths up to 50 mm, whereas the 6 mm deep cut is detectable at depths up to 90mm. Figure 7. The handheld sensor moving over a 16 mm diameter bar with a 5 mm deep v-cut on the surface, sensor height is 60 mm. 1570 ~ ... ~ 05 o ! 10 20 30 40 so 60 Hasht of S<ruOr abo •• bar defea (mm) Figure 8. The change in coil 3 output voltage versus sensor height over a 2.5 rnrn deep cut on a 16 rnrn diameter bar .. ~ IS "8 '0 ! os 60 80 100 120 1I',sht of S<ruOr abo,. bar d.fea (mm) Figure 9. The change in coil 3 output voltage versus sensor height over a 6 rnrn deep cut on 16 rnrn diameter bar. DISCUSSION At present the scanning sensor is able to detect bars at depths up to 160 rnrn for a I A 1.3 kHz bar current. As one would expect the ability of the sensor to distinguish defects on the surface of the bar decreases as the bar depth increases. On a 16 rnrn diameter bar 3 rnrn deep defects are visible at scan heights up to 40 rnrn, whereas larger defects such as those 8 mm deep are visible up to scan depths of 70 rnrn. For flaw detection the sensor operates best ifit is aligned with the direction of the bar, but the sensor can cope with the bar being set at an angle. However, the larger the angle between bar and sensor the less effective is the flaw detection and as the angle approaches 45°, the reconstruction algorithm produces a single valued image. The sensor can also cope with small bends in the bar, though it would not show the bar and defects at the site of a right angled bend. When using the handheld sensor it is necessary for the sensor to be accurately aligned with the bar, but by monitoring the output of each coil this is possible. At present defects as small as 2.5 rnrn by 5 rnrn wide can be seen at bar depths up to 50 rnrn, 6 rnrn deep defects are seen at bar depths up to 90 rnrn. In order to make the handheld sensor easier to use the sensor may be developed to run along a track placed on the concrete surface, if the 1571 sensor an be moved in a straight line at a constant velocity then its alignment with the test bar will become less critical. CONCLUSIONS The scanning sensor can detect and image bars at depths up to 160 nun for a lA, 1.3 kHz bar current. On a 16 nun diameter bar 3 nun deep defects are visible at scan heights up to 40 nun, larger defects are seen at correspondingly grater scan heights. The handheld sensor can detect bars at depths up to 600 nun for a 0.2 A 10 kHz bar current. On a 16 mm diameter bar 2.5 nun deep defects are visible at scan heights up to 50 nun. The depth measurement facility is accurate to > 90% for bar depths up to 400 nun. Work continues to increase the detection range, accuracy and easy of use of both sensors. ACKNOWLEDGEMENTS The authors wish to express their thanks to 'Radiodetection Ltd.' of Bristol, UK for fmancially supporting this work. REFERENCES 1. Dunker K.F. and Rabbat B.G. 'Why America's bridges are crumbling' Scientific American, 266 (March 1993) pI8-24. 2. White G.R. 'Concrete technology' (Van Nostrand Reinhold Co. 1977.), Chapters 2 & 3. 3. Murdock L.J and Brook K.M.l984. 'Concrete materials and practice' 5th Ed (Edward Arnold Ltd). Chapters 1,2,9. 4. The war office. 'Military engineering: Reinforced concrete design and Prestressed concrete design'. Vol. XIV (concrete) parts II & m. (Her Majesty's stationary office 1964). 5. Oately C.W. 1976. 'Electric and magnetic fields. Cambridge press'. 6. Coa J.Z. Gaydecki P.A. ' Imaging rebars in concrete.' Internal report, DIAS, UMlST 1993. 7. Ida, N. and Bastos J. P.A. ' Electromagnetics and calculation of fields' (Springer-Verlag New York; London, 1992). 8. Vector Field Ltd. 'Opera-3d: reference manuals' Oxford England 1996. 1572 

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Defect Visualization and Location of Reinforcing Bars and Cables in Concrete Using Active Electromagnetic Induction Imaging

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