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Structural Health Monitoring

DOI: 10.1177/14759217060579792006; 5; 17Structural Health Monitoring Fan Wu and Fu-Kuo Chang

II: Analysis and Algorithmebond Detection using Embedded Piezoelectric Elements for Reinforced Concrete Structures - Part

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17

Debond Detection using Embedded

Piezoelectric Elements for Reinforced ConcreteStructures Part II: Analysis and Algorithm

Fan Wu 1, * and Fu-Kuo Chang 2

1 Risk Management and Solutions, Inc.7015 Gateway Blvd, Newark, CA 94560, USA2 Department of Aeronautics and Astronautics, Stanford UniversityDurand 385, Stanford, CA 94305, USA

An investigation to detect debond in steel-reinforced concrete (RC) using built-in piezoelectricelements has been conducted. Results of the experimental work are presented in Part I of the study.In Part II, numerical solutions using a finite element method are applied to simulate the test results.Parametric studies are performed to evaluate the behavior of the sensor response to the various typesof RC structures with debonding damage. Based on the extensive parametric studies, a debonddetection algorithm for RC structures is established.

Keywords embedded piezoelectric actuators and sensors reinforced concrete debondingdamage detection stress waves finite element analysis

1 Introduction

Debonding of steel reinforcing bars from theconcrete matrices in reinforced concrete (RC)structures could weaken or destroy the struc-tural integrity, reduce the tensile resistance of the structures, and further lead to catastrophicfailure of the structures, when subjected todynamic forces such as earthquake groundmotions.

A method using embedded piezoelectric ele-ments as actuators and sensors to detect thedebonding damage in RC structures has beenstudied by the authors [1]. In this method, piezo-electric material elements, as actuators andsensors to generate and receive signals, areembedded into RC structures. A specific kind of

diagnostic signal, five-peak burst waves, is appliedon the actuators and received by the sensors. Bymonitoring the changes of sensor measurements,debonding damage of the structures can bedetected (Figure 1).

Three types of tests were conducted tensiletests for reinforcement bars, debonding testsfor RC beams, and bending tests for RCbeams. Results show that debonding damage inRC structures is detectable using embeddedpiezoelectric actuators/sensors. The amplitude of the sensor output increases as the extent of debond increases. The travel time of the signalsis found to be sensitive to the elongation of the rebar.

The mechanics of the testing wave propaga-tion in piezoelectric transducers and in the RC

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*Author to whom correspondence should be addressed.E-mail: [email protected]

Copyright 2006 SAGE Publications,Vol 5(1): 001712[1475-9217 (200603) 5:1;1712 10.1177/1475921706057979]

Copyright 2006 SAGE Publications,Vol 5(1): 001712[1475-9217 (200603) 5:1;1712 10.1177/1475921706057979]

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structure is very complicated. The testing systeminvolves comprehensive activities the conver-sion of electrical energy to mechanical energy inthe input actuators, the conversion of mechan-ical energy to electrical energy in the outputsensors, and the propagation of both electro-magnetic (EM) waves and stress waves in theconcrete structure. In the case here, stress wavesalong the steel reinforcing bar and inside theconcrete structure are the most important, sincethey carry the information about the rebarsbonding condition. For the piezoelectric actua-tors and sensors, energy conversion (or piezo-electricity) is more important. Notice that bothpiezoelectricity on piezoelectric transducers and

stress waves in the RC structure involve three-dimensional (3D) transfer functions. This leadsto a complicated analysis. A solution by directdeduction is extremely difficult. A numericalmodeling tool the finite element method withPZFlex is used to assist the modeling andanalysis.

2 Modeling and Simulation

To explain the phenomena observed in thetest results, modeling of the test system wasperformed. The basic wave algorithms relatedto the system were explored. In the system,the components related to energy conversionare the piezoelectric actuators and sensors.The epoxy layer, the steel rebar, and the con-crete components are related to stress wavepropagation.

For stress wave propagation, the equation of straindisplacement relation for a certain point isin the form of [18,20]

e 12 r u r u 0 1

where e is a 3 3 strain tensor, u is a displace-ment vector, and u 0 is the transpose of u , forelastic solid material.

If F is considered as the external load actingon the structure, then the equation of motionbecomes

r r @2u

@t2 F 2where r is a 3 3 stress tensor, is the materialdensity, and t is the time. Note that this is a 3D

transient wave equation. If F

is zero, thenEquation (2) becomes a free wave propagationequation.

One-dimensional (1D) longitudinal wave andshear wave equations can easily be deducted fromthe above equation. The wave propagation speedfor longitudinal wave is ffiffiffiffiffiffiffiffiffiffi E y=p , where E y is theYoungs modulus of the material, and the speedfor shear wave is ffiffiffiffiffiffiffiffiffi G=p , where G is the shearmodulus of the material.

Transient wave propagation will be dampedor attenuated inside the structure it travels in, soenergy will be lost along the path of wavepropagation. Hence, the damping factor has tobe considered also. The elastic constitutive equa-tion, with the damping factor, becomes:

r C : e :@"@t 3

where C is a tensor for elastic stiffness constants,and is a tensor which will be determined by thematerials damping value and the wave speed.

depends upon the time derivatives of the

strains, and it always has the same general formas the stiffness tensor C [2].The constitutive relation to couple the elec-

trical field with the acoustic field of piezoelectricmaterial can be expressed as the equations

D E d : ,e d 0 E s E :

4


Rebar

Concrete beam

Debond

Actuators Sensors

Figure 1 Debond detection using embedded PZT forRC structure.

18 Structural Health Monitoring 5(1)



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where D is an electrical displacement component,caused by both stress and electric field; E is theelectric field; " is a strain component produced byelectric field and stress field; d is the piezoelectricstrain constant tensor; d 0 is the transpose of piezoelectric strain constant tensor; s is the elastic

strain constant tensor; and is the permittivitytensor. Superscripts r and E have been added to

and s to show that these constants describedielectric and elastic properties measured underconditions of constant stress or constant electricfield. Because of coupling between electric andacoustic fields in a piezoelectric solid, measure-ment of the electrical properties depends uponthe mechanical constraints imposed on themedium, and vice versa [2].

PZFlex is a finite-element software programdeveloped by Weidlinger Associates, Inc. [3].The finite-element code uses explicit time-domain simulation for transient analysis of piezoelectric transducers and general wave pro-pagation. Although PZFlex was initially devel-oped for piezoelectric transducers in medicalapplications, it has been used in other applica-tions where piezoelectric material plays similarroles. Since PZFlex deals with both electromag-netic waves and transient waves for piezoelectricsolids and other structures, it is thus appropri-ate for the testing system here. However,

because such an analysis for RC structures hasnot been done prior to this study, carefulmodeling is required for all components of thesystem to ensure reasonable results. Componentmodeling includes the modeling of a steel rebar,piezoelectric ceramic material, the silver epoxylayer, and concrete. Details of the modelingprocess are introduced in the next section.

For numerical simulations, a steel reinforcingbar was modeled as an elastic, linear, isotropic,and homogeneous material. The steel density usedwas 7900kg/m 3 and the Youngs modulus was206 GPa. Based on the properties of steel, thelongitudinal (pressure) wave speed was 5100m/s.Ribs of the reinforcing bar were not modeled, andthe bar was simulated as a smooth bar to simplifythe modeling. The effect of the ribs on stress wavepropagation was not significant because the rele-vant wave length for 90kHz was about 60 mm [4],much longer than the rib width (23mm).

This means that the stress wave could propagatethrough the ridges easily with almost no wavedispersion, i.e., little energy loss. It was calculatedthat the amplitude drop of the stress wave due tothe bar ridges is about 14% [4,18]. Though verysmall compared to other energy losses, it has been

included in the results of the smooth bar analysisin numerical simulations.

The piezoelectric ceramic material for thePZT transducers was assumed to have linear,elastic, anisotropic, and homogeneous properties[2,17,27]. Since both stress waves and conversionsof electric energy and mechanical energy shouldbe considered, constants in Equations (3) and (4)need to be determined for the numericalsimulations. As mentioned in the authors firstarticle Part I: Experiment [1], the piezoelectricceramic used is good for both actuators andsensors. Parameters such as dielectric constants,and piezoelectric strain and stiffness constantsin Equations (3) and (4) were determined fromthe data provided by the manufacturer orfrom similar products of other manufacturers.Damping was also modeled for the PZT 850material according to the published data [5,26].Additionally, the silver epoxy which was used tobond the piezoelectric transducers to the steel barsurface was assumed to be a linear, elastic,isotropic, and homogeneous material.

Concrete material has often been modeled as alinear, elastic, and isotropic material [6,7,24].Since concrete is a composite material that con-sists of sand, cement, and various sizes of aggre-gates, the scatter of ultrasonic waves normallyprecludes the use of frequencies higher than150 kHz [8,15], and in practice, frequencies below100kHz are generally employed [7,23]. The testsystem used ultrasonic waves with a frequency of 90 kHz. Even so, during the wave propagation, alarge amount of energy from the source wasdissipated. To simplify the analysis, a homoge-neous model for the concrete material was used,and damping was applied to compensate for theinhomogeneity [5]. Damping values were based onthe test results from ultrasonic wave attenuationin concrete [8,9]. Rayleigh damping, which com-bines the effects of mass and stiffness damping,was applied [10,17]. Other parameters such asconcrete density, modulus of elasticity, and


Wu & Chang Debond Detection Using Piezoelectric Elements Part II 19



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Poissons ratio were defined as 2400kg/m 3 ,28GPa, and 0.2, respectively [22,25]. All thematerial properties used in the simulations arelisted in Table 1.

A whole RC beam structure was modeled toget a complete time history of the transient waves.Since concrete is a material with high-energydissipation, it was found in the simulations thatfor the beam configuration, a beam with a depthof d and a column with a diameter of d had thesame sensor signals. After considering both thestudy requirements and the model performance,the RC beam structure was configured as an axis-symmetric model. In the modeling, several zoneswith different mesh densities were used for compu-tational efficiency. High density zones wereapplied for the PZT area, while lower densityzones were used for concrete and steel only area.The time step used was a default value generatedby PZFLex, which was smaller than one-tenth of the period of the highest frequency to be solved(90kHz). Detailed mesh methods are introducedin the section on model verification.

3 Model Verification

While carrying out numerical simulationsfor RC structure, a series of extraneous low

amplitude, high frequency noises occurred, simi-lar to ringing effects. This is the so-called zeroenergy or hourglass modes [8,11,16]. Thoughthe PZFlex software was supposed to handlethis problem, it still appeared while modelingthe RC beam with the embedded piezoelectricactuators and sensors. In the analysis, a certainamount of viscoelastic damping was added tothe steel bar to suppress the ringing. However,the signals from the sensor output were delayedby 7e 6 s, for the case of 90kHz. This timedelay was extracted from the results of thesimulations.

Verification was performed on the reinforc-ing bar and RC beams with debond by compar-ing the numerical simulation results with theexperimental test results. Numerical modelswere constructed and calibrated based on theexperimental data.


Table 1 Material data used for numerical simulations.

Piezoelectric coefficient d z1 (m/V) 175 1012

Piezoelectric coefficient d z3 (m/V) 400 10 12

Piezoelectric relative dielectric constant T xx = 0 1700Piezoelectric relative dielectric constant T zz= 0 1470Youngs modulus for piezoelectric material E pxx (N/m 2) 6.3 1010

Youngs modulus for piezoelectric material E pzz (N/m 2) 5.4 1010Poissons ratio for piezoelectric material xz , yz 0.31Stiffness constant for piezoelectric material C 11 (N/m 2) 12.6 1010

Stiffness constant for piezoelectric material C 33 (N/m2) 11.7 1010




Youngs modulus for steel E s (N/m 2) 20.6 1010

Material density for steel (kg/m 3) 7900Poissons ratio for steel s 0.3Damping attenuation at 90 kHz for steel s (dB/m) 10Youngs modulus for concrete E c (N/m 2) 2.8 1010

Material density for concrete (kg/m 3) 2400

Poissons ratio for concrete s 0.2Damping attenuation at 90 kHz for concrete c (dB/m) 100Youngs modulus for silver epoxy E x (N/m 2) 0.875 1010

Material density for silver epoxy (kg/m 3) 2800Poissons ratio for silver epoxy s 0.3




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3.1 Analysis of Steel Reinforcing Bar

Numerical analysis for a #6 (diameter0.75 in. (19mm)) reinforcing bar was performed.The mesh is shown in Figure 2, and the partaround a PZT transducer is also shown in thefigure. As mentioned before, the analysis usesaxis-symmetric modeling. In the analysis, PZT is

simulated as a strip wrapping around the outsideof the bar. Thus, the PZT-covered area in themodeling is larger than the area in the tests.Two element layers for PZT transducer and oneelement layer of silver adhesive epoxy (throughthe thickness) were selected. Compared to therest of the area, a relatively high density mesh(electric window) was applied around PZT. Thesize of the electric window was three times thePZT length, covering its front and back parts.In this window, numerical calculations used theimplicit method to satisfy the global boundaryconditions of the region. Time step was higher inthis region. The mesh outside the electricwindow was looser and the explicit methodwas applied. The fewer the elements required,the slower the time step used. For the wholetested bar structure, a total of 6600 elementswere used.

A comparison of results from the tests andthe analysis are plotted in Figure 3. In theanalysis, input signals were 200 V, 90kHz, andfive-peak burst waves, the same as signals in thetests. The amplitude of the sensor output fromthe analysis was 0.1 V, larger than the test resultsof 0.06V. Output signals from the analysiswere stronger because the area covered by theactuators was about 25% larger than the area


Void PZT Silver epoxy Steel bar

Electric window

Figure 2 Mesh of a steel rebar with PZT partial view.

0.8 1 1.2 1.4 1.6 1.8 2 0.1

0.08

0.06

0.04

0.02

0

0.02

0.04

0.06

0.08

0.1

TestAnalysis

Time (10 4 s)

S e n s o r o u

t p u

t ( V )

Figure 3 Comparison of analysis and test results forsteel rebar tests.




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in the tests. The smooth bar simulation contrib-uted partially to the increase of the sensoroutput, or about 14%. The above two majoreffects result in a 40% enlargement of the signalamplitude. After scaling down the amplitudebased on these factors, the output from the analy-

sis matches well with the test results (Figure 3).The time step for the bar tests was 1e 7s, andthe time step for the analysis was 8e 9 s, formost of the elements.

The velocity of wave propagation can becalculated from the output plot, based on thewaves traveling distance between the actuatorsand the sensors and the time used for thedistance. Calculation shows that the velocity isabout 5000m/s. This is quite close to the long-itudinal wave velocity in a steel bar (5100m/s).Further from the plot, the first burst wave receivedby the output sensors virtually maintains thesame shape as the input signals, i.e., a five-peakburst wave. This indicates that little dispersionoccurred during the wave propagation. Thereason for no dispersion is that the test wave-length (60mm for 90 kHz) was much longer thanthe cross-sectional diameter of the bar (19mm).Longitudinal waves dominated the wave propaga-tion, and the surface waves could be ignored.

Thus, the bar can be treated as a rod withoutconsidering the cross-sectional area.

3.2 Analysis of RC Beam withDebonding Damage

A mesh for a RC beam with debondingdamage and embedded piezoelectric elements wasgenerated. Debond was simulated as a layer of void between the concrete and the steel bar.The debond size (length) was changed from 1 in.(25.4 mm) to 8 in. (203.2mm), respectively, tosimulate different debonding damage situations.Only part of the mesh near the PZT transducersis shown in Figure 4.

After comparing different mesh densities andresult precisions, 16 layers were applied for theconcrete part. Propagating stress waves decayedvery rapidly in concrete due to the inhomogeneityof the concrete material. Therefore, a largedamping value of 100dB/m has been used on thelongitudinal direction of the bar [8]. Althoughshear waves were very small, they could still beobserved in the simulation. A shear damping of 2 dB/m was used to get cleaner output signals.The type of damping used for both stress wavesand shear waves was Rayleigh damping.


Concrete Steel bar

Void

PZT

Silver epoxy

Figure 4 Mesh of a reinforced concrete beam with PZT partial view.




8/13

Figure 5 shows the waveform comparison of the sensor output between the tests and the

numerical analysis for different debond sizes.Results from the analysis have been adjusted inthe same way as in the steel-reinforced bar case,i.e., scaling down of the amplitude and exclusionof time delay. It shows that plots from the testshave more noises. The sharp noises at thebeginning were likely caused by the resonance of noises among built-in PC electrical parts and testcircuits. These noises were small in magnitude.Since the test signals were small, the electricalnoise became noticeable. Four sets of data fordebond sizes ranging from 1 in. (25.4 mm) to 8 in.(203.2mm) are shown in the plots, and thesimulations match well with the test results. Thefigure also shows that the amplitude of the sensoroutput increases with the increment of thedebond size. Again, a time step of 1e 7s wasapplied for the RC beam tests, and a time step of 8e 9 s was used for most of the elements in theanalysis.

Figure 6 is a set of output waveforms fordebond cases. From low to high, it shows the

results of debond sizes ranging from 1 in.(25.4 mm) to 8 in. (203.2mm). Wave dispersioncan be observed in no debond or small debondcases. Dispersion is attributed to the concretesurrounding the bar, which serves as an elasticfoundation of the bar. As waves start to propa-gate from the actuators, a large portion of energygoes into the concrete. Because of propertydifferences between the bar and the concrete,wave propagating velocity in the concrete isslower than in the steel. The superposition of twodifferent speed waves causes the dispersion of waves on the steel bar surface. As the debondsize increases, less energy goes into the concreteand more energy propagates along the bar.Therefore, the interference of waves from theconcrete to the bar surface becomes less signifi-cant, resulting in increased signal amplitude andless wave dispersion [18,27]. The speed of the 8 in.(203.2mm) debond case is about 4100 m/s, which


0.7 1 1.3 1.60.7 1 1.3 1.6-0.015

-0.01

-0.005

0

0.005

0.01

0.015

S e n s o r o u

t p u

t ( V )

-0.01

-0.005

0

0.005

0.01

0.015

DS= 2 DS= 1

DS=4 DS=8

TestAnalysis

Time (10 4 s)

Debond size (DS)

Figure 5 Comparison of waveforms for debond cases.




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is between the speed of a steel bar (5100 m/s) andthat of concrete (3400 m/s) [25]. The figure alsoshows that for different debond cases, the time of arrival of the output signals is almost constant.

The amplitude comparison of the signaloutput between the tests and the analysis isplotted in Figure 7. Amplitude in the analysis isslightly larger than in the tests because of larger

actuators simulated, as discussed before. Datafrom the analysis and the tests show the sameincrement trends as the debond size increases.Further, it is clear from the analysis that theamplitude increases exponentially with thedebond size. Based on the model of signalattenuation [9], the amplitude of signals attenu-ates exponentially along with the travel distance.In the case here, as the debond size increases,energy damping due to concrete bounded on thebar decreases, which causes the signals to increaseexponentially.

From the verification cases discussed above,numerical results using the finite element methodwith PZFlex match well with the test results. Thisproves the effectiveness of the numerical modelsestablished for RC structures and test compo-nents. Further, from the numerical analysis, it canbe concluded that the changes of sensor signalsare caused by energy loss of the propagating

waves. The increased signal amplitude forthe increment of debonding damage relates to thedecreased energy loss of the signals in thestructure. The amplitude of the signals increasesexponentially with the debond size.

4 Parametric Studies

Parametric studies would be a huge amountof work if they were solely based on the resultsof experimental tests. Numerical simulationsprovide a convenient tool for the parametric


Figure 6 Sensor output for debond cases using numerical solutions.

0.00E+00

5.00E-03

1.00E-02

1.50E-02

0 1 2 3 4 5 6 7 8 9Debond size (in.)

S e n s o r o u

t p u

t ( V )

Test- Analysis

Figure 7 Amplitude comparison for debond cases.




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studies. Since the numerical results matched wellwith the test results, the models for testingcomponents have been used for studies of differ-ent combinations of RC structural geometriesand configurations.

In parametric studies, a matrix that covers a

broad range of parameters of the structures andthe system has been made. It includes a reinforc-ing bar with different sizes and material proper-ties, a RC beam with various types of damagessuch as cracks and debond, a RC beam withmultiple cracks or debond regions, and a RCbeam with different bar sizes or different concretematerials. The results for crack studies areillustrated and discussed in the following sections.Other study results are mentioned in the conclud-ing section.

4.1 Cracks in RC Beam

Several types of crack geometries have beensimulated to determine the crack effects on thesensor output. Cases include a single crack,multiple cracks, and a single crack plus debonddamage, in a RC beam.

In the single crack study, a crack was createdin the middle of a RC beam (Figure 9). Thebeam thickness was 4 in. (101.6mm) and therebar used was a #6 bar. The crack width refers

to the crack opening and is represented by w.The crack length refers to the depth of a crackfrom the surface to the middle of the RC beam.In the simulation, first the width w was fixedand the length was increased until the crack tiptouched the bar, and then the crack length wasfixed while w was changed. The results are shownin Figure 8. The crack length increase does notsignificantly affect the sensor output, and thedata sets from the sensors for different crackwidths are nearly overlapping as w increasedfrom 1 to 6 mm.

An explanation for the observation follows.Since the crack is very small and the wavelengthof the testing signals is much longer thanthe crack width, the propagating waves will passover the crack without seeing it, under mostcircumstances. When the crack length is relativelylong (1.5 in. (38 mm)), the crack forms a wall inthe concrete, which prevents the waves from

propagating from one side to the another, includ-ing the portion that would originally be super-imposed to the waves on the bar. This leads to aslight decrease of the sensor output. However, thewave portion from the concrete to the bar is verysmall, so the changes can hardly be noticed. Onthe other hand, when a crack goes deep to thesurface of the rebar (crack length is about 1.7 in.(43.2 mm)), debond is formed. More wave energytravels on the rebar due to the release of thebonding, despite the energy blockage by thewall. These two parts offset each other and

result in no changes of the sensor output (refer tothe points with the crack length of 1.7 in.(43.2 mm) in the chart). Therefore, for the singlecrack case, while a crack in concrete structuredoes produce a minor change of output signals,the change is too small to be considered.

Studies of a case of crackdebond combina-tion further support the ideas discussed.Simulation of a single crack combined withdebonding damage was performed using the samesize of the RC beam. The crack has a width of 3 mm and goes through the concrete cross section.The debond size was changed from 1 in. (25.4mm),2 in. (50.8mm), 4 in. (101.6 mm) to 8 in. (203.2 mm),respectively. Figure 9 shows the results from thesimulation, that a single crack does not affect theoutput as long as there is debond. Becausethe debond width is much wider than a crack,the debond dominates the sensor output anddebonding damage becomes a major problem.


0.000

0.002

0.004

0.006

0.008

0.010

0 0.5 1 1.5 2Crack length (in.)

S e n s o r o u

t p u

t ( V )

w = 1 mm, 3 mmw = 6 mm

Crack width wCrack length

Rebar Concrete

Figure 8 Crack with different widths and differentlengths.




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Cases of multiple cracks have also beenstudied. When the cracks are small (the sum of the crack width is less than 1 in. (25.4 mm)), twoor even more cracks do not affect the waveenergy distribution in the RC structure, sincemajority of the energy is still dissipated into theconcrete. When the sum of the crack width is1 in. (25.4 mm) or more, the sensor is able todetect the damage in the same way as in thedebond case, and this type of crack case can betreated as the debond case.

5 Debond Diagnostic Algorithm

A debond diagnostic algorithm can be devel-oped based on the previous studies:

1. Amplitude of the sensor signals increasesexponentially with debond size. The relation-ship can be described by the debond detectingformula.

y ce x 5where x is the debond size, y is the signalamplitude, c is related to the base line, and is related to the rebar depth d in concrete.When d 1.5 in. (38 mm), 0.33.

2. Wave forms of the sensor signals are notaffected by the rebar position when the con-crete coverage depth is greater than 2 in.(50.8 mm).

3. The sensor signals are not affected by cracks(normal to the rebar) in concrete when cracksare small (sum of the crack widths is less than1 in. (25.4mm)).

4. The debond detection formula is valid forconcrete with different sizes of aggregates andrebars.

The debond diagnostic algorithm was appliedto the test data. As introduced in Part I:Experiment [1], the bending tests were done onRC beams. If the above formula (Equation (5)) isused on the tested beams, then the extent of debonding damage can be found by solving theproblem inversely. Figure 10 shows the predicted

debond sizes inside the beams using Equation (5).The dots in the debonding curve chart are thevalues recorded under different loads on thebeam. They are mapped to the correspondingdebond size in Figure 10. For example, forbeam 2, the dot with a load of 2 kip correspondsto about 1 in. (25.4 mm) of debond size; the dotwith the load of 3 kip corresponds to about2.8 in. (71.1 mm) debond size, etc. To prove thisdebond detection method [4], an analytical esti-mation was made based on the bond stress andsplit relation theory [1214,19,27] combined withthe bended beams conditions.

6 Conclusions and Further Work

A debond detection method that usesembedded piezoelectric ceramics as actuating andsensing elements was described. The method used


debond size

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0 2 4 6 8 10Debond size (in.)

S e n s o r o u

t p u

t ( V )

No crack

w=3 mm

Width wCrack

Debond size

Figure 9 Crackdebond combined study.

0.00

2.00

4.00

6.00

8.00

10.00

0 1 2 3 4 5 6 7

Debonding size x (in.)

S e n s o

r o u

t p u

t / b a s e

l i n e y

* Beam 1Beam 2

y* = e 0.33x

Figure 10 Debond detection for bended beams.




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high frequency transient stress waves to detectthe location and the extent of steel-reinforcedbars debonding from the concrete matrix in a RCstructure. Experimental test results showed thatthe method could detect debonding damage. Theamplitude of the sensor output increased as the

extent of debond increased. The time of signalarrival was found to be quite sensitive to therebars elongation.

Modeling and analysis was performed todelineate the phenomena observed in the testresults. Since the analysis involved both 3D stresswave propagation and piezoelectricity, numericalsimulations were applied to solve the modelingand analysis for the bar, the RC structures, andthe embedded piezoelectric components. With theproper selection of element sizes, element meshconfigurations, and artificial damping, waveformsobtained from the numerical solutions closelymatched the waveforms observed in the experi-mental tests. It was found that the changes in thesystem output were caused by the energy distribu-tion in the detected RC structure. Debondingdamage in the structure caused the decrease of energy dissipation, which led to an increase of thesystem output.

Further extended parametric studies werecarried out to simulate various RC structureconfigurations and damage situations. The studies

show that a high Youngs modulus of the PZT,materials, and a thin epoxy layer will increase thesensor signals. Bars of different sizes can betreated as an ideal bar without considering theircross section. Only longitudinal stress waves haveto be considered. Small sized cracks (normal tothe bar) inside the concrete structure do notaffect the sensor output. When multiple cracksexist and the sum of the crack sizes is greaterthan 1 in. (25.4mm), the output behaves almostthe same as the equivalent debond size. When acrack and debond damage exist in an RCstructure, the debond damage dominates thesignal changes. When the length of debonddamage increases, the output of the sensorsincreases exponentially. The exponential curvewill change modestly when the concrete coverageon the bar changes. When the coverage depth isgreater than 2 in. (50.8 mm), the exponentialdamage curve remains unchanged. Multiple

debond regions behave the same as a debondregion with the equivalent debond size. Thedebond thickness does not affect the sensoroutput. Aggregate sizes of concrete materials donot affect the sensor output.

A debond diagnostic algorithm based on

parametric studies is described by the debonddetecting formula (Equation (5)) the amplitudeof the sensor signals increased exponentially withdebond size. To obtain good detection results, thesensor output of the system needs to be increasedand the system noises need to be controlled andminimized. Tests show that the minimum sensorto noise ratio, which can clearly distinguishsignals from noise, is 150%. A ratio lower than itcould cause an accuracy problem for debonddetection. Improvement on sensor output can bemade with different actuator configurations. Forinstance, mount actuators at the end of a rebarso that they cover the whole cross section of thebar and reduce energy losses during the energyconversions by using a better property matchedbonding epoxy or a thinner adhesive layer andmaking a better bond between the piezoelectricmaterial and the rebar.

Further study could also focus on practicalissues, such as rebar corrosion effects, the agingof the epoxy, sensors, and actuators. The currentdebond detection method cannot determine if the

signal changes are caused by regular debond ordebond caused by corrosion. A good way topractice debond detection might be to combinethe method studied in this article with othercorrosion detection methods. Though PZT mate-rial has very stable properties for long periodsand in harsh environments, aging of such compo-nents still has to be studied before the methodcan be applied in real practice. The ultimate useof the detection method is to form a network of sensors and actuators embedded in a structurewith the ability to detect structure debondingdamage smartly.

Acknowledgments

The authors acknowledge the financial support of theNational Science Foundation under grant CMS-9812574and the technical support from Mr D. Vaughan and





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Mr G. Wojcik of Weidlinger Associates, Inc. with PZFlez.The authors also thank Professor C. Steele of theDepartment of Mechanical Engineering, and ProfessorsH. Krawinkler and G. Deierlein of the Department of CivilEngineering at Stanford University for their valuable inputsto the study.

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