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DOI: 10.1177/1475921706057982

2006 5: 59Structural Health Monitoring

D. Lecompte, J. Vantomme and H. Sol

Crack Detection in a Concrete Beam using Two Different Camera Techniques

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59

Crack Detection in a Concrete Beam

using Two Different Camera Techniques

D.Lecompte,

1,

* J.Vantomme

1

and H.Sol

2

1

Royal Military Academy,Department of Materials and Construction

Renaissancelaan 30,PO Box 1000,Brussels,Belgium

2

Free University of Brussels,Mechanics of Materials and Constructions

Pleinlaan 2,PO Box 1050,Brussels,Belgium

The study presents an application of two different optical measurement techniques for the detection of

cracks at the surface of a realistically sized concrete beam subjected to flexural loading conditions.

Both techniques provide displacement measurements in a discrete number of points on the surface.

Based on these displacement fields,deformations are calculated by means of the Green–Lagrange

strain expression.The study deals with the relationship between cracks and the concept of

deformation and it examines which of the two methods presented appears to be the most suitable for

crack prediction or detection.The study shows that it is possible to detect the appearance and

evolution of cracks,even before the cracks become visually detectable,with both methods and reveals

their complementarities.

Keywords concrete beam damage-detection optical measurements non-contact

measurements

1 Introduction

The present study is defined in the framework

of the FWO-Flanders-project Nr G.0266.01,

where the objective is to evaluate the behavior of

a realistically sized prestressed concrete beam,

during static and dynamic loading,using a four-

point bending disposition in which the two

bending points (position of the loading jacks) are

decoupled (see Figure 6).

Classical experimental validation techniques

that use strain gauges,extensometers,etc.,do not

allow a correct apprehension of the local damage

mechanisms,which lead to the failure of the

structure,due to the heterogeneous nature of

concrete.A more relevant modeling of these

mechanisms can be assisted by a full field

measuring technique focused on a given region of

interest of the loaded specimen.The question is

whether it is possible to predict crack develop-

ment before a crack becomes visually detectable

and whether the crack evolution during the

process can be monitored.

In this article,two different measurement

techniques are used to measure the displacements

of a discrete number of points on the surface of a

beam during loading.

The first technique is based on the compar-

ison of a number of digital images,taken during

the loading of the beam.It uses a given speckle

pattern,exhibited by the surface of the specimen

or obtained by a paintspray,combined with

*Author to whom correspondence should be addressed.

E-mail:David.Lecompte@rma.ac.be

Copyright 2006 SAGE Publications,

Vol 5(1):0059–10

[1475-9217 (200603) 5:1;59–10 10.1177/1475921706057982]

Copyright 2006 SAGE Publications,

Vol 5(1):0059–10

[1475-9217 (200603) 5:1;59–10 10.1177/1475921706057982]

by guest on November 25, 2013shm.sagepub.comDownloaded from

digital image correlation (DIC) and a charge-

coupled device (CCD) camera.It will be referred

to as the ‘DIC technique’ [1].

The second technique is based on the locali-

zation in space of a number of light emitting

diodes (LEDs),fixed to the surface of the beam.

This technique uses the principle of space inter-

section combined with three linear CCD cameras

and a number of infrared (IR) light emitting

diodes.It will be referred to as the ‘LED-CCD

technique’ [2].

The article presents a theoretical overview of

both techniques,examines the measurement prin-

ciples,and discusses the obtained results with

respect to the determination of cracks.

2 Measurement Techniques

2.1 Digital Image Correlation (DIC)

The DIC technique is an optical-numerical

measuring technique,which offers the possibility

to determine displacement and deformation fields

at the surface of objects under any kind of

loading,based on a comparison between images

taken at different loading steps.This technique

has been used in different technological domains

and many of its applications have been reported

[3–6].It allows to study qualitatively as well as

quantitatively the mechanical behavior of materi-

als under certain loading conditions.

A measurement consists of taking several

pictures of the object of interest during loading,

with a CCD camera.Every pixel of the camera

stores a certain gray scale value from 0 to 255,

corresponding to the intensity of the light

reflected by the surface of the tested specimen.

The objective is to obtain an image of the object

with a varied and distinctive gray value pattern,

to enable differentiation between different square

groups of pixels (subsets).

The concept behind the DIC-software match-

ing algorithm is that the gray value distribution

of a subset in the image of the undeformed object

should correspond to the gray value distribution

of the same material area in the image of the

deformed specimen.To measure a displacement

field,the image is divided into a number of

subsets.The size of a subset can for example

be 77,1111,or 1515 pixels.The image

correlation routine permits to locate every subset

of the initial image in the deformed image.

Subsequently,the software determines the displa-

cement values of the centers of the subsets,which

yield an entire displacement field.

Figure 1 depicts the sequence of taking a

picture of an object before and after loading,

storing the images onto a PC through a frame

grabber,performing the correlation of both

images (i.e.,locating the different undeformed

subsets in the deformed image) and finally

calculating the corresponding displacement of the

centers of the subsets,which finally yields the

desired displacement field.

2.2 LED-CCD Technique

The camera system is based on three linear

CCD units (three circular apertures in Figure 2).

Each unit is composed of a 2048 pixel linear

CCD,optics,and a processing board.The optics

are composed of an infrared window,a normal

lens,and a cylindrical lens,which compresses the

viewing area to a line.The processor of each unit

calculates the position of the peak of a light

source on the CCD.

The basic unit acts like a goniometer.Each

unit is capable of measuring the angle that a light

source makes with the optical axis,which is

parallel to the z-axis in Figure 3.This light

source should be small and yet powerful enough

Figure 1 Working principle of the DIC-system.

60 Structural HealthMonitoring 5(1)

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to exceed the light level of the environment.

IR-emitting diodes are used for this task.They

are small and can emit enough IR light.The IR

windows ensure that environmental light does not

influence the measurement.

The three optical goniometers are mounted

on a rigid structure (Figure 2).Using triangula-

tion techniques,one 3D position can be calcu-

lated from three 1D positions or angles.In

Figure 3,a schematic top view of the system is

shown.Cameras 1 and 3 provide the Z-and the

Y-position.Camera 2 mainly measures the

X-position.Measuring the position of a LEDbefore

and after loading of an object,on which the LED

is attached,yields its displacement in 3D space.

Since the cylindrical lens projects the 2D

viewing area on a 1D line,not more than one IR

marker can be observed at the same time.To

track several markers simultaneously,the markers

need to be flashed sequentially.Since the cameras

are mounted on a rigid structure,calibration only

has to be performed once in the factory.

3 Measurement of Deformation

As the objective is to measure crack damage,

the first question is how to quantify this damage

based on displacement measurements.Therefore,

a concept from continuum mechanics is used

[7,8].

The behavior of a material can be character-

ized by the displacement of a discrete set of

points at the surface of the material.Figure 4

shows an object before and after deformation.

The points P and Q delimit a line segment dX

in the undeformed configuration.P

t

and Q

t

delimit the same line segment dx

t

in the deformed

configuration.The translation vector is called u.

In vector notation this means:

x þ

dx ¼

Xþ

dXþ

u

Xþ

dX

ð1Þ

The Taylor expansion of the last term of the

second member yields:

x þ

dx ¼

Xþ

dXþ

u

X

þ r

uð Þ

dX

þ

r

2

u

dX

2

2

þ ð2Þ

Figure 2 Coordinate measurement system K600 (by

Krypton).

Figure 4 Deformation principle in continuummechanics.

Y

1

2

3

z

x

LED

Figure 3 Measurement by spatial intersection.

Lecompte et al.Crack Detection in a Concrete Beam 61

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Knowing that:

x ¼

Xþ

u

X

ð3Þ

Equation (2) becomes:

dx ¼

dXþ r

uð Þ

dXþ ð4Þ

or in finite difference notation

x ¼

Xþ r

uð Þ

Xþ ð5Þ

When projecting these vectors onto an orthogon-

ally normalized coordinate system one obtains:

x

y

!

¼

X

Y

!

þ

@u

@X

@u

@Y

@v

@X

@v

@Y

0

B

B

@

1

C

C

A

X

Y

!

þ

ð6Þ

¼ I þ

@u

@X

@u

@Y

@v

@X

@v

@Y

0

B

B

@

1

C

C

A

2

6

6

4

3

7

7

5

|ﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄ{zﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄﬄ}

F

X

Y

!

þ ð7Þ

where F is the deformation gradient tensor,which

is defined as follows:

dx ¼ F dX ð8Þ

with:

F ¼

@x

@X

@x

@Y

@y

@X

@y

@Y

0

B

@

1

C

A

¼

@ Xþuð Þ

@X

@ Xþuð Þ

@Y

@ Yþvð Þ

@X

@ Yþvð Þ

@Y

0

B

B

@

1

C

C

A

¼ I þ

@u

@X

@u

@Y

@v

@X

@v

@Y

0

B

@

1

C

A

2

6

4

3

7

5

ð9Þ

The gradient deformation tensor is a linear

operator that transforms a rectangle into a

parallelogram.x and y,X and Y,and thus u and

v are measured.The remaining unknowns are the

four members of the deformation gradient tensor.

Figure 5 shows four arbitrary data points

(centers of facets or LEDs) in the initial and

deformed configuration.The result covers the

deformation of the zone between Points 1,2,3,

and 4.

It is possible to calculate the strain compo-

nents from the deformation gradient tensor:

E ¼

1

2

F

t

F I

¼

1

2

U

2

I

or in two-dimensional space:

E ¼

1

2

2

@u

@x

þ

@u

@x

2

þ

@v

@x

2

@u

@y

þ

@v

@x

þ

@u

@x

@u

@y

þ

@v

@x

@v

@y

@u

@y

þ

@v

@x

þ

@u

@x

@u

@y

þ

@v

@x

@v

@y

2

@v

@y

þ

@v

@y

2

þ

@u

@y

2

0

B

B

B

@

1

C

C

C

A

ð10Þ

Once the deformation gradient tensor F is calcu-

lated,the chosen strain tensor E can be deter-

mined.The Green–Lagrange strain is an objective

strain expression,which means that it takes rigid

body motions into account.Finally,the eigenva-

lues and eigenvectors of the strain tensor are

calculated to obtain the major and the minor

principal strain values and their corresponding

directions.

In practice,strain is calculated between four

adjacent data points (four LEDs in the case of

Figure 5 Deformation principle in discrete 2D case.

62 Structural HealthMonitoring 5(1)

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the LED-CCD system or the centers of four

subsets in the case of the DIC system).When the

initial positions and the displacement components

of these four data points are known,the discrete

form of Expression (10) is used to calculate the

strain value in the region between the four data

points considered.The distance between the

initial positions of the data points is called the

gauge length.

It is clear that in the present case,the strain

value calculated does not represent an evenly

distributed deformation but is more a measure of

damage due to the appearance of cracks.

4 Experimental

4.1 Introduction

The experimental principle consists in loading

the prestressed concrete beam up to different

loading values and subsequently unloading it.

After the static loading and unloading procedure,

a dynamic analysis of the beam – which is not

discussed here – is performed.

The measurements discussed in the present

article are performed during one of the loading–

unloading cycles shown in Figure 7.Four tests

had already been performed where maximum

loads of 45,65,76,95kN,respectively were

imposed per jack.This study is focused on the

exploitation of the images that were taken during

the fifth cycle.Figure 6 shows an image of the

experimental setup and Figure 7 represents the

deflection curve during the eight consecutive

loading and unloading cycles.

4.2 Setup of the Two

Measuring Systems

A given zone is selected to perform the static

displacement measurements.Figures 8 and 9

show the location of the IR-LEDs (represented

Figure 7 Load–deflection diagram.

Figure 6 Experimental setup.

Lecompte et al.Crack Detection in a Concrete Beam 63

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by the dots).Forty-nine of those markers are

used and placed over the entire height of the

beam.To enhance the resolution of the 2D CCD

camera,only the lower part of the zone covered

by the LEDs is taken into account.In Figure 8,

this zone is represented by the rectangle at the

bottom flange.

Both measurement systems (Figure 10)

provide displacement measurements in a discrete

number of points.For the LED-CCD system,

these points are represented by the IR-LEDs

(encircled in Figure 11).Only the diodes located

at the lower flange are used for the comparison.

As for the DIC-system,the data points are

represented by the centers of the facets (white

dots in Figure 11).

Figure 12 represents the six different zones

in which finally the deformation/damage is deter-

mined by the two techniques.During the previous

loading cycles,several cracks already occurred.

They are put in evidence by the black lines.

Zones 1,3,and 5 exhibit such cracks.

At the end of the loading (95kN),a visual

inspection of the beam is performed.It leads to

the detection of two new cracks in Zones 2–4

(Figure 13).This should be detected by both

measuring systems.

Figure 9 Field of view of the 2D CCD-camera with LED

positions.

Figure 10 Image of both camera systems.

Figure 8 Experimental setup with zones of interest.

Figure 13 Appearance of two new cracks during the

5th loading step up to 95kN per jack.

Figure 12 Zoom in on the zone of interest with

subdomains.

Figure 11 Zoom in on the zone of interest.

64 Structural HealthMonitoring 5(1)

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5 Results and Discussion

5.1 Results from the

LED-CCD System

During the loading of the beam,a number of

8000 displacement measurements are performed

for each LED with this system.

Figures 14 and 15 represent the horizontal

and vertical displacements of the lower seven

LEDs during the loading of the beam.The

purpose of showing these graphs is to demon-

strate that it is difficult to retrieve any informa-

tion concerning damage or deformation based on

the displacement values.

However,when calculating the major princi-

pal strains in the different zones during loading,

more information becomes visible.Figure 16

represents the major principal strain values in the

six different zones between 0 and 95kN.It is

important to know that the orientation corre-

sponding to the major strain is the horizontal

direction.It is,as one would expect,perpendicu-

lar to the crack direction.

The accuracy of the determined displacements

depends on the distance between the system and

the measured object.For a distance between 1.5

and 3m,which is the case in the present study,a

standard deviation on the measured displace-

ments of 15mm is obtained.The standard

deviation of the calculated strains is 50mstrain.

Because of the large number of measurements in

time,it is possible to quantify the accuracy in

terms of ‘standard deviation’ and not in terms of

a more severe ‘uncertainty value’.The standard

deviation is calculated based on a piecewise linear

regression of the displacement and strain curves.

At a load of 40kN,it becomes clear where

the original cracks are located.The higher strain

values are found in Zones 1,3,and 5.In these

zones,a major principal strain value of

200mstrain is calculated.This value corresponds

very well with the elastic limit of the reinforcement

steel.Beyond this value (marked by the thick

black horizontal line in Figure 16),the curves

show a steeper ascending path.Figure 16 clearly

Figure 14 Horizontal displacement of lower seven

LEDs.

Figure 16 Major principal strain (continuous) in the

different zones during loading (measured by Krypton

system).

Figure 15 Vertical displacement of lower seven LEDs.

Lecompte et al.Crack Detection in a Concrete Beam 65

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shows when yielding of the reinforcement bars

occur and cracks begin to form in every zone.

Between a load of 80 and 90kN,an impor-

tant increase in the strain plot is observed

in Zone 2.This corresponds to the appearance

of a new crack in that zone,as can be seen in

Figure 13.

When exceeding a load of 90kN,the curves

corresponding to Zones 3 and 4 ascend more

steeply.This corresponds to the existence of a

new crack between Zones 3 and 4.The strain in

both zones is influenced by the new crack.

At a load of 90kN,the slope of the curve

corresponding to Zone 6 becomes more impor-

tant.During the visual inspection however,no

new crack was detected.

The major principal strains based on the

measured displacements of the LEDs are princi-

pally the result of the opening of a crack

in a given region.The ductile strain of concrete

is 100mstrain,beyond which cracks start to

appear.The strains that exceed this value are no

longer a measure of continuous deformation,but

a measure of the crack opening.Hence,the

increasing strain in Figure 16 has to be attributed

to the widening of the crack in the corresponding

region.

To reveal the correlation between the calcu-

lated major principal strain and the size of the

cracks,a region with only one crack has to be

chosen.When more than one crack occurs in

the same region,it is no longer possible to derive

the width of every single crack from the calcu-

lated strain.At the end of the loading (at 95kN),

a single visible crack is present in

Zones 1,2,and 5.The measured crack openings

are 0.28,0.16,and 0.21mm respectively.Every

crack is measured in the middle of the bottom

flange with a crack microscope with an accuracy

of 0.01 mm.

The ratio between the crack width and the

corresponding strain value in Zones 1,2,and 5

makes it possible to estimate the crack width in

Zone 6.

Based on the results in Table 1,the strain

value of 380 mstrain in Region 6 is

associated with a crack opening with a width

between 33 and 39mm,which is not visible with

the naked eye.

5.2 Results from the DIC System

With this technique,a picture is taken every

10kN load.The displacements are measured in a

field of 1400 data points.This means that

1400 subsets are used in the correlation process.

Finally,the displacement components in the

centers of these subsets are determined.The

uncertainty on the measured displacements is a

value expressed in pixels and is accepted to be

0.07 pixel.This means that the uncertainty

expressed in units of length depends on the

resolution of the images and thus depends on the

experiment.In the present case,a resolution

of 0.79mm/pixel is obtained.Therefore,the

uncertainty on the displacements expressed in

units of length is 55mm.For this system,it is

more appropriate to express the accuracy in

terms of the uncertainty on the measured displa-

cements and not in terms of standard deviation.

First of all,the mean values of the displace-

ment components of the data points (white

markers in Figure 11) located in the vicinity of

the IR-LEDs are calculated.Subsequently,the

major principal strain values are determined as

for the LED-CCD data.The results are shown in

Figure 17.It is clear that this plot is less detailed

than the one based on the LED-CCD data

because of the limited number of load steps at

which the necessary pictures are taken.

Table 1 Comparison between measured crack width and strain.

Measured

crack width (mm)

Calculated

strain (strain)

Crack width/strain

(mm/strain)

Region 1 0.28 2700 10.3e 5

Region 2 0.16 1810 8.8e 5

Region 5 0.21 2230 9.4e 5

66 Structural HealthMonitoring 5(1)

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For the determination of the accuracy on the

calculated strains,basic error analysis was used.

The uncertainty on the strain value depends on

the uncertainty of the displacements but also

on the chosen gauge length (i.e.,the distance over

which the strain is calculated).The smaller

the gauge length,the higher the uncertainty on

the strains.For the strains in Figure 17,a gauge

length of 150mm is chosen (similar to the

horizontal distance between the LEDs).This

means that the uncertainty on the strains in

Figure 17 is 500mstrain.

However,the advantage and the complemen-

tarity of the DIC technique become clear in

Figures 18–20.These figures show plots of the

calculated major principal strain in the entire

zone of interest.For these plots,the entire

displacement field is used (all the centers of the

subsets) and the gauge length is chosen to be

much smaller than in the case of Figure 17.This

means that crack information can be revealed

more locally because it is less smeared out over

the corresponding zone.The disadvantage is that,

because of the smaller gauge length,the uncer-

tainty on the strains is even higher.However,the

benefit of these plots is their capacity to locate

the cracks in the corresponding zones.

Figure 18 shows the existence of the three

initial cracks.The situation corresponds to a load

of 80kN.

Figure 19 displays the major principal strains

at a load of 90kN.A fourth crack appears in

Zone 2.This can also be noticed in Figure 16,

Figure 17 Major principal strain (every 10kN load)

in different zones during loading (measured by DIC

technique).

Figure 18 Major principal strain at 80kN.

Figure 19 Major principal strain at 90kN.

Figure 20 Major principal strain at 95kN.

Lecompte et al.Crack Detection in a Concrete Beam 67

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where the moment of crack forming can be

determined very precisely.However,the determi-

nation of the exact location of the crack is only

possible in the full field plot.

Figure 20 corresponds to the end of the

loading with a load of 95kN.A last clear crack

appears between Zones 3 and 4.This can again

be verified in Figure 16,where an increase in the

slope of the corresponding curves becomes clear.

Finally,an augmentation of the curve corre-

sponding to Zone 6 becomes noticeable at the end

of the loading (in Figure 16).Visual inspection did

not reveal a crack in the sixth zone.However,in

Figure 20,a change in the strain field in Zone 6

becomes clear as well.It means that a combination

of both techniques reveals the existence of a begin-

ning crack,not yet visible with the naked eye.

5.3 Comparison between

Both Systems

The number of measurements performed by

the LED-CCD system in the same period of time

is much higher than the number of pictures taken

with the DIC system.It is of course possible to

increase the number of pictures.However,due to

the time-consuming post-processing of the DIC

technique,one has to limit the frequency with

which the pictures are taken.To give an idea,

during the loading cycle from 0 to 95kN (which

took 30

0

) about 9000 measurements are nearly

instantly performed with the LED-CCD system.

Only 11 images over that same period of time are

taken and used for post-processing by the DIC

system.The processing of these 11 pictures took

about 40

0

with a Pentium III processor.

Due to the limited number of IR-markers,

the obtained information with the LED-CCD

system is not very local.Every change in the

material is extrapolated to the motion of only a

few markers.The DIC technique offers numeri-

cally the possibility to increase the number of

data points for which the displacement compo-

nents are determined.It therefore generates a

more local view of the deformation.

The present study clearly shows that the

LED-CCD system should be used to indicate the

larger region in which a crack occurs.

Furthermore,the calculated strain values give a

good idea about the actual crack width,located

in the corresponding region.The DIC system on

the other hand can be used to more precisely

locate the crack in that same region.

As a conclusion,one can state that both

measuring techniques are complementary.On the

one hand,the LED-CCD technique permits to

perform a large number of measurements over a

given period of time in a few data points.On the

other hand,the DIC technique enables the user

to obtain a dense field of data points in which

the displacement values are expressed in a few

number of time steps.

Acknowledgment

The results for the LED-CCD technique have been kindly

put to our disposal by Krypton Industrial Metrology and

Prof Dr Ir G.De Roeck from the KUL.

This project is supported by the Belgian National Science

Foundation.

References

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fu¨r Optische Mebtechnik mbH),Gom mbH,

www.Gom.com.

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3.Chevalier,L.,Calloch,S.,Hild,F.and Marco,Y.(2001).

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