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Composites: Part B 69 (2015) 350–358
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Composites: Part B
journal homepage: www.elsevier .com/locate /composi tesb
Evaluation of gluing of CFRP onto concrete structures by infraredthermography coupled with thermal impedance
http://dx.doi.org/10.1016/j.compositesb.2014.10.0021359-8368/� 2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: LGCgE – Université Lille Nord de France F59000, FSA –Université d’Artois, Technoparc Futura, 62400 Bethune, France. Tel.: +33 3 21 63 7131; fax: +33 3 21 61 17 80.
E-mail address: [email protected] (C. Alexis).
Chauchois Alexis a,b,⇑, Brachelet Franck a, Defer Didier a, Antczak Emmanuel a, Choi Hangseok b
a LGCgE – Université Lille Nord de France F59000, FSA – Université d’Artois, Technoparc Futura, 62400 Bethune, Franceb School of Civil, Environmental, and Architectural Engineering, Korea University, Seoul, Republic of Korea
a r t i c l e i n f o
Article history:Received 29 July 2013Received in revised form 24 September 2014Accepted 1 October 2014Available online 14 October 2014
Keywords:B. DebondingB. DefectsD. Non-destructive testingD. Thermal analysis
a b s t r a c t
Carbon Fiber Reinforced Polymers (CFRPs) have been increasingly employed for structural strengthening,and are attached to structures using bonding adhesives. The aim of this work is to characterize defects inthe bond between CFRP and concrete (after they are located by pulse infrared thermography), and assignthe defects a ‘‘numerical value’’ (ranging from 0 for a complete air–gap to 1 for a fully glued bond). Quan-titative characterization is performed by measuring the thermal impedance, and then identifying thethermophysical parameters of the system through fitting the measured impedance to a theoreticalmodel. An inversion procedure is carried out to estimate the unknown parameters, without prior knowl-edge of sample properties. In particular, it is possible to estimate more accurately both the amount ofglue within a defect and the thermal contact resistance.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Aging civil engineering infrastructure is at the center of currentconcerns of many countries. Indeed, after World War II, construc-tion of concrete structures (such as road infrastructure, buildings,and parking garages) heightened. So, for over 60 years, these struc-tures have suffered the ravages of time and various attacks. Manyof these concrete structures show changes in material compositionor structural damage (caused, for example, by poor design, poorconstruction or changes in boundary conditions as a result of scour,landslides or other external agents). Some structures can no longermeet the requirements for safe use; in some cases the way thestructures are used has evolved over the years as well (for examplethe increased size and number of vehicles). The demolition andreconstruction solution is usually too costly. In fact, alternativesare possible [1–3], including: repair of the surface, concrete protec-tion, materials regeneration, strengthening and adding extra mate-rial. The best solution depends on the importance of the structureand the amount of damage or disorder that has occurred.
One way of strengthening structures is by reinforcing with acomposite material plate, which has the advantages of not
corroding, being lightweight and high-strength. These compositesare referred to as ‘‘FRP’’, fiber reinforced plastics or ‘‘CFRP’’, carbonfiber reinforced plastics/polymers. This process allows the strengthand stiffness of the structure to be increased by the combinedaction of the plate and the concrete. FRP are composite materialsmade of high-strength fibers (glass, carbon or aramid) impregnatedwith a matrix (such as polyester, vinyl ester, or epoxy).
As they are becoming less expensive, composites are an attrac-tive solution for strengthening buildings and other infrastructure[4]. The strengthening or retrofitting of reinforced concrete struc-tures by externally bonded FRP systems is now a widespread andaccepted technique [5,6], which has been widely studied, forexample in [7–15].
The quality of bonding at externally bonded CFRP–concreteinterfaces is very important. For external reinforcing with FRPcomposites to be most effective, the work must be very carefullycarried out [16–18], and, crucially, the adhesive layer must not suf-fer too much damage when exposed to aggressive environments[19]. Otherwise, voids or flaws at the interface can occur; thesedefects are quite common. In the first case; ‘‘flaws’’ are formedbecause of poor workmanship during the initial application ofthe CFRP strips onto the concrete surface. Adhesion problemssometimes occur because not enough glue is used, or because someareas have not been glued at all. The second type of defect is‘‘delamination’’, and occurs because of stress concentrationsassociated with physical/chemical degradation of the binding layer
C. Alexis et al. / Composites: Part B 69 (2015) 350–358 351
[20] (when the composite is exposed to aggressive environmentssuch as elevated temperatures, ultraviolet radiation, infiltrationof moisture or extreme temperatures caused by fire).
These invisible defects embedded in the interface between theCFRP and concrete can significantly reduce the effective contactarea, and therefore the overall bond strength. Ultimately, the dura-bility and service life of the CFRP-strengthened concrete structureis adversely affected.
This means that quality control though ‘‘in situ’’ verification ofbonded FRP systems is of paramount importance. Conventionalmethods for evaluating the bond quality of the CFRP–concretecomposites are hammer-tapping and pull-off tests. Hammer-tap-ping requires the inspectors to manually tap each point to bechecked, while the pull-off test is destructive. Both types of testsare regarded as local tests, and do not allow effective, large-scaleinspection.
The Infrared Thermographic (IRT) technique has been widelyaccepted as an effective means of identifying and quantifyingunseen surface flaws in a wide range of composite materials [21–23]. This is because the presence of a defect (i.e. an ‘‘air layer’’) atthe interface in a composite material reduces the rate of heat dif-fusion when thermal stimulus is applied. In fact, of all the Non-Destructive Testing Techniques (NDT) under investigation forbond-quality assessment, (active) infrared thermography is of par-ticular interest, because it is easy to deploy and can be used to rap-idly inspect large surfaces, where only one side is accessible.
However, even if these techniques are able to locate the defects,most of the time they are unable to provide sufficient specificinformation such as the depth and width of the disbonded area,or to assess the degree of adhesion between the composite andthe adhesive. Indeed, during the early applications of IRT, themajority of published work simply focused on locating defects ina qualitative manner. It is more useful to quantify the size of thedefects so that the extent of damage can be assessed. Quantitativedefect characterization using infrared data can be achieved on thebasis of direct analytical methods [24,25] and inverse methods[26]. Approaches by direct analysis can be used to model the char-acteristics of the defects, but these approaches can be very com-plex, even when the defect geometries are simple, and canbecome impossible once anisotropic properties and defects in thesurface are considered [24].
Consequently, in this work we will couple two methods: a pulsethermographic detection method to visually locate the defect, anda thermal method based on a heat flux sensor (also known as ‘‘flux-meter’’), to characterize it in more detail. Our aim is to quantify thequality of the bonding between CFRP and concrete.
Fig. 1. Schematic of the experimental setup with the five parameters to beidentified.
2. Theory
For several years, our research team has been developing ther-mophysical characterization methods based on the study of ther-mal impedance, as measured using heat flux sensors [27].Thermal impedance represents the relationship between the fre-quency components of temperature and the frequency-dependentflux density in the same surface. From an experimental point ofview, it is determined simply by measuring the flux density andtemperature simultaneously in a measurement plane surface. Inpractice, a heat flux sensor [28] in which a thermocouple has beenembedded is placed in contact with the sample. The changes in fluxdensity and temperature measured in this way are different fromthose in the material access plane. The reasons for this perturba-tion are the presence of the sensor and the sensor/material contactresistance. This original approach is based on a frequency-depen-dent study of the changes at the surfaces of materials.
2.1. Notion of thermal impedance
2.1.1. DefinitionBy definition, the thermal impedance Z [29] is a complex
parameter, defined in the frequency domain as the ratio of thetemperature and heat flux. Thus, Z(f) can be written as:
Zðf Þ ¼ hðf Þ/ðf Þ ¼
hðxÞ/ðxÞ ð1Þ
where Z is a function of the frequency f or of the pulse-duration x.Both quantities can be measured simultaneously using a heat
flux sensor fitted with a thermocouple. If the thermal impedanceis estimated for characterization purposes, both the imposed stressand temperature response are observed simultaneously, and theirinteraction can be analyzed. Such an analysis is carried out in thefrequency domain, enabling the system to be studied on the basisof random signals. The frequency approach facilitates an under-standing of the phenomena, and, along with the sensitivity study,transformation into the spectral domain enables us to target pre-cisely which frequency range should be selected to identify therequired parameters. The thermophysical parameters can be iden-tified by fitting a theoretical model to the experimentally deter-mined impedance.
The thermal impedance can be expressed theoretically, as willbe shown in this paper. The experimental procedure for calculatingthe apparent thermal impedance of the input side of a materialrequires both temperature and flux sensors to be positioned onthe sample. Then one may observe either the natural exchangesbetween the material and the environment, or as in our case, heatthe sensor/material assembly directly.
To ensure exchanges are one-directional heat flow, the sensorconsists of a sensitive measuring area surrounded by a ring witha similar but inert material acting as a guard ring. The sensor is atangential-gradient fluxmeter (heat flux sensor) [30], 0.5 mm inthickness, whose temperature is measured with an embedded T-type thermocouple.
Fig. 2. Expanded view of a bonding failure.
352 C. Alexis et al. / Composites: Part B 69 (2015) 350–358
Theoretically, the system that we study may be represented bya series connection of five elements (Fig. 1):
� The part of the sensor between the measurement plane and theoutput plane, which can be modeled by a thermal capacitance Cf
(J m�2 K�1) [28].� A thermal contact resistance Rc (K m2 W�1) between the sensor
and the composite plate.� The composite plate with known thermal characteristics (k and
qc) and width ‘.� A thickness made up of glue and air–gap in proportions depend-
ing on the quality of the bond, and modeled using a thermalresistance R (K m2 W�1) and a heat capacity C (J m�2 K�1).� A thick concrete layer acting as a semi-infinite medium, of ther-
mal effusivity characterized by b (J m�2 s�1/2 K�1).
Each part is associated with a matrix [31].Each layer can be described as follows:
2.2. The sensor
The measurements are performed by a heat flux sensor (inwhich a thermocouple is embedded), placed into contact withthe composite plate. We can assume that the plane in which themeasurements are performed corresponds to the median plane ofthe sensor (Fig. 1), and the portion between the measurementplane and the material is the part of the sensor system which needsto be included in the model.
Previous work [32] has shown that when working frequenciesare below 0.1 Hz, this part of the sensor behaves like a constantthermal resistance in parallel with a constant thermal capacity.The respective thicknesses and thermal characteristics of the mate-rials of this part of the sensor mean that Rf is very small comparedto the contact resistance, and can be neglected in the model. Thus,the sensor portion downstream of the measurement plane is repre-sented by the following matrix:
1 0jxCf 1
� �ð2Þ
where Cf [J/(K m2)] is the relevant capacity of the fluxmeter (i.e., thecapacity of the part between the measurement plane and the planeof the sensor output). It is a constant value, regardless of the mate-rial being tested. It depends on the materials (Kapton, copper, con-stantan) which constitute the sensor, and can be calculated fromthe properties of these materials and the geometry of the sensor:Cf = 650 J m�2 K�1.
2.3. The contact resistance
Although the heat flux sensor and the composite plate havevery smooth surfaces, there are imperfections and defects whichcreate a thermal contact resistance Rc (which relates the tempera-ture drop between the contacting surfaces to the flux density).However, the nature of the surfaces suggests that this resistancewill be small. The following transfer matrix is associated with thisresistance [33]:
1 Rc
0 1
� �ð3Þ
Usually, in different methods, the contact resistance is neglected,or its value is fixed at a nominal value in the model. In our case, thiscontact resistance can be identified as part of the parameter fitting;this has the huge advantage that the value of the contact resistancecan be compared with an expected value to help validate the otherparameters obtained from the fitting process.
2.4. Composite Reinforcement Plate (CFRP)
This plate is considered in this work to be a homogeneous med-ium. The general form of the transfer matrix associated with ahomogeneous medium can be written as:
ch lffiffiffiffiffijxa
q� �1
bffiffipp � sh l
ffiffiffiffiffijxa
q� �bffiffiffiffiffiffijx
p� sh l
ffiffiffiffiffijxa
q� �ch l
ffiffiffiffiffijxa
q� �264
375 ð4Þ
where a is the thermal diffusivity (m2/s) of the plate, b the thermaleffusivity (J/K m2 s1/2) and l the thickness of the plate (m).
2.5. The bonding layer
Characterization of this layer is the aim of the measurement.The layer consists of a portion of glue (30 SikaDur Epoxy resin),with vacuum (air) when the bonding is imperfect. To evaluatethe bonding quality, we have introduced a coefficient a (‘‘occu-pancy ratio’’), related to the volume ratio of glue in the spacebetween the reinforcing plate and the concrete block. This coeffi-cient a can vary between 0 and 1. a approaches 0 when there isno glue (i.e., when the medium is an air gap), and a tends to 1 whenthe space between CFRP and the concrete is filled with glue. Anal-ysis of the thermal impedance measurement will yield an estimateof the equivalent thermal resistance Ra (m2 K W�1) of the bondinglayer. The value of this resistance will be between the resistancesof a layer of air and of a thickness ‘‘e’’ filled with glue, and can beinterpreted as a ‘‘ratio of bonding’’ or an ‘‘occupancy ratio’’, makingthe assumption that the glue region and the air region are bothparallel to the composite surface. This configuration is obviouslyunrealistic, but the parameter a (as defined in Eq. (5)) is still a use-ful way to translate the quality of the bond into a number between0 and 1 (Fig. 2):
1Ra¼ 1
Rglueþ 1
Rair¼ kglue
a � eþkair
ð1� aÞ � e ð5Þ
This can be written as:
Ra ¼a � ð1� aÞ:ðkglue þ kairÞ
ðkglue � kairÞ � ð1� aÞ � kglue þ a � kair� � ð6Þ
We can introduce the corresponding thermal capacity Ca
Ca ¼ a � ðqcÞglue þ ð1� aÞ � ðqcÞair ð7Þ
Fig. 3. Modulus sensitivity to parameters as a function of frequency, case of ‘‘goodbonding’’.
Fig. 4.1. Evolution of the sensitivity to the occupancy ratio of the glue a dependingon the sensitivity to the thermal effusivity b.
Fig. 4.2. Evolution of the sensitivity to the resistance Rc depending on thesensitivity to the thermal effusivity b.
C. Alexis et al. / Composites: Part B 69 (2015) 350–358 353
The sensitivity analysis presented below shows that the value ofthe capacitance Ca has very little influence on exchanges in the sys-tem. The value of a may be identified from the thermal resistanceRa of the layer of glue. The value of the associated capacity Ca maynot be particularly accurate, but its value does not take part in theexchanges.
From the general relationship (as shown in the matrix for thehomogeneous medium) and the above equations, we can writethe matrix for the glue and air layer as follows:
chffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa � Ra � jx
p� � ffiffiffiffiRapffiffiffiffiffiffiffiffiffi
Ca �jxp � sh e �
ffiffiffiffijxa
q ffiffiffiffiffiffiffiffiffiCa �jxp ffiffiffiffi
Rap � sh e �
ffiffiffiffijxa
q ch
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa � Ra � jx
p� �26664
37775 ð8Þ
2.6. Concrete
The short test time allows us to treat the concrete as a semi-infinite medium. The thermal behavior of the concrete is character-ized by its impedance, Zc.
ZcðjxÞ ¼1
bffiffiffiffiffiffijx
p ð9Þ
The parameter b (J m�2 s�1/2 K�1) represents the thermal effu-sivity of the material, in other words the material’s ability toabsorb heat from a medium in contact with it, especially via tran-sient exchanges.
2.7. Overall impedance of the system (sensor + contact + compositeplate + glue/vacuum + concrete)
To determine the overall impedance, it is necessary to deter-mine the transfer matrix of the assembly sensor/contact/compositeplate/bonding layer and consider a semi-infinite boundary condi-tion imposed by the layer of concrete. The transfer matrix (M1) ofthe assembly is the result of the product of all the individual matri-ces characterizing each part:
½M1� ¼1 0
jxCf 1
� ��
1 Rc
0 1
� ��
ch lffiffiffiffijxa
q 1
bffiffiffiffijxp � sh l
ffiffiffiffijxa
q
bffiffiffiffiffiffijx
p� sh l
ffiffiffiffijxa
q ch l
ffiffiffiffijxa
q 26664
37775� . . .
. . .�ch
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa �Ra � jx
p� � ffiffiffiffiRapffiffiffiffiffiffiffiffiffi
Ca �jxp � sh e �
ffiffiffiffijxa
q ffiffiffiffiffiffiffiffiffiCa �jxp ffiffiffiffi
Rap � sh e �
ffiffiffiffijxa
q ch
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa �Ra � jx
p� �26664
37775
ð10ÞThe state vector heðjxÞ
/eðjxÞ
� �determined in the plane of tempera-
ture measurement can be written as:
heðjxÞ/eðjxÞ
� �¼ ½M1� �
1bffiffiffiffijxp � /sðjxÞ
/sðjxÞ
" #ð11Þ
where /e is the heat flux entering the system and he is the temper-ature response; we can take (/e, he) as the input parameters of sucha system. /s, is the heat flux entering the concrete slab.
This allows us to write a general expression for the inputimpedance:
ZeðjxÞ ¼heðjxÞ/eðjxÞ
ð12Þ
The impedance as described thus depends on three parametersthat must be identified (given that the thermal capacity of the sen-sor Cf is known):
354 C. Alexis et al. / Composites: Part B 69 (2015) 350–358
� The occupancy rate a which sets the values of the heat capacityCa and the thermal resistance Ra of the bonding layer.� The concrete effusivity b.� The contact resistance Rc between the sensor and the composite
plate.
3. Analysis of sensitivity of impedance to thermophysicalparameters
The objective of such a sensitivity study is to define the influ-ence of each system parameter on the impedance, and to optimizethe choice of frequency range to be used for identification purposes[34,35].
In an inverse technique procedure, thermophysical parametersare estimated by seeking the grouping in which the experimentalimpedance is best approximated by the theoretical impedance.The possibility of simultaneously identifying these parameterscan be discussed on the basis of a sensitivity study. This is per-formed by observing variations in the given function when sub-jected to a change in one of the parameters. Thus, the analysistakes into account the range of variation and finds the conditionswhere the quantities can be identified individually. In this way,the frequency range under study can be optimized as a functionof the required objectives. It may even enable the model to be sim-plified, if certain parameters are shown to have negligible influenceon the observation range.
The impedance is a complex function of frequency. The sensitiv-ity of the magnitude of the impedance to various parameters isstudied. The sensitivity functions Spi of the modulus of Z to theparameters pi are defined by:
SZ;piðf Þ ¼ DZ=Z
Dpi=pið13Þ
In this expression, Z represents the modulus of the impedance.To simplify interpretation over a wide frequency range, the ratio ofthe relative variations in the parameters to the response function iscalculated and expressed as a percentage of the tested function.Since SZ,p is defined as a ratio of two non-dimensional functions,it follows that it is also non-dimensional.
The calculation, numerically obtained, involves introducingnominal values of the various parameters. Introduction of thesevalues does not detract from the aim of determining these param-eters, because the sensitivity functions are only used qualitatively.They highlight which parameters most affect the behavior of theresponse function. Any correlations, or situations where an effectcould be attributed to a change in more than one parameter, arealso identified. The sensitivity analysis can allow us to choose opti-mum frequency ranges for identifying the main parameters. Only arough estimate of the parameter values is needed to determine thesensitivity functions. In this case, the following values were cho-sen: Cf = 650 J m�2 K�1 for the sensor capacity, Rc = 3 � 10�3 K m2 -W�1, b = 2000 J m�2 s�1/2 K�1 for the thermal effusivity of thematerial and a = 1 for a fully glued layer (which corresponds toR = 0.01 m2 K W�1 for the resistance of the glue, andC = 2930 J K�1 for the thermal capacity of the glue (becauseC = e�q�c). This situation represents the maximum value of the ther-mal conductivity of the glue layer.
Fig. 3 shows the sensitivity of the modulus to each parameter.The fully glued case with no air gap is presented; because thepoorly-bonded case (with a low value of a) leads to the same con-clusions. This example shows that the best choice of frequencyrange is between �10�4 Hz and some 10�2 Hz, because, in thisrange, the impedance is highly sensitive to the occupancy ratio ofthe glue, a, and to the thermal effusivity of the material. It shouldalso be noted that there is high sensitivity to contact resistance Rc
(which increases with frequency), and that none of the sensitivitycurves for b, Rc or a are proportional, indicating that the sensitivi-ties are not correlated in the frequency range studied, and thus allthree parameters can be determined simultaneously. It should benoted that the sensitivity to the sensor capacity is much lower,as expected.
If we took a lower frequency range, we could neglect the Rc
parameter; however, in such a case, the experimental time wouldbe much longer, and this may mean that the exchanges are nolonger unidirectional.
Finally, to check if parameters b, Rc or a are correlated, the sen-sitivities to Rc and to a are plotted as a function of sensitivity to b. Ifthose parameters are correlated, the graph will be a line goingthrough the origin.
Both sensitivities are non-dimensional and expressed as per-centages in Figs. 4.1 and 4.2.
Correlations were not found, so in this study, parameters b, Rc
and a can be simultaneously determined in the studied area, fromthe same test.
4. Determination of thermophysical parameters of system
4.1. Calculation of experimental impedance
The principle involves considering the material as a linear sys-tem that does not vary in time [36]. It may be assumed that, whenthe system is excited by a flux stress, its response is a change insurface temperature. The analogue signal of temperature T(t),observed at a constant rate, may be represented by the series{T(1); T(2); T(3);. . .; T(p)}.
It is assumed that this signal is the response to an excitation offlux F(t). F(t) is also observed at discrete times {F(1); F(2); F(3);. . .;F(p)}.
The samples of the two signals may be linked by the followinglinear relationship:
a0TðkÞþa1Tðk�1Þþ���þapTðk�pÞ¼b0FðkÞþb1Fðk�1Þþ���þbqFðk�qÞð14Þ
This equation constitutes a discrete linear model of order (p, q).It expresses the fact that the value of function T at a given instantdepends on the past and present excitation values F and on theprevious values of T. Normalizing with respect to a0 gives thecommonly used expression:
TðkÞ ¼ �Xp
i¼1
aiTðk� iÞ þXq
i¼0
biFðk� iÞ ð15Þ
Hence h(z) is the z transform of the sequence T(k) and /(z) thatof F(k):
hðzÞ ¼Xþ1
k¼�1TðkÞz�k ð16Þ
/ðzÞ ¼Xþ1
k¼�1FðkÞz�k ð17Þ
From the time-dependent equation linking the input and theoutput signals of the linear system, an equivalent equation maybe written connecting the various z transforms:
hðzÞ þ a1z�1hðzÞ þ � � � þ apz�phðzÞ¼ b0/ðzÞ þ b1z�1/ðzÞ þ � � � þ bqz�q/ðzÞ ð18Þ
or:
hðzÞ/ðzÞ ¼
b0 þ b1z�1 þ � � � þ bqz�q
1þ a1z�1 þ � � � þ apz�p ¼ ZðzÞ ð19Þ
Z(z) is referred to as the z transfer function of the discrete-time lin-ear system.
Fig. 5. Schematic of the tested experimental sample.
TabMa
C. Alexis et al. / Composites: Part B 69 (2015) 350–358 355
Assuming z = ejxTe (where Te represents the sampling rate)brings the calculation back into the Fourier domain, where theexperimental impedance of the system Z(f) is obtained.
Parameters ai and bj are determined by a least-squares errorestimation procedure that involves adopting the group that mini-mizes the magnitude of the difference between each value of theoutput signal and the predicted value of the impedance.
The thermophysical parameters of the system are estimated byfitting the theoretical model to the experimental impedance. Thetheoretical impedance is a non-linear function of the thermophys-ical parameters and frequency. It is adjusted using an iterative pro-cedure based on minimizing a least-squares error criterion (byfinding a minimum of the sum of squares of the differencesbetween the functions described).
5. Experiments
5.1. Sample characteristics
The constituents of the concrete used for the preparation of thespecimens consisted of ordinary Portland cement, sand with amaximum diameter of 5 mm, coarse aggregate with a maximumdiameter of 15 mm and tap water. The 28 days axial compressivestrength of concrete was found to be 24.0 MPa on an average.
le 1in characteristics and thermal properties of elements making up the sample.
Material Thickness e (m) k (W m�1 K�1) q (kg m�3)
SIKA Carbodur 1012 (CFRP) 1.2 � 10�3 0.7 1530Epoxy resin (glue) Sikadur 30 2 � 10�3 0.2 1200Air (defect) 2 � 10�3 0.026 1.184Cement concrete block Semi-1 1.8 2300
Fig. 6. Defect detection experimental dev
A sample (Fig. 5) was reinforced with CFRP plate (SIKA Carbodur1012), externally bonded onto a concrete block (50 � 50 � 4 cm).The CFRP plate was bonded by the manufacturer-specified glue(Epoxy resin Sikadur 30). An artificial defect was created on theconcrete block when gluing the laminates. This defect was an airgap, 5 � 5 cm in size. The implementation of the glue has beenmade with a ‘‘new’’ glue and according to the procedure indicatedby the SIKA specifications.
The main characteristics of the CFRP plates (Carbodur 1012),epoxy resin and cement concrete used are reported in Table 1, whichalso reports their thermal properties (and those of an air gap).
5.2. Active infrared thermography detection
NDT techniques using infrared thermography typically fall intothree categories: pulsed thermography [37], modulated thermog-raphy [38] and pulsed phase thermography [39], the latter beingoften presented as a further development of the first two tech-niques. These NDT techniques are distinguished by different ther-mal stresses and different data processing.
In this experiment, infrared thermography is not used as a tech-nique for quantitative NDT, but as a tool for defect viewing. So,unlike conventional NDT using infrared thermography, it is notnecessary here to control the amplitude or duration of the thermalexcitation signals. The thermal energy emitted by halogen lampsserves only to slightly heat the reinforcing plate. Under these con-ditions, the thermal resistance created by an air gap or bondingfailure causes a higher temperature rise.
The test specimen is placed in front of the infrared camera and aframe fitted with two halogen lamps. This is set at a distance of 2meters from the sample, to provide a field of view which allowsobservation of the whole specimen. The thermal stress is appliedby means of two halogen lamps with a maximum power of 1 kWeach. The infrared camera used in this experiment is a CEDIP Silver220 cooled camera equipped with a matrix of 320 � 256 InSbdetector elements which are sensitive to medium waves, 3–5 lm. It has a typical NETD of 20 mK and allows acquisition as afunction of time.
c (J kg�1 K�1) a (m�2 s�1) b (J K�1 m�2 s�1/2) R (m2 K W�1)
840 5.44 � 10�7 948 1.71 � 10�3
1220 1.36 � 10�7 541 1 � 10�2
1000 2.2 � 10�5 5.55 7.69 � 10�2
920 8.5 � 10�7 1952
ice and experimental results.
Fig. 7. Experimental set-up for thermal characterization of the defective bond.
Fig. 8. Data flowchart showing the parameter estimation procedure.
356 C. Alexis et al. / Composites: Part B 69 (2015) 350–358
The first part of the experimental test bench is shown in Fig. 6.The excitation signal is a rectangular function of 15 s duration.
Fig. 6 shows the changes in average temperatures of two zones.One (green curve)1 is at a healthy area and the other area (red)has a bonding failure. After 30 s the temperature difference betweenhealthy and defective areas can reach nearly 3 �C.
This defect detection made using the infrared camera is a firststep, and helps locate potential poorly-bonded areas.
5.3. Experimental set-up
Fig. 7 shows the experimental set-up for thermal characteriza-tion of the defective areas. The objective is to apply a thermalstress onto the surface by means of electrical resistance. To ensurethat the majority of the energy is dissipated through the fluxmeterand reaches the material, thermal insulation is placed above theresistance. The thermal flux and temperature values are recordedby the fluxmeter, located between the resistance and the samplesurface. The unidirectionality of transfer is ensured by an insulat-ing belt surrounding the material studied; the sensor is equippedwith a guard ring and its surface is less sensitive than that of thesample. Edge effects are avoided. The weight placed on the devicekeeps everything in a stable position and reduces contact resis-tance. The thermal stress imposed by the heater is based on apseudo-random binary signal generated by the computer.
We tested our experimental device on two separate cases:
1 For interpretation of color in Fig. 6, the reader is referred to the web version ofthis article.
� On an area where we had previously identified a lack of bondingusing the infrared camera (position 1 in Fig. 7).� On a fully bonded, sound area (position 2 in Fig. 7).� The heat flux values and temperature readings will allow us to
calculate the transfer function for each of the positions (thedefective and sound areas).
5.4. Excitation signal
This method has the advantage of not requiring any control ofthe boundary conditions and being able to exploit random signals.It is perfectly suited to in situ use under any disturbance condi-tions. The experiments for the present study were performed inthe laboratory and stress was imposed on the system by dissipat-ing heat through a flat resistance. Pseudo-random binary signals(PRBS) were used. This type of signal has the advantage of selectingand exciting a wide spectral band, while limiting the amount ofenergy introduced into the system.
5.5. Experimental results
As described previously, the z transfer function is determined.The impedance obtained is represented in Fig. 9 in the form of agraph showing the moduli of the impedance plotted as a functionof the frequency. Thermophysical parameters (b, Rc, a) are thusdetermined by adopting the group that minimizes the differencebetween each value of the output signal and the predicted value,following the flowchart shown in Fig. 8.
Fig. 9. Comparison of the experimental impedance moduli and the values found bythe theoretical study.
Tabl
e2
Ther
mop
hysi
cal
para
met
ers
deri
ved
from
the
theo
reti
cal
stud
y.
Sam
ple
wel
lad
her
ed(g
lue)
Inte
rmed
iate
stat
e:sa
mpl
ew
ith
defe
ct(a
ir)
clos
eto
5%In
term
edia
test
ate:
sam
ple
wit
hde
fect
(air
)cl
ose
to15
%Sa
mpl
ew
ith
defe
ct(a
ir)
clos
eto
100%
Un
its
Val
ues
resu
ltin
gfr
omth
eop
tim
izat
ion
Val
ues
from
labo
rato
ryte
stin
gV
alu
esre
sult
ing
from
the
opti
miz
atio
nV
alu
esfr
omla
bora
tory
test
ing
Val
ues
resu
ltin
gfr
omth
eop
tim
izat
ion
Val
ues
from
labo
rato
ryte
stin
gV
alu
esre
sult
ing
from
the
opti
miz
atio
n
Val
ues
from
labo
rato
ryte
stin
g
Con
tact
resi
stan
ce(R
c)K
m2
W�
10.
005
a0.
012
a0.
008
a0.
017
a
Occ
upa
ncy
rati
o(a
)0.
99a
0.94
a0.
85a
0.02
3a
Ther
mal
effu
sivi
tyof
the
con
cret
e(b
)Jm
�2
s�1
/2K�
12.
04�
103
1.95�
103
2.08�
103
1.95�
103
2.1�
103
1.95�
103
2.09�
103
1.95�
103
Dev
iati
onin
(b)
%4.
616.
677.
697.
18
aN
ote:
itis
impo
ssib
leto
dete
rmin
eth
ese
para
met
ers
from
ala
bora
tory
test
.
C. Alexis et al. / Composites: Part B 69 (2015) 350–358 357
The fitting of the optimized theoretical impedance with themeasured impedance (obtained from the temperature and fluxusing the z transfer function), gave the parameter values shownin Table 2.
The standard deviation of the estimated values of the thermaleffusivity b compared to the value of the thermal effusivity b fromthe laboratory study ([NF-EN 12664]) is quite low (<8%). We alsonote that the occupancy ratio is close to 1 when the reinforcingplate is well bonded, and close to zero in the case of the air gap.The method seems to be able to define quantitatively the bondquality of a concrete block reinforced with CFRP.
6. Conclusions
As we described previously, CFRP plates are used to reinforcecivil engineering structures. Infrared thermography is one of thetechniques currently used to examine bonding of phases in com-posites in situ, and more often as a qualitative analysis techniqueto locate defects. Here we have coupled infrared thermographywith thermal impedance characterization which allows us to esti-mate the occupancy ratio of the interface (a, which varies from 1for a fully glued interface to 0 for a complete void). This then pro-vides a numerical indication of the quality of the bond between thecarbon fiber plate and concrete block.
The method is easily applied on site, and quickly provides anestimate of the degree of bonding of reinforcing plates to a struc-ture. Indeed, the method developed does not require control ofboundary conditions; it can calculate the contact resistance andCFRP have the advantage of having a good surface (smooth). Theonly restriction for in situ application is in the first approach byinfrared thermography (not too much wind, rain, etc.).
In the future, various defects with different a values could becreated and thermally characterized. The thermal characterizationcould potentially be correlated with the mechanical properties ofthese assemblies. If a strong correlation was demonstrated, ther-mal characterization could then provide an indirect estimate ofthe mechanical effectiveness of the CFRP reinforcement.
Acknowledgement
The writers appreciate the financial support partially byNational Research Foundation of Korea Government (NRF-2014R1A2A2A01007883).
358 C. Alexis et al. / Composites: Part B 69 (2015) 350–358
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