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Healing performance on concrete under mode I fracture byultrasonic testing using embedded transducers, acoustic emission
and digital image correlation method
E. Tsangouri1,3, G. Karaiskos2, D. G.Aggelis1, D. Van Hemerlijck1, and A. Deraemaeker2
1Department of Mechanics of Materials of Constructions (MeMC) , Vrije Universiteit Brussel (VUB),Pleinlaan 2, B-1050 Brussels, Belgium,
2Architecture and Town Planning (BATir) , Universite Libre de Bruxelles (ULB)-Building, , 50 avF.D. Roosevelt, CP 194/02, B-1050 Brussels, Belgium ,
3SIM vzw , Technologiepark 935, BE-9052 Zwijnaarde, Belgium,
email:[email protected],[email protected],[email protected],[email protected],[email protected]
ABSTRACT: Crack formation is the main cause of concrete mechanical properties degradation. Autonomous self-healing(i.e.SH) encapsulation systems promise material recovery by filling the cracks with healing agent (providing healing andsealing). The cracking of concrete elements is experimentally investigated by mode-I failure tests in small-dimensionedconcrete beams subjected to three-point bending tests. The perplexing fracture process phenomena require an advancedmonitoring system. In this study, an optical method (i.e. Digital Image Correlation- DIC), Acoustic Emission (i.e. AE) andan ultrasonic wave propagation technique based on embedded piezoelectric transducers, are combined in order to evaluatethe crack formation and opening of three concrete beams in which embedded SH encapsulated system is applied. Theloading is applied in two stages. The initial loading is responsible for the crack initiation and formation as well as for theSH capsules breakage. After the healing agent is released and dried, the concrete samples are reloaded. The DIC techniquecalculates the crack opening and visualizes the crack formation and propagation path. In parallel, the features extracted bythe AE technique quantify the fracture process at different loading stages and detect the regions in which healing occursby indicating the SH capsule breakage. Finally, a few couples of low-cost PZT transducers based on the concept of SmartAggregates (i.e. SMAGs) are embedded in the concrete beams for the estimation of the state of damage. Incorporationof the aforementioned fracture analysis provides a full-field view of concrete failure performance and is the keystone tocharacterize healing performance.
KEYWORDS: concrete healing, encapsulated agent, release, acoustic emission, digital image correlation, smart aggregates
1 INTRODUCTION
Traditionally, epoxy and cementitious agent manual injec-tion into cracks of concrete elements provides local repairand assures material restoration [1]. Nowadays, the concreterepairing technologies pursues to substitute the manual in-jection process by encapsulation of the retrieval compounds.
Damage during service life is restricted when brittle cap-sules carrying adhesive material embedded into concretebreak as cracking occurs and release their filling. The crackplane is filled by a durable and resistant recovering mean
and the damaged region regains its original qualities. Theaforementioned pioneering approach is well-established asself-healing attribute [2].
In the last few years, the research of concrete is focused onthe performance of embedded into structure tubular carri-ers some centimetres long and a few millimetres thick filledwith healing chemical compounds [3]. Within this frame-work, our research deals with the healing efficiency of two-component expansive polyurethane agent placed into glasstubes and activated as cracking occurs (Figure 1). Heal-ing recovery is experimentally estimated by bending frac-
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014Porto, Portugal, 30 June - 2 July 2014
A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.)ISSN: 2311-9020; ISBN: 978-972-752-165-4
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ture tests in concrete beams. Damage is introduced by crackopening at a pre-notched specimen leading to capsules rup-ture and healing process activation (Figure 2). Followingthe agent curing, the crack is re-opened under bending andthe resistance to fracture performance measured is indica-tive of healing [4]. To date, the bending load response wasused as a quantitative healing criterion, as drawn in Figure2. However, the filling of the crack by healing agent andthe mechanisms of the fracture resistance at the healed re-gions remained unknown [5]. In this direction, the challeng-ing monitoring of the healing performance is discussed inthis study. More specific, combination of advanced opticaland acoustic experimental techniques contribute providingan overview of concrete damage reparation (Figure 3).
Figure 1: Concrete casting: Two-component capsules at-tached on plastic wires fixed into the wooden mould are pre-sented. Teflon slice used to form the pre-notch is shown atthe bottom of the mould. Embedded SMAG transducers arealso placed facing the pre-cracked notch.
2 EXPERIMENTAL STUDY OF THE PROPOSEDHEALING CONFIGURATION
2.1 Two-cycle three-point bending tests
A bending beam testing set-up is chosen to form a singlecrack initiating from a pre-cracked zone at the bottom ofthe specimen. Unreinforced concrete beams with dimen-sions of 840x100x100 mm are tested under slow speed (0.04mm/min) displacement control. The testing set-up followsthe Rilem Technical Report FMS-50 and is schematicallyshown in Figure 2. The recommended testing is formulatedto enlighten fracture response and determine the dominantdamage features [6]. In bending, crack initiates from thepre-cracked region and reaches the top of the beam. Thetesting is stopped when crack opening of 0.3 mm is reached.As damage occurs, capsules standing at the crack path breakand the healing agent is released. After a 24 hours agent cur-ing period of the healing agent, reloading of the specimens is
load
time/crack opening/deflection time/crack opening/deflection
load
loading cycle: crack formation
capsule breakage
reloading cycle: healed crack
reopening
0,04 mm/min
800 mm
100 mm
strength regain stiffness regain
24 hours of curing
Figure 2: Rilem FMC Technical Report recommendationson specimen geometry. The simplified drawings present theloading and reloading stages of testing and an overview ofthe potential healing response in terms of strength and stiff-ness recovery is schematically shown.
performed in order to assess potential regain of the originalstrength and stiffness due to healing.
2.2 Acoustic Emission damage capturingAcoustic Emission (AE) is a sound capturing technique inwhich PZT sensors placed at surface receive sound wavesproduced when the specimen undergoes fracture deforma-tion. Sound energy released travel through the material andreach the specimen surface. The sensors receive the soundwave transfer the information to a digitized software. Plac-ing four or more sensors provides three-dimensional con-tinuous monitoring of fracture evolution by means of AEwaveform hits. AE activity and signal features (amplitude,energy, duration, frequencies, etc.) are the main damage in-formation source [7].A series of eight AE sensors are attached to the concretesurface (Figure 3), distributed at all different sides of thebeam (all sensors are located at a distance smaller than 50mm from the pre-notched plane in order to eliminate disper-sion/attenuation effects). By introducing the attenuation andthe sound velocity (3800 m/sec) characteristics of concretea volumetric -3D location- analysis provides the AE sourcelocation and the sensors AE hits features[8]. Scattering ofAE hits energy captured during loading and the activity ofAE features as bending occurs is discussed in this research.
2.3 Ultrasonic testing using Smart AggregatesThe ultrasonic wave propagation technique used in thepresent study is based on embedded piezoelectric transduc-ers which follow the concept of low-cost smart aggregates -SMAGs. A high voltage short pulse excitation is imposed
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Figure 3: DIC speckle pattern is shown at the middle of theconcrete specimen. The CCD cameras setting, fixed on ametallic tripod, face the grey coloured area surrounding thepre-crack. AE sensors attached on magnetic base are at-tached on the surface at different sides of the beam. AEsensors signal is amplified by pre-amplifiers devices, shownat the figure, before reaching the AE digital board.
at one of them (i.e. transmitter) and then a compressivewave (P-wave) is generated. The wave is received by theremaining SMAGs (i.e. receivers) after travelling throughthe concrete. The wave features (e.g. travelling time) canbe used to deduce the mechanical properties of the concreteand to build a damage indicator representative of the evo-lution of damage between the transmitter and the receiver[9, 10]. Two series of SMAGs are embedded symmetricallyat both sides of the pre-cracked cross section with a betweendistance of 100 mm, as shown in Figure 4. Two sensors fixedat height of 20 mm from the bottom of the beam (series A).Another couple of sensors is placed higher, at the level of 50mm from the bottom of the beam (series B).
2.4 Optical monitoring by Digital Image Cor-relation
Digital Image Correlation (DIC) is a well-known non-contact full field optical technique that computes the dis-placement fields at the specimen surface. Two CCD cam-eras are placed facing a grey speckle-patterned area and pe-riodically capture high resolution images. Each crack de-formation obtained as variation at the pixels grey intensity.The principles of DIC displacement fields determination arewell-established in literature. Sequentially, strain fields areobtained by the derivatives of displacements. DIC is chosenin order to provide a continuous monitoring of crack ini-tial formation, closure and reopening during bending. Colormapping images visualise the healing resistance by meansof cracking fracture [11].The stereoscopic cameras system is attached on a tripodand face the speckled pattern area (100x100mm) at thenotched middle area of the beam. The well-focused and cal-
Aco
ustic
Em
issi
on sound velocity 3800 m/sec
attenuation 0.5 dB/m
sensor type R15 (5 sensors), R6 (3 senosrs)
location set-up 3D location
Dig
ital I
mag
e C
orre
latio
n
camera type CCD
camera resolution 2064pxs x 2506pxs
size of area of interest 100 mm x 100 mm
rate of capturing each 3 seconds
subset, step 21 pixels, 6 pixels
strain filter window 15 pixels
Smar
t A
ggre
gate
s
pulse wave
high voltage short rectangular shape
800 V/ 2.5 secs
SMAGs series B
capsules
100 mm
100 mm
SMAGs series A
Figure-Table 4: Experimental parameters chosen at the set-ting of the methods applied
ibrated DIC cameras set-up captures highly accurate images(20 px/mm) every 3 seconds during testing. Those imagesare analysed by Vic-3D software (analysis characteristics:subset=21, step=6, strain window size=15) and the full-fieldpatterns of deformation (measure in mm) and strain (mea-sure in %) are well derived. At this paper the fields ey ofstrain at the direction perpendicular to the loading are pre-sented. The εy color map chosen indicates the fracture zoneformed around the crack at the range of 0 to 3 % [12].An overall view of different AE, DIC, SMAG testing fea-tures are presented at Table and Figure 4.
3 EXPERIMENTAL ANALYSIS - RESULTS
3.1 Two-cycles loading response
Encapsulated healing material is embedded in a series ofthree specimens. The healing response is evaluated by com-paring the healed series with another series of reference con-crete beams (healing agent is not introduced) [13]. In thispaper, due to space limitations required, the results fromonly one healed concrete beam are presented. The speci-men is chosen since the healing efficiency is representativeof material performance. In particular, it worths mention-ing that 63 % and 85 % recovery of loading strength andstiffness respectively is achieved. At the following sessions,the healing activity is considered with AE, DIC and SMAGmethods at a loading time base. In other words, as shown inFigure 5, time series plots of load performance in two-cyclesof testing (loading and reloading case) are studied.The bending performance is the typical tension softeningcase in plain concrete. Linear loading initial part indicates
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0
0,5
1
1,5
2
2,5
0 500 1000 1500
load
(kN
)
loading time (sec)
loading
reloading
d c
b
a
d c
b a
Figure 5: Bending performance of the concrete beam asmeasured during loading time. The time based analysismonitors load tension softening phenomenon. The differ-ent stages of crack initiation, formation and propagation areindicated in the figure and are discussed at the following ses-sions
the material stiffness. As crack forms initiating from the pre-cracked region, a plateau around the load peak representsthe material resistance to crack propagation. The post-peakloading stage defines the extended crack opening and leadsto bending failure. In both cycles of loading in Figure 5 thetension softening behaviour is obtained indicating that thehealing partial strength and stiffness recovery is provided.
3.2 Monitoring of healing activity by AcousticEmission
Hits energy is the essential AE feature to differentiate thebending concrete fracture from the capsules breakage. Asshown in Figure 7 the aforementioned AE hits clusteringspecifies the time frame into which capsules healing releaseis activated. Previously, it has been proven that brittle and in-stant capsules rupture release high amount of sound energycaptured by AE sensors.By filtering out the AE hits originating from capsules break-age, the cumulative AE energy during bending representsthe material healing performance. More specific, as shownif Figure 7, the cumulative energy due to concrete cracking
0
1000
2000
3000
4000
0 200 400 600 800 1000 1200
AE
ener
gy
time (sec)
loading reloading
cracking
capsule breakage
Figure 6: Scattering plot of AE hits as captured during load-ing (black font) and reloading (grey font) stage. The hitsemitted due to capsule rupture are differentiated from thehits due to bending fracture. The clusters of AE hits obtainedare used to filter out the loading response of the healed ma-terial.
at the loading stage is compared to the energy of crack re-opening at the reloading stage. As presented in the figure,the initial rate of energy increase corresponds to the linearload rise prior to crack formation and is indicative of mate-rial stiffness response.In both loading stages, the slope of cumulative energy plot inFigure 7, changes as crack forms. The rate of AE energy in-crease remains similar in both cases substantiating the claimthat due to healing recovery crack reforms and a delay ofcrack propagation appears.AE provides a qualitative overview of healing activity. Re-grettably, the crack plane region covered by the agent is notspecified and the healed areas are not localised.
3.3 Digital image correlation monitoring ofcrack resistance due to healing
Linear loading response of concrete is followed by inter-nal micro-cracking and plastic deformation until the peakof loading in the case of healthy (formerly uncracked) spec-imens, as presented in Figure 5. Full-field DIC εy strainfields detect micro-cracking fracture initiation at the end ofthe linear loading stage and locates crack formation up to theload peak. The a to d labels shown in Figure 8 correspondsto the number indicator in Figure 5, since the strain fields areobtained from the respective loading stage.Healing of concrete can be deduced from the fact that thefracture resistance up to the peak-load is visualized by DICin both the loading and reloading case. By ensuring the samecolor map range (0-0.3 % of strain) the extent of fracture andthe damage recovered zone can be located. DIC confirms anoverall crack healing in concrete by global full-field moni-
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0
20000
40000
60000
80000
0 500 1000 1500
cum
ulat
ive A
E en
ergy
loading time (secs)
loading reloading
stiffness regain resistance
to crack formation
Figure 7: AE hits energy activity during loading and reload-ing stage. The cumulative plot of energy received by AEsensors is indicative of damage initiation, formation and ex-tension till failure. The variation at the rate of energy ariseprovides the limits between the fracture stages occurred.
toring of strain distribution.
3.4 Results obtained with Smart AggregatesIn figures 9 and 10 the early part of the signal received atseveral testing moments during loading and reloading arerespectively presented. The aim of this plotting is to capturethe potential stiffness recovery and the filling of the capsuleby healing material. It is chosen to investigate the wave ar-rival features of series A since the crack is expected to re-open at the region above the pre-crack zone during reload-ing stage of testing. The pulse is released from one of lowerSMAG (series A) and received by the opposite SMAGs.Figure 9 combines the early part of the signals received indifferent loading stages: 0 kN corresponding to bending startpoint, 2.5 kN regarding the maximum bending load obtainedand several post-peak loading stages from 2 kN up to 0.5kN (end of loading stage). The degradation of material me-chanical response is visualized by the amplitude decrease ascrack propagation occurs. The damage formed in betweenthe pulse transmitter and the receiver is qualitatively cap-tured. The recovery of the pulse signal follows when, afterhealing curing, the reloading stage initiates. As shown in
b a
a b
c d
c d
loading stage
reloading stage
3 %
0 %
ey
Figure 8: DIC strain fields as visualized in different loadingstages. The number indicators of the figures correspond tothe bending load shown in figure 5. The fracture is progres-sively formed and is given by the color levels.
Figure 9: Early part of signal as received by SMAG sensorstanding at the opposite side of the cracked cross section atdifferent stages of loading
Figure 10 the amplitude of the new recorded signal at 0 kNis almost four times higher than the one received at the laststage of the loading test. The results obtained with SMAGscontribute to confirming that healing is successfully applied.Further information regarding healing efficiency is obtainedwhen the damage index is plotted at a loading base. Thedaage index is defined as the RMSD value between thehealthy signal and a damaged signal computed in thetime window corresponding to the first half-period of theundamaged signal . The results regarding the reloadingresponse shown in Figure 11, relatively calculated to thedamaged state before loading, is used as a reference signalof the following analysis. The evolution of the damageindex can be decomposed in three phases: initially thedamage index value is low (non-zero due to certain noiselevel in the signals) indicating the material healthiness,the second phase in which the damage index increasescorresponds to the crack formation leading to the thirdstage of loading where the damage index value is almostunitary due to severe crack opening damage. In Figure11, the damage index is obtained from the signals receivedfrom both SMAGs standing at one side of the cracked
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Figure 10: Early part of signal as received by SMAG sensorstanding at the opposite side of the cracked cross section atdifferent stages of reloading
transmitter:series A receiver:series A loading
transmitter:series A receiver:series B loading
transmitter:series A receiver:series A reloading
transmitter:series A receiver:series B reloading
Figure 11: Damage index evolution at loading and reloadingstage for different combinations of SMAG pulse releaser-receiver sensors
cross section. The variation at the damage slope is relatedto the SMAG height position. The delay of the damageindex arise of series B is justified since crack initiates fromthe pre-notch and propagates up to the top of the beam.During the reloading phase, the same three phases can beconfirmed. It is worth mentioning that the index valuescorresponding to the pair of SMAGs of series A grow firstlyand before the respective values of the other pair of SMAGswhere the receiver stands higher across the crack height).This observation concerns the healing agent presence onlyat the higher level of the cracked plane. In contrast, the highvalue of damage index calculated from the signals receivedat the bottom of the beam corresponds to the limited healingrecovery.
4 CONCLUSIONS - DISCUSSION
In this paper, three advanced monitoring techniques havebeen applied to monitor the efficiency of healing capsulesto repair cracks in concrete. The three techniques have beenfound to provide very useful and complementary informa-tion about the efficiency of the healing. In particular, it was
demonstrated that coupling DIC with Smart Aggregates wasan efficient mean to obtain local information, which cannotbe obtained using the classical global loading-strain curve.
5 ACKWNOLEDGEMENTS
The research described in this paper has been performed inthe frame of the SIM program on Engineered Self-HealingMaterials (SHE). The authors are grateful to SIM (StrategicInitiative Materials Flanders) and FNRS (Fonds de la Re-search Scientifique) for providing financial support.
6 REFERENCES
[1] G. Smoak, Guide to Concrete Repair, United StatesDepartment of Interior, Books for Business, 1997
[2] S.R. White, N.R.Sottos, P.H. Geubelle, J.S.Moore, M.R. Kessler, S.R. Sriram, E.N. Brown,S. Viswanathan, Autonomic healing of polymercomposites, Nature, Vol. 409, 2001
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[8] E. Tsangouri, K. Van Tittelboom, D. G. Aggelis andD. Van Hemelrijck, Acoustic Emission Detection andEvaluation of Crack Opening and Healing by use ofEmbedded Capsules and Recovery of Cracking Dam-age in Concrete, KIFA-6 and RILEM TC 239-MCM,Kumamoto, Japan, September 17-18, 2013
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