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An experimental Study on Acoustic Emission Energy andFracture Energy of Concrete

R. Vidya SagarDepartment of Civil Engineering, I.I.Sc., Bangalore-560 012, India

E-mail : rvsagar@civil.iisc.ernet.in

AbstractAn experimental study has been carried out using geometrically similar notched plain concrete three-point bend (TPB) specimens of differentsizes and the acoustic emissions (AE) are monitored during fracture process. In the present study, measurement of the AE energy releasedduring the fracture process of plain concrete beams is carried out. A monotonically increasing load applied on a series of concrete beamspecimens of different sizes with different strength. While testing concrete specimens for fracture energy, AE energy release was monitoredat the same time. An attempt has been made to relate AE energy and size independent specific fracture energy of concrete. The experimentalstudy show that it may be possible to get an idea about the fracture energy from the measured AE energy.

may be supposed that AE energy also should be sizedependent to fracture energy. In order to obtain a sizeindependent AEE the concept adopted earlier by previousresearchers is used in the present study [2-3]. Havingobtained the energies, fracture energy and AE energy, bothsize independently their ratio is obtained, so that only byknowing the AEE the size independent fracture energy canbe determined.

2. Experimental data analysis

The experimental data analysis consisted of calculatingfracture energy for each specimen using the RILEM methodof Hillerborg, using fictitious crack model and analyzing sizeindependent AE energy of concrete [10,5].

Acoustic emission activity is attributed to the rapidrelease of energy in a material, the energy content of theacoustic emission signal can be related to this energy release[7-9]. The true energy is directly proportional to the areaunder the acoustic emission waveform[7-9]. In the presentexperimental study it was considered that the energy of anelectrical signal is proportional to the square of the voltage,so in a simplified analysis it is necessary to square andintegrate the recorded voltage transients for each channel[8-9].

(1)

i = the recorded voltage transient V(t) of a channel

0t = the starting time of the voltage transient record

1t = the ending time of the voltage transient record

In the experimental study the authors obtained AEenergy by summing up of all the AE energy release values

1. Introduction

Because cement concrete is an important material,fracture and failure of concrete has been the subject ofwidespread research[1]. Much has been said in literatureabout the fracture energy of concrete and its importance.Many important structures like nuclear containment vesselscontain very thick concrete components of the order of ametre in size. Certainly size effect should be present in suchstructures. It is known that size effect lowers the tensilestrength and the material tends to be brittle [1]. Thereforefracture energy needs to be determined in order to assess thebrittleness of such structures. The fracture energy of concreteis a basic material property needed to understand fractureinitiation and propagation in concrete. Whether fractureenergy is size dependent or not is being discussed earlier byresearchers [1-4]. Strictly the fracture energy if taken as amaterial property should be constant, and should beindependent of the method of measurement, test methods,specimen shapes and sizes. Earlier researchers concludedthat fracture energy varies with the size and shape of the testspecimen [1]. Researchers studied the size effect in concreteand various models are proposed [1]. To a large extent theseresearch works are aimed at the explanation why the fractureparameters such as fracture energy, CMOD vary with thespecimen size [1]. In a few models, for example “local fractureenergy model” proposed by previous researchers dealt withthe possibility of constant fracture energy of concrete[2-3].One important point related to fracture process in concreteis it is necessary to understand the relationships betweenmicro structural phenomena and corresponding effects onmacroscopic behaviour. And also microstructral performancerelationships are the important aspects to understand thematerial behaviour clearly under different loading conditions[6].

Acoustic emission energy (AEE) is related to fractureenergy by previous researchers in the past [6]. Therefore it

Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2009, December 10-12, 2009

226 Vidya Sagar : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

of 8 channels to get the cumulative AE energy release. It isobvious that the crack propagation starts from the pre-definednotch tip. So, the AE transducers are placed near to thenotch.

3. Experimental program

3.1 Materials

The compressive strength of six concrete mixes used is78 MPa. Crushed granite aggregates of maximum size 20 mmas coarse aggregate and river sand as fine aggregate andPortland cement of grade 43 conforming to IS: 8112-1689 wasused in the experimental program. A sulphonated naphthaleneformaldehyde type super plasticizer and silica fume wereused as admixtures in order to improve the workability ofconcrete. Crushed ballast Cubes and cylinders were also castalong with the TPB specimens for determination ofcompressive strength.

3.2 Specimen preparation

The dimensions of all TPB specimens used in the presentexperimental study conform to RILEM (ReunionInternationale des Laboratoires d’Essais et de Recherchessur les Matériaux et les Constructions) recommandations [1].Span to depth ratio of the TPB specimen is 3 and thicknessof the TPB specimen is 80 mm. Three TPB specimens weretested for each of the three notch to depth ratios (a0/d=0.15,0.30 and 0.50). The specimens of all sizes were cast from thesame batch of concrete. The quality of the concrete wasuniform as possible. Details of the specimens tested in thispresent experimental study were given in Table. 1. The mouldswere filled with concrete in three layers and each layer wascompacted well using a needle vibrator. The specimen wasloaded at mid-span by a concentrated load and was simplysupported over span S. A notch of depth a0 is cut into the

cured beam using a concrete cutting machine. Three identicalspecimens for each notch/depth ratio and same size weretested(total 27 specimens). The notch width was around7mm. The specimens were demoulded after 24 hours ofcasting and cured by immersing them in water.

3.3 Experimental setup

The experimental setup consists of a loading frame withdata acquisition system. Three point bend plain (TPB)concrete specimens of similar geometry with different (notch/depth) ratios are tested. The loading frame is a materialtesting system (MTS) with data acquisition and all thespecimens are tested under crack mouth open displacement(CMOD) control at a rate of 0.0004 mm/sec. The midspandownward displacement was measured using linear variabledisplacement transducer (LVDT), placed at center of thespecimen under bottom of the beam. A clip gauge was usedto measure the CMOD. The maximum load was reached inabout 5 minutes following RILEM recommendations (1991).The data acquisition records load, CMOD, midspandisplacement and time.

The AE instrument used in the present study is a 8channel AEwin for SAMOS (Sensor based Acoustic Multi-channel Operating System) E2.0 system. AE test set upconsists of AE sensors, preamplifiers, processinginstrumentation, and AE win SAMOS software supplied byPAC. The transducers used in the experimental study wereR6D resonant type, AE differential transducers [11]. In generaldifferential sensors are used in environments where very lowlevel AE signals need to be processed and is also veryapplicable in high noise environments. The AE transducers(R6D) have peak sensitivity at 75 dB with reference1V/ (m/s). The operating frequency is 35 kHz – 100 kHz. Anessential requirement in mounting a sensor is enoughcoupling between the sensor face and the concrete testspecimen surface [7,11]. Vacuum grease LR ( high vacuumsilicon grease) was used as couplant in the present

Table 1: Details of the TPB specimens tested

Concrete Speci- Notchdetails Speci- Sensor Dimensions of Sensors positionmix men mens type the specimens from centre of

(resonant) (mm) the notch (mm)

(a0),mm a0/d L S t d r s

Small 12 0.15 3 R6D 290 240 80 80 32 32

24 0.30 3 28 28

40 0.50 3 20 20

Medium 24 0.15 3 R6D 530 480 80 160 65 65

48 0.30 3 50 50

F-Mix 80 0.50 3 40 40

Large 48 0.15 3 R6D 1010 960 80 320 130 130

96 0.30 3 110 110

160 0.50 3 80 80

24 0.30 3 28 28

40 0.50 3 20 20

227NDE 2009, December 10-12, 2009

experimental study. Before mounting the sensors on testingspecimen, the sensor’s surface was cleaned so that to makesure about allowing maximum couplant adhesion. Applicationof a couplant layer was thin, so it can fill gaps caused bysurface roughness and eliminate air gaps to ensure goodacoustic transmission. And all sensors were held firmly tothe testing surface. The threshold value of 40 dB was selectedto ensure a high signal to noise ratio[7,11].

4. Calculation of size independence acousticemission energy of concrete (EF)

By using the model of Duan et al and concept given inreference[2-3] and using Eq.(2) and Eq.(3) which areanalogous to the method adopted by Abdalla andKarihaloo(2003), Duan et al(2001), the size independentacoustic emission energy (EF) can be calculated by using theacoustic emission energy (Ef) released during the concretefracture [2-3] which are shown in Table.2. The model assumedthat that fracture energy required to create a crack along thecrack path is influenced by the size of the fracture processzone and also boundary of test specimen influences the sizeof the fracture process zone [1-2].

for 1-(a0/W) >al/W (2)

for1-(a0/W)£ (al/W) (3)

The acoustic emission energy values recorded duringthe TPB tests were substituted into Eq.(2) and Eq.(3) in orderto determine the size-independent acoustic emission energyEF and transition ligament length (al)AE. There are threevalues Ef(a0/W) for different sizes at each notch/depth ratio.By knowing Ef(a0/W), the size independent fracture energyEF is obtained for different sizes of TPB specimens. As thenumber of results of Ef(a0/W) for each depth W and notchto depth ratio was 3, but the number of unknowns are EF and(al)AE, the system of three equations was solved by leastsquares method to get the best estimate of EF and (al)AE. Thetwo variables (al)AE and EF were obtained from the threeequations by the least squares method, because theequations set of were over determined. Fig.2(a) shows a plot

Table 2: Measured fracture energy and AE energy from three point bend test

Fracture Energy (N/m) AE energy(relative units)a0/W W

mean COV(%) Mean COV(%)

0.15 80 112.5 22 219.5 35.0

0.30 80 123.1 16.5 102.5 31.5

0.50 80 90.5 10.2 42.6 63.8

0.15 160 128.9 2.9 201.5 24.7

0.30 160 108.6 14.6 117.4 42.1

0.50 160 100.7 3.8 79.5 15.1

0.15 320 133.4 12.1 327.1 34.4

0.30 320 122.3 5.6 209.5 33.6

0.50 320 117.1 16.2 225.5 11.3

Fig. 1 : Typical recorded load-CMOD, load-time and load-displacement diagrams.

228 Vidya Sagar : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Ef(a0/W) versus depth ratio. EF is plotted with respect todepth as shown in Fig.2b. The trend line drawn horizontal isparallel to the depth axis. However, in-spite of all that the sizeindependent acoustic emission energy EF was not absolutelyconstant with depth. The mean line drawn horizontally, viz.,parallel to the depth axis did not pass through all the points.There were deviations however small and the presentexperimental study may be useful to quantitative measure ofsize independent specific fracture energy of concrete.

Conclusions

l The present experimental study makes clear some extentabout the fundamental differences between the twomethods namely fracture energy calculated using RILEMmethod of Hillerborg [10] and fracture energy calculatedusing fictitious crack model [5].

l It was observed that AE events containing higher energylocated around the notch tip.

l It may be useful to relate acoustic emission energy tofracture energy of concrete. This relationship may beuseful to develop a laboratory tool. This laboratory toolcould be useful to study the damage in concretestructures. The author’s opinion is that this laboratorytool can be used in a laboratory under controlledconditions, as opposed to a field technique. Regardingdamage models, there is a large class of concreteperformance models based on principles of continuumdamage mechanics. These models often use parameterssuch as crack density to predict changes in stiffnessdue to damage. Up to now crack density is a difficultthing to measure, but AE has the potential to provideinformation on crack density that could then be used totune the continuum damage models related to concrete

References

1. Bhushan L. Karihaloo, Fracture mechanics and structuralconcrete, Longman Scientific& technical, Longman grouplimited, Longman house, Essex, CM20 2JE, 1995 England.

2. Abdalla H M and Karihaloo B L, Determination of size-independent specific fracture energy of concrete from three-point bend and wedge splitting tests., Magazine of concreteresearch, 55(2) (2003) pp.133-141.

3. Duan K, Hu X Z and Wittmann F H, Boundary effect onconcrete fracture induced by non-constant fracture energydistribution, in fracture mechanics of concrete structures,Proceedings FRAMCOS-4, 49-55 A.A. Balkema Publishers,Rotterdam, (2001) pp.49-55.

4. Hu X Z and Wittmann F H Size effect on toughness induced bycrack close to free surface., Engineering fracture mechanics, 65(2000) pp. 209-211.

5. Hillerborg A, Modeer M, Peterson P E, Analysis of crackformation and crack growth in concrete by means of fracturemechanics and finite elements, Cement and Concrete Research6 (1976) pp. 773-782.

6. Landis, Eric N, and Baillon Lucie, Experiments to relate acousticemission energy to fracture energy of concrete. Journal ofEngineering Mechanics ASCE, 128 (2002) pp. 698-702.

7. Miller , Ronnie K, McIntire, Paul, Non-destructive TestingHandbook. Acoustic Emission Testing. American Society forNon-destructive Testing INC (1987).

8. Pollock, Adrian A, Acoustic emission inspection, Technicalreport, TR-103-96-12/89, Physical Acoustics corporation., 195Clarksville Road, Princeton Jct., NJ., USA (1989)

9. Ohtsu M, The history and development of acoustic emission inconcrete engineering. Magazine of concrete research, 48(147)(1996) pp. 321-330.

10. RILEM Committee 50-FMC. Determination of the fractureenergy of mortar and concrete by means of three-point bendtests on notched beams, Materials and structures 1985; Vol.18,No.106, pp. 285-290.

11. User’s Manual SAMOS AE System Rev 2, Physical AcousticCorporation, Princeton. NJ (2005)

Fig. 2 : Variation fracture energy (Gf) and acoustic emissionenergy with depth (Fig.a) and the independent fractureenergy GF as a function of beam depth (Fig.b).