Microcracking and Fracture Process in Cement Mortar and Concrete: A Comparative Study Using Acoustic...

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Experimental MechanicsAn International Journal ISSN 0014-4851Volume 53Number 7 Exp Mech (2013) 53:1161-1175DOI 10.1007/s11340-012-9708-z

Microcracking and Fracture Processin Cement Mortar and Concrete: AComparative Study Using AcousticEmission Technique

R. Vidya Sagar, R. V. Prasad,B. K. Raghu Prasad & M. V. M. S. Rao

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Microcracking and Fracture Process in Cement Mortarand Concrete: A Comparative Study Using AcousticEmission Technique

R. Vidya Sagar & R.V. Prasad &

B.K. Raghu Prasad & M.V.M.S. Rao

Received: 21 March 2012 /Accepted: 19 December 2012 /Published online: 29 January 2013# Society for Experimental Mechanics 2013

Abstract This article reports the acoustic emission (AE)study of precursory micro-cracking activity and fracturebehaviour of quasi-brittle materials such as concrete andcement mortar. In the present study, notched three-pointbend specimens (TPB) were tested under crack mouth open-ing displacement (CMOD) control at a rate of 0.0004 mm/sec and the accompanying AE were recorded using a 8 chan-nel AE monitoring system. The various AE statisticalparameters including AE event rate dn

dt

� �, AE energy release

rate dEdt

� �, amplitude distribution for computing the AE based

b-value, cumulative energy (ΣE) and ring down count(RDC) were used for the analysis. The results show thatthe micro-cracks initiated and grew at an early stage inmortar in the pre peak regime. While in the case of concrete,the micro-crack growth occurred during the peak load re-gime. However, both concrete and mortar showed threedistinct stages of micro-cracking activity, namely initiation,stable growth and nucleation prior to the final failure. TheAE statistical behavior of each individual stage is dependenton the number and size distribution of micro-cracks. Theresults obtained in the laboratory are useful to understandthe various stages of micro-cracking activity during the

fracture process in quasi-brittle materials such as concrete& mortar and extend them for field applications.

Keywords Acoustic emission . Micro-cracking . Fractureprocess . Concrete . Mortar

Introduction

A quasi-brittle material such as concrete is characterized bya gradually decreasing stress transfer capability after itstensile strength has been reached [1–4]. The mechanicalbehavior and fracture properties of cement mortar and plaincement concrete are required to be studied to characterizeand evaluate such engineering materials. The ductility of thereinforced concrete (RC) member depends not only on theductility of steel (low for high strength steel, high for mildsteel), but also on the contribution of plain concrete. Theductility of plain concrete is quantified by the size of thefracture process zone (FPZ) which is an offshoot of fracturestudies [1–6]. The study of fracture process in plain cementconcrete (PCC) is an important topic since the highways andconcrete bridges always have a layer of PCC and provide abase for the RC structure to be built above. For example, ina typical plain concrete pavement the maximum tensilestresses are caused by traffic loads and also due to temper-ature [7]. Hence it is important to investigate the influenceof the external load on the fracture behaviour of concrete tobe able to fairly comment on the integrity of the structure. Itis vital to study how the micro and macro cracks are prop-agated under loading than knowing how they are generatedfor ensuring the reliability of the concrete structure. It can bedone using the energy based propagation measure of crackgrowth [1].

R. Vidya Sagar (*) : R.V. PrasadDepartment of Civil Engineering, Indian Institute of Science,Bangalore 560 012, Indiae-mail: [email protected]

B.K. Raghu PrasadCenter for Campus Maintenance and Development, IndianInstitute of Science, Bangalore 560 012, India

M.V.M.S. RaoNGRI, Uppal, Hyderabad 500 606, India

Experimental Mechanics (2013) 53:1161–1175DOI 10.1007/s11340-012-9708-z

Author's personal copy

One of the main characteristics of quasi-brittle materialsis their heterogeneity as a result of which the damage occursalways in the form of micro-cracks leading to the formationof tortuous bands and structural instability in concrete understress. The AE technique is useful to detect and monitor theformation of micro-cracks and their growth during the entirefracturing process in quasibrittle materials [8–10]. Further-more, the published work had shown that many similaritiesare present between the statistics of AE occurrence duringthe fracture of concrete, rock and earthquake activity[10–13]. In recent years, the application of high-speedmulti-channel AE waveform recording and source locationsystems have led to many significant advances in the studyof fracture process in concrete, rock and other quasi-brittlematerials [8–12]. The statistical data of AE, namely theoccurrence rate of AE events, b-values, energy counts etc.which can be obtained with the help of single or dualchannel AE-monitoring systems have also yielded usefuland interesting results [8–12, 14]. Those AE parametershave been utilized for studying the evolution of pre-failuremicro-cracking, precursory sequences and fracture processin concrete and rocks [8–12, 14]. It became possible becauseof the fact that the number of AE events is more or lessproportional to the number of growing cracks, and the AEamplitudes (or energy) are proportional to the length of thecrack growth increments in materials such as concrete androck etc [12, 14].

Several researchers have applied the AE technique forthe detection, characterization, evaluation and assessmentof damage in concrete structures using both signal basedand parametric based techniques [8–13, 15–27]. In fact,research in the field of AE testing of concrete structuresand analytical and interpretation methods of AE are un-dergoing a drastic improvement due to increasing need forthe health monitoring and renovation of concrete struc-tures [9]. Rigorous research work has led to the realiza-tion of several practical applications of AE testing. Tofurther such research activities, the RILEM technical com-mittee: TC 212-ACD has established a standard method(NDIS-2421) for crack detection and damage evaluationin concrete [8, 15].

In spite of extensive work done on the development ofseveral numerical models related to fracture process in con-crete, the experimental work aimed at understanding ofprecursory micro-cracking and physical processes that areresponsible for the fracture behavior is relatively less [27].Even if researchers working in concrete engineering are ableto predict failure loads, it is very essential to understand therelation between nucleation of cracks, crack growth, crackbridging, friction between two cracked surfaces and thecorresponding effects on the macroscopic behavior andfracture of concrete. It logically follows that to formulatepredictive models for large scale structural performance and

reliability, it is essential to understand the micro-structureperformance relationships [27]. In this connection, the AEwhich are generated during the various stages of micro-cracking and fracture, if detected and analysed can lead tomany advantages. Because, the AE events which occurduring fracture process in quasi-brittle materials are moreor less proportional to the number of growing cracks. Fur-ther the AE amplitudes are proportional to the length of thecrack growth increments [10–12].

The goal is to study the various stages of micro-crackingactivity in both cement concrete and mortar three point bend(TPB) specimens under controlled laboratory conditions. Itis aimed to study the fracture process of cement mortar andplain concrete using AE statistics. The occurrence rate ofAE events, AE based b- values, ring down count (RDC) andAE energy and energy/event data have been used for theanalysis. The present work is just a beginning to come outwith standard monographs to classify the damage of theconcrete and concrete structures in terms of AE statisticalparameters with respect to time.

Experimental Procedure

Materials and Mix Proportions

The materials used for the preparation of test samples arecement, fine aggregate, coarse aggregate, microsilica (silicafume) and superplasticizer. We prepared 10 concrete speci-mens and 9 mortar specimens for the present study. Theconcrete specimens were prepared using ordinary portlandcement (OPC) conforming to IS 12119:1987, coarse aggre-gate (maximum size is 20 mm) and fine sand. The mortarspecimens were prepared using OPC and sand. The water/cement ratio of 9 concrete specimens was 0.539 for the Mix-B category and 0.335 for the other 9 specimens of Mix-Fcategory. The water/cement ratio of all the 9 mortar speci-mens was 0.4. The details of concrete mixes, cement mortarproportions and their properties are summarized in Table 1.The 28th day compressive strength of concrete was found tobe 51 MPa for the Mix-B category, 78 MPa for the Mix-Fcategory and 47 MPa for the mortar specimens (Table 1).

Preparation and Testing of Specimens

In the present study the authors used the RILEM draftrecommendations (TC-89 FMT) for the preparation andtesting of specimens [28]. A central notch was saw-cut ineach test specimen before testing. All specimens were castwith the same batch of concrete in specially made woodenmoulds and compacted using a needle vibrator. Three typesof notches, N1=0.15d, N2=0.3d and N3=0.5d for eachdepth were used where d is the depth of the TPB specimen

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and N1, N2, and N3 are the symbols used to indicate differ-ent notch depths. For each type of notch, three specimenswere tested. We have carried out three point bend (TPB)tests on all the specimens. The schematic diagram of theTPB test specimen is shown in Fig. 1. As indicated in Fig. 1,d is depth of the beam, L is the length of the beam, S is thespan of the beam, t is the thickness of the beam and a0 is thedepth of the notch. The geometric details of the specimensare given in Table 2. The data of fracture energy (Gf) andAE energy for each specimen were measured and the valuesobtained are presented in Table 3 for mortar specimens andin Table 4 for Mix-B concrete specimens and in Table 5 forMix-F concrete specimens.

Test Set-Up

The test set-up consisted of material testing system (MTS)of 1200 kN capacity with a servo controlled hydraulicloading frame, data acquisition unit and a AE monitoring

system. The mid-span downward displacement was mea-sured using a linear variable differential transducer(LVDT) placed at center of the specimen under bottom ofthe beam. A clip gauge was used to measure the CMOD.The range of the CMOD gauge was 14 mm, gauge lengthwas 12 mm and travel was ±7 mm. The AE monitoringsetup consisted 8 AE sensors and a PC based multi-channelmonitoring system SAMOS AEwin (Sensor based AcousticMulti channel Operating System) manufactured by Physicalacoustic corporation (PAC). The AE transducers (R6D)were resonant type with a natural resonant frequency75 kHz [29]. The diameter of AE sensor was 19.0 mm andits height was 22.0 mm. The operating frequency of the AEsensor is 35 kHz – 100 kHz. The AE transducers had peaksensitivity at -75 dB with reference 1 V/(m/s) as shown inFig. 2. The threshold for detecting AE was fixed at 40 dB toensure a high signal to noise ratio. Figure 2 shows thecalibration curve of the R6D sensor’s response (or sensitiv-ity) as a function of frequency and all the sensors have hadthe same response [29]. Vacuum grease LR (high vacuum

Table 1 Details of concretemixture proportions Property Mix-B Mix-F Mortar (1:4)

Water/cement ratio 0.539 0.335 0.4

Cement, kg/m3 350 450.0 432.6

Micro silica, kg/m3 17.5 56.2 –

Fine aggregate, kg/m3 671.5 706.9 1730.7

Coarse aggregate, kg/m3 1190.3 1058 –

Water, kg/m3 188.8 150.9 173.0

Superplasticiser, (% weight of Cement content in mix) 1.0 1.2 –

Compressive strength, MPa (28 day) 51.0 78.0 47.0

SL

r ao

s

t

1 1

2 3

3

4

3

4

7

8

1

2 6

5

right side view left side view

front view

top view

1 to 8 represents sensors

2 4

6 8

d d

tt

Fig. 1 Schematic diagram of specimen geometry

Exp Mech (2013) 53:1161–1175 1163


silicon grease) was used as a coupling agent to fix thesensors on the surface of the test specimen.

Results and Discussion

Micro-Cracking Activity as Inferred from AE

The recorded AE data files were replayed and processedusing AEwin SAMOS software for the identification andevaluation of various stages of fracture process using thedata of AE event counts (n), energy (E) counts and ampli-tude distribution data at the maximum possible digitizationrate of 200 bin resolution. The frequency of occurrence ofmicro-cracking activity has been studied using the event ratedndt

� �and energy count rate dE

dt

� �data plots. The AE cumu-

lative energy count data (∑E) were normalized with respectto the final energy count for obtaining the normalized micro-crack damage data of the test specimens following theprocedure reported in detail elsewhere [12]. The origin anddevelopment of micro-cracking activity in cement concreteas well as in mortar can be inferred from AE statistics, as thenumber of AE events is more or less proportional to thenumber of growing cracks and the AE amplitude or energy

count data are proportional to the crack growth incrementsin brittle and quasi-brittle materials like rock and concrete[10–12]. In fact, the formation of micro-cracks is ubiquitousin brittle and quasi-brittle materials at stresses close to theyielding point. The useful AE statistical parameters to iden-tify and characterize the various stages of micro-crackingactivity in concrete are: (i) The occurrence rate of event andenergy counts, (ii) Amplitude distribution (b-value) and itsstress-induced changes, and (iii) Cumulative energy or eventcounts as detailed in the following sections.

Figure 3 shows time on the load-time plot to study thevariation of AE events with time in concrete specimen.Figure 4 shows the locations of the recorded AE events atthese times. Most of the AE activity has taken place in frontas well as on either side of the pre-existing notch tip of theTPB specimen. The typical recorded load-time curve andthe locations of the AE event sources are compared at loadlevels identified in Fig. 3. It shows that the first AE eventtook place around 87 sec after the start of experiment. Thedevelopment of the microcrack could be due to the loadwhich exceeded the tensile strength of the concrete. Afterreaching the peak load considerable AE events took place.The fracture process zone extends only over the tensionsoftening region (defgh) in Fig. 3. During this extension

Table 2 Geometric and sensor position details of specimens

Concrete mix Notch details Sensor type(resonant)

Dimensions of thespecimens (mm)

Sensors position Fromcentre of the notch (mm)

Notch depth(a0), mm

Notch/depthratio

Length Span Thickness Depth r s

Concrete( 78 MPa)- MixF

24 0.5 R6D 530 480 80 160 65 65

Concrete( 51 MPa)- Mix-B

48 0.3 50 50

Mortar 80 0.15 40 40

Table 3 Measured fracture energy and acoustic emission energy for Mortar specimens

Specimen a0d Depth (mm) Pmax (N) Average

Pmax (N)Work offracture WF

(N-m)

Fractureenergy(N/m)

Averagefractureenergy (N/m)

Cumulative AEenergy (relativeunits)

Average cumulativeAE energy (relativeunits)

MMN1a 0.15 160 7088.38 6067.2 0.8495 78.07 76.87 16.0 215.5MMN1b 0.15 160 6991.02 0.9524 87.53 344.0

MMN1c 0.15 160 4122.44 0.7075 65.02 287.0

MMN2a 0.30 160 5953.70 5837.8 0.8634 96.36 67.20 16.0 148.33MMN2b 0.30 160 5646.53 0.3692 41.20 328.0

MMN2c 0.30 160 5913.42 0.5740 64.06 101.0

MMN3a 0.50 160 3110.29 2652.0 *** *** 56.15 *** 53MMN3b 0.50 160 *** *** *** ***

MMN3c 0.50 160 2193.82 0.3594 56.15 53

1164 Exp Mech (2013) 53:1161–1175


of the fracture process zone a lot of AE events werereleased. Due to micro cracking the concrete present inthe fracture process zone softens. During the pre-peaknonlinearity (ad) and region of tension softening afterthe attainment of peak load (df) are primarily result ofmicrocracking. The tail region of tension softening dia-gram (fh) results from the aggregate interlocking andfrictional effects between cement matrix and aggregatesand also between aggregates. The trend in the cumula-tive number of recorded AE events with load providethe information of the micro-cracks initiation andgrowth before and after peak loading to some extent.Because, an AE event is a local material change givingrise to acoustic emission. Figure 5 shows time on theload-time plot to study the variation of AE events withtime in mortar specimen. In fact Figs. 4 and 6 show theinitiation and growth of micro-cracks qualitatively inconcrete and mortar respectively.

Figure 7 shows variation of load and cumulative AEenergy counts with respect to time for both concrete andmortar specimens. It was observed that before peak loadvery little AE activity occurred. On the onset of crack

formation occurring at or near the peak load, the AE releasetook place. The cumulative AE energy curve shows variableslope as the load is applied monotonically. One can observefrom Fig. 7 that the curve related to the cumulative AEenergy count versus time plot initially starts with very lessslope and then the slope becomes steeper and as the loadreaches the maximum, gradually the slope decreases. In caseof concrete, in the post peak load region the cumulative AEenergy count tends towards a stable value. But in contrast tothis, the cumulative AE energy curve continues to rise incase of mortar. Hence one can conclude that fracture processin mortar is different, when compared with the fractureprocess in concrete. Mortar is generally recognized to havemore brittle characteristics than concrete [1–3]. This is dueto mechanisms like aggregate interlock, crack bridging andcrack face sliding of concrete [1–3].

Figure 8 shows the variation of AE signal duration withtime for both concrete and mortar. It is known that the AEsignal duration is the time between AE signal start and AEsignal end. It was observed that signal duration in concreteis high when compared that in mortar. Figure 9(a, b) shows thevariation of rise timewith time. It is known that AE signal

Table 4 Measured fracture energy and acoustic emission energy for Mix-B concrete specimens

Specimen a0d Depth

(mm)Pmax N Average peak

load, Pmax NWF N-m Fracture

energyN/m

Averagefractureenergy

NominalstrengthN/mm2

CumulativeAE energy(relative units)

Average cumulativeAE energy (relativeunits)

BMN1a 0.15 160 9847.87 10231.68 1.5419 141.71 133.09 0.7693 271 364.33BMN1b 0.15 160 10287.63 1.1303 103.88 0.8037 303

BMN1c 0.15 160 10559.55 1.67215 153.69 0.8249 519

BMN2a 0.30 160 7173.98 8537.49 1.10486 123.31 126.78 0.5604 474 336.67BMN2b 0.30 160 9194.92 ***** **** 0.7183 377

BMN2c 0.30 160 9243.59 1.1671 130.25 0.7221 159

BMN3a 0.50 160 3971.37 4115.16 0.43624 68.16 95.77 0.3102 286 184.00BMN3b 0.50 160 4439.68 **** **** 0.3468 82

BMN3c 0.50 160 3934.44 0.7897 123.39 0.3073 ***

Table 5 Measured fracture energy and acoustic emission energy for Mix-F concrete specimens

Specimen a0d Depth mm Load N Average

load, Pmax

WF N-m FractureenergyN/m

Averagefractureenergy

Nominalstrength,N/mm2

Cumulative AEenergy (relativeunits)

Average cumulativeAE energy(relative units)

FMN1a 0.15 160 8891.110 10173.04 1.4194 130.40 129.44 0.69030 270.43 201.5FMN1b 0.15 160 11144.000 1.4538 133.60 0.86521 180.54

FMN1c 0.15 160 10484.023 1.3525 124.30 0.81397 153.85

FMN2a 0.30 160 7583.544 7030.752 1.1548 128.80 109.34 0.58878 186.09 117.4FMN2b 0.30 160 6594.897 0.8093 90.32 0.51202 73.2

FMN2c 0.30 160 6913.815 0.9759 108.90 0.53678 92.1

FMN3a 0.50 160 4194.616 4266.79 0.6719 104.90 101.25 0.32566 91.05 79.5FMN3b 0.50 160 **** **** **** **** ******

FMN3c 0.50 160 4338.969 0.6251 97.60 0.33687 67.12

Exp Mech (2013) 53:1161–1175 1165


rise time is the time between AE signal start and the peakamplitude of that AE signal. The rise time is more inconcrete when compared with that in mortar.

Frequency of Occurrence

The AE event rate n: ¼ dn

dt refers to the frequency of occur-rence of micro-cracking activity while the energy or peakamplitude corresponding to the individual AE events repre-sents the size of micro-crack or its incremental growth inbrittle and quasibrittle materials [11, 12]. The rate at which

AE energy is released E: ¼ dE

dt

h iis a measure of size distri-

bution of micro-cracks in all such materials [12]. By replay-ing the AE data recorded, the log of energy release rateversus time and log of event rate versus time plots for bothconcrete and mortar specimens are obtained and these areshown in Fig. 10. Initially as loading commenced, hardlyany AE activity (energy per event or ring down count) wasobserved. With the formation of new micro-cracks, thespecimens began to release AE which increased steadily.The formation of AE (or micro-cracks) commenced at140 sec after the start of the experiment for concrete spec-imen (Mix-F), at 120 sec for Mix-B concrete specimen andrather early at 90 sec after start of the experiment for mortarspecimen as shown in Fig. 10.

From these plots it can be seen that the trend in AEenergy release rate versus time and AE event rate versustime is more or less similar in all the specimens. The AEincreased steadily with increase of load indicating the for-mation and growth of micro-cracks and continued furtherafter the peak load is attained upto nearly 450 sec from thestart of the experiment in concrete specimens as shown inFig. 10(a, b) and upto 300 sec in mortar specimen as shownin Fig. 10(c). The results of the present study show thatmicro-cracking activity occurs in three successive stages,namely, Initiation (I), Coalescence (C) and Nucleation (N)that would lead to the final failure of concrete as well asmortar specimens under bending stress. The statistics of AEoccurring during these stages are distinctly different for bothcement concrete and mortar (Table 6).

Region-I called initiation stage showing raising trend,region-II called coalescence stage which has a constanttrend and the region-III called the nucleation stage whichshows a decreasing trend. The steady increase in both the

parameters E�and n

�which indicates the formation of new

micro-cracks during the prepeak load and post peak loadregimes can be termed as stage-I or the initiation stage (I) ofmicro-crack damage in concrete and mortar. It is then fol-lowed by coalescence (C) and nucleation stage (N) of micro-crack damage in concrete. The coalescence stage (C) ischaracterized by nearly stable or with minor fluctuations inthe plot. It can be considered as quasi-static fracture involv-ing subcritical growth of micro-cracks. The nucleation stage(N) is characterized by a gradual decrease in both the

parameters E�and n

�indicating the progress towards final

failure due to loss of aggregate interlock and frictionaleffects in concrete [11]. Coalescence stage is a combinedprocess involving activities such as micro-crack deflection,aggregate bridging and crack face friction. These plots maybe useful in identifying the various stages of microc-rackdamage during the failure of concrete and mortar. Micro-crack density distribution represents the number of micro-cracks per unit of observation area [12]. It was observed that

the fluctuations of E�and n

�were more in case of the mortar

specimen when compared to that in concrete specimens.This indicates that the density and size distribution ofmicro-cracks in concrete specimens is more or less uniformand small. While in case of mortar specimen, the fluctua-tions as seen in the plot are relatively high indicating that thedensity and size distribution of the newly formed micro-cracks can be non-uniform and larger in size.

Fluctuations in Microcrack Damage

It is interesting to see from Fig. 10 that the mortar specimenswhich have a low compressive strength (~ 47 MPa) showed

Frequency, MHz

Fig. 2 Sensor’s response as a function of frequency [Reprinted withpermission from Physical Acoustics Corporation (PAC), NJ, USA,User’s Manual SAMOS AE System (2005)]

a

b

c

d

h

g

f

e

Fig. 3 A typical recorded load-time plot (concrete)

1166 Exp Mech (2013) 53:1161–1175


large fluctuations of E�versus time. The concrete specimen

(Mix-F) which is quite strong had shown relatively smallfluctuations. The concrete specimen (Mix-B) showed fluc-tuations somewhere in between. The reasons for the changesin fluctuations in addition to the compositional and prepa-ration differences could be many. Ultrasonic attenuationfrom AE source to the sensor (receiver) could reduce themeasured AE energy. Also there is attenuation caused bymaterial absorption. In the present study 3800 m/sec wasused as sound velocity in concrete. A wavelength of 20 mmwould correspond to a frequency of 190 KHz and thisfrequency is with in the range of 100 KHz – 500 KHz. Ingeneral for concrete AE signals are emitted with in the rangeof 100 KHz – 500 KHz. In the present study the maximumaggregate size is 20 mm. If the wave length is less than sizeof aggregate then there is higher probability of internal

reflection of the waves, which is called scattering. Thisscattering may absorb energy. The ultrasonic scatteringcauses additional signal AE wave attenuation. Since con-crete is known to be a highly attenuating material, lowerfrequency sensors are suitable for AE studies. The availablesensor though has a highest sensitivity between 35 KHz –100 KHz, it is still able to pickup the signals with frequencyhigher than 100 KHz. Since the maximum aggregate size inconcrete for the present study is 20 mm, the wavelengthshould be greater than 20 mm. Accordingly, using the rela-tion V=nλ the frequency of the AE sensor to be used isdesirable to be less than 190 kHz. Accordingly R6D sensorwhich has a highest sensitivity in the range 35 kHz –100 kHz has been used. However there is surely ultrasonicattenuation in mortar specimens. But when compared toconcrete specimens the effect of scattering is less evidentthan that of concrete with large size coarse aggregates.

Scattering (or diffusion) of AE waves strongly dependson the heterogeneity of cementitious composites like cementpaste, mortar and concrete and also depends on the sizedistribution of the inclusions. Among cement paste, mortarand concrete, the cement paste is least heterogeneous whileconcrete is highly heterogeneous. The phenomena of scatteris on account of AE waves hitting the inclusions when theyencounter. Further, scatter cannot be quantified and it canonly said qualitatively as either more or less. In the presentstudy the concrete specimen was made using M78 gradeconcrete with admixtures silica fume and high-range waterreducer (HRWR) agent super plasticizer. Such a concretewould be relatively less heterogeneous and scatter could beless compared to that of mortar. Yet another reason is that in

Fig. 4 The AE source locations in a concrete TPB specimen tested at time, t: (a) 87 sec, (b) 109 sec, (c) 125 sec, (d) 163 sec, (e) 434 sec, (f)625 sec, (g) 939 sec, (h) 1612 sec (X-axis represents the span and Y-axis represents the depth of TPB specimen tested)

b

a

c

d

h

g

f

e

Fig. 5 A typical recorded load-time plot (mortar)

Exp Mech (2013) 53:1161–1175 1167


such high grade concrete the matrix is much stronger andtherefore the waves may not find a difference between thematrix and the inclusions. This could be due to stronginterfaces between coarse aggregates and cement mortar.The concrete specimens contains micro-silica, whichreduces porosity at the cement- aggregate interface. Thisinterface plays an important role in both fracture propertiesand AE properties. Stronger interfaces generally lead tohigher strength but also higher brittleness (through a smallerfracture process zone). The stronger interfaces also maylikely alter the AE activity by leading to fewer AE events,

but more energy release per AE event. Thus in the presentstudy, authors opinion is that addition of micro silica may becausing one of many competing phenomena.

During the travel of these AE waves from the source tosurface will be subjected to the attenuation. The energy lossexperienced by these stress waves per unit distance travelwill be related to the damping capacity (internal friction) ofthe material. This also causes the reduction of the totalelastic wave energy that reaches the sensor. The signalattenuates as it progresses. It was well cited in the literaturethat attenuation due to material causes an exponential

Fig. 6 The AE source locations in cement mortar TPB specimen tested at time, t: (a) 72 sec, (b) 100 sec, (c) 119 sec, (d) 136 sec, (e) 317 sec, (f)475 sec, (g) 924 sec, (h) 1535 sec (X-axis represents the span and Y-axis represents the depth of TPB specimen tested)

Time (AE energy) (sec)

Time (load) (sec)

Cum

ulat

ive

AE

ene

rgy

(rel

ativ

e un

its)

Load

(kN

)

Fig. 7 Plot of load and acousticemission (AE) energy versustime

1168 Exp Mech (2013) 53:1161–1175


Time (sec)

Dur

atio

n (s

ec)

(1)

(2)

(1) Concrete

(2) Mortar

Fig. 8 Plot of acoustic emission(AE) duration versus time

(a)

(b)Mortar

Fig. 9 Variation of rise timewith time in (a) concrete(b) mortar

Exp Mech (2013) 53:1161–1175 1169


decrease in amplitude, A [9].

A ¼ e�a0d ð1Þ

Where d is the propagation distance and ά is the materialattenuation coefficient. Rajagopalan et al. studied correla-tion between ultrasonic pulse velocity and strength of con-crete. Figure 11 show variation of amplitude with respect todistance for Mix-F specimens [9]. It is interesting to see thatthese plots confirm the earlier results. Jacob et al. studiedeffect of aggregate size on attenuation of Rayleigh surfacewaves in cement-based materials and concluded that aggre-gate size does not dominate attenuation and the absorption isthe main cause of attenuation mechanism present in thecement based materials [9]. In the present study a test wasperformed with the sensor positions varying along the spanof the TPB specimen with respect to a fixed source of soundwave pulses at the left end of the beam. The amplitudevariation when the sensor position changed is plotted inthe Fig. 11. The amplitude reduction at the sensor positionin the experiments can be obtained from the Fig. 11. Thus ifthe sensor is at distance d from the source the reduction onaccount of attenuation can be obtained. The amplitude dis-tance relation for concrete is Amplitude=59461e-0.0027d andfor mortar is Amplitude=44620e-0.0027d where d is the dis-tance (mm) from source and amplitude in micro-volts.

In fact ultrasonic wave propagation is generally compli-cated in concrete which is a three phase material consistingof aggregate particles, cement matrix – paste and the inter-facial transition zone (ITZ). The presence of reinforcementis another aggravating factor that can significantly affectwave velocities and attenuation. Ultrasonic attenuationoccurs in concrete due to scattering and material absorptionwhich in turn are dependent on the size of the coarseaggregates present and the frequency (or wavelength) ofthe propagating wave. Further, the coarse aggregate size inconcrete is considered as the main factor that influencesfracture propagation. For example, the crack initiationthreshold is dependent on aggregate size, and the crack

logE

,lo

gnlo

gE,

logn

time (sec)

time (sec)

C

E

E

E

n

n

n

I

load

(kN

)

time (sec)

(a)

C

load

I

N

Mix F concrete specimen 78MPa

mortar specimen

load

(kN

)

(c) load

N

load

(kN

)

logE

,lo

gn

(b)

C

load

I

N

Mix B concrete specimen 51MPa

Fig. 10 The logarithm of AE event (n:) and AE energy count (E

:) data

obtained during the experiments on (a) concrete specimen [mix-F], (b)concrete specimen [mix-B] and (c) mortar specimen plotted againsttime. The load versus time plots are also included in the figure. I, C andN denote the initiation, coalescence and nucleation stages of micro-cracking activity in the test sample

Table 6 AE event count, RDCcount and Energy count dataduring the initiation (I), Coales-cense (C) and nucleation (N)stages of microcrack activity inconcrete Mix-F, concrete Mix-Band cement mortar specimens

Specimen Event count(n) RDC (N) Energy count (E) E/n

Mix-F Concrete (78 MPa) I 14151 309239 120547 8.52

C 36242 913049 454150 12.53

N 12186 335654 199493 16.37

Mix-B Concrete (51 MPa) I 21012 420872 225027 10.71

C 18032 494003 316916 17.58

N 13841 371333 231227 16.71

Mortar I 6055 75681 29407 4.86

C 2864 46312 24953 8.71

N 2316 19967 39896 17.22

1170 Exp Mech (2013) 53:1161–1175


damage threshold is not only dependent on the aggregatesize but also on the strength of the concrete. Fracture tendsto propagate along aggregate-matrix interface and canchange direction locally to avoid large aggregate particles.Microcracking, pre-existing cracks, pore space distribution,and air-voids are the other features that affect the fracturebehaviour as well as wave propagation in concrete. The AEequipments and sensors (for example R6D) are accordinglymade to work in the frequency band of 35 KHz to 100 KHzon highly attenuating materials like concrete by taking intoaccount all the above.

Table 6 gives information about AE features in differentstages. RDC and energy counts have the same trend in allthe three stages. It is less in the initiation stage and nucle-ation stage while it is large in the coalescence stage. Inter-estingly E

n continues to increase from initiation stage tonucleation stage. Also from Table 6, it was observed thatmicro-cracking during nucleation stage (N) was more in-tense in concrete specimen (Mix-F) while the micro-crackaccumulation in the other concrete specimen (Mix-B) wasfound to be more intense at an early stage of coalescence (C)stage which is represented by the E/n ratio. Hence it can beinferred that as the strength decreases, damage occurs at anearly stage. Also, micro-cracking in mortar specimen hadsuddenly increased during nucleation stage.

Cumulative Micro-Crack Damage

It is known that AE ring down count is defined as the numberof times the AE signal exceeds a preset threshold [8]. Duringthe experiment, plots of cumulative AE ring down count(ΣRDC) versus time and cumulative AE energy (ΣE) versustime were obtained using AEwin – PAC software by replayingthe recorded data for each specimen as shown in Fig. 12. Withreference to the typical load-time response of quasi-brittle

y = 59461e-0 .0 0 3 8 x

0

10000

20000

30000

40000

50000

60000

0 100 200 300 400 500 600 700

Distance (mm)

F-M ixExpon. (F-M ix)

Am

plitu

de (

mic

ro v

olts

)

Fig. 11 Variation of amplitude with respect to distance for concretespecimens tested

load

(kN

)

ΣRD

C ,

ΣE

Ci

Cd

time (sec)

E

mortar specimen

(a)

(b)

(c)

load

(kN

)

ΣRD

C,

ΣE

RDC

ECi

Cd

time (sec)

Mix F concrete specimen

E

Mix B concrete specimen

load

(kN

)

ΣRD

C ,

ΣE

time (sec)

Cd

RDC

Ci

Fig. 12 The cumulative ring down count (ΣRDC) and energy count(ΣE) data obtained during the experiment on (a) Concrete specimen(mix-F), (b) Concrete specimen (mix-B) and (c) mortar specimenplotted against time. Dotted circles on energy count plot indicate thesudden release of energy during crack bridging process

Fig. 13 Enlarged view of variation of b- values between time 250 secto 300 sec. b-value over time graph plotted with groups of 50 events(asterisk marker), 75 events (circle filled marker) and 100 events(diamond marker) to verify the independence of final results fromnumber of events chosen for calculation [8]

Exp Mech (2013) 53:1161–1175 1171


materials in tension, it can be observed from Fig. 12 that theAE ring down count increases slowly and steadily followingclosely trend of energy count plot for all the 3 specimens [1].In all the three specimens, the stage at which crack initiates(Ci) and the stage corresponding to the beginning of damageor crack propagation (Cd) can be observed (Fig. 12). Ci and Cd

stages correspond respectively to the micro-crack formation or‘initiation’ (I) and micro-crack bridging or ‘coalescence’ (C)stages of pre-failure.

Mortar is a mixture of cement, fine aggregate (sand) andwater. Concrete is a composite of fine aggregate, coarse ag-gregate, cement and water. Thus it can be said that concrete is

formed by the aggregate and cement paste matrix together.Since the aggregates and cement paste matrix both havedifferent thermal coefficient and modulus of elasticity; theirinterfacial transition zone is considered as weak. This interfa-cial transition zone has great influence on the properties ofconcrete and hence concrete can be regarded as a three stagematerial consisting of the matrix, the aggregates and the weaktransition zone. Thus the fracture process in case of concretewould primarily depend upon the stability of these interfacialcracks. Due to the presence of fine aggregates alone in mortar,the voids would be measured on a scale of micrometer. Hencein case of cement mortar, the fracture process would primarily

Mix F concrete specimen – 78 MPa

mortar specimen I C N

(c)

N CI

(a)

(b)

Mix B concrete specimen – 51 MPa

N

I

C

Fig. 14 Comparison of varia-tion of b-value with load andtime for (a) Concrete specimen-78 MPa, (b) Concrete specimen-51 MPa and (c) mortar specimen

1172 Exp Mech (2013) 53:1161–1175


involve initiation and propagation of internal voids on amicrometer scale [4]. Bridging occurs when a crack hasmoved beyond an aggregate and stresses are continuouslytransmitted across the crack until it gets ruptured. During thefracture process, when interlocking happens between micro-crack faces, sudden dissipation of energy is caused due tofriction and crack bridging occurs [1–4]. This sudden releaseof energy can be identified in Fig. 12 which signifies theoccurrence of crack bridging. This observation has been indi-cated as a dotted circle in Fig. 12.

Microcracking in cement based composite materialsoccurs due to self restraint and aggregate restraint stresses.Thus micro-crack propagation mainly occurs due to theaggregate interlock and related frictional effects. FromFig. 12, it can be inferred that in concrete, microcrackingcommences from pre-peak region and continues upto thetime corresponding to the end of region of tension softeningafter reaching the peak load with reference to the typicalload-time response of quasi-brittle materials in tension [1].Propagation of the micro-cracks occurs at the timecorresponding to the tail region of the tension softeningdiagram [1]. Due to the presence of aggregates, the strengthis more and hence the AE count rate for concrete is muchhigher than that of cement mortar.

b-Value Analysis

AE event–amplitude distribution plots were used for thecomputation of b-value as follows.

log10NðMÞ ¼ a� bAdB

20

� �ð2Þ

where AdB is the peak amplitude of the AE events indecibels, N is the incremental frequency (the number ofAE hits with amplitude greater than the threshold), M isthe Richter magnitude of the events. a is an empiricalconstant and b is the AE based b-value [11, 13, 18]. Duringthe experiment, AE amplitude data was acquired with thehelp of AEwin PAC system and stored. The recorded datawas analyzed using Matlab by following Colombo et al.[11]. By following Colombo et al., for each specimen, thetotal number of AE hits during the experiment was dividedinto different groups of 75 hits each and then each one ofthese groups was analysed to obtain the corresponding b-value as shown in Fig. 13. Later, all the obtained b-valueswere plotted against time (corresponding to final time of thetime range in the respective group) as shown in Fig. 14.

During the initial loading process the b-value increased toa value well above 1 for both concrete as well as mortarspecimens as shown in Fig. 14. The RDC and energy countdata also were relatively less until the peak load as shown inFig. 12. This is the stage when micro-cracks begin to form.

The b- value then began to fall owing to the formation andgrowth of more micro-cracks as the experiment progressedas shown in Fig. 14. The b-value trend is clearly moreconcentrated in the beginning due to the formation of largernumber of micro-cracks at and past the peak load for all thespecimens tested (Fig. 14). The plots shown in Fig. 14 areuseful in identifying the initiation (I), Coaescence (C) andnucleation stage (N) of fracture process. As the coalescencestage transits into nucleation stage, the b-value trend is morescattered and it indicates the formation of localized macro-crack leading towards final failure. This can be observedclearly for all the specimens tested as shown in Fig. 14. Thisis similar to the observations reported earlier by Colmbo etal. [11] for reinforced concrete beam.

Normalized Micro Crack Damage

As mentioned in the previous sections, AE energy count actsas a measure of the extent of damage caused by micro-cracks in terms of size and number of micro-cracks formedduring the fracture process in the cement concrete or cementmortar specimens. Cumulative energy count (ΣE) is also ameasure of the accumulated micro-crack damage in therespective specimens. These ΣE values can be normalizedwith respect to the final ΣE count and plotted againstnormalized failure time [12]. This procedure has been adop-ted in the present study and the results obtained are shown inFig. 15. It is helpful in not only characterizing the variousstages of micro-cracking activity, but also in identifyingtheir corresponding sub-stages based on the change in slopeof the curve marked as i and ii in Fig. 15. Furthermore, theenergy per event data can also be calculated using theenergy and event count database to identify and characterize

Fig. 15 The normalized values of micro-crack damage calculatedfrom AE cumulative energy data obtained during the experiment onconcrete specimen-78 MPa, concrete specimen-51 MPa and mortarspecimen plotted against normalized failure time [I = initiation; C =Coalescence; N = Nucleation]

Exp Mech (2013) 53:1161–1175 1173


the various stages and substages of crack damage in engi-neering materials.

It can be observed from the plot that in all the speci-mens the curve gradually increases with the increase oftime indicating the increase in the micro-crack damageoccurring upto the final failure. The curve is steeper andhigher in concrete specimen (Mix-B) than that in theother specimen (Mix-F) indicating that the micro-cracking activity is higher in the former. The number ofevents in the nucleation stage, though small, would havehigher amount of energy associated with them. Changesin the slopes of the curve are more frequent and greaterin mortar specimen showing that extensive micro-crackdamage occurred in this specimen when compared withconcrete specimens. The results show that from the ini-tiation stage itself, concrete specimen (Mix-B) has suf-fered more micro-crack damage than concrete specimen(Mix-F). Comparatively greater micro-crack damage hasoccurred in the mortar specimen.

From Fig. 15 it can be observed that in higher strengthcement concrete specimen (Mix-F), extensive damage hasoccurred in the 2nd half of the coalescence stage correspondingto a micro-crack damage increase from ~55 % to ~87 % whencompared to the other concrete specimen (Mix-B) which is oflower strength and which has suffered more damage due tomicro-cracks in the initial stage and 1st half of the coalescencestage from ~30 % to reach ~ 70 % of the normalized micro-crack damage. This tallies with the observations made in theprevious section referring to Table 6.

The nucleation stage has begun earlier in concrete speci-men (Mix-B) than that in concrete specimen (Mix-F). Thiswill tally with the observations made previously. The micro-crack damage has increased sharply after 2nd half of coales-cence stage in concrete specimen (Mix-B) while the change isgradual in concrete specimen (Mix-F) and mortar specimen.In mortar specimen more extensive damage and propagationof micro-cracks have occurred during the nucleation stage.

It is not easy to directly observe or photograph howmicrocracks form and develop, and how the microstructurechanges in concrete during the fracture process. The micro-structure, no doubt, would strongly influence the fracturebehaviour and the elastic wave propagation in concrete.Authors understand that if the photographs of microstruc-tures obtained from the core samples of the beams testedbefore and after the loading will be useful to support theresults obtained using the AE technique. Needless to say,that the AE technique is useful to detect, infer and study theformation and growth of microcracks and not the changes inmicrostructure in concrete under stress. Advanced AE tech-niques can provide a better insight into fracturing processesremotely and non-destructively and can reconstruct precise-ly the scenario of failure. The best way to study the micro-structure and the dynamics of fracture is through X-ray

inspection and computed tomography (CT) data collectionin concrete during loading. In fact the combination of AEand X-ray CT imaging techniques would be much moreuseful [8].

Conclusions

Based on the above results the following major conclusionscan be drawn.

1. The parametric analysis of acoustic emissions (AE) hasrevealed that micro-cracking occurs in three distinct andsuccessive stages namely initiation, coalescence andnucleation in both concrete and mortar specimens.

2. Both concrete and mortar test specimens have shownsimilar trends with regard to the changes in RDC andenergy count data of AE during the course of the experi-ments. The results have helped in identifying the crackinitiation and crack damage states for the samplestested.

3. The time history of b- value fluctuations are relativelylarge and more frequent in the initiation stage for bothconcrete and cement mortar. Using b-value analysis, thenucleation stage (N) could be identified more accurate-ly. As the nucleation stage commenced, fluctuationshave reduced and the b-value has decreased indicatingthe micro-cracking damage is more intense. In fact thelines connecting the b-values which were closer earlierbegan to expand and became farther. In other words, theb-values are more concentrated near the peak load andmore scattered in the post peak regime.

4. The nucleation stage in concrete specimen of higherstrength began only after ~90 % the total micro-crackdamage had occurred in it during the initial and coales-cence stages. Where as in the specimens of lowerstrength, it commenced rather early in both concreteand mortar as inferred from the normalized cumulativeenergy count data.

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