Cement and Concrete Research

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1 Cement nd Concrete Reserch 48 (213) 1 8 Contents lists ville t SciVerse ScienceDirect Cement nd Concrete Reserch journl homepge: Investigtion of different frcture modes in cement-sed mterils y coustic emission D.G. Aggelis,, A.C. Mplsks, T.E. Mtiks Deprtment of Mechnics of Mterils nd Constructions, Vrije Universiteit Brussel, Pleinln 2, 15 Brussels, Belgium Deprtment of Mterils Science nd Engineering, University of Ionnin, 4511 Ionnin, Greece rticle info strct Article history: Received 27 April 212 Accepted 12 Ferury 213 Aville online xxxx Keywords: Acoustic emission Chrcteriztion (B) Mortr (E) Finite Element Anlysis (C) Crck Detection (B) Chrcteriztion of the crcking mode in cementitious mterils llows evlutions concerning the remining life of the structure since in generl, sher-like phenomen occur fter tensile crcking. Individul modes of crcking cuse different motion of the crck tip dictting the wveforms emitted fter crcking events. In this study frcture experiments on cementitious specimens re conducted. The frcture mode is controlled y modifying the experiment geometry nd the process is monitored y coustic emission. The distinct signture of the crcking modes is reflected on coustic wveform prmeters like the mplitude, RA-vlue nd frequency. Signls emitted y the sher testing exhiit longer wveforms nd lower frequency thn those y the tensile testing. The influence of inhomogeneity is lso evident s signls cquired t different distnces exhiit distinct chrcteristics. Results show tht AE study leds to chrcteriztion of the dominnt frcture mode using only two AE descriptors nd offers the potentil for in-situ ppliction. 213 Elsevier Ltd. All rights reserved. 1. Introduction Chrcteriztion of the frcture mode in engineering mterils is tsk concentrting lot of effort in the engineering community. This is prticulrly importnt since during the frcture process, sequence of frcturing modes is generlly followed. Therefore, chrcteriztion of the dominnt mode highlights the stge of filure (initil, intermedite or finl). Specificlly for concrete, lot of work hs een done showing tht sher phenomen dominte t the lst stges of filure, while initilly, tensile mode is mostly ctive [1 5]. A method tht is suitle for the monitoring of the frcture phenomen is coustic emission (AE). AE gives the opportunity to highlight the moments of mjor or minor frcture occurrences nd follows the procedure until finl filure. Informtion of the popultion of the AE signls, s well s specific indices enlighten the understnding of the frcture process nd enle the evlution of the generl condition of the mteril ech moment. Concerning the frcture mode chrcteriztion, moment tensor nlysis (MTA) hs een developed nd successfully pplied in lortory experiments [6]. The frcture mode is evluted nd resonly correlted with the oservtion of the loction of the crcks fter filure [1,6]. However, the use of MTA is not esy in rel structures due to prcticl limittions. The most importnt is tht ech AE event needs to e recorded y s mny s eight individul sensors in order to supply enough informtion on the directionlity of the coustic wve. In lrge structures the sensors re distriuted in reltively long distnces in order to cover s much volume of the Corresponding uthor: Tel.: ; fx: E-mil ddress: dggelis@vu.c.e (D.G. Aggelis). structure s possile nd enle evlutions on the whole scle. Therefore, when using sensor seprtion distnces of severl meters [7,8] it is highly unlikely to record the trnsient wves of the sme event with the required numer of sensors due to inherent mteril ttenution. In light of the ove, relile chrcteriztion scheme tht requires less numer of sensors is desirle which however, cn provide useful nd relile informtion on the mteril's dmge sttus. The prolem of ccurte chrcteriztion of the crcking condition in mteril is nerly impossile tsk. In order to fully descrie the dmge condition within mteril, severl prmeters should e evluted. These include the size, shpe nd orienttion of ech crck, the frcture mode under which it propgtes s well s its propgtion rte under specific loding pttern. Considering tht ny lrge concrete structure includes numerous crcks either from construction or function lods, one relizes tht successfully deling with the prolem of dmge sttus chrcteriztion in its solute form is highly unlikely. Still, informtion on the generl condition is vitl. If this informtion derived y NDT monitoring is the sis for mintennce procedure tht potentilly extends the sfe service life for decdes, it is understood tht even rough evlutions ttin gret significnce. Additionlly, indices otined s function of time enle comprisons etween the ehvior nd performnce of the structure efore nd fter sustining dditionl loding. An effort hs een undertken in order to monitor dmge of cementitious mterils sed on the recording of simple AE prmeters. A description of AE monitoring nd the min signl descriptors follows. AE llows monitoring of crck growth using trnsducers plced on the mteril. These trnsducers record the trnsient response of the /$ see front mtter 213 Elsevier Ltd. All rights reserved.

2 2 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) 1 8 surfce fter ech crcking event (similr to seismic ctivity) nd trnsform it into electricl voltge due to their piezoelectric nture [9]. The informtion tht cn e drwn concerns the crcks' loction nd the totl ctivity, which is relted to the numer nd the nture of crcking events [1,11]. Detiled study of qulittive prmeters of the received wveforms leds to chrcteriztion of the frcture process [1 6] nd sustined dmge ssessment [1,12,13]. Certin AE fetures hve een successfully correlted to the frcture process. One is the mximum mplitude (A) of the wveform tht depends on the intensity of the crcking source, see Fig. 1. Another importnt descriptor is RA vlue which is the durtion of the first, rising prt of the wveform (rise time, RT) over A mesured in μs/v, nd hs een shown to e very sensitive to the frcture mode [4,14,15]. Frequency prmeters like the verge frequency, AF, re lso importnt. AF is the numer of threshold crossings over the durtion of the signl, mesured in khz. Another importnt frequency feture is the pek frequency, PF, which is the frequency with the mximum mgnitude fter fst Fourier trnsformtion of the wveform [16], see Fig. 1. The individul modes of filure led to wve emissions with different chrcteristics llowing chrcteriztion of crcks sed on their mode [17]. Severl studies hve shown tht tensile crcks led to AE with higher frequency chrcteristics nd lower RA vlues thn sher crcks [1 4,14,15]. Aprt from dmge, AE prmeters hve lso een used to highlight other processes like self-heling in concrete [18,19], melting of prticles in mtrix [2] or correlting with strength nd prticle/inclusion size [11,21]. In most of the ove works tensile nd sher phenomen re successfully distinguished y their AE ehvior. However, the distinct frcture signtures re exhiited y vrious kinds of phenomen nd not y the different frcture modes of the mteril itself. As n exmple, in steel fier reinforced concrete [3,4] under ending, tensile crcks re relted to mtrix crcking. On the other hnd sher phenomen re ssocited with fier-pull out. The first depends on the strength nd qulity of the concrete mtrix, while the second on the interfcil sher strength etween the mtrix nd the fiers. Amplitude (V) Dur RT 1/ RA A Similrly mtrix crcking nd delmintions re monitored in lminted composites [11,22,23]. The first is siclly ssocited to the tensile strength of the off-xis lyers, while delmintions depend gin on the interfcil sher strength etween successive lyers. In the present pper the modes of frcture re ssocited with the core mteril itself (cement sed mtrix), without other sources of inhomogeneity like fiers or lmins. The geometry of the testing experiment ws slightly modified in order to excite different frcture modes s expressed y the dominnt norml or sher stresses developed t the position of crcking s lso suggested y finite element simultion. The mteril ws cementitious mortr with smll ggregtes (snd) which is considered reltively homogeneous to concrete tht includes lrge ggregtes. AE ws monitored during frcture nd the differences etween the typicl signls received during sher nd tension re highlighted. It is seen tht, t lest in lortory conditions, chrcteriztion of frcture mode cn e relily conducted sed on the results of just one AE receiver. This is the continution of study concerning the effect of propgtion distnce on the cquired AE prmeters of pure ending [24]. 2. Experimentl detils 2.1. Mterils Two similr mortr mixtures were produced consisting of six specimens ech. The specimen size ws mm s typiclly used for three-point ending. The ggregtes consisted of 1% crushed snd with mximum ggregte size of 4.75 mm nd fineness modulus of 2.93, while the wter/cement rtio ws.55 y mss. The density nd the wter sorption of the snd were 25 kg/m 3 nd 2.44% respectively. The exct mix proportions were s follows: cement (type II 42.5 N) 44 kg/m 3, wter 242 kg/m 3, snd 1529 kg/m 3, nd super-plsticizer 4.5 kg/m 3. Six of the specimens were sujected to three-point ending ccording to EN :22 [25], see Fig. 2. A notch ws creted t the mid spn of the ottom (tensile) side in order to secure tht the crck would initite t the center of the specimen. The lod ws pplied t constnt rte of 5 N/s nd the loding ws utomticlly Sensors 4 mm Time (µs) 1E-1 PF 1 mm Sher zone Mgnitude 1E-2 1E-3 1E Frequency (khz) Sensors Loding t 4 mm Supporting ts Fig. 1. () Typicl AE wveform with min prmeters, nd () typicl frequency spectrum. Fig. 2. Experimentl setup for () three-point ending nd () sher mode with AE sensors.

3 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) terminted t the moment of lod drop. A slight modifiction ws done in the molding of the other six specimens tht were intended for the study of the sher frcture mode. Specificlly, metl t with length of 5 mm nd width of 4 mm ws plced inside the specimen just fter molding t the center of the top side (Fig 2). This modifiction of the geometry in comintion with the support ltered considerly the stress field s will e discussed in lter section Acoustic emission Concerning AE monitoring, two rodnd AE sensors with mximum sensitivity t 5 khz (Pico, PAC) were ttched to the side of the specimen s seen in Fig. 2 nd. Roller ering grese ws used to promote coustic coupling, while the sensors were secured y tpe (see gin Fig. 2). The horizontl distnce etween the sensors ws 4 mm nd the first ws plced t the horizontl distnce of 15 mm from the expected loction of the crck which ws secured y smll notches, s seen in Fig. 2 for the ending nd in Fig. 2 for the sher type. The signls were recorded in two-chnnel monitoring ord PCI-2, PAC with smpling rte of 5 MHz. The threshold ws set to 4 db to void mient noise nd the cquired signls were pre-mplified y nother 4 db. 3. Frcture detils nd FEM nlysis As forementioned, the trget of the study ws to develop nd pssively monitor different frcture modes in mortr specimens. In the simple 3-point ending test, it is esily understood tht frcture strts from the ottom due to high tensile stresses. However, concerning the modifiction of the geometry to test mixed frcture mode, simple FEM nlysis ws deemed necessry. Two dimensionl nlysis ws conducted for oth types of testing for comprison purposes. A free version of commercilly ville softwre ws used [26]. For the threepoint ending simultion under discussion, the model geometry ws 16 4 mm, similr to the front view of the specimen. Plin strin conditions were pplied. The lod ws plced t the centrl point of the top side nd support ws provided y two points t the ottom side (see Fig. 3) t the exct points s dictted y the experiment. For the slightly modified geometry intended for the sher-driven frcture, gin the front view of the specimen ws modeled. In this cse the lod is pplied in distriuted re on the top of the specimen due to metl t plced when the mteril ws fresh, while the ottom support is lso distriuted t two res (see Fig. 4). The supports were plced in such wy tht there is n overlp of opposing forces t the left side (re A in Fig. 4), while there is zone of 1 mm without overlpping t the right side etween the lod nd support distriutions (re B). The geometry cn lso e seen in the photogrph of Fig. 2. This free zone (B) in conjunction with smll notch tht ws creted t the end of the support (point C) led in the frcture of the specimen t this zone. Mteril elstic constnts were indictively ssigned the following vlues: Elstic modulus 2 GP nd Poisson's rtio.2 leding to sher modulus of 8.3 GP. These vlues re typicl for cementitious mterils without however eing the only possile vlues. The geometry ws divided into tringulr elements with nominl side of 6.5 mm, while the totl numer of nodes ws 163. Other mesh sizes were lso tested (element side of 8 mm) nd resulted in very similr stress vlues with 16 mm 4 mm Fig. 3. Stress field for three point ending test: () norml stress (σ xx ), nd () sher stress (τ xy ).

4 4 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) mm Lod ppliction D A 1 mm B C Supporting ts 5 mm 5 mm D B Fig. 4. Stress field for the sher test: () norml stress (σ xx ), nd () sher stress (τ xy ). the ones discussed elow. Results of the nlysis concerning norml nd sher stresses re seen in Figs. 3 nd 4 for the two types of specimens. As expected, the norml stresses in the pure ending cse, shown in Fig. 3, exhiit positive mximum t the mid spn of the ottom, where the specimens ctully roke. This hppens due to the wek tensile strength of cementitious mteril. Fig. 3 shows sher stresses with the sme loding condition. Mximum sher stresses re of lower level (pproximtely hlf of the corresponding norml), while t the mid-spn of the specimen sher stresses chnge sign, eing very close to zero. Therefore, the crck t the center of the specimen cn e considered quite close to pure tensile mode. Concerning the specimen intended for more sher frcture, corresponding results re seen in Fig. 4 nd. Forces nd supports were pplied y mens of ts insted of contct points. The lod ppliction re on top nd the left support were overlpped, while there is no overlp etween the lod ppliction nd the support t the right ottom (re B in Fig. 4). This led to n unsupported zone ville for shering of the specimen. Fig. 4 nd shows the norml nd sher stresses respectively. It is seen tht the mximum norml stresses re exhiited wy from the notch with the mximum tension ner the ottom mid-spn point. However, the tip of the notch clerly exhiits mximum sher stresses, s seen in Fig. 4, while the sher zone extends to the top in digonl direction until the norml ngle in point D. Although, low norml stresses my still e pplied on the sme zone, it cn e resonly rgued tht the initition nd propgtion of the specific crck include strong sher components which ws not the cse for the pure ending experiment. Fig. 5 shows frctured specimens fter oth types of tests. As the simultion results imply, the specimen tested in pure ending (Fig. 5) ws frctured t the mid spn where mximum tension is exhiited in Fig. 3. For the sher type of test (Fig. 5), the crck extends digonlly from the notch to the top within the zone sher stresses indicted y FEM nd eing in greement with the simultion results. Since the crcking events include different constituents of the stress tensor, monitoring y AE is expected to show distinct trends, something presented nd nlyzed in the following section. 4. AE results Results of the coustic monitoring concern the totl ctivity nd the qulittive fetures of the wveforms. Fig. 6 shows the ccumulted AE signls (hits) received y the ner-y sensors (15 mm from the crck) for typicl ending nd sher tests. For oth of them the popultion of hits is uilt grdully until the moment of finl filure where the mximum hit rte is exhiited. The totl numer of signls is limited elow 1 hits. The 2nd sensor (55 mm wy from the crck) follows the sme ehvior with the difference of slightly lower numer of hits (Fig. 6). This is resonle due to longer propgtion distnce which crucilly ttenutes some of the signls lredy cptured y the 1st receiver, elow the threshold level.

5 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) Fig. 5. Frctured specimens fter () ending, nd () sher. Fig. 7 shows the RA of the signls received y the 1st sensor for the sme experiments. Concerning the ending mode, RA rnges minly elow 1 ms/v except for the moment of finl filure when AE ctivity (hits) AE ctivity (hits) Sher Sher Fig. 6. Cumultive AE ctivity of typicl tests: () 1st sensor (15 mm from the crck), nd () 2nd sensor (55 mm from the crck). some RA vlues of more thn 1 ms/v re noted. The specimen under sher on the other hnd exhiited some RA vlues of level higher thn 1 ms/v, while t the end it showed certin increse similr or even higher thn the ending cse. The solid lines re the moving verge of the recent five vlues in order to show the trends in more cler wy. The corresponding RA vlues s cptured y the 2nd sensor re seen in Fig. 7. Agin the signls coming from the sher experiment exhiit generlly higher RA vlues, s reveled y the moving verge lines. Typicl results concerning the AF re included in Fig. 8 nd for oth frcture modes nd sensors. In this cse, the ending type of frcture exhiits higher frequency in verge, rnging from 5 khz to 4 khz while t filure AF vlues close to zero re lso noted. On the other hnd sher mode results in frequencies up to 25 khz, while t mcro-frcture they re limited elow 15 khz. AF mesured y the 2nd sensor follows similr trends with the sher eing gin notly lower thn ending (Fig. 8). These results re some typicl comprisons etween pirs of specimens while in the next prgrph the verge vlues of ll specimens re discussed. In the present cse, the mient noise level ws low nd the pplied threshold secured tht no externl signls re cquired. In order to increse the reliility of the otined informtion even more, the following nlysis concerns only the AE events. This mens tht for ny hit of the first sensor nlyzed, nother hit ws lso recorded y the second sensor which elonged to the sme source event. Typiclly for ech experiment numer of pproximtely 2 events were recorded. The AF nd RA vlue of these events re verged nd presented in Fig. 9. Tensile frcture mode specimens monitored y the 1st sensor, typiclly exhiit AF etween 11 khz nd 16 khz, while their RA is less thn 5 μs/v in verge. Fig. 9 contins lso the corresponding vlues for the sher type of testing. The verge vlues of AF of the five specimens s mesured y the 1st sensor show mximum AF of 15 khz nd minimum RA of 84 μs/v eing totlly seprted from the corresponding popultion of the ending (tensile) mode s monitored y the sensor t the sme distnce. Concerning the sme events monitored y the 2nd sensor (t 55 mm) distinct chnges re shown. Tensile events exhiit lower frequency (from 8 khz to 14 khz) nd higher RA, close to 1 μs/v, compred to the sensor t 15 mm. This is n indiction of wveform distortion tht occurs in heterogeneous medi like cement mortr.

6 6 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) RA (µs/v) 1 Sher AF (khz) 3 2 Sher RA (µs/v) 1 1 Sher AF (khz) Sher Fig. 7. RA vs. time for different frcture modes s monitored y: () 1st sensor (15 mm from the crck), () 2nd sensor (55 mm from the crck). Scttering on the oundries of the microstructure (snd grins, ir ules) reduces the representtive frequency, minly y more effectively ttenuting higher components, while it lso elongtes the pulse ecuse the energy is redistriuted to mny rndom wve pths fter ech impct nd does not rrive in stright line s would hppen in homogeneous medium [27,28]. Additionlly, the sensor t 55 mm exhiits much lower frequency (round 6 khz) nd higher RA vlues (2 to 55 μs/v) thn the 1st for the sher mode s well. These indictions show tht ny AE study should e comined with knowledge of wve propgtion in the specific mteril, since few centimeters of dditionl propgtion etween sensor 1 nd 2 force noticele chnges. The ove mentioned AE descriptors (AF nd RA) re two of the most powerful in discriminting dmge mode [1,4,17]. However, the individul frcture modes result in differences in other AE prmeters s well. Fig. 1 shows the verge vlues of pek frequency (PF) nd Amplitude (Amp) for oth modes nd sensor distnces. The tensile mode exhiits higher mplitude nd pek frequency thn sher in verge for the sme sensor distnces. Aprt from the effect of frcturing mode, the effect of distnce is gin highlighted since the sensor t 55 mm otins much lower mplitude nd pek frequencies thn the ner-y sensor for oth crcking modes. Tle 1 summrizes the verge vlues of sic AE prmeters for the two exmined frcture modes nd propgtion distnces. One conclusion from these dt concerns the discrepncies in the AE descriptors etween the different frcture modes monitored t stndrd distnce. Sher (or mixed mode) frcture emits signls of lower sic frequency chrcteristics nd mplitude ut longer in rise time thn tensile. The differences re visile in ll prmeters Fig. 8. AF vs. time for different frcture modes s monitored y: () 1st sensor (15 mm from the crck), () 2nd sensor (55 mm from the crck). ut greter chnges re seen in RA, for which the vlues of sher (1734 μs/v) re more thn six times higher thn those of tension (278 μs/v). The decrese in mplitude is lso lrge since sher emissions re lower thn tensile y lmost 5 db (58.4 db to 53.8 db). Additionlly, frequency chrcteristics mesured either y the AF or the PF show strong decrese for the sher mode y out 35% compred to tensile events. Overll, the clerly seprted verges nd the reltively limited vrince (presented s stndrd devition over verge, AF (khz) Tensile, 15 mm Tensile, 55 mm Sher, 15 mm Sher, 55 mm RA vlue (µs/v) Fig. 9. AF vs. RA vlue for different frcture modes nd sensor distnces (ech symol is the verge of ll AE events of ech experiment).

7 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) Tensile, 15 mm Tensile, 55 mm 3 µs 58 db 5 µs Pek Freq. (khz) Sher, 55 mm Sher, 15 mm 6 µs 53 db 8 µs type Sher type Amp (db) Fig. 1. PF vs. Amp for different frcture modes nd sensor distnces (ech symol is the verge of ll AE events of ech experiment) Time (µs) Fig. 11. Typicl wveforms ttriuted to different frcture modes (distnce 15 mm). coefficient of vrition, COV) enle quite good seprtion of the clusters without the need to employ pttern recognition pproches. It is seen therefore, tht in simple geometries in lortory conditions, the crcking mode cn e relily monitored y its AE fingerprint. However, the differences mesured y the two sensors show how much the results re dependent on the testing conditions nd specificlly the distnce etween the crcks nd the sensors. The dditionl distnce of 4 mm etween the two sensors forces decrese of more thn 2% in frequency prmeters (132 khz to 19 khz for tension nd 87 khz to 56 khz for sher) nd n increse y three to four times in RA (278 μs/v to 9 μs/v for tensile nd 1734 μs/v to 398 μs/v for sher). Indeed, scttering is quite strong in this kind of mteril due to inhomogeneity which distorts the shpe of the propgting pulse resulting lso in considerle ttenution especilly t the higher frequencies. Complimentry to scttering, viscous dmping of the mteril contriutes to loss of trnsmitted energy to het, resulting in decrese in mplitude of 5 to 6 db etween the first nd the second receiver. 5. Discussion Comining the informtion of Tle 1, n illustrtion of n AE wveform typicl of ech mode, s recorded y the sensor t 15 mm is shown in Fig. 11. The tensile wveform hs n mplitude much higher thn sher, while it is lso chrcterized y shorter rise time (3 μs compred to 6 μs). Additionlly the totl durtion of tensile signl is significntly shorter thn sher. The ove mentioned results show tht the seprtion of frcture modes is tngile in lortory conditions. The chrcteriztion in rel structures would lso e tngile if it weren't for the signl distortion nd ttenution tht occur in heterogeneous mterils. Along with the sensor's response this is nother prmeter tht renders the ppliction Tle 1 Bsic AE prmeters for different modes nd propgtion distnces to the sensor. Mode nd distnce AF (khz) RA (μs/v) AMP (db) P-Freq (khz) Tensile (15 mm) Averge COV 16.5% 33.2% 5.5% 12.5% Sher (15 mm) Averge COV 18.% 49.7% 2.7% 27.7% Tensile (55 mm) Averge COV 19.6% 12.2% 3.% 1.5% Sher (55 mm) Averge COV 17.8% 3.8% 2.1% 18.2% of AE cse-specific nd does not llow fully exploiting its potentil in rel structures [29]. If distortion is not tken into ccount, tensile signls monitored t longer distnce (55 mm) my well e overlpped with sher signls received t 15 mm from the source, see Figs. 9 nd 1. This would prevent from pplying chrcteriztion in rel structures where the distnces etween crcks nd sensors re typiclly of the order of meters nd highlights the importnce of incorporting the distnce tht the wve trveled from the crck tip to the sensor. Fortuntely contemporry AE equipment includes softwre with powerful loction lgorithms. Therefore, the time dely etween cquisition of wveforms of the sme event t different positions leds to clcultion of the crck loction in rel time. This enles n inverse procedure for evlution of the signl s-emitted y the crck efore distortion due to inhomogeneity or geometry is ccumulted in the s-received y the sensor wveform [16]. Another issue tht should e highlighted concerning the specific study is tht lthough cement mortr cn e regrded s mcroscopiclly homogeneous concerning the stress nlysis, still the mteril contins grins with n verge size of 2 mm. Therefore, it is resonle tht the crck propgtes on the interphse etween the mtrix nd the inclusions nd the mcroscopiclly simulted or expected stress fields my somehow differ from the ctul in the micro-scle of the grin mtrix interphse. If the mteril ws totlly homogeneous then the ctul stresses would possily e closer to the stresses simulted. However, this study is focused on cementitious mterils, which in ny cse include snd grins nd therefore, this is relistic wy to study modes of frcture in cement-sed mterils. As future tsk experiments on mrle specimens re scheduled. Also possile modifictions in the geometry will e studied in order to develop pure sher frcture, since currently low level norml stresses re lso developed on the tip of the notch. Upgrding the scle of the prolem is lso eing considered in order to fcilitte concrete testing. Finlly, the sensor response function should e discussed. The results otined refer to specific sensor type. This type my e considered rodnd compred to other resonnt AE sensors ut still its response is fr from eing flt. Therefore, the wveforms cptured re certinly influenced y the sensor preferences. The specific vlues otined would definitely differ if other pirs of sensors hd een used in the experiment. However, wht is emphsized herein is the chnge etween the emissions of the different frcture modes nd seprtion distnces. All the experiments were conducted y the sme pir of sensors so tht ny distinct trends re ttriuted directly to the frcture mode nd seprtion distnce. In ny cse, in the field of concrete AE, stndrdiztion is currently eing ttempted [17] mening for the proposed type of sensor similr procedure could

8 8 D.G. Aggelis et l. / Cement nd Concrete Reserch 48 (213) 1 8 e undertken to estlish dtse with the AE fingerprints of frcture modes t vrious propgtion distnces. 6. Conclusions The present pper occupies with pssive monitoring of the frcture process in cementitious mterils with the use of the coustic emission technique. The min im is to chrcterize the coustic signture of frcture modes. For this reson, tensile nd sher frcture modes were trgeted. Both types of experiments were conducted on the sme loding equipment with slight modifictions. Elementry finite element nlysis showed tht the modifictions were ctully effective to lter the stress field nd shift from the norml tensile stresses (responsile for frcture in three point ending) to dominnt sher. Monitoring with rodnd AE sensors reveled distinct trends for the different frcture modes. Specificlly AE signls from tensile tests exhiit higher mplitude nd frequency, while those from sher tests exhiit longer durtion nd lower rising ngle, s mesured y the RA vlue. The results show tht ccurte chrcteriztion of modes is possile in lortory conditions sed on simple fetures recorded y one AE sensor. This would e very eneficil for wrning ginst finl filure since crcking modes generlly follow sequence during frcture. Results lso highlight the need to jointly study AE with elstic wve propgtion since n dditionl distnce of few centimeters lters significntly the frequency content nd shpe of the cquired wveforms. This is n importnt prmeter tht needs to e mended in order to fully exploit the cpilities of AE in lortory conditions nd expnd relile chrcteriztion in rel structures. Acknowledgment The contriution of Dipl. Mterils Engineer Peli Ginnoultou in mking nd testing the mortr specimens is cknowledged. 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