- Email: [email protected]
Analysis of the process of crack initiation and evolution in concrete with acoustic emission testing
archives of civil and mechanical engineering 14 (2014) 134–143
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Analysis of the process of crack initiation and evolution in concrete with acoustic emission testing B. Goszczyn´skan Kielce University of Technology, Civil Engineering and Architecture Department, Al. 1000-lecia P.P. 7 25-314 Kielce, Poland
ar t ic l e in f o
abs tra ct
Article history:
Application of the acoustic emission method (IADP), to the analysis of crack initiation and
Received 31 January 2013
growth in concrete and reinforced concrete beams is presented in the paper. This method
Accepted 23 June 2013
is based on the idea that every active destructive process becomes a source of acoustic
Available online 18 July 2013
emission. Comparing AE signals, generated within structures under service load, with
Keywords:
previously created database, one can identify the processes of active deterioration
Reinforced concrete
occurring in an element. They can be located on the basis of the difference in the time,
Damage process
that AE signal reaches the sensors with known wave velocity.
Micro-cracks and cracks
Because the cracking process (micro-cracking) occurs in concrete already at the
identification and location
maturing stage, experiments were performed on unloaded concrete members just after
Acoustic emission
concreting (when shrinkage occur) as well as on concrete beams (in technical scale) subjected to continuous loading. It was found that using the IADP method, it was possible to detect and locate creation of micro-cracks (not visible on the member surface) and initiation and growth of cracks, which are visible on the element surface. & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Cracks come as concrete natural response to strain condition developed in it. Their occurrence is inherently connected with the reinforced concrete structure performance and it is considered to be one of the factors that determine structure durability. Advanced process of crack initiation always provides a warning that the structure health has been threatened. Crack distribution, specific to a given situation, is a perfect source of information on the structure condition. The analysis of the process of crack initiation and growth can therefore be applied to reinforced structure diagnostics.
Crack initiation and growth [1], one of the basic mechanisms in concrete mechanics, has been investigated, both experimentally and theoretically, for many years [2]. In the theoretical analysis, micro-cracks (defects) theories are relied on exact solutions and the numerical ones using various models, e.g. [3–5]. The accuracy of this type of description depends on the model itself, and on the experimentally obtained parameters that determine a given model. Cracking is most often characterised by three quantities: cracking moment, distance between cracks and crack width. Those depend on [6] direct actions, i.e. loads and indirect ones, including, among others, concrete shrinkage, which affects crack initiation and opening.
n
Tel.: +48 667904658. E-mail address: [email protected]
1644-9665/$ - see front matter & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.06.002
archives of civil and mechanical engineering 14 (2014) 134–143
All the actions are random in character, and it can also be indicated that the condition of cracking depends, to a far extent, on randomly varied values of concrete tensile strength and steel adhesion to concrete. Attempts have therefore been made to find random functions that would describe the process of crack initiation, e.g. [7], yet the reliability of those descriptions needs to be validated against large size samples. Although advanced and interesting, those methods are still far from being directly applicable to the diagnostics of reinforced concrete structures [3]. For that reason, construction diagnostics currently rely on the methods based on direct measurements of the extent of cracks and of other destructive processes. Widely varied experimental techniques, such as radiology, optical microscopy, ultra sounds, have been employed to conduct investigations. For many years attempts have been also made to use acoustic emission measurements [8]. Great advancement in information technologies made it possible to further develop research methods, including the ones already in use, e.g. the method of acoustic emission signal measurement [9–12]. Advantages offered by the AE method, which differentiate it from others, include the following:
its global character, i.e. the possibility of applying testing to
the whole of the structure, or to its most important member (e.g. as regards the structure load bearing capacity); the possibility of accounting for the structure service conditions, e.g. the real service load, temperature, humidity, etc.; and the possibility of applying long-term (continuous or periodic) monitoring of the whole structure.
The two best known AE procedures, that have been used until now for the evaluation of the technical condition of structures, are the so-called:
American Procedure [13,14]. Japanese Procedure [15–17]. They are based on the individual descriptors of the acoustic emission signal (wave). They describe the way tests are conducted with the AE method exclusively on prestressed pre-tensioned or reinforced concrete beams. Both procedures [18,19]:
do not allow for testing under service load, are based on accurate load measurements taken in accor
dance with a precisely defined scheme, which is possible to accomplish only under laboratory conditions, have been developed for selected elements examined under laboratory conditions, do not indicate how sensors should be spaced, and do not specify the conditions under which a tested object is to be loaded.
Hence in works [18,20–22], the acoustic emission method based on the identification and location of active destructive processes (IADP) was presented.
135
The principle behind the method is to compare acoustic signals generated by an object (member) under load with the database of signals produced by destructive processes. On such basis actually proceeding processes are identified. The location of defects is carried through measurements of differences in the generated signals arrival time at acoustic sensors spaced on the member. Investigations on real structures and those performed under laboratory conditions demonstrate that the method allows appropriate identification of the process of crack initiation in pre-stressed members. Works conducted recently [22,23] confirm that it is possible to use the method to diagnose reinforced concrete members (non-prestressed). The present paper describes the application of the acoustic emission method (IADP) to the analysis of the process of crack initiation and growth in concrete and reinforced concrete beams.
2. Fundamentals of the method of identification of active destructive processes (IADP) The acoustic emission method is based on examining acoustic emission signals that accompany internal changes in the structure of the material, i.e. concrete, which are generated by a local release of energy. Initial and following strains produced by external actions result in changes in energy, which can be recorded, as an acoustic wave, on the member surface. Modern measurement apparatus together with software make it possible to record many parameters of the acoustic signal. Taking into account qualitative and quantitative variety of processes that proceed in a given material, there is a real chance that a correlation between the type of signal-generating processes and their parameters will be found. It can be expected that the processes of the material structure destruction will be characterised by a higher level of released energy, which should facilitate the separation of the stages of the material performance. Separating and identifying the signals corresponding to a given process in the material can be significant for nondestructive tests on the strength of structural concrete.
2.1.
Concept of acoustic emission IADP method
Fig. 1 shows, in a form of a diagram, the concept underlying the IADP method (Identification of Active Destructive Processes). The acoustic signal generated by a destructive process is recorded by AE sensors located on the surface of the examined member, then registered and compared with reference signal database created earlier for destructive processes. That allows the identification of destructive processes in the member. Processes generating AE signals accompany only active defects, i.e. those which arise or develop under conditions prevailing at measurement taking. Reference signal databases for individual processes are established using material specimens and models in laboratory tests. It is done on the basis of twelve AE signal parameters: root mean square, reverberation frequency, average signal frequency, initiation frequency, average signal level, amplitude in mV or dB, number of counts to
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archives of civil and mechanical engineering 14 (2014) 134–143
peak amplitude, number of counts, energy count, rise time, duration and signal strength (true energy) with the use of image analysis method.
2.2.
Location of destructive processes
The location of signals that accompany arising destructions (defects) takes place on the basis of measurements of differences in time of generated signals arrival at acoustic sensors spaced over the member. It is so called zonal or linear location [21,22]. Fig. 2 shows as an example the diagram of the so-called linear location. On the basis of difference in the signal arrival time Δt at sensors 1 and 2, it is possible to determine the position of the plane (a) normal to line 1–2 connecting the sensors. That makes it possible to easily define the plane in which a destructive process occurs, also in real objects, and then to locate the place of its occurrence [22].
2.3.
Separation of destructive processes
In order to compile a reference signal database, it is necessary to establish destructive processes, relevant for the technical
state evaluation of reinforced concrete structure, which generate acoustic emission signals that are recordable on the member surface. Analysis of the processes that occur in concrete and reinforced concrete members (non-prestressed) as a result of direct and indirect actions led to distinguishing eight groups of destructive processes and corresponding signal Classes: Class 1: Initiation of cracking in the cement paste. Class 2: Initiation of cracking on the paste-aggregate interface. Class 3: Initiation of micro-cracks. Class 4: Growth of cracks. Class 5: Loss of adhesion in the crack vicinity. Class 6: Buckling of compressed bars. Class 7: Crushing of compressed concrete. Class 8: Steel yielding (plastic deformation)/reinforcement rupture.
Because investigation into the process of crack initiation and growth can provide basis for the assessment of the technical state of reinforced concrete structures, destructive processes and corresponding signal Classes 1, 2, 3 and 4 were subjected to detail comparative analysis.
3. Analysis of the process of crack initiation and growth
Fig. 1 – Diagram of AE wave generation by destructive processes, their recording and comparison with reference signal database.
Table 1 – Numbers and notation of reference signal classes in unloaded concrete.
Strains (and stress concentrations) occur in concrete already at the maturing stage. They are caused by hydration processes and local increases in temperature. The multi-phase material composition results in internal stresses leading to a number of mechanical phenomena, including mainly the formation of micro- and even macro-cracks. In fracture of concrete structures the following can be distinguished:
micro-cracking, which produces internal effects not man
ifested on the member surface in the form of cracks (more than 0.1 mm in width), and initiation and growth of cracks, the process visible on the member surface.
Notation
Class no.
1s
2s
3s
Taking into account the above [24] and the structure of the concrete [25], the analysis of the process of crack initiation and growth was performed on unloaded concrete members, which validates Classes 1, 2 and 3 destructive processes, and also on reinforced concrete beams loaded until failure, which allows the validation of Class 4.
3.1. Analysis of the process of micro-cracking in unloaded concrete
Fig. 2 – AE source location with accuracy to the perpendicular plane.
Tests were performed [26] on fresh concrete, at the first stage of its maturing, when the shrinkage effect is the greatest. 150 150 600 mm rectangular prism specimens of C20/25 class and C40/50 class concrete were made at the faculty
archives of civil and mechanical engineering 14 (2014) 134–143
Fig. 3 – Specimen diagram.
137
laboratory in accordance with the concrete mix design employed at the concrete mixing plant, using limestone aggregate. The specimens were concreted and transferred on the next day to a water container for 11 days. There the setting proceeded, which allowed AE sensors fixing, in addition due to specimen storing in the water environment, shrinkage did not occur. After specimens were removed from water, acoustic emission sensors were fixed with an adhesive on one of their walls, whereas on the remaining three walls metal
Fig. 4 – AE Classes (shown as number of hits) and medium specimen strain versus time (C20/25) – concrete without admixtures.
Fig. 5 – AE Classes (shown as signal strength) and medium specimen strain versus time (C20/25) – concrete with admixtures.
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archives of civil and mechanical engineering 14 (2014) 134–143
bench-marks (acting as reference points) were glued (Fig. 3) to take strain measurements with the extensometer, 8 in. in base (200 mm.). Specimens were arranged in the laboratory hall, base strain measurement readings were taken, then acoustic emission measurements started and continued for 14 days. The number of signals of a given class was counted for 24 h.
Strain measurements and also those of ambient humidity and temperature were taken every 24 h. Basing on previous research findings, three groups of destructive processes were differentiated, which could be sources of acoustic wave generation:
micro-cracks in the cement paste; micro-cracks at the cement paste/aggregate interface; and initiation of micro-cracks (cracks dimensions below 0.1 mm) on the concrete surface.
Recorded acoustic signals were grouped into three classes corresponding to the processes described above and reference signal database for those destructive processes was established.
Class 1s – micro-cracks in the cement paste. Class 2s – micro-cracks at the cement paste/aggregate interface.
Class 3s – initiation of micro-cracks on the member surface.
Fig. 6 – AE Classes (shown as mean strength of the signal) and medium specimen strain versus time (C20/25) – concrete with admixtures.
Classes were differentiated on the basis of 12 parameters (mentioned before) of the acoustic signal using commercial Noesis 4.0 software. Symbols and colours attributed to classes are shown in Table 1.
Fig. 7 – Diagram of the loading of Type-I reinforced concrete beam, with spaced sensors and marked deformation area measured with 3D scanner.
Fig. 8 – Diagram of the loading of Type-II reinforced concrete beam, with spaced sensors and marked deformation area measured with 3D scanner.
archives of civil and mechanical engineering 14 (2014) 134–143
Then, obtained reference signal database was used to analyse acoustic signals for the other specimens tested similar way for 68 days. Figs. 4–6 show the results of measurements for specimens of C20/25 class concrete using limestone aggregate without Table 2 – Colours and codes attributed to signal classes. Colour/shape Class no.
No. 3
No. 4a
No. 4b
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chemical admixtures. Recorded AE signal Classes are presented: as a function of number of hits (one of the twelve AE parameters that provide Class identification) versus time (Fig. 4), as a function of signal strength versus time (Fig. 5) and as a function of mean strength of the signal (signal strength/number of hits) versus time (Fig. 6). Mean strains obtained from measurements taken with the extensometer on three sides of individual specimens on the successive days of the experiment are also presented. It can be stated that Class 1s and 2s signals are mainly recorded. They result from micro-cracks in the cement paste and at the paste–aggregate interface respectively. The mean strength of Class 2s signals is twice higher as of Class 1s. In the first 5 days, the amount of signals increases with an increase in strains. After that, the process is substantially attenuated in spite of continued strain increase, and finally it becomes stabilised after approx. 20 days.
3.2. Analysis cracking process in reinforced concrete beams Experimental tests were conducted on two types of reinforced concrete beams [23] differed in geometrical dimensions and way of loading: Fig. 9 – Type-I beam, recorded signal Classes at the first stage of loading (0–8000 s) – zone of sensors 1–4.
Type I — 0.12 0.30 3.00 m beam loaded with two concentrated forces (Fig. 7) and
Type II — 0.20 0.45 6.00 m beam loaded with one concentrated force applied at a distance of 0.80 m from the support axis (Fig. 8)
Applied load was increasing until failure with a constant velocity. Such a mode of beam loading made it possible to differentiate the areas in the beam, where bending and shearing were dominant. In the tests, the following data were recorded:
Concrete deformation on the lateral surfaces of the beams, with the use of a 3D optical scanner.
AE signals accompanying destructive processes, with the Fig. 10 – Type-I beam, recorded signal Classes at the first stage of loading (0–8000 s) – zone of sensors 2 and 5.
use 55 kHz frequency AE sensors, the spacing of which is shown in Fig. 7 (numbers 1–8) and Fig. 8 (numbers 1–9).
Fig. 11 – Type-I beam surface deformations after 16,000 s loading.
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archives of civil and mechanical engineering 14 (2014) 134–143
The recorded AE signals were transferred to a computer using m-SAMOS processor and analysed with Noesis 4.0 software and reference signal databases. Due to the analysis of the recorded acoustic emission signals, two groups of destructive processes that can cause the generation of an acoustic wave were differentiated:
micro-cracks and crack initiation, and initiation and growth of cracks (larger than 0.1 mm) on the concrete surface. Based on the twelve parameters of the acoustic signals, the destructive processes were grouped into three classes:
Class 3 – micro-cracks and initiation of cracks, Class 4a– initiation and growth of cracks from shearing, Class 4b– initiation and growth of cracks from bending, which were marked with symbols and colours shown in Table 2. During the loading process, the 3D optical scanner, the acoustic emission system and the loading system were synchronised with each other. Consequently, deformations and AE signals, recorded in a given second, were measured simultaneously with the values of forces.
3.2.1.
Fig. 12 – Type-I beam, summation graph of the signal strength with division into Classes – zone of sensors 2 and 5.
Analysis of fracture process in Type I beams
Figs. 9 and 10 show colour-marked AE signal Classes in the case of the Type I beam, identified with reference signal databases, in the near-support and central zones (zones covered by AE sensors 1–4 (dominant shearing zone) and 2 and 5 (dominant bending zone)). They were recorded at the initial stage of the loading process (0–8000 s) and are presented as a function of signal strength (one of Class-defining parameters) versus time. Class 3 signals, defined as signals resulting from microcracks, appear from the beginning of the loading process. First Class 4b signals (the propagation of bending cracks) are recorded after approx. 1000 s. Class 4a signals (the propagation of shearing cracks) manifest themselves later than Class 4b signals, and more apparently after approx. 6000 s. Similar behaviour is observed using 3D scanner [23]. At the initial stage of the loading process (after approx. 2000 s),
Fig. 13 – Type-II beam surface deinitiations after 400 s.
Fig. 14 – Type-II beam surface deformations after 550 s.
archives of civil and mechanical engineering 14 (2014) 134–143
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Fig. 15 – Type-II beam surface after damage.
Fig. 16 – Recorded signal Classes at the first stage of the Type-II beam loading (0–350 s).
At the end, the damage proceeds through crack propagation in the zone where bending is dominant, in the beam central area (zone covered by sensors 2 and 5), up to the member failure. Fig. 11 presents the image of the beam surface deformations, obtained with 3D optical scanner after 16,000 s of monotonically increasing load. Results obtained with the optical scanner illustrate the deformations of the examined beam lateral surface. The brightenings that are visible correspond to a local accumulation of deformations, which clearly represent cracks being formed. Their colour depends on the cracks width: white/green – min, red – max. Fig. 12 shows summation graph [23] of the signal strength for the whole beam loading process, with division into Classes, for the area where bending is dominant (sensors 2 and 5). As it was mentioned before, bending dominates in the zone of sensors 2 and 5. Using 3D scanner it is seen that, cracks perpendicular to the member axis, are mainly observed. It is also shown by the energy summation graph with division into Classes (Fig. 12). The sum of Class 4b signal strength (the propagation of bending cracks) clearly increases after 10,000 s, which is not visible for the growth of shearing cracks (Class 4a). In [22,23] it is shown that using IADP method it is possible not only to identify the cracking process due to bending, but also to locate it.
3.2.2.
Fig. 17 – Recorded signal Classes for the whole time of Type-II beam loading (0–600 s) – summation graph. cracks perpendicular to the member axis appear, which result mainly from the action of the bending moment. Then, the growth of perpendicular cracks is accompanied by the initiation and growth of shearing (diagonal) cracks (after approx. 6000 s) in the zone where shearing is dominant (zone covered by sensors 1–4 – Fig. 7). Bending and shearing cracks propagate until the failure of the beam; however, at first, the growth of shearing cracks in the near-support zone (zone covered by sensors 1–4) is clearly manifested.
Analysis of fracture process in Type II beams
After the load is applied, in the area defined by sensors 1–9 (Fig. 8), cracks resulting from shearing and bending develop (Fig. 13). After approx. 450 s of loading, a crack from shearing is suddenly formed (Fig. 14 – red colour), the growth of which leads to the beam failure (Fig. 15). The signal strength, as a function of time, at the loading initial stage, is presented in Fig. 16, whereas the summation graph of the signal strength as a function of time for the whole loading process is shown in Fig. 17. As can be seen in Fig. 17, up to approx. 400 s, signals accompanying the initiation of cracks resulting from shearing and bending are recorded. After 430 s, a dramatic growth of cracks from shearing occurs, which finally leads to the beam failure. That confirms visual observation, described above, with the 3D scanner – Figs. 13 and 14 and view of the lateral surface of the beam after damage – Fig. 15.
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archives of civil and mechanical engineering 14 (2014) 134–143
Fig. 18 – Comparison of the location of shearing cracks (the IADP method) with observation with 3D scanner. The location of the source of the acoustic signal is based on the measurement of difference in the signal arrival time at AE sensors, the position of which is known, as it is presented in Fig. 2 for linear location. Fig. 18 shows the comparison of the location of Classes of AE signals (linear location) originating in crack growth in the near-support zone. The upper part of the figure illustrates the application of the linear location of AE signals, and the bottom part shows cracks observed with the 3D scanner. Therefore, it can be stated that the IADP method allows not only the identation of cracks, but also makes it possible to locate them.
the beam surface observations conducted with a 3D optical scanner show fairly good congruence.
Acknowledgement Research was executed under the development Project NR 04 0007 10 “Non-destructive system of monitoring and diagnosis of reinforced concrete structures with particular emphasis on engineering objects”.
r e f e r e n c e s
4.
Conclusions
The paper presents the validation of the IADP method, which was developed and applied to prestressed structures, for nonprestressed structures. It was demonstrated that:
the IADP method can be applied to reinforced concrete (non-prestressed) members;
it is possible to establish reference signal databases, where
the crack growth process can be divided into two signal Classes corresponding to crack growth due to shearing and bending. Thus, the analysis of acoustic signals generated by the impact of a load indicates not only the initiation and propagation of cracks, but also provides information on what kind of cracks they are; and as regards crack identification and location, the analysis carried out with the IADP acoustic emission method and
[1] T. Godycki-Cwirko, The Mechanics of Concrete, Arkady, Warszawa, 1982 (in polish). [2] M.F. Kaplan, Crack propagation and the fracture concrete, ACI Journal Proceedings 58 (11) (1961) 591–610. [3] Z.P. Bazant, Concrete fracture models: testing and practice, Engineering Fracture Mechanics 69 (2002) 165–205. [4] Z. Yang, J. Chen, Fully automatic modelling of cohesive discrete crack propagation in concrete beams using local arc-length methods, International Journal of Solids Structures 41 (2004) 801–826. [5] P. Rossi, S. Richer, Numerical modelling of concrete cracking based on a stochastic approach, Materials and Structures 20 (1987) 334–337. [6] M. Knauff, Calculations for Reinforced Concrete Structures in Accordance with Eurocode 2, PWN, Warszawa, 2012 (in polish). [7] B. Goszczyńska, Description of empirical process of crack initiation in framework of probabilistic theory, Archiwum Inżynierii Lądowej XLVIII (4) (2002) 405–423.
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[8] A. Mielnik, Tests stresses, microcracks and other parameters of reinforced concrete beams by help of sonic gauges (Electrophonoscope), in: Proceedings of the XI KN PZITB and KILW PAN Scientific Conference, 1965, pp. 145–150 (in polish). [9] J. Hoła, K. Schabowicz, State-of-the-art non-destructive methods for diagnostics testing of building structures – anticipated development trends, Archives of Civil and Mechanical Engineering 10 (3) (2010) 5–18. [10] T. Uomoto, T. Taketo, Application of acoustic emission to the field of concrete engineering, Journal of Acoustic Emission 6 (3) (1987) 137–144. [11] J. Ranachowski, F. Rajmund, Z. Librant, The investigation of Brittle Materials using the Acoustic Emission Method on the Example of Ceramics and Concrete, IPPT PAN, Warszawa32–102 (in polish). [12] J. Hoła, Acoustic-emission investigation of failure of highstrength concrete, Archives of Acoustics 24 (2) (1999) 233–244. [13] A Procedure for Acoustic Emission Monitoring of Prestressed Concrete Girders, Draft B, The Ferguson Structural Engineering Laboratory, The University of Texas at Austin in Texas Department of Transportation, Austin TX 2001, pp. 1–28. [14] B.V. Tinkey, T.J. Fowler, R.E. Klingner, Nondestructive Testing of Prestressed Bridge Girders with Distributed Damage, Research Report 1857-2, Center for Transportation Research, The University of Texas at Austin, 2002. [15] Monitoring Method for Active Cracks in Concrete by Acoustic Emission, Federation of Construction Materials Industries, JCMS-III B5706k, Japan 2003, pp. 23–28. [16] T. Suzuki, H. Watanabe, M. Ohtsu, Damage evaluation in concrete using acoustic emission method, in: Proceedings of the 6th Fareast Conference on Non-Destructive Testing, 2002, pp. 111–116. [17] S. Yuyama, T. Okamoto, M. Shigeiski, M. Ohtsu, T. Kisi, A Proposed Standard for Evaluating Integrity of Reinforced Concrete Beams by Acoustic Emission: Standard and Technology Update, ASTM STP 1353, American Society for Testing and Materials, West Conshohocken, 1999.
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[18] G. Świt, Predicting Failure Processes for Bridge – Type Structures Made of Prestressed Concrete Beams using the Acoustic Emission Method, Wydawnictwo Politechniki Świętokrzyskiej, Kielce, 2011 (in polish). [19] B. Goszczyńska, G. Świt, W. Trąmpczyński, A. Krampikowska, Application of the acoustic emission to bridge testing and diagnosis; comparison of procedures, in: IEEE Xplore— Proceedings of the IEEE, 2012, pp. 1–10, ISBN 0018-9219. [20] L. Gołaski, G. Świt, M. Kalicka, K. Ono, Acoustic non destructive techniques as a new method for evaluation of damages in prestressed concrete structures: failure of concrete structures, Journal of Acoustic Emission 24 (2006) 187–195. [21] L. Gołaski, B. Goszczyńska, G. Świt, W. Trąmpczyński, System for the global monitoring and evaluation of damage processes developing within concrete structures under service load, The Baltic Journal of Road and Bridge Engineering 7 (4) (2012) 237–245. [22] B. Goszczyńska, G. Świt, W. Trąmpczyński, A. Krampikowska, J. Tworzewska, P. Tworzewski, Experimental validation of concrete crack initiation and location with acoustic emission method, Archives of Civil and Mechanical Engineering 12 (1) (2012) 23–28. [23] B. Goszczyńska, G. Świt, W. Trąmpczyński, A. Krampikowska, J. Tworzewska, P. Tworzewski, Experimental verification of the acoustic emission (AE) method for the cracking process determination and location in concrete elements, in: IABSE Rotterdam Conference Report, vol. 99, 2013, pp. 150–151. [24] K. Flaga, Shrinkage Stress and Anti-Shrinkage Reinforcement in Concrete Structures, Politechnika Krakowska (Inżynieria Lądowa), Kraków, 2011 (in polish). [25] A. Brandt, Diagnosis of Concrete and High Performance Concrete by Structural Analysis, IPPT PAN, Warszawa13–39 (in polish). [26] B. Goszczyńska, G. Świt, W. Trąmpczyński, K. Bacharz, M. Godowska, A. Krampikowska, Identification of acoustic emission signals in unloaded concrete, in: Proceedings 58th Annual Conference on Scientific Problems of Civil Engineering, Krynica – Rzeszów 2012, pp. 202–203.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Analysis of the process of crack initiation and evolution in concrete with acoustic emission testing B. Goszczyn´skan Kielce University of Technology, Civil Engineering and Architecture Department, Al. 1000-lecia P.P. 7 25-314 Kielce, Poland
ar t ic l e in f o
abs tra ct
Article history:
Application of the acoustic emission method (IADP), to the analysis of crack initiation and
Received 31 January 2013
growth in concrete and reinforced concrete beams is presented in the paper. This method
Accepted 23 June 2013
is based on the idea that every active destructive process becomes a source of acoustic
Available online 18 July 2013
emission. Comparing AE signals, generated within structures under service load, with
Keywords:
previously created database, one can identify the processes of active deterioration
Reinforced concrete
occurring in an element. They can be located on the basis of the difference in the time,
Damage process
that AE signal reaches the sensors with known wave velocity.
Micro-cracks and cracks
Because the cracking process (micro-cracking) occurs in concrete already at the
identification and location
maturing stage, experiments were performed on unloaded concrete members just after
Acoustic emission
concreting (when shrinkage occur) as well as on concrete beams (in technical scale) subjected to continuous loading. It was found that using the IADP method, it was possible to detect and locate creation of micro-cracks (not visible on the member surface) and initiation and growth of cracks, which are visible on the element surface. & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Cracks come as concrete natural response to strain condition developed in it. Their occurrence is inherently connected with the reinforced concrete structure performance and it is considered to be one of the factors that determine structure durability. Advanced process of crack initiation always provides a warning that the structure health has been threatened. Crack distribution, specific to a given situation, is a perfect source of information on the structure condition. The analysis of the process of crack initiation and growth can therefore be applied to reinforced structure diagnostics.
Crack initiation and growth [1], one of the basic mechanisms in concrete mechanics, has been investigated, both experimentally and theoretically, for many years [2]. In the theoretical analysis, micro-cracks (defects) theories are relied on exact solutions and the numerical ones using various models, e.g. [3–5]. The accuracy of this type of description depends on the model itself, and on the experimentally obtained parameters that determine a given model. Cracking is most often characterised by three quantities: cracking moment, distance between cracks and crack width. Those depend on [6] direct actions, i.e. loads and indirect ones, including, among others, concrete shrinkage, which affects crack initiation and opening.
n
Tel.: +48 667904658. E-mail address: [email protected]
1644-9665/$ - see front matter & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.06.002
archives of civil and mechanical engineering 14 (2014) 134–143
All the actions are random in character, and it can also be indicated that the condition of cracking depends, to a far extent, on randomly varied values of concrete tensile strength and steel adhesion to concrete. Attempts have therefore been made to find random functions that would describe the process of crack initiation, e.g. [7], yet the reliability of those descriptions needs to be validated against large size samples. Although advanced and interesting, those methods are still far from being directly applicable to the diagnostics of reinforced concrete structures [3]. For that reason, construction diagnostics currently rely on the methods based on direct measurements of the extent of cracks and of other destructive processes. Widely varied experimental techniques, such as radiology, optical microscopy, ultra sounds, have been employed to conduct investigations. For many years attempts have been also made to use acoustic emission measurements [8]. Great advancement in information technologies made it possible to further develop research methods, including the ones already in use, e.g. the method of acoustic emission signal measurement [9–12]. Advantages offered by the AE method, which differentiate it from others, include the following:
its global character, i.e. the possibility of applying testing to
the whole of the structure, or to its most important member (e.g. as regards the structure load bearing capacity); the possibility of accounting for the structure service conditions, e.g. the real service load, temperature, humidity, etc.; and the possibility of applying long-term (continuous or periodic) monitoring of the whole structure.
The two best known AE procedures, that have been used until now for the evaluation of the technical condition of structures, are the so-called:
American Procedure [13,14]. Japanese Procedure [15–17]. They are based on the individual descriptors of the acoustic emission signal (wave). They describe the way tests are conducted with the AE method exclusively on prestressed pre-tensioned or reinforced concrete beams. Both procedures [18,19]:
do not allow for testing under service load, are based on accurate load measurements taken in accor
dance with a precisely defined scheme, which is possible to accomplish only under laboratory conditions, have been developed for selected elements examined under laboratory conditions, do not indicate how sensors should be spaced, and do not specify the conditions under which a tested object is to be loaded.
Hence in works [18,20–22], the acoustic emission method based on the identification and location of active destructive processes (IADP) was presented.
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The principle behind the method is to compare acoustic signals generated by an object (member) under load with the database of signals produced by destructive processes. On such basis actually proceeding processes are identified. The location of defects is carried through measurements of differences in the generated signals arrival time at acoustic sensors spaced on the member. Investigations on real structures and those performed under laboratory conditions demonstrate that the method allows appropriate identification of the process of crack initiation in pre-stressed members. Works conducted recently [22,23] confirm that it is possible to use the method to diagnose reinforced concrete members (non-prestressed). The present paper describes the application of the acoustic emission method (IADP) to the analysis of the process of crack initiation and growth in concrete and reinforced concrete beams.
2. Fundamentals of the method of identification of active destructive processes (IADP) The acoustic emission method is based on examining acoustic emission signals that accompany internal changes in the structure of the material, i.e. concrete, which are generated by a local release of energy. Initial and following strains produced by external actions result in changes in energy, which can be recorded, as an acoustic wave, on the member surface. Modern measurement apparatus together with software make it possible to record many parameters of the acoustic signal. Taking into account qualitative and quantitative variety of processes that proceed in a given material, there is a real chance that a correlation between the type of signal-generating processes and their parameters will be found. It can be expected that the processes of the material structure destruction will be characterised by a higher level of released energy, which should facilitate the separation of the stages of the material performance. Separating and identifying the signals corresponding to a given process in the material can be significant for nondestructive tests on the strength of structural concrete.
2.1.
Concept of acoustic emission IADP method
Fig. 1 shows, in a form of a diagram, the concept underlying the IADP method (Identification of Active Destructive Processes). The acoustic signal generated by a destructive process is recorded by AE sensors located on the surface of the examined member, then registered and compared with reference signal database created earlier for destructive processes. That allows the identification of destructive processes in the member. Processes generating AE signals accompany only active defects, i.e. those which arise or develop under conditions prevailing at measurement taking. Reference signal databases for individual processes are established using material specimens and models in laboratory tests. It is done on the basis of twelve AE signal parameters: root mean square, reverberation frequency, average signal frequency, initiation frequency, average signal level, amplitude in mV or dB, number of counts to
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peak amplitude, number of counts, energy count, rise time, duration and signal strength (true energy) with the use of image analysis method.
2.2.
Location of destructive processes
The location of signals that accompany arising destructions (defects) takes place on the basis of measurements of differences in time of generated signals arrival at acoustic sensors spaced over the member. It is so called zonal or linear location [21,22]. Fig. 2 shows as an example the diagram of the so-called linear location. On the basis of difference in the signal arrival time Δt at sensors 1 and 2, it is possible to determine the position of the plane (a) normal to line 1–2 connecting the sensors. That makes it possible to easily define the plane in which a destructive process occurs, also in real objects, and then to locate the place of its occurrence [22].
2.3.
Separation of destructive processes
In order to compile a reference signal database, it is necessary to establish destructive processes, relevant for the technical
state evaluation of reinforced concrete structure, which generate acoustic emission signals that are recordable on the member surface. Analysis of the processes that occur in concrete and reinforced concrete members (non-prestressed) as a result of direct and indirect actions led to distinguishing eight groups of destructive processes and corresponding signal Classes: Class 1: Initiation of cracking in the cement paste. Class 2: Initiation of cracking on the paste-aggregate interface. Class 3: Initiation of micro-cracks. Class 4: Growth of cracks. Class 5: Loss of adhesion in the crack vicinity. Class 6: Buckling of compressed bars. Class 7: Crushing of compressed concrete. Class 8: Steel yielding (plastic deformation)/reinforcement rupture.
Because investigation into the process of crack initiation and growth can provide basis for the assessment of the technical state of reinforced concrete structures, destructive processes and corresponding signal Classes 1, 2, 3 and 4 were subjected to detail comparative analysis.
3. Analysis of the process of crack initiation and growth
Fig. 1 – Diagram of AE wave generation by destructive processes, their recording and comparison with reference signal database.
Table 1 – Numbers and notation of reference signal classes in unloaded concrete.
Strains (and stress concentrations) occur in concrete already at the maturing stage. They are caused by hydration processes and local increases in temperature. The multi-phase material composition results in internal stresses leading to a number of mechanical phenomena, including mainly the formation of micro- and even macro-cracks. In fracture of concrete structures the following can be distinguished:
micro-cracking, which produces internal effects not man
ifested on the member surface in the form of cracks (more than 0.1 mm in width), and initiation and growth of cracks, the process visible on the member surface.
Notation
Class no.
1s
2s
3s
Taking into account the above [24] and the structure of the concrete [25], the analysis of the process of crack initiation and growth was performed on unloaded concrete members, which validates Classes 1, 2 and 3 destructive processes, and also on reinforced concrete beams loaded until failure, which allows the validation of Class 4.
3.1. Analysis of the process of micro-cracking in unloaded concrete
Fig. 2 – AE source location with accuracy to the perpendicular plane.
Tests were performed [26] on fresh concrete, at the first stage of its maturing, when the shrinkage effect is the greatest. 150 150 600 mm rectangular prism specimens of C20/25 class and C40/50 class concrete were made at the faculty
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Fig. 3 – Specimen diagram.
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laboratory in accordance with the concrete mix design employed at the concrete mixing plant, using limestone aggregate. The specimens were concreted and transferred on the next day to a water container for 11 days. There the setting proceeded, which allowed AE sensors fixing, in addition due to specimen storing in the water environment, shrinkage did not occur. After specimens were removed from water, acoustic emission sensors were fixed with an adhesive on one of their walls, whereas on the remaining three walls metal
Fig. 4 – AE Classes (shown as number of hits) and medium specimen strain versus time (C20/25) – concrete without admixtures.
Fig. 5 – AE Classes (shown as signal strength) and medium specimen strain versus time (C20/25) – concrete with admixtures.
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bench-marks (acting as reference points) were glued (Fig. 3) to take strain measurements with the extensometer, 8 in. in base (200 mm.). Specimens were arranged in the laboratory hall, base strain measurement readings were taken, then acoustic emission measurements started and continued for 14 days. The number of signals of a given class was counted for 24 h.
Strain measurements and also those of ambient humidity and temperature were taken every 24 h. Basing on previous research findings, three groups of destructive processes were differentiated, which could be sources of acoustic wave generation:
micro-cracks in the cement paste; micro-cracks at the cement paste/aggregate interface; and initiation of micro-cracks (cracks dimensions below 0.1 mm) on the concrete surface.
Recorded acoustic signals were grouped into three classes corresponding to the processes described above and reference signal database for those destructive processes was established.
Class 1s – micro-cracks in the cement paste. Class 2s – micro-cracks at the cement paste/aggregate interface.
Class 3s – initiation of micro-cracks on the member surface.
Fig. 6 – AE Classes (shown as mean strength of the signal) and medium specimen strain versus time (C20/25) – concrete with admixtures.
Classes were differentiated on the basis of 12 parameters (mentioned before) of the acoustic signal using commercial Noesis 4.0 software. Symbols and colours attributed to classes are shown in Table 1.
Fig. 7 – Diagram of the loading of Type-I reinforced concrete beam, with spaced sensors and marked deformation area measured with 3D scanner.
Fig. 8 – Diagram of the loading of Type-II reinforced concrete beam, with spaced sensors and marked deformation area measured with 3D scanner.
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Then, obtained reference signal database was used to analyse acoustic signals for the other specimens tested similar way for 68 days. Figs. 4–6 show the results of measurements for specimens of C20/25 class concrete using limestone aggregate without Table 2 – Colours and codes attributed to signal classes. Colour/shape Class no.
No. 3
No. 4a
No. 4b
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chemical admixtures. Recorded AE signal Classes are presented: as a function of number of hits (one of the twelve AE parameters that provide Class identification) versus time (Fig. 4), as a function of signal strength versus time (Fig. 5) and as a function of mean strength of the signal (signal strength/number of hits) versus time (Fig. 6). Mean strains obtained from measurements taken with the extensometer on three sides of individual specimens on the successive days of the experiment are also presented. It can be stated that Class 1s and 2s signals are mainly recorded. They result from micro-cracks in the cement paste and at the paste–aggregate interface respectively. The mean strength of Class 2s signals is twice higher as of Class 1s. In the first 5 days, the amount of signals increases with an increase in strains. After that, the process is substantially attenuated in spite of continued strain increase, and finally it becomes stabilised after approx. 20 days.
3.2. Analysis cracking process in reinforced concrete beams Experimental tests were conducted on two types of reinforced concrete beams [23] differed in geometrical dimensions and way of loading: Fig. 9 – Type-I beam, recorded signal Classes at the first stage of loading (0–8000 s) – zone of sensors 1–4.
Type I — 0.12 0.30 3.00 m beam loaded with two concentrated forces (Fig. 7) and
Type II — 0.20 0.45 6.00 m beam loaded with one concentrated force applied at a distance of 0.80 m from the support axis (Fig. 8)
Applied load was increasing until failure with a constant velocity. Such a mode of beam loading made it possible to differentiate the areas in the beam, where bending and shearing were dominant. In the tests, the following data were recorded:
Concrete deformation on the lateral surfaces of the beams, with the use of a 3D optical scanner.
AE signals accompanying destructive processes, with the Fig. 10 – Type-I beam, recorded signal Classes at the first stage of loading (0–8000 s) – zone of sensors 2 and 5.
use 55 kHz frequency AE sensors, the spacing of which is shown in Fig. 7 (numbers 1–8) and Fig. 8 (numbers 1–9).
Fig. 11 – Type-I beam surface deformations after 16,000 s loading.
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The recorded AE signals were transferred to a computer using m-SAMOS processor and analysed with Noesis 4.0 software and reference signal databases. Due to the analysis of the recorded acoustic emission signals, two groups of destructive processes that can cause the generation of an acoustic wave were differentiated:
micro-cracks and crack initiation, and initiation and growth of cracks (larger than 0.1 mm) on the concrete surface. Based on the twelve parameters of the acoustic signals, the destructive processes were grouped into three classes:
Class 3 – micro-cracks and initiation of cracks, Class 4a– initiation and growth of cracks from shearing, Class 4b– initiation and growth of cracks from bending, which were marked with symbols and colours shown in Table 2. During the loading process, the 3D optical scanner, the acoustic emission system and the loading system were synchronised with each other. Consequently, deformations and AE signals, recorded in a given second, were measured simultaneously with the values of forces.
3.2.1.
Fig. 12 – Type-I beam, summation graph of the signal strength with division into Classes – zone of sensors 2 and 5.
Analysis of fracture process in Type I beams
Figs. 9 and 10 show colour-marked AE signal Classes in the case of the Type I beam, identified with reference signal databases, in the near-support and central zones (zones covered by AE sensors 1–4 (dominant shearing zone) and 2 and 5 (dominant bending zone)). They were recorded at the initial stage of the loading process (0–8000 s) and are presented as a function of signal strength (one of Class-defining parameters) versus time. Class 3 signals, defined as signals resulting from microcracks, appear from the beginning of the loading process. First Class 4b signals (the propagation of bending cracks) are recorded after approx. 1000 s. Class 4a signals (the propagation of shearing cracks) manifest themselves later than Class 4b signals, and more apparently after approx. 6000 s. Similar behaviour is observed using 3D scanner [23]. At the initial stage of the loading process (after approx. 2000 s),
Fig. 13 – Type-II beam surface deinitiations after 400 s.
Fig. 14 – Type-II beam surface deformations after 550 s.
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Fig. 15 – Type-II beam surface after damage.
Fig. 16 – Recorded signal Classes at the first stage of the Type-II beam loading (0–350 s).
At the end, the damage proceeds through crack propagation in the zone where bending is dominant, in the beam central area (zone covered by sensors 2 and 5), up to the member failure. Fig. 11 presents the image of the beam surface deformations, obtained with 3D optical scanner after 16,000 s of monotonically increasing load. Results obtained with the optical scanner illustrate the deformations of the examined beam lateral surface. The brightenings that are visible correspond to a local accumulation of deformations, which clearly represent cracks being formed. Their colour depends on the cracks width: white/green – min, red – max. Fig. 12 shows summation graph [23] of the signal strength for the whole beam loading process, with division into Classes, for the area where bending is dominant (sensors 2 and 5). As it was mentioned before, bending dominates in the zone of sensors 2 and 5. Using 3D scanner it is seen that, cracks perpendicular to the member axis, are mainly observed. It is also shown by the energy summation graph with division into Classes (Fig. 12). The sum of Class 4b signal strength (the propagation of bending cracks) clearly increases after 10,000 s, which is not visible for the growth of shearing cracks (Class 4a). In [22,23] it is shown that using IADP method it is possible not only to identify the cracking process due to bending, but also to locate it.
3.2.2.
Fig. 17 – Recorded signal Classes for the whole time of Type-II beam loading (0–600 s) – summation graph. cracks perpendicular to the member axis appear, which result mainly from the action of the bending moment. Then, the growth of perpendicular cracks is accompanied by the initiation and growth of shearing (diagonal) cracks (after approx. 6000 s) in the zone where shearing is dominant (zone covered by sensors 1–4 – Fig. 7). Bending and shearing cracks propagate until the failure of the beam; however, at first, the growth of shearing cracks in the near-support zone (zone covered by sensors 1–4) is clearly manifested.
Analysis of fracture process in Type II beams
After the load is applied, in the area defined by sensors 1–9 (Fig. 8), cracks resulting from shearing and bending develop (Fig. 13). After approx. 450 s of loading, a crack from shearing is suddenly formed (Fig. 14 – red colour), the growth of which leads to the beam failure (Fig. 15). The signal strength, as a function of time, at the loading initial stage, is presented in Fig. 16, whereas the summation graph of the signal strength as a function of time for the whole loading process is shown in Fig. 17. As can be seen in Fig. 17, up to approx. 400 s, signals accompanying the initiation of cracks resulting from shearing and bending are recorded. After 430 s, a dramatic growth of cracks from shearing occurs, which finally leads to the beam failure. That confirms visual observation, described above, with the 3D scanner – Figs. 13 and 14 and view of the lateral surface of the beam after damage – Fig. 15.
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Fig. 18 – Comparison of the location of shearing cracks (the IADP method) with observation with 3D scanner. The location of the source of the acoustic signal is based on the measurement of difference in the signal arrival time at AE sensors, the position of which is known, as it is presented in Fig. 2 for linear location. Fig. 18 shows the comparison of the location of Classes of AE signals (linear location) originating in crack growth in the near-support zone. The upper part of the figure illustrates the application of the linear location of AE signals, and the bottom part shows cracks observed with the 3D scanner. Therefore, it can be stated that the IADP method allows not only the identation of cracks, but also makes it possible to locate them.
the beam surface observations conducted with a 3D optical scanner show fairly good congruence.
Acknowledgement Research was executed under the development Project NR 04 0007 10 “Non-destructive system of monitoring and diagnosis of reinforced concrete structures with particular emphasis on engineering objects”.
r e f e r e n c e s
4.
Conclusions
The paper presents the validation of the IADP method, which was developed and applied to prestressed structures, for nonprestressed structures. It was demonstrated that:
the IADP method can be applied to reinforced concrete (non-prestressed) members;
it is possible to establish reference signal databases, where
the crack growth process can be divided into two signal Classes corresponding to crack growth due to shearing and bending. Thus, the analysis of acoustic signals generated by the impact of a load indicates not only the initiation and propagation of cracks, but also provides information on what kind of cracks they are; and as regards crack identification and location, the analysis carried out with the IADP acoustic emission method and
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