UDK 691:620.1:691.3:620.179.1	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 49(5)703(2015)
MONITORING EARLY-AGE CONCRETE WITH THE ACOUSTIC-EMISSION METHOD AND DETERMINING THE CHANGE IN THE ELECTRICAL PROPERTIES
PREGLED SVEŽEGA BETONA Z METODO AKUSTIČNE EMISIJE IN DOLOČANJEM SPREMEMB ELEKTRIČNIH LASTNOSTI
Lubos Pazdera1, Libor Topolar1, Marta Korenska1, Tomas Vymazal1, Jaroslav Smutny1, Vlastimil Bilek2
1Brno University of Technology, Faculty of Civil Engineering, Veveri 331/95, 602 00 Brno, Czech Republic 2ZPSV a.s., Uhersky Ostroh, Czech Republic pazdera.l@fce.vutbr.cz
Prejem rokopisa - received: 2014-07-18; sprejem za objavo - accepted for publication: 2014-11-05
doi:10.17222/mit.2014.112
Concrete is the most popular building composite material (CM). Its long-term aging properties depend on the mixture, the setting and the curing. When concrete is produced and used in the place where we want it to be for the whole of its life, it is very sensitive and easily ruined. Curing is one of the things that we do to keep concrete protected during the first week or so of its life: we maintain proper temperature (neither too hot nor too cold) and dampness. The acoustic-emission method and the electrical-property measurement technique are applied to monitor early-age concrete. The relationships between the acoustic-emission activity, the temperature and the electrical properties, e.g., the resistivity and the capacity of concrete at an early age were studied in this research. W. Chen et al. studied the microstructure development of hydrating cement pastes during early ages using non-destructive methods including the ultrasound P-wave propagation-velocity measurement and non-contact electrical-resistivity tests. Nevertheless, there are not many references about the continuous study of the properties of concrete during an early age. A long-time monitoring of concrete properties is necessary to determine its lifetime and quality. The acoustic-emission method was proven to be a very advantageous tool for a non-destructive monitoring of structural micro-changes during the lifetime of concrete. The differences between hardened concrete mixtures detected with acoustic emission can partly determine their properties at the age of 28 d. The basic concrete property is its compressive strength after 28 d; nevertheless, this can change over a long time period, thus a continuous measuring needs to be applied.
Keywords: concrete, acoustic emission, electrical properties, early age, lifetime, curing
Beton je najbolj priljubljen gradbeni kompozitni material (CM). Njegove dolgoletne lastnosti so odvisne od mešanice, položitve in strjevanja. Ko je beton izdelan in ko ga položimo na mesto, kjer naj bi bil do konca trajnostne dobe, je zelo občutljiv in se lahko poškoduje. Strjevanje je ena od stvari, ki zahteva zaščito betona v prvih tednih njegovega trajanja s pravilno temperaturo (niti pretoplo niti prehladno) in vlažnostjo. Metoda akustične emisije in meritve električnih lastnosti so bile uporabljene za kontrolo mladega betona. Preiskovana je bila odvisnost med aktivnostjo akustične emisije in temperaturo, električnimi lastnostmi, kot sta upornost in kapacitivnost mladega betona. W. Chen s sodelavci je študiral razvoj mikrostrukture mlade hidra-tantne cementne zmesi z neporušnimi metodami, vključno z merjenjem hitrosti napredovanja ultrazvočnih P-valov, in naredil preizkuse brezkontaktne električne upornosti. Vseeno ni veliko literaturnih virov o kontinuirnem spremljanju lastnosti mladega betona. Za določitev zdržljivosti in kvalitete betona je potrebna dolgotrajna kontrola. Metoda akustične emisije se je izkazala kot napredno orodje za neporušno kontrolo mikrosprememb zgradbe med trajnostno dobo betona. Razlike, ugotovljene z akustično emisijo med mešanicami betona pri strjevanju, so delno določile njihove lastnosti po 28 d. Osnovna lastnost betona je 28-dnevna tlačna trdnost; vendar se ta lahko spreminja v daljšem časovnem obdobju, zato se uporabljajo kontinuirne meritve. Ključne besede: beton, akustična emisija, električne lastnosti, zgodnja starost, trajnostna doba, strjevanje
1 INTRODUCTION
the hydration process provides information on the concrete resistance and allows an assessment of its
Cement hydration plays a critical role in the	durability9. Several non-destructive techniques have been
temperature development of early-age concrete due to	developed and applied in that respect, most of them
the heat generation14. It also has significant effects on	based on ultrasonic-wave measurements10. The recording
the material properties and performances at an early age,	of passive energy using acoustic-emission techniques
including material strengths5, critical stresses6 and dis-	was used to evaluate the structural activity in concrete
tresses like cracking7. Therefore, it is essential to capture	during the early ages, showing periods of intense micro-
the hydration property and temperature development of	structural changes during the curing process11.
concrete at an early age to prevent premature failures. It	Microstructural changes occurring in freshly poured
is known that the durability of cement-based products	concrete during the curing were monitored on a labora-
and concrete structures is highly influenced by the early	tory scale using a combination of the acoustic-emission
stages of hydration8. A precise knowledge of the micro-	technique and the monitoring of electrical properties12.
mechanical properties during the successive phases of	The acoustic-emission method is a passive-ultrasonic-
signal recording technique allowing an online monitoring of the internal microstructural activity of young concrete during the hydration process8. The acoustic-emission technique, which is one of the non-destructive evaluation techniques, is a useful method for investigating local damage in concrete during its lifetime13. The micro-changes in concrete structures can be easily estimated, taking account of the number of acoustic-emission hits and counts of the acoustic-emission signal14. Acoustic-emission signals are detected, due to micro-cracks (i.e., structural changes in the material) by the acoustic-emission sensors attached to the surface of a concrete specimen. The acoustic-emission parameters such as the number of hits, counts, duration time, amplitude, energy and rise time are recorded by the acoustic-emission measurement system15.
Impedance spectroscopy, called also dielectric spec-troscopy, the monitoring of electrical properties and nuclear-magnetic-resonance spectroscopy have become promising techniques for probing cements and concretes16. Cement paste is electrically conductive due to its interconnected pore network filled with an aqueous phase containing mobile ions such as Na+, K+ and OH .17 Electrical conductivity and resistivity are effective parameters used to monitor the microstructural features of cement-based materials. The electrical conductivity of cement-based materials is dependent on the porosity, the pore connectivity and the conductivity of the pore solution, all of which are significant for the durability of the material18. The impedance characteristics and spectra recorded over a wide range of frequencies (if possible from 100 mHz to 10 MHz or higher) provide new information and an insight into the concrete microstructure and hydration19. Electrical resistivity is related to the reinforcement corrosion, moisture and heat transfer in concrete20.
2 EXPERIMENTAL SET-UP
An acoustic-emission measuring system called LOCation ANalyser (LOCAN) 320 made by PAC (France) was used for the measurements. Wide-band sensors (made by the 3S Sedlak Company) were used. The sensor output was connected to pre-amplifiers (the PAC Company) with a 2 kHz high-pass filter. The proprietary PAC program package was used to record the acoustic-emission parameters. The acoustic-emission signals were taken by a LOCAN 320 acoustic-emission localizer. The LOCAN 320 system was fine-tuned at a well site and with a sonic-logging tool in the wellbore, with the acoustic threshold set slightly above the background-noise level as measured by LOCAN 320 and the internal gain set according to the manufacturer's recommendations. This device was designed to record and evaluate ultrasonic signals. It was used to analyze and store the acoustic-emission parameters. It can process signals coming from four measuring points. Each
recorded event can be characterized with the following acoustic-emission parameters: recording date and time, (maximum) amplitude, acoustic-emission energy, count, rise time and mean frequency21.
The impedance of the material under investigation changes in accordance with the structural changes, particularly the water absorption and evaporation. A change in the resistance is obvious. The capacity C of a parallel-plate capacitor is computed with:
C (f) = £ , • £ 0 • -
(1)
where £r is the relative permittivity, £0 is the vacuum permittivity (8.854 ■ 10-12 F/m), S is the measuring-electrode area, d is the distance and f is the frequency. The resistance, R, of the electrodes is given by:
R( f) = P •	(2)
where p is the resistivity. Microstructural changes in a material make the material's permittivity change. The permittivity value can also be affected by macrocracks, depending on the frequency22.
Two cylindrical steel electrodes of a diameter of 6 mm, buried 65 mm under the specimen surface, were used for measuring the resistance. To measure the capacitance, two rectangular metal-plate electrodes with dimensions of 25 mm x 45 mm were applied. All the electrodes were fixed to a plastic slab so that their constant configuration was guaranteed. The electrodes and the temperature-sensor outputs were connected to an automated measuring device. The measurements of the capacitance, the temperature and the impedance were started within 15 min after the mixture preparation. This phase of the experiment was carried out using a TESLA BM 595 RLCG bridge and a selector switch. The measurement was carried out at selected points with frequencies in the range from 100 Hz to 20 kHz. Each of the electrical quantities (resistance, capacitance, temperature, etc.) was measured separately23.
Figure 1: Experimental set-up before unmoulding, (samples S1, S2 tested with the acoustic-emission method and S3, S4 with impedance spectroscopy)
Slika 1: Eksperimentalni sestav pred razdrtjem (vzorca S1, S2 preizkušena z metodo akustične emisije in S3, S4 z impedancno spektroskopijo)
Figure 2: Experimental set-up after unmoulding (sample S1 and S4 are covered, S2 and S3 are uncovered)
Slika 2: Eksperimentalni sestav po razdrtju (vzorca S1 in S4 sta pokrita in S2 ter S3 nepokrita)
Sleeper concrete with a 32 mm crushed aggregate attained a compression strength of 107 MPa. Two of the specimens with dimensions of 100 mm x 100 mm x 400 mm were mould cast. Steel-mould upper sides were covered with PE foil (Figure 1). The components of the mixture are shown in Table 1. The mechanical properties of the monitored samples are statistically processed in Table 2. When the specimens were unmoulded, 24 h after the mixing, one specimen was covered with foil and the other one was not (Figure 2). In Figures 1 and 2, samples S1 and S2 are tested with the acoustic-emission method and samples S3 and S4 with impedance spectroscopy; samples S1 and S4 are covered.
Table 1: Concrete mixture
Tabela 1: Mešanica betona
Composition
Cement CEM I 42 5R
Water
Plasticizer Stachement 2060
Sand 0/4
Coarse aggregate 16/32
Mass, m/kg
390
125
700
1275
3 RESULTS
The acoustic-emission activities of both samples during hydration, hardening and setting are shown in Figures 3 and 4. After the unmoulding, i.e., at the age of
24 h, when one sample was wrapped and the other one was left without any protection, the acoustic-emission
activities were different. The increase in the cumulative acoustic-emission-count curve of the protected sample is lower than that of the sample without protection during the whole 120 d period, as shown in Figure 4. It should
Figure 3: Dependence of cumulative acoustic emission (NC) and temperature difference (A#) on time (t)
Slika 3: Odvisnost kumulativne akustične emisije (NC) in temperaturne razlike (A#) od časa (t)
be noted that the acoustic-emission signals were picked up six hours after the mixtures were made.
The temperature of the samples after the unmoulding was kept close to the ambient temperature (Figure 3). The basic changes in the concrete structure during the hydration relate to the temperature peak, in this case after 12 h, as shown in Figure 4.24 The temperature curves for both samples are the same as before the un-moulding, i.e., slightly different25.
Electrical measurements show a higher resistivity and a lower capacity, i.e., a higher capacitance of the specimen without protection after the unmoulding (Figure 5).
Different changes of both samples are also shown in Figure 6. On the first day the frequency characteristics of resistance (R) and capacitance (Xc) are the same, but thereafter they are different. There are three types of curves described in the legend in Figure 6. Each curve, as described on the curve at the 6th h, consists of eight points whose frequencies from the left-hand side of the horizontal axis are 100 Hz, 200 Hz, 400 Hz, 1 kHz,
Figure 4: Details of the curves plotted in Figure 3 for the first two days
Slika 4: Detajl krivulje s slike 3 za prva dva dneva
A
Figure 5: Dependence of electrical resistance (R) and capacity (C) on time (t)
Slika 5: Odvisnost elektri~ne upornosti (R) in kapacitete (C) od ~asa (t)
2 kHz, 4 kHz, 10 kHz and 20 kHz 26. It is clear that the shapes of the curves are different after the unmoulding, therefore, the curves of both samples are the same on the first day (in Figure 6 marked with downward-pointing triangles).
Table 2: Main parameters of samples Tabela 2: Glavni parametri vzorcev
Compressive strength
Flexural strength
Modulus of elasticity (bending)
Fracture toughness
Fracture energy
Units
MPa
MPa
GPa
MPa m1
J/m2
Mean
107
10.0
37
1.8
200
Deviation
55
0.2
0.4
10
4 CONCLUSION
The acoustic-emission method and impedance spec-troscopy are powerful tools for determining the development of microcracks during the hardening and setting of concrete. It can be assumed that higher numbers of microcracks cause higher numbers of acoustic-emission events. The dependence of a cumulative acoustic-emission activity on the time shows the necessity of curing the concrete during its whole lifetime, especially during intense hydration and hardening. The big differences between the acoustic-emission activities of wrapped and unprotected samples clearly indicate the essentiality of the high moistness of hardened concrete. The number of microcracks in cement mortar affects the resulting mechanical properties of concrete.
Impedance, or dielectric, spectroscopy together with acoustic emission can also help with the monitoring and a detailed description of lifetime behaviours of concrete specimens during hydration.
As the acoustic emission (Figure 3) and the impedance (Figure 5) history of both samples are similar, the
Figure 6: Dependence of capacitance (XC) on resistance (R) Slika 6: Odvisnost kapacitivnosti (XC) od upornosti (R)
number of microcracks is higher in the samples without foil, which means that the curing of concrete during an early age is necessary.
Acknowledgement
This paper has been worked out under the project No. LO1408 "AdMaS UP - Advanced Materials, Structures and Technologies", supported by Ministry of Education, Youth and Sports under the "National Sustainability Programme I" and under the project No. 13-18870S "Assessment and Prediction of the Concrete Cover Layers Durability" supported by Czech Science Foundation.
5 REFERENCES
1	G. Venkiteela, Z. H. Sun, H. Najm, Journal of Materials in Civil Engineering, 25 (2013) 2, 30-38, doi:10.1061/(ASCE)MT.1943-5533.0000528
2	W. Chen, Y. Li, P. L. Shen, Z. H. Shui, Journal of Nondestructive Evaluation, 32 (2013) 3, 228-237, doi:10.1007/s10921-013-0175-y
31. M. Nikbin, M. Dehestani, M. H. A. Beygi, M. Rezvani, Construction and Building Materials, 59 (2014), 144-150, doi:10.1016/ j.conbuildmat.2014.02.008
4Q. Xua, J. M. Ruiza, J. Hub, K. Wangc, R. O. Rasmussena, Ther-mochimica Acta, 512 (2011), 76-85, doi:10.1016/j.tca.2010.09.003
5	W. M. Hale, T. D. Bush, B. W. Russell, S. F. Freyne, Journal of the Transportation Research Board, 1914 (2005), 97-104
6	T. Nishizawa, T. Fukuda, S. Matsuno, K. Himeno, Transportation Research Record, 1525 (1996), 35-43, doi:10.3141/1525-05
7	M. Gawlicki, W. Nocun-Wczelik, L. Bak, Journal of Thermal Analysis and Calorimetry, 100 (2010), 571-576, doi:10.1007/s10973-009-0158-5
8	K. V. D. Abeele, W. Desadeleer, G. D. Schutter, M. Wevers, Cement and Concrete Research, 39 (2009), 426-432, doi:10.1016/ j.cemconres.2009.01.016
9	H. W. Reinhardt, C. U. Grosse, A. T. Herb, Materials and Structures, 33 (2000) 233, 581-583, doi:10.1007/BF02480539
10V. A. Aleshin, K. V. D. Abeele, Journal of the Mechanics and Physics of Solids, 55 (2007) 2, 366-390, doi:10.1016/i.imps.2006.07.002
3
11 S. F. Huang, M. M. Li, D. Y. Xu, M. J. Zhou, S. H. Xie, X. Cheng, Research in Nondestructive Evaluation, 24 (2013) 4, 202-210, doi:10.1080/09349847.2013.789949
121. Gabrijel, D. Mikulic, N. Bijelic, Tehnicki Vjesnik-Technical Gazette, 17 (2010) 4, 493-497
13	K. Ohno, M. Ohtsu, Construction and Building Materials, 24 (2010), 2339-2346, doi:10.1016/j.conbuildmat.2010.05.004
14	P. Lura, J. Couch, O. M. Jensen, J. Weiss, Cement and Concrete Research, 39 (2009) 10, 861-867, doi:10.1016/j.cemconres.2009. 06.015
15	M. Tasic, Z. Andic, A. Vujovic, P. Jovanic, Mater. Tehnol., 44 (2010) 6, 397-401
16	M. Marinsek, Mater. Tehnol., 43 (2009) 2, 79-84
17	Y. M. Kim, J. H. Lee, S. H. Hong, Cement and Concrete Research, 33 (2003) 3, 299-304, doi:10.1016/S0008-8846(02)00944-4
18	K. B. Sanish, N. Neithalath, M. Santhanam, Construction and Building Materials, 49 (2013), 288-297, doi:10.1016/j.conbuildmat. 2013.08.038
19	J. M. Cruz, I. C. Fita, L. Soriano, J. Paya, M. V. Borrachero, Cement and Concrete Research, 50 (2013), 51-61, doi:10.1016/j.cemconres. 2013.03.019
20	R. Ranade, J. Zhang, J. P. Lynch, V. C. Li, Cement and Concrete Research, 58 (2014), 1-12, doi:10.1016/j.cemconres.2014.01.002
21	C. Jomdecha, A. Prateepasen, P. Kaewtrakulpong, NDT & E International, 40 (2007) 8, 584-593, doi:10.1016/j.ndteint.2007.05.003
22	S. R. Mahapatra, V. Sridhar, D. K. Tripathy, Polymer Composites, 29 (2008) 5, 465-472, doi:10.1002/pc.20421
23	M. Lunak, I. Kusak, L. Pazdera, L. Topolar, V. Bilek, Proc. of the 48th International Scientific Conference on Experimental Stress Analysis EAN 2010, Velke Losiny, Czech Republic, 2010, 233-240, Code 106809
24	A. Feylessoufi, F. C. Tenoudji, V. Morin, P. Richard, Cement and Concrete Research, 31 (2001) 11, 1573-1579, doi:10.1016/S0008-8846(01)00602-0
25	M. H. Cornejo, J. Elsen, H. Baykara, C. Paredes, European Journal of Environmental and Civil Engineering, 18 (2014) 6, 629-651, doi:10.1080/19648189.2014.897005
26	J. M. Deus, B. Diaz, L. Freire, X. R. Novoa, Electrochimica Acta, 131 (2014), 106-115, doi:10.1016/j.electacta.2013.12.012