ACOUSTIC EMISSION METHOD IN CIVIL ENGINEERING APPLIED TO MONITORING THERMAL LOADING CONCRETE SPECIMENS BY A THREE-POINT BENDING TEST

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1 More info about this article: h Czech Society for Nondestructive Testing 32 nd European Conference on Acoustic Emission Testing Prague, Czech Republic, September 07-09, 2016 ACOUSTIC EMISSION METHOD IN CIVIL ENGINEERING APPLIED TO MONITORING THERMAL LOADING CONCRETE SPECIMENS BY A THREE-POINT BENDING TEST Lubos PAZDERA, Libor TOPOLAR, Jaroslav SMUTNY, Karel MIKULASEK, Petr CIKRLE Brno University of Technology, Faculty of Civil Engineering Phone: , Fax: ; pazdera.l@fce.vutbr.cz, topolar.l@fce.vutbr.cz, smutny.j@fce.vutbr.cz, mikulasek.k@fce.vutbr.cz, cikrle.p@fce.vutbr.cz Abstract Although the acoustic emission method of the non-destructive testing group is amply employed in many engineering fields, this is substantially different in civil engineering constructions. A group of enthusiasts at Brno University of Technology is dedicated to this topic. The paper is concerned with selected applications and their evaluation focusing on the concrete properties after thermal loading. Concrete properties are changed by thermal effects on the concrete structure. The results of experiments, in which the specimens were heated to seven different temperatures and then cooled down to the room temperature, indicate structural changes taking place of concrete specimens. Inhomogeneous materials such as concrete and the complex structure and construction are the main problems encountered in using the acoustic emission method in civil engineering. Keywords: acoustic emission, civil engineering, material, concrete, evaluation, high temperature, three-point bending test 1. Introduction Fire will cause damage to commercial, domestic, industrial, and other constructions even if made from concrete parts. Significant changes in concrete properties can be predicted if temperature increases by several hundreds of degrees Celsius. [1] These changes due to the high temperature depend on the type of the coarse aggregate used. Carbonate, siliceous and lightweight are the three types of aggregate used in concrete. Carbonate aggregates include limestone and dolomite. Siliceous aggregate includes materials consisting of silica and as well as granite and sandstone. The rise in temperature causes a decrease in the strength and modulus of elasticity. Note that concrete does not burn. [2] The changing colours of concrete are a very important indicator of the fire effect. There are no colour changes up to 100 o C with concrete only drying. Cracks and dehydration of the cementation paste are the main processes taking place up to 300 o C. Cracking of the cementation paste and aggregates are due to an expansion at 600 o C when the concrete colour changes to pink. A gray colour of concrete indicates a temperature of up to 800 o C when concrete completely dehydrates and breaks down easily. The colour of concrete changes to buff if the temperature increases up to 900 o C. [3] The acoustic emission method was applied during a three-point bending test of the thermally loaded specimens. 2. Experimental set up The three-point bending tests were performed on notched beams of cross section 100 mm 100 mm and length 400 mm as shown in Fig. 1. [4] The notch depth of 33 mm was placed in the centre of the beam. Beams were made according to Tab. 1. The specimens of common 32 nd EWGAE 403

2 temperature were heated at a heating rate of 5 C/min up to a defined temperature, which was maintained for 60 minutes and then placed in a room with a normal temperature. The defined temperatures with the mark of the group of specimens are listed by Tab. 2. Mixture Table 1 Concrete mixture (weight [kg] per 1 m 3 ) Cement CEM I 42.5R 345 Sand 0/4 848 Coarse aggregate 8/ Water 160 Superplasticizer 3 A Table 2 The defined temperatures Mark Temperature [ C] Figure 1 Experimental set up The fracture tests were carried out using a Heckert FPZ 100/1 testing machine within the range of 0 to 10 kn. An acoustic emission measuring system XEDO made by DAKEL (Czech Republic) was used for the acoustic emission measurements. [5] Four acoustic emission sensors of type MIDI (made by DAKEL) were used all having the same frequency range and were attached to the surface (Fig. 2). [6] nd EWGAE

3 Figure 2 The acoustic emission equipment DAKEL-XEDO and the acoustic emission sensor MIDI (diameter 6 mm, height 6.3 mm) 4. Results The time history of the acoustic emission activity (AE) and force (F) are shown in Fig. 3 and Fig. 4. In each figure, the seven graphs show the behaviours of the samples preloaded at the temperature indicated in the chart title in three-point bending tests. The acoustic emission activities are shown as linear values (AE) in Fig. 3 and as logarithmic values (AE [db]) in Fig. 4 by the equation [7] AE[dB]=20 log10(ae) (1) The behaviour of the acoustic emission activity is very similar for samples of type 20 and 200 as the changes in the structure of the sample are minimal. Analogous results were observed in specimens of type 400 and 600. Thus, the structure of type 200 as compared to the structure of type 400 is already significantly different. The melted structure of specimens of type over 800 is demonstrated by a higher number of acoustic emission events during the three-point bending loading; however, the energy of events is smaller. The specimens of type 800 and 1000 are very cracked. The structure of specimens of type 1200 is amorphous like, thus, the acoustic emission activity is similar over the entire experiment. 4. Conclusion These experiments have demonstrated that acoustic emission methods can successfully be used to investigate the deterioration of concrete after fire load. These problems are going to be addressed and a series of experiments testing different concrete mixes are going to be conducted as part of the ongoing research project. To better understand the behaviour of concrete at high temperatures and to support the experiments, numerical analysis can be useful. [8] However, the numerical analysis needs to know the properties and effects of concrete dependence on temperature. The acoustic emission method can be a tool that provides a description of the process. 32 nd EWGAE 405

4 Figure 3 Acoustic emission activity (left axis) and force (right axis) dependent on time for seven types of thermal loading nd EWGAE

5 Figure 4 Acoustic emission activity (left logarithmic axis) and force (right axis) dependent on time for seven types of thermal loading 32 nd EWGAE 407

6 5. Acknowledgement This paper has been written under the project GAČR No S supported by Czech Science Foundation and under the project LO1408 "AdMaS UP - Advanced Materials, Structures and Technologies", supported by Ministry of Education, Youth and Sports under the National Sustainability Programme I".. References 1. M. Ozawa, S. Uchida, T. Kamada, H. Morimoto, Study of mechanisms of explosive spalling in high-strength concrete at high temperatures using acoustic emission, Construction and Building Materials, Vol. 37, pp , December C. Grosse, H.-W. Reinhardt, F. Finck, Signal-based acoustic emission techniques in civil engineering, Journal of Materials in Civil Engineering, Vol. 15, Iss. 3, pp , May 2003, 3. S. Cattaneo, L. Biolzi, Assessment of thermal damage in hybrid fiber-reinforced concrete, Journal of Materials in Civil Engineering, Vol. 22, Iss. 9, pp , 25 November L. Topolar, L. Pazdera, V. Bilek, L. Dedeckova, Acoustic emission method applied on four point loading of concrete structures with and without small wires, proc. of The 50th Annual Conference on Experimental Stress Analysis 2012, EAN 2012; Tabor; Czech Republic; 4 June L. Pazdera, L. Topolar, Application acoustic emission method during concrete frost resistance, Russian Journal of Nondestructive Testing, Vol. 50, Iss. 2, pp , February J. Fiala, P. Mazal, M. Kolega, 'Cycle induced microstructural changes', International Journal of Microstructure and Materials Properties, Vol. 6, Iss. 3-4, pp , October C. Grosse, M. Ohtsu, Acoustic Emission Testing in Engineering Basics and Applications pp , J. Smutný, L. Pazdera, The using wavelet transformation for acoustic response analysis, Akustika, Vol. 21, Iss. 1, pp , March nd EWGAE