Acta Polytechnica CTU Proceedings doi:10.14311/APP.2018.15.0099 Acta Polytechnica CTU Proceedings 15:99–103, 2018 © Czech Technical University in Prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app CHARACTERIZATION OF CEMENT-BASED COMPOSITE EXPOSED TO HIGH TEMPERATURES VIA ULTRASONIC PULSE METHOD Iva Rozsypalová∗, Michal Vyhlídal, Richard Dvořák, Tomáš Majda, Libor Topolář, Luboš Pazdera, Hana Šimonová, Zbyněk Keršner Brno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic ∗ corresponding author: Iva.Rozsypalova@vutbr.cz Abstract. In this paper, the attention is paid to the investigation of the influence of high tempera- ture acting on specimens made from specially designed cement-based composite. The experimental programme was carried out on six sets of beam specimens with the dimensions of 20 × 40 × 200 mm. The specimens were loaded to a pre-set temperature of 100, 200, 400, 600, 800 and 1000 ◦C and then the temperature was kept for 60 minutes. When the temperature loading had been done, the specimens were left to cool down to the ambient temperature. After that, the ultrasonic pulse method was used to determine the degree of damage of temperature loaded specimens. The measured data obtained by this non-destructive method are in high correlation with values of informative compressive strength of the composite obtained after the temperature loading of specimens. Keywords: Fine-grained cement-based composite, glass spherical aggregate, high temperature, ultrasonic pulse method. 1. Introduction Cement-based composites belong to the frequently used building materials for a wide range of applica- tions [1, 2]. Structures and construction components utilizing this composite were commonly designed as- suming normal service temperatures. However, high temperatures acting on cement-based composite – for example during the fire etc. – cause a wide range of physical and chemical processes, which result in changes in the structure of composite and thus affects its mechanical properties [3, 4]. According to the theory of fracture mechanics, cement-based composites, in this case fine-grained cement-based composite, show quasi-brittle behaviour. Specimens made from these materials have the ability to carry the load even after the deviation from a linear branch of load–displacement diagram until the peak point and then the decrease of the loading force follows until the failure – see e.g. [5, 6]. One of the reasons for this behaviour could be the existence of the interfacial transition zone (ITZ). This well-known zone around aggregate particles of a few micrometers in size has the specific features. These are mainly higher porosity and calcium hydroxide contents compared to the bulk matrix [7–12]. The scope of this paper is to show the results of a pilot study which deals with the characterization of the damage degree of special cement-based composite specimens exposed to high temperatures via ultrasonic pulse method. 2. Experimental programme – materials, specimens, methods Three-part silicone moulds were made in plywood frame and subsequently used to produce the test spec- imens, see Figure 1. The test specimens were manufac- tured from the special designed fine-grained cement- based composite. The fresh mixture was made using the spherical sodium-potassium glass aggregates with a diameter of 2 ± 0.2 mm, Portland cement CEM I 42.5 R (from cement plant Mokrá) and water in a ratio of 3:1:0.35. The components were mixed in laboratory conditions using the standard mixer. After pouring and compaction of the fresh mixture, the moulds was sealed with a thin PE foil and placed in the stabilized laboratory conditions for 1 day, see Figure 2. After demoulding the test specimens had been stored in a water bath until their testing. The experimental programme was carried out on six sets of specimens according to considered maxi- mum temperature. Always, three test specimens with the nominal dimensions of 20 × 40 × 200 mm were prepared per each testing set. A laboratory furnace Classic type 5013 with 3 sides of heating coils was used to heat of test specimens. The temperature loading started at the laboratory temperature of 20 ◦C. The specimens were loaded with the temperature rate of 5 ◦C/min up to a pre-set temperature. The pre-set temperatures were 100 ◦C (reference temperature), 200 ◦C, 400 ◦C, 600 ◦C, 800 ◦C, and 1000 ◦C. The next part of the temperature–time curve was constant, i.e. the maximum temperature was kept for 60 minutes. After that, the specimens were left to cool down to the ambient temperature and had been placed in a room 99 http://dx.doi.org/10.14311/APP.2018.15.0099 http://ojs.cvut.cz/ojs/index.php/app I. Rozsypalová, M. Vyhlídal, R. Dvořák et al. Acta Polytechnica CTU Proceedings Figure 1. Illustration of silicone moulds preparing. Figure 2. Illustration of manufacture of test specimens. with the temperature of 20 ◦C. All specimens were tested by ultrasonic pulse method before and after the temperature loading performed at the laboratory temperature. During the ultrasonic pulse method measurement, the specimens were excited by a sensor placed in the centre of side cross-section of specimen – transmitter T (see Figure 3). Impulses were generated by the pulse generator Agilent 33220A. The acoustic emission system XEDO was used to detect the waves. Three piezoelectric sensors were placed on different locations of the specimen surface, see Figure 3. The first sensor S1 was placed near the exciter on the perpendicular surface, the second one (S2) on the opposite surface to the transmitter near the edge and the third sensor (S3) was on the same surface as the first one but close to the opposite edge. The generated signal can be in either form of con- tinuous noise or harmonic signals [13]. The evaluation of the signals response to the transmitted pulse in the tested object can be performed in two basic systems of analysis. In the first case, the resulting shift in relation to time is evaluated, and in the second case, the signals are converted to the frequency domain and a frequency analysis is performed. The results used in this paper are from the time domain. Subsequently, all specimens were provided by an initial notch and selected specimens were tested in a three-point bending configuration. Selected specimens will be tested at the Centre of Excellence Telč, Czech Republic in the four-point bending configuration, and X-ray computed tomography of these specimens is also planned – specimens are bellow marked CT. Analysis of results of these fracture tests are being prepared and they are not the matter of this paper. The compressive strength value was determined at the parts of specimens obtained after the three-point bending fracture tests were performed. The auxiliary loading device with two (upper and lower) square steel auxiliary platens was placed in the hydraulic testing machine. The specimens were placed between the auxiliary plates in such a way that the load was applied perpendicularly to the compaction direction, as can be seen in Figure 4. The test was carried out in accordance with the principles of standards ČSN EN 196-1 [14] and BS 1881: Part 119 [15]. Increasing load was applied continuously until the failure when the loading force decrease. The maximum load applied to each specimen was recorded. The informative com- pressive strength value of material of the specimen was calculated by dividing the maximum load by the area of contact – nominal surface area 40 × 20 mm2. 100 vol. 15/2018 Characterization of cement-based composite Figure 3. Illustration of the ultrasonic pulse method experimental set-up (transmitter T, sensors S1, S2, and S3). Figure 4. The compression test of specimens: The auxiliary loading device with two auxiliary platen was placed in the hydraulic testing machine with selected specimens prepared for the test (left); specimen after testing. 3. Results All measured data, results of the ultrasonic pulse method and informative compressive strengths for test specimens are shown in Table 1. Basic statistics – mean values and standard deviations – of corrected selected data, as well as coefficients of correlation are introduced in Tables 2 and 3. The ratio between maximum amplitudes r21 and r31 is called signal at- tenuation at the constant length – numbers 1, 2 and 3 correspond to the sensors S1, S2, and S3, index “b” or “a” means measurement before or after tempera- ture loading. The outcome of applied procedure is a proportion of ratio r of temperature loaded specimen (r21,a or r31,a) to ratio r of the non-degraded specimen (r21,b or r31,b) – labelled as R21 and R31, respectively. 4. Conclusions After the exposure of cement-based composites to elevated temperatures their mechanical parameters are reduced [4]. Therefore, the presented paper was focused on the evaluation of the damage degree of fine- grained cement-based composite specimens exposed 101 I. Rozsypalová, M. Vyhlídal, R. Dvořák et al. Acta Polytechnica CTU Proceedings Specimen Bulk density γ Tempe- rature T Ratio r21,b Ratio r31,b Ratio r21,a Ratio r31,a Ratio R21 Ratio R31 Compres- sive strength fc,i kg·m−3 ◦C – – – – – – MPa 01-100 CT 1912 100 0.0140 0.0276 0.0406 0.0545 2.900 1.975 – 02-100 1875 100 0.0190 0.0527 0.0089 0.0146 0.468 0.278 4.61 03-100 1847 100 0.0651 0.0425 0.0074 0.0113 0.114 0.266 3.58 04-100 1906 100 0.0154 0.0154 0.0098 0.0153 0.640 0.993 8.21 25 1925 100 0.0949 0.1391 0.0578 0.0916 0.609 0.659 6.03 26 1958 100 0.3462 0.4216 0.1154 0.1450 0.333 0.344 – 27 2009 100 0.2550 0.4041 0.0833 0.0816 0.327 0.202 8.44 05-200 CT 1912 200 0.0120 0.0289 0.0026 0.0111 0.217 0.384 – 06-200 1988 200 0.0156 0.0206 0.0042 0.0121 0.270 0.587 6.91 07-200 1956 200 0.0262 0.0296 0.0081 0.0180 0.309 0.607 4.68 08-200 1844 200 0.0255 0.0216 0.0072 0.0071 0.282 0.327 7.73 09-400 CT 1958 400 0.0166 0.0200 0.0037 0.0025 0.223 0.125 – 10-400 1993 400 0.2805 0.4328 0.0375 0.0609 0.134 0.141 4.42 11-400 2020 400 0.1404 0.1638 0.0166 0.0239 0.118 0.146 4.29 12-400 1975 400 0.1546 0.1421 0.0232 0.0240 0.150 0.169 5.21 13-600 CT 1951 600 0.0792 0.0748 – – – – – 14-600 1972 600 0.0799 0.0974 – – – – 1.88 15-600 1997 600 0.1051 0.1930 – – – – – 16-600 2017 600 0.1448 0.1586 – – – – 3.93 17-800 CT 1974 800 0.1554 0.1405 – – – – – 18-800 1942 800 0.1017 0.2213 – – – – – 19-800 1887 800 0.2204 0.1781 – – – – – 20-800 1899 800 0.0895 0.1810 – – – – – 21-1000 CT 1925 1000 0.0866 0.0901 1.5438 1.1155 17.82 12.38 – 22-1000 1953 1000 0.1186 0.1631 0.3696 0.4316 3.118 2.647 1.96 23-1000 1958 1000 0.1208 0.1438 0.6280 0.9794 5.200 6.809 1.67 24-1000 1987 1000 0.1259 0.1394 0.3037 0.3264 2.413 2.341 1.45 Table 1. Measured data, results of the ultrasonic pulse method and informative compressive strengths. Temperature T Bulk density γ Ratio R21 Ratio R31 Compressive strength fc,i ◦C kg·m−3 – – MPa 100 1919 (53) 0.475 (0.148) 0.457 (0.308) 6.17 (2.15) 200 1925 (63) 0.270 (0.039) 0.476 (0.142) 6.44 (1.58) 400 1986 (26) 0.156 (0.460) 0.145 (0.018) 4.64 (0.50) 600 1984 (29) – – 2.91 (1.45) 800 1925 (40) – – – 1000 1956 (25) 3.577 (1.449) 3.933 (2.496) 1.69 (0.26) Table 2. Mean values (standard deviations) of selected data. to high temperatures via ultrasonic pulse method. Resulting data from this method are in quite high correlation with loading temperature (coefficient of correlation was in range 0.63 to 0.92), as well as with compressive strength of composite determined after the temperature loading (coefficient of correlation was in range –0.59 to –0.91). The very high correlation is between ratios R21 and R31 (coefficient of correlation was in range 0.97 to 1.00) which means that it is sufficient placed the sensors on the surface in the case of a large structure. The strong relationship between compressive strength and temperature for this special cement-based composite with spherical glass aggregates (coefficient of correlation was in range 102 vol. 15/2018 Characterization of cement-based composite γ T R21 R31 fc,i Bulk density γ 1.00 Temperature T 0.23 | 0.29 1.00 symm. Ratio R21 –0.06 | 0.14 0.63 | 0.92 1.00 Ratio R31 –0.03 | 0.12 0.70 | 0.92 0.97 | 1.00 1.00 Compressive strength fc,i –0.19 | –0.63 –0.80 | –0.97 –0.66 | –0.91 –0.59 | –0.90 1.00 Table 3. Coefficients of correlation (dimensionless) of selected data: all values (Table 1) | mean values (Table 2). –0.97 to –0.80) was confirmed in this study. The ultrasonic pulse method as a non-destructive method seems to be well-suited for determining the damage degree caused by elevated temperature in case of this special cement-based fine-grained composite. List of symbols γ Bulk density before temperature loading at tempera- tures higher than 100 ◦C fc,i Informative compressive strength r21,a The ratio of the maximum amplitude from sensor S2 to the maximum amplitude from sensor S1 after temperature loading at temperatures higher than 100 ◦C r21,b The ratio of the maximum amplitude from sensor S2 to the maximum amplitude from sensor S1 before temperature loading at temperatures higher than 100 ◦C r31,a The ratio of the maximum amplitude from sensor S3 to the maximum amplitude from sensor S1 after temperature loading at temperatures higher than 100 ◦C r31,b The ratio of the maximum amplitude from sensor S3 to the maximum amplitude from sensor S1 before temperature loading at temperatures higher than 100 ◦C R21 R21 = r21,a/r21,b R31 R31 = r31,a/r31,b T Temperature Acknowledgements This outcome has been achieved with the financial sup- port of the Czech Science Foundation under project No. 16-18702S “AMIRI – Aggregate-Matrix-Interface Related Issues in Silicate-based Composites”. Besides the funding project, the controlled temperature loading of the test spec- imen was carried out under the thorough supervision of technician Pavel Kropáček at Institute of Physics, Faculty of Civil Engineering, Brno University of Technology. References [1] P.-C. Aïtcin, S. Mindess. Sustainability of concrete. Modern concrete technology. Spon Press, New York, 2011. [2] A. M. Neville. Properties of Concrete. Pearson Education Limited, Essex, 2011. [3] I. Hager. Behaviour of cement concrete at high temperature. Bulletin of The Polish Academy of Science 61(1):145–154, 2013. [4] S. Lim. Effects of elevated temperature exposure on cement based composite materials (doctoral dissertation), 2015. [5] B. L. Karihaloo. Fracture Mechanics and Structural Concrete. Concrete Design and Construction. Longman Scientific & Technical, New York, 1995. [6] S. Kumar, S. V. Barai. Concrete Fracture Models and Applications. Springer, Berlin, 2011. [7] J. Farran. 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[14] ČSN EN 196-1. 2016 methods of testing cement - Part 1: Determination of strength. [15] B. 1881. Testing concrete - part 119 method for determination of compressive strength using portions of beams broken in flexure (equivalent cubemethod), 1983. 103 http://dx.doi.org/10.1061/40558(2001)15 Acta Polytechnica CTU Proceedings 15:99–103, 2018 1 Introduction 2 Experimental programme – materials, specimens, methods 3 Results 4 Conclusions List of symbols Acknowledgements References