Temperature and Strain Measurements in Concrete using Micro-Structure Optical Fiber Sensors

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1 More Info at Open Access Database Temperature and Strain Measurements in Concrete using Micro-Structure Optical Fiber Sensors Lou AREIAS 1,2, Thomas GEERNAERT 2, Sanne SULEJMANI 2, Francis BERGHMANS 2, Geert LUYCKX 3, Karima CHAH 4, Ioannis TROULLINOS 1, Erik COPPENS 5, Philippe Van MARCKE 5, Didier RAYMAEKERS 5, Bart CRAEYE 6,7 1 EURIDICE/SCK CEN Boeretang200, 2400 Mol, Belgium; Phone: ; lareias@sckcen.be; ioannis.troullinos@euridice.be 2 Vrije Universiteit Brussel Pleinlaan 2, Brussels, Belgium; lou.areias@vub.ac.be; tgeernae@b-phot.org; ssulejma@b-phot.org; fberghma@b-phot.org 3 COM&SENS Technologiepark 3, 9052 Zwijnaarde, Belgium; gluyckx@com-sens.eu 4 University of Mons Faculty of Engineering, Boulevard Dolez 31, 7000 Mons Belgium; Karima.CHAH@umons.ac.be 5 ONDRAF/NIRAS Belgian Agency for Radioactive Waste and Enriched Fissile Materials, Kunstlaan 14, 1210 Brussels, Belgium; e.coppens@nirond.be; p.vanmarcke@nirond.be; d.raymaekers@nirond.be 6 University of Antwerp Prinsstraat 13, 2000 Antwerp, Belgium 7 Odisee University College Kwalestraat 154, 9320 Aalst, Belgium; bart.craeye@odisee.be Abstract A recent test carried out to evaluate the construction feasibility of the Belgian supercontainer concept incorporated several types of state-of-the-art sensors and innovative monitoring techniques, including the use of different types of optical fiber sensors. One of these is a relatively new type of sensor developed by the Brussels Photonics Team (B-PHOT) of the Vrije Universiteit Brussel. The sensor uses highly birefringent microstructured optical fibers equipped with fiber Bragg gratings (MOFBGs) sensors. They were embedded in a carbon-fiber reinforced composite plate to provide protection against the concrete s highly alkaline environment, facilitate installation in the concrete mould and allow the transfer of strain onto the fiber. The double reflection spectrum of the MOFBGs allows monitoring strain and temperature simultaneously. This paper presents results of temperature and strain measurements obtained with MOFBG sensors during a ½- scale test performed in The results compare well with similar measurements obtained using conventional thermocouples and vibrating wire strain gauges. Keywords: Fiber Bragg gratings, temperature, strain, concrete, micro-structure optical fibers, photonic crystal fibers 1. Introduction The Belgian supercontainer (SC) is a reference concept advanced by the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) for the postconditioning of vitrified high-level radioactive waste (VHLW) and spent fuel (SF). An experimental program is underway to evaluate the construction feasibility of the SC. The experimental program includes tests using so called ½-scale models of the SC conceptual design. A ½-scale test has a true diameter but a reduced height of approximately half of that proposed for a SC. The main construction elements of the SC include an optional outer steel envelope, a concrete buffer and lid, and a carbon steel overpack. Figure 1 shows the layout of the supercontainer concept. One of the long-term functions of the concrete buffer is to provide a high-alkaline chemical environment to favor the formation of an adhering passive oxide film on the outer surface of the carbon steel overpack. The protection provided by the oxide film is believed to result in

2 very low and uniform corrosion rates [1] of the carbon steel. The long-term function of the overpack is to contain the radionuclides during the thermal phase, which will last several thousands of years. Two 1/2-scale tests have been performed so far to study the feasibility of constructing the SC concept at large scale. The first test was performed in 2009 [2]. The second, which is the subject of this paper, was carried out in The prime objective of this test was to advance our knowledge regarding the thermo-mechanical behaviour of the concrete materials. Figure 1. Overview of the Belgian supercontainer conceptual design Temperature and strain measurements offer important information to validate modelling studies [3] and contribute to the general understanding of the thermal-mechanical behaviour of the concrete materials. The use of optical fiber sensors offers distinct advantages over conventional sensors. These include a larger monitoring coverage of the concrete, increased sensitivity and minimum intrusiveness as compared to conventional methods. This article presents results of temperature and strain measurements obtained with an innovative FBG-based sensor developed by the Brussels Photonics Team of the Vrije Universiteit Brussel (Belgium) based on micro-structured optical fiber technology. The fibers were embedded in a carbon-fiber-reinforced epoxy plate (CFRP) prior to installation in the SC mock up. The CFRPs offered a high degree of resistance against breakage during installation as well as protection against chemical attack generated by the highly-alkaline conditions present in the concrete materials Experimental design The half-scale test comprised three main stages. These included the construction of the concrete buffer, a heating period to simulate the heat emitted by VHLW, and a cool down phase. Table 1 lists the main chronological events of the test. Table 1. Main chronological events of the 2 nd half-scale test Days Event 0 Cast concrete buffer & start Phase 1 6 Removal of inner mould 29 Removal of outer mould 56 Cast filler & lid & start heating Phase Shut OFF heater & start cool down phase 167 END of test

3 The buffer consists of a non-reinforced self-compacting concrete (SCC) made using ordinary Portland cement, limestone powder and aggregates with water/cement ratio of 0.5 [3]. The filler is a cement mortar of similar composition but with a smaller-size aggregate and a higher water/cement ratio of 1.0. The B-PHOT team of the Vrije Universiteit Brussel has developed an optical fiber sensor that can overcome some of the typical limitations of conventional fiber Bragg grating (FBG) sensors. To discriminate between temperature and strain effects with FBG sensors in conventional step-index optical fibers, an additional chain of FBGs, installed in a special protective packaging, is necessary to shield the sensors from any strain-induced effects, so that they measure only temperature. These measurements are then used to compensate for temperature effects in the FBG sensors that were in direct contact with the concrete to measure strain. In addition, conventional FBGs are in practice only sensitive to axial strain, directed along the fiber length. The sensor developed by B-PHOT (Figure 2) makes use of micro-structured optical fiber (MOF) technology [4]. Such optical fibers have a transverse wavelength-scale microstructure, which typically consists of air holes running along the entire length of the waveguide. The particular MOF used in this experiment is referred to as the Butterfly MOF [5]. Because the Butterfly MOF is highly birefringent, an FBG in this fiber yields a double Bragg peak reflection, with typical Bragg wavelengths around 1550 nm and a typical wavelength separation Δλ of the order of 1 nm [6]. The pattern of air holes in the Butterfly MOF has been further designed to equip the sensor with a very high transverse strain sensitivity to allow it to also measure strain in the direction orthogonal to the fiber s axis. The Butterfly MOF has a patent pending (EP , CA , US , Birefringent Micro-Structured Optical Fiber for Sensor Application ). Figure 2. [Left] 3D illustration of an FBG in the core region of the Butterfly MOF; [right] example of a reflection and transmission spectrum indicating the double peak structure of a Butterfly MOFBG Three of these sensors were installed in the second ½-scale test to measure temperature and total strain. The sensors were embedded in a CFRP measuring 7x10 cm prior to installation. They were installed under a controlled orientation to allow them to use a well-known strain transfer calculation between the CFRP package and the MOFBGs [7]. Measurements obtained using conventional thermocouples and vibrating wire (VW) strain gauges embedded in the concrete at nearby locations provided a reference for comparison with the temperature and strain measurements acquired with the optical fiber sensors.

4 3. Results Figure 3 shows the temperature and total strain profiles obtained with the three Butterfly MOFBG sensors 1-3. The strain profiles of all three sensors closely follow the temperature evolution, as expected. They record the three main deformation phases of the concrete buffer namely, the initial cement hydration phase, which occurs at approximately 48 hours after casting of the buffer, the start of the heating phase at 56 days, and the cool down phase at the end of the test. For comparison, Fig. 4 shows the strain and temperature profiles for Butterfly MOFBG sensor 2, together with VW CH (radial) and VW CT (tangential) strain gauges, and the temperature for thermocouple TC2. The strain levels measured by the three sensors in phase 1, ending at 56 days, show generally lower values than those of the VW gauges. During the heating phase, the Butterfly MOFBG sensor shows values of strain intermediate between those given by the two VW strain gauges. A possible reason for the observed lower strain values of the Butterfly MOFBG sensor is the relatively large surface area of its CFRP package, which limits the transfer of strain from the concrete to the sensors. Figure 3. [Left] Temperature; and [right] total strain profiles obtained with the Butterfly MOFBG sensors 1-3 Sensor 1 VW CH & CT TC2 Sensor 2 Sensor 3 Figure 4. Comparison of Butterfly MOFBG sensor 2 with reference thermocouple C2, and VW CH (radial) and VW CT (tangential) strain gauges 4. Conclusions The Butterfly MOFBG sensors used in the second ½-scale test make use of photonic crystal fiber technology developed at the Belgian Vrije Universitieit Brussel. A Bragg grating in

5 such a fiber yields a double Bragg-peak reflection, which eliminates the need for temperature correction generally associated with other optical fibers when measuring strain. The pattern of air holes in the Butterfly fiber has been further designed to equip the sensor with very high transverse-strain sensitivity, allowing the butterfly MOFBG sensor to measure strain in the direction orthogonal to the fiber s axis. The measurements of temperature and total strain obtained with the Butterfly MOFBG sensors agree well with reference values acquired using conventional sensor technologies. This is particularly true for the temperature measurements, which displayed a high degree of similarity with the values measured using conventional thermocouples embedded nearby. However, the values of total strain measured with the Butterfly MOFBG sensors were generally lower than those provided by the VW strain gauges. The relatively large size of the CFRP housing probably limited the transfer of strain from the concrete to the fibers, which could explain the generally lower values of strain measured by the MOFBG sensors when compared to the results of conventional VW sensors located nearby. To our knowledge, this is the first time that micro-structured optical fiber based sensors were used to measure strain and temperature in concrete. These results open up new possibilities for the exploitation of the sensing features of fiber Bragg gratings in dedicated microstructured optical fibers. Acknowledgements The authors would like to acknowledge financial support from the European Commission 7th Framework Programme, the Agency for Innovation by Science and Technology (IWT) through the program for Strategic Basic Research (SBO) under grant agreement n (Self Sensing Composites), the Research Foundation Flanders (FWO), the Methusalem as well as the Hercules Foundations Flanders. The authors also gratefully acknowledge the technical help provided by C. Lefèvre (ESV EURIDICE) for the sensor installation. References [1] B. Kursten, F. Druyts, L. Areias, Y. van Ingelgem, D. De Wilde, G. Nieubourg, G. S. Duffó and C. Bataillon, "Preliminary results of corrosion monitoring studies of carbon steel overpack exposed to supercontainer concrete buffer, Corrosion Engineering, Science and Technology, vol. 49, no. 6, pp , Sep [2] L. Areias,, B. Craeye, G. De Schutter, H. Van Humbeeck, W. Wacquier, L. Villers, and A. Van Cotthem, (2010). Half-scale test: an important step to demonstrate the feasibility of the Belgian superconatiner concept for disposal of HLW. Proceedings 13th International Conference on Environmental Remediation and Radioactive Waste Management, ICEM2010, Tsukuba, Japan. [3] B.Craeye, Early-age behaviour of concrete supercontainers for radwaste disposal, doctoral thesis, Ghent University, [4] Russell, P. Photonic crystal fibers, Science 299, (2003). [5] Martynkien T., Statkiewicz-Barabach, G., Olszewski, J., Wojcik, J., Mergo, P., Geernaert, T., et al., "Highly birefringent microstructured fibers with enhanced sensitivity to hydrostatic pressure," Opt. Express 18, (2010).

6 [6] Berghmans, F., Geernaert, T., Baghdasaryan, T., Thienpont, H., Challenges in the fabrication of fibre Bragg gratings in silica and polymer microstructured optical fibres, Laser & Photonics Reviews 8, (2014). [7] Sonnenfeld, C., Sulejmani, S., Geernaert, T., et al., "Microstructured Optical Fiber Sensors Embedded in a Laminate Composite for Smart Material Applications," Sensors 11, (2011).