Available online at ScienceDirect. Procedia Engineering 172 (2017 )

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1 Available online at ScienceDirect Procedia Engineering 172 (2017 ) Modern Building Materials, Structures and Techniques, MBMST 2016 Comparison between different fiber optical strain measurement systems based on the example of reinforcing bars Martin Weisbrich, Klaus Holschemacher, Stefan Kaeseberg Leipzig University of Applied Sciences (HTWK Leipzig), Structural Concrete Institute, Karl-Liebknecht-Str. 132, Leipzig 04277, Germany Abstract Fiber optical measurement systems come to the fore after having been intensively investigated during the last two decades. One example is Fiber-Bragg-Gratings (FBG). This technology allows a sufficient strain measurement at predefined points. Another new technique deals with Rayleigh backscatter to realize a distributed strain measurement over the total length of an optical fiber. Such measurement is recognized to provide a high potential in civil engineering, especially in reinforced concrete. This paper deals with some preliminary comparisons between Rayleigh backscatter and FBG based on tension test with reinforcing bars. Therefor optical fibers were placed in notches, milled into the reinforcing bars. Analysis of the test data showed a strong correlation between the optical measurement systems and theoretical results up to the yield point of the steel. Furthermore, the advantages of distributed strain measurement were uncovered. In a last step, the paper offers a prospect concerning future applications for the Rayleigh backscatter system Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier Ltd. ( Peer-review under responsibility of the organizing committee of MBMST Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: Optical fiber; Strain measurement; Reinforcing bar; Fiber-Bragg-Grating; Rayleigh backscatter. 1. Introduction The fields of activity in civil engineering are subject to permanent changes. Therefore, maintenance as well as strengthening and monitoring of existing buildings have become more important leading to new requirements in civil engineering. Investments for new buildings become smaller while the costs for maintenance and health monitoring arrangements increase significantly. This monitoring should start as early as possible and must be carefully planned and conducted. Recently, this convention has been more or less ignored in the majority of cases. Many of the damage incidences in 30 to 50-year-old buildings validate a certain misconception about the importance of early health monitoring. The reasons for damages are manifold and reach from faulty construction to unpredictable natural phenomena [1]. Among others, the modern publicity of natural events has led to a stronger focus on building Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of MBMST 2016 doi: /j.proeng

2 1236 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) monitoring in the last few years. Advanced measurement techniques can contribute to a reliable and cost effective structural health monitoring. Not only do they allow an easier assessment of the building's actual health status, but visual controls are possible as well. A strict distinction between temporary and permanent measurements is necessary in this context. Permanent measurements require rugged measurement systems. Conventional electric systems like strain gauges used to measure strain in building structures are not suitable for continuous structural monitoring. Hence, optical measurement systems come to the fore after having been intensively investigated during the last two decades Common information about optical fiber measurement technologies This paper presents the examination of two methods to measure strain or temperature in building materials or structures with optical systems based on silica fiber. On the one hand, the Fiber-Bragg-Grating (FBG) system is a tried and tested technology to measure strain or temperature at predefined points along an optical fiber. For this purpose, a Bragg reflector is welded in an area of an optical fiber that reflects a small amount of wavelengths. On the other hand, a distributed fiber optic strain measurement using Rayleigh backscatter represents a new technology. This method is based on an optical single-mode fiber bonded or embedded to the test specimen in order to test a structure at numerous points over a specific area. A coherent Optical Frequency Domain Reflectometer (cofdr) interrogates continuously the fiber to measure the reflected light [2]. In contrast to FBG s, this system is able to detect defects or cracks over the total length of an optical fiber. The possibility to use a common grade telecommunication optical fiber to reduce the sensor costs is another advantage in comparison with FBG. 2. Measurement theory 2.1. Fiber-Bragg-Grating Fiber Bragg Gratings are formed by a periodic sequence of refractive index changes in the core of an optical fiber [3]. Fig. 1. Fiber-Bragg-Grating. With an FBG inside, the propagating light is partially reflected at the transition points of areas having different refraction indices (Fig. 1. Fiber-Bragg-Grating. For the majority of wavelengths, the reflected light parts are generally out of phase and extinguish without any measurable effect. However, for one certain wavelength - called Bragg wavelength B the light portions reflected by the consecutive index changes are in an equal phase and hence constructively added up, leading to a significant amount of light being returned. (E.g. equation 1) B 2 neff (1) where λ B = Bragg wavelength, n eff = effective refraction index and Δ = period of the index changes along the grating. Due to the sensitivity of the fiber s refractive index n eff to strain and temperature changes, such variations will lead to a defined shift in Bragg wavelength (i.e. light color). So, if one can detect the exact Bragg wavelength value then a definite statement about the fiber s strain state and temperature can be made. The corresponding change of strain in the optical fiber can be described with equation (2). B B (1 pe) (2)

3 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) where Δλ B = change of Bragg wavelength, p e = photo elastic component 0.22 and Δε = change of strain. [4] 2.2. Rayleigh The system utilizes swept-wavelength interferometry to scan a fiber optic sensor. If the fiber is physically changed, the interrogator registers a change in the scattered light. Fig. 2. Operating principle of Rayleigh scatter. Before the test starts, the system registers a reference measurement of the fiber at an ambient state and stores it as a fiber key. If the strain,, or the temperature,, is altered, the scatter profile along the length of the fiber changes. These two sets, the reference state and the altered state, result in a spectral shift,, of the scattered light (Fig. 1 and Fig. 2). This spectral shift is equivalent to the shift in the resonance wavelength,, of a Bragg grating as shown above, and it can be used to calculate a strain or temperature difference (equation 3). KTT K (3) For the most common germanosilicate core fibers the temperature and strain calibration constants set at values: 6 o 1 KT 6, C and K 0,780. The references [5 10] provide additional information on this process Conclusion Distributed measurement along the total length of a fiber sensor offers a lot of possibilities concerning different problems in RC structures, for instance tension stiffening, bond between reinforcement and concrete, anchorage of bars and tendons, crack detection, fatigue, and so on. But, as of yet, the Rayleigh measurement technique has not been evaluated in the proper way. For meaningful measurements, the optical fiber needs a sufficient shelter. The direct arrangement inside of the concrete is difficult, for instance cracks could damage and destroy the measurement element. Furthermore, the precise arrangement and anchorage is challenging, because the fiber should be placed and integrated inside of the reinforcement. One possibility is a notch receiving the fiber. In this case, there are questions concerning the shape of such a notch and the connecting material that enables a strong, consistent, and reliable bond between optical fiber and reinforcement across the entire measurement length. To solve the open tasks, some very preliminary axial tension tests with reinforcing bars of different diameters with integrated optical fibers were conducted. The next chapter deals with the utilized test setup and the experimental approach.

4 1238 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) Experiments The preliminary test in this paper describes the experiment on three reinforcing bars with different diameters (8mm, 10mm and 12mm). At each bar, both measurement systems have been mounted to compare the strain under tension directly. For the purpose of receiving the sensor fibers, a notch was milled on both sides of the reinforcing bar (Fig. 3). On the one side a fiber for the Rayleigh scatter was implemented, and on the other side a fiber with two FBG s (position 26cm and 74cm) was arranged. The fibers were bonded with a two components epoxy resin. The prepared reinforcing bars were tested with a testing setup as can be seen in Fig. 4. Fig. 3. Reinforcing bar with milled notch. 4. Experimental results Fig. 4. Test setup of axial tension test. For discussion, three types of diagrams are utilized in this paper. First diagram type is a 3D surface plot. Example is given in Fig. 5. For better understanding and comparison of the measurement results force-strain diagrams and strain vs. position diagrams were utilized, too (e.g. Fig. 6 and Fig. 7). Additional to the strain (Rayleigh, FBG), the distance between the upper and lower restraint (Fig. 4) from the tensile testing machine was measured. This approach allowed a strain calculation, labeled as distance (Fig. 6, Fig. 7, Fig. 8, and Fig. 9). It should be noticed that such a strain calculation

5 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) forces a certain deviation due to slip between reinforcing bar and restraint. Moreover, it is possible to calculate a theoretical strain value (up to the yield point) via measured force. This value was utilized for comparison, too. Fig. 5 reveals that there is a linear strain distribution up to 1000 microstrain over the total length of the fiber. Afterwards, strain distribution was getting significantly nonlinear. In common, strain measurement with Rayleigh scatter was able up to approximately 3500 microstrain. This was due to the fact that the epoxy resin showed a premature failure at such a strain value. There was an epoxy bonding failure with a crystalline crack structure. Fig. 5. 3D surface plot; 8mm reinforcing bar; Rayleigh scatter. Fig. 6 compares the three deployed measurement systems at a designated position (26cm). In this case the strain is shown as a function of the force. Like expected, all measurement systems monitored a linear force-strain function up to the yield point. The diagram reveals the remarkable likeness between Rayleigh and FBG test results. This circumstance is a clear evidence for the high potential of the Rayleigh scatter. Fig. 6. Force-strain diagram 8mm at position 26 cm.

6 1240 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) Fig. 7 offers the measured strain distribution over the entire reinforcing bar at three load steps (8,9kN, 17,7kN, and 26,3kN). Once again, strong likeness between the distinguish measurement techniques is obvious, especially up to the yield point. Afterwards the differences increase. Fig. 7. Strain vs. position diagram; 8mm reinforcing bar. Fig. 8 explains the strain development during test with 10mm reinforcing bar. Unfortunately, technical problems prevented a proper FBG measurement. Hence, FBG measurement results were not observed. Nonetheless, Fig. 8 confirms the statements as have been noted above. Fig. 8. Strain vs. position diagram; 10mm reinforcing bar. The tests with the 12mm reinforcing bar provided sufficient measurement results with all strain monitoring systems. Once again, Rayleigh and FBG enabled equal findings. But the diagram in Fig. 9 uncovers the major drawback of

7 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) FBG s. It is the incapability to ensure a sufficient strain measurement at a random point at the reinforcing bar. At this point, the Rayleigh scatter shows its qualities therefore that it supports proper measurement at quasi every point over the total length of the bar. This is an essential aspect for crack detection in concrete structures. 5. Future prospects Fig. 9. Strain vs. position diagram; 12mm reinforcing bar. This paper presents the very first distributed fiber optic strain measurements during axial tension tests at different reinforcing bars. These experiments show the enormous capability of modern optical measurement systems. In comparison to FBG, Rayleigh enables a close view of the strain development of an entire reinforcing bar. The use of a notch to receive and shelter the fiber inside of the reinforcing element proves to be a proper way. During all tension tests, Rayleigh measurement provided sufficient findings up to the yield point. In all cases, there was a strong correlation between theoretical and experimental measurement results. The Rayleigh technique was just as reliable and precise as the FBG measurement technique. After steel yielding, cracks appeared in the resin, used to connect fiber and steel surfaces. Because of this circumstance, further investigations concerning the reliability of different resin systems under high mechanical exposure are planned. In a next step, the distributed strain measurement is going to be utilized in the examination of the bond behavior of reinforcing bars by conducting Pull-out tests. This should help us to understand the anchorage mechanism in new high performance concretes; for instance, lightweight concrete. It also supports the evaluation of the impact of hooks or loops. Furthermore, the distributed strain measurement is scheduled to integrate optical fibers in new reinforcing materials, like fiber reinforced polymers (sheets, plates) and technical textiles. References [1] S. Käseberg, T. Müller, H. Kieslich, K. Holschemacher, Faser-Bragg-Gitter-Sensoren: Einsatzmöglichkeiten im Betonbau, in: K. Holschemacher (Ed.), Betonbau im Wandel; Neue Aspekte zur Bemessung, Konstruktion und Bauausführung, Bauwerk Verlag, Berlin, 2009, pp (In German) [2] W. Eickhoff, Optical frequency domain reflectometry in single-mode fiber, Appl. Phys. Lett. 39 (9) (1981) 693. [3] S. Kurtaran, M.S. Kılıçkaya, The modelling of Fiber Bragg Grating, Opt Quant Electron 39 (8) (2007) [4] S. Käseberg, K. Holschemacher, E. Reuschel, M.B. Schaller, T. Thiel, Smart FRP Systems with embedded FBG for structural monitoring and retrofitting, in: SMART th ECCOMAS Thematic Conference on Smart Structures and Materials, 2013.

8 1242 Martin Weisbrich et al. / Procedia Engineering 172 ( 2017 ) [5] S.T. Kreger, D.K. Gifford, M.E. Froggatt, A.K. Sang, R.G. Duncan, M.S. Wolfe, B.J. Soller, Distributed Fiber-Optic Temperature Sensing using Rayleigh Backscatter, in: The 14th International Symposium on: Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring, San Diego, California, SPIE, 2007, pp R-65301R-10. [6] S.T. Kreger, D.K. Gifford, M.E. Froggatt, B.J. Soller, M.S. Wolfe, High Resolution Distributed Strain or Temperature Measurements in Singleand Multi-Mode Fiber Using Swept-Wavelength Interferometry, in: Optical Fiber Sensors, Cancún, Mexico, pp. ThE42. [7] D. Samiec, Verteilte faseroptische Temperatur-und Dehnungsmessung mit sehr hoher Ortsauflösung, Photonik (6) (2011) (In German) [8] E.E. Sanborn, A.K. Sang, E. Wesson, D.E. Wigent, G. Lucier, Distributed Fiber Optic Strain Measurement Using Rayleigh Scatter in Composite Structures, in: T.A. Proulx (Ed.), Experimental and applied mechanics: Proceedings of the 2011 Annual Conference on Experimental and Applied Mechanics Volume 6, Springer, New York, 2011, pp [9] B.J. Soller, D.K. Gifford, M.S. Wolfe, M.E. Froggatt, High resolution optical frequency domain reflectometry for characterization of components and assemblies, Opt. Express 13 (2) (2005) 666. [10] K. Yüksel, M. Wuilparat, P. Mégret, Optical-frequency domain reflectometry: roadmap for high-resolution distributed measurements, in: IEEE/LEOS Symposium Benelux Chapter Proceedings, Brüssel, 2007, pp