- Email: [email protected]
Application of optical fiber distributed sensing to health monitoring of concrete structures
Mechanical Systems and Signal Processing 39 (2013) 441–451
Contents lists available at SciVerse ScienceDirect
Mechanical Systems and Signal Processing journal homepage: www.elsevier.com/locate/ymssp
Application of optical fiber distributed sensing to health monitoring of concrete structures Sergi Villalba n, Joan R. Casas Technical University of Catalonia, Construction Engineering Department, UPC, Barcelona, Spain
a r t i c l e i n f o
abstract
Article history: Received 29 October 2010 Received in revised form 25 January 2012 Accepted 31 January 2012 Available online 19 February 2012
The use of Optical Backscatter Reflectometer (OBR) sensors is a promising measurement technology for Structural Health Monitoring (SHM) as it offers the possibility of continuous monitoring of strain and temperature along the fiber. Several applications to materials used in the aeronautical construction have demonstrated the feasibility of such technique. These materials (composites, steel, aluminum) apart from having a smooth surface where the bonding of the sensor is easily carried out, they also have a continuous strain field when subject to external loading and therefore the bonding of the OBR on the material surface is not in danger for high levels of loading as the OBR can easily follow the strain in the material. The application of such type of sensor to concrete structures may present some difficulties due to (1) the roughness of the concrete surface and the heterogeneity due to the presence of aggregates of several sizes, (2) the fact that reinforced concrete cracks at very low level of load, appearance of a discontinuity in the surface and the strain field that may provoke a break or debonding of the optical fiber. However the feasibility of using OBR in the SHM of civil engineering constructions made of concrete is also of great interest, mainly because in this type of structures it is impossible to know where the crack may appear and therefore severe cracking (dangerous for the structure operation) can appear without warning of the monitoring if sensors are not placed in the particular location where the crack appears. In order to explore the potentiality of detecting cracks as they appear without failure or debonding, as well as the compatibility of the OBR bonding to the concrete surfaces, this paper shows the test carried out in the loading up to failure of a concrete slab. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Distributed sensing Fiber optical sensors Optical backscatter reflectometry Strain monitoring Damage detection Health monitoring
1. Introduction The essence of Structural Health Monitoring (SHM) has been clearly reflected by Housner et al. [1], where it is defined as the continuous or regular measurement and analysis of key structural and environmental parameters under operating conditions, for the purpose of warning of abnormal states or accidents at an early stage. The technique of fiber optic sensors for structural health monitoring has been used along 30 years ago. Over these years, the technique has been developed until obtain measures with accuracy similar to the standard strain gages and extensometers. The current state of the art offers three types of fiber optic sensors for structural health monitoring: Local fiber optic sensors
n
Corresponding author. Tel.: þ34 934016516. E-mail address: [email protected] (S. Villalba).
0888-3270/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ymssp.2012.01.027
442
S. Villalba, J.R. Casas / Mechanical Systems and Signal Processing 39 (2013) 441–451
Fig. 1. Wavelengths of the backscattered radiation [5].
(interferometric FOSs local sensors), quasi-distributed sensors (Fiber Bragg grating (FBG) sensor) and distributed fiber optic sensors such as the optical time domain reflectometry (OTDR) or the Brillouin scattering. Leung [2] presents a review of the potential of fiber optic sensors for the monitoring of concrete structures. In addition, novel distributed fiber optic sensing techniques for concrete structures, such as detection and monitoring of flexural cracks, and the delamination detection, are presented. Li et al. [3] present an overview of current research and development in the field of structural health monitoring with civil engineering applications. More recently, Majumder et al. [4] have summarized the research and development activities in SHM using FBG. They present a complete state-of-the-art where FBG have been critically reviewed, highlighting the areas where further work is still needed. Distributed sensors are based on the modulation of light intensity in the fiber. Two major distributed sensor methodologies are the optical time domain reflectometry (OTDR) and the Brillouin scattering. In the OTDR, Rayleigh and Fresnel scatterings are used for sensing structural perturbations. On the other hand, Brillouin scattering detects the Doppler shift in light frequency which is related to the measurements. The concept is to send a narrow pulse of light through the optical fiber, and to measure the backscattered radiation. The detected signal provides a detailed picture about the local loss distribution or reflections along the fiber caused by any of the attenuation mechanisms or some other non-homogeneities on the fiber, the location of the defect may be calculated by the ‘time of flight’. Resolution is in the order of meters, but operating range was several km, so the technique has been found very useful to locate fiber breaks. The backscattered light consists of different spectral components due to different interaction mechanisms between the propagating light pulse and the optical fiber. These backscattered spectral components include Rayleigh, Brillouin and Raman peaks or bands (Fig. 1). The Rayleigh component includes the most of the backscattered light. Most of the backscattered light keeps the incident wavelength (elastic collisions photons–atoms) The intensity of Brillouin scattered light depends on temperature and strain, while the intensity of Raman light only on the temperature. Specifically, the intensity of the anti-Stokes component of the Raman radiation (scattering) increases with the temperature, while the Stokes component remains stable. The ratio of intensities for these two peaks, together with the time of the arrival, provides information about the local temperature and position, respectively. Also, Raman peak’s height is insensitive to strain; in the case of the Brillouin scattering, the wavelength drifting is related to the local temperature and strain in the fiber. Brillouin-based sensing techniques rely on the measurement of a frequency as opposed to Raman-based techniques which are intensity based. Brillouin based techniques are consequently inherently more accurate and ¨ more stable on the long term, since intensity-based techniques suffer from a higher sensitivity to drifts (Guemes et al.) [5]. Optical Backscatter Reflectometry (OBR) is based on a frequency-domain technique, optical frequency-domain reflectometry (OFDR) that uses a tunable laser and an interferometer to probe reflections. Frequency domain techniques are usually used to analyze systems on the component- or module-level when a very high-resolution (microns) analysis of the reflections in a system is required. Optical backscatter reflectometry differs from other frequency-domain techniques in that it is sensitive enough to measure levels of Rayleigh backscatter in standard single-mode fiber. The OBR uses swept wavelength interferometry (SWI) to measure the Rayleigh backscatter as a function of length in optical fiber with high spatial resolution. An external stimulus (like a strain or temperature change) causes temporal and spectral shifts in the local Rayleigh backscatter pattern. These temporal and spectral shifts can be measured and scaled to give a distributed temperature or strain measurement. The capabilities of the OBR represent at least an order of magnitude improvement in spatial and spectral resolution over the initial demonstration of the technology. The SWI approach enables robust and practical distributed temperature and strain measurements in standard fiber with millimeter-scale spatial resolution over tens to hundreds of meters of fiber with strain and temperature resolution as fine as 1 microstrain and 0.1 1C [5].
2. Application to civil structures Several applications to materials used in the aeronautical construction have demonstrated the feasibility of such technique. It is already used for structural tests of wind turbine blades and typical aeronautic structures (strain
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monitoring) [5]. These materials (composites, steel, aluminum) apart from having a smooth surface where the bonding of the sensor is easily carried out, they also have a continuous strain field when subject to external loading and therefore the bonding of the OBR on the material surface is not a problem at high levels of loading as the OBR can easily follow the strain in the material.
Fig. 2. Locations of fiber optic sensors for a concrete dam [6].
Fig. 3. Fabrication procedure. Up: view of the reinforcing details and the straing gauges attached to the reinforcing bars. Bottom: view of the construction joint. One part of the slab is poured and the second part is waiting for concrete placement.
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The OBR technique can deliver important benefits in civil engineering since it can be used for health monitoring of civil structures, such as big buildings, dams, bridges or pipelines. Currently, research is based on the cracking control, deformation and damage detection. The fiber optic sensors are used, for example, to detect deflections in the embankment dams, and to detect cracks in roller-compacted concrete dams. Other application is the control of leakage by temperature measurements. This technique allows the control of joint sealing and waterproofing. Fig. 2 shows possible locations of fiber optic sensors for a concrete dam (Fleitz, J. 2008) [6]. The present paper shows the possibility of SHM of structural concrete using OBR technique. The possibilities of the technique in both the current performance of the fiber when attached to a concrete surface and the accuracy in detection and locating cracks are demonstrated by the application in a laboratory test.
3. Experimental set-up 3.1. Test specimen, loading and measurement locations The checking of Optical Backscatter Reflectometer sensors was carried out in a concrete slab of an experimental campaign conducted in the Structural Technology Laboratory of the Technical University of Catalonia (UPC). Dimensions of the reinforced concrete slab were 5.60 m spam length, 1.60 m width and 0.285 m thickness. The slab was built up in two casting phases with a lag of 48 h between concreting. A construction joint was placed at a distance of 2.9 m from the slab edge ( Fig. 3). The slab was part of a research project with the objective to validate the use of loop joint in the
Fig. 4. Load arrangement and location of OBR sensors.
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reinforcing bars connecting two different concrete slabs poured at different times with a construction joint between them (Villalba) [7]. The slab was simply supported at both ends and the loading was applied using an actuator ‘‘MTS’’ of 1 MN capacity ‘‘P’’ in the middle of the slab (Fig. 4). The slab was monitored with OBR sensors at the top and bottom surfaces, exactly in the four stretches as shown in Fig. 4. The optical fiber used was a single-mode fiber (SMF) type with a 50 m length. A coating of a polymer (polyimide) was used to protect the fiber against scratches and environmental attack. Firstly, bond areas were cleaned and free from grease. All surfaces were cleaned with a commercial cleaning solvent and were allowed to dry. Then, a commercial glue was applied to the bond area (on the concrete surfaces), avoiding to apply adhesive excess. The glue technology used was a one part component (without mixing) chemical type ethyl cyanoacrylate, with low viscosity. The adhesive was applied to one of the bond surfaces, avoiding to use items like tissue or a brush to spread the adhesive. After this preparation, the assembling of the optical fiber on the concrete surface was carried out within a few seconds. The slab was also monitored in the reinforcing steel bars with dynamic strain gauges. Deflection was measured at the center and ends of the slab using linear displacement transducer (LVDT). Joint opening at the construction joint was measured from their initiation using magnetic transducer ‘‘Temposonics’’ (Figs. 4 and 5). LVDT are a type of electrical transformer used for measuring linear displacement, which can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. These are based in a contactless sensing technology. In particular, LVDT with 50 and 300 mm of stroke length range was used at ends and center slab, respectively. The linearity deviation was less than 0.1% for full stroke. A total of five LVDT sensors were used. Temposonics are linear-position sensors that use the time-based magnetostrictive position sensing principle developed by MTS. These are based on a contactless sensing technology, too. Specifically, Temposonics E-Series Model ER sensors were used, with 50–75 mm of stroke length range and with linearity deviation less than 0.02% for full stroke. A total of two Temposonics sensors were used.
Fig. 5. Monitoring set-up and location of OBR sensor on concrete slab.
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The mechanical properties of concrete and reinforcing steel used in the slab are shown in Table 1. The diameter of the reinforcing bars was 20 mm. The calculated cracking bending moment and the corresponding load at the actuator are also listed in Table 1.
4. Tests results and discussions The experimental ultimate load capacity of the slab was 255.15 kN. This load is 1.123 (12.30%) times higher than the calculated theoretical load of 227.20 kN. The fiber was bonded along the longitudinal direction, in top and bottom sides, and the strain field at different load levels can be observed. Strain grows uniformly, with irregularities near the load application region (Figs. 4 and 5). Figs. 6 and 7 present strain versus fiber length (third and fourth stretches) measured under incremental loading from 50 to 170 kN up to failure. Data measured with optical fibers detects strain concentration due to cracks, even at low load levels. It is possible to track crack appearance with increasing load levels and detect the cracks’ location even in several cracking conditions. Table 1 Material properties and characteristics. Yield limit of reinforcing bar (N/mm2)
Concrete compressive strength (N/mm2)
Additive
Cement
fckm (N/mm2)
E (N/mm2)
M_crack (kNm)
P_crack (kN)
500
35
Glenium C-355
I 52,5 R
51.31
33,147.63
89.721
43.30
Fig. 6. Strain along the fiber length (third stretch, bottom side) for increasing load level (50–170 kN). Upper-left: superposition of all load levels. Rest: load levels of 50, 70 and 111 kN.
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Fig. 7. Strain along the fiber length (fourth stretch, bottom side) for increasing load level (50–170 kN). Bottom left: superposition of all load levels.
Fig. 8. Peaks location in optical fiber and measured strain. Crack location (third stretch).
By superimposing several load levels, it is observed how the lengths (location and position) where the peaks due to cracking remain stable. In some cases, a single initial peak degenerates into a double because of the proximity of the two cracks. The behavior of the slab for the load levels of 50, 70 and 110 kN was specifically monitored. The location of the peaks obtained from the use of OBR sensors, is shown in Figs. 8 and 9 for the third and fourth stretches (for 50 and 110 kN load levels). These are coincident with the visually observed cracks in the surface (Figs. 8–10). The peaks obtained by the frequency signal indicate the strain change (strain increase) produced by the generation of cracks. In the center of the slab is where most of them are concentrated.
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Fig. 9. Peaks location in optical fiber and measured strain. Crack location (fourth stretch).
Fig. 10. Crack location (third stretch). Load level equal to 170 kN. The figure shows a bottom view of the slab. Inside the circle the fiber crossing the crack without breaking is visible.
Figs. 8 and 9 show some of the trigger cracks. These peaks correspond to the points located at 2.931 (concrete joints), 2.822, 2.674 and 2.548 m, for the fourth stretch, and the point located at 2.932 m, for the third stretch. It may be seen in Fig. 10 how at high-level loads (170 kN), the OBR sensors have continued to acquire information correctly and also to perform without breaking. This is of great importance regarding structural monitoring application in real concrete. Strain gage sensors are not commonly used in the surface of concrete slabs since material heterogeneity due to the presence of aggregates of several sizes promotes a nonuniform strain field in the surface. Fig. 11 shows the strains of top face for the first and second stretches. In this case, the signal does not provide noise perturbations, and, for different load levels up to failure, the microstrains values obtained have been low. Only, when the test arrives at load values next to the breaking load (compression failure in top face), the OBR sensor detects peaks in the strain field, that indicate the slab stars cracking due to excessive compression. At this high level of load and damage, the OBR is still showing a good performance and providing values of strain, even with important cracking present (Fig. 12). Fig. 13 shows the resulting damage at failure due to concrete cracking. 4.1. Comparison with strain gauges in the reinforcing bars The use of monitoring strain gauges is usual for testing discrete reinforcing bars in concrete structures. However its use directly embedded in concrete does not provide satisfactory results due to the relative dimensions of strain gauges to the normal size components of concrete mix (mainly aggregates) and the problems related to the pouring conditions. Fig. 14 presents the load versus microstrain curves of the specimen for the various bars tested. Four longitudinal reinforcing bars were monitored with strain gauges attached to them, in the middle of the slab and close to the construction joint. Fig. 15 shows the position of the strain gauges. The cracks began to develop close to an applied load of
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Fig. 11. Strain along the fiber length (first and second stretch) for increasing load level.
Fig. 12. OBR sensor across a crack in the top face of the concrete slab (first stretch).
Fig. 13. View of the slab at the end of the test. OBR sensor (Top face—second stretch). The failure was due to the concrete crushing and spalling.
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Fig. 14. Load vs. microstrain (me) in reinforcing bars.
Fig. 15. Location of strain gauges in the reinforcing bars. The figure shows the concreting joint between the two different concreting phases and close to the joint the strain gauges attached to the bars and encapsulated for protection during pouring of concrete.
Table 2 Load vs. microstrain specimen slab (a) strain gauges and (b) optical fiber OBR. Microstrain ‘‘average’’ (me) (50 kN load level)
Reinforcing bars strain gauges B6–B5–B3–B2
Microstrain ‘‘average’’ (me) (110 kN load level)
Stretch ‘‘length’’
Peak location in optical fiber length (m)
400 Peak location related to origin of stretch (m)
Microstrain (me) (50 kN load level)
1810 Microstrain (me) (110 kN load level)
3 4
16.551 21.123
2.991 2.987
– 400
1550 2250
41 kN, the time at which the gauges measured for each bar showed a sudden change (increase) of stress (tension). Table 2 shows the microstrain of the bars for 50 and 110 kN load levels. The location of the strain gauges is closer to the detected peaks between the lengths of 2.991 and 2.987 m of the third and fourth stretches, respectively. Obviously, the values obtained do not match exactly. The OBR measures the deformation of the extreme fiber of the section at a lower level than where the reinforcing bars are instrumented. However, the range of values obtained shows a satisfactory and successful performance of the OBR sensor in concrete.
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5. Summary and conclusions The results of the test carried out in the loading up to failure of a reinforced concrete slab where OBR was attached to the external surface make it possible to draw the following conclusions: – The OBR can be easily placed on a concrete surface despite the existing roughness, showing a good bonding even for loads close to failure. – The reliability of the experimental results through OBR sensors have been compared visually by the visual inspection of the cracking process as load increases. As shown in Figs. 8 and 9, the peaks in the strain diagram of the OBR sensor fit very well with the position of the cracks visually detected. The findings from the visual inspection are also confirmed by the additional standard sensors placed in the slab. In fact, as shown in Table 2, the comparison of OBR data with the strain measurements from the strain gauges placed in the reinforcing bars has confirmed this correct performance too. – The feasibility to use OBR in SHM for civil engineering structures (reinforced concrete, prestressed concrete) is also of great interest, when compared to other types of local fiber optic sensors. In the case of concrete structures it is impossible to know exactly where the crack may appear due to the heterogeneity of the material. Therefore, the opening of a crack in a different location from where a local monitoring sensor is placed is highly feasible. The cracking not being detected by the monitoring could lead to wrong decisions regarding the SHM performance. The use of OBR sensors, providing a continuous monitoring of strain solves this limitation as cracking will be detected as a peak or discontinuity in the strain field monitored along the fiber. – The results obtained show how the OBR sensor is not only capable to detect appearance of cracks that are hardly visible, but also to perform correctly up to load levels producing a crack width in the range of 1 mm. In fact, through the test procedure, and despite the high-level of load applied (above 60% of breaking load), optical fiber OBR frequency signal is acquired properly, and this provides correct strain values without breaks of the sensor. In summary, the experience shown in the present paper confirms the usefulness and effectiveness of OBR strain measurements in detecting and monitoring the presence of damage-induced cracks in concrete structures. The accuracy of the results obtained validates the use of this technique to know where the cracks may appear, their premature evolution and behavior up to failure.
Acknowledgments The authors would like to thank the Spanish Ministry of Education and Science for the financial support under research projects BIA2006-15471-C02-01 and BIA2007-28685-E. References [1] G.W. Housner, L.A. Bergman, T.K. Caughey, A.G. Chassiakos, R.O. Claus, S.F. Masri, et al., Structural control: past, present, and future, J. Eng. Mech. 123 (9) (1997) 897–971. [2] C. Leung, Fiber optic sensors concrete: the future? NDT&E Int. 34 (2001) 85–94. [3] H. Li, D. Li, G. Song, Recent applications of fiber optic sensors to health monitoring in civil engineering, Eng. Struct. 26 (2004) 1647–1657. [4] M. Majumder, T.K. Gangopadhyay, A.K. Chakraborty, K. Dasgupta, D.K. Bhattacharya, Fibre Bragg gratings in structural health monitoring-Present status and applications, Sens. Actuators, A 147 (2008) 150–164. ¨ [5] A. Guemes, A. Ferna´ndez, B. Soller, Optical fiber distributed sensing—physical principles and applications, Struct. Health Monit. 9 (3) (2010) 233–245. ˜ olas de Presas. Comite´ Nacional Espan ˜ ol de grandes Presas. [6] Fleitz, J., Hoppe S. (2008). Nuevas Tecnologı´as de Auscultacio´n. VIII Jornadas Espan JEPVIII_074: 1-10. ˜ o y validacio´n experimental de uniones mediante superposicio´n con lazos en viaductos de hormigo´n de seccio´n transversal [7] Villalba, S. (2010). ‘‘Disen evolutiva. Optimizacio´n del proceso constructivo’’. Ph. D. thesis. Civil Engineering Department, Technical University of Catalonia, Barcelona, Spain.
Contents lists available at SciVerse ScienceDirect
Mechanical Systems and Signal Processing journal homepage: www.elsevier.com/locate/ymssp
Application of optical fiber distributed sensing to health monitoring of concrete structures Sergi Villalba n, Joan R. Casas Technical University of Catalonia, Construction Engineering Department, UPC, Barcelona, Spain
a r t i c l e i n f o
abstract
Article history: Received 29 October 2010 Received in revised form 25 January 2012 Accepted 31 January 2012 Available online 19 February 2012
The use of Optical Backscatter Reflectometer (OBR) sensors is a promising measurement technology for Structural Health Monitoring (SHM) as it offers the possibility of continuous monitoring of strain and temperature along the fiber. Several applications to materials used in the aeronautical construction have demonstrated the feasibility of such technique. These materials (composites, steel, aluminum) apart from having a smooth surface where the bonding of the sensor is easily carried out, they also have a continuous strain field when subject to external loading and therefore the bonding of the OBR on the material surface is not in danger for high levels of loading as the OBR can easily follow the strain in the material. The application of such type of sensor to concrete structures may present some difficulties due to (1) the roughness of the concrete surface and the heterogeneity due to the presence of aggregates of several sizes, (2) the fact that reinforced concrete cracks at very low level of load, appearance of a discontinuity in the surface and the strain field that may provoke a break or debonding of the optical fiber. However the feasibility of using OBR in the SHM of civil engineering constructions made of concrete is also of great interest, mainly because in this type of structures it is impossible to know where the crack may appear and therefore severe cracking (dangerous for the structure operation) can appear without warning of the monitoring if sensors are not placed in the particular location where the crack appears. In order to explore the potentiality of detecting cracks as they appear without failure or debonding, as well as the compatibility of the OBR bonding to the concrete surfaces, this paper shows the test carried out in the loading up to failure of a concrete slab. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Distributed sensing Fiber optical sensors Optical backscatter reflectometry Strain monitoring Damage detection Health monitoring
1. Introduction The essence of Structural Health Monitoring (SHM) has been clearly reflected by Housner et al. [1], where it is defined as the continuous or regular measurement and analysis of key structural and environmental parameters under operating conditions, for the purpose of warning of abnormal states or accidents at an early stage. The technique of fiber optic sensors for structural health monitoring has been used along 30 years ago. Over these years, the technique has been developed until obtain measures with accuracy similar to the standard strain gages and extensometers. The current state of the art offers three types of fiber optic sensors for structural health monitoring: Local fiber optic sensors
n
Corresponding author. Tel.: þ34 934016516. E-mail address: [email protected] (S. Villalba).
0888-3270/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ymssp.2012.01.027
442
S. Villalba, J.R. Casas / Mechanical Systems and Signal Processing 39 (2013) 441–451
Fig. 1. Wavelengths of the backscattered radiation [5].
(interferometric FOSs local sensors), quasi-distributed sensors (Fiber Bragg grating (FBG) sensor) and distributed fiber optic sensors such as the optical time domain reflectometry (OTDR) or the Brillouin scattering. Leung [2] presents a review of the potential of fiber optic sensors for the monitoring of concrete structures. In addition, novel distributed fiber optic sensing techniques for concrete structures, such as detection and monitoring of flexural cracks, and the delamination detection, are presented. Li et al. [3] present an overview of current research and development in the field of structural health monitoring with civil engineering applications. More recently, Majumder et al. [4] have summarized the research and development activities in SHM using FBG. They present a complete state-of-the-art where FBG have been critically reviewed, highlighting the areas where further work is still needed. Distributed sensors are based on the modulation of light intensity in the fiber. Two major distributed sensor methodologies are the optical time domain reflectometry (OTDR) and the Brillouin scattering. In the OTDR, Rayleigh and Fresnel scatterings are used for sensing structural perturbations. On the other hand, Brillouin scattering detects the Doppler shift in light frequency which is related to the measurements. The concept is to send a narrow pulse of light through the optical fiber, and to measure the backscattered radiation. The detected signal provides a detailed picture about the local loss distribution or reflections along the fiber caused by any of the attenuation mechanisms or some other non-homogeneities on the fiber, the location of the defect may be calculated by the ‘time of flight’. Resolution is in the order of meters, but operating range was several km, so the technique has been found very useful to locate fiber breaks. The backscattered light consists of different spectral components due to different interaction mechanisms between the propagating light pulse and the optical fiber. These backscattered spectral components include Rayleigh, Brillouin and Raman peaks or bands (Fig. 1). The Rayleigh component includes the most of the backscattered light. Most of the backscattered light keeps the incident wavelength (elastic collisions photons–atoms) The intensity of Brillouin scattered light depends on temperature and strain, while the intensity of Raman light only on the temperature. Specifically, the intensity of the anti-Stokes component of the Raman radiation (scattering) increases with the temperature, while the Stokes component remains stable. The ratio of intensities for these two peaks, together with the time of the arrival, provides information about the local temperature and position, respectively. Also, Raman peak’s height is insensitive to strain; in the case of the Brillouin scattering, the wavelength drifting is related to the local temperature and strain in the fiber. Brillouin-based sensing techniques rely on the measurement of a frequency as opposed to Raman-based techniques which are intensity based. Brillouin based techniques are consequently inherently more accurate and ¨ more stable on the long term, since intensity-based techniques suffer from a higher sensitivity to drifts (Guemes et al.) [5]. Optical Backscatter Reflectometry (OBR) is based on a frequency-domain technique, optical frequency-domain reflectometry (OFDR) that uses a tunable laser and an interferometer to probe reflections. Frequency domain techniques are usually used to analyze systems on the component- or module-level when a very high-resolution (microns) analysis of the reflections in a system is required. Optical backscatter reflectometry differs from other frequency-domain techniques in that it is sensitive enough to measure levels of Rayleigh backscatter in standard single-mode fiber. The OBR uses swept wavelength interferometry (SWI) to measure the Rayleigh backscatter as a function of length in optical fiber with high spatial resolution. An external stimulus (like a strain or temperature change) causes temporal and spectral shifts in the local Rayleigh backscatter pattern. These temporal and spectral shifts can be measured and scaled to give a distributed temperature or strain measurement. The capabilities of the OBR represent at least an order of magnitude improvement in spatial and spectral resolution over the initial demonstration of the technology. The SWI approach enables robust and practical distributed temperature and strain measurements in standard fiber with millimeter-scale spatial resolution over tens to hundreds of meters of fiber with strain and temperature resolution as fine as 1 microstrain and 0.1 1C [5].
2. Application to civil structures Several applications to materials used in the aeronautical construction have demonstrated the feasibility of such technique. It is already used for structural tests of wind turbine blades and typical aeronautic structures (strain
S. Villalba, J.R. Casas / Mechanical Systems and Signal Processing 39 (2013) 441–451
443
monitoring) [5]. These materials (composites, steel, aluminum) apart from having a smooth surface where the bonding of the sensor is easily carried out, they also have a continuous strain field when subject to external loading and therefore the bonding of the OBR on the material surface is not a problem at high levels of loading as the OBR can easily follow the strain in the material.
Fig. 2. Locations of fiber optic sensors for a concrete dam [6].
Fig. 3. Fabrication procedure. Up: view of the reinforcing details and the straing gauges attached to the reinforcing bars. Bottom: view of the construction joint. One part of the slab is poured and the second part is waiting for concrete placement.
444
S. Villalba, J.R. Casas / Mechanical Systems and Signal Processing 39 (2013) 441–451
The OBR technique can deliver important benefits in civil engineering since it can be used for health monitoring of civil structures, such as big buildings, dams, bridges or pipelines. Currently, research is based on the cracking control, deformation and damage detection. The fiber optic sensors are used, for example, to detect deflections in the embankment dams, and to detect cracks in roller-compacted concrete dams. Other application is the control of leakage by temperature measurements. This technique allows the control of joint sealing and waterproofing. Fig. 2 shows possible locations of fiber optic sensors for a concrete dam (Fleitz, J. 2008) [6]. The present paper shows the possibility of SHM of structural concrete using OBR technique. The possibilities of the technique in both the current performance of the fiber when attached to a concrete surface and the accuracy in detection and locating cracks are demonstrated by the application in a laboratory test.
3. Experimental set-up 3.1. Test specimen, loading and measurement locations The checking of Optical Backscatter Reflectometer sensors was carried out in a concrete slab of an experimental campaign conducted in the Structural Technology Laboratory of the Technical University of Catalonia (UPC). Dimensions of the reinforced concrete slab were 5.60 m spam length, 1.60 m width and 0.285 m thickness. The slab was built up in two casting phases with a lag of 48 h between concreting. A construction joint was placed at a distance of 2.9 m from the slab edge ( Fig. 3). The slab was part of a research project with the objective to validate the use of loop joint in the
Fig. 4. Load arrangement and location of OBR sensors.
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reinforcing bars connecting two different concrete slabs poured at different times with a construction joint between them (Villalba) [7]. The slab was simply supported at both ends and the loading was applied using an actuator ‘‘MTS’’ of 1 MN capacity ‘‘P’’ in the middle of the slab (Fig. 4). The slab was monitored with OBR sensors at the top and bottom surfaces, exactly in the four stretches as shown in Fig. 4. The optical fiber used was a single-mode fiber (SMF) type with a 50 m length. A coating of a polymer (polyimide) was used to protect the fiber against scratches and environmental attack. Firstly, bond areas were cleaned and free from grease. All surfaces were cleaned with a commercial cleaning solvent and were allowed to dry. Then, a commercial glue was applied to the bond area (on the concrete surfaces), avoiding to apply adhesive excess. The glue technology used was a one part component (without mixing) chemical type ethyl cyanoacrylate, with low viscosity. The adhesive was applied to one of the bond surfaces, avoiding to use items like tissue or a brush to spread the adhesive. After this preparation, the assembling of the optical fiber on the concrete surface was carried out within a few seconds. The slab was also monitored in the reinforcing steel bars with dynamic strain gauges. Deflection was measured at the center and ends of the slab using linear displacement transducer (LVDT). Joint opening at the construction joint was measured from their initiation using magnetic transducer ‘‘Temposonics’’ (Figs. 4 and 5). LVDT are a type of electrical transformer used for measuring linear displacement, which can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. These are based in a contactless sensing technology. In particular, LVDT with 50 and 300 mm of stroke length range was used at ends and center slab, respectively. The linearity deviation was less than 0.1% for full stroke. A total of five LVDT sensors were used. Temposonics are linear-position sensors that use the time-based magnetostrictive position sensing principle developed by MTS. These are based on a contactless sensing technology, too. Specifically, Temposonics E-Series Model ER sensors were used, with 50–75 mm of stroke length range and with linearity deviation less than 0.02% for full stroke. A total of two Temposonics sensors were used.
Fig. 5. Monitoring set-up and location of OBR sensor on concrete slab.
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The mechanical properties of concrete and reinforcing steel used in the slab are shown in Table 1. The diameter of the reinforcing bars was 20 mm. The calculated cracking bending moment and the corresponding load at the actuator are also listed in Table 1.
4. Tests results and discussions The experimental ultimate load capacity of the slab was 255.15 kN. This load is 1.123 (12.30%) times higher than the calculated theoretical load of 227.20 kN. The fiber was bonded along the longitudinal direction, in top and bottom sides, and the strain field at different load levels can be observed. Strain grows uniformly, with irregularities near the load application region (Figs. 4 and 5). Figs. 6 and 7 present strain versus fiber length (third and fourth stretches) measured under incremental loading from 50 to 170 kN up to failure. Data measured with optical fibers detects strain concentration due to cracks, even at low load levels. It is possible to track crack appearance with increasing load levels and detect the cracks’ location even in several cracking conditions. Table 1 Material properties and characteristics. Yield limit of reinforcing bar (N/mm2)
Concrete compressive strength (N/mm2)
Additive
Cement
fckm (N/mm2)
E (N/mm2)
M_crack (kNm)
P_crack (kN)
500
35
Glenium C-355
I 52,5 R
51.31
33,147.63
89.721
43.30
Fig. 6. Strain along the fiber length (third stretch, bottom side) for increasing load level (50–170 kN). Upper-left: superposition of all load levels. Rest: load levels of 50, 70 and 111 kN.
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Fig. 7. Strain along the fiber length (fourth stretch, bottom side) for increasing load level (50–170 kN). Bottom left: superposition of all load levels.
Fig. 8. Peaks location in optical fiber and measured strain. Crack location (third stretch).
By superimposing several load levels, it is observed how the lengths (location and position) where the peaks due to cracking remain stable. In some cases, a single initial peak degenerates into a double because of the proximity of the two cracks. The behavior of the slab for the load levels of 50, 70 and 110 kN was specifically monitored. The location of the peaks obtained from the use of OBR sensors, is shown in Figs. 8 and 9 for the third and fourth stretches (for 50 and 110 kN load levels). These are coincident with the visually observed cracks in the surface (Figs. 8–10). The peaks obtained by the frequency signal indicate the strain change (strain increase) produced by the generation of cracks. In the center of the slab is where most of them are concentrated.
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Fig. 9. Peaks location in optical fiber and measured strain. Crack location (fourth stretch).
Fig. 10. Crack location (third stretch). Load level equal to 170 kN. The figure shows a bottom view of the slab. Inside the circle the fiber crossing the crack without breaking is visible.
Figs. 8 and 9 show some of the trigger cracks. These peaks correspond to the points located at 2.931 (concrete joints), 2.822, 2.674 and 2.548 m, for the fourth stretch, and the point located at 2.932 m, for the third stretch. It may be seen in Fig. 10 how at high-level loads (170 kN), the OBR sensors have continued to acquire information correctly and also to perform without breaking. This is of great importance regarding structural monitoring application in real concrete. Strain gage sensors are not commonly used in the surface of concrete slabs since material heterogeneity due to the presence of aggregates of several sizes promotes a nonuniform strain field in the surface. Fig. 11 shows the strains of top face for the first and second stretches. In this case, the signal does not provide noise perturbations, and, for different load levels up to failure, the microstrains values obtained have been low. Only, when the test arrives at load values next to the breaking load (compression failure in top face), the OBR sensor detects peaks in the strain field, that indicate the slab stars cracking due to excessive compression. At this high level of load and damage, the OBR is still showing a good performance and providing values of strain, even with important cracking present (Fig. 12). Fig. 13 shows the resulting damage at failure due to concrete cracking. 4.1. Comparison with strain gauges in the reinforcing bars The use of monitoring strain gauges is usual for testing discrete reinforcing bars in concrete structures. However its use directly embedded in concrete does not provide satisfactory results due to the relative dimensions of strain gauges to the normal size components of concrete mix (mainly aggregates) and the problems related to the pouring conditions. Fig. 14 presents the load versus microstrain curves of the specimen for the various bars tested. Four longitudinal reinforcing bars were monitored with strain gauges attached to them, in the middle of the slab and close to the construction joint. Fig. 15 shows the position of the strain gauges. The cracks began to develop close to an applied load of
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Fig. 11. Strain along the fiber length (first and second stretch) for increasing load level.
Fig. 12. OBR sensor across a crack in the top face of the concrete slab (first stretch).
Fig. 13. View of the slab at the end of the test. OBR sensor (Top face—second stretch). The failure was due to the concrete crushing and spalling.
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Fig. 14. Load vs. microstrain (me) in reinforcing bars.
Fig. 15. Location of strain gauges in the reinforcing bars. The figure shows the concreting joint between the two different concreting phases and close to the joint the strain gauges attached to the bars and encapsulated for protection during pouring of concrete.
Table 2 Load vs. microstrain specimen slab (a) strain gauges and (b) optical fiber OBR. Microstrain ‘‘average’’ (me) (50 kN load level)
Reinforcing bars strain gauges B6–B5–B3–B2
Microstrain ‘‘average’’ (me) (110 kN load level)
Stretch ‘‘length’’
Peak location in optical fiber length (m)
400 Peak location related to origin of stretch (m)
Microstrain (me) (50 kN load level)
1810 Microstrain (me) (110 kN load level)
3 4
16.551 21.123
2.991 2.987
– 400
1550 2250
41 kN, the time at which the gauges measured for each bar showed a sudden change (increase) of stress (tension). Table 2 shows the microstrain of the bars for 50 and 110 kN load levels. The location of the strain gauges is closer to the detected peaks between the lengths of 2.991 and 2.987 m of the third and fourth stretches, respectively. Obviously, the values obtained do not match exactly. The OBR measures the deformation of the extreme fiber of the section at a lower level than where the reinforcing bars are instrumented. However, the range of values obtained shows a satisfactory and successful performance of the OBR sensor in concrete.
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5. Summary and conclusions The results of the test carried out in the loading up to failure of a reinforced concrete slab where OBR was attached to the external surface make it possible to draw the following conclusions: – The OBR can be easily placed on a concrete surface despite the existing roughness, showing a good bonding even for loads close to failure. – The reliability of the experimental results through OBR sensors have been compared visually by the visual inspection of the cracking process as load increases. As shown in Figs. 8 and 9, the peaks in the strain diagram of the OBR sensor fit very well with the position of the cracks visually detected. The findings from the visual inspection are also confirmed by the additional standard sensors placed in the slab. In fact, as shown in Table 2, the comparison of OBR data with the strain measurements from the strain gauges placed in the reinforcing bars has confirmed this correct performance too. – The feasibility to use OBR in SHM for civil engineering structures (reinforced concrete, prestressed concrete) is also of great interest, when compared to other types of local fiber optic sensors. In the case of concrete structures it is impossible to know exactly where the crack may appear due to the heterogeneity of the material. Therefore, the opening of a crack in a different location from where a local monitoring sensor is placed is highly feasible. The cracking not being detected by the monitoring could lead to wrong decisions regarding the SHM performance. The use of OBR sensors, providing a continuous monitoring of strain solves this limitation as cracking will be detected as a peak or discontinuity in the strain field monitored along the fiber. – The results obtained show how the OBR sensor is not only capable to detect appearance of cracks that are hardly visible, but also to perform correctly up to load levels producing a crack width in the range of 1 mm. In fact, through the test procedure, and despite the high-level of load applied (above 60% of breaking load), optical fiber OBR frequency signal is acquired properly, and this provides correct strain values without breaks of the sensor. In summary, the experience shown in the present paper confirms the usefulness and effectiveness of OBR strain measurements in detecting and monitoring the presence of damage-induced cracks in concrete structures. The accuracy of the results obtained validates the use of this technique to know where the cracks may appear, their premature evolution and behavior up to failure.
Acknowledgments The authors would like to thank the Spanish Ministry of Education and Science for the financial support under research projects BIA2006-15471-C02-01 and BIA2007-28685-E. References [1] G.W. Housner, L.A. Bergman, T.K. Caughey, A.G. Chassiakos, R.O. Claus, S.F. Masri, et al., Structural control: past, present, and future, J. Eng. Mech. 123 (9) (1997) 897–971. [2] C. Leung, Fiber optic sensors concrete: the future? NDT&E Int. 34 (2001) 85–94. [3] H. Li, D. Li, G. Song, Recent applications of fiber optic sensors to health monitoring in civil engineering, Eng. Struct. 26 (2004) 1647–1657. [4] M. Majumder, T.K. Gangopadhyay, A.K. Chakraborty, K. Dasgupta, D.K. Bhattacharya, Fibre Bragg gratings in structural health monitoring-Present status and applications, Sens. Actuators, A 147 (2008) 150–164. ¨ [5] A. Guemes, A. Ferna´ndez, B. Soller, Optical fiber distributed sensing—physical principles and applications, Struct. Health Monit. 9 (3) (2010) 233–245. ˜ olas de Presas. Comite´ Nacional Espan ˜ ol de grandes Presas. [6] Fleitz, J., Hoppe S. (2008). Nuevas Tecnologı´as de Auscultacio´n. VIII Jornadas Espan JEPVIII_074: 1-10. ˜ o y validacio´n experimental de uniones mediante superposicio´n con lazos en viaductos de hormigo´n de seccio´n transversal [7] Villalba, S. (2010). ‘‘Disen evolutiva. Optimizacio´n del proceso constructivo’’. Ph. D. thesis. Civil Engineering Department, Technical University of Catalonia, Barcelona, Spain.