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Full-scale test of a concrete box girder using FBG sensing system
Engineering Structures 30 (2008) 643–652 www.elsevier.com/locate/engstruct
Full-scale test of a concrete box girder using FBG sensing system Wonseok Chung, Donghoon Kang ∗ Korea Railroad Research Institute, #360-1, Woram-dong, Uiwang-Si, Gyenggi-Do, 437-757, Republic of Korea Received 21 December 2006; received in revised form 9 April 2007; accepted 7 May 2007 Available online 18 June 2007
Abstract For the structural monitoring of railway bridges, electromagnetic interference (EMI) is a significant problem as modern railway lines are powered by high-voltage electric power feeding systems. Fibre optic sensing systems are free from EMI and have been successfully applied in civil engineering fields. This study presents the application of fibre Bragg grating (FBG)-based sensing systems to precast concrete box railway bridges. A 20 m long full-scale precast concrete box railway girder was fabricated and tested in order to identify its dynamic and static performance. The experimental programme involved two phases. First, the dynamic characteristics of the test were identified using a digitally controlled exciter. An FBG-based accelerometer was installed on the surface of the structure. The second phase involved the measurement of the nonlinear static behaviour until failure. Multiplexed FBG strain sensors were embedded along the length of steel rebar and a strain-induced wavelength shift was measured in order to monitor internal strains. The measured values from the FBG-based sensors are compared with the results using electric signal-based sensors. The results show that the FBG sensing system is promising and can improve the efficiency of structural monitoring for modern railway bridges. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Railway bridge; Fibre Bragg grating (FBG); Monitoring; Full-scale; Strain
1. Introduction In the structural monitoring of railway bridges, electromagnetic interference (EMI) is a significant problem as modern railway lines are powered by high-voltage electric power feeding systems. Moreover, the fibre optic sensors are free from EMI and have been successfully applied in civil engineering applications. There has been growing recognition of the potential use of fiber Bragg grating (FBG) sensors among several types of fibre optic sensors. FBG sensors have significant more advantages than other fibre optic sensors, as to include immunity to EMI, multiplexing capability, absolute measurement, high temperature endurance, and the added convenience of embedment. FBG sensors are suitable for internal strain measurements as they are made of fibre with a small diameter (125 µm) and thus do not affect the stress and strain state of a solid [1]. Recently, fibre optic sensors have been successfully integrated with many civil engineering fields ranging from concrete structures [1–7] to soil problem [8]. A number ∗ Corresponding author. Tel.: +82 31 460 5760; fax: +82 31 460 5759.
E-mail address: [email protected] (D. Kang). c 2007 Elsevier Ltd. All rights reserved. 0141-0296/$ - see front matter doi:10.1016/j.engstruct.2007.05.003
of bridges have been successfully integrated with the fibre optic sensing system in order to the monitor static and dynamic response on bridge decks and columns [9–15]. It has been reported that the physical quantities which are able to be determined by fibre optic sensors include temperature, acceleration, corrosion, inclination, and crack [9]. In recent years, there has been an increasing number of research [10–15] towards the structural health monitoring (SHM) of bridges using FBG sensors due to its several practical advantages. Chan et al. [15] reported that FBG sensors have been successfully integrated in the SHM system of the Tsing Ma Bridge (the world longest suspension bridge) to monitor several key parts including hanger cable, rocker bearing and truss girders. The applicability of fibre optic sensor to the internal strain measurement in concrete structures was successfully verified through the strain monitoring in concrete specimen [1–3] and the interface strain measurement of FRP-concrete composite structures [1]. Slowik et al. [4] embedded FBG sensors in cement paste specimen to investigate early age shrinkage of cement paste. Kuang et al. [5] proposed a relatively inexpensive intensity-based plastic fibre optic sensing system for crack detection in concrete structures. Ansari [6] and Leung [7]
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W. Chung, D. Kang / Engineering Structures 30 (2008) 643–652
Fig. 1. FBG sensor wavelength-encoding operation.
provided an extensive review on the application of fibre optic sensors to concrete structures. A majority of previous research on embedded FBG sensors was conducted on small scale specimens. Little attention has been paid to the embedding of multiplexed FBG sensors for application in large concrete structures where conditions are harsh. The purpose of this study is to evaluate the applicability of embedded FBG sensors to the full-scale railway concrete girder. For this purpose, multiplexed FBG sensors were embedded into the reinforcing rebar in the test girder. The test girder was then statically loaded until failure in order to measure the internal strain of the girder and to ensure the durability of the FBG sensors. Another objective is to assess an FBG-based sensing system for dynamic monitoring of modern railway bridges. As described earlier, high electric power feeding systems in modern railway transportation generate noise signals into the structural frequency content. Thus, an EMI-free sensing system is necessary to acquire successful measurement data. The dynamic tests were performed to identify the modal parameters based on the FBG-based sensing system. In this paper, the principles of FBG sensors and interrogator are discussed first. The description of the full-scale specimen is then presented followed by a detailed description of the test programme. Finally, the applicability of the FBG sensing system to the full-scale concrete girder against the results of traditional sensing system is presented. 2. FBG sensing system
condition, is reflected at the Bragg grating part and the other wavelengths pass through it as illustrated in Fig. 1. The Bragg wavelength is a function of the refractive index of the fibre core and the grating period, as given in Eq. (1). If the grating is exposed to external perturbations, such as strain and temperature, the Bragg wavelength changes. By measuring the wavelength change accurately, it is possible to measure physical properties such as strain and temperature. This is a fundamental principle that allows the fibre Bragg grating to be used as a sensor. The shift of a Bragg wavelength due to strain and temperature can be expressed as ∆λ B = λ B α f + ξ f 1T + (1 − pe ) ε (2) where ε is the strain, 1T is the temperature change, α f is the coefficient of thermal expansion (CTE), and ξ f is the thermo-optic coefficient. The coefficient pe is the strain-optic coefficient of an optical fibre and can be determined from Eq. (3). 2 n { p12 − ν ( p11 + p12 )} pe = (3) 2 where n is the refractive index of the fibre core, ν is the Poisson’s ratio. p11 and p12 are the components of the strainoptic tensor. The strain-optic coefficient of an optical fibre must be measured in order to measure the accurate strains. Using the above equations with the assumption of no temperature change (1T = 0 in Eq. (2)), it is possible to measure the strain from the wavelength shift as
2.1. Principles of FBG sensor The basic operation principle of fibre Bragg grating (FBG) sensors involves monitoring of the wavelength shift in the reflected wavelength spectrum. A FBG is composed of periodic changes of the refractive index that are formed by the exposure to an intense UV interference pattern in the core of an optical fiber. This grating structure results in the reflection of the light at a specific narrowband wavelength, known as the Bragg wavelength. The Bragg condition is expressed as λ B = 2n e Λ
(1)
where λ B is the Bragg wavelength of the FBG, n e is the effective refractive index of the fibre core and Λ is the grating period. The wavelength, which is determined by the Bragg
ε=
1 ∆λ B . 1 − pe λ B
(4)
The strain can be calculated solely by measuring the wavelength shift. Furthermore, if it is assumed that FBG sensors are under strain-free conditions, Eq. (2) can be rewritten as 1T =
1 ∆λ B . α + ξ λB
(5)
The temperature can be calculated by simply measuring the wavelength shift in the reflected wavelength spectrum. If FBG sensors are used to practical applications where strain and temperature are coupled, thermal effect shown in Eq. (2) can be eliminated by decoupling 1T using Eq. (5). For this purpose,
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Fig. 2. Wavelength division multiplexing of FBG sensor.
it is recommended that the temperature should be measured simultaneously. When an FBG sensor is applied to a structure that is under only a thermal strain, the net wavelength change of the FBG sensor can be expressed as the sum of two terms, the first a refractive index change and the second the thermal strain of the structure itself, as shown in Eq. (6) [16]. ∆λ B = ξ 1T + (1 − pe ) (αstr 1T ) . λB
(6)
From Eq. (6), αstr 1T denotes the pure thermal strain of the structure itself, and the equation can be rewritten as ∆λ B 1 − ξ 1T . (7) εth = 1 − pe λ B As mentioned earlier, multiplexing capability for distributed measurement is the most practical advantage of the FBG sensing system. Multiplexing enables several FBG sensors available in a single optical fibre. In other words, the multiplexed arrays of FBG sensors allow for measuring strain at discrete locations on a given structure. The wavelength-encoded nature of the source spectrum facilitates the wavelength-division multiplexing by allowing each sensor to be assigned a different part of the available source spectrum as shown in Fig. 2. This makes the overall sensor system simple and optimal to use with respect to electronic strain gauge systems, which require individual connections between every strain gauge and data acquisition system. 2.2. Interrogator Various interrogation schemes are used to detect the wavelength shift of the Bragg wavelength. They are fundamentally based on a combination of a broadband light source and a receiver that is wavelength-dependent. In many cases, light emitting diodes (LEDs), amplified spontaneous emission (ASE) sources, erbium doped fiber (EDF) sources, and ultrashort-pulse lasers are used as a broadband source. For wavelength-dependent receivers, scanning tunable filters, detector-array spectrometers, and unbalanced interferometers
Fig. 3. Schematic diagram of wavelength swept fibre laser [17].
have been employed. However, these schemes have shown shortcomings associated with low signal powers, poor spectral resolution, and relatively complex signal processing. In order to successfully contend with those problems, a wavelengthtunable narrow-band laser is a good alternative for the light source of a fibre Bragg grating interrogation system. In this study, an interrogation system with a wavelengthswept fibre laser (WSFL) [17] is used. The WSFL has a scanning tunable filter in the cavity to sweep the laser output wavelength in time continuously and repeatedly over a range of a few tens of nanometers. This system offers several attractive features compared to previous systems. First, it provides for high signal powers, as the full source output is available during the measurement of a given grating’s Bragg wavelength. Second, the wide tuning range of the source and its narrow instantaneous spectral line width allow for a large number of individual sensors within the array. Fig. 3 shows a schematic diagram of the wavelength-swept fibre laser. Table 1 shows detailed specifications of the FBG interrogation system used in this paper. The interrogation system manufactured by FiberPro [18] includes a WSFL as a broadband light source and has advanced characteristics, such as a wide wavelength range and a high resolution. The
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(a) Front view (units in mm).
(b) Cross-section. Fig. 4. Layout of a concrete box girder. Table 1 Specifications of an FBG interrogation system Parameter
Specifications
Wavelength range Avg. output power Wavelength accuracy Wavelength resolution Measurement speed Max. # of channels Operating temperature
38 nm (1532–1570 nm) 3 mW < 5 pm (4.15 µε) < 1 pm (0.83 µε) 200 Hz 8 10–40 ◦ C
resolution of the sensing module is less than 1 pm (0.83 µε) and the wavelength measurement accuracy is typically less than ±5 pm (4.15 µε). The maximum speed of measurement is 200 Hz. For the signal processing of acquired data, it is supported by R LabVIEW software with a graphic user interface. The strain curves from the FBG sensor arrays can be plotted and stored simultaneously by a PC running the software. 3. Full-scale prestressed concrete box girder A full-scale prestressed concrete box girder with dimensions of 2050 mm × 1400 mm was fabricated using standard procedures. The girder was originally designed for a railway bridge. The length of the test girder was 20 m. Fig. 4(a) presents
a front view of the test girder under investigation and Fig. 4(b) shows a cross-section and the reinforcement of the test girder. The material properties of test girder are summarized in Table 2. Six FBG sensors were multiplexed in a single optical fibre and installed into a steel rebar. The FBG sensors were installed in a groove, as shown in Fig. 5. The FBG sensors and connecting optical fibre lines were placed straight along the groove. The optical fibre was then coated with epoxy glue, R as shown in Fig. 5(c). A fast curing epoxy (Araldite ) which is curable at room temperature is used to fill a groove along steel rebar and also to transfer mechanical stress from the structure to the sensor. The total length of the FBG embedded rebar is 20 m. The FBG embedded rebar was then positioned at the lower part of the girder before the pouring of the concrete. Several conventional strain gauges were also installed in order to compare the performance of the FBG sensors. In order to evaluate dynamic characteristics of the test girder, the modal test was conducted. Monitoring of acceleration is widely used not only identifying the system characteristics but also assessing the serviceability of the bridge. However, the FBG-based accelerometer is not widely available. In this study, an FBG-based accelerometer [19] was used to pick up the response of the test girder under forced vibration. Five surface-mounted accelerometers were also installed at several
Table 2 Material properties of test girder Material property
Concrete
Strands (MPa)
Reinforcing bar
Young’s modulus (GPa) Yield stress (MPa) Ultimate stress (MPa) Compressive strength (MPa) Tensile strength (MPa)
29.3 – – 38.4 3.45
20,000 1,600 1,900 – –
200 300 450 – –
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(a) Installation of FBG sensor line to a steel rebar.
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(b) Groove.
(c) With epoxy glue. Fig. 5. FBG embedded rebar.
(a) Position of traditional accelerometer (units in mm).
(b) FBG based and traditional accelerometer at mid-span. Fig. 6. Layout of accelerometers.
longitudinal locations, as shown in Fig. 6(a). Both an FBG accelerometer and a traditional accelerometer were installed at the same location (at mid-span) for comparisons, as shown
in Fig. 6(b). The FBG-based accelerometer used in this study (BPS-700, Tokyo Sokushin Co.) consists of a cantilever beam, a mass, a spring, and an FBG element, as shown in Fig. 7. This
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Fig. 7. FBG based accelerometer [19].
Fig. 9. Maximum acceleration at different exciting frequencies.
Fig. 8. Vibration exciter.
system is modelled by an undamped single-degree-of-freedom (SDOF) system. A more detailed description of an FBG-based accelerometer is available in the reference [19]. A series of dynamic tests with increasing input frequencies was carried out in order to examine the modal parameters using the FBG-based accelerometer. A digitally controlled vibration exciter was used to obtain precise frequency response functions. The exciter generated the input frequency to the target structure by means of kinematic, harmonic motions in a vertical direction. During the tests, the frequency of excitation was slowly increased from 0 Hz to a prespecified frequency. The amplitudes from all sensors were recorded with a sampling rate of 200 Hz. Fig. 8 shows a photograph of the vibration exciter on the girder. 4. Experimental results 4.1. Dynamic resonance test In the SHM and the assessment of railway bridges, the dynamic effects resulting from the running trains should be taken into account to avoid excessive vertical deflection and acceleration. This can be occasioned if the exciting frequency generated by equidistant train wheel loads coincides with the natural frequency of the railway bridge. UIC (International Union of Railways) code [20] provides the guideline of the natural frequency for railway bridges to avoid the aforementioned problem. Thus, the evaluation of the modal properties is essential for the structural monitoring of railway bridges. Meanwhile, high electric power feeding systems and subsequent electric systems in modern railway transportation generate noise signal to the structural frequency content. Thus, an integrated FBG sensing system is required to minimize the
electromagnetic interference for the SHM of railway bridges. Under this circumstance, the dynamic applicability of the integrated FBG sensing system should be evaluated for further applications. The first phase of the test was to evaluate modal parameters of the girder including the natural frequencies and damping ratios using a commercial FBG-based accelerometer. A series of dynamic tests was conducted using the vibration exciter. Fig. 9 shows the maximum vertical acceleration recorded in FBG-based accelerometer for different input frequencies. It is observed that the large amplitude of vibration is occurred between 7 and 8 Hz. Fig. 10 shows the response acceleration time history for different input frequencies (6.0, 7.4, and 9.0 Hz). It is clear that the FBG-based accelerometer produces results close to a conventional accelerometer for all cases. Fig. 10(a) shows that the amplitude of the detected vibration is not significant for the input frequency of 6 Hz. When the input excitations reach the value of 7.4 Hz, the vibration amplitude clearly increases to approximately 0.55g, as shown in Fig. 10(b). Fig. 10(c) shows the acceleration time history when the girder is excited at an input frequency of 9.0 Hz. As can be seen in Fig. 10(c), both accelerometers provide two peaks as the input frequency coincides with the natural frequency as the processes of the exciter are decreased and increased. The natural frequency of the test girder was determined in such a manner that the time history of free vibration range is extracted and converted into the frequency domain using Fourier transform. Fig. 11 shows the comparison results in frequency domain. It was found that the FBG-based accelerometer and the conventional accelerometer provide the same natural frequency of 7.42 Hz for the test girder. This confirms that Fig. 10(b) shows the resonance behaviour of the test girder. It is commonly known that an estimation of the damping ratio is critical before the dynamic behaviour of bridge structures can be known. For practical structures, the damping ratio is generally determined experimentally rather than analytically. For lightly damped systems, the damping ratio (ξ ) over jth decayed cycles can be determined from ξ=
1 u¨ i ln . 2π j u¨ i+ j
(8)
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(a) Input frequency = 6.0 Hz.
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Fig. 11. Natural frequency of first bending mode.
(b) Input frequency = 7.4 Hz. Fig. 12. Comparison of damping ratio.
Fig. 13. Layout of load frame. (c) Input frequency = 9.0 Hz. Fig. 10. Acceleration time history.
Fig. 12 shows a comparison of a damping ratio estimated from the FBG-based accelerometer and that from a traditional accelerometer. As can be seen, both accelerometers present a damping ratio that ranges between 1.5%–2%. 4.2. Static failure test The second phase of the test was to investigate the performance of the embedded FBG sensors in the girder. A
full-scale prestressed concrete box girder was subjected to conditions of four point bending and loaded until failure. The test was conducted in order to monitor the flexural behaviour and to measure the internal strains of the girder. Fig. 13 shows the test unit and the load frame. Two vertical servo-controlled hydraulic actuators were used to apply external loads. The test girder was loaded to the point at which all load carrying capacity is lost. The experiment was controlled using the force control method for a low load level and switched to the displacement-control method after initial cracks were detected by an unaided naked eye. The final applied load was 3700 kN.
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Fig. 14. Load–deflection curve at the mid-span.
The measured load–deflection curve at mid-span is shown in Fig. 14. It becomes apparent that the response is linear until the first crack has formed at the bottom part of the girder at approximately 1500 kN. Beyond this point significant nonlinear behaviour occurred due to the propagation of concrete cracks. Fig. 15 shows the crack configuration at various load levels during the static load test. The embedded FBG sensors continuously monitored straininduced wavelength shifts throughout the entire experiment. Fig. 16 shows the reflected wavelength spectrum for the multiplexed FBG sensors before and after the load test. Straininduced wavelengths are clearly shifted to the positive direction during the loading process, which implies the structures are in the state of tension according to Eq. (4). It is observed that the fourth peak (FBG 4 in Fig. 4(a)) is affected by
birefringence which split the peak signal in the reflected wavelength spectrum. This seems to be caused by the local transverse stress of the grating part which is induced by microcracking of the epoxy glue under the severe load condition. Kang et al. showed that the signal stability of FBG sensors can be influenced by strain gradients by structural geometry or cracks on the surface when FBG sensors are embedded into or attached on the structure [21] and proposed a novel method of fabricating FBG sensors by reducing the grating length to diminish the birefringence effect [22]. Work is currently under way to incorporate the effect of the grating length for the embedded FBG sensors in the larger-scale civil structures. Fig. 17 shows the load–strain response at the mid-span position. The initial yielding of the reinforcing rebar at the midspan position was measured at approximately 2200 kN for the FBG sensor and 2000 kN for the strain gauge, respectively. This difference seems reasonable given the many uncertainty associated with concrete material. While the central portion of the reinforcing rebar initially yielded, prestressing tendons still carried the load up to the load level of 3500 kN through the interaction with concrete and reinforcing rebar. Thus, a further increase in the load produces the reduced stiffness as can be seen in Fig. 16. Fig. 18 shows the load–strain response at the quarter-span position. It is observed that after the yielding of the reinforcing rebar at approximately 33,000 kN for the FBG sensor, the load carrying capacity of the test girder lost abruptly due to the yielding of prestressing tendons. In general, the results of FBG sensors are in close agreement with the corresponding strain gauge results. The failure behaviour of a large concrete structure is clearly captured by the FBG sensor, as can be seen in Figs. 17 and 18. Therefore, the embedded FBG sensor is deemed capable of measuring the plastic behaviour of a full-scale prestressed concrete girder.
(a) 1500 kN.
(b) 2000 kN.
(c) 2500 kN.
(d) 3000 kN.
(e) 3700 kN. Fig. 15. Crack pattern.
W. Chung, D. Kang / Engineering Structures 30 (2008) 643–652
Fig. 16. Reflected wavelength spectrum before and after the loading test.
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girder. First, the dynamic properties of the test girder were identified using an FBG-based accelerometer. A static failure test was then carried out on the test girder. Multiplexed FBG sensors were embedded and internal strains were measured from the fabrication process of the girder. It was found that the FBG sensing system is capable of measuring the dynamic properties as well as the internal strains of the girder. Moreover, the embedded FBG sensors successfully captured the plastic behaviour of the test girder. The success of the test encourages the use of the FBG sensing system as an alternative to a conventional system based on electric-type sensors. It is expected that the FBG sensing system can be effectively applied to modern railway bridges that suffer from electro-magnetic interference. References
Fig. 17. Load–strain curve at centre bottom.
Fig. 18. Load–strain curve at quarter bottom.
It should be noted that none of FBG sensors were damaged during loading test. On the other hand, some conventional strain gauges were not stable even though cautious installation is conducted on the surface of the reinforcing rebar. A side-byside comparison of the stain gauge result and the FBG result is shown in Fig. 17. It is clear that the reliability of the FBG sensor is better than that of the conventional sensors. 5. Summary and conclusions This paper presents the applicability and reliability of an FBG sensing system to a full-scale prestressed concrete box
[1] Bonfiglioli B, Pascale G. Internal strain measurements in concrete elements by fiber optic sensors. Journal of Materials in Civil Engineering 2006;15(2):125–33. [2] Li Q, Li G, Yuan L. Calibration of embedded fiber optic sensor in concrete under biaxial compression. Measurement 2004;35:303–10. [3] Li Q, Li G, Wang G, Ansari F, Liu Q. Elasto-plastic bonding of embedded optical fiber sensors in concrete. Journal of Engineering Mechanics 2001; 128(4):471–8. [4] Slowik V, Schlattner E, Klink T. Experimental investigation into early age shrinkage of cement paste by using fibre Bragg gratings. Cement and Concrete Composites 2004;26:473–9. [5] Kuang KSC, Akmaluddin, Cantwell WJ, Thomas C. Crack detection and vertical deflection monitoring in concrete beams using plastic optical fiber sensors. Measurement Science and Technology 2003;14(2):206–16. [6] Ansari F. State-of-the-art in the applications of fiber-optic sensors to cementitious composites. Cement and Concrete Composite 1997;19:3–19. [7] Leung CKY. Fiber optic sensors in concrete: The future? NDT & E International 2001;34(2):85–94. [8] Lee W, Lee W, Lee S, Salgado R. Measurement of pile load transfer using fiber Bragg grating sensor system. Canadian Geotechnical Journal 2004; 41(6):1222–32. [9] Casas JR, Cruz PJS. Fiber optic sensors for bridge monitoring. Journal of Bridge Engineering 2003;8(6):362–73. [10] Tennyson RC, Mufti AA, Rizkalla S, Tadros G, Benmokrane B. Structural health monitoring of innovative bridges in Canada with fiber optic sensors. Smart Materials and Structures 2001;10:560–73. [11] Maaskant R, et al. Fiber-optic Bragg grating sensors for bridge monitoring. Cement and Concrete Composites 1997;19(1):21–33. [12] Moyo P, Brownjohn JMW, Suresh R, Tjin SC. Development of fiber Bragg grating sensors for monitoring civil infrastructure. Engineering Structures 2005;27(12):1828–34. [13] Lin YB, Chang KC, Chern JC, Wang LA. The health monitoring of a prestressed concrete beam by using fiber Bragg grating sensors. Smart Materials and Structures 2004;13:712–8. [14] Kister G, et al. Structural health monitoring of a composite bridge using Bragg grating sensors. Part 1: Evaluation of adhesives and protection systems for the optical sensors. Engineering Structures 2007;29(3):440–8. [15] Chan THT, Yu L, Tam HY, Ni YQ, Liku SY, Chung WH, Cheng LK. Fiber Bragg grating sensors for structural health monitoring of Tsing Ma bridge: Background and experimental observation. Engineering Structures 2006;28(5):648–59. [16] Sirkis JS. Unified approach to phase–strain–temperature models for smart structure interferometric optical fiber sensor: Part I, development. Optical Engineering 1993;752–61. [17] Yun SH, Richardson DJ, Kim BY. Interrogation of fiber grating sensor arrays with a wavelength-swept fiber laser. Optics Letters 1998;23(11): 843–5.
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[18] FiberPro, Inc. IS7000 user’s manual. Daejeon (Republic of Korea); 2006. [19] Mita A, Yokol I. Fiber Bragg grating accelerometer for structural health monitoring. In: Fifth international conference on motion and vibration control. 2000. [20] Union Internationale des Chemins de fer (UIC), 776-1. Loads to be considered in railway bridge design. Paris. 1994.
[21] Kang DH, Park SO, Hong CS, Kim CG. The signal characteristics of reflected spectra of fiber Bragg grating sensors with strain gradients and grating lengths. NDT & E International 2005;38(8):712–8. [22] Kang DH, Hong CS, Kim CG. Characteristics of fiber Bragg grating sensors with various grating lengths embedded in composite materials. Key Engineering Materials 2006;321–3:152–5.
Full-scale test of a concrete box girder using FBG sensing system Wonseok Chung, Donghoon Kang ∗ Korea Railroad Research Institute, #360-1, Woram-dong, Uiwang-Si, Gyenggi-Do, 437-757, Republic of Korea Received 21 December 2006; received in revised form 9 April 2007; accepted 7 May 2007 Available online 18 June 2007
Abstract For the structural monitoring of railway bridges, electromagnetic interference (EMI) is a significant problem as modern railway lines are powered by high-voltage electric power feeding systems. Fibre optic sensing systems are free from EMI and have been successfully applied in civil engineering fields. This study presents the application of fibre Bragg grating (FBG)-based sensing systems to precast concrete box railway bridges. A 20 m long full-scale precast concrete box railway girder was fabricated and tested in order to identify its dynamic and static performance. The experimental programme involved two phases. First, the dynamic characteristics of the test were identified using a digitally controlled exciter. An FBG-based accelerometer was installed on the surface of the structure. The second phase involved the measurement of the nonlinear static behaviour until failure. Multiplexed FBG strain sensors were embedded along the length of steel rebar and a strain-induced wavelength shift was measured in order to monitor internal strains. The measured values from the FBG-based sensors are compared with the results using electric signal-based sensors. The results show that the FBG sensing system is promising and can improve the efficiency of structural monitoring for modern railway bridges. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Railway bridge; Fibre Bragg grating (FBG); Monitoring; Full-scale; Strain
1. Introduction In the structural monitoring of railway bridges, electromagnetic interference (EMI) is a significant problem as modern railway lines are powered by high-voltage electric power feeding systems. Moreover, the fibre optic sensors are free from EMI and have been successfully applied in civil engineering applications. There has been growing recognition of the potential use of fiber Bragg grating (FBG) sensors among several types of fibre optic sensors. FBG sensors have significant more advantages than other fibre optic sensors, as to include immunity to EMI, multiplexing capability, absolute measurement, high temperature endurance, and the added convenience of embedment. FBG sensors are suitable for internal strain measurements as they are made of fibre with a small diameter (125 µm) and thus do not affect the stress and strain state of a solid [1]. Recently, fibre optic sensors have been successfully integrated with many civil engineering fields ranging from concrete structures [1–7] to soil problem [8]. A number ∗ Corresponding author. Tel.: +82 31 460 5760; fax: +82 31 460 5759.
E-mail address: [email protected] (D. Kang). c 2007 Elsevier Ltd. All rights reserved. 0141-0296/$ - see front matter doi:10.1016/j.engstruct.2007.05.003
of bridges have been successfully integrated with the fibre optic sensing system in order to the monitor static and dynamic response on bridge decks and columns [9–15]. It has been reported that the physical quantities which are able to be determined by fibre optic sensors include temperature, acceleration, corrosion, inclination, and crack [9]. In recent years, there has been an increasing number of research [10–15] towards the structural health monitoring (SHM) of bridges using FBG sensors due to its several practical advantages. Chan et al. [15] reported that FBG sensors have been successfully integrated in the SHM system of the Tsing Ma Bridge (the world longest suspension bridge) to monitor several key parts including hanger cable, rocker bearing and truss girders. The applicability of fibre optic sensor to the internal strain measurement in concrete structures was successfully verified through the strain monitoring in concrete specimen [1–3] and the interface strain measurement of FRP-concrete composite structures [1]. Slowik et al. [4] embedded FBG sensors in cement paste specimen to investigate early age shrinkage of cement paste. Kuang et al. [5] proposed a relatively inexpensive intensity-based plastic fibre optic sensing system for crack detection in concrete structures. Ansari [6] and Leung [7]
644
W. Chung, D. Kang / Engineering Structures 30 (2008) 643–652
Fig. 1. FBG sensor wavelength-encoding operation.
provided an extensive review on the application of fibre optic sensors to concrete structures. A majority of previous research on embedded FBG sensors was conducted on small scale specimens. Little attention has been paid to the embedding of multiplexed FBG sensors for application in large concrete structures where conditions are harsh. The purpose of this study is to evaluate the applicability of embedded FBG sensors to the full-scale railway concrete girder. For this purpose, multiplexed FBG sensors were embedded into the reinforcing rebar in the test girder. The test girder was then statically loaded until failure in order to measure the internal strain of the girder and to ensure the durability of the FBG sensors. Another objective is to assess an FBG-based sensing system for dynamic monitoring of modern railway bridges. As described earlier, high electric power feeding systems in modern railway transportation generate noise signals into the structural frequency content. Thus, an EMI-free sensing system is necessary to acquire successful measurement data. The dynamic tests were performed to identify the modal parameters based on the FBG-based sensing system. In this paper, the principles of FBG sensors and interrogator are discussed first. The description of the full-scale specimen is then presented followed by a detailed description of the test programme. Finally, the applicability of the FBG sensing system to the full-scale concrete girder against the results of traditional sensing system is presented. 2. FBG sensing system
condition, is reflected at the Bragg grating part and the other wavelengths pass through it as illustrated in Fig. 1. The Bragg wavelength is a function of the refractive index of the fibre core and the grating period, as given in Eq. (1). If the grating is exposed to external perturbations, such as strain and temperature, the Bragg wavelength changes. By measuring the wavelength change accurately, it is possible to measure physical properties such as strain and temperature. This is a fundamental principle that allows the fibre Bragg grating to be used as a sensor. The shift of a Bragg wavelength due to strain and temperature can be expressed as ∆λ B = λ B α f + ξ f 1T + (1 − pe ) ε (2) where ε is the strain, 1T is the temperature change, α f is the coefficient of thermal expansion (CTE), and ξ f is the thermo-optic coefficient. The coefficient pe is the strain-optic coefficient of an optical fibre and can be determined from Eq. (3). 2 n { p12 − ν ( p11 + p12 )} pe = (3) 2 where n is the refractive index of the fibre core, ν is the Poisson’s ratio. p11 and p12 are the components of the strainoptic tensor. The strain-optic coefficient of an optical fibre must be measured in order to measure the accurate strains. Using the above equations with the assumption of no temperature change (1T = 0 in Eq. (2)), it is possible to measure the strain from the wavelength shift as
2.1. Principles of FBG sensor The basic operation principle of fibre Bragg grating (FBG) sensors involves monitoring of the wavelength shift in the reflected wavelength spectrum. A FBG is composed of periodic changes of the refractive index that are formed by the exposure to an intense UV interference pattern in the core of an optical fiber. This grating structure results in the reflection of the light at a specific narrowband wavelength, known as the Bragg wavelength. The Bragg condition is expressed as λ B = 2n e Λ
(1)
where λ B is the Bragg wavelength of the FBG, n e is the effective refractive index of the fibre core and Λ is the grating period. The wavelength, which is determined by the Bragg
ε=
1 ∆λ B . 1 − pe λ B
(4)
The strain can be calculated solely by measuring the wavelength shift. Furthermore, if it is assumed that FBG sensors are under strain-free conditions, Eq. (2) can be rewritten as 1T =
1 ∆λ B . α + ξ λB
(5)
The temperature can be calculated by simply measuring the wavelength shift in the reflected wavelength spectrum. If FBG sensors are used to practical applications where strain and temperature are coupled, thermal effect shown in Eq. (2) can be eliminated by decoupling 1T using Eq. (5). For this purpose,
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Fig. 2. Wavelength division multiplexing of FBG sensor.
it is recommended that the temperature should be measured simultaneously. When an FBG sensor is applied to a structure that is under only a thermal strain, the net wavelength change of the FBG sensor can be expressed as the sum of two terms, the first a refractive index change and the second the thermal strain of the structure itself, as shown in Eq. (6) [16]. ∆λ B = ξ 1T + (1 − pe ) (αstr 1T ) . λB
(6)
From Eq. (6), αstr 1T denotes the pure thermal strain of the structure itself, and the equation can be rewritten as ∆λ B 1 − ξ 1T . (7) εth = 1 − pe λ B As mentioned earlier, multiplexing capability for distributed measurement is the most practical advantage of the FBG sensing system. Multiplexing enables several FBG sensors available in a single optical fibre. In other words, the multiplexed arrays of FBG sensors allow for measuring strain at discrete locations on a given structure. The wavelength-encoded nature of the source spectrum facilitates the wavelength-division multiplexing by allowing each sensor to be assigned a different part of the available source spectrum as shown in Fig. 2. This makes the overall sensor system simple and optimal to use with respect to electronic strain gauge systems, which require individual connections between every strain gauge and data acquisition system. 2.2. Interrogator Various interrogation schemes are used to detect the wavelength shift of the Bragg wavelength. They are fundamentally based on a combination of a broadband light source and a receiver that is wavelength-dependent. In many cases, light emitting diodes (LEDs), amplified spontaneous emission (ASE) sources, erbium doped fiber (EDF) sources, and ultrashort-pulse lasers are used as a broadband source. For wavelength-dependent receivers, scanning tunable filters, detector-array spectrometers, and unbalanced interferometers
Fig. 3. Schematic diagram of wavelength swept fibre laser [17].
have been employed. However, these schemes have shown shortcomings associated with low signal powers, poor spectral resolution, and relatively complex signal processing. In order to successfully contend with those problems, a wavelengthtunable narrow-band laser is a good alternative for the light source of a fibre Bragg grating interrogation system. In this study, an interrogation system with a wavelengthswept fibre laser (WSFL) [17] is used. The WSFL has a scanning tunable filter in the cavity to sweep the laser output wavelength in time continuously and repeatedly over a range of a few tens of nanometers. This system offers several attractive features compared to previous systems. First, it provides for high signal powers, as the full source output is available during the measurement of a given grating’s Bragg wavelength. Second, the wide tuning range of the source and its narrow instantaneous spectral line width allow for a large number of individual sensors within the array. Fig. 3 shows a schematic diagram of the wavelength-swept fibre laser. Table 1 shows detailed specifications of the FBG interrogation system used in this paper. The interrogation system manufactured by FiberPro [18] includes a WSFL as a broadband light source and has advanced characteristics, such as a wide wavelength range and a high resolution. The
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(a) Front view (units in mm).
(b) Cross-section. Fig. 4. Layout of a concrete box girder. Table 1 Specifications of an FBG interrogation system Parameter
Specifications
Wavelength range Avg. output power Wavelength accuracy Wavelength resolution Measurement speed Max. # of channels Operating temperature
38 nm (1532–1570 nm) 3 mW < 5 pm (4.15 µε) < 1 pm (0.83 µε) 200 Hz 8 10–40 ◦ C
resolution of the sensing module is less than 1 pm (0.83 µε) and the wavelength measurement accuracy is typically less than ±5 pm (4.15 µε). The maximum speed of measurement is 200 Hz. For the signal processing of acquired data, it is supported by R LabVIEW software with a graphic user interface. The strain curves from the FBG sensor arrays can be plotted and stored simultaneously by a PC running the software. 3. Full-scale prestressed concrete box girder A full-scale prestressed concrete box girder with dimensions of 2050 mm × 1400 mm was fabricated using standard procedures. The girder was originally designed for a railway bridge. The length of the test girder was 20 m. Fig. 4(a) presents
a front view of the test girder under investigation and Fig. 4(b) shows a cross-section and the reinforcement of the test girder. The material properties of test girder are summarized in Table 2. Six FBG sensors were multiplexed in a single optical fibre and installed into a steel rebar. The FBG sensors were installed in a groove, as shown in Fig. 5. The FBG sensors and connecting optical fibre lines were placed straight along the groove. The optical fibre was then coated with epoxy glue, R as shown in Fig. 5(c). A fast curing epoxy (Araldite ) which is curable at room temperature is used to fill a groove along steel rebar and also to transfer mechanical stress from the structure to the sensor. The total length of the FBG embedded rebar is 20 m. The FBG embedded rebar was then positioned at the lower part of the girder before the pouring of the concrete. Several conventional strain gauges were also installed in order to compare the performance of the FBG sensors. In order to evaluate dynamic characteristics of the test girder, the modal test was conducted. Monitoring of acceleration is widely used not only identifying the system characteristics but also assessing the serviceability of the bridge. However, the FBG-based accelerometer is not widely available. In this study, an FBG-based accelerometer [19] was used to pick up the response of the test girder under forced vibration. Five surface-mounted accelerometers were also installed at several
Table 2 Material properties of test girder Material property
Concrete
Strands (MPa)
Reinforcing bar
Young’s modulus (GPa) Yield stress (MPa) Ultimate stress (MPa) Compressive strength (MPa) Tensile strength (MPa)
29.3 – – 38.4 3.45
20,000 1,600 1,900 – –
200 300 450 – –
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(a) Installation of FBG sensor line to a steel rebar.
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(b) Groove.
(c) With epoxy glue. Fig. 5. FBG embedded rebar.
(a) Position of traditional accelerometer (units in mm).
(b) FBG based and traditional accelerometer at mid-span. Fig. 6. Layout of accelerometers.
longitudinal locations, as shown in Fig. 6(a). Both an FBG accelerometer and a traditional accelerometer were installed at the same location (at mid-span) for comparisons, as shown
in Fig. 6(b). The FBG-based accelerometer used in this study (BPS-700, Tokyo Sokushin Co.) consists of a cantilever beam, a mass, a spring, and an FBG element, as shown in Fig. 7. This
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Fig. 7. FBG based accelerometer [19].
Fig. 9. Maximum acceleration at different exciting frequencies.
Fig. 8. Vibration exciter.
system is modelled by an undamped single-degree-of-freedom (SDOF) system. A more detailed description of an FBG-based accelerometer is available in the reference [19]. A series of dynamic tests with increasing input frequencies was carried out in order to examine the modal parameters using the FBG-based accelerometer. A digitally controlled vibration exciter was used to obtain precise frequency response functions. The exciter generated the input frequency to the target structure by means of kinematic, harmonic motions in a vertical direction. During the tests, the frequency of excitation was slowly increased from 0 Hz to a prespecified frequency. The amplitudes from all sensors were recorded with a sampling rate of 200 Hz. Fig. 8 shows a photograph of the vibration exciter on the girder. 4. Experimental results 4.1. Dynamic resonance test In the SHM and the assessment of railway bridges, the dynamic effects resulting from the running trains should be taken into account to avoid excessive vertical deflection and acceleration. This can be occasioned if the exciting frequency generated by equidistant train wheel loads coincides with the natural frequency of the railway bridge. UIC (International Union of Railways) code [20] provides the guideline of the natural frequency for railway bridges to avoid the aforementioned problem. Thus, the evaluation of the modal properties is essential for the structural monitoring of railway bridges. Meanwhile, high electric power feeding systems and subsequent electric systems in modern railway transportation generate noise signal to the structural frequency content. Thus, an integrated FBG sensing system is required to minimize the
electromagnetic interference for the SHM of railway bridges. Under this circumstance, the dynamic applicability of the integrated FBG sensing system should be evaluated for further applications. The first phase of the test was to evaluate modal parameters of the girder including the natural frequencies and damping ratios using a commercial FBG-based accelerometer. A series of dynamic tests was conducted using the vibration exciter. Fig. 9 shows the maximum vertical acceleration recorded in FBG-based accelerometer for different input frequencies. It is observed that the large amplitude of vibration is occurred between 7 and 8 Hz. Fig. 10 shows the response acceleration time history for different input frequencies (6.0, 7.4, and 9.0 Hz). It is clear that the FBG-based accelerometer produces results close to a conventional accelerometer for all cases. Fig. 10(a) shows that the amplitude of the detected vibration is not significant for the input frequency of 6 Hz. When the input excitations reach the value of 7.4 Hz, the vibration amplitude clearly increases to approximately 0.55g, as shown in Fig. 10(b). Fig. 10(c) shows the acceleration time history when the girder is excited at an input frequency of 9.0 Hz. As can be seen in Fig. 10(c), both accelerometers provide two peaks as the input frequency coincides with the natural frequency as the processes of the exciter are decreased and increased. The natural frequency of the test girder was determined in such a manner that the time history of free vibration range is extracted and converted into the frequency domain using Fourier transform. Fig. 11 shows the comparison results in frequency domain. It was found that the FBG-based accelerometer and the conventional accelerometer provide the same natural frequency of 7.42 Hz for the test girder. This confirms that Fig. 10(b) shows the resonance behaviour of the test girder. It is commonly known that an estimation of the damping ratio is critical before the dynamic behaviour of bridge structures can be known. For practical structures, the damping ratio is generally determined experimentally rather than analytically. For lightly damped systems, the damping ratio (ξ ) over jth decayed cycles can be determined from ξ=
1 u¨ i ln . 2π j u¨ i+ j
(8)
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(a) Input frequency = 6.0 Hz.
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Fig. 11. Natural frequency of first bending mode.
(b) Input frequency = 7.4 Hz. Fig. 12. Comparison of damping ratio.
Fig. 13. Layout of load frame. (c) Input frequency = 9.0 Hz. Fig. 10. Acceleration time history.
Fig. 12 shows a comparison of a damping ratio estimated from the FBG-based accelerometer and that from a traditional accelerometer. As can be seen, both accelerometers present a damping ratio that ranges between 1.5%–2%. 4.2. Static failure test The second phase of the test was to investigate the performance of the embedded FBG sensors in the girder. A
full-scale prestressed concrete box girder was subjected to conditions of four point bending and loaded until failure. The test was conducted in order to monitor the flexural behaviour and to measure the internal strains of the girder. Fig. 13 shows the test unit and the load frame. Two vertical servo-controlled hydraulic actuators were used to apply external loads. The test girder was loaded to the point at which all load carrying capacity is lost. The experiment was controlled using the force control method for a low load level and switched to the displacement-control method after initial cracks were detected by an unaided naked eye. The final applied load was 3700 kN.
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Fig. 14. Load–deflection curve at the mid-span.
The measured load–deflection curve at mid-span is shown in Fig. 14. It becomes apparent that the response is linear until the first crack has formed at the bottom part of the girder at approximately 1500 kN. Beyond this point significant nonlinear behaviour occurred due to the propagation of concrete cracks. Fig. 15 shows the crack configuration at various load levels during the static load test. The embedded FBG sensors continuously monitored straininduced wavelength shifts throughout the entire experiment. Fig. 16 shows the reflected wavelength spectrum for the multiplexed FBG sensors before and after the load test. Straininduced wavelengths are clearly shifted to the positive direction during the loading process, which implies the structures are in the state of tension according to Eq. (4). It is observed that the fourth peak (FBG 4 in Fig. 4(a)) is affected by
birefringence which split the peak signal in the reflected wavelength spectrum. This seems to be caused by the local transverse stress of the grating part which is induced by microcracking of the epoxy glue under the severe load condition. Kang et al. showed that the signal stability of FBG sensors can be influenced by strain gradients by structural geometry or cracks on the surface when FBG sensors are embedded into or attached on the structure [21] and proposed a novel method of fabricating FBG sensors by reducing the grating length to diminish the birefringence effect [22]. Work is currently under way to incorporate the effect of the grating length for the embedded FBG sensors in the larger-scale civil structures. Fig. 17 shows the load–strain response at the mid-span position. The initial yielding of the reinforcing rebar at the midspan position was measured at approximately 2200 kN for the FBG sensor and 2000 kN for the strain gauge, respectively. This difference seems reasonable given the many uncertainty associated with concrete material. While the central portion of the reinforcing rebar initially yielded, prestressing tendons still carried the load up to the load level of 3500 kN through the interaction with concrete and reinforcing rebar. Thus, a further increase in the load produces the reduced stiffness as can be seen in Fig. 16. Fig. 18 shows the load–strain response at the quarter-span position. It is observed that after the yielding of the reinforcing rebar at approximately 33,000 kN for the FBG sensor, the load carrying capacity of the test girder lost abruptly due to the yielding of prestressing tendons. In general, the results of FBG sensors are in close agreement with the corresponding strain gauge results. The failure behaviour of a large concrete structure is clearly captured by the FBG sensor, as can be seen in Figs. 17 and 18. Therefore, the embedded FBG sensor is deemed capable of measuring the plastic behaviour of a full-scale prestressed concrete girder.
(a) 1500 kN.
(b) 2000 kN.
(c) 2500 kN.
(d) 3000 kN.
(e) 3700 kN. Fig. 15. Crack pattern.
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Fig. 16. Reflected wavelength spectrum before and after the loading test.
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girder. First, the dynamic properties of the test girder were identified using an FBG-based accelerometer. A static failure test was then carried out on the test girder. Multiplexed FBG sensors were embedded and internal strains were measured from the fabrication process of the girder. It was found that the FBG sensing system is capable of measuring the dynamic properties as well as the internal strains of the girder. Moreover, the embedded FBG sensors successfully captured the plastic behaviour of the test girder. The success of the test encourages the use of the FBG sensing system as an alternative to a conventional system based on electric-type sensors. It is expected that the FBG sensing system can be effectively applied to modern railway bridges that suffer from electro-magnetic interference. References
Fig. 17. Load–strain curve at centre bottom.
Fig. 18. Load–strain curve at quarter bottom.
It should be noted that none of FBG sensors were damaged during loading test. On the other hand, some conventional strain gauges were not stable even though cautious installation is conducted on the surface of the reinforcing rebar. A side-byside comparison of the stain gauge result and the FBG result is shown in Fig. 17. It is clear that the reliability of the FBG sensor is better than that of the conventional sensors. 5. Summary and conclusions This paper presents the applicability and reliability of an FBG sensing system to a full-scale prestressed concrete box
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