Engineering Structures 27 (2005) 1828–1834

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Development of fiber Bragg grating sensors for monitoring civilinfrastructure

P. Moyoa,∗, J.M.W. Brownjohnb, R. Sureshc, S.C. Tjinc

aDepartment of Civil Engineering, University of Cape Town, Rondebosch 7701, Cape Town, South AfricabDepartment of Civil and Structural Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK

cSchool of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Available online 11 July 2005

Abstract

The concept of structural health monitoring has been the subject of research over the last few years, particularly in civil and structuraengineering where ageing infrastructure is of major concern. These studies have led to initiatives towards the development and deof new sensing technologies. Owing to the harsh environments found in the construction industry, and the large size of civil enstructures, such sensors should be robust, rugged, easy to use and economical. Fiber Bragg grating (FBG) sensors offer a viable sapproach with a number of advantages over traditional sensors. These include immunity to electromagnetic interference, light wesize, multiplexing capabilities, ease of installation and durability. This paper reports some results from a multi-disciplinary researchon FBG sensors involving the School of Civil and Structural Engineering and the School of Electrical and Electronic Engineering atTechnological University in Singapore. Novel FBG strain sensors have been developed and deployed on highway bridges to measustrain, static strain, and temperature. Results of these studies indicate that, if properly packaged, FBG sensors can survive the severassociated with the construction environments of civil infrastructure.© 2005 Elsevier Ltd. All rights reserved.

Keywords: Fiber optic sensors; Fiber Bragg grating sensors; FBG strain sensors; FBG temperature sensors; Structural health monitoring

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1. Introduction

Socio-economic pressure to install structural heamonitoring (SHM) systems on civil infrastructureorder to improve safety, and to optimize maintenastrategies, is growing. The demand for SHM systemhas largely been driven by deficiencies in structuperformance arising from ageing phenomena, and the neeto increase the load-carrying capacity of structures tenable them to cope with increased loading regimes. Bcontinuously or periodically measuring structural respoto live loading and environmental changes such as strestrain, temperature, and vibration, and applying relevadata analysis procedures, one can detect anomalie

∗ Corresponding author. Tel.: +27 21 6502592; fax: +27 21 6897471.E-mail address: pmoyo@ebe.uct.ac.za (P. Moyo).

0141-0296/$ - see front matter © 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2005.04.023

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structural behaviour and performance. Anomalies incluand are often defined as, deterioration and damage resufrom changes in material properties, geometric properboundary conditions, system connectivity and the loadenvironment of the structure.

A typical SHM system comprises an array of senssensor excitation hardware, a host computer and commucation hardware and software. Sensors play the imporole of providing information about the state of strain, strand temperature of the structure. Their selection for aticular application is governedby application, sensor senstivity, power requirements, robustness and reliability. MSHM systems reported in the literature have focused onditional sensing technologies such as electrical resisttrain sensors, vibrating wire strain gauges and piezoelectraccelerometers. While the traditional sensors are robusstrong enough for civil engineering applications they of

P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834 1829

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require many cables to support them, and for long distamonitoring these cables suffer from electromagnetic intference (EMI). Also only sparse populations of electricresistance (ER) sensors canbe installed due to costs. Inrecent years there have been a number of research initiatowards thedevelopment and deployment of fiber Braggrating sensors (FBG) for sensing applications in civil astructural engineering [1–3]. The interest in FBG sensoras a viable sensing approach for civil infrastructure hbeen motivated by the advantages they offer over traditiosensors. FBG sensors are: immune to electromagneticinter-ference, lightweight, small insize, easy to install, corrosioresistant, durable and can be multiplexed. The multiplexcapabilities of FBG sensors mean that many sensorsbe installed on a structure with minimum wiring. Howevethese sensors are fragile and their packaging should enthat the sensors are robust and rugged enough to withsthe harsh environment found in the construction industA number of FBG based strain monitoring programs hbeen reported in the technical literature. Todd [4] demon-strated the sensing capabilities of FBG sensors by instaa number of sensors on a steel girder in a bridge. Schulz5]reported the application of long gauge FBG sensors for mitoring civil structures. The performance of FBG sensorhas also been demonstrated in the laboratory [6–8]. Mostof these studies focus on sensing and multiplexing capaities of FBG sensors. Little attention has been paid topackaging of these sensors for application in civil engineing where conditions are harsh. This paper reports thesign and experimental evaluation of FBG sensors carried oat Nanyang Technological University (Singapore), with pticular emphasis on packaging of sensorsfor applications inconcrete structures.

2. Principle of FBG sensors

A Bragg grating is a periodic structure fabricated bexposing photosensitized fibercore to ultraviolet light(Fig. 1). When light from a broadband source interawith the grating a single wavelength, known as the Bragwavelength, is reflected. The Bragg wavelength is related tothe grating period,Λ, and the effective refractive index ofthe fiber by

λB = 2Λneff. (1)

Both the effective refractive index and the grating perivary with changes in strain,ε, temperature,T , andpressureP, imposed on the fiber. An applied strain and pressureshif t the Bragg wavelength through expansion or contracof the grating periodicity and through the photoelaseffect. Temperature affects the Bragg wavelength throthermal expansion and contraction of the grating periodicand through thermal dependence of the refractive indThese effects are well understood and, when adequamodelled, provide a means for predicting strain, pressuretemperature. If only the dominant linear effects of these thr

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factors on an FBG are considered, neglecting higher-ocross-sensitivities, then the amount of Bragg wavelenshift is given by [9]

�λB = Kεε + KT �T + K P P (2)

where Kε, KT and K P are coefficients of wavelengtsensitivity to strain, temperature and pressure (force per unarea) for an FBG given by

Kε = [1 − 0.5neff(ρ12 − ν(ρ11 − ρ12))]λB (3)

KT = [1 + ξ ]λB (4)

K P =[−1 − 2ν

E+ n2

2E(1 − 2ν)(2ρ12 + ρ11)

]λB (5)

whereρ11 and ρ12 are the components of the fiber opstrain tensor andν is Poisson’s ratio for the fiber,ξ isthe fiber thermo-optic coefficient,E is Young’s modulusfor the fiber,n is the refractive index of the fiber. Mostof these properties are known experimentally and remfairly constant for the different fibers. For a common baFBG, they areKε = 1.15 pm/µε, KT = 11 pm/◦C andK P = −3.0 pm/MPa, respectively, at 1.55µm wavelength.

3. FBG sensing systems

Installation of bare FBG sensors on componentscivil structures may result in a high rate of sensor failowing to thefragile nature of the glass fiber and the harenvironment of the construction industry. Also handlproblems resulting from the small physical size of the fiadd to the difficulties associated with attaching bare Fsensors to structural components. As noted above, aFBG sensor is sensitive to temperature, strain and pressurAn ideal sensor should be sensitive to only one paramand be immune to others. Thus there is a need to devtechniques for minimizing cross-effects in FBG sensoThe elimination of cross-sensitivity may be achievedmeasurements at two different wavelengths or two diffeoptical modes, for which the strain and temperatresponses are different [10–13]. Fernández-Valdivielsoet al. [14] proposed temperature discrimination based onFBG sensor anda thermochromic material where the strais measured through the variations of the FBG wavelenwhile the temperature is measured using the changoptical power reflected by the thermochromic materHowever these techniques suffer from system complexitlimited measurement range and limited multiplexicapabilities. A simpler way to correct a strain measuremfor the effect of temperature is using physically separasensors, where the one for temperature compensatioisolated from the strain field. The following sectiodescribe novel strain FBG sensors and temperaturesensors designed for application in civil and structuraengineering. The important aspect of the design of thFBG sensors is their packaging for civil engineeringapplications.

1830 P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834

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Fig. 1. FBG fabrication process.

Fig. 2. FBG temperature sensor.

Fig. 3. Calibration of a temperature sensor.

3.1. Temperature sensor

The temperature sensor consists of an FBG senpackaged into a 35 mm long metal tube (Fig. 2). Themetal tubing protects the FBG from external stress aincreases the temperature sensing range and sensitivity byenhancing the FBG dilation [15]. The Bragg wavelengthshift of the FBG temperature sensor is only relatedthe change of ambient temperature. A typical calibratiocurve for the temperature sensor is shown inFig. 3. Thewavelength–temperature coefficient is 25 pm/◦C, with asensitivity of 0.04 ◦C, and the accuracy is 0.2 ◦C.

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Fig. 4. Embedded FBG strain sensor.

3.2. FBG strain sensor

The embedded strain sensor consists of an Fsandwiched between layers of carbon composite mate(Fig. 4) and is about 50 mm long and 0.5 mm thicThe accuracy and sensitivity of the sensor are depenupon the optical interrogation system. The function ofoptical interrogating system is to detect the wavelengshift in relation to external perturbation and to deduce thmeasurands using Eq. (2). The interrogating system usefor strain measurements described here had a sensitivity o1 µε and an accuracy of 5µε. It should be noted that thestrain sensoris not immune to temperature variations awould therefore indicate strain due to thermal expansThe sensor described inSection 3.3allows for minimizationof strains related to temperature change.

3.3. Temperature compensated FBG strain sensor

The temperature compensated sensor consists ofFBGs written in close proximity. One FBG is encaseda metal tube to form a temperature sensor while the otis sandwiched in carbon composite as described above.assembly of the two sensors is then encased in a specdesigned dumb-bell (Fig. 5). The sandwiched sensorbonded to the dumb-bell using an epoxy glue andtemperature sensor is left loose. The design of the dubell is such that the strain sensor is isolated from presseffects. This sensor was designed for strain and temperameasurements in mass concrete.

4. Experimental evaluation of FBG sensors

Embedding the FBG sensors in carbon compomaterial or encasing the FBG in a metal tube alters itssensitivity coefficients making it necessary to calibratesensors. To this end experiments were carried out whthe embedded FBG strain sensors were installed along

P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834 1831

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Fig. 6. Tensiletest load–strain curve.

conventional foil strain gauges and then subjected to varkinds of loads. In addition experimental evaluation soughto establish simple procedures for installing FBG sensorconcrete structures.

4.1. Tensile tests

Tensile tests were performed on a steel rebar instmented with both FBG sensors and electrical resistancestrain gauges (ERS) attached to the rebar in close proxity. Fig. 6 shows the load–strain curves for both FBG anelectrical strain gauges. Clearly there is a linear variatbetween electrical FBG sensor strains and electrical retance strains. The correlation between FBG sensor strand electrical resistance strains was about 0.99. Onbasis of the tensile experiments, the wavelength–stcoefficient of the embedded FBG strain sensor was foto be 1.06 pm/µε (Fig. 6).

4.2. Static tests on concrete beams

The objective of this experiment was to study tperformance of FBG sensors in quantifying strai

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Another aim of the experiment was to establish a simprocedure for installing FBG sensors in concrete structurImportant issues considered were: bonding sensors to rebprotecting sensors during casting and isolating pressureeffects.

Two 3000 mm long, 300 mm deep and 150 mwide doubly reinforced concrete beams were used (Fig. 7).Each beam was instrumented with six FBG strain sensalongside ERS gauges on top and bottom rebars and atspans andquarter-spans. All FBG sensors survived concrpouring and compaction.

The beams were simply supported and first loadedthe serviceability limit state and then to failure. Test resushow good linearity (Fig. 8) between FBG sensor strains anelectrical resistance strains.

4.3. Monitoring the curing process of a concrete beam

Two FBG temperature sensors described inSection 3.1were attached to top and bottom rebars of the 3000 m300 mm deep, 150 mm wide reinforced concrete bedescribed inSection 4.2alongside FBG strain sensors. Thprocess of curing of the beam was monitored over a fdays.Fig. 9 shows the temperature changes from the FBGtemperature sensors attached to top and bottom rebartogether with air andconcrete surface temperature chang(measured using a thermistor) during the curing process.temperature increased rapidly and reached a maximumapproximately 8 h after casting. The maximum temperaturecorded for the top and bottom rebars are 39.3 ◦C and40.2 ◦C respectively.

4.4. Dynamics tests

A reinforced concrete beam 5500 mm long, 150 mm deand 300 mm wide was used for these vibration experime(Fig. 10). Six strain FBG sensors were installed in the beaMinimum protection was provided for a set of three sensoOf these sensors only one was damaged during castinset of four FBG sensors and four demountable electristrain gauges was installed on the surface of the beam. Thebeam was excited using an instrumented impulse hamand signals acquired by relevant interrogating systems.sampling rates for the sensors were FBG sensors at 17.5dynamic strain gauges (DSGs) at 100 Hz. The natufrequency of the beam was then extracted from the strsignals.

The maximum amplitude of the dynamic strain recordby DGSs at beam mid-span is 58 microstrains while tFBG sensor recorded 55 microstrains (Fig. 11). This slightdifference can be attributedto the low sampling rate usedfor FBG sensors. The power spectrum density of all signshows very close agreement with the maximum power aabout 7.9 Hz (Fig. 12).

1832 P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834

Fig. 7. Reinforced concretebeam sensor arrangement.

Fig. 8. RC beam load–strain curves at beam mid-span.

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Fig. 9. Monitoring concrete curing process.

4.5. Tests on a temperature compensated sensor

The temperature compensated FBG sensor was placan oven and temperature increased from 22 to 85◦C. Thewavelength shift was measured every 5◦C change (Fig. 13).The sensitivities of the strain and temperature sensors wfound to be 13.74 pm/◦C and11.67 pm/◦C respectively. Thesensor was embedded in a concrete cylinder and testecompression.Fig. 14shows the response to the compressiload of the temperature compensated sensor.Clearly, only

Fig. 10. Dynamic test set-up.

the strain sensor indicatesa change in wavelength of theFBG with increasing compression.

5. Concluding remarks

The results of these studies demonstrate that, if propinstalled, FBG sensors can survive the severe conditionsassociated with the construction of concrete structuresyield accurate measurementsof strains and temperatureThe potential applications for these FBG sensors inclu

P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834 1833

sity.

Fig. 11. Dynamic strains: (a) dynamic strain signal recordedby the FBG; (b) dynamic strain signal recorded by the DSG.

Fig. 12. Beam natural frequency based on strainsignals: (a) FBG signal power spectral density; (b) DSG signal power spectral den

nsor.

Fig. 13. Thermal response of temperature compensated FBG sensor: (a) thermal response of strain sensor; (b) thermal response of temperature se

1834 P. Moyo et al. / Engineering Structures 27 (2005) 1828–1834

Fig. 14. Response of temperature compensated sensor to loading: (a) strain sensor response to compression; (b) response of temperature during load testing.

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long term structural health monitoring, short term conditiassessment of structures, vibration and seismic respstructures and traffic loading assessment on bridges.deployment of these sensorsis currently limited by lack ofportable, rugged data acquisition systems with low powconsumption.

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