Fibre-Bragg-gratings-in-structural-health-monitoring—Present-status-and-applications_Sensors-and-

Sensors and Actuators A 147(2008)150–164Contents lists available at ScienceDirectSensors and Actuators A:Physicalj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s naReviewFibre Bragg gratings in structural health monitoring—Present status and applicationsMousumi Majumder ∗,Tarun Kumar Gangopadhyay,Ashim Kumar Chakraborty,Kamal Dasgupta,D.K.BhattacharyaCentral Glass &Ceramic Research Institute (CSIR),196Raja S.C.Mullick Road,PO:Jadavpur University,Kolkata 700032,Indiaa r t i c l e i n f o Article history:Received 23August 2007Received in revised form 15April 2008Accepted 15April 2008Available online 22April 2008Keywords:Fibre-optic sensor Fibre Bragg gratings FBG strain sensorSensor for structural monitoring Sensor for compositea b s t r a c tIn-service structural health monitoring (SHM)of engineering structures has assumed a significant role in assessing their safety and integrity.Fibre Bragg grating (FBG)sensors have emerged as a reliable,in situ,non-destructive tool for monitoring,diagnostics and control in civil structures.The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field.In this article,the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed,highlighting the areas where further work is needed.A few packaging schemes for FBG strain sensors are also discussed.Finally a few limitations and market barriers associated with the use of these sensors have been addressed.©2008Elsevier B.V.All rights reserved.Contents 1.Introduction .........................................................................................................................................1512.Principle of operation of FBG sensors...............................................................................................................1512.1.Strain measurement using FBG sensors .....................................................................................................1512.2.Strain–temperature cross-sensitivity .......................................................................................................1523.FBG interrogation techniques.......................................................................................................................1524.FBG encapsulation techniques......................................................................................................................1525.Applications of FBG strain sensors in structural sensing...........................................................................................1545.1.Strain monitoring in civil infrastructure.....................................................................................................1545.1.1.Strain monitoring in reinforced concrete beams...................................................................................1555.1.2.Strain monitoring in smart beams.................................................................................................1565.1.3.Pile load monitoring ...............................................................................................................1565.1.4.Early-age cement shrinkage........................................................................................................1575.1.5.Moisture/humidity measurement in civil applications............................................................................1585.1.6.FBGs in geodynamic studies .......................................................................................................1595.1.7.Ultrasonic non-destructive testing of structural health using FBG................................................................1606.Conclusion...........................................................................................................................................161Acknowledgements.................................................................................................................................161References...........................................................................................................................................161∗Corresponding author.Fax:+913324730957.E-mail address:mousumi@cgcri.res.in (M.Majumder).0924-4247/$–see front matter ©2008Elsevier B.V.All rights reserved.doi:10.1016/j.sna.2008.04.008M.Majumder et al./Sensors and Actuators A147(2008)150–1641511.IntroductionStructural integration offibre-optic sensing systems represents a new interdisciplinary branch of engineering which involves the unique combination of laser-optics,fibre-optics,optoelectronics, microelectronics,artificial intelligence,composite material science and structural engineering.Fibre-optic sensors have a number of advantages over their electrical counterparts and are the primary candidates for complete sensing systems.Fibre Bragg grating(FBG)sensors have undergone a rapid development in the recent years following the observation of very-narrow-band reflection in the photosensitive core-region of Ge-doped silica opticalfibres[1]and itsfirst successful fabrication onfibre-core by exposure of a coherent two-beam UV interference pattern in1989[2].FBGs are immune to electromagnetic inter-ference(EMI)and ground loops.They are lightweight and have small physical dimensions,suitable for being embedded into,or attached to a structure.No wires are required to connect sensors to the control system as thefibres themselves act as both the sensing elements and the signal propagation conduit.FBG sensors offer a unique advantage of single ended connection to control systems because only reflected signals from the FBGs are important for demodulation.FBGs possess excellent resolution and range,water and corrosion resistance,ability to be multiplexed,immunity to harsh weather conditions,compact sensor and harness size,and reasonable cost per channel.Besides,wavelength encoded infor-mation is given by FBGs.Since wavelength is an absolute parameter, signal from FBG may be processed such that its information remains immune to powerfluctuations along the optical path.Thus,they offer a self-referencing,absolute measurement scheme.The FBG sensing technology shows great potential for applications within a variety of industries[3].FBG sensors have attracted interest from the civil structure communities over the past decade for structural health,vibration and seismic response monitoring.FBG sensors have been embedded in concrete for deformation monitoring and traffic load assessment in bridges and buildings.FBG sensors have been established as a major leading technology as compared to other competingfibre-optic sensor technologies.A major share of the papers presented at the17th Optical Fibre Sensors Conference(OFS-18)held at Cancun,Mexico in2006were onfibre Bragg gratings.The main advantages of FBGs over other fibre sensor schemes are its low cost,good linearity,wavelength multiplexing capacity,resistance to harsh environments and the transduction mechanism,which eliminates the need for referenc-ing as in interferometric sensors.FBG sensor technology is now on the verge of maturity after almost two decades of active research and development in thisfield.Efforts are now concentrating on delivering complete FBG sensor systems including front-end elec-tronics.Strain studies in civil structures are pivotal in avoiding unex-pected catastrophic failures.Long-term strain study of structures also helps in freezing the design limits of similar structures.Con-ventionally,most structures rely on maintenance schedules,visual inspection and a few conventional sensors for the purpose of dam-age monitoring.But the high cost of maintenance,lack of precision in visual inspection and susceptibility of sensors to harsh environ-mental conditions has made structural health monitoring(SHM) a necessity.Over the past few decades,Fibre Bragg grating sen-sors have emerged as a suitable,accurate and cost-effective tool in SHM of civil structures like high-rise buildings,bridges,tunnels and dams.For existing structures,FBG sensors can be attached onto the structure surface,whereas for new structures,these sensors can be embedded into the structure during the construction phase with-out any serious effect on the structural integrity.The information from such SHM systems can provide early warning forcompro-Fig.1.Transmission and reflection spectra from an FBG.mised integrity of structures and thus help avoid severe losses.Such information is also helpful to adapt and update newer designs of similar structures.Several review papers on applications of FBGs have been published[4–11].Strain and temperature have so far been the dominating measurands of interest.These reviews have primarily focused on the various usages of the FBG sensors in different sens-ing areas.This paper aims to provide an overview of the application of FBG sensors for strain measurement,particularly in thefield of structural sensing.Various applications as in structural health mon-itoring for bridges and concrete structures,moisture sensing,strain sensing of smart structures using FBG–FRP bars and ultrasonic non-destructive testing using FBG sensors have been discussed.Practical aspects like packaging of the sensors and demodulation techniques suitable for the use of FBG sensors in structural health monitoring have been discussed in detail.This review is expected to provide useful insight to researchers in thefield of structural sensing usinga reliable and effective strain sensing platform.2.Principle of operation of FBG sensorsFBGs are obtained by creating periodic variations in the refrac-tive index of the core of an opticalfibre[2].Fig.1shows the internal structure of an opticalfibre with an FBG written in it.When light is made to pass through the grating,at a particular wavelength,called the Bragg wavelength,the light reflected by the varying zones of refractive indices will be in phase and amplified. The Bragg wavelength is expressed asB=2n eff (1) where B is the Bragg wavelength,n eff is the effective refractive index of the FBG and is the grating period.2.1.Strain measurement using FBG sensorsWhen strain is induced in an FBG,the relative change in Bragg wavelength is expressed asBB=(1− e)ε(2) whereεis the longitudinal strain on the FBG and e is the effective photo-elastic constant of thefibre core material.e=n2eff2[p12−v(p11+p12)](3) where p ij are the silica photo-elastic tensor components and is the Poisson’s ratio.For an FBG of central wavelength of1550nm, typical strain sensitivity is approximately1.2pm/microstrain[9].152M.Majumder et al./Sensors and Actuators A147(2008)150–164Table1Temperature compensation techniques in FBG interrogationSchemes of temperature compensation RemarksTwo separate FBGs(one for strain measurement and the other for temperature measurement) [12,13]The most simple and straightforward technique for temperature compensation.However,resolution of the technique is low and interrogation of twofibres is cumbersomeTwo closely spaced gratings of differentwavelengths inscribed in the samefibre[14]Interrogation is easier.In a measurement range of0–900microstrain and25–120◦C,error reported is5%One LPG and two FBG scheme[15]This technique relies on the fact that LPGs have a much higher temperature response but lower strain responsecompared to short period FBGs.Resolution reported is±9microstrain and±1.5◦CTwo FBGs with entirely different wavelengths [16]In a measurement range of600microstrain and50◦C,error reported is10microstrain and±5◦C.However,this technique requires two light sources and two interrogation units,thereby increasing the overall cost of the systemTwo FBGs of varying diameter spliced together [17,18]System is illuminated from a single broadband source and interrogated using a single interrogator.In a measurement range of2500microstrain and120◦C,maximum error reported is17microstrain and1◦CSingle FBG with an EDFA[19]The amplified spontaneous emission power of the EDFA source has a linear relation with temperature and is used tomeasure temperature.This reading is subtracted from the combined strain temperature response of the Braggwavelength to get pure strain readings.Experimental r.m.s.deviation values of strain and temperature reported were18.2microstrain and0.7◦C,respectivelySingle FBG inscribed in erbium–ytterbiumfibre[20]In a measurement range of1100microstrain and50–180◦C,error reported is55.8microstrain and3◦CFBG embedded in composite material[21,22]This technique utilizes the birefringence property of the gratings when embedded.Wavelength separation of the twopeaks provides an estimate of temperature,whereas average wavelength between such separations provide the strainvalueAthermal packaging of thefibre using metal coating of the FBG[23]By controlling the thickness of the metal coating,the effective thermal expansion coefficient and the Young’s Modulus are controlled.For a temperature change30–80◦C,Bragg wavelength shifts by about50pmFBG glued to a bimetal alloy strip used as acantilever beam[24]In a temperature range of−20to60◦C,thermo-opto coefficient reported is−0.4pm/◦C2.2.Strain–temperature cross-sensitivityThe Bragg wavelength B is also affected by temperature changes.The relative change in the Bragg wavelength due to tem-perature change is expressed asBB=(˛+ ) T(4)where T is the change in temperature experienced at the FBG location,˛is the thermal expansion and is the thermo-optic coefficient.For an FBG of central wavelength of1550nm,typical temperature sensitivity is approximately13pm/◦C[9].Combining Eqs.(2)and(4),we get the effective Bragg wavelength shift due to strain and temperature and are expressed asBB=(˛+ ) T+(1− e)ε(5)For pure strain measurements,effects of temperature change on the Bragg wavelength has to be suitably compensated.Several techniques to offset this behavior are available in literature.Table1 shows a few competing techniques to achieve the purpose of tem-perature compensation of FBGs.3.FBG interrogation techniquesVarious interrogation techniques of FBGs have been proposed in literatures[25–60].The interrogation units are responsible for reading the Bragg wavelength shift of the FBGs induced by various physical parameters like strain,temperature,etc.Optical spectrum analyzers are unsuitable for this purpose due to their high cost and low scanning speed.The choice of the interrogation method depends upon several factors like type and range of strain being measured,accuracy and sensitivity required,number of sensors being interrogated and cost of the instrumentation.The FBG interrogation schemes commonly used are tabulated (Table2).4.FBG encapsulation techniquesOwing to the large span and long service period of civil structures,the durability,reliability and robustness of its health monitoring system is a key issue.Bare FBGs being very fragile in nature,require suitable encapsulation before being put into regular monitoring service.Various encapsulation techniques for FBG strain sensors have been proposed in literatures[61–67]. Fig.2(a)–(e)shows some of the encapsulated FBGs from literature as referenced and used for strain measurement on concrete surfaces and mental surfaces,respectively.Fig.2(a)shows a capillary encapsulated FBG sensor for use in concrete structures.The mental holder ring is attached on the surface of the concrete structure and it faithfully transmits the deformation of the structure to the FBG sensor.Fig.2(b)shows an FBG sensor encapsulated in slice base by gluing.This type of encap-sulated FBG sensors can be very well used in mental and concrete structures.Fig.2(c)shows smart FRP–FBG sensors.The FRP bars are embedded with FBG sensors thus utilizing the dual properties of FRP’s mechanical strength and the FBG’s sensing ability.Fig.2(d) shows another encapsulated FBG for use in cement structures.The fibre is placed inside a steel tube,which is further enclosed in a concrete-proof plastic hose.This protects thefibre from strong alkaline environment of the cement as well is effective in faith-ful transmission of strain to the sensor.Fig.2(e)shows a packaged Table2FBG interrogation schemesTypes Technologies ReferencePassivedetec-tionschemeLinearly wavelength-dependent device[25–29]CCD spectrometer[30–33]Power detection[34–37]Identical chirped-grating pair[38] Activedetec-tionschemeFabry-perotfilter[39–41]Unbalanced Mach-Zehnder interferometer[42–49]Fibre Fourier transform spectrometer[50,51]Acoustic-optic tunablefilter[52–54]Matched FBG pair[55–58]Michelson interferometer[59]LPG pair interferometer[60]M.Majumder et al./Sensors and Actuators A147(2008)150–164153Fig.2.(a)Mental capillary encapsulated FBG sensor[62].(b)Mental slice encapsulated FBG sensor[62].(c)FRP–FBG sensors[62].(d)FBG encapsulated in a steel tube[92].(e)Long-gage FBG sensor with surface mounting facility[61].(f)Schematic of FBG embedded coaxially in a cylindrical polymer package[63].(g)Athermally packaged FBG strain sensor.FBG for civil structure applications where we require to monitor a considerably macrostrain value.The FBG is encapsulated in a tube where the distance between the tie points define its effective gage. Optional brackets are also provided to enable surface mounting of the sensors.This technique thus,is effective in increasing the overall gage of the sensor.Another packaging technique proposed by Ngoi et al.[63]is to embed the FBG coaxially in a cylindrical silicone rubber tubing as shown in Fig.2(f).This packaging is specifically useful for sensing lateral loading.FBGs have been known to suffer from a phenomenon called peak splitting under the influence of a lateral load.Peak splitting occurs due to the effect of birefringence of the FBGs when subjected to lateral loading,i.e.unequal load-ing along the two perpendicular axes of thefibre.This issue has been taken care of by packaging the FBG in silicone rubber which is known to have a low elastic modulus and high Poisson’s ratio. It is also thermally stable in the temperature range from−100to 320◦C.Experimental data obtained from FBG sensors reveal agree-ment with thefinite element simulation results.This packaging has served to increase the lateral pressure sensitivity of the FBG sensor without inducing birefringence.However,the system suffers from an inherent time lag due to the typical viscoelastic nature of silicone rubber.Upon applying a load,the packaged FBG sensor shows an abrupt rise in wavelength,which gradually settles down to a stable, lower value within a few seconds.Authors propose readings to be taken once this spurious response settles down,approximately5s after the application of the load.Leng et al.in2005have proposed various designs of FBG sensor protection depending upon the usage area.These include sensors154M.Majumder et al./Sensors and Actuators A147(2008)150–164for metallic surfaces,CFRP composites and concrete structures[64]. Designs include FBGs embedded inside steel tubes,steel rebars, CFRP prepegs,etc.The packaged sensors have been evaluated for optimum strain transfer between the sensor and test specimen by using non-linearfinite element analysis.Concrete cylinders instru-mented with FBG sensors and electrical resistance strain gauges have been subjected to compressive loading and the results found from both type of sensors to be in proper agreement.Authors also claim that due to higher resolution,FBG strain sensors would be able to detect the initiation of failure of structures earlier than the strain gauges.Dawood et al.have described in detail a procedure to embed FBG sensors between the foam core and cross-ply laminate of GFRP sandwich material using vacuum infusion technique[65]. The sandwich structure consisted of a single layer of polymer foam sandwiched between two layers of GFRP skins.An array of six mul-tiplexed FBG sensors was used.The area of the gratings was left uncoated to provide better mechanical coupling to the GFRP.The FBGs were to be embedded between the core and the skin of the sandwich.The opticalfibre was then aligned and laid up along the center line of the foam core.The prepared sandwich specimen was then placed in a vacuum bag and resin/hardener mixture infused inside.The specimen was cured for15h at room temperature.This type of packaging is useful in sensing microscopic localized defects like debonding of the GFRP material.The FBG being fully embedded in the test specimen is able to detect the internal defects of the spec-imen at an early stage.However,the embedding process is involved and requires a greater degree of precision and care.Accuracy and repeatability have been found to be satisfactory under static and dynamic loading conditions.Lu and Xia have used FBG sensors directly embedded into CFRP sheets for real-time monitoring of RC beams[66].The authors claim that in this case there is no need of a protective coating or adhe-sive layer between the bare FBG and the CFRP sheets.Thus,the measured strain from the FBG–CFRP composite provides the actual strain measured without incurring any dampening effect.This is a distinct advantage over other encapsulation technique.RCC beams instrumented with FBG-embedded CFRP sheets were subjected to compressive load.A theoretical calculation of strain at different loading conditions using the dimensional values and Young’s Mod-ulus of the RCC beams was done.The measured values were in good agreement with the theoretical strain values.Another very commonly used and simple packaging technique of FBGs for strain monitoring in concrete structures is to install the sensors in a steel rebar and then use the rebar at the site of measurement.Chung and Kang[67]have used such a technique where they have placed a six-FBG multiplexedfibre in a groove cut in a steel rebar and used a fast curing adhesive to bond thefibre to the rebar. This FBG embedded rebar has been used at the site of strain mon-itoring.However,before using this type of packaging it is essential to study the strain transfer characteristics of the adhesive and the rebar material for accurate strain measurement.Two major issues associated with the use of FBG sensors as health monitoring tool in civil structures are their high fragility and cross-sensitivity to more than one measurand.Special ather-mal encapsulated FBGs that take care of the strain–temperature cross-sensitivity are available[23,68,69].Lo and Kuo have proposed athermal packaging of FBGs using a metal coating that serves as thermal compensator[23].An FBG1cm in length is written using a phase mask.It has a central wavelength of1532.93nm at30◦C.The fibre substrate is quartz and a copper coating of5-␮m thickness is deposited onto the substrate using electroless plating technique. Quartz having a much lower thermal expansion coefficient than copper,any rise in ambient temperature results in a greater expan-sion of the copper than that of the FBG.This compresses the FBG and creates a negative strain on it,thereby compensating for the temperature-induced wavelength shift of the FBG.This proposed technique of temperature compensation thus involves a simple bi-material that is reliable and feasible for mass production.Moyo et al.have reported a packaged FBG that is suitable for use in the harsh conditions of the construction industry and also takes care of the temperature compensation of the sensors[69].The pack-aged sensor is dumb-bell shaped and consists of two FBGs placed closely.One FBG,sandwiched between two layers of carbon com-posite material,is epoxied on the dumb-bell surface and is prone to both strain and temperature changes.Another FBG,encased in a metal tube is prone only to temperature perturbations.Several tests were performed on these packaged FBG sensors and the data compared against conventional foil strain gauges.Tensile tests were reported on steel rebars and the sensor response was found to be linear and closely correlated to those of foil gauges.Static test on simply supported reinforced concrete beams instrumented with the sensors also showed approximately linear response,thus jus-tifying the packaging and installation procedures of the sensors. Dynamic tests on the beam were carried out using an impulse hammer and the maximum strain thus recorded by the FBG and foil gauges were respectively55and58microstrain.The packaged sensor was also embedded inside a concrete cylinder,which was subjected to compressive load.Only the strain sensor showed a high sensitivity whereas the temperature-monitoring sensor was almost unaffected.It may be noted that in most cases,the strain sensitivity of an encapsulated FBG is significantly different from that of the bare FBG. Hence calibration of the encapsulated FBG sensor must be carried out before it is put into real-world application.5.Applications of FBG strain sensors in structural sensing5.1.Strain monitoring in civil infrastructureA major application of FBG strain sensors is in thefield of real-time online health monitoring of bridges and civil structures [70–73].FBG sensors have a major advantage over conventional non-destructive techniques in that they are capable of remotely monitoring the condition of the test structure.The interrogating instrumentation being located off-site results in higher efficiency of the system and better safety of the personnel.Twenty-six FBG strain sensors have been reported to be moni-toring the Horsetail Falls Bridge in Oregom successfully for2years [74].The bridge was originally built in1914and in1998it was strengthened by placing composite wraps over the concrete beams. Long-gage FBG strain sensors are placed in grooves cut into the con-crete and in the wraps as well.This is done with a view to assess the actual strengthening provided by the wraps to the bridge.The long-gage sensors provide a relatively macroscopic strain value that can measure several kHz of dynamic strain with a resolution as low as0.1microstrain and hence are very useful in structural monitor-ing.A similar test is reported on the Sylvan Bridge with fourteen long-gage sensors.Saouma et al.[75]have used FBG sensors on aluminum as well as concrete specimens to monitor the strain and have validated the results against electrical strain gauges.They have extended the laboratory work by instrumenting six beams and six columns of a newly constructed building with FBG sensors that can be used to monitor the strain of the beams and columns online.Monitoring of the prestressing tendons of the Beddington Trail Bridge,Canada using Bragg grating strain sensor array has been done by Maaskant et al.[76].Three types of prestressing tendons have been used in this bridge,namely steel strand,carbonfibreM.Majumder et al./Sensors and Actuators A147(2008)150–164155Fig.3.Packaged FBG-based strain sensor used in the health monitoring of West Mill Bridge[78].composite cable and leadline rod.The main objective behind this work was to study the long-term losses in the tendons due to stress relaxation and creep.FBG sensors were bonded to each type of ten-don and then embedded in concrete girders.A total of18sensors were placed strategically on the girder in order to monitor the point of maximum strain generation.Proper cabling of the sensors min-imizes the effect of moisture and alkalinity of the medium on the sensors and also reduces pinching and microbending phenomena of thefibre.Static strain measurements of the girders with a precision of±40microstrain have been done.A comparison of data collected over19months reveal that the loss of prestress in CFRP tendons is almost25%lesser compared to that in steel tendons thereby jus-tifying some merit in the use of CFRP material in bridges.Besides, dynamic strain monitoring on various positions of the girder were studied by passing vehicles with known loads.Strain resolution of 1microstrain over a range of10,000microstrain had been achieved. This information can be useful from the end of traffic monitoring, bridge designing and its maintenance.FBGs have been used in the structural health monitoring of the Tsing Ma Bridge in China[77].At1377m,this is the longest sus-pension bridge in the world.It has a double deck configuration,one for highway traffic and the other for railway.The deck is of hybrid arrangement using both truss and box forms.This bridge is in ser-vice from1997and a structural health monitoring system,wind and structural health monitoring(WASHM)has been monitoring its health from inception.In this report,the response of FBG sensors has been checked against the WASHM system under specific load-ing conditions.FBGs were fabricated in-house,suitably packaged and installed on the bridge.Tests were carried out at three specific locations viz.,hanger cables,rocker bearing and supporting struc-ture on a section of the lower deck using21FBG sensors.Separate FBGs were used to monitor the instantaneous temperature of the structure and compensate accordingly in strain readings.When-ever a heavy traffic load was subjected on the bridge,the response of the FBG monitoring system peaked which was in close agree-ment to the response of the resistive strain gauge sensors of the WASHM system.Gebremichael et al.in2005have reported the use of40FBG sensors to remotely monitor the real-time strain on Europe’sfirst all-fibre reinforced composite bridge,The West Mill Bridge[78]. Fig.3shows the packaged FBG sensor used in the structural health monitoring of this bridge.The main objective behind this study was to collect real-time,in situ strain data from the bridge and to analyze this data for assessment of its structural integrity,main-tenance scheduling and validation of design codes.The authors have developed a dedicated FBG interrogation system based on the WDM technique for use in this work.Cost per channel of the instru-mentation is reported to be comparable to those of conventional strain sensors,if used in multiplexed sensor scheme.The moni-toring system wasfirst tested inside the laboratory on structural test elements instrumented with FBG and strain gauge sensors. The structural elements were subjected to both quasi-static and dynamic loading.The tests reveal that response of the FBG sen-sors was linear,repeatable and without any significant hysteresis. Finally the two-lane bridge,which is fully made of glass and carbon fibre reinforced polymer,is carefully and strategically instrumented with40FBG and11strain gauge sensors.The FBG sensors were coated with moisture-proofing silicon compound and consecu-tively with composite stripes that minimize birefringence effect in fibres due to transverse loading.On-site testing was done with a 30-ton lorry positioned at different points on the bridge.The strain monitored in the different channels conforms to the relative posi-tioning between the sensors and the loading point.The strain data obtained from FBGs and resistive gauges were found to be closely correlated and the technique has been established as a long-term condition monitoring of the all-fibre composite bridge.As a very significant continuation of this work,Kister et al.have evaluated the performance of the adhesive and protection system used with the FBG sensors installed on the West Mill Bridge[79]. Unpackaged FBG sensors were installed on the bridge structure using cynoacrylate glue,which is the primary adhesive.Beads of epoxy adhesive were also deposited on top of thefibres to ensure added anchorage.Strips of glassfibre composite material were bonded on both sides of thefibres using the two adhesives.Silicone sealant was then applied from top to seal the package from posite covers were then used to envelope the complete sensor package.The dimensions of thefinal packaged sensor were 1.8mm in thickness and maximum1m in length.The interfacial bond strength developed between the adhesive layers and the opti-calfibre were evaluated by the modifiedfibre pull-out test[80].It was reported that whereas unstrippedfibres failed due to debond-ing and sliding of cladding,strippedfibres failed due to fracture of the cladding close to the glue edge or due to rupture of the glue length.It was also observed that cynoacrylate-gluedfibres could withstand a higher failure load than the epoxy-gluedfibres.Hence cynoacrylate had been chosen as the primary adhesive for bond-ing the optical sensors on to the bridge structure.The durability of the sensor protection system was assessed by immersing coupons embedded with packaged sensors in water for a duration of90days. Results show negligible influence of water absorption on the sen-sors.Sensor survival rate while bonding on the bridge structure has been reported as100%.Integrity of adhesives and durability of the sensors has been assured and the sensors on the West Mill Bridge have been providing continuous satisfactory performance over the last3years.5.1.1.Strain monitoring in reinforced concrete beamsNumber of strain studies of reinforced concrete beams instrumented with FBG sensors have been reported in liter-atures[81–83,64,67].Maher and Nawy[82]have compared the response of FBG strain sensors and conventional resistive strain gauges on reinforcing bars.The bars having dimension of 305cm×25.4cm×30.5cm were subjected to three-point bending tests.Few FBG sensors were carefully embedded into V-grooves cut into the reinforcing bars and a few others were simply epox-ied on the back surface of the bars alongside the conventional strain gauges.For test duration of7days,the nominal compressive strength of the concrete was found to be76MPa and data collected from FBG sensors and strain gauges were in agreement.Davis et al.have reported the use of embedded wavelength divi-sion multiplexed FBG sensors in monitoring the strain on reinforced concrete beams and decks till their failure[83].An8-ft long beam was instrumented with the Bragg sensors at different strategic loca-tions and subjected to four-point bending.Thefibre sensors were bonded to rebars using ordinary foil gauge adhesives and coated。