4 th International Conference on Short & Medium bridges, Halifax, 8-11, August, 1994.

Fiber Optic Sensing for Bridges

R. M. Measures, T. Alavie, R. Maaskant, S. Karr, S. Huang, L Grant\ A Guha-Thakurta*, G. Tadros**, and S. RizkaIla#

University of 1bronto Institute for Aerospace Studies FIBER OPTIC SMART STRUCTURES LABORATORY

4925 Dufferin St., Downsview, Ontario, CANADA

" Conforce Construction Ltd., Calgary, Alberta, CANADA * City of Calgary, Structures and Facilities, Calgary, Alberta, CANADA ** Straight Crossing Inc., 233 - 19th St., N.E., Calgary, Alberta, CANADA #University of Manitoba, Engineering Bldg., Winnipeg, Manitoba, CANADA

ABSTRACf There is growing interest in the use of carbon fiber based composite materials in large concrete

structures, such as bridges, where corrosion is a serious problem. Recent, developments in fiber optic sensors has made it possible to monitor the use of these advanced composite materials and attaining a better understanding of their potential role in bridge: design, construction, maintenance and repair. A new two span concrete highway bridge built in the city of Calgary in 1993 is the first in the world to use two types of carbon fiber composite prestressing tendons in several of its precast concrete deck support girders. We have instrumented several of these girders with 1S-fiber optic Bragg grating sensors in order to monitor the changes in their internal strain over an extended period of time. A multichannel fiber laser demodulation system was develOped for interrogating this network of fiber optic sensors embedded within the bridge girders. This demodulation system is rugged, compact and transportable to the bridge construction site where it allowed changes in the internal strain on all three types {steel and two types of carbon fiber composite} of prestressing tendons to be tracked over several months. The same set of structurally integrated fiber optic sensors has also be used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truck. _

1

INTRODUCTION Large concrete structures like:. bridges. ov~ multilevel pa~ing garages.._ represent

a large investment. TIle COO'OSlOI1 of steel {used for prestressIng tendons and other reinfofcements, such IS stirrups} is now sceft IS a serious mode of degradation in such structures and bas raised abe prospect of replacing this steel with carbon fiber based composite materials. 1llese fiber reinforCed polymers are ,Practically immune to corrosion, have favourable structural ~ and lend thCmsetvcs 10 mtemal monitoring by means of embedded fiber optic sensors. In cold-climatel this ~Iaccment is dcemecllO be even more important and urgent due to the accelerated deterioratiOn of atcel reinforced concrete structures by de-icing safts. Since composite materials are unproven in there substitution for steel in concrete sttuctures there is considcrlble motivation to instrument such lest structures in order to gauge how well composite materials serve in this new role.

The last couple of years bas ICCIl the deYclopment of an optical fiber strain sensor that is very well suited for thlS .. 1t. This new fonn of opticaJ fiber strain gauge, termed an intracore Bragg grating. is well suited for use with com~aterials IS it can readily be embedded within composite structures and as we shaD te can track changes in strain over extended peIiods of time. An addjtional feature that is attractive for use in large open structures like bridges is the invulnerability of fiber optic sensors to electrical interference. particularly storms, when compared to electrical baed sensors. A structurally integrated fiber optic sensing system could monitor the .tate of such a structure thl'O~ut its working liCe. It could determine the: strain. deformation or deflection, load distributiOn, temperature or environmental degldatiOn, expcrienccd by this valuable asset and warn if any exCUllaons in system parameters deviate from aCcepted values. Such an ~1ec1 tensing ~ could also monitor the various structural components during construction. possibly lcadmg to improved quality controL In cenain fastancCs this resident leaSing ayatan could also priwide valuable fnf'onnation on the usaF of the atructute. For example, In the case of a bricfge abe leaSing system could provide information on the eraff'1C itself. It is QUite conceivable that a suitably instrumented bridge could monitor not only the number of vchlc1es pet minute. but also determine the weight, and the s~ of each wbiclc. .

. 1\vo types of carbon ~inI tendons were used In I new twtwpan COIlCIete highway bridF that was construc:ted In the City of Calgary. Canada. In 1993. Four of the twenty six bulb .:r section precast concrete girdcta &hat ~ the decIc of tho Beddingtoft 1nill centre Street road bridge were preatn:accI with 'lbIcYO Itope cable [supplier: Tokyo Rope Manuf., Japan], while two 0Cben were ~ with 1.c:adIine cable (supplied by Mttsubjahi Kasei of Japan]. The remaining twenty jirderI relied Oft steel cabk:& for preaUelSing. We instrument five of these c:onc:rete ~ with fiber op!k Intracote Braa grating IenSOII {supplied by. Dr. K. Hill of the Communication Research Centre. Ottawa}. two ptcstrc:ssed with 1bkyo rope. one with Leadline and two with slCcL In each case the fiber optic strain sensors were adberc(t to the surface of the tendon and the specially protcctecI leadin/oul optical CibcII egressed through recessed ports In the side of the concrete lirders. Flg.l. These optical fibers were then fusion spliced to long armoured optical fibers that ran through special conduits to I central junction box, Flg.2, where they were c:onncc:toti=d for interrogation.

FmER OPTIC SENSING SYSTEM The tibet optic intracxxe Bragg grating is I segment of an optical fiber that has been

internally modiCted by ~ to UV light 10 that it reflects light at one wavelength {actually a ~ narrow bind o( aboUt 0.2 nm half1ieight full width}. The key to the use of a liber optic Bragg gratina as a sensor Is that this Bragg wavelength depends upon the strain add temperature impOsed on iIie optical fibei It the location of the Bragg grating. Thus as the strain changes the waVelength at w6ich the light Is reflected sbifts. We have also shown that I gieatly improved performance can be obtainCd if the sensing Braq grating is made to serve as the second mirror of a laser. In this a~ the sensing fiber OptiC 1Jragg grating tunes the laser IS indicated in Fig 3, (MCISUl'tS et al. 1992.1: have also developed I simple, passive. ratiometric method of wavelcnath demodulating sensors, that Is to say measuring their wavelength. (Melle et a~ 1992]. Tn this arrangement the light emitted from the other side of the laser {i.e.. opposite from the BraiJ grating .ide l is 'plit and sent to two detectors. One detector hIS a spectral filter {the transmisston of wlllch fs a function of wavelength} lhead of it so that the magnitude of the detected signal depends upon the wavelength of the optical signal, Fig.· 4. The second detector

Measure.f et aL, Fiber Optic Sensing for Bridges

provides a reference to cancel variations in intensity of the optical signal. We have demonstrated that a Bragg grating fiber laser sensor {BGFlS} may be packaged to form a compact. robust and sensitive sensing system fMel1e et all993 .. Alavie et al .. 1993). Fig. 1, displays a photograph of the four channel Bragg grating fiber laser sensor system, shown schematically in Fig. 5, we developed for the Beddington 'Irail Bridge project being connected to the optical fibers egressing from the recessed port in one of the precast concrete girders.

In developing this fiber optic sensin, system for the Calgary bridge we had to address a number of issues: the load induced strams would generally be small {within 2500 JAStrain} although, the pmstressing tendons are subject to a tensile strain of around 8000 J.lStrain; the environment is harsh in terms of high moisture and large temperature excursions and the optical fibers were subject to rough handling. Also large dimensions {- SO ftl were involved and at least two intracavity connectors were interposed between each Bragg gratin~ and its demodulating fiber laser, see Fig. s. The insertion loss and backreflection 8SSOCIated with connectors would normally create serious problems when ptaocd inside a lasing cavity. Fortunately, we were able to make Angle Polished-Physical Contact {APIPC} type connectors with less than -60 dB backreflection and only 0.2 dB insertion loss.

Our prototype system was designed with a high pass filter which allows transmission over a 6 nm window. This corresponds to a strain dynamic range of approximately 5000 J.lStrain. For 12-bit digital processing the strain resolution of the system is approximately at 1 f'Strain. The system dynamic response depends entirely on the processing electronics and in the early version was about 700 Hz. The strain ac:x:uracy of the system depends on the stability of the fiber laser and is limited by laser jitter. In this early arrangement, a repeatable ac:x:uracy of better than 20 f.lStrain was achieved. Since the unit is intended for raeld use, temperature stability of the system i.e. filter temperature drift, must be considered. Also embedded within the concrete of each intrumented girder was a fiber optic intracore Bragg grating that was mounted within a small steel tube and only supported at one end. This grating Servccf as a temperature sensor by being strain decouplccl from the girder. This is necessary to compensate for thermal induced apparent strain [Measures et at 1993].

In order to cbaractcri%c the Bragg grating sensor demodulation syscem, a Bragg grating was mounted onto a cantilever beam adjacent to a msistive strain foil gauge, and connectorized to channel one of the demodulation system. The cantilever beam was then loaded in flexure and both the foil strain gauge response and the filtered response of the optical system were recorded. This procedure was repeated for all four channels. An imJ)Ortant objective of our program was to investigate the strength and durability of the fiber optic Bragg ~tings at the prestress levels likely to be cncounteml in the bridge. Consequently, an optICS I fiber Bragg grating was adhered in the middle section of a carbon fiber /PEEK specimen. A resistive foil ga~ge was attached along side the gratina to provide a reference strain signaL The specimen was then loaded in tension using In M'fS-88O and a white light souroe and an Hewlett-Packard optical spectrum lnalyzer were used to follow the behavior of the Bragg grating sensor with loading. During the test, the specimen was loaded in 250 JAStrain increments to 8000 J.lStrain and the data recorded both in tenns of spectral content and center wavelength. While the experimental results may largely depend on sensor orientation and installation on the fest sample, the gratings surviJ tensile strains of greater than 8000 JAStrain. Fig. 'is a representative plot of grating sensor center wavelength with applied strain. In most of our test samples the fiber sensors only survived strains of less than 1%. This is mainly attributed to the handling which is generally required in the present fabrication process and is not a characteristic of the grating itself. In fact, recent results indicate that much higher ultimate strength values can be attained with proper handling and methods of producin~ the Bragg gratings. A practical resident sensor must provide reliable readings over the life of the structure. This IS especially important in civil engineering applications where knowledge of structural fatigue can be used in scheduling preventive maintenance and repair aimed at prolonging the useful life of the structure. Using the ~me experimental setup the carbon fiber/PEEK specimen {instrumented with the grating sensor and a resistive foil gaujlC} was cyclically loaded from 0 to -2000 fAStrain over 320,000 times. Fig. 7 is a plot of gratmg center wavelength at low and high strain values with number of load cycles. The inset shows the grating spectral msponse before and after the test and suggests very little change in Bragg gratin, spectrum over 320,000 cycles. The slight slope in the intracore grating center wavelength With increasing number of loading cycles is, however, an indication of ereep in the carbon fiberlPEEK specimen.

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Measures el aL. Fiber Optic Sensing for Bridges

FIBER OPTIC SENSOR INSTALLATION A cross sectional view of one of the bulb-tee precast prestressed deck support girders is

displayed in Fig. 8. The location of the prestressing tendons, the duct for post-tensioning, and (he site of the fiber optic Bragg grating sensor is also revealed. Installation of (he fiber optic sensors required consideration of: their long term strength, the proper transfer of strain from the prestressing tendon to the adhered sensor, and protection, routing and access of the optical fiber leads. Conventional strain gauge technology offers techniques which can be readily transfered to fiber o{>tic strain sensors. BOnding agents have been developed which yield stable gluelines for effective Ion~ term strain transfer. In this regard care must be taken to ensure that the glueline is suffICIently thin or that the bonding length is suffICiently long such that the strain in the host is fully developed in the sensor region.

Of signifrcant interest in the instrumentation of bridges and similar structures is their expected long service life. This poses a serious challenge as the prestressing tendons, onto which the strain senso~ are attached, are subject to tensile strains in excess of 8000 f,lStrain during initial stressing. A simplified illustration of the girder fabrication and prestressing process is provided in FJ,.'. After placing the tendons <and the associated reinforcement) in tbe fonnworlc. a specifacd prestress is applied to the tendons and the concrete is poured in place. The girder is alloWed to cure until it meets the requisite strength requirements. The applied stress is then released from the tendons. This tensile strain is then resisted by the concrete which in tum is compressed. The development of the compressive stress in the concrete, and the associated tensife stress in the tendon occurs over a finite length as shown. The corresponding strain development in the pmmessing tendon is depicted in Flg.10. Also shown in this figure is the strain monit~ by the surfac:e adhered fiber optic sensor. The sensor was adhered after prestressing to avoid imposing this large tensile strain on the fiber optic Bragg grating. Stress corrosion should therefOre be minimized and a long operational life can be expected for the sensor. The sensor thus reads zero strain until the girder is destressed, whereupon it would read a sudden compression {of -1000 lASlrain in Fig. to} corresponding to the rapid reduction in the tensile strain. Subsequent concrete shrinkage and creep slowly increase the compressive strain monitored by the sensor.

Avoidin, the destruction of the sensor and its leads during concrete placement and compaction IS one of the most daunting tasks in the instrumentation of concrete structures with sensors. Proper cabtca need to be used which both protect the optical fibers from tbe moisture and the surrounding alkaline environment. and minimize pinching and microbending which can compromise the integrit1 of the lead fiber and produce a loss of ~I signal strength. The lead fibers need to be properly routed so as to be protcctcd from the Vibrating probes which are used to distribute the concrete through the maze of reinforcement bars. With increased usage of carbon fiber composite materials in the construction industry it will become highly desirable to structurally int~te the sensors with the tendons and reinforcement bars during their manufacture. Thll would provide excellent protection for the sensors and their leads, and yield a very convenient means of instrumenting concrete structures. Such a development could playa crucial role in the aoc:eptance of fiber optic sensors as a broadly used built-in diagnostic tool for continuous bridge surveillance and maintenance.

The fiber optic strain sensing network installed in the Beddington Thlil Bridge in Calgary consists of IS.8ragg gratin~ sensors that were positioned at a number of locations along the length of the bridge as indrcated in Fig. 11. A strain-decoupled temperature sensor was installed within each girder to allow for correction of thermal apparent strain. Some of the sensors were placed at the position of maximum sensitivity to bridge loads, approximately 37.5% of the girder length from tbe abutment end [locations 2 and 4]. For the carbon fiber composite tendons. sensors were also placed in the stress transfer zone. These locations are also indicated in Fig. 11 [locations 1 and 3]. The optical fiber leads are routed to a single junction box placed at the top of one of the abutments and shown in Fig. 2.

STRAIN MEASUREMENTS and DISCUSSION Important information can be deduced from the prestressing tendon strain measurements.

From the point of view of precast prestressed concrete girder productivity it is desirable to destress and remove the girder from the form as quickly as possible. This sometimes occurs while the concrete is not properly cured and this can lead to problems. The fiber optic strain sensors that are embedded within the girder and adhered to the prestressing tendons offer a

Measures et at, Fther Optic Sensing for Bridges

direct measurement of the stress relaxation which occurs upon destressing and the ensuing shrinkage and creep of the concrete as curing continues. The adequacy of the tendon/concrete bond can also be determined by such sensors if they are strate$ically placed in relation to the stress transfer length. The presence of these sensors could provide an opportunity to assess the condition of a girder and determine the suitability of scheduled events such as girder installation, deck pour and post-tensioning.

In the Beddington Trail Bridge project. an important motivation for the use of strain sensors was the desire to monitor the peifmnance of the Tokyo Rope and Leadline carbon fiber composite prestressing tendons in relation to traditional steel strand prestressing tendons. Some preliminary strain malts available form the Beddington Trail Bridge are provided in Fig. U. The first set of strain data were obtained after the deck pour and immettiate1y following the post-tensioning of the girders. These mcumements represent the stress relaxation in the tendons from the combined effects of de-stressing, concrete shrinlcage, ~, post-tensioning of the girders, and the dead load of the girders and bridge deck assembly. The strain values presented have been conected for the thermal apparent strain caused by the thermal output of the sensor itself and the mismatch between the expansion coefficient of the sensor (CI'E=O.5B-6/C) and that of the tendon to which it is bonded. The second and thUd measurements were taken approximately one month and five months after the bridge opening. The results show that the relaxation in the steel tendons is substantially larger than In the Tokyo Rope and l.eadline tendons. A small amount of further relaxation has Occurred during the period from October to March. The thennally induced variation in presuas is seen to be much more pronounced in the steel tendons as evident from the November ~ment (temperature change -23 C).

Monitoring of: traffIC usage, extreme load events and load history may also be possible with built-in fiber optic strain sensors. Such information may prove to be useful in bridge maintenance procedures and scheduling, in assessing bridge designs. and comparing actual loading to the design loads. This information might also be specified in bridge design codes and procedures. We have been able to demonstrate that our structurally integrated fiber optic sensors can measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truclc. Fig. 13, displays the transient change in the strain within one gitder as a large truck passes over tbe bridge. The static change in the strain when the truck was parked on the bridge is also seen and was of comparable magnitude. Although this preliminary test involved a 21 ton truck we expect to be able to improve our strain resolution by a factor of 10 which should allow detection of individual cars crossinJ the bridge. We thus have a glimpse of what may be possible with the marriage of photonlc technology and material and structural engineering.

ACKNOWLEDGMENTS We acknowledge support from the many a~ncies involved in this project. These include:

Canadian Space Agency, aty of Calgary, Instttute for Space and 'terrestrial Science, Natural Science and Engineering Research Council, Ontario Centre for Materials Research, and the Ontario Laser and Lightwave Research Centre. This project was greatly helped by the efforts of many members of the tJTIAS Fiber Optic Smart Structures Laboratory, especially: G. FIShbein, S. Huang, S. Karr. D. Glennie and M. Ohn.The authors would also like to thank K. Hill of Communications Research Centre for providing the intracore gratings and D. J. DiGiovanni of AT&T Laboratories (or the Erbium doped fiber.

REFERENCES --A.T Alavie, S.B.Karr, A. Othonas, and R.M.Measures, 1993, " fiber Laser Sensor Array", SPIE-Vol .. 1918, Smart Structures and Materials Conference, Albuquerque, february 1 -4. --R.M. Measures, S.M. Melle and K. Liu, 1992, "Wavelength Demodulated Bragg Grating Fiber Optic Sensing Systems for Addressing Smart Structure Critical Issues", Smart Materials and Structures, \b1.1 .. 36-44. --S.M. Melle, K. Liu and R. M. Measures, 1992, "A Passive Wavelength Demodulation System for Guided-Wave Bragg Grating Sensors", IEEE PhoLTech. Lett. volA, 516-518. --S. Melle, 'I Alavic, S. Karr, T. ColO),. K. Liu, and R.M. Measures, 1993," A Bragg Grating-Thned Fiber Laser Strain Sensor System", Photonics 1echn. Letts. Vol. S, 263-266. --R. Maaskant, T. Alavie, R. Measures, M. Ohn, S. Karr. C.Wade, G. Tadros and S. Rizkal\a, 1994, "Fiber Ootic Bragg Gratting Sensor Network Installed in a Concrete Road Bridge," SPIE· Vol., 2191-53, ~mart Structures and Materials Conference, Orlando, 13-18, february.

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Measures et aL. Fiber Optic Sensing for Bridges

Figure 1. Lead-in/out optical fibers from embedded Bragg grating sensors egressing from receacd port in one of the instrumented precast concrete girders for the Boddington 'nail bridge. The multichannel Bragg grating sensor demodulation system is seen to be connected to these lead-in/out optical fibers.

Figure 2. View of special conduits used to bring all of the optical fibers from the embedded Bragg grating sensors to one junction box mounted on one of the abutments for the 8eddington "Irail bridge.

Measures el aL, Fiber Optic Sensing for Bridges

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Measures et aL, Fiber Optic Sensing for Bridges

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Measures et oL, Fiber Optic Settsiltg for Bridges

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Figure 8. A cross sectional view of one of the bulb-tee precast prestressed deck support girders for the Beddington Trail bridge:.

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Measures et aL, Fiber Optic Sensing for Bridges

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Figure 11. The fiber optic sensor locations for the Beddington Trail Bridge in Calgary.

Measures et aL, Fiber Optic Sensing for Bridges

. .

Sensor Location

Figure 12. Preliminary internal strain measurements from the Beddington Trail Bridge revealing, the stress relaxation in the carbon and steel tendons from the combined effects of: destressing, concrete shrinkage, dead loading of the bridge deck, and the post-tensioning applied across the two spans.

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Figure 13. Preliminary mea.c;urement of the tra'.'sient chan~e in. the strain within on~ girder as. a large truck passes over the Bcddmgton Trail Bridge. Also shown IS the static loading associated with parking the truck on the bridge.

Measures et aL, Fiber Optic Sensing for Bridges