Applications of Composites, Optical Fibre Sensors and Smart Composites for Concrete Rehabilitation: An Overview

Applied Composite Materials 9: 221–247, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Applications of Composites, Optical Fibre Sensorsand Smart Composites for Concrete Rehabilitation:An Overview

KIN-TAK LAU, LI-MIN ZHOU and PING-CHEUNG TSEDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong,People’s Republic of China. e-mail address: [email protected]

LI-BO YUANDepartment of Physics, Harbin Engineering University, Harbin 150001,People’s Republic of China

(Received 9 January 2001; accepted 17 May 2001)

Abstract. Many researches and developments in the field of concrete rehabilitation and repair byusing fibre reinforced plastic (FRP) materials have been successfully carried out in the last decade.There is no doubt that these materials have been promisingly used to replace conventional steelreinforcements for concrete structure with providing excellent structural durability and margin ofsafety. Until the last few years, researchers from diverse disciplines have drawn into vigorous effortsto develop a structure, which is able to measure its own structural condition by using embeddedoptical fibre sensors. This structure was appreciated as “Smart structure”. However, the smart struc-tures have not yet been adopted by the civil engineering applications. In this paper, a brief reviewon the applications of the FRP and optical fibre sensor in concrete repair and structural health mon-itoring, respectively, is given. Further discussions on the potential use of smart composites for civilengineering applications are also addressed.

Key words: concrete rehabilitation, fibre reinforced plastic, smart structures, optical fibre sensors.

1. Introduction

The demand on rehabilitation and renewal of aged and deteriorated civil concretestructures has been swiftly increased in recent years. Continuing deterioration ofthe world’s infrastructure elements highlights the urgent need for an effective re-habilitation technique with low material and maintenance costs, short installationtime and minimum traffic interruption. It is now evident that the deterioration ofworld-wide infrastructure elements has been caused by a variety of factors includ-ing time dependent material degradation, widespread fatigue damage, initial useof poor materials, flaws in design and the use of deicing salts. In addition, therapid increase of population in many developing countries enhances the requestsof upgrading and retrofitting the existing concrete structure as well as an increasein the traffic volume, which is beyond the original design limit. Besides, the world

222 KIN-TAK LAU ET AL.

Figure 1. Severe cracks appeared at the tops of the concrete column after earthquake in thecity of Nantao in 1999.

scientists and engineers are required to find the solution immediately to reinforcethe old concrete structures, which are most susceptible to seismic damages. Fig-ures 1 and 2 show common damages of concrete structures due to nature calamityand corrosion of steel reinforcements [1]. An estimated need for infrastructureconstruction and rehabilitation in the Asian areas alone is almost US $2 trillionin the early twenty-first century [2]. In the United States, 80 billion will be paid forthe repair of current deficiencies in civil infrastructure. In California alone, over3.5 billion is required for seismic retrofitting of concrete bridges.

Over the past several decades, an extensive research and development in thefield of material engineering and science have been carried out with fibre-rein-forced plastic (FRP) composites, which have a wide range of practical applications.It has been reported that concrete bridges remained standing after the 6.6 mag-nitude earthquake in Los Angeles after jacketing by FRP composite wrap [3].Mirmiran and Shahawy [4] indicated that a glass fibre composite jacket had beenused confidently as an insulator for hot oil pipeline in Gulf of Mexico. It is reportedthat the jacket has withstood a harsh salty environment with over 140◦F, directsunlight and high moisture conditions for eleven years without any substantialdamages and degradations.

The conventional rehabilitating techniques using stitching and steel reinforcingpatch design provide promising strengthening solutions in civil concrete applica-tions. Unfortunately, the weight penalty, labour-intensive and subsequent corrosionof the steel material may eventually increase the overall maintenance cost. Thereby,this requires the development and application of new materials and technologies,

COMPOSITES, OPTICAL FIBRE SENSORS AND SMART COMPOSITES 223

Figure 2. The concrete spalling happened due to the volumetric expansion of corroded steelreinforcement.

which can extend the service life of the constructed facilities with reducing the timeand cost for maintenance, time for traffic disturbance and improving durability.Thus, the use of FRP materials such as glass or carbon fibres in polymer matrices isbecoming increasingly important to extend the service life of our civil constructionfacilities in the 21st Century.

Due to the increases of public and political concerns about the decay of nationalinfrastructure, it is also required for an effective mean to monitor the condition andstrain history of the strengthened and repaired concrete structures [5]. In the pastdecade, a multi-disciplinary field of engineering known as ‘Smart Structure’ hasrepresented one of the major research topics in the World. It is formed by the mar-riage of engineering materials with a structurally-integrated fibre-optic sensor andactuation control system. The system is capable of assessing damage and warningof impending weakness in the structural integrity of the structure [6].

The smart structure technology requires the development of materials with self-optical nerves. Structures constructed from such materials can monitor their ownmechanical properties such as strain and thermal states by some measuring means.In civil engineering construction, buildings, bridges, tunnels, highways, and rail-ways, the use of embedded optical fibres can improve concrete evaluation, whichprovides real-time structural health monitoring and enables immediate conditionawareness for service assessment of structural integrity. Also, optical fibre is in-sensitive to electromagnetic interference and can be configured to sense a varietyof physical effects, which include temperature changes, mechanical properties of


host materials, electric field strength and magnetic field strength with minimumambient interference.

In aerospace industries, embedded optical fibres are being used in a compositewing for the in-flight monitoring purpose. The periodical manual inspection ofcrack and delamination of composite structures can be greatly reduced and onlyrequired when it is needed. In addition, the duration for which the structure isrequired to withstand damage would be reduced because damage would be detectedearlier. Unfortunately, the realistic application of the embedded optical fibre is stilllimited in civil engineering applications.

In this paper, a brief review on the applications and researches of FRPs inconcrete rehabilitation and repair is presented. The use of the embedded opticalfibre sensor in FRP and concrete materials to measure strain and temperature isalso discussed. Finally, the potential applications of smart composite technology incivil engineering are also given.

2. Advanced Composites for Concrete Rehabilitation and Retrofitting

A fibre reinforced plastic (FRP) has been recognised as one of the most innova-tive materials in the applications ranging from the aerospace industry to prevalentsport goods and facilities for more than 30 years. It has been demonstrated thatthe FRP could be successfully used to replace conventional materials for mostprimary structural elements in modern aircraft with safety and durability [7]. Themost attractive benefits of using the FRP in real-life applications include its highstrength to weight ratio, resistance to corrosion, ease of moulding into a complexshape without extensively increasing the manufacturing cost, good adhesion prop-erties to most common materials and ability to be pre-fabricated in the factory.Furthermore, the risk on the job accidents and injuries is considerably reducedbecause of lightweight equipment used. Unfortunately, the utilization of the FRPin civil construction is still limited so far. The serious concerns regarding the useof the FRP as reinforcements or substitutes for concrete structures are lack ofexperimental history, international design code and recognised specifications, andeconomics related to fabrication [8].

Many extensive researches have paid much attention to the developments ofusing the FRP in retrofitting and strengthening civil concrete structures in recentyears. The implementation of the researches are mainly grouped into five areas,which include:

(1) concrete confinement by FRP wrap and grid systems,(2) beam strengthening in flexure by using an externally bonded FRP patch,(3) seismic damage resistance of a concrete column,(4) investigation on the failure mechanisms, and(5) environmental effects of FRP strengthened structures.


2.1. COMPOSITE WRAPPED CONCRETE COLUMNS

Many experimental and theoretical studies have been conducted by a number ofresearchers and construction engineers on the use of steel-based isotropic materi-als for concrete rehabilitation and retrofitting. These materials have been widelyaccepted in most industrial applications. However, there exist the negative aspectssuch as the corrosion caused by harsh environments and a need for large liftingequipment to hold the steel plate in place during the installation process [9], which,in turn, subsequently increases the time of traffic interruption.

Earlier research regarding the confinement of laboratory-sized Portland cementconcrete columns by using FRP laminates was conducted in 1992 [10]. Plain con-crete columns were wrapped by glass fibre composites and subsequently subjectedto a uni-axial compressive load in order to investigate the mechanical responsesof the column after confining. In recent studies, remarkable results were foundthat the ultimate compressive strength of the confined column was substantiallyincreased by increasing the number of wrapping laminates [11–14]. Toutanji [14]has found that the use of carbon fibre composites to confine the concrete columnappeared more reliable than that using glass fibre composites when exposed toharsh environments.

Karbhari et al. [2] found that the relation of the compressive stress and strain ofthe confined concrete column responded bi-linearly in the uni-axial compressiontest. He also addressed that the use of a FRP wrap was an effective means interms of cost and speed, and overall fabrication process for the purpose of concreterehabilitation. Mirmiran et al. [15] concluded that the confinement effectivenessfor a square concrete column was dependent on the corner radius of the columnand hoop strength of the wrapping materials.

Extensive practice in large columnar concrete structures has been performed insitu by using the automated FRP wrapping system [2] which was called “Compos-ites jacketing system”. FRP prepreg materials were laid-up and cured automaticallyby applying pressure and heat simultaneously through the “Robo-Wrapper” ma-chine (see Figure 3). It should be noted herein that the major function of the FRPlaminates for the concrete in a state of triaxial compression is to restrain its trans-verse dilation without involving heavy materials and equipment, which can causethe disturbance of traffic operation. However, the latest research found that thecomposite wrapped concrete column presented reduction in strength and stiffness,and behaved more catastrophic failure under freeze-thaw cycles conditions [16].

Rochette and Labossiere [17] have used carbon and aramid sheets to confineconcrete columns. They found that the strength of the confined column was directlyrelated to the column shape. They concluded that the most effective confinementswere achieved for a circular section. A column with a sharp corner is discouragedsince premature failure by punching of the fibre may result. Lau et al. [18] foundthat the use of high modulus wrapping materials for concrete confinement mightresult in increasing the hoop stress in the wrapping material at both ends of bond.


Figure 3. The composite jacketing system (ROBO-Wrapper IITM)(extracted from Xxsys Technologies, Inc.).

Negative hoop stress may exist in the wrapping sheet and eventuate in causing ten-sile failure in a concrete column. In Figure 4, a schematic illustration of the failuresequence of the concrete columns wrapped by glass fibre composite laminates isgiven [19].

FRP grid and tube systems have been recently developed to replace steel re-inforcement for reinforced concrete columns [1, 4, 20]. Experimental results in-dicated that the compressive strength and toughness of the concrete column rein-forced by the FRP grid were increased compared to those of the fresh column. Leeet al. [1] pointed out that the use of a PVC tube as an insulated layer betweenthe concrete and the external FRP filament could prevent fibre damage due tothe chemical attack from the concrete and provide the confining action for thewhole system. However, the FRP grid and tube methods are only suitable fornew construction elements since the concrete is required to cure inside the pre-fabricated FRP/PVC tube. In Figure 5, two types of composite grid-reinforcedconcrete columns are shown [1]. Mirmiran and Shahawy [4] have used a glass fibre


Figure 4. Failure mechanism of wrapped concrete cylinder subjected to compression load.

Figure 5. Composite grid-reinforced concrete columns: (left) previous design made by glassfibre composites; (middle) finished hybrid column after filling with concrete; (right) compositegrid tube with orthogonal and helical carbon fibre composite winding [2].

composite tube to reinforce a concrete column. The fibres were aligned in both thecolumn longitudinal and circumferential directions. The longitudinal fibres aimedto restrict buckling of the column while circumferential fibres are used to confinethe column for Poisson expansion under the compressive load condition.


2.2. COMPOSITE STRENGTHENED CONCRETE BEAMS

The steel plate bonding method for a rectangular concrete beam has been world-wide used for over twenty years to strengthen concrete members [21]. The dis-advantages of this method include inconvenient transportation and installation ofthis heavy material and the subsequent corrosion due to exposure to harsh environ-ment [14].

The use of FRP materials as an external reinforcement for bridge retrofit hasbeen represented as a noteworthy improvement in terms of cost-effective, lesseffort and time than traditional methods [21–25]. The FRP materials have a lowspecific mass, and are easy to handle, particularly when operating in constrainedand enclosed situations. Since the FRP have a high tensile strength with a rela-tively low weight, it allows significant reductions in plate dimensions and instal-lation facilities when used as an externally bonded plate reinforcement. Due tothe corrosion-free property of the FRP materials, they become practice in use forreal-life applications [26–28].

Ritchie et al. [25] has observed experimentally that the failure of a FRP strength-ened beam occurred catastrophically, which was caused by linear-elastic prop-erties of the FRP. It has been shown that the ultimate flexural strength of theFRP plate bonded beams was higher than that of the beam without plate bond-ing [29, 30]. FRP strips can be effectively used for controlling shear cracking,shear strength and stiffness of the beam by bonding the stripes on both sidesof the beam and aligning all fibres in the direction normal to the shear crack[14, 29, 31, 32]. In general, the FRP plate bonding technique is catalogued intothree strengthening patterns, which include (a) tension face strengthening; (b) shearsurfaces strengthening and (c) tension and shear surfaces strengthening methodsas shown in Figure 6. A continuous wrapping technique around the sharp bot-tom corners of the beam is discouraged to avoid punching failure of the fibreswhen the beam is subjected to bending. Recent developments on the FRP platebonding technique have extended to bond a pre-stressed composite plate onto RCstructure. It therefore imposes compressive residual stress into concrete in orderto improve over flexural strength of the structure associated with lower structuraldeflection [21].

The debond and peel-off failures of the FRP plate of strengthened beams wouldresult in a sudden drop in load and brittle failure of concrete [33]. In certain aspects,anchoring mechanisms have been used in practice to avoid the peel-off and slidingof the plate at plate end regions [34]. Recently, it has been found that plate peelingat the plate/glue/concrete interface is, indeed, very rare due to strong chemicalbonding of adhesive materials to the concrete. This is attributed usually due tobad workmanship [35]. However, a high peel-off stress may cause the plate andconcrete is separated as a unit from the underside of the steel reinforcement. Lauet al. [36] has developed a simple analytical model to evaluate the stress intensityfactor of a cracked concrete beam after being strengthened by an externally bonded


Figure 6. Experimental results of composite strengthened RC beam after being submerged intodifferent solutions.

composite plate on the beam tension surface. He found that the use of high modulusand a thick composite plate would reduce the stress intensity factor inside the con-crete beam. However, a negative stress intensity factor may result if the concretebeam has a large crack after being strengthened by an externally bonded plate onits tension surface.

For the purpose of strength restoration of damaged structures, several experi-mental studies have been performed to investigate the mechanical performance ofthe cracked-concrete structures after being strengthened by the externally bondedFRP plate. Buyukozturk et al. [37], and Lau and Zhou [19] found that the flexuralstrength of an FRP bonded cracked-concrete beam was increased as compared tothe cracked-beam without strengthening. Surface treatment of the bonding areais essential in order to provide somewhat rough surface to improve the bondingquality.

Lau et al. [19] also found that the glass fibre composites strengthened beam wassusceptible to acidic environment. The overall flexural strength of the compositestrengthened RC beam dropped about 30% after the beam has been submergedinto acidic solution with a PH level of 4.01 for six months. In Figure 7, the ex-


Figure 7. E-glass/epoxy composite for column strengthening in real-life practice [43].

perimental results of the composite-strengthened RC beam after being submergedinto different solutions for six months are plotted. However, all the FRP strength-ened concrete beams exhibited catastrophic failures. Therefore, the design of thestructure requires a higher factor of safety [30].

2.3. SEISMIC RESISTANCE

Recent earthquakes such as the one in Northridge in 1994, Kobe in 1995, and Tai-wan in 1999, have repeatedly demonstrated the vulnerabilities of old infrastructureelements to the demands of seismic deformation. In the existing reinforced concretecolumns with insufficient transverse reinforcement and/or design for seismic de-tailing, three different types of failure modes are generally observed under seismicload and deformation inputs. They are:

(1) column failure by shear,

(2) column failure at the flexural plastic hinge region, and

(3) failure at the lap splice region.

These problems under seismic vibration result in the concrete spalling and crush-ing, and concrete cracking followed by column rupture.

The development of a new seismic retrofit system for the reinforced concretecolumns has been demonstrated by jacketing technique with continuous carbon fi-bre reinforced composites [3, 38]. Carbon fibre composites were wrapped in either


the whole RC column or the lower column end to form the connection between thefooting and column. Experimental study was scaled down to laboratory-sized RCspecimen. Load was applied repeatedly in the lateral direction to simulate the realearthquake conditions. The results indicated that the retrofitted RC column withthe surface wrapped composite could improve seismic response characteristics.The ultimate load that the strengthened structures would carry was increased upto 40% and the allowable deformation was more than 180% compared to that of itsunstrengthened status. However, the supporting facilities and the time for installingthe FRP reinforcement were greatly reduced.

Unreinforced masonry (URM) constructions have been recognised as one ofthe most poorly performed constructions under the seismic loading and also iden-tified as the main cause of loss of life in recent earthquakes [39]. The glass fibrecomposite plate has demonstrated a superior performance in increasing the flexuralstrength of the walls by adhering vertically to the surface of a URM wall under thesimulated earthquake loading.

2.4. CURRENT RELATED RESEARCHES IN REAL-LIFE APPLICATION

Many researches have been conducted in the United Kingdom, Japan, and Canadato investigate the use of a carbon fibre composite plate for the real infrastructureretrofits [34, 40, 41]. Strips of the carbon fibre composite plate were bonded onthe bottom surfaces of existing bridges and wrapped around the circumferenceof aged columns to improve overall mechanical performance of the structures.A jack-and-anchor arrangement has been used to secure the plate at both ends [41].Aramid fibre (Kevlar) possesses many advantages over carbon fibre as reinforce-ment for concrete, which includes (i) low density; (ii) non-catastrophic failure,which can reduce the risk of sudden damage; (iii) better impact resistance (it is,suitable for structures liable to collision damage and seismic disturbances) and(iv) electric insulator (so it can be used close to power lines or communicationfacilities) [42].

E-glass fibre composites have been used to reinforce the structure, which is de-signed for accommodating the addition of the helipad on the top of hospital build-ing [43]. Total 15 columns were wrapped by a varied number of layers of SHE 51E-Glass fibre with saturated in an epoxy matrix at the top of two floors. A finallayer of UV protective coating was applied to ensure long term performance. Theproject is aimed at investigating the mechanical behaviour of the column after beingwrapped by FRP laminates.

The official data issued by the Federal Highway Administration on the use ofFRP materials for bridge rehabilitation in 1998 showed that the overall cost forstrengthening the bridge by using the FRP bonding technique instead of recon-struction was saved up to 91% [41]. The time required for completing the job wasonly 2 weeks which caused but a slight impact on traffic interruption. However,the time used to reconstruct the bridge was more than 15 months. In Table I,


Table I. Comparison for bridge rehabilitation by reconstruction and FRP strengtheningmethods.

Total cost Duration Strength gain Impact on

traffic

1986 $ 151,000 15 months 100 % of original Major

reconstruction restrictions

1998 FRP $ 12,000 2 weeks 18% increase Little

strengthening interruption

Table II. Economic analysis of concrete rehabilitation by using conventionalsteel and FRP materials.

Economic parameters Steel FRP

Expected life of repair (years) 12 20

Initial cost (ratio) 1 1.3

Maintenance cost (ratio) 1 0.25

Construction duration (months) 6 3#

# It requires two months for design and fabrication of composite materials.

The installation time only needs for one or two days [4].

the results issued by the New York State Department of Transportation (NYSDOT)and Federal Highway Administration are shown. They have used the carbon fiberlaminates (CFRP) for bridge rehabilitation in 1998, the strength of the strength-ened structures could be restored to its original status. According to the economicconcern, EI-Mikawi and Mosallam [44] found that the use of the FRP offereda cost-effective alternative for concrete repair and rehabilitation. In Table II, thecost breakdown based on cost data from recent repairs conducted in Washington isshown. The data show that the expected lifetime for repair for a reinforced concrete(RC) structure is only about 12 years. The embedded steel reinforcements in the RCstructure are easy to corrode, which resulted in volume change of the structure. Thecorrosion is mainly due to the moisture, use of deicing salts, exhaust fumes fromthe surrounding environment. However the use of advanced composite materialscan minimise such effects.

XXsys Technologies, Inc. California has developed a Robo-WrappedTM auto-wrapping machine for strengthening the existing column where corrosion andcracks are found by using the carbon fibre prepreg. They found that the load car-rying capacity of deteriorated and overloaded bridges was increased after beingstrengthened by carbon fibre reinforced composites. The degree of debonding wasalso much smaller than that of using steel as reinforcement. Consequently, a farsmaller reduction in strength of the structures was expected.


Figure 8. Structure possibilities created by the confluence of four disciplines: materi-als and structures, sensing system, actuator control systems, and adaptive learning neuralnetwork [71].

3. Optical Fibre Sensor for Structural Health Monitoring

A new development of “Smart materials and structures” has been driven by astrong demand for high performance in recent years. A system integrated intostructures and being able to monitor the physical and mechanical properties suchas temperature and strain during the service condition is appreciated as a “Smartstructural health monitoring system”. The term of smart material and structure iswidely used to describe the unique marriage of material and structural engineer-ing by using fibre optic sensors and actuation control technology [45]. The smartstructure is constructed from the materials which could continuously monitor theirinternal strain and thermal status, and therefore, capable of assessing damage andwarning of impending weakness in structural integrity. This design results improvesafety and economic concerns regarding weight saving and avoidance of over-designing of the structure. In Figure 8, it is shown that the confluence of fourdifferent fields – structures, sensor systems, actuator control systems, and neuralnetwork systems can be appreciated as a potential of a broad class of structures.The built-in sensor and actuator systems can be used for sensing and controllingsome aspect, respectively, of the structure. The neural network system can be usedto analyse the condition of the structure according to the information received fromthe sensors and transmit the control signal to the actuation unit. The structure


with self-accommodated sensors, actuators, and a micro-manipulation unit couldbe called an “Intelligent adaptive structure”. This structure can be used to detectdamage and provide a diagnostic action to the structure to maintain that the struc-ture is operated in safety conditions. Recent researches have shown a great interestin using optical fibres as embedded sensors for measuring the temperature variationand strain profile during the manufacturing and loading processes of advancedcomposite materials [46].

3.1. OPTICAL FIBRE SENSORS

Optical fibres have been developed for long-distance data transmission in the tele-communication industry. However, in their earliest application, optical fibre wasconceived as a medium for transmission of light in medical endoscopy. The useof optical fibres for applications in the telecommunication industry actually startedin the mid 1960s, and ever since has gone through a tremendous growth and ad-vancement. The development of optical fibre sensors started in earnest in 1977 eventhough some related demonstrations preceded this date [47–50]. The increased useof advanced composites in aeronautics instigated the need for new damage detec-tion techniques, which can monitor the integrity of structural components duringthe service period. Green et al. [51] and Chan et al. [52] have successfully demon-strated that the embedded optical fibre sensors could be used to measure strain ofadvanced composites with a high accuracy and resolution. Du et al. [53] and Chanet al. [52] also demonstrated that the embedded fibre-optic Bragg grating sensorscould be multiplexed to measure strain at different locations of the composite plateby occupying only one single optical fibre. Therefore, the optical fibre sensors havebeen extensively employed as real-time damage detection tools in advanced aircraftand space vehicles.

In Figure 9, an optical fibre sensor embedded into glass fibre composite ma-terials is shown. External perturbations such as strain, pressure or temperaturevariations induce changes in the phase, intensity, or wavelength of light wavespropagating through the optical fibres. The changes in one or more of the prop-erties of light can then be related to the parameter being measured. Optical fibresare geometrically versatile and can be configured to arbitrary shapes. The smartstructure concept takes advantage of the geometric adaptability of optical fibres. Inthis technology, optical fibre sensors are embedded within the structural materialor bonded on the structure surface for the purpose of real-time damage assessment.The most attractive feature of fibre optic sensors is their inherent ability to serveas both the sensing element, and the signal transmission medium, allowing theelectronic instrumentation to be located remotely from the measurement site. Thisis especially useful for remote monitoring of the condition of bridges. Moreover,the advantages of using embedded optical fibre sensors in composite materials aredimensional and material compatibilities. The fibres do not degrade during curing,


Figure 9. Cross section view of a glass fibre composite with an embedded optical fibre.

they do not corrode, and bond strongly to the matrix. Incorporation of the fibresduring the processing stage also offers an opportunity to monitor the condition ofstructural elements during fabrication [54–56].

The success of optical fibre sensor technology in the condition monitoring ofcomposite materials led to a limited number of research and development ac-tivities in the civil engineering discipline. A number of researchers realised thatthis emerging field of technology could have impact on the condition monitoringof civil structures, so that durability, safety, and efficiency of the infrastructuresystem could be improved. The proper application of optical fibre sensors to con-crete structure repair monitoring requires understanding of certain fundamentalmethodologies pertaining to sensor mechanisms as well as sensor multiplexingstrategies.

Uptill to now, enormous types of optical fibre sensors have been developed.The sensors have been classified in a number of different ways. For instance, theycan be categorised basing on the monitoring applications. The classification isdepicted as

Optical fibre sensor applications:

1. Localised,

2. Multiplexed,

3. Distributed.


Figure 10. Intensity type fibre optic micro-bending sensor.

3.1.1. Localised Sensors

Localised fibre optic sensors determine the measurement over a specific segmentof the optical fibres, and are similar in that sense to conventional strain or temper-ature gauges. Sensing based on the intensity modulation pertains to light intensitylosses that are associated with bending or micro-bending of optical fibres alongany portion of their length. In Figure 10, a fibre-optic microbend sensor with anelastic and enclosed diaphragm is shown [57]. Sensors taking advantage of thisphenomenon are termed intensity- or amplitude-type sensors. The advantages ofintensity-type sensors are the simplicity of construction, and compatibility withthe multi-mode fibre technology. Phase sensors cover a broad range of opticalphenomena for sensing purposes. Figure 11 demonstrates a number of differentconfigurations that can be employed for measuring the change in the phase of lightby an interferometric sensor.

A fibre optic Bragg grating (FBG) type sensor is one of the most exciting tech-nologies in the field of optical fibre sensors in recent years. The FBG sensor appearsto be ideally suited for structural health monitoring of composite materials and civilengineering application [58, 59]. The principle of the FBG sensor technique hasbeen discovered by Hill et al. in 1978 [60]. It was found that the reflective gratingcould be photorefractively formed in the core of the gramanium doped silicatefibres. A schematic illustration of the FBG strain measuring system is shown inFigure 12. The FBG technology is defined as change of the core refractive index(ncore) of the optic fibre at a specified point (grating) and acts as a mirror forreflecting the light signal from the source. The reflective wavelength of the lightfrom the grating depends on the variation of the core refractive index, which is


Figure 11. Fibre-optic interferometric sensors.

caused by changing the physical dimension or thermal deformation at the gratingregion. For the structural strain monitoring, load is directly transferred from thehost material at a particular position to the fibre-core section by shear. This causesthe length of the grating region to be changed and the resultant refractive index ofthe core section to vary in due course. The mechanical properties of the structureare simply determined by measuring the reflective wavelength change from thesystem due to this transformation of the refractive index.

Udd and Du et al. [53, 61] presented that the FBG sensor was ready to beembedded into the material without any degradations of the structure. The sensoralso provides an effective means to monitor the manufacturing process and internalhealth conditions by distributing the sensors at different locations inside/on thestructure. From the view of applications in civil structure monitoring, these sensorsare intended for use as a localised fibre optic sensor.

3.1.2. Multiplexed Sensors

Multiplexed sensors are usually constructed by combining a number of individualsensors for measurement of perturbations over a large structure. Theoretically,it is possible to use optical switching and other innovative ideas for this pur-


Figure 12. Fibre-optic Bragg grating (FBG) strain measuring system.

pose. A number of researchers have developed innovative methods for the develop-ment of multiplexed fibre optic sensors. The most widely employed multiplexedsensing technique is based on the measurement of propagation time delays of lighttravelling in the fibre, based on the measurand-induced change in the transmissionof light. An optical time-domain reflectometer (OTDR) is used for this purpose[62, 63]. A pulsed light signal is transmitted into one end of the fibre, and lightsignals reflected from a number of partial reflectors along the fibre length arerecovered from the same fibre end as shown in Figure 13. By using this con-cept, it is possible to determine the distance to the strain field, d, by means ofthe two-way propagation time delay 2t through the simple relationship (relatingvelocity and distance): d = 2t × v. Here v is the velocity of light in the fi-bre, and 2t is the time required for the two-way travel of the signal from indi-vidual reflectors. Since the velocity of light is known, an OTDR is capable ofdetecting the location of strain fields through the measurement of reflected timesignals.


Figure 13. Multiplexed fibre optic white light Michelson interferometer strain sensors inseries.

A promising technique is based on wavelength division multiplexing by usingBragg gratings [64, 65]. In this technique, a broad-band light source, defined aslight, containing a number of wavelengths within a certain region of the spec-trum, is employed for scanning a number of Bragg grating-type sensors in seriesand/or in parallel. The reflectance wavelength of each Bragg grating is slightlydifferent from the others. In this way, wavelength shifts of individual sensors arerecognised, detected and then related to the magnitude of strain at specific sensorlocations.

3.1.3. Distributed Sensors

Distributed sensors make full use of optical fibres, in which each element of theoptical fibre is used for both the measurement and data transmission purposes.The purpose of making measurements by distributed or multiplexed optical fibresis to determine locations and values of measurands along the entire length of thefibre. These sensors are most appropriate for application to large structures ow-ing to their multi-point measurement capabilities. A distributed sensor permits themeasurement of a desired parameter as a function of length along the fibre. Oneway is based on the Michelson interferometric technique by using white light (lowcoherence light) as the interferometer source. The multiplexed strain sensors inseries configured by using a switch and in parallel by a 1 × N star coupler aredemonstrated [66]. In Figure 14, the distribution of an optical fibre strain-sensingsystem is shown.


Figure 14. Distribution of optical fibre strain-sensing system: PIN, detector; FCs, fibreconnector.

3.2. APPLICATIONS OF OPTICAL FIBRE SENSORS IN CIVIL CONCRETE

STRUCTURES

Since advanced composite materials (ACM) are now routinely being used as sec-ondary and primarily structural members for aircraft and automotive structures,the use of the composite material with the embedding of the optical fibre sensorscan lead to reductions in weight, inspection intervals, and maintenance cost of thestructures and consequently, improvement in overall performance. Du et al. [53]and Chan et al. [67] have used the embedded FBG sensors, which are locatedbetween the composite laminates as internal strain measuring devices. They foundthat the embedded FBG sensors could provide a strain distribution along the struc-ture without influencing the mechanical properties of host material. Dewynter-Marty et al. [68] embedded the FBG sensors into the glass fibre/epoxy laminateto monitor thermal and residual strain properties during the curing process in au-toclave. The temperature measured gave a good agreement with that measured byautoclave temperature probe. The FBG sensor could sustain compressive strainup to 3100 µε strain without failure [19]. Lo et al. [69] developed a single-pitchFBG sensor for corrosion detection by coating copper onto a pre-loaded opti-cal fibre. The principle of corrosion sensors is that the environment corrosionwould change the coating thickness and eventually cause the changes of residualstrain inside the fibre grating region. A parameter, which represents the corrosionrate can be measured according to the reflected wavelength change in the grat-ing. A schematic diagram of the corrosion detected FBG sensor is illustrated inFigure 15. Kalamkarov [70] found that the FBG sensor embedded in the carbonfibre reinforced composite plastic (CFRP) rod is unaffected by fatigue load andcreep effects.


Figure 15. Corrosion detection FBG sensor.

Figure 16. A schematic diagram of distributed FBG sensor for bridge strain monitoring.

The FBG sensor has been used in civil construction industry only in recentyears. The first use of FBG sensors as a structural health monitoring device inbridge structure was demonstrated in 1994 [71]. The array of FBG sensors wasadhered on the surface of a carbon fibre composite tendon to measure strain anddeformation of the structure. In Figure 16, a schematic diagram of FBG sensorlocations for strain monitoring along the bridge is shown. They found that themeasuring range of this interrogation system was within 5 mε and 1 µε. The ac-curacy of ∼20 µε was demonstrated. Saouma et al. and Lau et al. [72–74] haveused the embedded FBG sensors to monitor strains of laboratory-sized concretebeams. They found that the results gave a good agreement with those measured


by externally bonded electric strain gauges. The Naval Research Laboratory in theUSA [75] performed an experimental study for a bridge, which is in the ¼ scale ofreal structure with the embedding of 60 FBG sensors to measure strain. They foundthat the FBG sensors could measure the strain in real-time. However, due to thelimit of scanning speed of the Fabry–Perot tunable filter used for wavelength-shiftmeasurement, only static strain could be measured. In Canada, multiplexing FBGsensors were embedded during the construction along a pre-stressed girders for abridge, which was made of FRP materials to monitor strain response at differentpositions during service [76].

3.3. PROBLEMS IN APPLICATION

The acceptance of these materials in civil engineering industry is still very lim-ited due to lack of sufficient design codes, material standards and specifications.Moreover, the quality control of the FRP materials used for concrete strengtheningis difficult since it is very dependent on several environmental parameters such ashumidity and temperature [34]. Long-term durability in harsh environments is alsoan important aspect to be concerned for the current strengthening method beforefurther applying to real practise.

4. Potential Applications of Smart Composites in Civil Engineering

A term of “Smart Composites” implies that the composites have their ability todetect damage and impending failure under certain marginal conditions [77]. Thesensor, like a nervous system in a human body, can sense or forecast a hiddentrouble for global and local deformations, and damages in the structures. Mostlatest publications have addressed that the use of optical fibre sensors associatedwith other actuators could build a “live” structure, i.e., the structure can sense,manipulate and response to any undesired structural deformation or damage inorder to recover strength of the structure [78–80].

The applications of the FRP and optical fibre sensor in the civil engineeringindustry have been discussed independently in the previous sections. However,the use of the smart composite in the civil concrete structure has not yet beenadopted in all real-life infrastructure structure elements. It has been proved thatthe use of the FRP can substantially increase the strength of concrete structures byusing simple composite plate bonding or hand lay-up techniques. The optical fibresensor, however, also presents its ability to measure strain remotely and preciselywithout imposing any strength degradation of the structure. Thereby, the use thesmart composites for concrete strengthening as well as structural health monitoringdevice can definitely improve the durability and safety of the structure.

Since the technology of smart composites, in which the optical fibre sensorsare integrated into the composites to form a single part compound, has maturelydeveloped, the smart composites can be used as reinforcement as well as sensors for


Figure 17. Smart composites for concrete rehabilitation.

repairing and strengthening damaged concrete structures, as shown in Figure 17.The composite can be formed as a patch to bond on the concrete surface to improvethe tension properties of the concrete structure. The sensors are integrated into thecomposite, which results in reducing the risk of damage of the fibre and can bemanufactured in-house without being affected by situ-environment.

5. Conclusions

This paper reviews a number of research and practical applications in the utilisa-tion of FRP and optical fibre sensors for concrete repair and rehabilitation, andstructural health monitoring, respectively. The FRP can be confidently used basingon many research achievements and practical use for strengthening, repairing, andupgrading the existing damaged concrete structures. The optical fibre sensors alsoexhibit their potential in real-life application. Due to the physical size of the opticalfibre sensor that is relatively small as compared with the infrastructure element,embedding the sensor into the structure would not involve any bad effect on thestructure. Smart composites have been widely adopted in most high-tech engineer-ing applications, such as a wing in aerospace and aircraft industry. The resultsfrom all research achievements showed that the smart composite could be usedas a primary structure as well as a sensor to monitor strain or temperature of thestructure. However, this innovative concept has not yet been adopted in all civilengineering applications.

In the future, the development of smart materials and structures will play amajor role in all engineering disciplines. The optical fibre sensors will be one ofthe most structural health monitoring devices in achieving this goal. The use of


smart composite materials as concrete reinforcement as well as a real-time struc-tural health monitoring device can greatly improve the structural durability andsubstantially improve the safety of the structure.

Acknowledgement

This research project was funded by The Hong Kong Polytechnic Universitythrough grants G-YW60 and G-YC64, and The Research Grant Council of HongKong through a grant PolyU 5160/99E.

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Documents

Applications of Composites, Optical Fibre Sensors and Smart Composites for Concrete Rehabilitation: An Overview