Progress on understanding debonding problems in reinforced ...

Construction and Building Materials 18(2004) 9–19

0950-0618/04/$ - see front matter� 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0950-0618(03)00094-1

Progress on understanding debonding problems in reinforced concrete andsteel members strengthened using FRP composites

Oral Buyukozturk*, Oguz Gunes, Erdem Karaca

Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139,USA

Received 5 July 2003; received in revised form 23 August 2003; accepted 24 August 2003

Abstract

Use of fiber reinforced plastic(FRP) composite materials for strengthening and repair of structural members has become anincreasingly popular area of research and application in the last decade. However, the method is yet to become a mainstreamapplication due to a number of economical and design related issues. From a structural mechanics point of view, an importantconcern regarding the effectiveness and safety of this method is the potential of brittle debonding failures. Such failures, unlessadequately considered in the design process, may significantly decrease the effectiveness of the strengthening or repair application.In recent years, there has been a concentration of research efforts on characterization and modeling of debonding failures. Thispaper provides a review of the progress achieved in this area regarding applications to both reinforced concrete and steel members.� 2003 Elsevier Ltd. All rights reserved.

Keywords: Fiber reinforced plastics; Repair; Strengthening; Debonding

1. Introduction

FRP composite materials have experienced a contin-uous increase of use in structural strengthening andrepair applications around the world in the last decade.High stiffness-to-weight and strength-to-weight ratios ofthese materials combined with their superior environ-mental durability have made them a competing alterna-tive to the conventional strengthening and repairmaterials. Local and federal agencies faced with thechallenge of economically upgrading the ever-increasingnumber of aging and substandard structures have invest-ed in this area leading to numerous research studies andapplications. It has been shown through experimentaland theoretical studies that externally bonded FRP com-posites can be used to improve the desired performanceof a structural member such as its load carrying capacityand stiffness, ductility, performance under cyclic andfatigue loading and environmental durability. However,the method is yet to become a mainstream applicationdue to a number of economical and design related

*Corresponding author.E-mail addresses: [email protected](O. Buyukozturk),

[email protected](O. Gunes), [email protected](E. Karaca).

issues. From a structural mechanics point of view, animportant concern regarding the effectiveness and safetyof this method is the potential of brittle debondingfailures. Such failures, unless adequately considered inthe design process, may significantly decrease the effect-iveness of the strengthening. In recent years, manyresearchers have focused on this important issue throughboth experimental and theoretical investigations. Thispaper provides a review of the progress on understandingand modeling of debonding failures in FRP strengthenedreinforced concrete(RC) and steel members.

2. FRP strengthening of RC and steel members

Applicability and effectiveness of strengthening withFRP composites depend largely on the material and thetype of the member to be strengthened. Research up todate in this area has mainly focused on applications toRC members. In a strengthening application, thestrengthening material is generally expected to have asimilar or higher stiffness compared to the base materialof the member being strengthened. While this is gener-ally the case for concrete and soft metals such asaluminum, the stiffness of most FRP composite systems

10 O. Buyukozturk et al. / Construction and Building Materials 18 (2004) 9–19

Fig. 1. Elastic and strength properties of FRP composites compared with conventional construction materials,(a) Elastic(Young’s) modulus(b)Stress–strain behavior.

Fig. 2. Failure modes of FRP strengthened flexural(a) RC and(b) steel members.

are considerably less than that of structural steel. Fig.1a compares the elastic modulus of concrete, aluminumand steel with those of several commercially availableFRP composite systems and Fig. 1b shows a comparisonof strength–strain behavior in tension. It can be inter-preted from this comparison that strengthening of steelmembers with FRP composites may be mechanicallyless advantageous and economically less feasible com-pared to concrete and aluminum members. Nevertheless,a specific type of application involving steel structures,which is both mechanically and economically welljustified, is repair of fatigue damaged steel memberswith FRP composites. Recently, research and applica-tions in other types of applications involving steelmembers have also increased due to continually decreas-ing costs of FRP materials, ease of installation and thepotential of eliminating welded and bolted repairs.Strengthening with FRP composites can be applied to

various types of structural members including beams,columns, slabs and walls. Depending on the membertype, the objective of strengthening may be one or acombination of several of the following:(1) to increase

axial, flexural or shear load capacities;(2) to increaseductility for improved seismic performance;(3) toincrease stiffness for reduced deflections under serviceand design loads;(4) to increase the remaining fatiguelife; (5) and to increase durability against environmentaleffects. In general, applications where the accessibilityconditions allow wrapping of the member with FRPcomposites, such as FRP wrapping of RC columns, maynot usually suffer from debonding problems, and thus,are not included in this paper. Instead, emphasis is givento shear andyor flexural strengthening of RC and steelbeams and repair of fatigue damaged steel memberswhere debonding problems are frequently encounteredand play an important role in the behavior and perform-ance of the member.Failure of FRP strengthened RC and steel flexural

members may take place through several mechanismsdepending on the beam and strengthening parameters.In the case of RC beams, failure may take place through(1) concrete crushing before yielding of the reinforcingsteel, (2) steel yielding followed by FRP rupture,(3)steel yielding followed by concrete crushing,(4) cover

11O. Buyukozturk et al. / Construction and Building Materials 18 (2004) 9–19

Fig. 3. Types of debonding in FRP strengthened RC members.

Fig. 4. Types of debonding in FRP strengthened steel members.

delamination,(5) FRP debondingw2,12x. In addition tothese, shear failure occurs if the shear capacity of thebeam cannot accommodate the increase in the flexuralcapacity. These failure modes are illustrated in Fig. 2a.Similarly, Fig. 2b shows the failure modes of an FRPstrengthened steel member as(1) top flange buckling incompression,(2) web buckling in shear,(3) FRP rup-ture, (4) FRP debonding. An investigation of each ofthese failure modes is required in the design process toensure that the strengthened member will perform sat-isfactorily. Knowledge provided in this paper is limitedto debonding problems.

3. Debonding problems in FRP strengthened RC andsteel members

Debonding in FRP strengthened members takes placein regions of high stress concentrations, which are oftenassociated with material discontinuities and with thepresence of cracks. Propagation path of debondinginitiated from stress concentrations is dependent on theelastic and strength properties of the repair and substratematerials as well as their interface fracture properties.The term debonding failure is often associated with asignificant decrease in member capacity due to initiationor propagation of debonding.Theoretically, debonding in FRP strengthened mem-

bers can take place within or at the interfaces of

materials that form the strengthening system, favoring apropagation path that requires the least amount of energy.Crack propagation in one of the constituent materials isgenerally preferred over interface debonding in designof structural joints; however, the latter is often encoun-tered, especially in cases of poor surface preparation orapplication. Fig. 3a shows possible types of debondingin FRP strengthened RC members. A majority of thedebonding failures reported in the literature took placein the concrete substrate. However, depending on thegeometric and material properties, other debondingmechanisms can also be observed. Fig. 3b shows that acombination of different debonding types and mecha-nisms can take place in a single experiment. Types ofdebonding in FRP bonded steel members shown in Fig.4a are similar to those in RC, in this case involving thesteel substrate. Fig. 4b shows the failure and debondingsurfaces on an FRP strengthened notched steel specimenfailed under tensile fatigue loading.

4. Experimental research on debonding problems

Early experimental observations of debonding in FRPstrengthened RC and steel flexural members were report-ed after studies at the Swiss Federal Materials Testingand Research Laboratories(EMPA) in Switzerland.Kaiser w35x showed that CFRP plates can be used to


Fig. 5. Influence of shear strengthening and anchorage on strengthened beam behavior under monotonic loading.

strengthen RC beams and identified different failuremodes. Among these, debonding of the FRP reinforce-ment from the concrete substrate was identified as animportant mode of failure since it could take place atpremature load levels and the failure was generally verybrittle w47,48x. This is followed by numerous studiesreporting various aspects of FRP debonding problemsin FRP strengthened RC beams. It was suggested thatimproper selection of adhesives might promote debond-ing failuresw68x. Debonding failure behavior of strength-ened beams was shown to be highly dependent on theexisting steel reinforcement ratio and the type andamount of external FRP reinforcementw25,43,60,63,67,72,74,86x. For a fixed FRP ratio, debonding poten-tial was shown to increase significantly with increasingFRP thicknessw25x. Experiments on simply supportedbeams strengthened with different lengths of FRP rein-forcement have revealed that debonding failure load andductility decreases with decreasing lengths of the FRPreinforcement. The general conclusion of these studiesis that by extending the FRP reinforcement to thesupports as much as possible, potential of debondingfailures may be decreased, although not eliminatedw80,32,4,23,54x. Laboratory test studies on FRP strength-ened beams with notches in the shear span or mid-spanrevealed that unstable debonding may also originatefrom flexural and flexuralyshear cracksw88,31,70x. Anumber of researchers have investigated debondingproblems in beams precracked before strengthening.Mixed conclusions were drawn from these studies,calling for further research on this issuew6,7,32,60x.Role of bond anchorage in increasing debonding resis-tance of strengthened beams was demonstrated byseveral studiesw25,26,50,56,74,78,79x. A recent experi-mental investigation by the authors has revealed thatboth failure load and ductility of precracked beamsstrengthened with FRPs can be significantly increasedthrough shear strengthening and bond anchorage, as

shown in Fig. 5w14,15x. Performance of FRP strength-ened beams under fatigue loading was investigatedthrough several experimental studies, which generallyconcluded that unless adequate bond anchorage is pro-vided, debonding problems may become significantunder fatigue loadingw9,20,33,45,73x. Experimentalinvestigations regarding durability of FRP strengthenedconcrete systems under various environmental exposureconditions including freeze-thaw, wet-dry and tempera-ture cycles and various aqueous solutions have shownthat deterioration of the concrete–FRP interface maylead to debonding problemsw16,18,27,28,38,85x. Relia-ble quantification of such effects is not possible at thecurrent state of experimental and theoretical research.Although the experimental research on FRP strength-

ened steel members has been limited compared to RCmembers, there has been a significant increase ofresearch interest in this area in recent years. Initialexperimental study in this area explored use of highmodulus CFRP plates to repair a real scale fatiguedamaged steel box girderw21,22x. This study revealedthat FRP composites can increase the remaining life offatigue-damaged members and that debonding was thegoverning failure mechanism. Basetti et al. performedlaboratory tests on FRP repaired center-notched couponsand later implemented the method on a riveted bridgew10,11x. Both laboratory and field studies showed thatFRP bonded repair of fatigue-damaged steel membersis a powerful technique that provides high structuralefficiency and extends the life of flawed structuralcomponents at an economical cost. An experimentalstudy by the authors involving fatigue testing of side-notched steel specimens repaired with FRP patches invarious configurations have confirmed the effectivenessof the technique by increasing their fatigue lives, whiledrawing attention to research issues in design anddurability of the repairw13x. Results from this study,part of which are shown in Fig. 6, indicates that fatigue


Fig. 6. Fatigue performance of FRP strengthened 3y8 inch-thick A36 notched steel specimens.

life of notched specimens, which would fail underapplied stress in unrepaired configuration, can be signif-icantly increased with the application of FRP compos-ites. Fundamental experimental research on durability ofFRP bonded steel have shown that surface preparationand environmental exposure conditions can significantlyaffect bond durability and drew attention to potentialgalvanic corrosion problems in carbon FRP-steel sys-tems w13,36,46,83x. Proven effectiveness of the repairtechnique and better understanding of the durabilityissues lead to consideration of the method for strength-ening of steel members. Laboratory tests on FRPstrengthened large-scale steel members with and withoutnotches have shown that the method can be used toincrease the stiffness, load capacity and ductility of steelmembersw42,71x. A field application of the method wasperformed to strengthen a bridge girder in Newark,Delawarew49x.

5. Modeling research on debonding problems

Characterization and modeling of debonding in struc-tural members strengthened with externally bonded rein-forcement has long been a popular area of inter-disciplinary research due to critical importance ofdebonding failures in bonded joints. In the last decade,there has been a concentration of research efforts in thisarea with respect to FRP strengthened flexural members,and considerable progress has been achieved in under-standing the causes and mechanisms of debonding fail-ures. Research studies in this area can be classified ingeneral terms by their approach to the problem asstrength and fracture approaches. In addition to these, anumber of researchers have proposed relatively simplesemi-empirical and empirical models that avoid thecomplexities of stress and fracture analyses and can berelatively easily implemented in design calculations.

5.1. Strength approach

Prediction of debonding failures through strengthapproach involves calculation of the interfacial or bondstress distribution in FRP strengthened members basedon elastic material properties. Calculated stresses arecompared with the ultimate strength of the materials topredict the mechanism and load level of debondingfailures. Fig. 7a shows a conceptual illustration of thebond stresses at the concrete–FRP interface and in theFRP reinforcement in a RC beam strengthened in flex-ure. Similarly, Fig. 7b shows the stresses developed ina steel beam strengthened in flexure. For illustrativepurposes, fatigue cracks are introduced in the bottomflange. As seen from these figures, high shear andnormal stresses develop at the laminate ends and atcrack locations, which lead to interfacial debonding andpotential debonding failures.Strength based research on debonding problems in

FRP strengthened RC beams have produced severalsolution methods that predict bond stress distributionsin plated beams, based on the common assumption thatall materials are linearly elastic, although concrete crack-ing is considered. A majority of these methods give arelatively simple and approximate solution while othersinvolve a higher-order analysis, yielding more accuratebut also more involved solutions. The key differencebetween the approximate and higher-order solutions isthat the former assume constant shear and normalstresses in the adhesive layer, whereas the latter takesthe stress variations across the adhesive thickness intoaccount. Due to constant shear assumption, the approx-imate solutions do not satisfy the zero shear boundarycondition at the ends of the adhesive layer. Both solu-tions give close results except for a very small zonenear the ends of the adhesive layer, in the order of theadhesive thickness.


Fig. 7. Approximate and actual stress distributions in FRP strengthened(a) RC and(b) steel flexural members.

An approximate staged solution of bond stressesdeveloped by Robertsw64x, which is a simplified versionof a more rigorous solution by Roberts and Haji-Kazemiw65x, is frequently used for its simplicity and applicabil-ity to general loading conditions. Modified versions ofthis solution were used by other researchers for FRPand steel plated beamsw19,87,92x. Alternative solutionsdeveloped for FRP strengthened beams consider defor-mation compatibility condition and give similar resultsalthough they show differences in their solutionapproaches and applicability to different loading config-urations w39,44,76,82x. A conceptual and numericalcomparison of some of these solutions is provided inw76x. Rigorous higher-order solutions of the bond stress-es in plated beams were also developedw58,75x. Acomparison of the results obtained from higher-orderand approximate solutions shows close agreement exceptfor the previously mentioned small end zonesw75x.Following the calculation of bond stresses, a failure

criterion dependent on the material strength propertiesis needed for prediction of debonding failures. Someresearchers suggested that debonding failure takes placewhen the maximum shear and normal stresses reachcertain valuesw57,65x. Alternative failure criteria includeconcrete failure under biaxial stressesw19,44x or aMohr–Coulomb type criterionw87,92x.Determination of bond stress distributions in FRP

strengthened steel members is considerably easier com-pared to RC members. Development of solution methodsfor stresses in bonded joints with elastic adherends hasbeen a popular area of research since the 1930s due totheir use in diverse industries. A review of the developedsolution methods for various cases of geometry andloading conditions are reviewed in various textsw3,84x.A general numerical solution procedure was developedby Yuceoglu and Updikew90x for stress analysis of

bonded plates and joints. Besides these methods, thesolution procedures mentioned above can be used forstrengthened steel members upon certain simplifications.Failure loads of these systems can be predicted basedon point-based, zone-based or strain energy density-based criteriaw3,84x.

5.2. Fracture approach

The fact that debonding is essentially a crack propa-gation promoted by local stress intensities has raisedinterest among some researchers to take a fracturemechanics approach to the problem and develop predic-tive models that utilize elastic and fracture materialproperties. Hamoush and Ahmadw29x used Linear Elas-tic Fracture Mechanics(LEFM) and finite elementmethod to model debonding in steel plated concretebeams with no internal reinforcement. Taljstenw81xderived linear and non-linear mode II fracture equationsfor symmetric and asymmetric overlap joints, which canbe used in bond shear tests. Fukuzawa et al.w24xmeasured mode II fracture toughness of interfacialdebonding between FRP and mortar by use of doubleshear specimens pulled in tension. In a similar study,Neubauer and Rostasyw53x performed bond strengthtests to determine the FRP–concrete interface fractureenergy. Karbhari and Engineerw37x used a modifiedshear test to measure the mixed mode interface fractureenergy between concrete and FRP. Wu et al.w88xperformed an experimental and numerical study toinvestigate debonding in plain concrete beams strength-ened in flexure with FRP composites. Beams with amid-span notch, simulating a flexural crack, were loadedin three-point bending and initiation and propagation ofdebonding around the notch was measured. Results werevalidated by the compliance method through a finite


element analysis. Yuan and Wuw89x later developed ananalytical methodology to model debonding initiated atthe FRP concrete interface at either middle or end ofthe laminate by use of fracture mode partitioning.Hearing and Buyukozturkw32x employed a fractureenergy based criterion to predict debonding failures inreinforced concrete beams. Leungw41x derived the shearstress distribution at the FRP–concrete interface arounda flexural crack in plain concrete beam based on linearelastic fracture mechanics(LEFM) and superpositionmethod. Using a similar approach, Lau et al.w40xestimated the modeI stress intensity factor,K , at theI

tip of a flexural crack in a concrete beam strengthenedin flexure with FRP composites. Neubauer and Rostasyw52x developed a fracture mechanics based bond strengthmodel to investigate the effects of the differential dis-placements at the shear crack mouths on the overallbond strength and suggested a bond strength reductionfactor of 0.9 to include such effects. Rabinovitch andFrostig w59x extended their high-order bond strengthmodelw58x to a fracture mechanics approach and includ-ed the effect of debonding around flexural and shearcracks. The few fracture models proposed so far havelimited success in predicting the failure load for FRPstrengthened beams and need further improvement.Service life prediction of fatigue damaged steel mem-

bers repaired with FRP patches calls for a fractureapproach since the system behavior is based on fatiguecrack propagation. There is an extensive research back-ground in this area due to common use of the crackpatching technique in the aircraft industry since 1970sw8x. Various researchers have developed analytical andfinite element methods for the determination of stressintensities in the repaired system and for prediction ofthe fatigue life w51,66,69x. However, characteristics ofthe problem in structural strengthening applications aresignificantly different than those in the aircraft applica-tions due to complexity of load conditions and thicknessof the substrate material. Adoption of the existingmodeling techniques to structural strengtheningyrepairapplications is needed.

5.3. Semi-empirical and empirical models

Empirical models referred to in this paper are thosethat do not involve a systematic stress or fractureanalysis, but involve the use of simplified relations ona phenomenological basis to predict failure. The generalobjective of these models is to provide a simple meth-odology to predict debonding failures without going intocomplex stress or fracture analyses. Several such modelswere proposed for FRP strengthened beams based oncertain parameters that influence their debonding behav-ior. A group of proposed models are based on the shearcapacity of the strengthened beamw4,5,34,55,77x. Failureis assumed to take place when the external shear acting

on the beam at the plate ends exceeds a predefinedvalue. The so-called concrete tooth models consider theconcrete cover between two adjacent cracks as cantile-vers extending from the flexural steel reinforcement tothe bottom of the beamw30,61,62,91x. Cover debondingis assumed to occur when the stresses at the root of thetooth reach the tensile strength of the concrete. Analternative model based on truss analogy predicts failureload by means of bond yield condition for the plate–concrete interfacew17x. Investigation on the validity ofthe proposed semi-empirical and empirical models haveshown that each model may hold for a group of beamswith certain strengthening and load conditions, however,none has yet been proven to hold for the general caseof FRP strengthened beams.

6. Cyclic loading effects on debonding

Seismic retrofitting of existing structures comprises amajor portion of structural strengthening applications.Thus, performance of strengthened members undercyclic loading must be thoroughly investigated withemphasis on brittle debonding failures to ensure theseismic safety of strengthened systems. Although severalresearchers have studied the performance of strength-ened beams under fatigue loadingw9,20,33,45,73x, highamplitude cyclic load performance of strengthenedbeams remains virtually uninvestigated. Fig. 8 showsselected results from a comprehensive experimentalstudy performed by the authors on high-amplitude cyclicload behavior of beams strengthened in various config-urationsw15x. It is seen from the figure that performanceof FRP strengthened beams under cyclic loading mayfall below that under monotonic loading, the degree ofwhich is dependent on the strengthening parameters andanchorage conditions.Potential effects of cyclic loading on retrofitted beam

performance can be conceptually deduced from knownmechanical behavior of reinforced concrete beams undercyclic loading. Increase of plastic deformations in rein-forced concrete beams under cyclic loading is a well-known phenomenon. Knowing that FRP materialsdisplay a linearly elastic behavior, the stresses both inthe FRP composite reinforcement and at the concrete–FRP interface are likely to increase with the increasingbeam plastic deformation under cyclic loading. Depend-ing on the FRP reinforcement ratio and anchorageconditions, increase in interfacial and normal stressesmay promote debonding and unexpected FRP rupturefailures. Thus, these effects must be properly consideredin the design process.An important aspect of debonding failures in FRP

strengthened beams under cyclic loading is the post-failure behavior. Although the load capacity of the beammay decrease due to a premature debonding failure, itis required that the beam displays a ductile post-failure


Fig. 8. Influence of shear strengthening and anchorage on FRPstrengthened beam behavior under cyclic loading(a) flexural(b) flex-uralqshear(c) flexuralqshearqanchorage.

behavior so that it can contribute to the structuralperformance at the pre-retrofit level. Experimental stud-ies have shown that in some cases a brittle shear failure

follows the debonding failure at load levels below thecalculated shear capacity of the beamw15x. In suchcases, not only the retrofitting is ineffective, but alsoharmful to the structure since the ductility of the memberis decreased significantly. ACI 440Fw2x requires thatfor beams strengthened only in flexure, it must beensured that the shear capacity of the beam, calculatedin accordance with ACI 318w1x, can accommodate theincreased shear demand. However, experimental evi-dence indicates that a special consideration must begiven to calculation of the shear capacity of beams tobe strengthened in flexure with FRP composites.Repair of fatigue-damaged steel members with FRP

composites was shown to be an effective technique.Although the system is likely to perform satisfactorilyunder low amplitude fatigue loading, performance ofsuch systems under high amplitude cyclic loading con-ditions is unknown. Considering that partial debondingin such systems is expected under extended fatigueloading, and that environmental exposure may adverselyaffect bond integrity, it is apparent that a thoroughinvestigation of the high-amplitude cyclic load perform-ance of these systems is of critical importance. Currently,no reports of experimental or analytical research studieson this issue exist.

7. Environmental effects on debonding

FRP composites are known for their resistance toenvironmental exposure conditions, however, durabilityof the FRP strengthening systems, as a whole, is a majorconcern in structural rehabilitation applications. Behav-ior of FRP strengthened beams subjected to freeze-thaw,wet-dry and temperature cycles or various aqueoussolutions prior to loading have been studied by a limitednumber of researchers and varying degrees of strengthdeterioration have been observedw16,18,27,28,38,85x.Similarly, studies on bond durability of FRP bondedsteel systems have reported deterioration caused bytemperature and moisture cyclesw13,36,46,83x. In addi-tion, potential galvanic corrosion problems associatedwith strengthening of steel members with FRPs contain-ing conductive fibers such as carbon raises durabilityconcerns. Current state of knowledge on bond durabilityof FRP strengthened systems is limited to a few exper-imental studies. Theoretical modeling studies focused inthis area are yet to be performed.

8. Research needs

A review of the experimental and modeling studieson debonding problems in FRP strengthenedyrepairedRC and steel members show that despite considerableprogress, research in this area is still very young.Continued experimental and theoretical research at bothmaterial and structural level is needed in various aspects


of debonding problems in FRP strengthened systems.Existing debonding models based on strength and frac-ture approaches and empirical relations must be furtherdeveloped and improved for better prediction of debond-ing failure loads. Continued experimental studiesfocused on specific problem areas are needed for vali-dation and calibration of developed models. Specificresearch need in strength-based models is the develop-ment of better failure criteria based on the calculatedstresses, material properties and strengthening configu-ration. For better understanding of the brittle fracturephenomena in the debonding process, fracture tests onmaterials, systems and subsystems is needed for accuratecharacterization of the interface fracture energy andfracture criteria, including the effects of mixed fracturemodes.The modeling efforts described for monotonic load

conditions are yet to be extended for cyclic loadsconditions. Considering the ever-increasing field appli-cations of the method, there is an urgent need to fill theresearch gap in this area. Experimental and theoreticalresearch on cyclic load performance of FRP strengthenedRC and steel members must be performed to ensure theseismic safety of existing and future applications.Bond durability in strengthening systems must be

thoroughly investigated both experimentally and theo-retically in order to properly account for environmentaldegradation effects in the design and life cycle costestimation procedures. Influence of environmental expo-sure on strength and fracture properties of the interfacesmust be characterized through targeted experimentalinvestigations.Quality assurance of bond during installation and in

service is another major area where immediate attentionand increased emphasis is needed. Considering thelimited knowledge on mechanics and durability of thestrengthening applications, reliable NDT methods mayplay a vital role in assuring bond integrity and structuralsafety.

9. Conclusion

Debonding problems stand as a critical barrier againstwide range use of FRP composites in structural strength-ening and repair applications. Proper characterization ofdebonding problems and their inclusion in the designcodes is essential for common use of the technique. Areview of up to date research progress on debondingproblems is made with presentation of recent researchresults supporting the discussions, and future researchneeds specific to debonding problems are stated.

Acknowledgments

This material is partially based upon work supportedby the National Science Foundation under Grant No.0010126.

References

w1x ACI 318-99. Building Code Requirements for Structural Con-crete and Commentary. American Concrete Institute, Detroit,1999.

w2x ACI 440F. Guide for the Design and Construction of ExternallyBonded FRP Systems for Strengthening Concrete Structures.ACI Committee 440-F Draft Document, American ConcreteInstitute, Detroit, 2000.

w3x Adams RD, Comyn J, Wake WC. Structural adhesive joints inengineering. New York: Chapman and Hall, 1997.

w4x Ahmed O, Gemert DV, Vadewalle L. Improved model forplate-end shear of CFRP strengthened RC beams. CementConcrete Compos 2001;23:3–19.

w5x Ali MSM, Oehlers DJ, Bradford MA. Shear peeling of steelplates adhesively bonded to the sides of reinforced concretebeams. Proc Institution Civ Eng Struct Build 2001;140:249–59.

w6x Arduini M, Di Tommaso A, Nanni A. Brittle failure in FRPplate and sheet bonded beams. ACI Struct J 1997;94(4):363–70.

w7x Arduini M, Nanni A. Behavior of precracked RC beamsstrengthened with carbon FRP sheets. J Compos Constr1997;1(2):63–70.

w8x Baker AA, Jones R, editors. Bonded repair of aircraft struc-tures. Boston: Martinus Mijhoff Publishers, 1988.

w9x Barnes RA, Mays GC. Fatigue performance of concrete beamsstrengthened with CFRP plates. J Compos Constr1999;3(2):63–72.

w10x Bassetti A, Liechti P, Nussbaumer A. Fatigue resistance andrepairs of riveted bridge members. In: Proceedings of theInternational Conference Fatigue Design. Espoo, Finland,1998:535–546.

w11x Bassetti A, Nussbaumer A, Hirt MA. Crack repair and fatiguelife extension of riveted bridge members using compositematerials. In: El-Sheikh S, editor. Proceedings of the Interna-tional Bridge Engineering Conference. Tampa, FL, 2000:227–238.

w12x Buyukozturk O, Hearing B. Failure behavior of precrackedconcrete beams retrofitted with FRP. J Compos Constr1998;2(3):138–44.

w13x Buyukozturk O, Gunes O. Use of fiber reinforced plastics(FRP) in repair and strengthening of railroad bridges. AARAffiliated Lab Research Progress Report. Association of Amer-ican Railroads’ Technology Scanning Committee, 2000.

w14x Buyukozturk O, Gunes O, Karaca E. Characterization andmodeling of debonding failures in RC beams strengthened withFRP composites. In: Symth AW, editor. Proceedings of the15th ASCE Engineering Mechanics Conference. New York:ASCE, 2002a.

w15x Buyukozturk O, Gunes O, Karaca E. Debonding problems inseismic retrofitting of RC beams using FRP composites. In:Proceedings of the 7th US National Conference on EarthquakeEngineering. Boston: EERI, 2002b.

w16x Chajes MJ, Thomson TA Jr, Farschman CA. Durability ofconcrete beams externally reinforced with composite fabrics.Constr Build Mater 1995;9(3):141–8.

w17x Colotti V, Spadea G. Shear strength of RC beams strengthenedwith bonded steel or FRP plates. J Struct Eng2001;127(4):367–73.

w18x Di Tommaso A, Neubauer U, Pantuso A, Rostasy FS. Behaviorof adhesively bonded concrete–CFRP joints at low and hightemperatures. Mech Compos Mater 2001;37(4):327–38.

w19x El-Mihilmy MT, Tedesco JW. Prediction of anchorage failurefor reinforced concrete beams strengthened with fiber-rein-forced polymer plates. ACI Struct J 2001;98(3):301–14.


w20x El-Tawil S, Ogunc C, Okeil A, Shahawy M. Static and fatigueanalysis of RC beams strengthened with CFRP laminates. JCompos Constr 2001;5(4):258–67.

w21x EMPA. Internal Test Report 148’181. Swiss Federal Labora-tories for Materials Testing and Research. Zurich, Switzerland,1994a.

w22x EMPA. Internal Test Report 148’181y1. Swiss Federal Labo-ratories for Materials Testing and Research. Zurich, Switzer-land, 1994b.

w23x Fanning PJ, Kelly O. Ultimate response of RC beams strength-ened with CFRP plates. J Compos Constr 2001;5(2):122–7.

w24x Fukuzawa K, Numao T, Wu Z, Yoshizawa H, Mitsui M.Critical Strain Energy Release Rate of Interface DebondingBetween Carbon Fiber Sheet and Mortar. In: Non-Metallic(FRP) Reinforcement for Concrete Structures, Proceedings ofthe Third International Symposium. Sapporo, Japan, 1997:295–302.

w25x Garden HN, Hollaway LC, Thorne AM. A preliminary evalu-ation of carbon fibre reinforced polymer plates for strengthen-ing reinforced concrete members. P I Civil Eng–Str B1997;123:127–42.

w26x Garden HN, Hollaway LC. An experimental study of thefailure modes of reinforced concrete beams strengthened withprestressed carbon composite plates. Compos Part B1998;29(4):411–24.

w27x Green MF, Bisby LA, Beaudoin Y, Labossiere P. Effect offreeze–thaw cycles on the bond durability between fibrereinforced polymer plate reinforcement and concrete. Can JCiv Eng 2000;27(5):949–59.

w28x Hamilton HR III, Dolan CW. Durability of FRP reinforcementsfor concrete. Prog Struct Eng Mater 2000;2:139–45.

w29x Hamoush SA, Ahmad SH. Debonding of steel plate-strength-ened concrete beams. J Struct Eng 1990;116(2):356–71.

w30x Hassanen MAH, Raoof M. Design against premature peelingfailure of RC beams with externally bonded steel or FRPplates. Mag Concrete Res 2001;53(4):251–62.

w31x Hearing B. Delamination in Reinforced Concrete Retrofittedwith Fiber Reinforced Plastics. Ph.D. Thesis, MassachusettsInstitute of Technology. Cambridge, MA, 2000.

w32x Hearing B, Buyukozturk O. Delamination in Reinforced Con-crete Retrofitted with Fiber Reinforced Plastics. ResearchReport, Department of Civil and Environmental Engineering,Massachusetts Institute of Technology, Cambridge, MA, 2000.

w33x Inoue S, Nishibayashi S, Yoshino A, Omata F. Deformationcharacteristics, static and fatigue of reinforced concrete beamsstrengthened with carbon fiber-reinforced plastic plate. TransJpn Concrete Institute 1996;18:143–50.

w34x Jansze W. Strengthening of Reinforced Concrete Members inBending by Externally Bonded Steel Plates. Netherlands: DelftUniversity Press, 1997.

w35x Kaiser HP. Strengthening of Reinforced Concrete with Epoxy-Bonded Carbon Fibre Plastics. Doctoral Thesis. Diss. ETH Nr.8918, ETH Zurich, Switzerland,(in German), 1989.

w36x Karbhari VM, Shulley SB. Use of composites for rehabilitationof steel structures–determination of bond durability. J MaterCiv Eng 1995;7(4):239–45.

w37x Karbhari VM, Engineer M. Investigation of bond betweenconcrete and composites. J Reinf Plast Compos1996;15(2):208–27.

w38x Karbhari VM, Zhao L. Issues related to composite plating andenvironmental exposure effects on composite–concrete inter-face in external strengthening. Compos Struct 1998;40(3–4):293–304.

w39x Lau KT, Dutta PK, Zhou LM, Hui D. Mechanics of bonds inan FRP bonded concrete beam. Compos: Part B 2001a;32:491–502.

w40x Lau KT, Shi SQ, Zhou LM. Estimation of stress intensityfactor (KI) for an FRP bonded concrete beam using thesuperposition method. Mag Concrete Res 2001b;53(1):31–41.

w41x Leung CKY. Delamination failure in concrete beams retrofittedwith a bonded plate. J Mater Civ Eng 2001;13(2):106–13.

w42x Liu X, Silva P, Nanni A. Rehabilitation of Steel BridgeMembers with FRP Composite Materials. In: Figueiras J,Juvandes L, Furia R, editors. Proceedings of CCC 2001Composites in Construction, Porto, Portugal, 2001:613–617.

w43x Maalej M, Bian Y. Interfacial shear stress concentration inFRP-strengthened beams. Compos Struct 2001;54:417–26.

w44x Malek AM, Saadatmanesh H, Ehsani MR. Prediction of failureload of RyC beams strengthened with FRP plate due to stressconcentration at plate end. ACI Struct J 1998;95(2):142–52.

w45x Masoud S, Soudki K, Topper T. CFRP-strengthened and cor-roded RC beams under monotonic and fatigue loads. J ComposConstr 2001;5(4):228–36.

w46x McKnight SH, Bourban PE, Karbhari VM, Gillespie JW.Surface Preparation of Steel for Adhesive Bonding in Rehabil-itation Applications. In: Basham KD, editor. Infrastructure:New Materials and Methods of Repair, Proceedings of theThird Materials Engineering Conference. San Diego, CA:ASCE, 1994. pp. 1148-1155.

w47x Meier U, Kaiser H. Strengthening of Structures with CFRP. In:Iyer SL, Sen S, editors. Advanced Composite Materials inCivil Engineering Structures. Las Vegas, NV, 1991:224–232.

w48x Meier U. Carbon fiber reinforced polymers, modern materialsin bridge engineering. Struct Eng Int 1992;2(1):7–12.

w49x Miller T, Chajes MJ, Mertz DR, Hastings JN. Strengtheningof a steel bridge girder using CFRP laminates. J Bridge Eng2001;6(6):514–22.

w50x Mukhopadhyaya P, Swamy RN, Lynsdale CJ. Optimizingstructural response of beam strengthened with GFRP plates. JCompos Constr 1998;2(2):87–95.

w51x Naboulsi S, Mall S. Modeling of cracked metallic structurewith bonded composite patch using the three layer technique.Compos Struct 1996;35:295–308.

w52x Neubauer U, Rostasy FS. Bond Failure of Concrete FiberReinforced Polymer Plates at Inclined Cracks—Experimentaland Fracture Mechanics Model. In: Dolan DW, Rizkalla SH,Nanni A, editors. Proceedings of the Fourth InternationalSymposium. Non-Metallic(FRP) Reinforcement for ConcreteStructures. Baltimore, USA, 1999:369–382.

w53x Neubauer U, Rostasy FS. Design Aspects of Concrete Struc-tures Strengthened with Externally Bonded FRP-Plates. In:Forde MS, editor. Proceedings of the 7th International Confer-ence on Structural Faults and Repair. Edinburgh, UK,1997:109–118.

w54x Nguyen DM, Chan TK, Cheong HK. Brittle failure and bonddevelopment length of CFRP–concrete beams. J ComposConstr 2001;5(1):12–7.

w55x Oehlers DJ. Reinforced concrete beams with plates glued totheir soffits. ASCE J Struct Eng 1992;118(8):2023–38.

w56x Pareek S, Kurata M, Sotoyama R. Flexural strengthening ofreinforced concrete beams by continuous fiber sheets. TransJpn Concrete Institute 1999;21:201–8.

w57x Quantrill RJ, Hollaway LC, Thorne AM. Predictions of themaximum plate end stresses of FRP strengthened beams: partII. Mag Concrete Res 1996;48(177):343–51.

w58x Rabinovitch O, Frostig Y. Closed-form high-order analysis ofRC beams strengthened with FRP strips. J Compos Constr2000;4(2):65–74.

w59x Rabinovitch O, Frostig Y. Delamination failure of RC beamsstrengthened with FRP strips—a closed-form high-order andfracture mechanics approach. J Eng Mech 2001;127(8):852–61.


w60x Rahimi H, Hutchinson A. Concrete beams strengthened withexternally bonded FRP plates. J Compos Constr 2001;5(1):44–56.

w61x Raoof M, Zhang S. An insight into the structural behaviour ofreinforced concrete beams with externally bonded plates. ProcInstitution Civ Eng Struct Build 1997;122:477–92.

w62x Raoof M, Hassanen MAH. Peeling failure of reinforced con-crete beams with fibre-reinforced plastic or steel plates gluedto their soffits. Proc Institution Civ Eng Struct Build2000;140:291–305.

w63x Ritchie PA, Thomas DA, Lu LW, Connely GM. Externalreinforcement of concrete beams using fiberglass reinforcedplastics. ACI Struct J 1991;88(4):490–500.

w64x Roberts TM. Approximate analysis of shear and normal stressconcentrations in the adhesive layer of plated RC beams. StructEng 1989;67(12):229–33.

w65x Roberts TM, Haji-Kazemi H. A theoretical study of thebehavior of reinforced concrete beams strengthened by exter-nally bonded steel plates. P I Civil Eng Pt 2 1989;87:39–55.

w66x Rose LRF. A cracked plate repaired by bonded reinforcements.Int J Fract 1982;18(2):135–44.

w67x Ross CA, Jerome DM, Tedesco JW, Hughes ML. Strengtheningof reinforced concrete beams with externally bonded compositelaminates. ACI Struct J 1999;96(2):212–20.

w68x Saadatmanesh H, Ehsani MR. Fiber composites can strengthenbeams. Concrete Int: Des Constr 1990;12(3):65–71.

w69x Schubbe JJ, Mall S. Investigation of a cracked thick aluminumpanel repaired with a bonded composite patch. Eng Fract Mech1999;62:305–23.

w70x Sebastian WF. Significance of midspan debonding failure inFRP-plated concrete beams. J Struct Eng 2001;127(7):792–8.

w71x Sen R, Liby L, Mullins G. Strengthening steel bridge sectionsusing CFRP laminates. Compos: Part B 2001;32:309–22.

w72x Shahawy MA, Arockiasamy M, Beitelman T, Sowrirajan R.Reinforced concrete rectangular beams strengthened with CFRPlaminates. Compos: Part B 1996;27B:225–33.

w73x Shahawy MA, Beitelman TE. Static and fatigue performanceof RC beams strengthened with CFRP laminates. J Struct Eng1999;125(6):613–21.

w74x Sharif A, Al-Sulaimani GJ, Basunbul IA, Baluch MH, GhalebBN. Strengthening of initially loaded reinforced concrete beamsusing FRP plates. ACI Struct J 1994;91(2):160–8.

w75x Shen HS, Teng JG, Yang J. Interfacial stresses in beams andslabs bonded with thin plate. ASCE J Eng Mech2001;127(4):399–406.

w76x Smith ST, Teng JG. Interfacial stresses in plated beams. EngStruct 2001;23(7):857–71.

w77x Smith ST, Teng JG. FRP-strengthened RC beams II: assessmentof debonding strength models. Eng Struct 2002;24(4):397–417.

w78x Spadea G, Swamy RN, Bencardino F. Strength and ductility ofRC beams repaired with bonded CFRP laminates. J BridgeEng 2001;6(5):349–55.

w79x Swamy RN, Mukhopadhyaya P. Debonding of carbon-fibre-reinforced polymer plate from concrete beams. P I Civil Eng–Str B 1999;134:301–17.

w80x Taljsten B. Concrete Beams Strengthened for Bending UsingCFRP-Sheets. In: Forde MC, editor. Proceedings of the EighthInternational Conference on Extending the Life of Bridges,CivilqBuilding Systems. Structural FaultsqRepair-99. Lon-don, UK, 1999.

w81x Taljsten B. Strengthening of concrete prisms using the plate-bonding technique. Int J Fract 1996;82:253–66.

w82x Taljsten B. Strengthening of beams by plate bonding. J MaterCiv Eng 1997;9(4):206–11.

w83x Tavakkolizadeh M, Saadatmanesh H. Galvanic corrosion ofcarbon and steel in aggressive environments. J Compos Constr2001;5(3):200–10.

w84x Tong L, Steven GP. Analysis and design of structural bondedjoints. Boston: Kluwer Academic, 1999.

w85x Toutanji H, Gomez W. Durability characteristics of concretebeams externally bonded with FRP composite sheets. CementConcrete Compos 1997;19(4):351–8.

w86x Triantafillou TC, Plevris N. Strengthening of RC beams withepoxy-bonded fibre-composite materials. Mater Struct1992;25:201–11.

w87x Varastehpour H, Hamelin P. Strengthening of concrete beamsusing fiber-reinforced plastics. Mater Struct 1997;30:160–6.

w88x Wu Z, Matsuzaki T, Tanabe K. Interface Crack Propagation inFRP-Strengthened Concrete Structures. In: Proceedings of theThird International Symposium. Non-Metallic(FRP) Rein-forcement for Concrete Structures. Sapporo, Japan, 1997:319–326.

w89x Yuan H, Wu Z. Energy release rates for interfacial crack inlaminated structures. Struct Eng Earthquake Eng2000;17(1):1–12.

w90x Yuceoglu U, Updike DP. Stress analysis of bonded plates andjoints. J Eng Mech Div 1980;106(EM1):37–56.

w91x Zhang S, Raoof M, Wood LA. Prediction of peeling failure ofreinforced concrete beams with externally bonded steel plates.P I Civil Eng–Str B 1995;110:257–68.

w92x Ziraba YN, Baluch MH, Basunbul IA, Sharif AM, Azad AK,Al-Sulaimani GJ. Guidelines toward the design of reinforcedconcrete beams with external plates. ACI Struct J1994;91(6):639–46.

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