2004_progress on understanding debonding prob using frp_ob, ek, & og

8/10/2019 2004_Progress on Understanding Debonding Prob Using FRP_OB, EK, & OG

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Construction and Building Materials 18 ( 2004 ) 919

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 of these 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


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10 O. Buyukozturk et al. / Construction and Building Materials 18 (2004) 919

Fig. 1. Elastic and strength properties of FRP composites compared with conventional construction materials, ( a) Elastic ( Youngs ) modulus ( b)Stressstrain 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 strengthstrain 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 well

justified, 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 tovarious 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 and yor flexural strengthening of RC and steel

beams 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 flexuralmembers 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


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11O. Buyukozturk et al. / Construction and Building Materials 18 (2004) 919

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

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

delamination, (5) FRP debonding w2,12x. 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 of these 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 place

in 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 of debonding 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


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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 w47,48 x. 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 failures w68x. 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 reinforcement w25,43,60,63,67,72,74,86 x. For a fixed FRP ratio, debonding poten-tial was shown to increase significantly with increasingFRP thickness w25x. 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 eliminatedw80,32,4,23,54 x. Laboratory test studies on FRP strength-ened beams with notches in the shear span or mid-span

revealed that unstable debonding may also originatefrom flexural and flexural yshear cracks w88,31,70 x. Anumber of researchers have investigated debondingproblems in beams precracked before strengthening.Mixed conclusions were drawn from these studies,calling for further research on this issue w6,7,32,60 x.Role of bond anchorage in increasing debonding resis-tance of strengthened beams was demonstrated byseveral studies w25,26,50,56,74,78,79 x. 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. 5 w14,15 x. 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 loading w9,20,33,45,73 x. 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 concreteFRP interface maylead to debonding problems w16,18,27,28,38,85 x. 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 of research 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 girder w21,22 x. This study revealedthat FRP composites can increase the remaining life of fatigue-damaged members and that debonding was thegoverning failure mechanism. Basetti et al. performedlaboratory tests on FRP repaired center-notched coupons

and later implemented the method on a riveted bridgew10,11 x. 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 repair w13x. Results from this study,part of which are shown in Fig. 6, indicates that fatigue


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Fig. 6. Fatigue performance of FRP strengthened 3 y8 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 of FRP 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 w13,36,46,83 x. 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 steelmembers w42,71 x. A field application of the method wasperformed to strengthen a bridge girder in Newark,Delaware w49x.

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 of debonding 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 concreteFRP interface and in theFRP reinforcement in a RC beam strengthened in flex-ure. Similarly, Fig. 7b shows the stresses developed in

a 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 inFRP 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.


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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 Roberts w64x, 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 of this solution were used by other researchers for FRPand steel plated beams w19,87,92 x. 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 w39,44,76,82 x. 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 developed w58,75 x. Acomparison of the results obtained from higher-orderand approximate solutions shows close agreement exceptfor the previously mentioned small end zones w75x.

Following the calculation of bond stresses, a failurecriterion 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 reach

certain values w57,65 x. Alternative failure criteria includeconcrete failure under biaxial stresses w19,44 x or aMohrCoulomb type criterion w87,92 x.

Determination of bond stress distributions in FRPstrengthened 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 texts w3,84x.A general numerical solution procedure was developedby Yuceoglu and Updike w90x 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 criteria w3,84x.

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 Ahmad w29x used Linear Elas-tic Fracture Mechanics (LEFM ) and finite elementmethod to model debonding in steel plated concretebeams with no internal reinforcement. Taljsten w81xderived 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 double

shear specimens pulled in tension. In a similar study,Neubauer and Rostasy w53x performed bond strengthtests to determine the FRPconcrete interface fractureenergy. Karbhari and Engineer w37x 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 of debonding around the notch was measured. Results werevalidated by the compliance method through a finite


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element analysis. Yuan and Wu w89x later developed ananalytical methodology to model debonding initiated atthe FRP concrete interface at either middle or end of the laminate by use of fracture mode partitioning.Hearing and Buyukozturk w32x employed a fractureenergy based criterion to predict debonding failures inreinforced concrete beams. Leung w41x derived the shearstress distribution at the FRPconcrete 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 mode I stress intensity factor, K , at the I 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 reduction

factor of 0.9 to include such effects. Rabinovitch andFrostig w59x extended their high-order bond strengthmodel w58x 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 crack patching 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 of the fatigue life w51,66,69 x. However, characteristics of the 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 strengthening yrepairapplications 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 beam w4,5,34,55,77 x. 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 beam w30,61,62,91 x. 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 plateconcrete interface w17x. Investigation on the validity of the 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 loading w9,20,33,45,73 x, 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-urations w15x. It is seen from the figure that performanceof FRP strengthened beams under cyclic loading mayfall below that under monotonic loading, the degree of which is dependent on the strengthening parameters andanchorage conditions.

Potential effects of cyclic loading on retrofitted beamperformance 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 in

the FRP composite reinforcement and at the concreteFRP 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 FRPstrengthened 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


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

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 beam w15x. In suchcases, not only the retrofitting is ineffective, but alsoharmful to the structure since the ductility of the memberis decreased significantly. ACI 440F w2x requires thatfor beams strengthened only in flexure, it must beensured that the shear capacity of the beam, calculatedin accordance with ACI 318 w1x, 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 FRPcomposites was shown to be an effective technique.Although the system is likely to perform satisfactorilyunder low amplitude fatigue loading, performance of such systems under high amplitude cyclic loading con-ditions is unknown. Considering that partial debonding

in 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 observed w16,18,27,28,38,85 x.Similarly, studies on bond durability of FRP bondedsteel systems have reported deterioration caused bytemperature and moisture cycles w13,36,46,83 x. In addi-tion, potential galvanic corrosion problems associatedwith strengthening of steel members with FRPs contain-ing conductive fibers such as carbon raises durability

concerns. 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 strengthened yrepairedRC 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


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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 loadconditions are yet to be extended for cyclic loads

conditions. 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 bethoroughly 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 inservice 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 of debonding 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.

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2004_progress on understanding debonding prob using frp_ob, ek, & og

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