STEEL-FRP COMPOSITE STRUCTURAL SYSTEMS Kent A. Harries University of Pittsburgh, Civil and Environmental Engineering Pittsburgh, PA USA [email protected] Sherif El-Tawil University of Michigan, Civil and Environmental Engineering Ann Arbor MI USA [email protected]

ABSTRACT Most extant fiber reinforced polymer (FRP) applications in the structural engineering field involve concrete-FRP composite systems, where FRP components are attached to or embedded into concrete structures to improve their structural performance. A relatively small and innovative body of work focusing on steel-FRP composite structural systems is beginning to develop. A state-of-the-art review of progress in this emerging and rapidly developing area is presented. The review is divided into applications: strengthening steel structures with FRP, stabilizing (or bracing) buckling-critical steel elements with FRP and relieving fatigue or fracturecritical conditions. Discussions of concerns pertaining to joining FRP and steel as well as the potential and future of this technology are also presented.

INTRODUCTION The use of fiber reinforced polymer (FRP) composite materials has become relatively common in infrastructure applications. Most existing applications involve concrete-FRP composite members. Nonetheless a relatively small and innovative body of work is developing focusing on steel-FRP composite structural systems. Steel-FRP composite systems are almost exclusively aimed at retrofit methods for the underlying steel material or structure. Whether strengthening, repairing fractures, relieving stress to enhance fatigue performance or enhancing local or member stability, steel-FRP systems leverage the unique material properties of each material in establishing a composite member or structure. The objective of this paper is to synthesize existing information on the use of composite FRP/steel structural systems and to establish the state-of-the-art and state-of-practice in this nascent field. Composite bridge structures comprising FRP decks on steel girders are not discussed in the present work. Such systems amount to little more than direct replacement of a concrete deck with a FRP deck and there is a significant body of work reporting this application. FIBER REINFORCED POLYMER (FRP) COMPOSITE MATERIALS Fiber reinforced polymer (FRP) composite materials combine high-modulus, high strength fibers in a low-modulus polymeric matrix which ensures load transfer between the fibers. The strength and stiffness of an FRP composite is largely determined by the fiber type and fiber architecture. FRP materials that are suited to civil infrastructure and structural engineering applications

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typically have high fiber-volume fractions. Orientation of the fibers is controlled so that the resulting FRP is anisotropic. FRPs enable high loads to be carried safely and allow the material to be tailored to suit the local structural demands in the component to be retrofitted. The inservice performance of a composite is influenced both by the fiber and matrix material. Carbon (CFRP) and glass (GFRP) FRP materials are ubiquitous in the field of structural retrofit. CFRP may be high strength (hsCFRP), high modulus (hmCFRP) and, most recently, ultra-high modulus (uhmCFRP). Generally an increase in CFRP stiffness is accompanied by a reduction in strength and rupture strain of the fiber. GFRP (most typically based on E-glass fibers) have a much lower modulus than CFRP and are somewhat less expensive on a unit stiffness basis. To be effective in strengthening applications, the modulus of the FRP selected for a particular application should be compatible with the substrate material. For this reason, CFRP materials have often been used with a steel substrate. A polymeric matrix binds and protects the fibers of an FRP, transferring force into, and between, fibers through interfacial shear. The matrix also provides stability and environmental protection to the slender, delicate fibers. Epoxy resin systems are most commonly used as the matrix in hand lay-up applications and as the adhesive in plate bonding techniques. Polyester resin systems are often used as the matrix material in preformed composite materials such as those used for plate bonding applications. In terms of ease of handling, installation and quality control, preformed CFRP plates or strips are rapidly becoming the preferred products for structural retrofit. The exception is that wet layup fabrics remain appropriate for applications involving retrofit forming around corners. In either case the resulting system has a steel-adhesive-FRP interface region. Table 1 provides a summary of representative basic material properties for each layer in the system. The FRP properties are given for the composite strip product rather than for the raw fibers. Hand lay-up products will typically have lower strengths and stiffnesses than those given in Table1 since the resulting fiber volume ratio is typically lower than in preformed systems. Table 1 – Typical properties of steel-adhesive-FRP systems.

tensile modulus, GPa tensile strength, MPa ultimate strain, % 3 density, kg/m -6 o CTE, 10 / C a o Tg , C shear strength, MPa bond strength, kPa a

Preformed FRP Strips

Mild Steel

hsCFRP

hmCFRP

uhmCFRP

GFRP

200 276-483 18-25 7530 21.6 -

166 3048 1.8 ~1618 ~0 149 -

207 2896 1.4 ~1618 ~0 149 -

304 1448 0.5 ~1618 ~0 149 -

42 896 2.2 ~2146 8.8 resin -

Adhesive high low modulus modulus 4.5 0.4 25 4.8 1.0 >10 ~1201 ~1201 162 n.r. 63 24.8 9.0 ~20.7 ~5.0

Tg = glass transition temperature – associated with vitrification of polymer matrix

STRUCTURAL RETROFIT WITH FRP A great deal of work has been conducted on the use of externally bonded FRP systems for structural strengthening of building and bridge systems and components. The overwhelming majority of this work has focused on the retrofit of reinforced concrete structures. In virtually every existing application, FRP materials are used to supplement steel reinforcement. Indeed, early FRP external bonding applications were developed as an alternative to heavy and awkward steel plate bonding techniques. In some applications, the analogy to steel is appropriate; however this paradigm is restrictive and does not always result in an efficient use of the unique properties of FRP materials: their high stiffness, strength and linear behavior.

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There is a variety of design guidance available for the use of externally bonded FRP materials for the retrofit and rehabilitation of concrete structures (ACI 2002 in the USA). Current research in the area has a strong emphasis on issues of the durability of the bonded FRP and the nature and behavior of the bonding mechanism itself. The bond of FRP to concrete can only be as strong as the substrate concrete and may fail through a variety of mechanisms, some associated with classical debonding mechanisms and others associated with the behavior and condition of the concrete substrate. Provided adequate quality control is executed, the behavior of externally bonded FRP will be largely governed by the substrate concrete. This will not be the case for a stronger steel substrate, allowing more conventional bond mechanics to be used to describe debonding behavior. RETROFIT OF STEEL MEMBERS WITH FRP There is comparatively little work investigating the use of bonded FRP materials for the retrofit of steel members. Most available research and guidance focuses on the use FRP for flexural retrofit: generally, applying FRP materials to the tensile flange of a section to increase its capacity. In Great Britain, work has been done in using externally bonded FRP to rehabilitate degraded or understrength (often due to increased load demands) cast and wrought iron structures (Moy 2001). Hollaway (2004) reported on European applications of CFRP to strengthen cast iron bridges including the first application on the Bures Bridge in Suffolk, UK and another on the Hythe Bridge, Oxford, UK. Both applications required the pre-fabrication of custom CFRP components to be fitted to the bridges to increase their load carrying capacity. Garden (2001) reports the case of a curved steel beam strengthened using a low temperature moulding hybrid carbon/glass fiber reinforced polymer composite prepreg materials with a unidirectional fiber orientation along the direction of the beam and a 0o/90o orientation employed to resist the shear and torsional loading. Cadei et al. (2004) reports on thirteen applications of strengthening cast iron structures and six instances of strengthening steel structures; no applications on wrought iron are known. The selection of FRP for cast or wrought iron is largely for aesthetic purposes. It is believed that the capacity of the member can be increased using the high performance FRP materials while maintaining the architectural integrity of the member and limiting the increase in member dimension and structural weight. It is noted that while Cadei et al. is prepared as a guideline, the document presents design from a theoretical point of view and notes: “Experimental testing is recommended to validate the design procedures presented herein. Appropriate test programs should also be undertaken as a part of major strengthening schemes.” A lengthy list of requirements for future research and a sparse list of demonstration applications illustrate that the use of FRP to retrofit steel members is very much in its infancy. Nonetheless, the Cadei et al. document represents the state-of-the-art for FRP strengthening of steel structures. STRENGTHENING OF FLEXURAL ELEMENTS In the United States, two NCHRP-IDEA projects (IDEA-011 (Mertz and Gillespie 1996) and IDEA-051 (Mertz et al. 2002)) were performed at the University of Delaware in the late 1990’s. These projects focused on the flexural strengthening of corroded bridge girders and addressed the use of bonded FRP materials on only the tension flange of simple girders. The rationale was that the bottom flanges of bridge girders typically see the greatest level of corrosion, largely due to debris accumulation. In the earlier study, Mertz and Gillespie (1996) report on six small scale tests of 1525 mm long W200x15 members retrofit with the five different adhesively bonded schemes; shown in Figure 1 (the fifth scheme was similar to that shown in Fig 1(a) using a different CFRP material.) All

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specimens demonstrated an increase in flexural stiffness and strength compared to the unstrengthened control specimen. As might be expected, the “sandwich-reinforced” specimen (Fig. 1(b)), having the additional FRP material located a distance below the flange showed the greatest improvement in stiffness and elastic strength (although the second CFRP strip retrofit Fig 1(a) was comparable for both parameters). The “composite-wrapped” specimen (Fig. 1(c)) showed the greatest increase in ultimate capacity since the composite wrap surrounding the foam core provided better shear transfer through higher loads than the aluminum honeycomb (Although, again, the second CFRP strip method was comparable.) All specimens tested failed in a debonding mode of failure, with the CFRP peeling from the flange at its ends. The second CFRP strip specimen, and the specimen having a pultruded channel retrofit (Fig. 1(d)), did not debond until loads greater than the unstrengthened beam failure load were exceeded.

Figure 1 – Small-scale specimens tested by Mertz and Gillespie. Two corroded 600 mm deep girders (similar to W610x125 but with tapered flanges), recovered from a 1940-era Pennsylvania bridge, were tested using a strengthening scheme similar to that shown in Fig 1(a), having CFRP strips applied to both the top and bottom of the tension flange. The CFRP strengthening was able to increase the stiffness and moment capacity of the corroded girders. The first girder had a stiffness and strength, prior to strengthening, of approximately 87% of that expected for a new, uncorroded girder. In this case, the stiffness was increased to that of the uncorroded girder and the strength was found to exceed that of the uncorroded girder. The second girder had an original condition resulting in about 62% of its uncorroded stiffness. In this case the strengthening significantly improved the behavior but was unable to restore the uncorroded stiffness or strength. The ultimate capacity of both specimens was controlled by buckling of the compression flange which was not addressed in the strengthening scheme. Nonetheless, the tension flange did yield and no debonding of the CFRP was observed in either specimen. Having demonstrated the viability of strengthening steel with CFRP strips, Mertz et al. (2002) extended their initial work with pilot studies investigating the transfer length of bonded CFRP and the fatigue performance of FRP strengthened girders. Simple steel tensile specimens having CFRP strips bonded to both faces were used to investigate the force transfer between steel and CFRP. In all specimens, regardless of load level considered, approximately 99% of the total force transfer occurred within the first 75 to 100 mm (3 to 4 in.) of the bonded reinforcement. This result is consistent with standard calculations of “effective bond length” which is considered to be a characteristic property of thin bonded FRP systems (Teng et al. 2002). Mertz et al. recognize the importance of a thin adhesive layer in allowing stress transfer between the steel substrate and CFRP. However, an efficient stress transfer exacerbates debonding stresses. Despite this, Mertz et al. only report debonding in one tension specimen at strains corresponding to yield of the underlying steel specimen. They attribute the lack of debonding in the other specimens to the use of adhesive having a low shear modulus and high

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elongation properties. Fatigue performance of CFRP bonded to the tension flange of simply supported specimens revealed little degradation of specimen stiffness or strain carried by the CFRP and no apparent debonding after several million cycles of loading. Mertz et al. (2002) and Miller et al. (2002) report on a field installation on a single girder of a bridge carrying I-95 Southbound over Christina Creek outside Newark Delaware. This installation was a demonstration of the techniques developed in the IDEA projects. A single W610x125 girder spanning 7.5 m was retrofit with CFRP on the bottom of the tension flange. Load tests indicated a reduction in tension flange strains of 11.4% under a vehicle approximately equivalent to an AASHTO H32 loading. Chacon et al. (2004) report a related demonstration project involving the strengthening of two W24x100 floor beams of the Ashland Bridge carrying State Route 82 over Red Clay Creek in Delaware. In this application a decrease in tension flange strain of 5.5% was observed under service load conditions. Neither of these demonstration strengthening projects were strictly necessary – the strengthened girder and floor beams did not require strengthening. Rather, the projects are intended to serve as test beds to investigate the long term performance and durability of CFRP strengthening techniques for steel bridge girders. Patnaik and Bauer (2004) report on an experimental program of CFRP strengthened steel beams. This study also addresses flexural strengthening by adhering CFRP strips to the tension flange. As expected, the moment capacity of the beam was increased - in this case by about 14%. Both strengthened beams are reported to have failed due to lateral torsional buckling, however the authors report a final failure involving rupture of the CFRP. This latter observation demonstrates that in this application, bond of the CFRP to the steel substrate was adequate. A second series of tests reported by Patnaik and Bauer (2004) involved 350 mm deep beams having intentionally slender – 3 mm wide by 325 mm tall – webs intended to investigate shear strengthening with CFRP. As expected, the unstrengthened beam failed due to elastic web buckling prior to flexural yielding. The application of vertically oriented, unidirectional CFRP to both sides of the web is reported to have allowed the section to yield prior to the onset of inelastic web buckling. Furthermore, although significant debonding of the short CFRP strips was evident, the failure is reported to have been ductile and “it was possible to sustain the load for a short time even after the initiation of web shear buckling.” Schnerch et al. (2004) investigated resin and adhesive selection for wet lay-up of carbon fiber sheets and bonding of pre-cured laminate plates and their effect on the flexural behavior of a steel monopole. Resin selection for the wet lay-up process was determined through testing of double lap shear coupons using ten different resins. Test results showed a 25% increase in the elastic stiffness of the steel monopole resulting from the application of a limited number of CFRP sheets. Other similar investigations of the use of CFRP strips attached to the tension flange of I-girders have demonstrated generally improved flexural capacity – proportional to the CFRP applied – although little improvement to girder stiffness (Sen et al. 2000; Tavakkolizadeh and Saadatmanesh 2003a; Lenwari et al. 2005). In such applications, Lenwari et al. (2006) demonstrated that the stress intensity at the ends of CFRP plates governs the debonding strength and that this region was critical for the initiation of debonding. Colombi and Poggi (2006a) observed similar behavior but also demonstrated a substantial increase in the post-yield stiffness provided CFRP debonding could be mitigated. Al-Saidy et al. (2004) investigated the effect of CFRP plates on the behavior of composite steelconcrete beams. The investigation focused on the behavior of beams damaged intentionally at their tension flange to simulate corrosion and then repaired with CFRP plates attached to the tension area. Damage varied between no damage and a loss of 75% of the bottom flange. The

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test results showed that the elastic flexural stiffness of damaged beams can be partially restored (up to 50%); whereas the strength of damaged beams can be fully restored to their original, undamaged state using the particular CFRP plates investigated. Similarly, Photiou et al. (2006) reported a series of tests on artificially degraded flexural beams where the failure load for all specimens exceeds the plastic collapse load of the undamaged beam. Furthermore, by using Ushaped FRP applications extending up the web to the neutral axis, composite action was provided between the steel member and fiber layer leading to better performance and mitigating debonding even at failure levels. STRENGTHENING OF TENSILE ELEMENTS Jiao and Zhao (2004) tested 21 butt-welded very high strength (VHS) steel tubes strengthened with CFRP in axial tension. Three types of failure were reported: adhesive failure, fiber rupture and mixed failure. The authors concluded that significant strength increase was achieved using the epoxy bonded CFRP strengthening technique and proposed a theoretical model to estimate the load carrying capacity of butt-welded VHS tubes strengthened with CFRP An experimental and numerical study was carried out by Colombi and Poggi (2006b) to verify the effectiveness of CFRP pultruded plates to reinforce steel tensile members. Three different sets of specimens were tested under uniaxial tension. In the first group, CFRP plates were used as double side reinforcement for continuous steel plates. In the second group, double lap specimens were tested to study the force transfer capability of the plates. Bolted joints were strengthened with CFRP in the final group. Force transfer and failure mechanisms were evaluated from both analytical and numerical models. The results were mixed and showed that the strengthening methods used for all three test groups lead to a mild increase in capacity for some specimens but did not realize any benefit for other cases. RETROFIT OF STEEL CONNECTIONS Mosallam et al. (1998) present a pilot study investigating the use of CFRP T-sections for strengthening steel moment connections in seismic regions. The proposed detail resembles a typical welded haunch bracket (AISC 1999) installed on both top and bottom flanges. The CFRP haunch bracket is reported to have exhibited improved rotational capacity over a comparable welded steel haunch. It is believed that the improved deformability results from the greater flexibility of both the CFRP haunch itself and the bonding adhesive which permits a more uniform force transfer than a fully welded connection. REPAIR OF FATIGUE DAMAGE AND ENHANCING FATIGUE LIFE USING FRP MATERIALS FRP materials, particularly CFRP, exhibit excellent performance when subject to fatigue loads. In conditions of tension fatigue where environmental effects are not affecting behavior, CFRP behavior is dominated by the strain-limited creep-rupture process. Plotted on a semi-log S-N curve, CFRP composites exhibit strength degradation due to tensile fatigue on the order of 5 to 8% per decade of logarithmic cycles (Curtis 1989). Additionally, CFRP composites do not generally exhibit a clearly defined endurance limit under conditions of tension fatigue. Fatigue performance of FRP composites may be affected by a variety of factors (Agarwal and Broutman 1990). Fatigue performance of CFRP has been shown to be relatively unaffected by changing fiber type but the S-N response may shift significantly as the matrix composition is changed (Curtis 1989; Boller 1964). Fiber orientation relative to loading direction (Boller 1964) and laminate architecture in multi-directional FRP composites (Davis et al. 1964) can also significantly affect fatigue behavior. Although clearly fiber-volume ratio of an FRP composite will

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affect the gross section stress capacity, reduced fiber volume has been shown to result in a reduced rate of strength degradation with cycling (Tanimoto and Amijima 1975). It has also been shown that the fatigue life of CFRP bars depends on the mean stress and the stress ratio (minimum-to-maximum stress). Higher mean stress or a lower stress ratio causes a reduction in fatigue life (Rahman and Kingsley 1996; Saadatmanesh and Tannous 1999). Increased ambient temperature, even within a range typical of infrastructure applications (20oC to 40oC) is known to be detrimental to the fatigue performance of FRP materials, affecting the stress range although not typically the rate of degradation (Adimi et al. 1998; Agarwal and Broutman 1990). Jones and Civjan (2003) used fracture specimens having CFRP applied to one or both sides to evaluate the ability of a CFRP overlay to enhance fatigue performance. Center-hole specimens with crack initiators and edge-notched specimens made of cold-rolled A36 steel having measured yield and ultimate stresses of 345 MPa and 490 MPa, respectively, were used. Increases in fatigue life were reported for all specimens tested, although the effectiveness of the FRP (and thus the fatigue life enhancement) was dominated by the adhesive behavior. Tavakkolizadeh and Saadatmanesh (2003b) presented the results of a study on the retrofitting of notched steel beams with CFRP patches for medium cycle fatigue loading. Twenty one S127x4.5 A36 steel beams were prepared and tested under four-point bending. The number of cycles to failure, changes in the stiffness and crack initiation and growth in the specimens were compared to unretrofitted specimens. The authors concluded that the CFRP patches not only extended the fatigue life of the notched detail more than three times, they also decreased the crack growth rate significantly. Nozaka et al. (2005) report a fundamental study of the use of CFRP strips for the repair of fatigue-damaged tension flanges of steel I-girders. The focus of this study was to establish appropriate values for the effective bond length for such repairs. A variety of repair configurations were tested including providing a gap (bonded and unbonded), no gap, and fully bonded or partially bonded CFRP in the region of the existing fatigue crack. Additionally two CFRP systems and five adhesive systems were tested. The results reported the greatest increase in strength resulting from the system using both the CFRP and adhesive with the lowest moduli of elasticity of those considered. Liu et al. (2006) report a study of the direct tension fatigue behavior of bonded CFRP sheets used to create “strap joints” between two steel plates. This study reported an apparent fatigue limit of 40% of the ultimate static strength of the strap joint specimens. Below this limit specimen failure and steel-CFRP bond behavior was not affected by the applied fatigue loads. ENHANCING STABILITY OF STEEL ELEMENTS FRP composite materials have recently been used to enhance the stability of steel members. In this application, the high stiffness and linear behavior of FRP materials are utilized to provide “bracing” that improves the buckling and post buckling behavior of steel components. Recent research has demonstrated that the application of FRP reinforcement can lead to improvements in the flange local buckling (FLB), web local buckling (WLB) and lateral torsional buckling (LTB) behavior of steel members. This application is not aimed at increasing the load carrying capacity of the steel section, per se; although this may certainly be accomplished if desired. Rather, the application is aimed at providing stability (in the sense of bracing) to the steel member through the addition of supplemental stiffness at strategic locations. Ekiz et al. (2004) demonstrated the use of CFRP wraps to enhance the plastic hinge behavior of double-channel members modeled on chord members of a special truss moment frame. Two cases are considered, one where the entire gross cross section is wrapped, the second where

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only the extending flanges are wrapped; both methods exhibited improved behavior of the hinge as compared to unwrapped specimens. Ekiz et al. report that the presence of the CFRP wrap increased the size of the yielded plastic hinge region, inhibited the occurrence of local buckling and delayed the onset of lateral torsional buckling. These effects resulted in reduced strain demands, increased rotational capacity, and improved energy dissipation capacity in the plastic hinge region. In a study investigating the use of CFRP to strengthen hollow structural square (HSS) columns, Shaat and Fam (2006) report on concentric axial load tests of squat HSS 88.9x88.9x3.2 sections wrapped with both longitudinal and transversely oriented CFRP sheets. Axial compression strength increases on the order of 8% to 18% and axial stiffness increases (resulting from the longitudinally oriented CFRP) of between 4% and 28% are reported. They suggested that the transverse CFRP can help restrain outward directed local buckling of the HSS walls. Similar tests on long HSS columns did not show similar results since behavior was dominated by initial eccentricities rather than local buckling. Accord and Earls (2006) present an analytical study wherein the effects that bonded low modulus GFRP strips have on the inelastic cross-sectional response of I-shaped sections developing plastic hinges under a moment-gradient loading is investigated. This work demonstrated that the presence of the GFRP strips enhanced the structural ductility of the cross-section as a result of providing effective bracing of the flange outstands, and thus inhibiting the formation of the local buckles in the compression flange of the cross-section. As the location of the GFRP strips was adjusted to increase their efficacy as bracing elements, a concomitant increase in structural ductility was noted; thus supporting the notion that the GFRP employed in this fashion enhances the overall performance of the steel member through the bracing that it provides against dominant plate buckling modes. Harries et al. (2008) demonstrated the concept of strategically applying FRP material to a steel compression member in order improve global and local buckling behavior. Improvement in loadcarrying capacity was found to be proportional to the increase in effective radius of gyration (ry) affected by the presence of the FRP. For elastic buckling, the entire section is considered in which case the increase in ry is nominal. For local (plastic) buckling, however, only the outstanding plate element (WT stem, in the cases tested) is considered in which case the proportional improvement in capacity is greater. Harries et al. showed that prior to FRP debonding, the presence of the FRP controls the plastic buckling and delays the formation of the plastic “kink”. The formation of this kink affects the cyclic compressive capacity of the section upon subsequent reloading, the tensile stiffness of the section, and can lead to section fracture in relatively few loading cycles. Thus the application FRP may represent a viable option for improving the energy absorption and ultimate cyclic ductility of elements susceptible to plastic buckling in a seismic lateral force resisting system. Ekiz and El-Tawil (2006) report on an analytical and experimental research program conducted to investigate the buckling behavior of compressive steel braces strengthened with CFRP laminates. To improve the effectiveness of the CFRP wraps, the steel member is first sandwiched within a core (comprised of mortar or PVC blocks) prior to attaching the external CFRP sheets. The authors derived expressions for requirements to prevent buckling of the steel braces from equilibrium considerations and verified the expressions with test results. Small scale tests show that significant improvements can be achieved in the inelastic axial deformation reached prior to buckling and load carrying capacity after buckling when CFRP wrapping is used. In related research, Ekiz and El-Tawil (2007) showed that the same CFRP strengthening technology can be scaled up. They demonstrated large improvements in the buckling and post-buckling response of full-scale double angle brace members subjected to

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reversed cyclic loading. The authors proposed that CFRP wrapping could be used to make steel braces behave in a buckling restrained manner for seismic retrofit purposes. CHALLENGES TO THE USE OF FRP BONDED TO STEEL Behavior of FRP-to-Steel Bond There has been relatively little research conducted on the bond behavior of FRP-steel joints in the context of civil engineering applications. In addition to conventional modes of failure, FRPstrengthened steel members may exhibit debonding of the FRP laminate. In considering debonding failures, the thickness of the adhesive layer plays a significant role in the failure mode (Xia and Teng 2005). Typically a thin uniform adhesive layer is desirable. Such adhesive layers of reasonable thickness (say less than 2 mm thick) will exhibit relatively ductile debonding failures within the adhesive layer. Thicker adhesive layers (as may result when the adhesive is used to make up for dimensional changes in the substrate) exhibit brittle delamination failures along the steel-adhesive interface. Additionally, Xia and Teng (2005) have shown that FRPsteel interfacial behavior is accurately modeled using relatively simple load-slip relationships. Indeed, for thin adhesive layers, a bilinear load-slip relationship approximates observed experimental behavior well. Additionally, debonding behavior in such cases is closely related to adhesive tensile properties and is relatively independent of adhesive layer thickness. Stratford and Chen (2005) report that interfacial stress analysis to predict shear and throughthickness peel stress distributions for FRP-steel adhesive joints is easily and accurately accomplished using low-order linear elastic stress analysis such as that recommended by Cadei et al. (2004). Interfacial stress discontinuities occur at the termination of the adhesive layer. Based on experience gleaned from the aerospace industry, a variety of termination details may be used to reduce these discontinuities including spew, convex or concave fillets, tapers and reverse tapers, stepped FRP plates and external clamps (Stratford and Chen 2005). Substrate Preparation Bond to steel, regardless of the application, requires a clean and sound substrate, and practical field application requires a relatively simple procedure. The typical application (Cadei et al. 2004) involves abrasive (grit) blasting followed within a few hours with a primer/conditioner to ensure that corrosion product does not form and contaminate the newly exposed steel. Since epoxy adhesives will be used, the primer will typically be a (matching) silane-based product which can also serve as an “adhesion promoter”. The adhesive, protective GFRP layer (see below) and CFRP are then installed. Research associated with the previously discussed NCHRP-IDEA program investigated the quality of steel-to-CFRP bonds (Bourban et al. 1994; McKnight et al. 1994; Karbhari and Shulley 1995) and recommend the use of a silane primer; although no specific mechanical surface preparation was recommended. The results from the research however are inconclusive as to whether the silane primer itself improved (promoted) bond performance. It is possible that the primer enhanced bond performance simply by inhibiting the formation of corrosion product between the time of surface preparation and that of CFRP application. Garden (2001) reports a curved I-girder completely wrapped in CFRP; in this case silica gel packs were used to protect the prepared surface from moisture and thus corrosion. Thermoset epoxy adhesives are developed specifically to offer good adhesion to metals. They interact strongly with the adherands and promote excellent bonding. Bourban et al. (1994) indicates a clear benefit from curing the epoxy adhesive at elevated temperatures (around 93oC) during the initial cure (10 to 20 minutes). The resulting bond is stronger, tougher and more durable when subject to adverse environments (Karbhari and Shulley 1995). Furthermore,

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because the epoxy cures faster, it is less likely to sag (requiring falsework) or be affected by vibrations or loading that may be present during an in situ application (Moy 2007). To this end, Karamuk et al. (1995) have proposed the concept of using induction heating of the steel substrate to assist in the accelerated cure of the epoxy adhesive. Environmental Exposure, Creep and Fatigue Behavior Moisture, humidity and elevated temperature can all affect the behavior of a bonded FRP system, regardless of the substrate material to which it is applied. Some FRP materials are additionally susceptible to creep due to sustained loads and adhesive bondlines are susceptible to damage from cyclic (fatigue) loads (Harries 2005). Research efforts associated with the use of FRP materials in concrete infrastructure offer some relevant guidance as to the effect of typically experienced environmental and mechanical loading conditions. When used in conjunction with a steel substrate, some additional environmental protection may be accorded the CFRP by the presence of fireproofing materials or topcoat or finishing systems. Galvanic Corrosion Galvanic corrosion occurs when two different metals are electrically coupled in the presence of an electrolyte (surface moisture or condensation). The corrosion potential is “measured” by how far away on the electropotential series the two conductors are. Carbon is widely separated from steel making galvanic corrosion of the steel likely. Suitable design and detailing is sufficient to mitigate galvanic corrosion as evidenced in the aerospace industry that has been successfully joining aluminum and carbon fibers (even more widely separated than carbon and steel on the electropotential series) for many years. Although the adhesive layer itself or a coating applied to the steel substrate should be adequate to mitigate galvanic corrosion, relying on these methods is a risk in infrastructure applications where rugged handling may affect the quality of the insulating barrier. For infrastructure applications, the inclusion of a GFRP (E-glass is an insulator) layer between the steel and CFRP is suggested (Cadei et al. 2004; Tavakkolizadeh and Saadatmanesh 2001). SUMMARY AND CONCLUSIONS This paper reviews the existing state-of-the-art in the application of steel/FRP composite structures for civil infrastructure. Rehabilitation of steel members and structures using FRP materials is a relatively nascent field, building largely from the success of FRP rehabilitation methods for concrete structures. Studies have shown that strengthening steel flexural elements with FRP is viable. Improving fatigue or fracture propagation conditions have also proven successful in laboratory studies. An application unique to steel structures involving the use of FRP materials to provide stability or buckling restraint has been proposed and demonstrated by a few researchers. As the reports reviewed in this paper suggest, the application of FRP in steel structures is currently in its infancy, much like concrete/FRP applications were in the late 1980s. Concrete/FRP technology has since burgeoned into a vibrant and rapidly growing industry. Drawing parallels from the concrete/FRP experience and the fact that steel/FRP applications show great promise, steel/FRP technology appears to be also poised for explosive growth in the near future. This growth will require extensive research to identify new applications, develop the means by which to fully realize the potential of steel/FRP composite systems, and standardize the technology so that it can ultimately be codified and widely adopted by practicing engineers.

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