STEEL-FRP COMPOSITE STRUCTURAL SYSTEMS
Kent A. Harries
University of Pittsburgh, Civil and Environmental Engineering
Pittsburgh, PA USA
kharries@engr.pitt.edu
Sherif El-Tawil
University of Michigan, Civil and Environmental Engineering
Ann Arbor MI USA
eltawil@umich.edu
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
5
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
6
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
7
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
8
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,
9
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.
10
ACKNOWLEDGEMENTS
This paper was prepared as part of the activities of the ASCE Committee on Composite
Construction (formerly chaired by the second author) Task Group on Steel-FRP Composite
Structures chaired by the first author. The comments and review provided by the entire
committee are gratefully acknowledged. Dr. Judy Liu of Purdue University and Mr. Ekin Ekiz, a
PhD candidate at the University of Michigan working under the supervision of the second
author, both contributed to the manuscript.
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