ASCE CCC – Task Group on Steel-FRP
ASCE COMMITTEE ON COMPOSITE CONSTRUCTION
TASK GROUP ON STEEL-FRP COMPOSITE CONSTRUCTION
Task Group Members
Kent A. Harries (chair)
Amit Varma
Sherif El-Tawil
Judy Liu
Yan Xiao
Stuart Moy
Dennis Mertz
Xiao-Ling Zhao
Len Hollaway
University of Pittsburgh
Purdue University
University of Michigan
Purdue University
University of Southern California
University of Southampton, UK
University of Delaware
Monash University, Australia (IIFC Liason)
University of Surrey
Objective of Task Group:
1. Synthesize existing information on the use of composite FRP/steel structural systems.
2. Establish the state-of-the-art and state-of-practice and summarize existing projects in this
nascent field.
Deliverables of Task Group:
1. Internal report to committee at 2006 meeting in St Louis.
2. Establish sufficient material and interest for session(s) at 2007 Structures Congress in
Long Beach.
3. Produce synthesis document aimed at ASCE Journal of Composites for Construction as
the established material warrants.
4. Participate in appropriate IIFC Conference
Scope of Task Group
Arguably, the most mature application of FRP/Steel composite construction is the use of GFRP
bridge decks acting in a composite manner with steel girders. A great number of demonstration
projects have been executed primarily through the FHWA IBRC project. These have been
largely summarized by Hooks and O’Connor1 (2004). There are a number of ongoing and
published studies addressing such systems. Harries is currently serving as editor for a special
issue of the ASCE Journal of Bridge Engineering focusing on FRPs in bridge infrastructure. This
issue will be largely populated by papers discussing FRP bridge decks. It is for this reason and
the fact that this is largely a mature application that it is proposed that the Task Group NOT
address issues of FRP bridge decks on steel girders.
Resolution - Scope of Task Group
The task group will address FRP bridge deck systems by citation of key documents and reviews.
This will be included in the scope but will not be a focus since excellent reviews have already
been completed.
Hooks, J., and O’Connor, J., 2004. A Summary of Six Years Experience Using GFRP Composites for Bridge
Decks, Proceedings of the 21st International Bridge Conference, June 12-14, 2004, Pittsburgh, PA.
1
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ASCE CCC – Task Group on Steel-FRP
The Task Group should focus on innovative, emerging and under-reported applications of the use
of steel-FRP composite systems. The following topics have been identified in this regard:
1. Increasing flexural strength of steel beams by bonding FRP sheets in the tension zone.
2. Inhibiting local and global buckling of steel members using FRP.
3. Increasing fatigue life or repairing fatigue cracks using FRP.
4. Using FRP tendons to prestress steel girders.
5. Increasing torsional strength of steel girders using FRP.
6. Improving behavior of conventional steel and concrete composite members by using FRP
Task Group Communication
The task group will communicate by email.
All editing and comments should be executed using the MSWord “track changes” tool and
forwarded to Kent. Please ensure that your “track changes” settings include your user name and
that your user name will make sense to those reading your comments (mine is “harries”). Kent
will compile changes and distribute updated versions as “clean copies”.
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ASCE CCC – Task Group on Steel-FRP
STEEL-FRP COMPOSITE STRUCTURAL SYSTEMS
ASCE Committee on Composite Construction
Task Group on Steel-FRP Composite Construction
Sherif El Tawil*
Kent A. Harries (chair)*
Len Hollaway
Judy Liu*
Dennis Mertz
Stuart Moy
Amit Varma
Yan Xiao
Xiao-Ling Zhao (IIFC Liason)
University of Michigan
University of Pittsburgh
University of Surrey
Purdue University
University of Delaware
University of Southampton, UK
Purdue University
University of Southern California
Monash University, Australia
1. Introduction
1.1 Steel-FRP and Metal-FRP Composite Systems in Other Arenas
2. Fiber Reinforced Polymer (FRP) Composite Materials
3. GFRP Bridge Deck Systems Acting Compositely with Steel Girders
3.1 Fiber Reinforced Polymer Bridge Decks
3.2 Composite Behavior of GFRP Bridge Decks on Steel Girders
4. Retrofit Systems for Structural Strengthening
4.1 Structural Retrofit with FRP
4.2 Retrofit of Steel Members with FRP
4.3 Strengthening of Flexural Elements
4.4 Strengthening of Tensile Elements
4.5 Retrofit of Steel Connections
5. Repair of Fatigue Damage and Enhancing Fatigue Life using FRP Materials
6. Enhancing Stability of Steel Elements
7. Challenges to the Use of FRP Bonded to Steel
7.1 Substrate Preparation
7.2 Behavior of FRP-to-Steel Bond
7.3 Environmental Exposure, Creep and Fatigue Behavior
7.4 Galvanic Corrosion
8. Future Directions
9. References
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ASCE CCC – Task Group on Steel-FRP
1. INTRODUCTION
The use of fiber reinforced polymer (FRP) composite materials has become relatively common
in infrastructure applications. Most extant applications involve concrete-FRP composite
members. Nonetheless a relatively small and innovative body of work is developing focusing on
steel-FRP composite structural systems. With the exception of FRP bridge decks, steel-FRP
composite systems are exclusively aimed at retrofit methods for the underlying steel. 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 report 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. Finally, the authors present their position in terms of what future directions work in
steel-FRP composite structural systems may take that appear most promising.
1.1 Steel-FRP and Metal-FRP Composite Systems in Other Arenas
Beyond civil infrastructure applications there are few applications of steel (or ferrous)/FRP
composite structural systems. Other metal/FRP systems are relatively common in other
industries, however. Most are familiar with the use of FRP materials in the automotive and
aerospace industries. Typically applications in this case are not “composite” per se but involve
simply mating FRP or plastic components with metallic components. True metallic/FRP
composites, called fiber metal laminates (FML), are common in modern aerospace applications
(Vlot and Gunnink 2001). FML materials are composed of thin sheets of (most commonly)
aluminum alternating with traditional FRP materials. The first FML was an aluminum/aramid
laminate known as ARALL. The most common FML today is an aluminum/glass laminate
known as GLARE. FML materials offer greater damage tolerance and corrosion resistance with a
lower specific weight and are thus finding substantial use in high performance applications.
Repair of metallic aerospace and marine structures using FRP materials also has an established
industry presence, although in most cases, such repairs are superficial (skin or envelope) rather
than structural (airframe or hull). The aerospace and marine industries provide a great deal of
background for the adoption of FRP/steel composites in civil applications. Guidance on substrate
preparation, FRP geometry and terminations and joining are all available. Nonetheless, it must be
recalled that these industries (and more so, biomedical applications) are largely performance
driven with cost being of secondary consideration. In civil applications, clearly cost is the driving
factor. Thus paradigms of the design of FRP/metallic composite systems and their use will shift
as civil applications are explored and considered.
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1.2 International Research
The International Institute for FRP in Construction (IIFC) has initiated a task group on FRPStrengthened Metallic Structures. The membership of this task group, chaired by Xiao-Ling
Zhao of Monash University, has assembled a bibliography of 118 international journal papers,
conference articles and theses/dissertations related to the topic. Additionally, this task group is
convening a special Workshop in Sydney in December 2007.
We may provide a link to this bibliography or, with Dr. Zhao’s blessing include it as an
Appendix to the present document.
Question to TG: any thoughts?
Clarification: there is no planned IIFC report at this time. There will be a special conference
however – December 2007 in Australia.
2. 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
typically use high strength, high modulus fibers in relatively high fiber-volume fractions.
Orientation of the fibers is controlled so that the resulting FRP is anisotropic. FRPs enable high
mechanical stress to be carried safely and allow the material to be tailored to suit the local stress
patterns in the component to be retrofit. The in-service 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 lay-up
fabrics remain appropriate for applications involving retrofit forming around corners (column
wrapping, shear strengthening of beams using “U-wraps”). In either case the resulting system has
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a steel-adhesive-FRP interface region. Table 2.1 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 Table 2.1 since the resulting fiber volume ratio will
typically be lower.
Table 2.1 Typical properties of steel-adhesive-FRP systems.
Mild
Steel
tensile modulus
MPa (Msi)
tensile strength
MPa (ksi)
ultimate strain, %
density
kg/m3 (lb/ft3)
CTE
10-6/oC (10-6/oF)
strip thickness
mm (in.)
strip width
Tg4
o
C (oF)
shear strength
MPa (psi)
bond strength
kPa (psi)
200
(29)
276483
(40-70)
18-25
7530
(490)
21.6
(12)
-
FRP Strips
Adhesive1
high
low
modulus modulus3
4.5
0.4
(0.65)
(0.06)
hsCFRP1
hmCFRP1
uhmCFRP1
GFRP2
166
(24)
207
(30)
304
(44)
42
(6)
3048
(442)
2896
(420)
1448
(210)
896
(130)
25
(3.6)
4.8
(0.7)
1.8
~1618
(~101)
1.4
~1618
(~101)
0.5
~1618
(~101)
~0
~0
1.0
~1201
(~75)
162
(90)
>10
~1201
(~75)
~0
2.2
~2146
(~134)
8.8
(4.9)
1.5
(0.06)
-
-
-
-
63 (145)
-
24.8
(3600)
~20.7
(~3000)
9.0
(1300)
~5.0
(~725)
1.3
(0.05)
-
1.3
1.3
(0.05)
(0.05)
typically up to 150 mm (6 in.)
-
149 (300)
149 (300)
149 (300)
resin
-
-
-
-
-
-
-
-
-
-
n.r.
1
representative data from single manufacturer (SIKA Corporation); a number of companies provide similar products
data from single manufacturer (Tyfo), there is only one known preformed GFRP product offered in the infrastructure market
traditionally, high modulus adhesive systems are used in strengthening applications; an example of a very low modulus adhesive is provided to
illustrate range of properties
4
Tg = glass transition temperature
n.r. = not reported
2
3
2.1 Relation between Individual Fiber Strength and Fiber Strength in the Composite
Many researchers have investigated the relationship between individual fiber strength and fiber
strength in the composite product and presented techniques for estimating composite strength
from fiber characteristics (e.g. Bullock 1974, Harlow and Phoenix 1981, Yushanov and
Bogdanovich 1998). The common finding from this research is that the strength of a fiber within
the composite product is lower than that of the individual fibers as typically reported by the
manufacturers, in some cases, as low as 40-50%. While some engineers investigating uses of
FRP materials for strengthening applications have performed coupon tests to estimate the tensile
strength of FRP reinforcement, there are some instances in the literature where composite fiber
strength was assumed equal to the manufacturer reported fiber strength, which is unconservative.
Nevertheless, an unconservative estimate of laminate strength is inconsequential if the modes of
failure under study do not involve composite rupture, but rather substrate and/or bond related
failures. In an attempt to develop a design oriented technique for specifying composite product
strength as a function of manufacturer reported fiber strength, Okeil et al. (2001) used the
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Weibull theory to establish a relationship between the fiber tensile strength and the tensile
strength of CFRP laminates. They showed that the proposed model reasonably replicates test
results for reinforced concrete beams strengthened with CFRP laminates. They also pointed out
that FRP products have a size effect associated with them and so FRP strength derived from
small coupon tests could be misleading. In practice, FRP strips are becoming the dominant
product. In this case, manufacturers are generally reporting in situ strengths and stiffnesses based
on appropriate coupon testing. Examples of such values are given in Table 2.1.
3. GFRP BRIDGE DECK SYSTEMS ACTING COMPOSITELY WITH STEEL GIRDERS
Arguably, the most mature application of FRP/steel composite construction is the use of glass
FRP (GFRP) bridge decks acting in a composite manner with steel girders. A great number of
demonstration projects have been executed primarily through the FHWA IBRC project. These
have been largely summarized by Hooks and O’Connor (2004).
GFRP decks are attractive because of their minimal installation time, high strength-to-weight
ratios, and excellent tolerance to frost and de-icing salts. Their light weight, and thus reduced
dead load, is particularly attractive for rehabilitating posted structures since the replacement of
heavy conventional concrete decking with lighter weight GFRP decking may translate to
additional live load carrying capacity of the bridge system.
Curiously however, despite this latter advantage, most of the bridges currently employing GFRP
decks in their designs are new superstructure (girders and deck) as opposed to rehabilitation
applications. Using GFRP deck panels in a new design may not be as practical as using them for
rehabilitation projects. The deck panels themselves are initially more expensive than their
equivalent concrete slab counterparts and there is not likely to be a great savings to be had from
the reduced dead load. The installation time, however, can be as little as one working day for a
relatively short span bridge. This is a significant time savings over placing a concrete deck and
may warrant the selection of a GFRP deck in some circumstances.
The eventual use of GFRP decks for new bridge construction is unclear. Light-weight GFRP
decks are attractive for certain applications such as moving spans (bascules, etc.) and bridges
located in remote areas where placing a concrete deck may be impractical (Moses et al. 2006).
Currently, there are no AASHTO guidelines for the application of GFRP decks and designers
appear to treat these decks as “equivalent to concrete”. Based on the observations of studies
reported above, this is not the case.
For rehabilitation, however, the reduced dead load of a GFRP deck may represent a significant
advantage possibly allowing load posting to be removed or an increase in the bridge rating to be
made. For example, a typical commercially available deck system weighs approximately one
fifth of what a comparable concrete deck would weigh. Similarly, for historic bridge structures,
the reduced deck load may permit increased load rating without altering the original state of the
bridge. Nonetheless, there are implications to the use of GFRP decks associated with reduced
apparent effective width and increased distribution factors as discussed in the following sections.
The rapid installation of a GFRP deck also reduces bridge closure time for a rehabilitation
project.
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3.1 Fiber Reinforced Polymer Bridge Decks
GFRP bridge decks capable of replacing typical 200 to 250 mm (8 to 10 in.) thick reinforced
concrete decks (weighing 4.8 to 6.0 kPa (100 to 125 psf)) typically weigh about 0.96 kPa (20
psf) without a wearing surface, representing a significant savings in the dead load of the
superstructure.
Although there have been a variety of proprietary GFRP bridge deck systems that have been
proposed and demonstrated, this work focuses on those systems designed to span transversely
between longitudinal bridge girders, typically acting in a composite manner with the girders.
Such decks carry traffic loads in a manner similar to a “one way” concrete deck and typically
take one of three fundamental forms:
1. A series of closed-shape pultruded GFRP tubes that are sandwiched between top and bottom
face plates. The tubes and face plate components are assembled using a structural adhesive.
(examples include Crocker et al. 2002 and Zhou et al. 2002). A variation on this form using
an arrangement of perpendicular tubes and no face plates is presented by Chandrashekhara
and Nanni (2000).
2. A series of interlocking pultruded shapes which include both face plates and web elements
(examples include Motley et al. 2002 and GangaRao et al. 1999). Such systems are generally
more versatile and have better final product quality control than adhesively assembled tubes.
3. Sandwich panel structures consisting of face plates and a core section. The core configuration
may vary but examples include fiber reinforced foam (Stoll and Banerjee 2002) and
vertically (through the deck thickness) oriented pultruded fiber shapes or plates typically
referred to as “honeycomb panels” (Gillespie et al. 2000; Kalny et al. 2002).
According to the American Composites Manufacturers Association (ACMA), there are (as of
July 2005) 83 existing road bridges in the United States having GFRP decks. Of these,
approximately 38 are GFRP deck-on-steel girder structures having girder spacing over which the
GFRP deck spans ranging from 800 mm (31.5 in.) to 2840 mm (112 in.). The majority of these
applications are represented by the second deck type described above.
3.2 Composite Behavior of GFRP Bridge Decks on Steel Girders
A number of in situ tests of GFRP decks on steel girders have indicated that the decks can be
made to behave in a composite manner with the girders at service load levels. The limited data
available from most in situ tests have led researchers to report the composite action as an
apparent effective width of GFRP deck acting in full composite action with the girder. The
degree of composite action available, however, is greatly reduced due to the relatively low axial
stiffness (EA) of the GFRP.
Keller and Gurtler (2005) demonstrated that degree of composite behavior that may be achieved
is affected by the in-plane longitudinal shear stiffness of the GFRP deck. In a study of GFRP
deck adhesively connected to steel girders while there was no strain discontinuity reported at the
steel girder-FRP deck interface, there was significant through-deck shear lag between the top and
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bottom face plates of the GFRP deck resulting top plate strains of only approximately 55% of
those observed in the bottom face plate.
Keller et al. (2004) report results using two different GFRP deck systems designed to carry
comparable loading. Nonetheless, the deck cell geometry differed such that the in-plane shear
stiffness of the decks differed by a factor of eight. The deck having lower in-plane shear stiffness
exhibited a composite stiffness at service load level of only 74% of that of the stiffer deck.
Additionally, the ultimate failure mode of the two deck types differed: the deck having lower inplane shear stiffness failed in the bottom face plate while the stiffer deck exhibited a failure that
engaged the entire deck depth. The same study also clearly demonstrated a transverse shear lag
across the deck at the ultimate limit state, although a relatively uniform transverse stress
distribution at service load levels. The presence of a transverse shear lag reduces the efficiency
of the GFRP deck acting compositely with the girder. The transverse results suggest a notable
different behavior, in this case, between service and ultimate load levels. These observations
point to the relative importance of the through-deck and transverse shear lag phenomenon when
evaluating compositeness in FRP-steel bridge girder systems.
Keller’s research involved GFRP decks adhesively connected to steel girders. This is a unique
application which is believed to result in a more uniform (and therefore efficient) shear transfer
between the girder and deck than the more conventional use of shear studs. Shear stud
connections of steel to GFRP decks typically involve the use of multiple-stud groups embedded
in discrete cells of the GFRP deck which are then locally grouted. In the most common
application, the resulting grouted pocket is approximately 180 mm (7 in.) long and 450 mm (18
in.) wide, relative to, and centered on, the underlying girder flange. The spacing between grout
pockets is most typically 610 mm (24 in.).
3.2.1 Stud Capacity Push-Off Tests
Standard push-off style tests are used to determine the shear capacity of embedded studs
connecting steel girders and GFRP decks. Both Moon et al. (2002) and Turner et al. (2003)
report the average capacity of shear studs embedded in a GFRP deck grout pocket to be
approximately 70-80% of the capacity of a comparable shear connection in a continuous
concrete deck. Similar tests conducted by Yulismana (2005) showed that the shear studs were
only able to achieve 52-58 % of their nominal capacity (defined by the shear capacity of the
stud). Only tests conducted by Drexler (2005) report studs actually achieving their nominal
capacity, although these tests utilized two sets of shear stud connections on each side of the test
specimen. Such a non-traditional test arrangement is believed to result in greater observed
capacities.
The observed inability to achieve the stud shear capacity was due to failure modes associated
with crushing (Moon et al. 2002) or delamination (Turner et al. 2003 and Yulismana 2005) of the
GFRP decks. The failure reported by Turner et al. involved the web of the GFRP deck separating
from the face plate.
3.2.2 Moment Distribution Factors
Moment distribution factors (DF) are design tools used to determine the maximum expected
moment each supporting girder must be able to resist given the strength contributions of adjacent
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superstructure elements. Alternatively, they may be seen as factors describing the manner in
which the design loads will be distributed to the supporting girders making up the superstructure.
These factors are given as a fraction of the design load. An accurate assessment of moment
distribution factors is critical for bridge design. The use of GFRP decks requires an evaluation of
distribution factors for these decks so that girder design forces are appropriately evaluated. For
bridge deck replacement applications, the behavior of the replacement deck should approximate
that of the original deck with regard to distribution. If, for instance, the distribution is more
critical (less transverse distribution of wheel loads) for the replacement GFRP deck, some girders
will see proportionately greater stress due to a given live load; as compared with the previous
condition with the replaced deck system. It is also important to note that the reduced dead load of
the GFRP deck does have the effect of reducing overall girder stresses but may adversely affect
the live load-induced stress range which may be critical for fatigue considerations. Thus,
distribution factors for GFRP replacement decks that differ significantly from those of the
original concrete decks may, in fact, aggravate fatigue-sensitive details (Harries and Moses
2006).
A study of moment distributions factors determined from in situ service load tests of multiple
bridge structures indicates that the “lever rule” (AASHTO 2004) results in adequately
conservative distribution factors for use in design (Moses et al. 2006). Refining the calculation of
distribution factors beyond this approximation will require a significantly broader database to be
established.
4. RETROFIT SYSTEMS FOR STRUCTURAL STRENGTHENING
4.1 Structural Retrofit with FRP
A great deal of work has been conducted on the use of externally bonded FRP systems for the
structural strengthening of building 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 being used “in place of steel reinforcement”. Indeed, early FRP
external bonding applications were developed as an alternative to very heavy and awkward steel
plate bonding techniques (Meier et al. 1993; McKenna and Erki 1994). 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 relatively unique properties of FRP materials: their high stiffness, strength
and linear behavior.
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; fib 2001; ISIS 2001; JSCE 2001,
among others). 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 (an overview of debonding
mechanisms and a summary of a number of debonding models are provided by Teng et al.
(2002)) 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.
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4.2 Retrofit of Steel Members with FRP
There is comparatively little work done 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. The first application was the Bures Bridge in Suffolk, UK, which is
an arch bridge having a 18.3 m (60 ft) span. The structure consists of five arched cast-iron bolted
girders, which support 1525 mm (5 ft) wide brick jack arches spanning transversely between
them. The CFRP composite plates were pre-curved to fit each arch and these plates were
attached to the bottom flanges and the web of the steel girders to complete the rehabilitation. The
second application was the Hythe Bridge, Oxford, UK, which was constructed in 1874 and
divided in two square clear spans of 7.3 m (24 ft). The deck was comprised of eight inverted Tee
section cast iron beams, with cast iron channel section edge beams supporting a decorative
parapet; which had a capacity of 73.4 kN (16.5) kips. The objective of the strengthening scheme
was to raise its capacity from 73.4 kN to 400 kN (90 kips). Lastly, a curved steel beam was
strengthened using a low temperature moulding advanced polymer composite prepreg material
(Garden 2001). The fibers of the unidirectional prepreg were aligned along the direction of the
beam and the 0o/90o arrangement was employed to resist the shear and torsional loading. The
prepreg composites were based upon carbon and glass fibers.
Cadei et al. (2004) reports on thirteen applications of strengthening cast iron structures and six
instances of strengthening steel structures (including those discussed above); 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 (often intricately shaped) 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.
4.3 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 by the University of Delaware in the late 1990’s.
These projects focused on the flexural strengthening of corroded bridge girders and addressed
only the use of bonded FRP materials on the tension flange of simple girders. The rationale was
that the bottom flange of bridge girders typically see the greatest level of corrosion, largely due
to debris accumulation.
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Figure 4.1 Small-scale specimens tested by Mertz and Gillespie.
In the earlier study, Mertz and Gillespie (1996) report on six small scale tests of 1525 mm (60
in.) long W200x15 (W8x10) members retrofit with the five different adhesively bonded
schemes; shown in Figure 4.1 (the fifth scheme was similar to that shown in Fig 4.1(a) using a
different CFRP material.) All specimens demonstrated an increase in flexural stiffness and
strength compared to the un-strengthened control specimen. As might be expected, the
“sandwich-reinforced” specimen (Fig. 4.1(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 4.1(a) was comparable for both parameters). The
“composite-wrapped” specimen (Fig. 4.1(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 from its ends. The second CFRP strip specimen, and the specimen
having a pultruded channel retrofit (Fig. 4.1(d)), did not debond until loads greater than the
unstrengthened beam failure load were exceeded. Based on the results of the small-scale
program, the second CFRP strip method was selected for application to full-scale specimens.
Two significantly corroded 600 mm (24 in.) deep girders (similar to W610x125 (W24x84) with
tapered flanges), recovered from a 1940-era Pennsylvania bridge, were tested using a
strengthening scheme similar to that shown in Fig 4.1(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 second 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 first 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
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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 occurs 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 a thin bonded FRP system (Teng et al. 2002). Mertz et al.
recognize the importance of a thin adhesive bondline in allowing stress transfer between the steel
substrate and CFRP. An efficient stress transfer, however exacerbates debonding stresses.
Despite this, Mertz et al. only report debonding in one tension specimen at strains corresponding
to yield of the steel specimen. They attribute the lack of debonding in the other specimens to the
use of adhesive with a low shear modulus and high elongation properties (properties were not
provided in the report).
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 under the following test conditions:
a) 2.55 million cycles at 83 MPa (12 ksi); corresponding to a maximum shear stress in the
adhesive of 6895 kPa (1000 psi).
b) 10 million cycles at 34.5 MPa (5 ksi); corresponding to a maximum shear stress in the
adhesive of 1234 kPa (179 psi).
Finally, Mertz et al. (2002) and Miller et al. (2002) report on a field installation on a single girder
of a bridge carrying I-95S over Christina Creek outside Newark Delaware. This installation was
a demonstration of the techniques developed in the IDEA projects. A single W610x125
(W24x84) girder spanning 7.5 m (24.5) feet 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
equivalent to approximately an AASHTO H32 loading. Chacon et al. (2004) report a related
demonstration project involving the strengthening of two W24x100 (historic shape) 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.
Deng et al. (2003) presented analytical work where the stresses in steel beams reinforced with a
bonded CFRP plate under mechanical and thermal loads were calculated. Finite element analysis
was employed to validate the analytical results. A parametric study was carried out to show how
the maximum stresses were influenced by the dimensions the material properties of the adhesive
and the adherends.
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.
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A second series of tests reported by Patnaik and Bauer (2004) involved 350 mm (14 in.) deep
beams having intentionally slender – 3 mm by 325 mm tall (1/8 inch wide by 13 in.) – 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.”
In a project funded by the Federal Highway Administration’s Innovative Bridge Research and
Construction (IBRC) program, Phares et al. (2003) attempted to use CFRP post-tensioning rods
to improve the structural response of a steel bridge. A CFRP post-tensioning system was
installed in the positive moment region of an existing bridge in an effort to improve its live load
carrying capacity. Although the system provided little enhancement to the stiffness of the bridge,
approximately 5 to 10 percent of the live load moment at critical sections was reduced by the
post-tensioning system. It is not clear from the study whether such an approach provides benefits
over the more traditional bonded reinforcement.
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. A Super-Light Beam (SLB) with an
additional steel plate welded along the length of the compression flange was used to determine
the development length of the strengthening technique (Schnerch et al. 2006). It is reported that
the addition of the steel plate in the compression region simulates the presence of a concrete slab
by decreasing the neutral axis depth such that the strain profile of the cross-section would be
similar to that in a bridge girder acting compositely with a concrete deck. The 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.
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 carbon fiber-reinforced polymer
plates attached to the tension area. Damage to the beams was induced by removing part of the
bottom flange; varying between no damage and a loss of 75% of the bottom flange. All beams
were tested to failure to observe their behavior in the elastic, inelastic, and ultimate states. The
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.
Other similar investigations of the use of CFRP strips attached to the tension flange of I-girders
have demonstrated generally improved flexural capacity although little improvement to girder
stiffness (Sen et al. 2000; Tavakkolizadeh and Saadatmanesh 2003a). Both studies utilized
moderate scale rolled sections (W200x35.9 (W8x24) and W360x101 (W14 x 68), respectively)
having composite concrete slabs. Various amounts of CFRP were utilized and proportional
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increases in flexural capacity of the composite girders were observed accordingly. Regardless of
the amount of CFRP used, little increase in elastic stiffness is observed. This is attributed to the
relatively small contribution of the CFRP to the section modulus and the relative flexibility of
the adhesive (Tavakkolizadeh and Saadatmanesh 2003).
Lenwari et al. (2005) reported on the flexural behavior of steel beams that were strengthened
with partial-length, adhesive-bonded CFRP plates. Seven steel beams were strengthened with
three different CFRP lengths, attached to the bottom flange of the beam and tested under fourpoint loading. Two different failure modes were observed: plate debonding at their ends in
beams with short plates; and plate rupture at midspan in beams with long plates. The authors
concluded that the attached CFRP plates significantly increased the strength and extended the
elastic range of the beams. An analytical method was also proposed to evaluate the flexural
behavior of the strengthened beams.
Photiou et al. (2006) investigated the effectiveness of C/GFRP hybrid prepreg materials having
two different moduli in strengthening artificially degraded steel beams of rectangular crosssection under four-point loading. First, two beams were upgraded with U shaped prepreg units,
which extended up the vertical sides of the beam to the neutral axis height. Another two beams
were strengthened using prepreg strips attached only to their soffit. The hybrid prepreg units
were composed of carbon and glass fiber reinforced polymers and all the specimens had the
same hybrid lay-up although two different moduli CFRP were used. The authors reported that
the failure load for all specimens exceed the plastic collapse load of the undamaged beam and by
using the U-shaped configuration for the fibers, composite action was provided between the steel
member and fiber layer leading to better performance even at failure levels. Debonding was
observed for the specimens which had fiber layers only on the soffit
Lenwari et al. (2006) investigated the debonding strength of partial-length, adhesively bonded
CFRP plates used to strengthen steel beams. Static and fatigue tests were conducted with steel
beams strengthened with CFRP to investigate possible reasons of debonding under four-point
loading. Variables studied were CFRP plate thickness and modulus, adhesive bondline thickness,
adhesive modulus, and adhesive spew-fillet angle. The authors concluded that the stress intensity
at the end of CFRP plates governs the debonding strength and that this region was critical for the
initiation of debonding.
Nozaka et al. (2005) proposed an alternative technique to single and double lap specimen testing
for obtaining the effective bond length of CFRP strips. Since most of the civil engineering
applications do not have an orientation similar to single or double lap geometry, stress
distribution in the adhesive layer was analyzed and a more suitable specimen model and test
setup were proposed. The proposed alternative for assessing the effective bond length for the
adhesive/CFRP system was demonstrated by comparing results with a prototype repaired bridge
with CFRP strip. Weaknesses and strengths of standard single-lap and double-lap specimens
were also discussed. They reported that the new test setup which accounts for flexure together
with tension would be more accurate for obtaining the effective bond length for polymers in civil
engineering applications
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Colombi and Poggi (2005a) discussed the results of an experimental and numerical program to
characterize the static behavior of steel beams strengthened with pultruded CFRP strips. Hshaped steel beams with different CFRP reinforcement geometries bonded to the tension flanges
using different epoxy adhesives were tested under three point bending. Force transfer
mechanisms, strength and stiffness of the beams were the main interest of the study. Results
were validated with different analytical and numerical models and with a finite element model
developed by the authors. (comment on conclusions of this study required)
4.4 Strengthening of Tensile Elements
Jiao and Zhao (2004) tested 21 butt-welded very high strength (VHS) steel tubes strengthened
with CFRP in axial tension. Lap shear strength between CFRP and VHS tubes was obtained as a
preliminary study before testing the welded connections. Three types of failure were reported:
adhesive failure, fiber tear and mixed failure. The authors concluded that significant strength
increase was achieved using the CFRP-epoxy 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 (2005b) to verify
the effectiveness of CFRP pultruded plates to reinforce tensile steel members. Three different
sets of specimens were tested under axial 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 last group. Force transfer and failure mechanism were evaluated from both
analytical and numerical models. (comment on conclusions of this study required)
4.5 Retrofit of Steel Connections
Mosallam et al. (1998) present a pilot study investigating the use of CFRP T-sections for
strengthening steel moment connection 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 exhibit 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.
5. 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
composite behavior is dominated by the strain-limited creep-rupture process. Plotted on a semilog S-N curve (S is linear scale; N is log scale), 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 (an excellent
summary is provided in Agarwal and Broutman 1990). Fatigue performance of CFRP has been
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shown to be relatively unaffected by changing fiber type (e.g.; one carbon fiber for another) 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 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 (68oF to 104oF)) 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 the CFRP overlay to enhance the fatigue performance of the specimens.
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 (50 ksi and 71
ksi), 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
(non-standard shape) 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 extend the fatigue life of the notched detail more than three times, it also
decreases 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 (i.e., cracked) 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. It is noted that although the test specimen girders were intended
to be “fatigue-damaged,” no fatigue tests of the CFRP-repaired specimens are reported.
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.
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6. 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. The 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 only
the extending flanges are wrapped; both methods exhibited improved behavior of the hinge as
compared to unwrapped specimens (Figure 6.1). 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 (Figure 6.1a and c) 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.
Figure 6.1 Effect of FRP reinforcement on plastic hinge response.
FRP wrapped around flanges of channels. (Ekiz et al. 2004)
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Accord et al. (2006 and Accord 2005) present an analytical study wherein nonlinear finite
element modeling strategies were employed to examine 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. The modeling involved the discretization of the
I-section by shell finite elements placed at constituent plate mid-surfaces and the modeling of the
GFRP, and the flexible adhesive located along the steel-GFRP interface, using continuum
elements. Both geometric and material nonlinearity were considered in all of the modeling.
Accord et al. 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
(Figure 6.1). As the location of the GFRP strips was adjusted to increase their efficacy as bracing
elements, a concomitant increase in structural ductility was noted (Figure 6.2); 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. An
experimental study is based on this work has been initiated.
90
applied load at beam tip (kN)
80
X = 64 mm
70
60
cantilever
steel section
X = 38 mm
50
40
GFRP strip:
t = 6.4 mm
w = 25 mm
30
20
steel beam:
d = 381 mm
b = 152 mm
tf = 10 mm
tw = 6.4 mm
X = 13 mm
no GFRP
GFRP
10
X
0
0
100
200
300
deflection at beam tip (mm)
400
transverse location of GFRP, X
Figure 6.2 Analytical load-deflection behavior of
GFRP stabilized steel cantilever (Accord 2005).
Sayed-Ahmed (2004) proposed the use of CFRP strips applied horizontally to the compression
zone of slender-webbed Class 3 and Class 4 (ECS 2002) steel sections in an effort to improve the
web buckling behavior of the section. In this analytical study, the author reports an increase in
the theoretical critical load applied to a beam in four-point flexure, of between 20% and 60%,
and for the ultimate capacity, between 2 and 9%. There is however, some ambiguity in this
study: although stating that the CFRP is intended to stabilize the compression zone of the web,
the CFRP is applied at the mid-height of the web. In the calculation of the critical load, it appears
that it is assumed that the applied CFRP simply halves the slenderness (hw/tw) of the web in the
same manner a horizontal stiffener would. Nonetheless, the concept of FRP-stabilized steel is
considered albeit with an unclear application.
In a study investigating the use of CFRP to strengthen hollow structural square (HSS) columns,
Shaat and Fam (2004) report on concentric axial load tests of squat HSS 88.9x88.9x3.2 (HSS3½
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x 3½ x 1/8) sections wrapped with both longitudinal and transversely oriented CFRP sheets.
Axial strength increases on the order of 8% to 18% are reported and axial stiffness increases
(resulting from the longitudinally oriented CFRP) of between 4% and 28% are reported. It is
reported that the transverse CFRP can help to restrain outward directed local buckling of the
HSS walls.
Ekiz and El-Tawil (2006a) report on an analytical and experimental study 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 of
wrapped steel members 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 (Figure 6.3). In related research, Ekiz and El-Tawil (2006b) showed that the
same CFRP strengthening technology could be scaled up. They demonstrated large
improvements in the buckling and post-buckling response of full-scale double angle brace
members subjected to reversed cyclic loading (Figure 6.4). The authors proposed that CFRP
wrapping could be used to make steel braces behave in a buckling restrained manner for seismic
retrofit purposes.
Figure 6.3 Improving the compressive response of steel braces
using CFRP wrapping (Ekiz and El-Tawil 2006a).
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Axial Strain (%)
-1
0
Axial Strain (%)
1
2
-2
120
120
80
80
40
40
Load (kips)
Load (kips)
-2
0
-1
0
1
2
0
-40
-40
Mortar Blocks
Angle Section
2.5x2.5x3/16
-80
Fiber
Wrap
-80
-120
-120
-3
-2
-1
0
1
2
3
-3
-2
-1
0
1
2
Axial Displacement (in)
Axial Displacement (in)
(a) Bare specimen
(b) FRP wrapped specimen
3
Figure 6.4 Improving the compressive response of full-scale steel double angle
members using CFRP wrapping (Ekiz and El-Tawil 2006b).
7. CHALLENGES TO THE USE OF FRP BONDED TO STEEL
7. 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 (0.08 in.) 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
FRP-steel 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 through
thickness 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). Although such analyses do not strictly satisfy the zero shear boundary condition at
the end of the adhesive and implicitly rely on stress redistribution within the adhesive, they
overcome the complexity of higher-order analyses (Yang et al. 2004) without an appreciable loss
of accuracy.
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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). Low order analysis has been
shown to accurately capture the effects of such details (Stratford and Chen 2005).
7.2 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 (typically to SSPC-SP5 standard (2000)) 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 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
(200oF)) 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,
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 (this is addressed by S.S.J.
Moy in an unpublished paper reported by Cadei et al. (2004)). 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.
7.3 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.
7.4 Galvanic Corrosion
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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 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 a number of 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 most 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).
Torres-Acosta (2002a, 2002b) reported on the galvanic corrosion effect of carbon-polymer
composites on steel. In a mortar and concrete environment, he indicated that there might be some
adverse effects when CFRP is in contact with the steel. However, this depends on the epoxy
properties. The results were not conclusive and the author recommended that further works was
needed to validate the drawn conclusions.
8. FUTURE DIRECTIONS
Identify key infrastructure needs that may benefit from steel-FRP composite systems. Bridge
deterioration issues; optimization of HPS; improvement of strength/ductility/redundancy in
extreme events seem to be areas where seminal work may come at a reasonable price.
Specialty applications (cel towers, power grid towers, tainter gates, etc.) also hold promise
for directed research. Design guidelines are also needed.
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