Anchorage Devices Used To Improve The Performance of Reinforced Concrete Beams Retrofitted With FRP Composites: State-of-the-Art Review
Anchorage Devices Used To Improve The Performance of Reinforced Concrete Beams Retrofitted With FRP Composites: State-of-the-Art Review
Anchorage Devices Used To Improve The Performance of Reinforced Concrete Beams Retrofitted With FRP Composites: State-of-the-Art Review
Abstract. The anchorage of fiber-reinforced polymer (FRP) composites when applied to reinforced concrete (RC) structures as externally
bonded reinforcement is an effective means to achieve higher levels of fiber utilization prior to premature debonding failure. Commonly
documented anchorage methods for FRP-to-concrete applications demonstrating encouraging results include FRP U-jackets, FRP anchors
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(also known as spike anchors, among other names), patch anchors (utilizing unidirectional and bidirectional fabrics), nailed metal plates (also
known as hybrid bonding), near-surface mounted rods, mechanical fastening, concrete embedment, and mechanical substrate strengthening.
Anchorages applied to FRP systems have been verified through experimental testing and numerical modeling to increase the ductility,
deformability, and strength of the member and also prevent, delay, or shift the critical mode of FRP debonding failure. Although the benefits
of anchorage solutions have now been widely acknowledged by researchers, further studies are required in order to establish reliable design
formulations to negate the requirementsdfFRPanchoragesystemsappliedtoFRP-strengthenedRCflexuralmembers.Availableexperimentaldat
aarecompiledandcataloguedandananchorageefficiencyfactorforeachanchoragetypeunderinvestigationisassignedinordertoquantifytheanchor’s
efficiency.Finally,currentshortcomingsinknowledgeareidentified,inadditiontoareasneedingfurtherinvestigation.DOI:10.1061/(ASCE)CC.
1943-5614.0000276.©2013AmericanSocietyofCivilEngineers.
CE Database subject headings: Fiber reinforced polymer; Anchors; Fastening; Concrete beams; Rehabilitation; Composite materials;
State-of-the-art reviews.
Author keywords: Fiber-reinforced polymer (FRP); U-jackets; Anchor; Spike; Mechanical fastening; Bidirectional fabric; Substrate
strengthening.
Introduction However, FRP solutions are not without their inherent short-
comings. For instance, it is widely recognized that failure of RC
The retrofitting of existing reinforced concrete (RC) structures has structures retrofitted with FRP almost always occurs by debonding
become necessary due to environmental degradation, changes in of the FRP from the concrete substrate. To prevent this type of fail-
usage, and heavier loading conditions. In the forefront of retrofit- ure, national standards and design guidelines impose strict limita-
ting technology is the use of advanced fiber-reinforced polymer tions on the allowable strain level in the FRP which may be safely
(FRP) composites applied to structural members as externally utilized in design. To achieve acceptable levels of concrete-FRP
bonded reinforcement (Bank 2006; Hollaway and Teng 2008; contact bond stress, allowable strains are lower in cases where a
Karbhari and Abanilla 2007). The suitability of this material when higher degree of strengthening is required and can be as low as
compared, for example, to structural steel is largely due to its light 10–25% of the material rupture strain (Kalfat and Al-Mahaidi 2011).
weight, superior tensile strength, and its resistance to corrosion. Low levels of efficiency are often the result of using higher modulus
These FRP materials are typically applied to the concrete surface fibers and multiple layers of FRP. In practice these limitations result
using epoxy resin after adequate surface preparation of the con- in severe underutilization of the FRP material properties. Anchor-
crete, typically involving sandblasting, water jetting, and the appli- age of the FRP is one means to significantly improve the efficiency
cation of a suitable primer. Once applied, up to seven days of curing of FRP systems and hence provide a solution to these shortcomings.
is typically required to achieve the full bond strength of the system Extensive research has been undertaken to understand the mech-
(Hag-Elsafi et al. 2001). anisms of FRP application and failure and has resulted in design
guidelines being published all around the world within the last
1 decade [e.g., International Federation for Structural Concrete (fib)
Ph.D. Candidate, Swinburne Univ. of Technology, Melbourne,
Australia (corresponding author). E-mail: rkalfat@swin.edu.au 2001; Japan Society of Civil Engineers (JSCE) 2001; Concrete
2
Professor of Structural Engineering, Swinburne Univ. of Technology, Society 2004; American Concrete Institute (ACI) 2008; Oehlers
Melbourne, Australia. E-mail: ralmahaidi@swin.edu.au et al. 2008]. It is understood that the bond strength of FRP materials
3
Associate Professor, Dept. of Civil Engineering, Univ. of Hong Kong, can be improved when sufficient anchorage is provided and such
Pokfulam, China. E-mail: stsmith@hku.hk
provisions have been acknowledged to delay or prevent the critical
Note. This manuscript was submitted on June 29, 2011; approved on
December 15, 2011; published online on December 20, 2011. Discussion
mode of FRP debonding failure (Galal and Mofidi 2010). In addi-
period open until July 1, 2013; separate discussions must be submitted for tion, anchorage devices can be essential to transfer the stress from
individual papers. This paper is part of the Journal of Composites for Con- one structural component to another where application is limited
struction, Vol. 17, No. 1, February 1, 2013. © ASCE, ISSN 1090-0268/ by the geometrical configuration. A popular example is the shear
2013/1-14-33/$25.00. strengthening of T-shaped sections (Ceroni et al. 2008).
age concepts by drawing upon a wide selection of publications. The tions suggest that as the plate end moves further away from the
paper assumes a largely qualitative style by physically explaining support, cover separation failure becomes the controlling mode,
each anchor concept with the aid of appropriate diagrams. Informa- whereas IC debonding governs when the distance between the plate
tion about typical experimental investigations undertaken on each end and support is relatively small (Yao and Teng 2007). In addi-
anchor type and descriptions of behavior and failure are given. tion, the probability of debonding initiating near the plate end has
Databases are also assembled from available test results and effi- been found to be the highest when the ratio of maximum shear
ciency factors are calculated for each anchor concept. Such calcu- force to bending moment is high, such as the higher peeling stresses
lations represent the quantitative aspect of the paper. While it generated at the ends of the external plate. Therefore, slender beams
is recognized that anchorages can be of benefit to a variety of with high shear span/depth ratios do not present a need for plate end
FRP-strengthened elements such as connections, wall, and beams anchorage because failures are initiated in regions of high bending
members, emphasis has been given in this paper to flexural mem- moment well away from the plate ends (e.g., Garden and Hollaway
bers strengthened in flexure and shear because these constitute 1998). These are just some of many qualitative observations to be
the most common strengthening situations. Finally, the terms retro- found in the published literature.
fitting and strengthening are used interchangeably throughout
the paper.
Anchorage Devices for FRP Reinforcement Used to
Strengthen Members in Flexure
Mechanisms of FRP Failure and Debonding for
Flexurally Strengthened Members Three general categories of anchorage type have been investigated
to date to prevent debonding in RC members strengthened in
To date, several failure modes for RC beams strengthened in flexure flexure with FRP, namely
with FRP plates have been identified from experimental investiga- 1. U-jacket anchors (Smith and Teng 2003; Al-Amery and
tions and these are shown in Fig. 1. The modes are summarized as Al-Mahaidi 2006; Pham and Al-Mahaidi 2006; Yalim et al.
(1) concrete crushing, (2) FRP rupture, (3) shear failure, (4) con- 2008);
crete cover separation failure (Yao and Teng 2007), (5) plate end 2. Mechanically fastened metallic anchors (Garden and Hollaway
interfacial debonding (Leung and Yang 2006), (6) intermediate 1998; Spadea et al. 1998; Jensen et al. 1999; Duthinh and
flexural or flexural-shear crack-induced interfacial debonding Starnes 2001; Wu and Huang 2008); and
Fig. 1. Types of FRP debonding (adapted from Pham and Al-Mahaidi 2004)
along the length of the beam reduced the interfacial slip between the
CFRP flexural fiber and the concrete section by up to one-tenth. In
FRP U-Jacket Anchors this study, the U-jackets lead to the full utilization of the CFRP flexu-
FRP U-jacket anchors involve the application of unidirectional ral tensile capacity. The results demonstrated an increase in flexural
or bidirectional fiber to the ends of flexural FRP reinforcement strength of up to 95% when using CFRP U-jackets to anchor the
(Fig. 2) to prevent or delay debonding initiating from the plate CFRP fiber. However, when using conventional CFRP fibers alone,
end. U-jackets can also be placed along the length of the member an increase of only 15% was achieved.
to prevent or delay debonding initiating away from the plate end. Yalim et al. (2008) also conducted investigations on the effects of
The ultimate function of a U-jacket is to provide the confinement U-jacket configurations placed throughout the span as opposed to
necessary to resist the tensile peeling stresses and longitudinal only the plate ends. A total of 26 beams were tested in 3-point
crack propagation at fiber termination points or intermediate loading with 4, 7, 11, and continuous U-jacket arrangements. The
cracks. Khan and Ayub (2010) investigated anchorage heights study utilized FRP U-jackets to anchor both FRP laminates (modulus
ranging from 100–200 mm and suggested that U-shaped ancho- of elasticity of 131 GPa) and FRP sheets (modulus of elasticity of
rages were effective irrespective of their height. The study deter- 70.6 GPa). In addition, three alternative surface profiles were inves-
mined that 100 mm partial-height U-wraps delivered the same tigated: smooth, intermediate, and rough. However, each surface pro-
effectiveness as full-height U-wraps because in both cases failure file was not appropriately defined (except by broad definition) and as
was by concrete crushing. Because concrete crushing was observed a result, the categorization is not an appropriate definition of surface
for the shorter length jackets, the true potential of full-height jackets roughness. The use of four U-jackets at the FRP ends was successful
could not be utilized. in preventing the end interfacial debonding failure that was observed
Debonding failure modes can change due to the addition of FRP in unanchored specimens, and failure was shifted to IC debonding,
U-jackets. For example, Smith and Teng (2003) showed that with confirming the findings of earlier researchers. The beams with seven
the addition of plate-end U-jackets, the critical debonding failure jackets failed in the same way at a higher load together with U-jacket
mode could be shifted from concrete cover separation to IC de- debonding. Specimens with eleven jackets and full continuous jack-
bonding. Therefore, in an effort to prevent failure by IC debonding, ets failed by rupture of FRP. Although the strain utilization levels and
the placement of U-jackets throughout the span or in the flexural- ultimate load capacity were improved with the addition of U-jackets
shear zones (at certain spacings) has been investigated by several throughout the span, it was found that a higher level of anchorage
researchers to date (Al-Amery and Al-Mahaidi 2006; Khan and improved the ductility more than it did the strength. However, the
Ayub 2010; Pham and Al-Mahaidi 2006; Yalim et al. 2008). ductility measurements were solely based on the maximum vertical
Although lacking in material efficiency, this method has been deflection for the beams prior to failure. Ductility can be defined as
proven to result in FRP rupture. Such an arrangement of U-jackets the RC beam’s ability to deform under tensile stress and can be de-
is also used for shear strengthening applications. Selected studies termined by monitoring deflection, beam curvature, or strain in the
are summarized in the following. tensile reinforcement. Monitoring beam deflection may be indicative
IC debonding in beams retrofitted with U-jacket anchors was re- of ductile behavior, but the method fails to consider deformability in
ported by Pham and Al-Mahaidi (2006). The experimental program terms of beam curvature and cracking (measured from tensile
comprised 260 × 140 mm RC beams tested under three-and four- reinforcement strain). In addition, most FRP design guidelines check
point bending. Anchorages encompassing unidirectional fibers of strain of the the tensile reinforcement to ensure ductility. Although
209 GPa modulus were placed at the carbon FRP (CFRP) plate ends the benefit of U-jacket anchors in flexural retrofitting applications is
or at a spacing of 180 mm within the shear zone. Each jacket com- evident, the provision of U-jackets throughout the span to prevent the
prised two plies of fabric that was 0.175 mm thick and 50 mm wide, mechanisms of plate end and IC debonding may not be a materially
which was bonded to the sides and the soffit of the concrete beam to efficient method to improve the efficiency of FRP strengthening ap-
form a U-shape. While the end U-jacket proved to be effective in plications because additional material is required to reach a given
limiting both forms of end debonding, i.e., end cover separation fail- strength (Orton et al. 2008).
ure and end interfacial debonding, the critical failure mode was seen
to shift to intermediate-span debonding at a higher load, and it often
Inclined U-Jacket Orientations
occurred together with rupture of the end U-jacket. Such behavior
was also observed in Smith and Teng’s (2003) study. The rupture Promising results have been achieved based on the limited research
was due to a sliding action of the CFRP reinforcement underneath conducted on inclined U-jackets at the FRP ends only (Fig. 2).
spectively, where no anchorage was provided. defined as the ratio of ultimate midspan deflection to yield midspan
Of the numerous anchor configurations tested, it was found that deflection, where as curvature ductility was considered in a similar
U-jackets placed at the FRP plate-end locations 200 mm from sup- fashion but utilized the midspan curvature values. Although all
ports failed by premature concrete crushing and intermediate span specimens strengthened with both perpendicular and inclined shear
debonding, while U-jackets placed 420 mm away from supports jackets exhibited greater load-carrying capacity, deflections, and
failed by premature concrete crushing and concrete cover separa- ductility, it was found that perpendicular orientations of U-jacket
tion failure. The influence of end termination distance on end de- anchors provided the most noticeable improvement, with increases
bonding failure is consistent with current debonding models (Smith in curvature ductility of 45% and 24% for deflection ductility. The
and Teng 2002; Smith and Teng 2003). Inclined and X-shaped an- improvements were less obvious in the inclined U-jacket anchors.
chor arrangements all failed by concrete crushing. Interestingly, the This may be due to the higher postcracking stiffness exhibited due
authors point out that the CFRP plate experienced the highest con- to the inclined U-jacket anchors. Strain in the tensile reinforcement
finement near the side faces of the beam and less restraint in the is usually the most common measure of ductility utilized by FRP
central zone. This implies that U-jacket anchorages lose effective- design guidelines such as ACI 440.2R-08 (2008). It may be more
ness with increasing beam width. Although the authors concluded beneficial for future researchers to measure the tensile reinforce-
that the inclined and X-shaped anchors successfully prevented both ment strain to quantify ductility performance.
forms of plate end and IC debonding, premature concrete crushing
failure prevented the occurrence of FRP rupture, masking the full
Prestressed U-Jackets
potential of the anchorages from being realized.
Duthinh and Starnes (2001) also confirmed that concrete crush- Prestressed U-jackets are a method of anchorage on which little
ing was the controlling failure mode in two out of the three research has been conducted. The advantages of prestressing stem
specimens that they tested, and the other mode was a combination from the increased level of confinement and the higher shear resis-
of U-jacket rupture and intermediate flexural-shear crack debond- tance provided by the prestressed U-jackets. In practical applica-
ing. The laboratory program comprised 2–6 layers of 200 mm wide tions, prestressing was introduced onto the sides of the CFRP
CFRP jackets placed diagonally on each plate end. The inclined U-jackets by Pham and Al-Mahaidi (2006) by introducing a gap
fibers effectively prevented cover separation failure at the plate between the jacket and the concrete soffit, as presented in Fig. 3.
ends. It was found that two and six layers of jacket anchored A prestressing strain of 500 με was introduced into the jacket
the carbon plate to strain levels of 8,260 and 11;000 με, respec- sides by inserting wedges into a preformed gap. Beams with pre-
tively, without slippage. The above research demonstrates the clear stressed jackets showed no evidence of slippage in the anchorage
advantages of using inclined U-jackets as opposed to perpendicular zone at failure. This was attributed to an increase in concrete shear
orientations at the CFRP plate ends. In addition to the jackets capacity in the anchorage zone as a result of the compressive stress
providing confinement, an improvement of bonding and resistance induced by the U-jackets. The legs of the prestressed U-jackets did
Fig. 3. Two anchorage systems used by Pham and Al-Mahaidi (2006, © ASCE)
(Fig. 4), adhesively bonded metallic U-jackets, and U-jackets with cracking. The authors stipulated that clamping combined with
end clamping. Researchers such as Garden and Hollaway (1998), adhesion can double or triple the anchorage capacity that can be
Spadea et al. (1998), Duthinh and Starnes (2001), and Wu and expected from the bond alone. However, no investigations were
Huang (2008) have found that the use of metallic anchorages carried out using bolted anchorages without torque to assess the
provides a significant increase in anchorage strength in addition contribution of clamping force on anchorage enhancement within
to ductility enhancement. the context of the test setup.
Previous experimental testing demonstrated the ineffectiveness Spadea et al. (1998) attempted to improve the performance of
of bonded angle sections for plate-end anchorage due to the lack of CFRP-strengthened RC beams by using external steel anchorages
a secure plate end fixing to the concrete. Experiments were con- designed to control and minimize the bond-slip between the
ducted by Garden and Hollaway (1998) with a number of 1.0 m concrete beam and the CFRP plate. The anchorages consisted of
long plated beams tested in four-point bending. Cantilevers were U-shaped steel anchors installed at the plate ends, together with
also tested to demonstrate that the structural benefit of plate-end four to eight U-shaped steel anchorages distributed throughout
anchorage diminishes as the shear span/depth ratio of the beam the span, The plates were bonded to the concrete using epoxy resin
increases. Each beam was strengthened with 67 mm wide and and contained no external bolts or mechanical fasteners. Experi-
0.87 mm thick, 111–115 GPa modulus CFRP plates. The bolted mental testing measured maximum fiber strain utilizations of
plate-end anchorage system used comprised a 40 mm long steel 80% (12;000 με) for beam specimens with end anchorages at
anchorage block of the same width as the composite plate. The the plate ends, together with eight U-shaped anchorages distributed
block was secured to the composite plate using laminate adhesive throughout the span, corresponding to a 67% enhancement over the
and two mild steel bolts. corresponding unanchored specimen. In addition to the enhanced
A comparison was made between the mechanically fastened fiber utilization and strength enhancement provided by the steel
steel anchorages and where the bonded plate was continued under anchorages, greater ductility and gradual debonding of the plate
the supports of the beam, resulting in a clamping force applied nor- over an extended time increment were also observed.
mal to the plate. The authors concluded that the main requirements Ductility was evaluated through an examination of deflection
of bolted plate-end anchors were the shear resistance of the anchor (deflection ductility), curvature (curvature ductility), and the
bolts and the FRP-steel adhesive bond. The conclusion was based area-under-the-load deflection curve at yielding of the tension steel
Fig. 4. (a) Typical FRP plate anchored using permanent mechanical anchorage device [Reprinted with permission from Kalfat (2008)]; (b) schematic
of typical test setup
hesive, special mechanical fasteners are installed longitudinally crete substrate and the dowel length can be confined to the cover
along the FRP reinforcement at a specified spacing. Insertion region of the member. The other end of the anchor is epoxied onto
of the mechanical fasteners follows the same procedure as the the surface of the FRP plate. The ends of the fibers which are
MF-FRP method. The fasteners do not carry any bearing forces, but splayed and epoxied onto the surface of the plate in order to disperse
act to increase the bond strength between the FRP and the concrete local stress concentrations are herein referred to as the anchor fan.
by resisting the tensile peeling stresses which can initiate a debond- A convenient means by which to determine the fundamental
ing failure. strength and behavioral characteristics of FRP anchors is to test
Wu and Huang (2008) observed two distinct failure modes of them in FRP-to-concrete joint assemblies such as that shown in
the hybrid system, namely (1) CFRP rupture at midspan, which Fig. 6(d), from Zhang et al. (2012) and several researchers have
occurred with specimens strengthened with 2- and 4-ply strips, investigated such joints to date (e.g., Zhang et al. 2012; Zhang
and (2) complete strip debonding, which was observed for the and Smith 2012a, b; Niemitz 2008). A generic load-slip response
specimen strengthened with 6-ply strips, indicating that the bond of single fan and bow-tie anchors is shown in Fig. 6(e). The three
strength had been exhausted. Considerable increases in flexural main stages of the load-slip response are denoted by A (i.e., debond-
capacity and bond strength were observed as a result of the hybrid ing and activation of FRP anchor), B (i.e., postpeak reserve of
plate-bonding system. A 79% increase in moment resistance was strength offered by completely intact FRP anchor and frictional
attributed to the addition of the fasteners alone for the same area of resistance of debonded plate), and C (i.e., postpeak reserve of
CFRP. However, the increase in bond strength was even higher than strength offered by partially intact FRP anchor and frictional resis-
the moment increase. This resulted in specimens mechanically tance of debonded plate). Ongoing research is establishing the key
fastened with 4-and 6-ply strips splies reaching flexural strengths loads (P) and slips (δ) for varying anchor material and geometric
of 184.9% and 268.2%, respectively, higher than the 2-ply speci- properties (e.g., Kim and Smith 2009; Smith 2010; Zhang et al.
men with no fasteners. 2012). A review by Smith (2010) reported that FRP spike anchors
The application of steel anchorages to CFRP-strengthened with a single fan component increase the shear strength and slip
members is limited by factors such as cost, practicality, labor capacity of FRP-to-concrete joints by up to 70% and 800%, respec-
intensiveness, and durability. Drilling threaded rods or expansion tively, over unanchored control joints. Of particular interest in
anchors into existing structures is time-consuming and has the po- Fig. 6(f) is the significant effect of dowel angle on the joint strength
tential to damage existing reinforcement. In addition, long-term enhancement over the unanchored control joint (Zhang and
durability is a concern and is aggravated by the galvanic coupling Smith 2012a).
with the carbon fiber, which must be mitigated by use of a glass One of the earliest reported tests on FRP anchors in a concrete
fiber layer between the steel and the concrete. Research has dem- member was by Lam and Teng (2001). In their work, RC cantilever
onstrated that steel anchorages generally provide higher anchorage slabs of 700 mm span strengthened with glass FRP (GFRP) plate
strength than nonmetallic anchors because of their metallic rigidity bonded to the tension face of the slabs were tested. The use of a
and the ability of mechanical fasters to effectively resist tensile and GFRP anchor as a mechanical anchorage system can also prevent
shear forces. premature peeling of CFRP laminates in the presence of curvature.
Fig. 5. (a) Mechanical fastener; (b) predrilled holes; (c) details of the HB-FRP system; (adapted from data from Wu and Huang 2008)
FRP
anchor
dowel
region
146%
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150%
125%
50%
24%
0%
DA-45 DA-90 DA-112.5 DA-135
(d) (e) (f)
Fig. 6. (a, b, c) Anchor construction and installation of FRP anchors (reprinted from Engineering Structures, Vol. 33, No. 4, Smith, ST, Hu, S, Kim,
SJ & Seracino, R 2011, “FRP-strengthened Rc slabs anchored with FRP anchors”, Pages 1075–1087, April 2011, with permission from Elsevier);
(d) test setup (single lap) (reprinted from Construction and Building Materials, FRPRCS9 Special Edition, H.W. Zhang, S.T. Smith, S.J. Kim,
“Optimisation of carbon and glass FRP anchor design”, Pages 1–12, June 2012, with permission from Elsevier); (e) generic load-slip response
of FRP-to-concrete joint anchored with bow-tie anchor; (f) joint strength enhancement (above unanchored control) [modified from Zhang and
Smith (2012b)]
Eshwar et al. (2005) investigated 200 × 400 mm RC beams span- smaller anchors and reduced spacings were more effective in fully
ning 5.5 m with both straight and curved beam soffits (curvature developing the capacity of the FRP fiber, as larger spacings did not
5 mm over 1 m). A single row of 10 mm FRP spike anchors was anchor the entire width of the FRPs, resulting in partial debonding
embedded 76 mm into the concrete beam at 500 mm spacings. (Orton et al. 2008).
Reductions in strength of 20% and 30% were observed in beams Lam and Teng (2001) conducted investigations on improving
strengthened with wet lay-up fibers and precured laminate due to the strength of wall cantilever slab connections using GFRP strips.
curvature and premature peeling. Inclusion of the anchor FRPs with Fiber anchors were installed to anchor the GFRP strips into the RC
the wet lay-up system applied to the curved-soffit specimen led to wall. The authors observed that debonding was stopped by the fiber
the strength being increased by 35% compared to the unanchored anchors and the slabs finally failed by tensile rupture of the FRP. In
specimen. This resulted in the strength of the curved-soffit beam tests on similar slabs simply bonded with two 80.5-mm wide GFRP
containing the anchor FRPs being higher than that of the flat soffit strips without the use of fiber anchors, debonding between the FRP
beam strengthened with wet lay-up fibers. Others have investigated and the slab occurred in all cases (Teng et al. 2000).
the performance of FRP anchors in flexural members (e.g., Micelli
et al. 2010). In most cases, the addition of FRP anchors was found
to increase the strength and ductility of the FRP-strengthened Evaluation of FRP Anchors Used to Strengthen
members. However, this is not always the case and reasons why Members in Flexure
remain to be addressed.
Further research has shown that the use of FRP anchors is an Grelle and Sneed (2011) recently established the need for a large
effective way to improve the strength of reinforced concrete database of anchorage test results. This section therefore presents a
members. Orton et al. (2008) determined that two rows of three database of selected strain data for FRP anchorage systems, in
10 mm diameter anchors were able to develop the FRP tensile which each anchorage type can be compared using a common
capacity and led to fracture of the entire width of the FRP. They correlation parameter. In order to comparatively assess each
reported that FRP anchors increased the efficiency of material us- anchorage, the concrete strength ( fc0 ), fiber modulus (Ef ), number
age of the FRP retrofit to 57%, indicating that FRPs with anchors of plies (n), and fiber thickness (tf ) were used to standardize the
are able to achieve a given strengthening capacity and require less strain data from experimental results collected from a number of
material than unanchored FRPs. In this case, the strength of researchers. Fiber modulus, number of plies, and fiber thickness
the member increased by 270%, with only a 175% increase in the all affect the magnitude of FRP-to-concrete bond stresses at the
FRP material. In addition, it was found that a greater number of interface at a given level of FRP strain, whereas concrete strength
jected to combined shear and torsion in a configuration similar to that The effect of using continuous and discontinuous steel/CFRP
shown in Fig. 7. The experiments utilized a bidirectional carbon plates bonded to the top and bottom of shear reinforcement was
composite fiber with 45° fiber orientation and a modulus of elas- investigated by Ortega et al. (2009). The steel/CFRP plate anchors
ticity of 63.3 GPa. In this technique, the U-jacket was bonded to the were fixed using concrete wedge anchors and steel bolts. A typical
web of the beam and anchored 50 mm below the intersection of the representation is shown in Fig. 8. In this study, continuous mechan-
web and the flange. An additional steel angle fastened to the beam ically fastened steel plate anchorages were ineffective because the
flange with 20 mm diameter steel threaded rods was used at the en- continuous plate exhibited a bucking failure mode due to the cur-
trance of the flange and the web to delay end-jacket debonding fail- vature of the beam at failure. The fasteners exhibited bearing failure
ure. Using the extended U-jacket together with mechanically in some locations. In addition, slippage of the CFRP prevented the
fastened steel angles was found to be more effective than using CFRP shear reinforcement from reaching its full capacity. This was
the U-jacket anchored to the beam web with 20 mm rods only. solved by the development of a modified anchor bolt system, which
A 23% increase in strength and an enhanced ductility of 38% consisted of wrapping the CFRP composite around the first plate
were achieved compared to that of the web-anchored U-jacket tech- and overlapping with the second plate, creating a three-layer
nique. Ductility was measured by considering both deflection and connection.
twist ductility (monitoring the maximum angle of twist) and the This behavior was also verified by Aridome et al. (1998), who
maximum strain level of the steel reinforcement. The authors sug- concluded that continuous steel plate anchors separated prema-
gested that the enhanced torsion capacity was because of an in- turely due to in-plane bending stresses within the steel anchorage.
crease of the enclosed area inside the expected critical shear Staggered plate anchors were found to provide the highest beam
flow path induced by the mechanical anchorage provided into ductility, which was measured by monitoring beam deflections.
the beam flanges. However, no comparisons with unanchored To equate vertical deflections with ductility is not representative
U-jacketed specimens were made to assess the contributions of of the beam’s ability to undergo sufficient cracking and deformabil-
the steel anchorages. ity prior to failure. Cracking and deformability are the current mea-
Mechanically anchored U-jackets have achieved greater sures used to ensure ductility in FRP-strengthened members in FRP
effectiveness in pure shear applications (Aridome et al. 1998; design guidelines monitored by the strain level in the tensile
Maeda et al. 1997; Ortega et al. 2009; Tanarslan et al. 2008). reinforcement. The staggering of steel anchorages within the com-
An investigation into the shear behavior of concrete T-beams pression zone was important to reduce the overall compression
strengthened with alternative CFRP schemes was conducted by block stiffness, resulting in higher deflections. However, as a result
Tanarslan et al. (2008). The study encompassed specimens of plate staggering, the compression block stiffness shifts the
retrofitted with CFRP side bonding, L-wrapping (leg of L devel- neutral axis of the section toward the bottom fiber, resulting in lower
oped beneath flange), U-jacketing, and extended U-jacketing. Steel strain in the tensile reinforcement and a lower degree of cracking.
anchorages were applied to CFRP sheets in both top and bottom Alternative variations of metallic anchorage devices were used
locations for four of the specimens tested. In addition, 10 mm by Aridome et al. (1998), The configurations investigated are
threaded rods were used to fasten the 50 × 50 × 5 mm steel plates shown in Fig. 9. Although strengthened beams without any
Fig. 7. Implemented strengthening schemes: (a) U-jacket; (b) extended U-jacket (adapted from Deifalla and Ghobarah 2010)
Fig. 9. Steel anchorage schemes for strengthening of T-beams in shear (adapted from Aridome et al. 1998)
anchorage at the underside of the flange were not tested, the re- capacity of the FRP, while a 200 mm embedded length is sufficient
searchers reported yielding of the main flexural reinforcement in to develop the full tensile strength of the FRP. Although these
all the strengthened beams with steel anchorages. It was also found figures show significant promise, the test ignores the high compres-
that the strengthened beams with angles bolted into the flange sive forces in the direction of the beam’s length which are present
reached a higher load than bolting angles into the web. This has in the flange. These forces may in turn affect the strength of the
been consistently verified by many researchers. anchorage.
Lee and Al-Mahaidi (2008) and Lee (2003) conducted large
scale experimental investigations on the shear-strengthening of
Anchorage of FRP through Concrete Embedment reinforced concrete T-beams using two L-shaped shear jackets
Embedment of the L-shaped or U-shaped fibers within the flange 40 wide and 1.2 mm thick. The shear jackets were embedded
of the T-beam is a form of anchorage involving local cutting, 100 mm into the flange of the beam for suitable anchorage. Photo-
breakout, and removal of concrete to the underside of the beam grammetry was used to record deformation measurements. Anchor-
flanges. The breakouts are typically filled with epoxy resin after age failure was initiated at the beam soffit by an abrupt ripping of a
embedment with composite fiber ligatures, as presented in Fig. 10. concrete portion at the CFRP bend zone, resulting in separation
Although lacking the inherent drawbacks of full wrapping because failure of the CFRP laps at the beam soffit (Lee 2003). Measure-
no access is required to the top of the slab, embedment can be ments of average strains indicated that 5;500–8;884 με was
a labor-intensive, destructive process, particularly where a small achieved prior to the occurrence of this failure. Because no observ-
ligature spacing is required. able CFRP pull-out from the flange was recorded, it is difficult
Pull-out tests reported by Swiss Federal Laboratories for Mate- to assess the residual capacity of the top embedment anchorage.
rials Science and Technology (EMPA) (1998) have revealed that a It is believed that the use of the rigid L-plates may have been
100 mm embedment is sufficient to develop 60–80% of the tensile responsible for the initial debonding due to peeling stresses being
Fig. 10. (a) Typical FRP plate embedded 150 mm into beam side with epoxy resin; (b) schematic of typical test setup
chorages used (Ceroni et al. 2008). the fan opening angles should be limited to less than 90° to
Experimental testing to determine the improvement from the use limit stress concentrations and prevent premature fracture of the
of such anchors has been limited to date. In the context of the an- FRP fiber.
chor pull-out scenario shown in Fig 11, experiments have been con- FRP spike anchors have also been successful in strengthen-
ducted to date. Investigations have been carried out by Ozdemir ing L-shaped concrete specimens confined with FRP jackets.
(2005) to determine the required embedment depth into the con- Karantzikis et al. (2005) concluded that a limited strength increase
crete to achieve full development of the anchor under pull-out con- is observed in the use of jackets without anchors, regardless of the
ditions. Ozdemir determined that there is an effective embedment FRP thickness used. This was due to poor utilization of the FRP as
depth after which the capacity of the anchor no longer increases. a result of premature debonding at the reentrant corner. Partial
Tests were conducted using 10–20 MPa concrete with 14–20 mm depth FRP anchors were found to allow the jacket to deform sub-
diameter anchors, and the embedment depth was suggested as stantially and even approach its tensile capacity. Increases in
100 mm. Ozbakkaloglu and Saatcioglu (2009) also conducted a strength of 20–30% were seen due to the anchors only. The use
large number of pull-out tests with 25–100 mm embedment and of full-depth anchors resulted in increased strength (49% increase
concluded that an increase in embedment length results in a de- due to anchors only) but marginal benefits in deformability. Further
crease in the average bond strength. This implies that the bond research has demonstrated that FRP jackets and anchors effectively
stress distribution decreases with increasing bond length. Tests confine deficient column lap splices and successfully alter the col-
and modeling of FRP anchors subjected to pull-out forces have also umn failure mode from brittle splice failure to yielding of column
been undertaken by Kim and Smith (2009a, b, 2010). reinforcement (Kim et al. 2009). It was found that increasing the
An important characteristic of FRP anchors is the bend that ex- spacing of anchors improved the strength of the splice, while de-
ists between the braided fiber toe embedded in the concrete and the formation capacity was improved by using a greater number of
fanned portion of the anchor in shear applications. This bend is smaller anchors. There is currently a lack of available data in which
typically 90 degrees. ACI 440.2R-08 (2008) states that where fibers FRP anchors have been applied to anchor FRP shear fibers, where
wrap around the corners of rectangular cross sections, the corners sufficient measurements were reported. This should be a focus for
should be rounded to a minimum 13 mm radius to prevent stress future studies.
concentrations in the FRP system. Specimens tested by Pham and
Bayrak (2009) utilized a bend radius ranging from 0–12 mm and
Anchorage Improvement through a Mechanically
recorded a 23% reduction in anchor strength when no bend radius
Strengthened Substrate
was used. Based on previous research by the Japan Society of Civil
Engineers (JSCE) (2001), anchors could lose about half of their It is presently understood that the strength of the concrete substrate
tensile capacity due to the stress concentration caused by the anchor is a key factor affecting the debonding mode and overall bond
bend. Orton et al. (2008) suggested that anchors with two times the strength of FRP-to-concrete joints. However, increasing the
cross-sectional area of the longitudinal CFRP should be used in strength of the concrete substrate has experienced little investiga-
practice. Ozbakkaloglu and Saatcioglu (2009) also investigated tion to date. Research conducted by Al-Mahaidi and Sentry (2009)
Fig. 11. Typical details of FRP spike anchors applied to shear applications
amount of steel reinforcement in the web-flange joint and aid rectional carbon fiber wrap applied horizontally across the laminate
the flow of shear and torsional forces into the flange. The omission strip. The direction of the fibers was perpendicular to the direction
of the bar in future specimens is not expected to adversely affect the of the strip. The first sheet overlayed the second, sandwiching the
substrate properties but this remains to be verified by experimental laminate strip in between. Anchoring the ends of CFRP laminates
testing. Specimens consisted of a single CFRP laminate of dimen- in this manner was effective in increasing the ultimate failure load
sions 120 × 2 × 1000 mm bonded to the surface of the concrete by 39–43% and resulted in an increase in the maximum laminate
block with a bond length of 500 mm. strain of 19–28%. The authors concluded that carbon fiber fabrics
The introduction of the mechanical chase was observed to shift applied horizontally across the laminate strip did not provide an
the debonding failure mode from within the concrete cover zone to effective level of confinement to uniformly increase the bond
the CFRP-adhesive interface. An examination of the laminate after strength between the adhesive and concrete layer. This was verified
failure revealed the majority of the surface exposed with little through an examination of the bond-slip relations and the fact that
epoxy bonded to it. In addition to almost doubling the anchorage no increase in bond stress was observed as a result of the anchor-
capacity, significantly higher bond stresses of up to 11 MPa were age. It was stipulated that the use of a 50 mm wide adhesive tapper,
recorded in the strengthened substrate specimens, while only introduced to the laminate sides to provide a smooth transition for
5.0 MPa was achieved in control models. This corresponded to the unidirectional fibers along the length of the laminate, assisted in
a 95–100% increase in ultimate capacity, a 118% increase in bond the distribution of laminate-adhesive stresses to a greater width of
stress, and a 83–93% increase in the maximum strain level reached concrete.
Fig. 12. Anchorage type 1 specimen geometry: (WG1 and WG2) (a) configuration of strain gauges; (b) chase details and installation of N24
reinforcement bar; (c) section through chase [Kalfat and Al-Mahaidi (2011); reprinted from Composite Structures, Vol. 92, No. 11, R. Kalfat,
R. Al-Mahaidi, “Investigation into bond behaviour of a new CFRP anchorage system for concrete utilising a mechanically strengthened substrate”,
Pages 2738–2746, October 2010, with permission from Elsevier]
Fig. 13. Anchorage types 0 and 2–6 specimen geometry and material properties: (a) type 4 (WG12); (b) type 5 (WG10, WG11); (c) type 6 (WG8);
(d) anchorage types 2–5 applied to a box girder bridge [Al-Mahaidi and Kalfat (2011); reprinted from Composite Structures, Vol. 93, No. 4,
R. Al-Mahaidi, R. Kalfat, “Investigation into CFRP plate end anchorage utilising uni-directional fabric wrap”, Pages 1265–1274, March 2011, with
permission from Elsevier]
Anchorage type 3 was developed for use in combined shear and Anchorage type 6 was later developed to improve the perfor-
torsional strengthening applications to adequately anchor external mance of the type 3 anchor by adding a single layer of bidirectional
laminates applied to the outer webs to the beam soffit. Type 3 fiber to the unidirectional fiber, which continued around the corners
anchors utilized L-shaped lengths of CFRP unidirectional fibers of the concrete prism. The anchorage utilizes the combined benefits
applied to the corners of a box section. These were indented of types 3 and 5 and results in a distribution of fiber-to-adhesive
to be appropriately lapped with a CFRP laminate applied to the bond stresses over a greater length and width of concrete, achieving
main faces of the concrete prism. The overall increase in strength an increase in failure load of 195% and resulting in laminate
(46–57%) of this anchorage system was attributed to the transfer rupture. The above mentioned results suggest that the use of uni-
of bond stress further away from the loaded edge, which was directional and bidirectional fiber as a means of creating a greater
facilitated by the anchoring effect of the unidirectional fiber curved bond area with the concrete substrate allows substantially higher
and anchored around the end of the concrete block. In order to utilization of carbon fiber laminates beyond a standard codified
achieve a more efficient distribution of fiber-adhesive stresses over design approach.
a greater area of concrete, two layers of a bidirectional fiber were
implemented in anchorage types 4 and 5 to anchor the CFRP lam-
inate. The results demonstrate that bidirectional fiber (45°) ap- Evaluation of FRP Anchors Used to Strengthen
plied to the ends of CFRP laminates resulted in a more efficient Members in Shear
distribution of CFRP-adhesive stresses over a greater width of
concrete and was effective in providing a 93–109% increase in In order to evaluate the various types of anchorages used to
failure load. increase the effectiveness of FRP shear strengthened members,
Table 1. (Continued.)
f c0 tft Ef εf;max
Author Specimen Comments MPa mm GPa με kfa Failurea
(Pan et al. 2010) B4 Double notched beam with side plates 49.2 0.22 235 6,492 0.51 IC
(Pan et al. 2010) B5 Double notched beam with FRP plate 49.2 0.22 235 10,217 0.81 IC
(Pan et al. 2010) B6 Unnotched beam with FRP plate 49.2 0.22 235 10,489 0.83 IC
(Pan et al. 2010) B7 Precracked bonded with FRP plate 49.2 0.22 235 9,399 0.74 IC
(Pan et al. 2010) B8 Unnotched beam with FRP plate 49.2 0.22 235 9,954 0.79 IC
Prestressed U-jacket Anchor 0.78 (Average)
(Pham and Al-Mahaidi 2006) A2a 1 prestressed U-jacket-3 × 12 mm dia bars 53.7 1.056 209 4,571 0.71 IC
(Pham and Al-Mahaidi 2006) A2b 3 prestressed U-jackets at 180 mm c=c − 3 × 12 mm dia bars 53.7 1.056 209 5,416 0.85 IC
Inclined FRP U-jacket Anchor 1.36 (Average)
(Sagawa et al. 2001) U1-45-1 Inclined U-jacket anchor, 1 place 27.3 0.165 230 15,000 1.36 FR
(Sagawa et al. 2001) U1-45-2 Inclined U-jacket anchor, 2 places 27.3 0.165 230 15,000 1.36 FR
FRP + steel anchorage 1.87 (Average)
(Spadea et al. 2000) A1.2 Steel anchorages Type A/Type B 30 1.2 152 9,600 1.83 ED
(Spadea et al. 2000) A1.3 Steel anchorages Type A/Type B/Type C 30 1.2 152 10,500 2.00 ES/ED
(Spadea et al. 2000) A2.2 Steel anchorages Type A/Type B-Arr1 30 1.2 152 10,000 1.90 ES/ED
(Spadea et al. 2000) A2.3 Steel anchorages Type A/Type B-Arr2 30 1.2 152 11,000 2.09 ES/ED/CC
(Spadea et al. 2000) A3.2 Steel anchorages Type A/Type B 30 1.2 152 10,200 1.94 ED
(Spadea et al. 2000) A3.3 Steel anchorages Type A/Type B/Type C 30 1.2 152 12,000 2.28 ES
(Duthinh and Starnes 2001) B4a Steel clamp at laminate ends, 400 N.m 42.3 1.2 155 10,070 1.63 ED
(Duthinh and Starnes 2001) B6 Steel clamp at laminate ends, 400 N.m 41.3 1.2 155 7,800 1.28 ES
FRP Anchors 1.14 (Average)
Smith et al. (2010) S3 FRP anchors along whole span (Type A) 41.4 0.498 239 7,676 1.00 IC
Smith et al. (2010) S4 FRP anchors along whole span (half no. anchor as S3) (Type A) 44.1 0.498 239 8,025 1.02 IC
Smith et al. (2010) S5 Shear span FRP anchors (Type A) 44.1 0.498 239 8,884 1.13 IC
Smith et al. (2010) S6 Plate end FRP anchors (Type A) 45.4 0.498 239 6,696 0.84 IC
ARS = anchorage failure at soffit; ASF = adhesive separation failure; CSF = concrete separation failure; FF = flexural failure; FR = fiber rupture; PFR = partial fibre rupture; CPO = concrete pull-out failure;
Failure Typea
a classification and evaluation approach is adopted based on the
(Average)
(Average)
PASF/LR
effective strain approach given in ACI 440.2R-08 section 11.4.1
ASF
ASF
CSF
CSF
(2008) for shear-strengthened members, the results of which are
presented in Table 1. The FRP effective strain is used to determine
the anchorage effectiveness factor (kfas ), using Eq. (2):
3.15
3.11
2.63
2.84
4.02
2.55
2.49
2.62
εf;max
kfas
kfas ¼ ð2Þ
kv εfu
Shear
εf;max
5,800
4,900
5,300
7,500
4,640
4,881
k1 k2 L e
kv ¼ ≤ 0 · 75 ð3Þ
11;900εfu
GPa
Ef
210
210
210
210
210
210
23;300
Le ¼ ð4Þ
ðnf tf Ef Þ0·58
mm
tft
2
2
2
2
2
2
2=3
f c0
Downloaded from ascelibrary.org by MISSOURI, UNIV OF/COLUMBIA on 03/26/13. For personal use only.
MPa
k1 ¼ ð5Þ
fc0
62
62
62
62
62
62
27
8 9
< dfv −Le =
WG8-bidirectional fiber (1 ply, 45) + unidirectional fiber (2 ply), 0°
dfv u-wraps
k2 ¼ dfv −Le ð6Þ
: ;
dfv side-bonded
WG12-bidirectional fiber (1 ply), 45 þ50 mm lip
Type 1
Type 1
of the anchorages, some of their data has been omitted from Table 2.
Type
Type
Type
Type
The data for flange embedment anchors are currently limited and
more data are required to establish statistical reliability, the anchor-
age effectiveness factor of 4.27 being well above the other forms of
CFRP + Mechanical Substrate strengthening
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