Strengthening of rc beams using prestressed fiber reinforced polymers – a review

Review
Strengthening of RC beams using prestressed fiber
reinforced polymers – A review
Muhammad Aslam 1
, Payam Shafigh, Mohd Zamin Jumaat ⇑
, S N R Shah
Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
h i g h l i g h t s
Compared to prestressed steel, prestressed FRPs exhibited good structural properties.
CFRPs are suitable prestress materials, it covers all the advantages of other FRPs.
The NSM technique offers a shield to the prestressed material against the environment.
The 40% prestressing in NSM CFRP rod can give maximum flexural strength of the beam.
Among different CFRP shapes, CFRP rods are the most effective type under prestressing.
a r t i c l e i n f o
Article history:
Received 11 April 2014
Received in revised form 28 January 2015
Accepted 18 February 2015
Available online 10 March 2015
Keywords:
Strengthening
Composite
RC beam
Flexure
Anchorage
Fiber reinforced polymer
Prestressing
Near surface mounted
a b s t r a c t
The use of steel and fiber reinforced polymers (FRPs) for strengthening RC beams can significantly
improve the flexural strength, fatigue life and the serviceability of the beams compared to un-strength-
ened beams. Prestressing materials enable the material to become more efficient since a greater portion
of its tensile capacity is employed. Investigations have shown that prestressed FRPs are effective materi-
als for strengthening deteriorated structures. This paper presents a comprehensive review on the flexural
behavior of strengthened RC beams using prestressed FRPs. The review covers the near surface mounted
(NSM), externally bonded reinforcement (EBR) and externally post-tensioned techniques (EPT) and the
corresponding advantages and disadvantages are highlighted. Anchorage systems and the effect of pre-
stressing levels on the ductility, deformability and bond behavior of prestressed FRPs are also addressed.
Recommendations for the future research are also presented.
Ó 2015 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
2. Prestressed strengthening materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
2.1. Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
2.2. Fiber reinforced polymers (FRP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
3. Research progress on prestressed near surface mounted CFRP reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
3.1. Anchorage and prestressing systems for NSM CFRP reinforcement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
3.2. Failure modes of prestressed CFRP reinforcement in NSM technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
3.3. Bond behavior of prestressed NSM CFRP reinforcement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
3.4. Effect of prestress level on ductility and deformability in NSM CFRP strengthened RC beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
3.5. Finite element validations in prestressed NSM CFRP reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
http://dx.doi.org/10.1016/j.conbuildmat.2015.02.051
0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +60 379675203; fax: +60 379675318.
E-mail addresses: aslam_bhanbhro13@yahoo.com, bhanbhroma@gmail.com (M. Aslam), pshafigh@gmail.com (P. Shafigh), zamin@um.edu.my (M.Z. Jumaat),
naveedshah@muetkhp.edu.pk (S N R Shah).
1
Tel.: +60 1123246915; fax: +60 379675318.
Construction and Building Materials 82 (2015) 235–256
Contents lists available at ScienceDirect
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4. Research progress on prestressed externally bonded (EB) CFRP reinforcement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
4.1. Anchorage and prestressing systems used for CFRP in EBR technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
4.2. Failure modes of prestressed CFRP reinforcement in EBR technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
4.3. Effects of externally bonded prestressed CFRPs on bond behavior, ductility and deformability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
4.4. Prestressed NSM CFRP strips versus external prestressed CFRP strips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
4.5. Finite element validations in prestressed externally bonded CFRP reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
5. Research progress on externally post-tensioned CFRP behavior (EPT CFRP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
5.1. Advantages and disadvantages of external post-tensioning (EPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
5.2. Failure modes of prestressed CFRP reinforcement in EPT technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
5.3. Effects of externally post-tensioned steel and FRP tendons on ductility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
5.4. Behavior of the deviators and the second-order effects in externally Post-tensioned tendons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
5.5. Finite element validations of externally post-tensioned CFRP reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
6. Advantages of NSM technique over EBR and EPT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
7.1. Future research recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
1. Introduction
The service life of concrete structures mainly depends upon
both the satisfactory design and use of appropriate construction
materials. Any flaw in these factors may result in the early degra-
dation and deterioration of the structure. The replacement of such
structures is not always possible due to the high operational
expenditure and their usage limitations. The only way to retain
the structure in a safe working mode is to strengthen or renovate
the structures. Consequently, the strengthening of deficient struc-
tures needs a complete understanding of the material and the
adopted strengthening techniques. A number of attempts have
been undertaken by the researchers to identify the most suitable
materials and appropriate techniques to strengthen the deficient
structures, which resulted in an abrupt increase in the use of the
prestressing technique for this purpose. Nowadays, strengthening
has widely been done using prestressed materials, such as steel
tendon and fiber reinforced polymers [1–7]. Prestressing creates
a negative moment in the member and enhances its properties,
which enables the member to sustain higher loads.
The post-tensioned steel reinforcement is widely recognized for
its satisfactory performance in strengthening concrete structures
[8,9]. Though prestressing steel tendons, usually high tensile steel
rods (tendons), are used to sustain clamping load which creates
compressive stress equivalent to the tensile stress due to bending
load. However, it is a fact that steel is relatively weak in respect of
environmental impacts. Additionally, due to more complex behav-
ior, the strengthening of structures using prestressing materials
needs a careful design approach and a full understanding of the
behavior of both the materials and elements.
The development in experimental research has determined that
alternative strengthening materials such as fiber reinforced poly-
mers can increase the flexural strength of the structure, provides
resistance against higher temperature, greater corrosion resistance
and minimize the weight of the structure. In general, beams
strengthened with FRP composites significantly contributed to
enhance the flexural strength, fatigue life and the serviceability
of the beams over un-strengthened beams [10]. Prestressing the
FRPs allows the material to efficiently utilize its tensile capacity
which enhances its ultimate and serviceability limit capabilities
Fig. 1. Chart of strengthening process.
236 M. Aslam et al. / Construction and Building Materials 82 (2015) 235–256

[11]. Investigations have shown that prestressed FRPs are effective
materials for strengthening deteriorated structures [9,12]. The
popular types of FRP identified by researchers include aramid fiber
reinforced polymers (AFRP), carbon fiber reinforced polymers
(CFRP) and glass fiber reinforced polymers (GFRP) in the form of
rods, strips, plates and laminates. All of these materials have been
investigated under a variety of strengthening techniques that
include near surface mounted (NSM), externally bonded reinforce-
ment (EBR) and external post-tensioning (EPT) using anchorage
and non-anchorage systems. The strengthening process is shown
in Fig. 1.
In this paper, the literature about different prestressed materi-
als was reviewed for strengthening purposes. The aim of this study
is to find out that which material has received considerable atten-
tion from researchers using different prestressing techniques. The
use of prestressed CFRP under near surface mounted, externally
bonded reinforcement and externally posttensioned techniques is
presented and the corresponding advantages and disadvantages
are highlighted. The influence of the anchorage system on pre-
stressed materials, major failure modes and the serviceability of
the strengthened beams are also discussed. The effect of the pre-
stressing level on the ductility, deformability and bond behavior
of prestressed CFRP is also addressed. Conclusions were made on
the basis of the information extracted from the literature and
future recommendations are provided accordingly.
2. Prestressed strengthening materials
Contemporarily, steel and FRPs have been used in the strengthening of dete-
riorated structures. Steel is the traditional and commonly used material. Though
all the materials are adequate for strengthening, however, the passage of time
and the advancement in the subject have led researchers to explore strengthening
materials that have the ability to provide maximum benefits in terms of strength,
serviceability and construction, as well as maintenance cost. These efforts revealed
that prestressing can considerably increase the limit state properties of strengthen-
ing materials. In this section, the behavior of the different prestressing materials is
discussed and various advantages and disadvantages are also highlighted.
2.1. Steel
The ductility, good strength to weight ratio and low fabrication and erection
costs make steel a suitable material for strengthening purposes in both normal
and prestressed conditions. Strengthening by prestressed steel tendons is a popular
method due to its availability, uniform material properties, easy of working, high
ductility and high strength. Strengthening reinforced concrete beams with steel
tendons is an efficient technique. Tan and Tjandra [13] concluded that the life inter-
val of the prestressed steel tendons may be smaller, due to the fact that the steel
was stressed up to half of its ultimate strength value. Moreover, post-tensioned
steel tendon is more sensitive to corrosion than the internal steel reinforcement,
because, firstly, the steel tendons relatively have small diameter and, secondly high
strength steel is more susceptible to corrosion compared to normal internal steel
reinforcement. Even a small corroded spot or uniform corrosive layer can sig-
nificantly reduce the cross sectional area of the steel tendon. The exposition of
unprotected steel tendons to the environment, even for a few months, can cause
a large reduction in its mechanical properties. If un-bonded cables are used, they
must be protected by anti-corrosive material such as asphalt, oil, grease, or a com-
bination of plastic tubing and grease [9].
The disadvantages exhibited by prestressed steel tendons motivated the
researchers to identify a better replacement for steel for the purpose of strengthen-
ing. Importantly, the upgrading of concrete structures is not easy in most cases as
structures pose a difficult and different set of problems. Nordin [9] reported that
prestressed steel can be replaced by prestressed FRP materials to improve dura-
bility, first rate creep and relaxation behavior.
2.2. Fiber reinforced polymers (FRP)
The weak resistance against adverse environmental aspects, higher flexural
strengths and corrosion problems has given rise to the use of prestressed FRPs,
especially in the case of bridge structures. Moreover, when the length of the
strengthening components and handling arrangements on construction sites are a
significant consideration, FRPs, again, provide better performance than steel [14].
Prestressed FRPs compensate the weaknesses displayed by prestressed steel
and have been introduced in recent decades as a more suitable strengthening mate-
rial than steel. Prestressed FRPs have recently been used in concrete structural
members as external or internal reinforcement instead of conventional steel
tendons. FRPs have good potential of use due to their desirable properties in
prestressed conditions. These properties include high performance, high strength-
to-weight ratio, high stiffness to weight ratio, high energy absorption, corrosion
resistance and high fatigue resistance. When used for strengthening, prestressed
FRPs can control the aging of construction materials and can sustain the impacts
of vehicles and fire far better than prestressed steel. Seismic upgrading and changes
in the use of the structure are also more easily accommodated through the use of
these polymers.
Studies have shown that the ultimate load carrying capacity and serviceability
of prestressed FRP flexural strengthened beams are higher than that for their pre-
stressed steel counterparts [9,15–19]. Tables 1 and 2 showed the typical tensile
properties of various types of prestressed strengthening materials [20–22].
Prestressed FRPs have limited strain capacity and premature debonding failure
may occur in the earlier stages [16]. The deformability index can be used as a major
tool to avoid this phenomenon. By prestressing the FRP reinforcement, the stress in
the internal reinforcing steel and deflections will decrease and there will be a higher
utilization of the FRP materials. The prestressed FRP reinforcement and the internal
reinforcing steel work together directly from zero loading leading to higher con-
crete cracking, yielding and ultimate loads. Both the higher temperature and higher
levels of stress may result in creep deformation in the structure. This phenomenon
can be avoided by adopting the load range recommended by the fabricator [23].
The available forms of prestressed FRPs are AFRP, GFRP and CFRP. A prestressed
AFRP performs very well with a good fatigue properties, high strength to weight
ratio, excellent resistance to corrosion, quick and easy construction and low labor
costs [24]. Compared to prestressed steel, it also possesses a strong shield against
corrosion, high temperature and adverse environmental effects. At the same time
it has a lower weight than steel and a longer service life. However, AFRP shows
some non-negligible disadvantages. AFRP exhibits poorer performance in resisting
acid and alkaline media which can be more efficiently resisted by CFRPs [25].
The literature contains rare information about the strengthening of structures
using prestressed GFRP material. Lin et al. [26] recommended that the prestressed
GFRP is a powerful tool to produce a considerable increase in the load carrying
capacity of RC beams. GFRP has a modulus of elasticity closer to concrete but it
has lower elasticity and ductility as compared to steel, AFRP and CFRP, which mini-
mizes the usage of GFRP in the strengthening of RC structures.
Table 1
Typical uniaxial tensile properties of prestressing tendons (ACI Committee 215).
Properties Steel
tendon
7 wire steel
tendon
AFRP
tendon
GFRP
tendon
CFRP
tendon
Tensile strength
(MPa)
1400–
1900
1725 grade 1 1200–
2100
1400–
1700
1650–
24001860 grade 2
Density (Ib/ft3
) 490 N.A 75–90 75–130 90–100
Table 2
Typical uniaxial tensile properties of prestressing tendons (CAN/CSA-S806-02).
Mechanical properties Prestressing
steel
AFRP
tendon
GFRP
tendon
CFRP
tendon
Nominal yield stress (MPa) 1034–1396 N/A N/A N/A
Tensile strength (MPa) 1379–1862 1200–2068 1379–1724 1650–2410
Elastic modulus (GPa) 186–200 50–74 48–62 152–165
Density (kg/m3
) 7900 1250–1400 1250–2400 1500–1600
0 10000 20000 30000 40000
0
1000
2000
3000
4000
CFRP
AFRP
GFRP
Steel Bar
Steel Tendon
Strains ( ε )
Tensilestrength(Mpa)
Fig. 2. Stress–strain behavior of prestressed materials (tendons).
M. Aslam et al. / Construction and Building Materials 82 (2015) 235–256 237

Carbon fibers are sometimes called graphite fibers due to the percentage of car-
bon above 95% by weight [27]. Prestressed CFRPs are suitable where the strength,
stiffness, lower weight and fatigue are critical issues. Moreover, CFRPs are useful
in applications that require high temperature resistance, chemical attack resistance
and damping resistance. Keeping in view these significant facts, researchers have
primarily focused on investigating the prestressed CFRP behavior under NSM, EBR
and post-tensioning techniques. Tables 1 and 2 indicate that CFRP possesses almost
all the qualities of the other FRPs along with increased flexural strength and duc-
tility. CFRPs can strengthen the structure in shear by wrapping fibers round the sec-
tions better than other FRPs. It efficiently helps in the repair of damaged structures
and seismic retrofitting by increasing their load capacity and minimizing the cost of
replacing the deficient structure [28]. Kobraei et al. [29] investigated the effects of
using CFRP bars as shear reinforcement instead of steel stirrups in RC beams. It was
observed that the CFRP shear reinforced beams showed good crack extension in the
whole beam, however, the width of cracks is larger than the steel reinforced beams.
While by decreasing the distance of the CFRP shear reinforcement, the number of
the cracks will be increased but the width of the crack is thinner.
The development in the CFRP prestressing sector has increased with the passage
of time. Advance adhesives were also utilized to create a bond between CFRP and
structure to achieve an optimal strengthening method provided that the CFRPs
can be positioned and isolated easily [30]. The main advantages of using prestressed
CFRPs in the strengthening of beams are its low weight, high tensile strength, cor-
rosion resistance, good resistance to degradation and creep, electromagnetic neu-
trality, quick and easy construction, low relaxation losses, low labor costs and has
an ability to modify the elastic modulus [2,9,31]. Several concrete bridges in
Japan, UK, China, Europe and Canada have been strengthened using prestressed
CFRP [32]. Among the available FRPs, prestressed CFRP accounts for 95% usage in
applications adopted for the strengthening of structures [12]. At this stage, it is
essential to validate the suitability of prestressed CFRP usage under different pre-
stressing techniques. The stress strain behavior of prestressed materials (tendons)
is shown in Fig. 2.
3. Research progress on prestressed near surface mounted CFRP
reinforcement
The NSM method was initially presented in 1940 [33]. Steel
cables were applied but high corrosion affect led the replacement
of steel cables with FRPs. In 1960 scientists developed a strong
epoxy adhesive, and used it in the construction industry for bond-
ing the steel or FRP reinforcement in the sawed grooves on the con-
crete surface [16,34,35]. In the NSM technique, prestressed FRP
rods or strips are inserted into grooves on the concrete surface
and bonded to the concrete using epoxy adhesive [8]. Flexural
strengthening using prestressed NSM FRP reinforcement can
increase the ultimate strength of an RC member quite remarkably.
It also significantly changes the behavior of the member under ser-
vice loads or substantially increases the stiffness of the member.
Prestressed FRPs using the NSM technique can also reduce the duc-
tility of the strengthened beam [36]. It should also be noted that in
non-prestressed NSM FRP strengthening application only a portion
of the strength of the FRP reinforcement is effective and the system
is a passive strengthening technique that remains inactive until
additional loads are applied. Therefore, improving the
serviceability performance can be achieved by means of prestress-
ing the NSM FRP reinforcement. The application of NSM method
using prestressed FRP bar for strengthening RC beam is shown in
Figs 3 and 4.
Fig. 3. Schematic diagram of near surface mounted (NSM) prestressed FRP reinforcement.
Fig. 4. The application of NSM method using prestressed FRP bar for strengthening RC beam [37].

The following factors should be taken into account when
strengthening RC structures using the NSM system [19,38]:
Type of FRP reinforcement: The choice of FRP material with
higher tensile strength and modulus of elasticity such as CFRP
instead of GFRP and AFRP would allow the use of smaller FRP
and groove cross sectional areas, hence, less risks of interfering
with the internal reinforcement.
Groove dimension: The groove width, depth, clear distance
between two adjacent grooves, and the net distance between
a groove and the edge of beam are all relevant construction
parameters that can affect the bond performance. The depth
of the groove is limited by the depth of the concrete cover.
Groove filler: In terms of structural behavior, it’s most relevant
mechanical properties are the shear and tensile strengths; it can
be epoxy adhesive or cement mortar.
To validate the suitability of prestressed CFRP in the strengthen-
ing of RC beams, Nordin and Täljsten [12] performed fifteen full-s-
cale RC beam tests with different bond lengths. Two types of CFRPs
were investigated. It was found that using prestressed quadratic
CFRP rods increase the cracking, yield and ultimate loads of the
strengthened beams with respect to the reference beam. Later
on, El-Hacha and Gaafar [2] studied the advantages of non-corro-
sive CFRP reinforcement compared to the effectiveness of external
prestressing. Higher opening cracking and yielding of steel was
occurred in prestressed CFRP strips strengthened beams.
Increasing the prestressed level resulted in nominal moments.
Several other researchers also concluded the same that prestressed
CFRP composites can be used more efficiently due to their higher
tensile capacity, and contribute to sustain higher loading capacity
under both service and ultimate conditions [2,39].
The load–deflection relationship of a typical reinforced concrete
(RC) beam under static loading come across three stages: a pre-
cracking stage, a post-cracking/pre-yielding stage, and a post-yield-
ing stage. Fig. 5 shows that a beam strengthened with CFRP rod exhi-
bits similar load–deflection behavior, but higher cracking, yielding,
and ultimate loads [2]. El-Hacha and Gaafar [2] and El-Hacha et al.,
[11] proposed a load–deflection curve for an un-strengthened and
strengthened RC beams with non-prestressed and various levels of
prestressed FRP reinforcement (Fig. 6). It is evident that as the pre-
stressing level in the strengthened beams increased the service-
ability and ultimate strengths improved but the ductility is
reduced. The decrease in the ductility of the strengthened beams is
due to the prestressing effects that lead to less energy dissipation
and due to the increased tension reinforcement ratio as a result of
the supplemental NSM FRP reinforcement to the internal reinforcing
steel bars in the concrete member.
El-Hacha and Gaafar [31] and El-Hacha et al. [40] studied the
flexural behavior of the beams strengthened with prestressed
CFRP strips. The strips did not show any initial debonding and
the strips rupture was the governing failure mode. Prestressed
strengthening minimize the global deflection of the structure at
different load levels. Badawi and Soudki [41] concluded that, in
NSM technique, prestressed CFRP rods strengthened beams sub-
jected to static load could increase the yield load and ultimate load
capacity of the control beam by 26% and 50%, respectively. The ulti-
mate and serviceability condition of prestressed beams was found
significantly better than reference beam by using a proper
mechanical anchorage [35]. Badawi et al. [42] provided an equa-
tion to compute the prestressing stress in the NSM CFRP rod
employed at full length of the beam. A few past results proved that
all prestressed strengthened structures suffered with lower deflec-
tion as compared to reference beam, hence damage growth was
lower [43].
Hassan and Rizkalla [44] established a quantitative method by
providing two analytical models to control the debonding failure
of NSM CFRP strips and rods. They concluded that when NSM
CFRP bars were analyzed under static loading at different embed-
ment lengths, it can increase both the stiffness and flexural
strength of the concrete beams. In addition they determined that
the rupture of the NSM CFRP bar is not likely to occur regardless
of the embedment length used. The efficiency of using NSM CFRP
reinforcement is primarily controlled by the bond characteristics
of the bars as well as by the bond between the concrete and the
adhesive material.
Fig. 5. Beams strengthened with CFRP, (a) un-strengthened, (b) strengthened without
prestress, and (c) strengthened with prestress, (A) Elastic stage, (B) yielding stage [2,11].
Fig. 6. Comparison of the typical load–deflection of the beams strengthened with
various levels of prestressing (P1, P2 P3), non-prestressed beams, and un-
strengthened beams [2,11].
45
60
75
90
105
120
135
150
MaxLoad(KN)
Prestressing level (%)
Badawi Soudki [16]
Choi Soudki [35]
Nordin Carolin [45]
Hacha Gafaar [2]
C.B S.B P-20% P-40% P-60%
Fig. 7. Graph showing the maximum load at different prestressing levels in NSM
(CFRP rods) (C.B: control beam, S.B: strengthened beam, P-20%, P-40%, P-60%:
prestressing level at 20%, 40% and 60%).

Table 3
Research progress in prestressed CFRPs flexural strengthened beams under static loadings
Beams specifications Concrete Steel x-tension y-compression FRP Adhesive Loading condition Refs.
Beam’s
names
Length
(m)
Anchorage/bonded
or un bonded
Strength
(Mpa)
No: bars
(x *
y)
Diameter of steel
reinforcement (x *
y)
Strength
(Mpa)
Prestressing
level (%)
Type Prestressing
technique
No: of
layers
ftu
(MPa)
Ef
(GPa)
Type Type of
load
Loading
rate (mm)
Load
(KN)
Failure mode
Control-1 2.7 – 16.4 3 *
3 D10 tension D13
compression D22 T
D19 C
G420 – – – – – – – Static 5.08 51 FRP flexure [46]
NFCB-1 2.7 – 16.4 3 *
3 G420 0 CFRP
Strip
EBR 1 2161 165 Epoxy Static 5.08 77 CFRP
debonding
NFCBW2 2.7 – 16.4 3 *
3 G420 0 CFRP
Strip
EBR 2 2161 165 Epoxy Static 5.08 97 CFRP
debonding
PFCB1-0R 2.7 Anchorage + bonded 16.4 3 *
3 G420 0 CFRP
Strip
EBR 1 2161 165 Epoxy Static 5.08 122 CFRP rupture
3 G420 20 CFRP
Strip
3 G420 40 CFRP
Strip
3 G420 60 CFRP
Strip
3 G420 70 CFRP
Strip
Control-2 6.8 – 20.7 5 *
3 G420 0 – EBR – – – – Static 5.08 329 CFRP rupture
3 G420 50 CFRP
strip
B00 5.15 – 40 3 *
2 15M tension 10M
compression
500 (10M)
475 (15M)
– – – – – – – Static 1–3 84 Concrete
crushing
[2]
B2-0 5.15 Anchorage + bonded 40 3 *
2 0 CFRP
rod
NSM 1 2068 124 Epoxy Static 1–3 137 CFRP rupture
2 20 CFRP
rod
2 40 CFRP
rod
2 60 CFRP
rod
NS-NP 3.30 – 31 5 *
2 U20 tension U14
compression
423 – – – – – – – Static – 255 Concrete
crushing
[34]
S-NP 3.30 Bonded 31 5 *
2 423 0 CFRP
lam:
NSM 1 2066 140 Epoxy Static – 281 Pre: debonding
S-P5 3.30 Bonded 31 5 *
2 423 5 CFRP
lam:
NSM 1 2066 140 Epoxy Static – 284 CFRP rupture
S-P20 3.30 Bonded 31 5 *
2 423 20 CFRP
lam:
S-P30 3.30 Bonded 31 5 *
2 423 30 CFRP
lam:
Control 3.50 – 52.5 2 *
4 15M tension 9M
compression
473 – – – – – – – Static 1 80 Concrete
crushing
[35]
PBFB 3.50 Fully bonded 52.5 2 *
4 473 40 CFRP
rod
NSM 1 2800 155 Epoxy Static 1 124 CFRP rupture
PBPB2 3.50 Partially bonded 52.5 2 *
4 473 40 CFRP
rod
4 473 40 CFRP
rod
4 473 40 CFRP
rod
4 473 40 CFRP
rod
NSM 1 2800 155 Epoxy Static 1 110 CFRP
debonding
PBFB-60 3.50 Fully bonded 52.5 2 *
4 473 60 CFRP
rod
240M.Aslametal./ConstructionandBuildingMaterials82(2015)235–256

Table 3 (continued)
Beam’s
names
Length
(m)
Anchorage/bonded
or un bonded
Strength
(Mpa)
No: bars
(x *
y)
Diameter of steel
reinforcement (x *
y)
Strength
(Mpa)
Prestressing
level (%)
Type Prestressing
technique
No: of
layers
ftu
(MPa)
Ef
(GPa)
Type Type of
load
Loading
rate (mm)
Load
(KN)
Failure mode
PBPB4-60 3.50 Partially bonded 52.5 2 *
4 473 60 CFRP
rod
RC-1 2.70 – 46.9 3 *
2 500 (U6) – – – – – – – Static – 27 Steel
yield + crushing
[17]
RC-2 2.70 Bonded 46.9 3 *
2 298 (U8) 0 CFRP
lam:
EBR 1 2500 150 Epoxy Static – 41 CFRP
debonding
RC-3 2.70 Bonded 46.9 3 *
2 340 (U12) 0 CFRP
lam:
debonding
RC-4 2.70 Bonded 46.9 3 *
2 U16 tension, U14
tension,
U12 tension, U6 comp,
U8 shear
270 (U14) 0 CFRP
lam:
debonding
PC-1 2.70 Anchorage + bonded 53.2 3 *
2 300 (U16) 57.3 CFRP
lam:
debonding
2 58.5 CFRP
lam:
EBR 1 2500 150 Epoxy Static – 49 CFRP rupture
crushing
2 57.7 CFRP
lam:
crushing
2 48.7 CFRP
lam:
debonding
2 54.5 CFRP
lam:
crushing
2 U12, U14, U16 Tension
U6 compression
6 (500) 42.1 CFRP
plate
debonding
[73]
2 8 (298) 44.1 CFRP
plate
EBR 1 2500 150 Epoxy Static – 44 Crushing
rupture
2 12 (340) 50.6 CFRP
plate
EBR 1 2500 150 Epoxy Static – 42 Debonding
rupture
2 14 (270) 31.5 CFRP
plate
debonding
2 16 (300) 43.5 CFRP
plate
CFRP rupture
RB-0 1.80 Bonded + U-Wrapped 32 2 *
2 U12 TC 515 – – – – 2510 155 Epoxy Static 30 103 Concrete
crushing
[74]
2 U12 TC 515 5 CFRP
lam:
EBR 1 2510 155 Epoxy Static 30 101 Rupture
debonding
2 U12 TC 515 10 CFRP
lam:
debonding
2 U12 TC 515 15 CFRP
lam:
debonding
2 U12 TC 515 20 CFRP
lam:
debonding
Control 3.5 – 45±2.9 2 *
2 15M tension 10M
compression
440±4 – – – – – – – Static 1.5 65 Concrete
crushing
[16]
Pretress-
0
3.5 Bonded 45±2.9 2 *
2 440±4 0 CFRP
rod
NSM 1 1970 136 Epoxy Static 1.5 97 Concrete
crushing
Prestress-
40
3.5 Bonded 45±2.9 2 *
2 440±4 40 CFRP
rod
NSM 1 1970 136 Epoxy Static 1.5 115 CFRP rupture
Prestress-
60
3.5 Bonded 45±2.9 2 *
2 440±4 60 CFRP
rod
NSM 1 1970 136 Epoxy Static 1.5 112 CFRP rupture
Control 2.70 – 18 3 *
3 D13 tension D10
compression
D13 (466.2) – – – – – – – Static – 50 Flexural [71]
NFCB1 2.70 – 18 3 *
3 D10 (475.2) 0 CFRP
plate
debonding
(continued on next page)
M.Aslametal./ConstructionandBuildingMaterials82(2015)235–256241

Table 3 (continued)
Beam’s
names
Length
(m)
Anchorage/bonded
or un bonded
Strength
(Mpa)
No: bars
(x *
y)
Diameter of steel
reinforcement (x *
y)
Strength
(Mpa)
Prestressing
level (%)
Type Prestressing
technique
No: of
layers
ftu
(MPa)
Ef
(GPa)
Type Type of
load
Loading
rate (mm)
Load
(KN)
Failure mode
NFCBW2 2.70 – 18 3 *
3 0 CFRP
plate
Delamination
PFCU1-
0R
2.70 Anchorage + unbounded 18 3 *
3 0 CFRP
plate
PFCU1-
2R
3 20 CFRP
Plate
PFCU1-
4R
3 40 CFRP
plate
PFCU1-
6R
3 60 CFRP
plate
PFCB1-0R 2.70 Anchorage + bonded 18 3 *
3 0 CFRP
plate
rupture
3 20 CFRP
plate
rupture
3 40 CFRP
plate
rupture
3 60 CFRP
plate
rupture
PFCU1-4L 4.80 Anchorage + unbounded 18 3 *
3 60 CFRP
plate
PFCU1-6L 6.30 Anchorage + unbounded 18 3 *
3 60 CFRP
plate
B00 5.15 – 46 3 *
2 15M tension 10M
compression
475 – – – – – – – Static – 84 Concrete
crushing
[31]
B1 5.15 Anchorage + bonded 46 3 *
2 475 0 CFRP
strips
2 475 20 CFRP
strips
2 475 40 CFRP
strips
2 475 60 CFRP
strips
MU-III 3.3 – 26.4 3 *
2 D13, D16, D19 tension
D10 compression
– – – – – – – Static 60 75 Flexural [72]
M0-III 3.3 Anchorage + bonded 26.4 3 *
2 D10 (500.7) 0 CFRP
plate
EBR 1 2850 165 Epoxy Static 60 90 CFRP
debonding
2 D13 (498.7) 40 CFRP
plate
EBR 1 2850 165 Epoxy Static 60 121 Tension
2 D16 (476.2) 60 CFRP
plate
2 And 80 CFRP
plate
M4-1 3.3 Anchorage + bonded 26.4 3 *
2 D19 (465.8) 40 CFRP
plate
2 60 CFRP
plate
2 80 CFRP
plate
EBR 1 2850 165 Epoxy Static 60 91 –
2 60 CFRP
plate
EBR 1 2850 165 Epoxy Static 60 110 –
2 80 CFRP
plate
M6-IV 3.3 Anchorage + bonded 26.4 3 *
2 60 CFRP
plate
242M.Aslametal./ConstructionandBuildingMaterials82(2015)235–256

Table 3 (continued)
Beam’s
names
Length
(m)
Anchorage/bonded
or un bonded
Strength
(Mpa)
No: bars
(x *
y)
Diameter of steel
reinforcement (x *
y)
Strength
(Mpa)
Prestressing
level (%)
Type Prestressing
technique
No: of
layers
ftu
(MPa)
Ef
(GPa)
Type Type of
load
Loading
rate (mm)
Load
(KN)
Failure mode
M8-IV 3.3 Anchorage + bonded 26.4 3 *
2 80 CFRP
plate
L6-III 3.3 Anchorage + bonded 20.6 3 *
2 60 CFRP
plate
L8-III 3.3 Anchorage + bonded 20.6 3 *
2 80 CFRP
plate
H6-III 3.3 Anchorage + bonded 35.6 3 *
2 60 CFRP
plate
H8-III 3.3 Anchorage + bonded 35.6 3 *
2 80 CFRP
plate
U7-I 3.3 Anchorage + bonded 44.1 3 *
2 70 CFRP
plate
U7-II 3.3 Anchorage + bonded 44.1 3 *
2 70 CFRP
plate
U7-III 3.3 Anchorage + bonded 44.1 3 *
2 70 CFRP
plate
U7-IV 3.3 Anchorage + bonded 44.1 3 *
2 70 CFRP
plate
A 2.44 – 41 2 *
2 D13 tension
D10 comp
415 – – – – – – – Static 8.9 68 Concrete
crushing
[75]
B 2.44 Bonded 41 2 *
2 415 0 CFRP
lam:
EBR 1 3972
3972
228 Siga-
mate
Static 8.9 89 CFRP rupture
C 2.44 U-Wrapped + bonded 41 2 *
2 415 15 CFRP
lam:
EBR 1 228 Siga-
mate
Static 8.9 112 Rupture
debonding
Ref 4.00 – 61 2 *
2 u16 both TC 500 – – – – – – – – 60 75 Concrete crush: [45]
NP 4.00 Bonded 64 2 *
2 500 0 CFRP
rod
NSM 1 2800 160 BPE465 Static 55 118 CFRP fracture
P1 4.00 Bonded 68 2 *
2 500 40 CFRP
rod
P2 4.00 Bonded 68 2 *
2 500 40 CFRP
rod
M.Aslametal./ConstructionandBuildingMaterials82(2015)235–256243

It was observed from the literature that, in the case of CFRP rods
and strips, the results have shown a direct proportionality between
an increment in prestressing level and the failure load up to a 40%
prestressing level (Fig. 7) [2,16,35,45,46]. After this level, it was
also found that there is a decline in the strength capacity of the
object by increasing the prestressing level. The failure of the pre-
stressed strengthened beams occurred due to yielding in the ten-
sion reinforcement steel followed by a rupture of the CFRP rod.
Increasing the prestressing limits of the CFRP strips improved the
behavior of the beams at service conditions but at the same time
the beams were losing their ductility.
The use of prestressed CFRP laminates in the NSM technique is
rarely reported in the literature. Only one significant study by
Hajihashemi et al. [34] is available. This research proved that the
prestressed strengthened beams have 15% higher ultimate load-
bearing capacity than the non-prestressed strengthened specimen,
and their failures corresponded to lower deflections. The crack
characteristics were similar in both control (un-strengthened)
and non-prestressed strengthened beams; while, the prestressed
strengthened beams limited the crack distribution. Furthermore,
compared to the control beam, the number and width of cracks
in the prestressed strengthened specimens were reduced to 5–
30% and 22–52%, respectively. The most important thing associat-
ed with this strengthening technique is that the ductility of the
strengthened RC beams also decreased. The details about pre-
stressed NSM CFRP are mentioned in Table 3.
3.1. Anchorage and prestressing systems for NSM CFRP reinforcement
Several anchorages and prestressing systems have been devel-
oped to grip the FRP reinforcement (strips and bars) to induce a
prestress in the NSM CFRP reinforcement for flexural strengthen-
ing. In direct prestressing system, the NSM CFRP (bars and strips)
are tensioned by reacting against the beam ends. The installation
practice initiates by first filling half of the groove with epoxy adhe-
sive for a length of the groove necessary to assure that the pre-
stressed CFRP rebar is able to transfer the forces into the beam;
then the rebar is inserted into the groove for the entire length of
the beam; finally, by reacting against both ends of the beam, the
CFRP bar was tensioned using a hydraulic jack bearing against steel
wedge anchors used to grip the CFRP bar [47,48]. The developed
steel wedge anchor system mentioned in Fig. 8.
It is easy to use the above mentioned mechanical anchorage
systems in the laboratory testing works and its approach to pre-
stress the NSM FRP reinforcement against both the ends of the
beam. But in the field applications this anchorage system (tension-
ing device) cannot be used because there is no access at the ends of
the beams. Considering this drawback, De-Lorenzis and Nanni [49]
proposed an alternative tensioning-anchoring device for prestress-
ing NSM FRP rods against the tension face of the beam (Fig. 9). The
FRP rod is inserted into the steel tube filled with epoxy. The other
end of the steel tube is externally threaded which is inserted into
another steel tube welded to one or more stainless steel anchors
which can be substituted by reinforcing steel bars. These bars are
long enough to be grouted into the holes drilled in the tension face
of the beam and the steel tubes are positioned in the groove. Then,
at one end of the beam, the internal threaded steel tube is forced to
move with respect to the external one by tightening a nut on the
internal tube. The prestressing system allows prestressing the
FRP reinforcement against the beam itself, hence, improved
efficiency.
Several researchers have arranged an indirect prestressing sys-
tem, the prestressing force is induced into the embedded NSM FRP
reinforcement inside grooves filled with epoxy by tensioning the
ends of the FRP on a stressing bed by jacking against an external
reaction steel frame, independent of the strengthened beam, to
the required stress or strain level and left until the epoxy cures.
After the epoxy gains its strength, the prestressing system is
released and the effect of prestressing is transferred indirectly to
the concrete beam. Practically, this prestressing system for
strengthening RC members cannot be implemented easily in the
field and requires specialized equipment (Fig. 10) [1,12,50].
Badawi [51], Badawi and Soudki [16] developed a direct pre-
stressing system. They also used mechanical steel clamp anchors
to grip the CFRP bar at both ends, and 6 days curing time was
allowed for the epoxy adhesive before the prestressing force was
released. Then the clamp anchors were slowly loosened to give a
slow transfer of the force to the beam and the adhesion of the
epoxy between the CFRP bar and the epoxy interface. Thereafter
all the temporary components were removed. The details about
developed mechanical system was given in Fig. 11.
Gaafar [52] developed the direct prestressing system and prop-
er anchorage system (two steel anchors) were bonded with CFRP
reinforcement at both fixed and jacking ends with epoxy. This sys-
tem was successfully used by El-Hacha and Gaafar [2], Oudah [53],
Fig. 8. Schematic diagram and picture of the steel-wedge anchorage system (a)
parts of the Waterloo anchor; (b) the assembled Waterloo anchor, proposed by Al-
Mayah et al. [47,48].
Fig. 9. Schematic diagram of the prestressing and anchorage system proposed by
De-Lorenzis et al. [49].
Fig. 10. Prestressing and anchorage system proposed by Nordin et al. [1,12,50].

Oudah and El-Hacha [54], and Yadollahi Omran [55]. The beam end
with jack was bolted to steel brackets. First bracket was used to
transfer the load from hydraulic jack to adjustable anchor. The
other bracket was used to support the jack. The details of the
selected anchorage and prestressing system are shown in Fig. 12.
These researchers have reported outstanding results in terms of
efficiency, ease of working operation and minimum prestressing
losses [52].
3.2. Failure modes of prestressed CFRP reinforcement in NSM
technique
The majority of the failure modes have occurred under the NSM
technique is associated with the arrangement of the experimental
setup; however, the failure modes in the prestressing phenomenon
are also registered in the literature. Two different types are global-
ly adopted for the NSM technique with and without end anchorage.
The key failure modes in the NSM method, without end anchorages
are concrete crushing, CFRP debonding and CFRP delamination.
While the failure modes achieved in prestressed NSM CFRPs with
end anchorages are concrete crushing and CFRP rupture.
Researchers have used CFRP rods under the NSM technique with
and without end anchorage systems. The results of the studies in
both categories have shown similar types of failure. In the studies
with the end anchorage system, structural failure due to concrete
crushing and CFRP rupture was detected [2]. While in studies with-
out end anchorage system, the failure was observed due to con-
crete crushing; CFRP debonding and CFRP rupture [16,35,45,56].
El-Hacha and Gaafar [31] used prestressed CFRP strips in the
NSM technique with a proper anchorage system and experienced
failure through concrete crushing and CFRP rupture. Hajihashemi
et al. [34] applied the bonded prestressed CFRP laminates in the
NSM technique for strengthening beams in which they observed
failure through concrete crushing, CFRP debonding and CFRP rup-
ture. It is found from the literature that many studies have been
performed to explain premature debonding failure mode.
However, the exact mechanism of this failure has not yet been
established due to numerous factors affecting the bond strength
at the FRP-concrete interface. A comprehensive detail of the failure
modes achieved in prestressed CFRPs used for strengthening of RC
beams was presented by Badawi [51] as shown in Fig. 13. Failure
modes faced from researchers are explained in Table 3.
3.3. Bond behavior of prestressed NSM CFRP reinforcement
The RC beams strengthened with prestressed NSM CFRP require
control of the bond failure to develop the full flexural capacity of
the beam. The bond of the prestressed CFRP plays a major role in
the effectiveness of the NSM as a suitable strengthening technique.
The transfer length of a prestressed NSM CFRP rod, defined as the
distance over which the rod must be bonded to the epoxy to devel-
op the prestressing force in the rod, is intended to provide bond
integrity for the strengthened beam. The bond strength decreases
with an increase in the bond length as a result of the non-uniform
Fig. 11. (a) RC beam with anchorage and prestressing system. (b) Prestressing system developed by Badawi and Soudki [16,51].
Fig. 12. (a) Elevation view at jacking end. (b) Isometric view of prestressing system developed by Gaafar [52].
Fig. 13. Effect of prestressing level on the failure modes [51].

distribution of bond stresses. The transfer length of CFRP rods
embedded in epoxy is expected to be different from the behavior
of CFRP rods in concrete [36]. Badawi et al. [42] suggested an
empirical equation based on curve fitting of the measured data
to estimate the transfer length of prestressed NSM CFRP bar:
fs ¼ fpreð1 À expÀBx
Þ ð1Þ
where fs (prestressing stress) in the CFRP bar at a given distance (x)
from the end of the bonded length, fpre (maximum prestressing
stress), B is a factor to account the rod type, type and the thickness
of the epoxy, and the method of force releasing [42].
The test variables were the type of CFRP rod and the level of
prestressing force in the rods (40%, 45%, 50%, and 60% of the tensile
capacity of the CFRP rod). The average transfer length of the NSM
CFRP spirally wound rod was found to range from 190 mm to
350 mm for the 40–60% prestressing levels, and 160 mm at 40%
prestressing level for sand blasted rods [36].
Wahab et al. [57] conducted bond tests on RC strengthened
beams with non-prestressed and prestressed NSM CFRP rods under
static loading to understand the mechanism of bond failure. The
test variable were the presence of tension steel, type of CFRP rod
(sand blasted and spirally wound), and the level of prestressing
force (0%, 40%, 45%). They found that the transfer length of the pre-
stressed CFRP rod was found to be 150 mm and 210 mm for the
prestressed sand blasted and spirally wound rods, respectively.
The failure mode for non-prestressed NSM CFRP strengthened
beams was pull-out between the CFRP rod and the epoxy. The fail-
ure mode for the prestressed NSM CFRP strengthened beams
depended on the type of rod used. For the sand blasted rods, the
failure was the same as the non-prestressed strengthened beams.
For the spirally wound rods, failure was by pull-out of the rod from
the epoxy in the region close to the support.
3.4. Effect of prestress level on ductility and deformability in NSM CFRP
strengthened RC beams
The serviceability condition in terms of reduced crack width
and deflection, and the ultimate load carrying capacity of pre-
stressed NSM CFRP may improve with an increase in the level of
the prestressing force, while ductility (defined as the ratio of
deflection at ultimate load to deflection at yield load) and deforma-
bility (defined as the ratio of the deflection at failure to the deflec-
tion at steel yielding) of the strengthened beams may decrease
with an increase in the prestressing force, and the RC beams may
fail due to rupture of the CFRP [2,16,58]. The reduction in the duc-
tility is due to the increase of the tension reinforcement (steel and
CFRP) and prestressing, which leads to less energy dissipation.
Structures with low deformability may fail suddenly without cau-
tion, resulting in catastrophic failure [2,16,58].
Thus, in general, prestressing is used to enhance the flexural
behavior of reinforced concrete members under service loads espe-
cially in bridges and (or) beams that have large spans and there is a
limitation on the deflection and serviceability conditions. Because
of their high tensile strength properties, FRP materials have great
advantages for using in prestressing and post-tensioning strength-
ening applications. Beside their economic benefits, prestressed FRP
systems provide the following benefits and advantages [36]:
It can improve the serviceability of the beam.
Reduce the dead load deflections.
It can reduce the crack widths and delay start of cracking.
It can relieve the strains in the internal steel reinforcement.
It can increase the yielding of internal steel reinforcement at a
higher proportion of the ultimate load.
It can provide more efficient use of the concrete and the FRP
materials.
3.5. Finite element validations in prestressed NSM CFRP reinforcement
The complexity of the various issues that exist in strengthening
the RC beams by prestressed NSM CFRP, the non-linear behavior of
different materials and elements, and the possibility of systematic
and random errors during experimental procedure, motivated sci-
entists to use computer modeling for analysis of such structures.
With the passage of time, numerical analysis has proven itself to
be a trusted method to get more precise results. The strengthening
of RC beams using prestressed CFRP is quite a modern approach
and a few studies are available on this topic. Mostly, numerical
modeling has been used by researchers to validate their
experimental results under fatigue loading. Neto et al. [59] pre-
pared an elasto-plastic FE model to analyze the behavior of RC
beams with pre-stressed NSM CFRP. They mainly focused on iden-
tifying the failure mechanisms and the complexity of the multiple
nonlinear aspects of the behavior of the structure, the numerical
and experimental results showed good relationship.
A 3D nonlinear FE analysis of strengthened RC beams with pre-
stressed NSM CFRP strips was prepared by Omran and El-Hacha
[60]. The effect of debonding at the epoxy concrete interface was
considered in the model by identification of the fracture energies
of the interfaces and appropriate bilinear shear stress slip and ten-
sion stress gap models. The CFRP strips can be prestressed by using
the temperature equivalent method. The constitutive confined
concrete model for the flexural members was created from an
un-confined concrete curve and assigned to the concrete materials.
The comparison between test results and FE results confirms the
Fig. 14. Isometric view of the beam [53]. Fig. 15. 3D Finite Element Model [61].

excellent accuracy of the suggested model. Oudah [53] developed
FEM models of strengthened RC beams using prestressed NSM
CFRP rods and strips using Abaqus FEM software to predict the
flexural behavior during prestressing and cyclic loading. In addi-
tion to the experimental and analytical investigations, the results
were properly validated the experimental findings during the un-
cracked and post-cracked regions (Fig. 14).
Echeverria and Perera [61] performed beam pull out tests in slip
control mode and followed the NSM technique. A non-linear 3D FE
model, according to the experimental setup, was then prepared to
simulate the beam test configuration (Fig. 15). Their model focused
on the numerical model of the different materials involved in the
test and their interaction and the way of controlling the load appli-
cation in the slip-control mode. The CFRP rod was glued to the con-
crete using an adhesive material. The experimental and numerical
results showed perfect collaboration in the findings.
4. Research progress on prestressed externally bonded (EB)
CFRP reinforcement
The concept of using the externally bonded reinforcement (EBR)
technique for prestressing has been examined by researchers for
many years. In the EBR technique, prestressed FRP plates or lami-
nates are bonded to the external surface of the strengthened beam
using strong epoxy adhesives. Prestressing the FRP plates or lami-
nates prior to bonding allows the high tensile strength of the mate-
rial to be exploited resulting in improvements in the working load
range of the beam. The act of prestressing induces a compressive
strain in the bottom fiber of the section, resulting in an upward
camber that reduces the deflection of the member throughout
the loading as shown in Fig. 16. The real life application of EBR
technique using prestressed CFRP bar for strengthening some
structures are shown in Figs. 17 and 18.
Garden and Hollaway [14] used the bonded prestressed and
non-prestressed CFRP plates. The prestressing was applied
between 25% and 50% of the plate strength. They found that the
plate fracture and breaking of bond between beams and plates
were the governing failure modes. Progressive failure of plates
was occurred due to longitudinal splitting and inter laminar frac-
ture. Later, Kadhim et al. [64] presented a comparison between
the EBR technique with and without prestressing in materials.
They explored the best suitable location for the placement of pre-
stressed CFRP plates in RC beams and concluded that the use of
prestressed CFRP plates on the edges increased the load carrying
capacity of RC beams strengthened with prestressed CFRP plate
by about 11% more than when used in the center of RC beam.
Shang et al. [65] found that beams prestressed with CFRP are
effective and economic tool to achieve higher cracking load, mini-
mized deflection and optimal flexural stiffness as well as reduce
the risk of debonding. Reza Aram et al. [66] applied the gradient
method to anchor the prestressed CFRP strips. They concluded that
the prestressed CFRP strips caused no significant reduction in beam
deflection and crack width if compared to the non-prestressed beam,
and further, this method is not effective because the gradient anchor-
age was in the region of shear stresses from loading. This method
might be more useful for large span beams like bridge girders.
Kim et al. [67] investigated the flexural behavior of the pre-
stressed concrete beams strengthened with prestressed CFRP
sheets and mainly focused on ductility and cracking behavior.
They developed an analytical FEM model to predict the flexural
behavior and recommended that CFRP sheets can be prestressed
up to 20% of the strength using reliable anchorage system. Later,
they applied non-metallic anchorage system and minor prestress
loss was observed while transmitting the stress in the CFRP sheets.
A closed-form solution for the transfer of prestress was developed
and compared to the experimental results [68].
Fig. 16. Externally bonded (EB) prestressed FRP reinforcement.
Fig. 17. Real life picture of externally prestressed CFRP bonded strengthened
building, Ebikon, Switzerland (CH) [62].
Fig. 18. Real life strengthened slab view with prestressed laminates (with plates)
and non prestressed laminate (without plate) [63].

Figeys et al. [69] minimized the inconvenience in present day
prestressing method. They developed two simplified systems to
test the feasibility of the novel prestressing system. A first test ser-
ies showed that the anchorage to the concrete is feasible. A suffi-
cient anchorage capacity can be realized by gluing and bolting
the anchor block. Tensile tests on the basic clamping system
showed that a clamping capacity can be obtained by pushing the
wedges into the anchor block, with a high force. Woo et al. [70]
investigated the strengthening effect of prestressed CFRP plates
on RC beams and proposed a strength prediction method. They
found that prestressed CFRP plates can increase the cracking, yield-
ing and ultimate load. Furthermore, the serviceability of the beam
is also enhanced.
Very few studies are available that concentrate on CFRP plates
with prestressing at different levels using the anchorage system
with and without epoxy bonding. In the EBR technique, Yang
et al. [71] concluded that when prestressed CFRP plates are applied
with a bonded anchorage system it can sustain a maximum load at
the 40% prestressing level. In contrast when a prestressed CFRP
plate is used with the unbounded anchorage system it can sustain
a maximum load at the 60% prestressing level. They did not recom-
mend an increase in the prestressing procedure with CFRP plates
beyond the load range of 125KN. Following the same pattern
Woo et al. [72] and Xue et al. [73] used the anchorage system with
epoxy bonding. In this study, as compared to Yang’s study, the
maximum strength level was achieved at the 40% prestressing
level and the strength gradually decreased with the increase in
prestressing level (Fig. 19). This clearly shows the effect of epoxy
bonding in providing the maximum strength at a lower prestress-
ing level; hence, producing a more economical design of beam. Past
experimental details using prestressed CFRP plates with the EBR
technique are mentioned in Table 3.
The studies discuss the use of prestressed CFRP laminates in
EBR (Small Scale) and define different experimental setups
[74,75], they found that the failure load (ultimate load) varied,
being 85KN, 90KN, 101KN, and 116KN, and 120KN in the reference
beam (un-strengthened), strengthened beams (non-prestressed)
and prestressed strengthened beams at 5%, 20% and 30%, respec-
tively. When CFRP laminates were used in externally post-ten-
sioned technique (EPT) the observed failure load varied, being
47.3KN, 77KN, 78KN and 80KN, and 82KN in the reference beam
(un-strengthened), strengthened beams (non-prestressed) and
prestressed strengthened beams 5%, 20% and 30%, respectively
[76]. Although, laminates have not been found to be sufficiently
effective in EBR and post-tensioning techniques, as they do not
provide extraordinary strength in beams, they demonstrate that
even increasing the load laminates perform the best job using
the NSM technique. Furthermore, the lower cost of prestressing
is achieved with no harm to the increased strength of the beam.
4.1. Anchorage and prestressing systems used for CFRP in EBR
technique
The usage of prestressed EB-CFRP plates and laminates, as a
strengthening technique, has been investigated the most. Mainly,
three prestressing systems have been inspected for the strengthen-
ing of the structural members. These are:
The strengthening will be done after the unloading of the struc-
tures using hydraulic jacks.
Prestressing the fiber reinforced polymers (FRP) against the
external independent structural members.
Prestressing the fiber reinforced polymers (FRP) against the
strengthened structure itself.
The use of prestressed FRP to strengthen beams is difficult to
apply in the field as it can be applied in the laboratory. Yu et al.
[75] suggested a mechanical device indirect prestressing system,
made of one steel beam. They arranged a simple way to anchor
the FRP sheets and to apply the prestressing force. The laminates
were bonded to removable steel plates, which were fixed to the
40
60
80
100
120
140
MaxLoad(KN)
Prestressing level (%)
Bonded Anchorage [71]
Unbonded Anchorage [72]
Bonded Anchorage [73]
C.B S.B P-20% P-40% P-60%
Fig. 19. Graph showing the maximum load at different prestressing levels in EBR
(CFRP Plates) (C.B: control beam, S.B: strengthened beam, P-20%, P-40%, P-60%:
prestressing level at 20%, 40% and 60%).
Fig. 20. Prestressing system developed by Yu et al. [75].
Fig. 21. Prestressing system arranged by Mukherjee and Rai [74].
Fig. 22. Prestressing system arranged by Yang et al. [71].

mechanical device by threaded rods and nuts. The prestressing
force was applied manually by twisting the steel nuts, which were
tightened against the mechanical device (Fig. 20). Later, the same
prestressing system was used for strengthened RC beams using
prestressed CFRP laminates. Prestressed CFRP laminates consider-
ably enhanced the ultimate strength and serviceability of the retro-
fitted beam with the help of mechanical device. They also
presented a theoretical formulae based on the equilibrium of the
forces and the compatibility of strains. The results showed higher
tensile strength values as compared to ACI [77].
Later Mukherjee and Rai [74] investigated the flexural behavior
of the retrofitted RC beams with externally prestressed CFRP lami-
nates. They designed an indirect prestressing system, in that the
CFRP laminate were mounted on two drums (one fixed and other
rotating) of the prestressing machine and secured at the ends to
prevent slippage of the laminate. The setup is shown in Fig. 21.
They observed the effects of variation in prestressing force on the
behavior of CFRP sheets. The results specified that rehabilitation
of knowingly cracked beams by bonding prestressed CFRP lami-
nates is structurally efficient. Yang et al. [71] investigated the
bending of RC beams strengthened using prestressed CFRP plates,
employing different FRP bonding and prestressing methods. They
designed a direct prestressing system and suggested that the rough
surface of the anchorage system is treated, and then fixed to the
beam with anchor bolts. In order to prevent a load concentration,
the anchorage system of the CFRP plate was attached to a GFRP
tab (Fig. 22). The concluded that the ductility of the beams
strengthened with CFRP plates having the anchorage system was
considered to be high with a ductility index above 3.
Xue et al. [73] experimentally investigated the RC beams
strengthened with prestressed CFRP plates and recommended three
possible flexural failure modes (the compression failure, tension
failure and debonding failure) according to the CFRP reinforcement
ratio. They developed a direct prestressing system with a set of
anchorages to hold the prestressed CFRP plate, which consists of
the steel plates, aluminum laminates and a series of bolts. The alu-
minum sheets were placed between steel plates. The steel plate
was bolted to the CFRP plate at its one end. A prestressing frame
was used for the application of the load as shown in Fig. 23. They pre-
dicted the bending strength of beams under different failure modes.
Siwowski et al. [78] studied the application of the new system
for the prestressing of CFRP strips and practical aspects of on-site
works. They proposed the Neoxe Prestressing System (NPS) contains
two main elements; special steel anchorages mounted on both
ends of a single CFRP strip and a stretching device. The system con-
tained two kinds of steel anchorages: an active anchorage combin-
ing with a stretching device and a passive one. The entire system
with the CFRP strip prepared with the steel anchorages and mount-
ed on experimental pilot beam (Fig. 24). Their studies showed the
effectiveness and reliability of the new prestressing system, which
seems to be universal for both steel and concrete applications.
You et al. [46] investigated the effectiveness and the feasibility
of the strengthened RC beams externally bonded prestressed CFRP
Fig. 23. Prestressing system arranged by Xue et al. [73].
Fig. 24. Prestressing system arranged by Siwowski et al. [78].
Fig. 25. Prestressing system arranged by You et al. [46].

strips. Two large-scale and eight small-scale RC beams strength-
ened with different arrangement of prestressed CFRP strips were
tested under static loading conditions up to failure. They recom-
mended a prestressing system with a novel anchorage system
allowing the utilization of the full capacity of the prestressed
CFRP strips. The mechanical anchorages consisted of three indi-
vidual anchors; two fixed grip anchors which permanently
attached the end of CFRP strips to the beam and one jacking anchor
which was used to apply the tension force to the CFRP strips. The
anchorage and jacking assemblage organized were directly apply
the tension to the CFRP strips by jacking and reacting against per-
manent anchors fixed on the RC strengthened beam itself (Fig. 25).
The specimens experienced early initial cracking and yielding of
steel.
4.2. Failure modes of prestressed CFRP reinforcement in EBR technique
The number of failure modes in the EBR method is higher than
for the NSM technique. The studies have thoroughly discussed var-
ious types of failure in both end anchorage and non-anchorage sys-
tems. Jumaat et al. [8] classified the failure modes in this type of
technique into two kinds. The first category was comprised of fail-
ures that occur under full composite action, which include con-
crete crushing and FRP rupture, while the other category covers
the premature failures without reaching full composite action.
These failures include end cover segregation, CFRP end delamina-
tion, shear and bending crack debonding. Among these cracks,
the CFRP delamination due to an abrupt drop in the load consti-
tutes a brittle failure regardless of whether the tension steel rein-
forcement has yielded (Fig. 13) [14,79].
Triantafillou and Deskovic [80] reported an analysis of the prob-
lem of providing the maximum achievable prestress level without
experiencing a de-bonding failure in the end zone. They found that
a higher prestress level can be achieved by increasing the length of
the bond. It was also concluded that for prestressed CFRP strength-
ened RC beams, an additional mechanical anchor at the ends would
increase the potential of using prestressing technique for external-
ly bonded CFRP materials. Later, Triantafillou et al. [7] verified their
analytical model by performing an experimental test. A reasonable
agreement was achieved between their model and the obtained
experimental results. Details of the failure modes are explained
in Table 3. It was also found that excellent flexural behavior was
obtained in terms of strength, stiffness, and ductility [80].
A significant study on prestressed CFRP plate behavior was per-
formed by Garden and Hollaway [14]. The results of this study
proved that the prestressed CFRP plate prevents cracking of the
adhesive layer. A phenomenon associated with shear cracking in
the concrete and the progressive failure occurred suddenly due
to longitudinal splitting and inter-laminar fracture.
4.3. Effects of externally bonded prestressed CFRPs on bond behavior,
ductility and deformability
The condition of the original structure must be examined prop-
erly before the strengthening is performed. Poor concrete does not
allow easy strengthening of beams using prestressed FRPs. The
presence of impurities on the surface should be avoided. A satisfac-
tory bond between the materials demands a clean and levelled
concrete surface. Impurities should not be allowed on the struc-
tural surface at the time of bonding. Manufacturer’s guidelines
should be followed to select weather conditions. And when the
strengthening process will be finished, the FRP composites must
be properly examined [81].
Quantrill and Hollaway [82] identified the prestressing tech-
niques for both small and large scale beams. The evaluations were
made for externally reinforced beams. They concluded that beams
strengthened with a prestressed plate produce an equivalent, or
slightly higher level of ductility. Later, Kim et al. [67] recommend-
ed that the structural ductility of the strengthened beams is a cri-
tical issue, considering the abrupt and brittle failure of the
prestressed CFRP laminates themselves. They further recommend-
ed that CFRP sheets could be prestressed up to 20% of the ultimate
design strain with adequate anchorages. It is further computed
from the past literature that the cracking may also affect service-
ability of a strengthened beam, and may be especially important
for durability.
4.4. Prestressed NSM CFRP strips versus external prestressed CFRP
strips
The performance of the strengthened RC beams using pre-
stressed and non-prestressed NSM CFRP strips were compared to
the prestressed and non-prestressed CFRP strips externally applied
on the tension face of the beams. It should be noted that research-
ers have used the prestressed CFRP strips at different prestressing
levels in both NSM [31] and EBR [46] techniques. Fig. 26 shows the
comparison of performance of CFRP strips in both techniques. It is
evident from the graph obtained mentioned below that the NSM
technique is more effective and gives relatively better results in
terms of ultimate load capacity. The comparison study between
the NSM and EBR systems showed that the NSM system displayed
better load carrying capacity due to higher bond strength at the
concrete-FRP interface [38,83]. The NSM FRP composites are also
safe from external mechanical damage, since these are placed in
the grooves on concrete surface and are completely covered by
the epoxy adhesive.
4.5. Finite element validations in prestressed externally bonded CFRP
reinforcement
Previous studies [59–61] have shown that the FEM offers a
powerful and general analytical tool for investigating the structural
behavior of RC structures strengthened with prestressed CFRP
under EBR technique. The consistency of the technique is principal-
ly reliant on the precision of the simulation of actual behavior and
geometrical characteristics of the prototype structure under
experimental investigations into the FE model. In the case of com-
plicated civil engineering problems where the simple analytical
methods are not practicable, the FEM provides a practical, multi-
purpose, and consistent methodology to handle. However, there
is a large research gap regarding the prediction of the behavior of
RC beams strengthened with prestressed CFRP under EBR tech-
nique. A few studies are available focusing on FEM of prestressed
EBR system.
40
60
80
100
120
140
160
MaxLoad(KN)
Prestressing levels (%)
CFRP strips (NSM)
CFRP strips (EBR)
C.B S.B P-20% P-40% P-60%
Fig. 26. Graph showing maximum load at different prestressing levels in NSM
EBR (CFRP strips) (C.B: control beam, S.B: strengthened beam, P-20%, P-40%, P-60%:
prestressing level at 20%, 40% and 60%) [31,46].

Pellegrino [84] developed a numerical analysis method to appro-
priately evaluate the flexural behavior of strengthened RC beams
with prestressed CFRP under EBR technique. Two dimensional elas-
to-plastic FE analysis was carried out using a commercial FE package
in order to represent the beam crack pattern, to predict ultimate
loads reasonably and the strain of CFRP at failure loads. The concrete
and CFRP laminate was modeled using eight-node quadrilateral iso-
parametric plane stress element. Each element having sixteen
degrees of freedom (dof) with two displacements, Ux and Uy, at each
node. The rebar and stirrups was modeled as embedded reinforce-
ment elements with perfect bond between rebar and concrete. The
embedded model overcame the concrete mesh restriction because
the stiffness of the reinforcing steel was evaluated separately from
the concrete elements. With this approach, the reinforcing layer
was aligned with one of the local iso-parametric element coordinate
axis. In this study the FE analysis in [84] had two limitations.
Microcracks produced by drying shrinkage and handling were pre-
sent in the concrete to some degree. These would reduce the stiff-
ness of the actual beams, while the FEM did not include
microcracks. Secondly, perfect bond between the concrete and steel
reinforcing was assumed in the FEM. However, this assumption is
not always acceptable for actual beams. An applicability of the FE
analysis method was confirmed with experimental results obtained
from four points bending test in the study.
5. Research progress on externally post-tensioned CFRP
behavior (EPT CFRP)
Initially, in 1950s external post-tensioning was utilized to
strengthen bridges only however, modern day construction applies
this technique for both deficient and newly built structures, adding
minor increase in the weight of original structure while allowing
the re-stressing, monitoring and replacement of the external ten-
dons. However, corrosion demands protection of steel tendons.
FRP materials in the form of CFRP, GFRP and AFRP are considered
as a better solution for steel corrosion problems as well as to
achieve higher strength and desired stiffness. Figs 27 and 28 have
shown the real life strengthened bridges using externally post-
tensioned technique (EPT).
External prestressing method is used for repairing and strength-
ening of structural members in flexure and shear under service
load conditions both in positive and negative moment regions
[87]. It also used to control the short and long-term deformations
caused by the applied loadings. In reinforced concrete structures,
overlaying the compressive forces in the tension faces reduces
the existing crack widths, thus mitigating the effects of corrosion
on internal steel reinforcement, and possibly reducing the magni-
tude of vibrations caused by the live loadings. These major issues,
which are frequently faced by all bridge engineers, can be solved
with the help of the EPT system. By utilizing small effort, this sys-
tem can be used to enhance the torsional and shear strength of the
members [87–92]. Chen et al. [93] investigated the cracking behav-
ior, and buckling of the post tensioned strengthened beams. There
proposed a design based on the moment redistribution to estimate
the strength of continuous composite beams with external post-
tensioned tendons.
Matta et al. [94] presented a novel external post-tensioning sys-
tem for CFRP tendons. The basic arrangement of this EPT system con-
tains a set of two pultruded CFRP bar assemblies, each with a live and
dead end. Stainless steel couplers were used for each bar to allow the
CFRP rods to improve the ultimate strength. Intermediate deviators
can be extended vertically to impart additional prestressing force
(PT) and achieved a profile bar configuration. They recommended
that in this system, post-tensioning force can be applied by pulling
the tendon at one end and by pushing down an extendable deviator.
The length of tendons and the location of anchors along the flexural
member in addition to the dual mode to generate force in the bar
(pushing and pulling) allow for the optimization of an EPT system.
Geometry and degree of end constraint of the flexural member are
also critical for the design (Fig. 29).
Wang et al. [76] investigated the behavior of the long term
prestress losses in post-tensioned CFRP laminates under different
prestressing levels and strengthening ratios. They proposed that
the prestressing losses in the post tensioned CFRP laminates are
mainly attributable to the anchorage system, however, the time
dependent losses caused by the shrinkage and creep of the con-
crete and the relaxation of the CFRP laminates are relatively small.
5.1. Advantages and disadvantages of external post-tensioning (EPT)
Picard et al. [95] and Nordin and Täljsten [12] listed some
advantages for using external prestressing for both new and exist-
ing structures:
The dimensions of the structural section can be condensed due
to the less space required for the internal steel reinforcement.
The assembly of the external steel tendons are easier and sim-
pler to check during and after installation.
The external steel tendons can be replaced and removed if the
corrosion protection of the tendons allows for the release of
the prestressing force.
The frictional losses are considerably reduced because the
external steel tendons are only connected to the structural
member at the deviation and the anchorage zones.
Fig. 27. Real life strengthened bridge (EPT Technique) [85].
Fig. 28. Real life strengthened bridge (EPT Technique) [86].
Fig. 29. Prestressing systems for externally post-tensioning CFRP Steel (Rods)
[94].

The main structural operations, prestressing and concreting, are
more independent of one another; therefore the effect of work-
manship on the overall quality of the structure can be reduced.
But it is also necessary to understand the drawbacks of this EPT
technique. The following are some of the disadvantages must be
kept in mind, Picard et al. [95] and Nordin and Täljsten [12]:
The external post-tensioned tendons are more easily accessible
than internal ones and, subsequently, are more exposed to dam-
age and fire.
The external post-tensioned tendons are exposed to vibrations
and, consequently, their free length should be inadequate.
In the deviation zones, high transverse pressure acts on the
post-tensioned steel tendon. The deviation zones should be
properly mounted to decrease the friction as much as possible
and to avoid damage to the prestressed steel tendon.
At the ultimate limit states, the flexural strength is reduced due
to the external tendons as compared to the internal tendons.
The stress variations between the cracking and ultimate load
may not be assessed at the critical section only, as is done for
internal tendons.
At ultimate limit states, the main concern for the externally pre-
stressed structures is the failure with little warning due to
inadequate ductility.
5.2. Failure modes of prestressed CFRP reinforcement in EPT technique
Various studies have thoroughly discussed various types of fail-
ure in the EPT technique. Wang et al. [76] studied the post-tensioned
CFRP laminates in EPT and observed that types of failure that
occurred under full composite action were concrete crushing and
CFRP rupture. The observed ultimate load varied, being 47.3KN,
77KN, 78KN and 80KN, and 82KN in the reference beam
(un-strengthened), strengthened beam (non-prestressed) and pre-
stressed strengthened beams at 5%, 20% and 30%, respectively. El-
Refai [96] studied the post-tensioned CFRP rod under static loading
in which he observed that all failures are flexural and that the failure
modes were concrete crushing, steel yielding and CFRP tendon
rupture.
Furthermore, the testing of FRP tendons has shown that, often,
there is brittle behavior that causes a sudden failure at high levels
of force. The weak part for the external cables, when using FRP
materials, is found to be the anchorage. It can therefore be difficult
to distinguish between the failure of a tendon or an anchorage.
Often the anchorages have a problem to handle the high tensile
stresses that would validate the use of FRP materials in prestress-
ing. However, this is changing with a number of research projects
around the world focusing on the anchorage issue. This means that
a fracture that seems to have been caused by a tendon failure could
easily have been caused by failure of the anchorage instead.
5.3. Effects of externally post-tensioned steel and FRP tendons on
ductility
It is important to mention that strengthened beams with pre-
stressed FRP tendons have different ductility behavior as compared
to the prestressed steel tendons strengthened beams. Usually, a
steel post-tensioned strengthened RC beam elastically deforms
up to cracking and an increase in deflection occurs. In contrast,
FRP post-tensioned strengthened beams continue to deform with
increase in load up to FRP rupture or concrete failure.
Grace and Abdel-Sayed [97] used the post-tensioned steel ten-
dons and suggested that prestress loss can affect the ductility of
the strengthened beams. They concluded that the prestress loss
in the steel tendon varies between 5% and 7%. However, Stoll
et al. [98] recommended that the CFRP tendons have less ductility
than steel tendons when subjected to extreme loading. It shows
that post-tensioned CFRPs may contribute with lower energy
absorption and greater ultimate load capacity than the similar steel
post-tensioned strengthened beam.
5.4. Behavior of the deviators and the second-order effects in
externally Post-tensioned tendons
Generally, the connections of the external prestressed tendons
to the structural member are at the deviators and the anchorages.
The tendons (FRP/Steel) are free to move between those relative
points (deviators) to the section of the structural member as
shown in Fig. 30. If the deviators were not applied properly the sec-
ond order effects due to the tendon eccentricity, lead to a lower
load carrying capacity Tan and Ng [99]. They further recommended
that the deviators use along the span length of the member (beam)
can effectively reduce the second-order effects. Their test results
showed that the single deviator at the section of maximum deflec-
tion resulted in satisfactory service and ultimate load behavior.
Matupayont [100] analyzed strengthened beams with different
arrangement of deviators to investigate the decrease in the ulti-
mate flexural strength due to the loss of tendon eccentricity.
They concluded that the large beam deflections cause secondary
effects due to the fact that tendons only follow the concrete deflec-
tion at the deviator points.
Harajli et al. [101] investigated the influence of the deviators by
performed three different arrangements. Three strengthened
beams were used, using without deviator, with one deviator and
two deviators. Their test results showed that the performance of
the members with one and two deviators was almost same, and
that the responses for those arrangements were even similar to
the response of beams with internal tendons.
5.5. Finite element validations of externally post-tensioned CFRP
reinforcement
In the case of FE modeling, when concrete beams are post-ten-
sioned with prestressed unbonded CFRP tendons the deflection of
the whole system becomes a controlling parameter in the analysis.
Moreover, the model should be capable of the treatment of nonlinear
material properties, where concrete is capable of directional crack-
ing and crushing besides incorporating plastic and creep behavior.
El-Aziz [102] has developed a FE model to calibrate the tension stiff-
ening characteristic of concrete in order to take into consideration
the comparison of the bond behavior of the post-tensioned CFRP
tendons and the post-tensioned steel tendons strengthened beams.
The calibrated tension stiffening was used in the analytical model for
the proper modeling of the cross-section curvatures and deflection
calculations. Fathelbab et al. [103] performed FE modeling of
Fig. 30. Behavior of deviators and the second-order effects in externally post-
tensioned tendons [99].

strengthened simple beams using prestressed CFRP techniques and
presented parametric studies. For post-tensioning tendons, the FE
model captured the cracking in tension and the crushing in
compression. An eight noded model with translations in three nodal
directions; x, y, and z was developed. An initially isotropic material
was assigned to the element. The study assumed linearly elastic
stress–strain behaviorfor the CFRP laminates. The FEM showed good
understanding with experimental results.
6. Advantages of NSM technique over EBR and EPT
Strengthening using prestressed NSM CFRP technique is grow-
ing widely, and offers an alternative to the EBR and EPT prestress-
ing systems. The following are some of the advantages of NSM over
EBR and EPT [36,49]:
Excellent for strengthening in the negative moment regions,
where EBR would be subjected to mechanical and environmen-
tal damage.
Feasibility of anchoring into members adjacent to the one to be
strengthened.
Less likely to debond near ultimate capacity.
Protection of the embedded FRP in the grooves from external
damage, such as vehicle impact, better fire performance, resis-
tance to moisture and avoids freeze–thaw problems.
The choice of FRP material with higher strength and modulus of
elasticity such as CFRP instead of GFRP and AFRP, would allow
the use of smaller FRP and groove cross sectional areas; hence,
there is less risk of interfering with the internal reinforcement.
In terms of structural behavior, it’s most relevant mechanical
properties are the tensile and shear strengths; therefore the
grooves can be properly filled with epoxy adhesive or cement
mortar.
It should be noted that in non-prestressed NSM FRP strengthen-
ing applications only a portion of the strength of the FRP reinforce-
ment is effective and the system is a passive strengthening
technique that remains inactive until additional loads are applied.
Therefore, improving the serviceability performance can be
achieved by means of prestressing the NSM FRP reinforcement.
7. Conclusions
This paper presents a detailed literature review concerning the
flexural performance of strengthened RC beams using prestressed
FRP reinforcement. The research progress, design approach and
research gaps are addressed. The conclusions made from this study
suggest that:
(1) As compared to FRPs the prestressed steel has comparatively
heavier weight and low resistance against adverse environ-
mental conditions that can produce a larger reduction in
its mechanical properties.
(2) As compared to prestressed steel, prestressed FRPs exhibited
good potential and desirable structural properties including
high flexural strength, enhance the ultimate load carrying
capacity, reduce the deflections, high strength to weight
ratio, high stiffness to weight ratio, high energy absorptions,
corrosion resistance and high fatigue resistance. Moreover,
the debonding resistance of prestressed FRP is higher than
for prestressed steel.
(3) Among the types of FRPs, CFRPs are more likely to contain all
the advantages of other FRPs, and, in addition, it was observed
that prestressed CFRPs increase the flexural strength and
ductility of the structure. The other main advantages of pre-
stressed CFRPs are the low weight, resistance to corrosion,
high tensile strength, low relaxation losses, good resistance
to creep and degradation, electromagnetic neutrality, quick
and easy construction, low labor costs and have an ability to
change the modulus of elasticity. But the use of prestressed
CFRPs also have some disadvantages, as it has higher risk of
initial debonding, extreme brittle behavior and high cost in
retrofitting as compared to steel. Additionally, although it
provides an increase in strength there is less increment in
stiffness.
(4) Both the AFRP and the GFRP show an important decrease in
their tensile strength when they are subjected to a long-
term constant load, while studies showed that CFRP has very
little loss in tensile strength.
(5) The major failure modes observed under flexural loading in
prestressed CFRP strengthened beams are concrete crushing,
steel fracture, CFRP delamination, CFRP rupture and shear,
and bending crack debonding in CFRP.
(6) The near surface mounted (NSM) technique provides a
shield to the prestressed strengthening material against
the environment, and, at the same time, provides an opti-
mum and quick experimental setup. In addition, it may
improve the cracking, yielding and ultimate loads more
effectively. Besides that, the prestressed strengthened
beams under this technique have failures that corresponded
to lower deflections.
(7) By prestressing the NSM FRP reinforcement, the cracking
load in the strengthened prestressed beams increases sig-
nificantly and the deflection reduces at service load levels
in comparison to the non-prestressed NSM strengthened
beams. This improvement is more pronounced as the pre-
stressing force is increased. However, the ductility and
deformability of the strengthened beam are reduced as the
level of prestressing is increased.
(8) The maximum flexural strength of the strengthened RC
beam can be achieved at 40% prestressing of the CFRP rod.
The beams strengthened with 40% prestressing in CFRP rods
under NSM technique can even enhance twice the flexural
strength as compared to the un-strengthened beam.
(9) Among the different shapes of the CFRPs (plates, laminates,
strips and rods), CFRP rods were considered as being the
most effective type under prestressing.
(10) More prestressing not in any way guarantee more flexural
strength. An interesting aspect was found in the case of
CFRP strips, irrespective of whether the NSM or EBR tech-
nique is applied, no significant change occur in the flexural
strength of RC beam at higher level of prestressing.
(11) The effect of using epoxy bonding with the anchorage sys-
tem ensures maximum strength at a lower prestressing level
and produces a more economical design of beam.
(12) Increasing the prestressing level of CFRPs may improve the
flexural strength and serviceability of prestressed CFRPs
strengthened beams. However, the ductility and deforma-
bility may decrease with an increase in the prestress level.
(13) By prestressing the externally bonded CFRPs (plates and
laminates), the failure was observed with little warning
due to insufficient ductility.
7.1. Future research recommendations
Further research is required in order to encompass the different
aspects of the behavior of RC beams strengthened using pre-
stressed FRP. The following are some recommendations for future
investigations and research:

Strengthening of rc beams using prestressed fiber reinforced polymers – a review

Strengthening of rc beams using prestressed fiber reinforced polymers – a review

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