Strengthening of Reinforced Concrete Beams by Using Fiber-Reinforced Polymer Composites - A Review

Journal of Building Engineering 25 (2019) 100798
Contents lists available at ScienceDirect
Journal of Building Engineering

journal homepage: www.elsevier.com/locate/jobe
Strengthening of reinforced concrete beams by using fiber-reinforced T

polymer composites: A review
Ayesha Siddikaa,∗, Md. Abdullah Al Mamunb, Rayed Alyousefc, Y.H. Mugahed Amranc,d
a
Department of Civil Engineering, Pabna University of Science and Technology, Pabna, 6600, Bangladesh
b
Department of Civil Engineering, Rajshahi University of Engineering and Technology, Rajshahi, 6204, Bangladesh
c
Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, 11942, Alkharj, Saudi Arabia
d
Department of Civil Engineering, Faculty of Engineering, Amran University, 1 Quhal, Amran Province, Yemen
ARTICLE INFO ABSTRACT
Keywords: Fiber-reinforced polymer (FRP) composites are extensively used in advanced concrete technology given their
FRP superiority over traditional steel reinforcements. These materials possess high strength capacity and corrosion
Beam strengthening resistance and can be employed as the main reinforcements in combination with adhesives and anchorages to
Fatigue strengthen reinforced concrete (RC) beam members. RC beams are designed to provide resistance against
Impact and blast
flexure, shear, torsion, fatigue, impact, and blast loading. The strength and ductility of RC beams can be im-
Mode of failure
proved via FRP strengthening techniques with a combination of fibers. The overall strength of FRP composites in
RC beams is controlled by fiber type, configuration, and materials and strengthening technique. This review
focuses on the characteristics and behaviors of FRP-strengthened RC beams under various loading conditions. It
also presents the typical FRP composites with the properties, features, and applications. This review demon-
strates that FRP composites can be used to recover the strength of damaged and corroded beams and exhibit
good durability and insulation performance. It also provides a straightforward perspective of strengthening and
retrofitting techniques for RC beams using FRP composites.
1. Introduction to-weight ratio, insect and fungal resistance, and chemical attack re-
sistance; low thermal transmissibility; and facile installation [4–10].
Fiber-reinforced polymer (FRP) resembles a thin polymer layer that FRPs are typically used to toughen RC members that have failed to meet
may comprise different fibers. Various typical FRP materials, such as obsolete building codes or that must withstand increased static loading
glass FRP (GFRP), carbon FRP (CFRP), aramid FRP (AFRP), and basalt and to repair damaged members after environmental degradation,
FRP (BFRP), are available [1]. FRP composites are applied in the form corrosion, earthquakes, or storms [11]. FRP-strengthened RC members
of laminates, rods, dry fibers, or sheets bonded to concrete with ad- exhibit enhanced ductility, flexural strength, shear and torsion capa-
hesives and fasteners (Fig. 1). Reinforced concrete (RC) beams are FRP city, seismic resistance, and durability [9,12,13]. Previous studies have
composites that have been formulated to carry service loads and pro- exerted considerable effort to develop various suitable methods and
vide stress protection against bending, shear, torsion, vibration, impact, techniques for strengthening existing RC structures by using FRPs.
and fatigue under specific conditions. RC beams may require These studies have also established guidelines for the application of
strengthening to prolong service life by increasing durability or to FRPs under critical environment and loading conditions. This review
withstand sustained loads that exceed design loads by increasing provides a straightforward perspective of current issues related to FRP-
loading capacity. Strengthening is also commonly performed to in- based strengthening and retrofitting techniques for RC beams.
crease beam capacity to meet current code requirements and to re-
habilitate beams with corroded reinforcements [2]. 2. Background of FRP
Strengthened undamaged RC beams or damaged beams retrofitted
with FRP composites have gained popularity because they feature FRP possesses special characteristics, such as noncorrosiveness; high
noncorrosiveness; high longitudinal tensile strength, stiffness, strength- strength-to-weight ratio, tensile strength, and durability; good fatigue
Corresponding author.
∗
E-mail addresses: ayesha.ruet@yahoo.com, ayeshace@pust.ac.bd (A. Siddika), mamun_05ce7@yahoo.com (Md. A.A. Mamun),
r.alyousef@psau.edu.sa (R. Alyousef), mugahed_amran@hotmail.com (Y.H.M. Amran).
https://doi.org/10.1016/j.jobe.2019.100798
Received 14 February 2019; Received in revised form 27 April 2019; Accepted 6 May 2019
Available online 10 May 2019
2352-7102/ © 2019 Elsevier Ltd. All rights reserved.
A. Siddika, et al. Journal of Building Engineering 25 (2019) 100798
Fig. 1. Different FRP reinforcements [3]. Annotations: a) unidirectional carbon, glass, and aramid fiber sheet, (b) cured transparent bidirectional GFRP sheet, (c)
basalt fiber sheet, (d) fractured confining GFRP jacket, (e) CFRP tendons, (f) CFRP laminate, (g) CFRP strip for NSM, (h) CFRP bars, (i) GFRP bars, (j) polypropylene
fiber rope, (k) vinylon rope.
Table 1
Properties of FRP composites.
Refs. FRP Type Unit weight (g/m2) Thickness of CFRP (mm) Tensile strength (MPa) Elastic modulus (GPa) Rupture strain (%) Ultimate strength (MPa)
[23] GFRP bar – – 483–1600 35–51 1.2–3.1 –

CFRP bar – – 600–3690 120–580 0.5–1.7 –
AFRP bar – – 1720–2540 41–125 1.9–4.4 –
[24] CFRP sheet – 0.4 – 84 1.25 1050
[15] CFRP sheet 200 0.115 3790 230 – –
GFRP sheet 915 0.36 3240 72.4 – –
[25] CFRP 340 0.45 1548 89 1.74 –
[26] CFRP sheet 300 0.165 4800 230 – –
[27] CFRP sheet – 0.86 609 63.3 0.96 –
[2] CFRP sheet – 0.165 3550 235 1.50 –
GFRP sheet – 0.353 1700 71 2.0 –
[28] CFRP – 1.4 2850 168 –
[29] CFRP sheet – 0.131 – 238 0.015 4300
CFRP Strip – 1.2 – 165 0.017 3100
GFRP sheet – 0.131 – 72.5 0.04 2276
[30] CFRP sheet – 0.111 – 242 1.7 4103
GFRP sheet – 0.273 – 73 2.7 3400
[31] CFRP sheet – – 212 1.58 3350
[32] CFRP sheet – 0.167 3522 258.9 – –
[33] BFRP sheet 300 0.12 1684 – 2.1 –
CFRP sheet 340 0.45 1500 – 1.65 –
[34] CFRP sheet – 0.348 2089.4 119.25 1.7 –
GFRP sheet – 0.352 786.5 34.13 3.5 –
properties; lower relaxation losses than mild steel bars; minimal dis- strength can be used in construction. The properties of FRP that have
ruption; and facile handling. FRP features irregular surfaces and elec- been used for different research purposes are listed in Table 1.
trical insulators and lacks a magnetic signature [14–17]. FRP-based The selection of strengthening materials generally depends on the
materials have superior properties. The unit weight of CFRP ranges types of materials used in the existing structure, strength requirements,
from 1.5 g/cm3 to 1.6 g/cm3, that of GFRP ranges from 1.2 g/cm3 to environmental conditions, availability, and cost. The beneficial per-
2.1 g/cm3, and that of AFRP ranges from 1.2 g/cm3 to 1.5 g/cm3 [18]. formances of different types of FRP in strengthening have been reported
CFRP is 5 times lighter than conventional steel; moreover, the tensile but have not been compared. BFRP reinforcements are more suitable
strength of CFRP is 8 times to 10 times higher than that of conventional than CFRP components because the former shows good strain control
steel [19]. FRP composite systems cannot be used as compression re- capability at failure [35]. GFRP is highly vulnerable in alkaline en-
inforcements in structural applications because their compressive vironments given that it contains a high amount of silica, whereas CFRP
strength is drastically lower than their tensile strength (20%–50%) is highly resistant to all forms of alkali [36–38]. CFRP has been selected
[18]. FRP reinforcements exhibit a low modulus of elasticity and linear over GFRP or AFRP to improve the strength and expansion resistance of
elastic behavior before failure because of their poor plasticity and concrete because of its high stiffness. Although CFRP is commonly used
brittle nature [20]. CFRP fibers have a modulus of elasticity of 165 GPa and provides higher stiffness than GFRP while requiring low amounts of
and a Poisson's ratio of 0.183 [21], whereas CFRP sheets have a mod- resin [39], it is more expensive than GFRP [36]. The rupture strength of
ulus of elasticity of 230 GPa and an ultimate tensile strength of CFRP sheets with vertical fibers is higher than that of CFRP with hor-
3500 MPa [22]. Therefore, FRP fibers and sheets with moderate to high izontal fibers [40]. The orientation and alignment of fibers are crucial
2
factors that should be considered in the selection of retrofitting mate- strength [2]. Rounding sharp edges before wrapping can reduce ex-
rials. The torsion capacity of CFRP beam elements is more advanta- cessive stress concentration along edges and increase the confinement
geous for strengthening than that of GFRP beam elements [41]. How- effect necessary for enhancing compressive strength [27,50]. This
ever, CFRP-strengthened beams rapidly fail once reaching the ultimate modification increased impact resistance by approximately 10% after
point, whereas GFRP-strengthened beams exhibit residual strength for a strengthening [50]. In the NSM technique, grooves with specific widths
considerable duration after peaking. GFRP has good energy absorption and thicknesses are drilled through the concrete face. The concrete is
capacity and is recommended for use in earthquake-strengthening then filled with FRP strips and concrete/epoxy sealing. The NSM
projects. Mustafa and Hassan [42] observed that beams strengthened technique is superior to other techniques because it protects the FRP
with GFRP composites exhibited a drastic reduction in stiffness and components from harsh environments and prevents premature de-
remarkable deflection that immediately resulted in crack initiation. bonding. The effect of surface preparation on the shear strengthening of
CFPR-strengthened beams, however, showed better performances than 32 small-scale concrete beams was investigated [46]. Four-point
GFRP-strengthened beams after crack production until failure. Hybrid loading test results revealed that the performance of the grooving
FRPs comprising a combination of CFRP and GFRP are novel con- method was better than that of other surface preparation methods.
struction materials used to meet strength requirements. These materials Surface preparation often does not prevent debonding but can delay
have high ductility, deformability, and break resistance and are in- debonding and can increase beam carrying loads by 12%. CFRP strips
expensive. They exhibit the high impact resistance of GFRP and the have different configurations and are subjected to different surface
high tensile modulus and strength of CFRP [34,43,44]. The thermal treatments [49]. Theoretical and experimental results have shown that
resistivity and ductility of BFRP are better than those of CFRP laminates the tensile strength of the NSM FRP strengthening system is better than
[45]. A hybrid combination of CFRP and BFRP is recommended to that of the EB-reinforced system. Therefore, the selection of the surface
strengthen beam elements that will be subjected to high temperatures. preparation method is dependent on the types and configurations of the
Therefore, the selection of FRP for strengthening is crucial and requires FRP material and loading conditions.
considerable attention and further research. Moreover, the selection of
this material depends on variations in structural systems, loading 3.2. Adhesive requirement
characteristics, and environmental situations.
High-quality adhesive resins are used to achieve the perfect bonding
3. Typical techniques for the strengthening of RC beams of FRP composites and concrete surface necessary to ensure proper
strength. Resins are polymers that may be solid and extremely viscous
Numerous techniques are used to strengthen RC beams with FRP in nature. They are used as adhesives for perfect bonding with other
materials. These techniques involve the use of externally bonded (EB) polymeric materials and may have polyester, epoxy, or phenolic forms
laminates, near-surface mounted (NSM) bars/strips, mechanical an- [51]. Different types of epoxy resins are available. Most epoxy resins
chorage systems, or grooving methods with or without adhesives comprise epoxy and hardener components that are mixed in requisite
[46,47]. EB FRP strengthening can be performed with the desired proportions in accordance with the manufacturer's specifications. For
number of strengthening layers on the beam in any configuration, such example, the epoxy and hardener components are mixed at 1:1 to 5:1
as side bonding, U-wrapping, or full wrapping. NSM strips can be in- ratios unless specified. Epoxy resins have unit weights of 1.1 kg/m3 to
serted into grooves cut along the tension faces of beams and covered 1.4 kg/m3 and weights per unit surface area of approximately 0.5 kg/
with concrete layers with sufficient thicknesses. The ends of fibers must m2 [44,51]. Epoxies are applied along the treated surface of RC beams,
be anchored effectively to increase the efficiency of FRP in improving and FRP sheets/strips are directly attached along the indicated space.
overall strength and to reduce the probability of FRP strip debonding. Epoxies are sometimes injected into RC members through drilled holes
FRP anchors, mechanical fasteners, or other special systems are gen- for crack repair or FRP anchor or strip insertion when NSM strength-
erally applied as anchoring systems. The anchors can be spike anchors, ening techniques are applied [22,52]. Epoxy resins have tensile
powder-actuated fasteners, straps, or any other suitable configurations. strengths of 30 MPa–90 MPa, maximum elongation at failure of
FRP fabrics are sometimes connected [45] through threading or 0.9%–4.5%, and elastic modulus of 1.1 GPa–6 GPa [2,25,32,33,51]. The
stitching along the thickness of the concrete member to be strength- required epoxy curing period are ranging from 3 days to 14 days at
ened. External FRP strips are inserted into drilled holes to improve temperature 16 °C–23 °C [25,33,34]. Epoxies should have high glass-
flexural and shear capacity. Additional FRP sheets are used to close transition temperatures to withstand elevated temperatures and should
holes after filling with a binder, which may be mortar or an adhesive. be durable to adapt to critical environment and loading conditions.
The overall strength of RC beams is controlled by the anchoring system.
3.3. Anchorage provision
3.1. Surface preparation
Anchorage methods are classified on the basis of the use of addi-
Surface preparation remarkably changes the overall bond strength tional metallic elements or FRP anchors. FRP sheets can be anchored
of strengthening systems. Concrete surface preparation involves the with concrete elements by using transverse sheets or straps along with
removal of the weak external layer and impurities hidden on the con- the reinforced concrete or by inserting FRP strips or mechanically
crete surface and is necessary to prevent the premature failures of FRP- bonded anchors into grooves cut into the concrete. The use of U-shaped
retrofitted structures. In EB FRP strengthening, surface preparation can transverse straps of FRP sheets anchored with flexible FRP reinforce-
be conducted by removing concrete mortar and pore openings through ments confers resistance against delamination under intense loads [24].
innovative techniques. Cleaning and roughening the surfaces of com- Other anchorage techniques implemented to strengthen load capacity
posites can enhance bond strength. Sandblasting, water jetting, include the application of metallic anchorages with plates and bolts
grinding, brushing, air pressure, and nylon peel-ply techniques are [53] and the insertion of powder-actuated fasteners into predrilled
commonly used to promote bond formation between FRP and concrete holes in RC beams [54]. The efficiency of each method differs and
[6,34,46,48,49]. The effectiveness of different surface preparation depends on appropriate arrangements and loading conditions. Spike
processes on strength behavior has also been investigated. Mechanical anchors are practical and advantageous anchor systems [15,55]. The
abrading or sandblasting with an appropriate primer is recommended strengths of nearly vertical inclined anchors are almost twice those of
for the preparation of concrete surfaces for EB FRP application [48]. horizontal anchors [15]. The effectiveness of the anchorage provided by
The sharp corners of RC beams, however, should not be directly FRP fans and FRP NSM bars has been tested, and concrete can provide
wrapped with FRP sheets because this approach can reduce fiber efficient bonding and increase debonding load by 25% [26]. The proper
3
execution of anchorage with long FRP laminates increases beam concrete cover control the effectiveness of NSM strengthening [61,62].
strength. Several studies have demonstrated that bond stress does not Another work has investigated the flexural capacity of beams
entirely develop along the whole bond length of FRP sheets and con- strengthened with mechanically fastened FRP strips and found that the
crete when tensile loads are applied but instead develops within a short ultimate moment capacity of the strengthened beams increased by
distance from the loaded ends of FRP sheets [56]. Therefore, long FRP 20.1% [54]. The effects of two different types of FRP-wrapping condi-
bond lines can reduce ultimate bond strengths under tension. Effective tions (GFRP and CFRP) with three different concrete cover thicknesses
bond length is highly dependent on the thickness and elastic modulus of (20, 30, and 50 mm) have been examined. The ratios of the actual bond
FRP and the strength of concrete [56]. FRP EB bars with a minimum strength to the predicted bond strength for all concrete cover and
diameter of 13 mm should be wrapped on the external corners of wrapping condition combinations varied from 96% to 134%. The effect
structural elements with sufficient overlap to ensure proper anchorage of FRP wrapping intensified as the thickness of the concrete cover de-
and to avoid internal corner wrapping [18]. Meanwhile, though anchor creased. Recent works on FRP applications have revealed remarkable
system enhanced the capacity of the strengthened beam significantly, it improvements in the overall strength of retrofitted beams. Another
reduces the overall ductility [57]. Therefore the chances of brittle state study showed that replacing one corroded steel reinforcement out of
of failure could be a risk that arises from this types of system. The time three with a NSM BFRP strip increased flexural strength by 14% and
required for FRP strips to bond with concrete is a crucial factor of the deflection carrying capacity by 36.2% and enhanced ductility and strain
entire strengthening process. Fast-working systems are always bene- capacity [58]. The load capacity of the control specimen increased by
ficial. The mechanical fastening method is faster than the conventional 35% when two of its corroded steel reinforcements were replaced with
bonding method. Specifically, mechanical fasteners can bond with FRP two BFRP strips. Therefore, the flexural strengthening or repair of RC
strips in only 30 min, whereas conventional fasteners bond with FRP beams using FRP composites is highly effective and reliable. The ef-
strips in 4 h [54]. The application of CFRP fabric to a beam with a cross fectiveness of RC beam reinforcement or retrofitting depends on the
section of 254 mm × 165 mm and effective span of 1.829 m required quality of materials and the conditions of the damaged beams.
45 min whereas that of CFRP-procured laminates required 15 min per
beam [55]. The NSM bonding technique requires less installation labor, 4.2. Shear strengthening
time, and surface preparation and has lower debonding probability than
the EB technique [58]. The shear capacity of beams with insufficient shear reinforcement
or cracked concrete must be increased. EB FRP systems have been
4. Typical strengthening applications successfully used in the shear strengthening of RC beams over the past
few years. Additional FRP web reinforcements can be applied as shear
RC beams must be strengthened to increase their flexural strength, reinforcements with vertical, inclined, side-bonded, U-wrapped or an-
shear strength, fatigue life, seismic resistance, and impact and blast chored configurations to beams through the EB technique. The EB FRP
resistance. Beams should be strengthened after a certain period of strengthening technique has been proven to increase the shear strength
loading because damage, which may be in the form of structural da- of RC beams, and its effectiveness under corresponding loading condi-
mage or corrosion, may have been initiated. tions depends on the types and orientations of FRP reinforcements [24].
Additionally the effectiveness of shear strengthening using EB FRP
4.1. Flexural strengthening system also controlled by the percent of FRP, internal steel reinforce-
ment, concrete quality even longitudinal tensile steel [63]. The ap-
RC beams commonly undergo flexural failure. Most RC beams must proximation on EB FRP design for shear strengthening system was
be strengthened to resist failure and prolong their lifetime. The flexural proven very conservative, because EB FRP cannot be treated as internal
capacity of RC beam elements can be improved through strengthening stirrup. Additionally the full strength of internal stirrup cannot use to
or retrofitting via EB and NSM FRP techniques. The performance of predict the capacity, because all stirrup does not meet the yielding
strengthened beams depends on material quality, strengthening tech- during concrete cracks widened [63,64]. However, shear strengthening
nique, and loading and environmental conditions. The application of system through FRP was proved as effective in several researches. A 2 m
FRP laminates along the tension face (generally soffit) of beams is an RC beam was strengthened by using an EB U-wrapped CFRP sheet with
effective method for increasing flexural strength [33,59]. Although a width of 50 mm, thickness of 0.176 mm, and spacing of 187.5 mm
beams that have been U-wrapped or fully wrapped with FRP sheets [60]. The load capacity of the strengthened beam increased by 32.4%.
demonstrate excellent performances, the practical application of fully Moreover, the deformability of the beam was 114.7% higher than that
wrapped FRP is complicated. U-wrapping with FRP can enhance the of the unstrengthened beam, and the failure mode of the strengthened
flexural strength, stiffness, and deflection resistance of RC beams [33]. beam changed from shear tension failure to shear compression failure.
FRP thickness controls strengthening performance. Bending tests have Hussein et al. [22] used CFRP to strengthen a RC beam that had been
revealed that the deflection (39%) of FRP-strengthened RC beams U- previously damaged by excessive shear. They closed shear cracks in the
wrapped with two-layer FRP sheets reduced by 39% relative to that of damaged beam through the application of external prestress force and
FRP-strengthened RC beams U-wrapped with one layer of FRP [30]. A U-wrapping with a single-layered CFRP sheet. The prestress force was
high flexural reinforcement ratio could attenuate strengthening [60]. removed after epoxy hardening. The load-carrying capacity of the re-
The use of the U-wrapped hybrid composite of CFRP and GFRP layers paired beams increased by 57%. The repair of shear cracks through
can enhance the ultimate load capacity of RC beams by 103% [43]. The epoxy injection before EB FRP sheet wrapping can also increase the
overall performance of EB wrapping is dependent on the bonding be- ultimate capacity of rehabilitated beams effectively [52]. Moderate
tween the FRP sheet and concrete [60]. Intermediate anchorage, me- shear-span-to-depth ratio, low FRP spacing, and ignoring transverse
chanical fastening, and NSM FRP strip strengthening are recommended reinforcement can enhance the effectiveness of strengthening [40].
to overcome premature bond failure, which includes critical and brittle- Hybrid FRP systems are more effective than unidirectional FRP systems
type failure [24,58]. The NSM technique for strengthening can enhance [52]. The additional intermediate anchorage of FRP laminates in any L-
beam ductility and confer reliable flexural strength. The results of a or U-wrapping system with flexural strengthening reinforcements can
four-point bending test performed in a previous study revealed that the be performed to maintain ductility and provide adequate shear capa-
flexural strengths of EB- and NSM FRP-strengthened beams increased city. A previous study applied EB CFRP and GFRP to strengthen the
by 74% and 111% [28]. NSM strip strengthening increased strength by shear of beams with and without anchors [65]. In this study, shear was
66% [61]. The selection of appropriate fibers, the positions of FRP in- strengthened by applying 200 mm-wide CFRP sheets at a spacing of
sertion, the application of the appropriate resin, and the quality of the 275 mm and 100 mm-wide GFRP sheets at a spacing of 200 mm in a U-
4
wrapped system along a 2.2 m effective span of the RC beam. Anchors shear-torsion capacity of the RC beam by 97% and improved the
fabricated from 10 mm-diameter CFRP and GFRP bars were used. The strength of shear-cracked beams by 60% [70]. Ductility also improved
load capacity of the CFRP-anchored beam increased by 75% relative to as torsion capacity increased [41]. Ultimate strain did not exceed
that of the control beam. GFRP-strengthened beams in anchored and 0.0035 [2,71]. The increment in strain level indicates enhanced duc-
nonanchored systems underwent shear failure after peak loading, tility and delayed shear failure in FRP-strengthened beams [27]. Fully
whereas CFRP-strengthened beams underwent flexural failure. Another wrapped EB FRP-strengthened steels with internal longitudinal and
investigation demonstrated that the ultimate load-carrying capacity of transverse steels are more likely to experience strain than U-wrapped
shear-strengthened beams using mechanically fastened hybrid FRP in- EB FRP with internal longitudinal and transverse steels. The torsion
creased by 82.2% relative to that of the unstrengthened beam in the capacity of FRP-strengthened beams can be expressed in terms of twist
absence of a stirrup within the critical shear span [66]. But, the strength angle and is dependent on the spacing and width of wrapping FRP
of the beam with internal steel stirrup was increased by approximately sheets. Increased ductility is a sign of the increased twisting capacity of
69.2% after applying same strengthening. This result indicates that the strengthened beams [27,70]. NSM techniques are less reliable than EB
internal steel stirrup interacted with EB FRP. The adverse relation be- techniques in increasing torsion capacity. In addition, using anchorage
tween the internal stirrup and EB FRP was introduced into some of the with FRP wrapping helps to reduce debonding failure, and strain level
model to develop a proper design criteria for shear strengthening [67]. increases along anchorage points because of stress concentration [27].
Because, presence of stronger steel stirrup inside beam causes ob-
struction to the effective utilization of capacity of EB FRP [64]. This, 4.4. Blast resistance
because the strengthened beam cannot reach its ultimate limit when
internal steel stirrup transected by the critical shear cracks [67]. The AFRP is highly resistant to blast damage and can be used to prevent
direction of FRP wrapping controls the cracking pattern of the spall and fragmentation completely because of its high energy absorp-
strengthened beam. Beams with FRP wrapping along the 45° direction tion capacity [72]. Trinitrotoluene or a mixture of ammonium nitrate
could resist the formation of diagonal cracks, whereas beams with fi- and fuel oil is generally used to simulate explosive charge weight in
bers wrapped along the 0° and 90° directions could not [40]. Shear blast tests. Several researchers have reported that composite laminates
cracks progress downward in diagonally arranged FRP-strengthened of carbon and glass fibers can protect against massive explosive blasts
beams after reaching ultimate load. and that NSM techniques show negligible efficiency when blast re-
In most of the cases, the shear strengthening design are based on an sistance is equivalent to 6 kg of explosive charge weight [73]. Advanced
approximation that the shear cracks will produce along 45° angle, but techniques and superior combinations of FRP with RC beams are
the researches on shear strengthened beams reveals that the angle needed to resist highly explosive charge weight in practical circum-
varies in between 30°-60° which depends on the parameters relating to stances. Overall ductility is enhanced by using other polymers in ad-
shear strength of FRP system [63,64,68]. The cracking size, shape and dition to FRP or hybrid FRP. A combination of CFRP with sprayed
inclination must be need to analyzed before design for shear strength- polyurea is also used to strengthen RC structures by enhancing ducti-
ening, because both have unique influence on the design strength and lity, stiffness, and overall resistance under blast loading [74].
failure characteristics [67,68]. Moreover, increasing the FRP-covered
area could enhance capacity. For example, load capacity increased by 4.5. Impact strength
17.5% after the U-wrapped FRP area was increased by 33% [66]. The
use of NSM strips also noticeably increased shear strength. A large bond Impact forces could be generated in RC structures from moving
interface area has been utilized to resist shear with respect to the strip loads, ice falls, accidental falling loads, explosion, and tornados [75].
cross-sectional area in NSM-strengthening systems [69]. The successful Point loads are transiently concentrate on structures at a loading rate of
implementation of NSM-strengthening systems depends on the amount up to 10 s−1 [76] with higher strain rates than static and seismic loads
and orientation of FRP strips, the strength of the concrete, and the type [75]. The impact strengthening of RC structures is necessary given the
of steel strips used. The change in failure mode from brittle shear failure increased incidences of global terrorism, explosions, and accidents
to ductile flexural failure after ultimate loading is indicative of the ef- [50]. The FRP-strengthening technique can efficiently improve the
fectiveness of the strengthening scheme [66]. The highly brittle shear impact resistance and shear strength capacity of structures [25]. Shear
failure mode of RC beams strengthened through U-wrapping with FRP failure in RC beams is critical, and failure frequently occurs under
transforms into ductile failure mode [60]. impact loading wherein the same beam fails in flexure under any static
loading [6,50]. The impact strength of strengthened RC beams under
4.3. Torsion strengthening shear increased by 5%–15% [75,77]. The impact load capacity of beams
strengthened by using two layers of CFRP in the longitudinal direction
Torsional resistance controls the overall design of spandrel beams, along the soffit and one CFRP strip U-wrapped in the transverse di-
curved beams, beams that support spiral slabs, secondary beams, and rection increased by 1.5%, and the impact resistance of beams
eccentrically loaded bridge girders [2,27]. Therefore, these types of strengthened with seven U-wrapped strips increased by approximately
beams should possess high torsional resistance to avoid high brittle 9% [50]. These results indicate that shear strengthening successively
torsional failure [27]. The torsion-deficient beam elements of structures increased impact strength. Under impact force, an element needs to
can be strengthened by using EB U-wrapped or fully wrapped FRP la- withstand concentrated force and free-vibration effects within elas-
minates or sheets along the beam span. The torsional moment re- tic–plastic deformation for a period beyond impact time [50]. Tough
sistance of RC beams fully wrapped with GFRP increased by 72% re- and stiff materials are needed to resist impact forces. Therefore,
lative to that of the control beam [2]. The ability of a fully wrapped strengthened beams should have sufficient toughness. AFRP has higher
strengthening system to improve torsion capacity is better than that of a impact resistance than other materials and has high energy absorption
U-wrapped system [2,41] because full wrapping could restrain crack capacity [1,72]. Additionally, the performance of a 45° angular wrap
widening [2]. FRP strips wrapped in the vertical direction may reduce system involving FRP strips is better than that of a vertical U-wrapping
crack widths by up to 20% [70]. FRP wrapped along the 45° direction system under impact loading [25]. Insufficiently strengthened beams
rather than the 90° direction efficiently improved ductility and in- develop numerous inclined shear cracks and diagonal cracks along their
creased torsional strength [2]. Fiber strips wrapped along the 45° di- midspans where impact force is applied [6]. These cracks could be
rection reached their ultimate tensile strength at failure, thus proving minimized by using sufficient amounts of FRP strips wrapped in the 45°
the effectiveness of strengthening [2]. Strengthening from the begin- direction. Given that debonding strain in FRPs is lower under impact
ning by wrapping with FRP strips along the 90° direction increased the force than under static force, FRP strain should be considered carefully
5
in design [50]. Moreover, the faces of structures wherein positive and using CFRP. The two faces of a column had been strengthened by using
negative bending can occur simultaneously must be strengthened be- a single-layer CFRP sheet, and the bottom surface of a beam was
cause impact could induce negative bending [76]. The top fiber zone of strengthened by one CFRP layer [92]. The strengthened frame with-
U-wrapped beams could experience brittle debonding and failure, and stood a seismic test with a 3% drift ratio. A similar frame was
fully wrapped beams show improved performance under impact [4]. strengthened through the installation of NSM CFRP strips along the tops
The bond characteristics of FRP-strengthened concrete are drastically and bottoms of beams and the two sides of columns. Under 0.1%,
affected by loading rate because of uncertain strain distribution under cracking initiated in the control structure under 0.1% drift but not in
impact loading [78,79]. The ductility and failure mode of FRP- the strengthened specimen. Beams strengthened with NSM strips
strengthened concrete, however, remain unaffected [77]. showed high strength, whereas those strengthened with EB CFRP sheets
showed high energy absorption. Shear failure can dominate in RC
4.6. Fatigue life beams deficient in seismic resistance during seismic events [93]. The
critical location of shear failure in beams is near the support/column,
Bond strength is the major point of concern in the performance of which could be reinforced through FRP wrapping. The main aim of the
FRP-strengthened beams under fatigue loading [80]. The loss of bond seismic strengthening of structures is to provide adequate ductility and
strength shortens the fatigue life of structures subjected to cyclic energy absorption capacity with high lateral stiffness. FRP wrapping
loading during adhesive curing. Structures begin to lose their overall enhances the flexural capacity and shear strength of beams. This ap-
stiffness and member ductility and tend to experience permanent de- proach improves ductility and increases the energy dissipation capacity
formation under fatigue loading [55]. FRP composites can improve the of the beam–column joint [91].
fatigue resistance of strengthened structures given their high tensile
moduli of elasticity [81]. CFRP is suitable for improving the fatigue 4.8. Retrofitting of RC beams
resistance of structures because of its high elastic modulus and good
fatigue strength [1]. Several researchers [82–86] studied more than 45 4.8.1. Corrosion affected RC beams
reinforced concrete beams spliced on their mid-spans with different Corrosion can severely damage RC beams. The most common cor-
wrapping conditions and different concrete cover thicknesses under rosion damages include concrete cracking caused by expansive stresses
fatigue loading. Unwrapped beams underwent failure in the form of induced by corrosion by-product deposition and mass loss along the
concrete cover splitting, whereas FRP-wrapped beams underwent steel–concrete interface [94]. The load capacity and service lives of
combined splitting and pull-out failure. These results showed that the beams with corrosion damage could be increased through repair with
influence of the bond strength of CFRP wrapping was higher than that FRP laminates or strips. The strength of corrosion-damaged beams re-
of GFRP wrapping. Structural strength did not vary under a 1 Hz si- covered to its predamage level through the deployment of CFRP sheets
nusoidal transient load of up to 50% of the ultimate capacity of the non- along the tension sides of beams [24]. The influence of corrosion on RC
strengthened RC beam during the installation and curing of FRP in EB beams strengthened by FRP sheets along the tension face and two
CFRP-strengthened beams [31]. Therefore, the appropriate execution of partially U-wrapped FRP sheets at two loading points was studied
the FRP strengthening technique can improve a structure's fatigue through the four-point bending test. The beams lost approximately 5%,
performance. The application of prestressed steel anchorage NSM CFRP 10%, and 15% of their steel mass. The strengthened corroded beams
strips in a 5 m long RC beam increased strength by 54% [80]. Tension showed increments of 23.9% and 9.3% in ultimate flexural strength and
steel specimens exhibited linear strain and debonding in the early a reduction of 2% in strength when compared with the noncorroded
stages of fatigue loading for 3 million cycles at a frequency of 2.0 Hz to beam. Premature debonding reduced beam strength and decreased
develop 125 MPa stress. Debonding resistance can be improved by in- corrosion strength by 15%. EB FRP and two NSM strips were used to
creasing prestress levels. Alternative anchorages applied through the repair a severely corroded beam that had lost approximately 24% of
NSM technique are needed to resist slip under fatigue loading. The fa- steel mass and that had developed cracks with widths of 1.4 mm [95].
tigue life of FRP-strengthened beams can be divided into three stages Cracked concrete that had been previously patch-repaired with NSM
[87]. The first stage is spent in the crack initiation process and accounts was repaired with the EB wrapping system. The EB wrapping system
for 3%–5% of fatigue life. In the second stage, damage stabilizes, and provided higher strength gain (approximately twice) and better stress
deformation patterns negligibly change. This stage accounts for 90% of distribution than the NMS system [95]. The ductility and ultimate de-
fatigue life. It is succeeded by rapid failure and tensile steel rupture flection of beams with damaged concrete covers repaired with FRP
followed by FRP debonding. Therefore, the total fatigue life is con- were lower than those of the control beam [24,62,96]. FRP repair
trolled by the tensile strength of steel and FRP bond strength. The stress without the removal of cracked concrete covers resulted in debonding
limit considered for the fatigue life of FRP reinforcements and concrete failure [94,95]. The removal of the damaged concrete cover improved
under compression or steels under various loading is 80% of yield stress distribution and increased strain utilization [95]. A 23-year-old
strength [88]. Reinforcing materials can resist fatigue failure that is less RC beam was repaired [62] by using NSM CFRP strips with diameters of
than 60% of their yield strength [89]. A fatigue limit of 55% of ultimate 6 mm. The beam was previously corroded in a chloride environment
strength is proposed as the threshold for applied load on BFRP using and had lost 36% of the cross-sectional area of its steel reinforcement in
beams fabricated with sea sand concrete [90]. the tension zone. In addition, the corroded beam had lost 36% of its
moment capacity and 47% of its stiffness and had developed cracks
4.7. Seismic resistance with widths of 3 mm in its tension zone. The original strength of the
repaired beam was restored. However, its stiffness was insufficiently
RC building structures rarely fail in seismic events, and the restored. The fatigue strength of a mildly corroded RC beam had been
beam–column joint is the most influential factor of the failure of RC sufficiently recovered through repair with one layer of CFRP sheets,
building structures. Joint failure could lead to global structural failure whereas that of a moderately to severely corroded RC beam was re-
[71]. Thus, the critical regions of beams must be strengthened to pre- covered through repair with a 20%–30% prestressed CFRP sheet [97].
vent joint failure. The application of FRP laminates along the tension The strain at failure in a beam repaired with EB FRP was considerably
faces of beams could provide adequate ductility [91]. The FRP U- lower than ultimate strain because the repaired beam failed as a result
wrapping system can provide seismic resistance within reasonable of the yielding of corroded steel and damaged concrete [96]. Non-
limits. FRP-wrapped beams have sufficient energy absorption capacity structural cracks resulting from sulfate attack reduced the flexural
and load-carrying capacity [72] that meet the crucial requirements of a strength and shear carrying capacity of the overall structure. Sulfate-
seismically resistant structure. RC frames had been strengthened by damaged shear-deficient beams had been repaired with CFRP strips,
6
CFRP sheets, or GFRP sheets wrapped in the 45° and 90° directions
[29]. The load capacity of the strengthened beams was 30% higher than
that of the damaged beams. Moreover, the strain capacity of the
strengthened beams increased by 26%–37%. Therefore, FRP can be
used to repair corrosion damage, improve performance, and extend
service time.
4.8.2. Crack repaired RC beams

Beams undergo initial cracking due to high temperature, shrinkage,
shear load, and minor flexural cracking. FRP can be used to repair Fig. 2. Potential failure zones in a FRP-strengthened concrete system [21].
beams with initial cracks effectively. Cracks could be repaired by using
epoxy adhesives and wrapping with EB FRP sheets [94]. A concrete is debonding, which can be classified into several types. FRP fracture is
bridge girder that had undergone shear damage and minor cracking in expected to fail with steel yielding to increase strain utilization.
its end regions recovered its strength when repaired with wrapped FRP Common failure patterns and their behaviors require detailed discus-
sheets used as shear reinforcement [98]. The loads of the damaged sion.
beam were approximately 73% of those of the undamaged control
beam, whereas those of the beam repaired using CFRP and GFRP sheets
were 120% and 102%, respectively. AFRP wrapping had also been used 5.1. Debonding failure
to strengthen RC beams that had developed initial cracks under loading
with approximately 70% of failure load [10]. The strength of the re- One of the most commonly reported disadvantages of the FRP
paired beam increased by 66.4% relative to that of the non-strength- strengthening system is its brittle debonding failure mode [4]. This
ened beam. Moreover, crack width decreased by 82.7% under failure failure mode is common in side-bonded, U-wrapped EB FRP systems.
load. Diagonal wrapping performed better than vertical wrapping in Concrete cover separation, plate-end debonding, flexure, and shear-
this experiment. induced debonding are premature debonding modes [103]. Except for
Steel reinforcements in concrete lose their stiffness and tensile concrete cover separation, which predominantly occurs along the in-
strength when exposed to high temperature or fire. Thermally damaged ternal steel level, most brittle debonding failure modes occur along the
concrete beams could be repaired by using FRP. Cracks with an average FRP–concrete interface. Concrete cover and shear reinforcement have
width of 0.8 mm developed in the vicinity of steel reinforcements in RC negligible influences on the delamination of FRP strips [104]. However,
beams exposed to 600 °C for 2 h. In addition, the steel reinforcements excessively thick EB FRP layers promote brittle failure [104]. Different
lost 30%–40% of their tensile strength and 73% of their compressive types of intermediate anchorages, including U-jacket anchors, me-
strength [99]. The beams were strengthened by applying three 8 mm chanically fastened anchors, and FRP anchors, have been used to pre-
NSM strips along their soffits. The stiffness and load capacity of the vent early debonding failure [103]. Metallic clamps have been used to
repaired beams increased by 67% and 40%, respectively, relative to prevent delamination and to increase ductility [104]. These anchors
those of heat-damaged beams. Moreover, the performance factor of the delay debonding by enabling the continuation of the load path between
beams reached 0.8–1.05 [61]. The capacity of a beam exposed to 800 °C FRP concrete and increasing bond strength [105]. Moreover, the use of
increased by 33% after repair. The suggested spacing for NSM strips any typical fiber in the horizontal and vertical directions could prevent
used to repair heat-damaged RC beams that had been exposed to tem- debonding failure effectively [106]. The NSM strengthening technique
peratures exceeding 500 °C should be less than 100 mm [100]. The could delay the debonding failure of FRP. Concrete cover separation is
strength of a prestressed concrete bridge girder that had lost approxi- the most common failure observed in NSM systems [107]. Therefore,
mately 50% of its strands was restored through retrofitting with the EB the concrete cover and bond between concrete and NSM strips control
FRP system along its midspan [101]. In this system, the allowable strain failure mode. Moreover, ductile side NSM-strengthened beams fail in
in FRP laminates was 1.5%. Ultimate moment capacity increased by concrete crushing through intermediate crack-induced debonding ra-
50% and midspan deflection reduced by 60% after retrofitting. The ther than through concrete cover separation [107]. The smooth surfaces
retrofitted girder, however, failed through premature debonding. of NSM strips can reduce adhesion between the strip and adhesives. The
Therefore, the efficiency of FRP in the repair of RC beams is unques- reduction in adhesion will induce the formation of longitudinal cracks
tionable and likely depends on fiber selection and orientation, appli- in the adhesive layer and is attributed to the radial component of bond
cation technique, and loading, as well as the environmental condition stress and the occurrence of progressive failure in the interface of the
under which damage had previously developed. The previous damage FRP adhesive [69]. By contrast, cohesive shear failure within the ad-
state of the retrofitted beam controls failure mode because severely hesive can occur in stiff concrete when adhesive stress exceeds the
damaged beams undergo intense steel yielding, which influences duc- tensile strength limit. Cohesive shear failure within concrete is the most
tility. Cover separation debonding is the most prominent failure mode common debonding failure experienced by the NSM system. Therefore,
of damaged beams repaired through EB FRP. the thickness of adhesive layers should be sufficient to resist these types
of debonding. Debonding is generally initiated from the end of FRP
5. Analysis of failure of FRP-strengthened RC beam strips, where flexural cracks that introduce drastic variations in strain
level in FRP strips are most prominent [108].
The effectiveness of strengthening could be reflected by the failure
mode of strengthened structures. Most strengthened structures are ex- 5.2. Rupture of FRP
pected to undergo ductile failure with the maximum utilization of strain
capacity. EB FRP could help change the failure mode of RC structures The high strength of concrete–adhesive bonds, the effective utili-
from brittle failure to ductile failure under static and dynamic loading zation of stress, and the inferior quality of fibers within high-strength
[4]. Potential failure zones are described as areas of weak linkages concrete and internal steel could induce the occurrence of FRP rupture
among all components such that the whole system fails when any as the ultimate failure. The most common failure experienced by
component fails, as depicted in Fig. 2 [21]. The failure zones of RC strengthened RC beams with full wrapping is FRP rupture after loca-
beams strengthened by EB FRP are shown in Fig. 3 [102]. This failure lized debonding, which may be ascribed to the effective utilization of
mechanism has also been described in several studies [13,103]. The strain [103]. EB FRP systems with fibers in the horizontal and vertical
most common failure associated with beams strengthened with EB FRP directions show higher strain utilization than EB FRP systems with
7
Fig. 3. Typical failure mode of a RC beam strengthened by EB FRP [102].
fibers in the vertical direction. Nevertheless, debonding occurs when strengthened beams are shown in Table 2, where the typical mode of
fibers tear under failure load. FRP rupture occurs because of the de- failure described in this section can be seen for varying strengthening
velopment of excessive diagonal shear cracks under peak load when a system.
partially side-wrapped shear strengthening system is used in RC beams
under ultimate loading. These ruptures occur in the strong direction
6. Sustainability of FRP-strengthened beams
parallel to fiber alignment [106]. The maximum FRP rupture strain is
obtained in the horizontal direction for the beam strengthened with a
6.1. Durability
single layer of CFRP sheet instead of a double layer sheet. The allowable
strain in FRP is approximately 10%–25% of the rupture strain [103].
The sustainability of FRP-strengthened beams under critical ex-
Multiple layers of FRP can reduce the possible utilization of strain and
posure conditions is a matter of concern given the susceptible nature of
the brittle rupture of FRP layers at levels below their effective stress.
polymer resins. Composite FRP in RC beams is durable under mild
Anchors can prevent premature fracture effectively with the help of
exposure conditions, such as moisture, acid–alkaline environments,
effective bond stress distribution along the width of concrete.
freeze–thaw cycles, and temperature. Research on the durability of
FRP-strengthened structures has shown that EB FRP could resist cor-
5.3. Failure of anchorage system rosion up to a reliable limit because it could minimize chloride diffusion
into concrete [3,62]. FRP used with a concrete cover is safe under
FRP-strengthened beams without any end anchorage exhibit re- moderate moisture and temperature conditions. The NSM technique for
duced flexural strength capacity because of premature failure through FRP strengthening is advantageous because it prevents FRP from
plate end debonding [13]. A study on the failure of anchorage perfor- coming into direct contact with the environment [99]. The FRP con-
mance showed that the use of anchors can prevent debonding failure in crete interface degrades during the first 3 months of exposure to
partially wrapped GFRP-strengthened beams and increase capacity moisture and does not considerably degrade after this period. However,
[65]. Anchorage can increase strain levels in FRP before failure. The use the bond interface and shear strength of concrete drastically decreased
of the appropriate types of anchorages increased strength by up to 95% after water immersion [109,110]. The bond strength of FRP-strength-
and reduced slip level [103]. Given the presence of the anchor system, ened concrete elements decreased after 18 months of continuous im-
the chance of debonding reduces stress concentration along the anchor mersion in water [111]. A properly applied resin matrix can isolate and
zone in most cases. Therefore, primary failure occurs in the form of FRP protect fibers from alkaline or acidic environments and retard dete-
rupture with concrete crushing or with partial or full debonding [105]. rioration [37]. Petersen et al. [36] reported that strength did not
FRP anchors develop fractures when used in connection with FRP drastically decrease during the lifetime of CFRP in an alkaline en-
wrapping. The location of NSM strips controls stress distribution and vironment. Another study revealed that glass fibers with vinyl–ester
crack patterns within the anchorage zone. NSM strips along the side of resin possesses better degradation resistance than basalt fibers im-
the beam could better enhance the utilization of FRP strain than NSM mersed in a highly alkaline solution for up to 5000 h at 60 °C [37,38].
strips along the bottom of the beam [107] because the bottom NSM This study confirmed that carbon fibers with epoxy resin are highly
strip-strengthened beam fails through concrete cover separation at the resistant to degradation in alkaline environments and that their bond
anchorage zone. The different failure modes observed in FRP strength increased when immersed in seawater at 60 °C. BFRP bars
Table 2
Failure modes of FRP-strengthened beams.
EB/NSM FRP Type System Failure Mode Reference
EB CFRP 1L along soffit DB with SY [34]

GFRP 1L along soffit Local DB with CC [34]
Hybrid 1L CFRP and 1L GFRP Partial DB, major FC and CC [34]
CFRP 1L parallel CC and RFRP [44]
1L perpendicular to beam axis
Hybrid Pultruded CFRP and GFRP plate DB with SY [66]
CFRP Fully wrapped strips spaced at half of depth RFRP strip [40]
CFRP U-wrapped strips spaced at half of depth DB [40]
CFRP U-wrapped strip with straight CFRP anchor RFRP with CC [105]
CFRP Two vertical side wraps with 1L FRP up to half of beam depth SC with RFRP in horizontal direction [106]
NSM CFRP Side NSM strips CC by IC [107]
CFRP Bottom NSM strips CCS [107]
CFRP Bottom NSM strips at midspan only CCS [108]
CFRP Bottom NSM strips along 97% of span CC with CCS [108]
Annotations: L = layer, DB = debonding, SY = steel yielding, CC = concrete crushing, FC = flexural crack, RFRP = rupture in FRP, IC = intermediate crack in-
duced debonding, CCS = concrete cover separation, SC = shear crushing.
8
showed lower strength retention than GFRP under the same sustained exposed to high temperatures [45]. Given the excessive brittleness of
pressure and exposure [112]. Epoxy resins absorbed more water than hardened FRP composites, some accidental damages could arise during
vinyl–ester resins. The bond stability of FRP composites with epoxy installation or drilling. CFRP must be drilled to anchor and fasten the
resin was better than that of vinyl–ester resin [37,38]. FRP-strength- strengthening system. Nevertheless, drilling poses the risks of fiber pull-
ened beams have good freeze–thaw resistance. The flexural strength of out or breakage, crack formation in the matrix, thermal degradation,
FRP-wrapped beams was 36% higher than that of the control beam after and delamination [5,129]. During the installation of FRP composites,
freezing–thawing [113]. The properties of FRP laminates were negli- increments in temperature resulted in premature debonding by redu-
gibly affected by freeze–thaw cycles of less than 80; however, the cing bond strength and introducing interlaminar faults. Special atten-
tensile strength of FRP laminates decreased by 10% when the number tion is needed during the drilling and installation of fasteners to prevent
of freeze–thaw cycles exceeded 80 [114]. The selection of the appro- unusual degradation. Metal will undergo accelerated galvanic corrosion
priate FRP composite and wrapping system controls the behavior of during prolonged exposure to carbon in the presence of an electrolyte
laminates. The strengths of BFRP and GFRP composites negligibly de- solution, which may be salt, acid, or combustive materials [130].
creased under freezing–thawing, whereas the tensile strength of CFRP Therefore, using CFRP in strengthening systems with metal anchorages
reduced by approximately 16% [113]. The stress generated from cyclic or in direct contact with steel can reduce the durability and strength of
freezing–thawing is only slightly destructive but can induce micro- structural systems. The ACI code recommends avoiding contact be-
cracking and delamination in fiber matrixes [115,116]. FRP–adhesi- tween steel and CFRP in RC members to prevent galvanic corrosion
ve–concrete bonds are susceptible to freeze–thaw conditions in moist [131]. The use of epoxy resin as a nonconductive barrier can reduce but
environments and can cause corrosion and expansion/contraction stress not completely prevent corrosion [132]. The intermediate layer of
[117]. Water ingress into concrete at freezing temperatures can result GFRP is used as a buffer between steel and the CFRP layer [133].
expansion of water volume and concrete stress. These phenomena result However, FRP strengthening requires a high initial financial investment
in the formation of microcracks that then propagate toward FRP la- given that it necessitates the use of a highly specialized operating
minates and the concrete interface and finally cause debonding. The system. Moreover, FRP production lacks specifications and standar-
working temperatures of fibers range from −50 °C to 700 °C [114,118]. dized and appropriate guidelines. Thus, FRP materials produced by
Polymeric matrixes could lose their stiffness, viscosity, and mechanical different companies show different characteristics. This problem should
properties at temperatures of 60 °C–80 °C, which exceed the glass be considered promptly to overcome production difficulties and fulfill
transition temperature [115,119–121]. For example, CFRP laminates the growing demand for FRP. Cost will be optimized is production is
lost 30% of their tensile strength when the temperature changed from increased. The life cycle assessment of FRP should consider not only the
20 °C to 70 °C under 65% relative humidity [114]. Given the drastic materials, energy, and resources needed for the production of FRP but
variation in the thermal expansion coefficient of concrete and FRP also the beneficial uses of FRP. FRP is sustainable in accordance with
composites, a slight increment in temperature resulted in the fluctu- construction, cost, and environment [122,134].
ating expansion of FRP and concrete and the development of stress due
to the swelling of FRP in the transverse direction [122]. Debonding 7. Conclusions
among fibers inside the FRP is the most frequent failure mode at high
temperatures [123] given the high thermal expansion coefficients of In modern concrete technology, researchers continuing research for
polymeric matrixes [124]. Positive result obtained from the study an advanced material combination for the construction of a light
conducted by Al-Tamimi et al. [125], where concrete-CFRP bond structure with increased stability, resistance against various loading
gained strength when exposed to sun and saline environment. The au- conditions, and ability to withstand environmental barriers for a de-
thors explained that, the elevated temperature was caused a greater sired service life. FRP could meet the requirements for strengthening RC
polymer crosslinking and complex polymeric interaction happened elements under controlled conditions. This review focused on the be-
within the matrix, therefore bond strength increased. havior of FRP-strengthened RC beams under various loading conditions
and presented information on typical FRP materials and their proper-
6.2. Uncertainties ties, features, and applications. As shown in this review, FRPs demon-
strate superior strength development and durability performance. This
The most crucial limitation of FRP strengthening is the loss of the review also provided a straightforward perspective of improving the
mechanical properties of composites and bond strength at elevated application and performances of RC beams through strengthening and
temperatures [118,126]. The elastic moduli of CFRP and BFRP lami- retrofitting with FRP. The following conclusions can be drawn and
nates decreased by 90% and 27%, respectively, at 250 °C [45]. There- specific recommendations are provided to future studies related to the
fore, improving the fire resistance of FRP strengthening systems is strengthening of RC beams with FRP:
crucial. Several studies [127,128] have focused on the fire resistance of
FRP-strengthened RC beams. Covering the bonded layer of FRP com- 1. FRP possesses high tensile strength and can increase the ultimate
posites with a layer of any flame-resistant material can protect the strength of RC beams up to twice that of the original RC beam when
strengthened beam but may be impractical given the excessive thick- strengthened, though FRP has low elastic modulus. The NSM tech-
ness of the cover layer [118]. Designing FRP strengthening structures nique is more reliable than other techniques in terms of some spe-
through a three-level procedure can ensure sufficient fire resistance cific case as strength gain and delays to form debonding failure.
while optimizing cost [127]. Bare RC beams with high load ratios have Additionally, RC beams demonstrate superior shear and torsion re-
been designed through a level-I design approach, a level-II design sistance capacities when wrapped in EB FRP layers at an angle and
method based on the 500 °C isotherm method (illustrated by the au- when additional anchorages are applied. The type of anchorage used
thors), and a level-III design procedure based on threshold temperature. in FRP strengthening is dependent on the strengthening scheme,
The presence of a thick layer of insulation in the CFRP anchorage zones loading conditions, and practical considerations.
in beams strengthened with FRP through the EBR or NSM method en- 2. The fatigue life of FRP-strengthened RC beam is primarily depends
sured that the whole system retained considerable bond strength and on effective tensile reinforcements and the bond strength between
fire resistance even if the CFRP–concrete bond had been lost along its FRP-concrete interface. FRP possesses high stiffness, resistance to
central length [28]. Several researchers have investigated the appro- spalling and fragmentation, good energy absorption capacity, and
priateness of using hybrid FRP systems to strengthen beams exposed to ductility. Therefore, FRP-strengthened concrete elements can be
high temperatures. The hybrid combination of BFRP and CFRP lami- used to develop structures with high impact and blast resistance
nates conferred fire resistance to strengthened structural elements along with seismic resistance.
9
3. Predamaged and cracked RC beams that have been undergoes shear [15] L. Koutas, T.C. Triantafillou, Use of anchors in shear strengthening of reinforced
loading or have lost steel mass because of corrosion can be effec- concrete T-beams with FRP, J. Compos. Constr. 17 (2013) 101–107, https://doi.
org/10.1061/(ASCE)CC.1943-5614.0000316.
tively repaired by using FRP. The performance of repaired and ret- [16] L. Nguyen-Minh, P. Phan-Vu, D. Tran-Thanh, Q. Phuong Thi Truong, T.M. Pham,
rofitted beams using FRP system are significant and can prolonged C. Ngo-Huu, M. Rovňák, Flexural-strengthening efficiency of cfrp sheets for un-
the service life of structures for a reliable limit, though the repaired bonded post-tensioned concrete T-beams, Eng. Struct. 166 (2018) 1–15, https://
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beams undergo failure below the ultimate stress limit of FRP. [17] J.G. Teng, J.F. Chen, S.T. Smith, L. Lam, FRP : Strengthened RC structures, Front.
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technique, quality of materials and loading condition. Premature [18] ACI 440.2R-02, Guide for the Design and Construction of Externally Bonded FRP
Systems for Strengthening Concrete Structures, Am. Concr. Institute, Farmingt.
debonding should be try to resist up to a certain level at least by Hills, MI, USA, 2002.
addressing preventive measures to increase the strength capacity [19] M. Garcez, L. Meneghetti, L.C. da Silva Filho, Structural performance of RC beams
and lifetimes of FRP-strengthened beams. Careful surface prepara- poststrengthened with carbon, aramid, and glass FRP systems, J. Compos. Constr.
12 (2008) 522–530, https://doi.org/10.1061/(ASCE)1090-0268(2008)12:5(522).
tion, NSM or anchorage with EB FRP can be useful in this case.
[20] R. El-Hacha, K. Soudki, Prestressed near-surface mounted fibre reinforced polymer
5. Although the durability of FRP under unfavorable environmental reinforcement for concrete structures — a review, Can. J. Civ. Eng. 40 (2013)
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cohesive near-bond failure of fiber-reinforced polymer-plated concrete, J. Compos.
on the durability of FRP have not yet been investigated. Such an Constr. 8 (2004) 528–538, https://doi.org/10.1061/(ASCE)1090-0268(2004)
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may arise during the installation, curing, and service period of FRP [22] M. Hussein, H.M.E.-D. Afefy, A.-H.A.-K. Khalil, Innovative repair technique for RC
beams predamaged in shear, J. Compos. Constr. 17 (2013) 04013005, , https://
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Reinforced with Fiber-Reinforced Polymer (FRP) Bars, Am. Concr. Institute,
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Acknowledgment [24] A.H. Al-Saidy, A.S. Al-Harthy, K.S. Al-Jabri, M. Abdul-Halim, N.M. Al-Shidi,
Structural performance of corroded RC beams repaired with CFRP sheets, Compos.
The authors gratefully acknowledge the financial support provided Struct. 92 (2010) 1931–1938, https://doi.org/10.1016/j.compstruct.2010.01.001.
[25] T.M. Pham, H. Hao, Impact behavior of FRP-strengthened RC beams without
by the Department of Civil Engineering, College of Engineering, Prince stirrups, J. Compos. Constr. 20 (2016) 04016011, , https://doi.org/10.1061/
Sattam Bin Abdulaziz University, Saudi Arabia and Department of Civil (ASCE)CC.1943-5614.0000671.
Engineering, Faculty of Engineering, Amran University, Yemen. [26] F. Ceroni, M. Pecce, Evaluation of bond strength in concrete elements externally
reinforced with CFRP sheets and anchoring devices, J. Compos. Constr. 14 (2010)
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Strengthening of Reinforced Concrete Beams by Using Fiber-Reinforced Polymer Composites - A Review

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Journal of Building Engineering 25 (2019) 100798

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Journal of Building Engineering

Strengthening of reinforced concrete beams by using fiber-reinforced T

ARTICLE INFO ABSTRACT

[23] GFRP bar – – 483–1600 35–51 1.2–3.1 –

4.8.2. Crack repaired RC beams

Fig. 3. Typical failure mode of a RC beam strengthened by EB FRP [102].

EB CFRP 1L along soffit DB with SY [34]

6 (2012) 41–47, https://doi.org/10.1007/s40069-012-0004-1. compositesb.2015.09.031.

structures strengthened with FRP laminates using a reliability-based advanced https://doi.org/10.1016/j.corsci.2018.04.002.

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