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Developments in the Built Environment 18 (2024) 100394
Available online 4 March 2024
2666-1659/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-
nc/4.0/).
Novel approach for strengthening T-beams decient in shear with
near-surface mounted CFRP ropes in form of closed stirrups
Adamantis G. Zapris
a
, Violetta K. Kytinou
b
, Viktor Gribniak
b
, Constantin E. Chalioris
a
,
b
,
*
a
Laboratory of Reinforced Concrete and Seismic Design of Structures, Department of Civil Engineering, School of Engineering, Democritus University of Thrace (DUTН),
Xanthi, 67100, Greece
b
Laboratory of Innovative Building Structures, Vilnius Gediminas Technical University (VILNIUS TECH), Sauletiekio av. 11, Vilnius, LT, 10223, Lithuania
ARTICLE INFO
Keywords:
Reinforced concrete (RC)
T-beam
Shear strengthening
Carbon ber-reinforced polymer (CFRP)
CFRP rope
CFRP rope anchorage
U-shaped sheet
ABSTRACT
Existing reinforced concrete buildings frequently require shear-strengthening to ensure structural integrity, and
externally bonded carbon ber-reinforced polymers (EB-CFRP) have become a wide-applicable approach.
However, slabs and geometric restrictions impede wrapping the element, necessitating incomplete U-shaped
applications vulnerable to debonding failures. This experimental study introduces a novel approach for shear-
strengthening nine full-scale T-beams using near-surface mounted CFRP ropes forming closed stirrups. Their
effectiveness as shear reinforcement is investigated compared to an alternative method incorporating EB-CFRP
sheets with CFRP rope anchorage. Test results indicate that both techniques enhance the beams’ strength and
performance. The load capacity increased 2 times for the reference specimen and 1.4-1.9 times for the
strengthened beam with FRP sheets. The displacement at the maximum load increased 5.2-7.8 and 6.6-7.5 times,
respectively. The rope stirrup arrangement outperforms the rope anchorage system in mechanical and con-
struction efciency. A thorough protocol for executing these processes to mitigate construction-related defects is
introduced.
1. Introduction
The gradual deterioration of existing reinforced concrete (RC)
buildings requires rehabilitation, repair, and strengthening to preserve
structural integrity. The susceptibility to shear failure during seismic
events, known for its brittle nature, often causing sudden collapse with
catastrophic consequences, determines the particular strengthening
issue. Moreover, many existing RC frame structures have a decient
arrangement of transverse reinforcement and inadequate shear capacity
of structural components (Dong et al., 2023) while in other cases the
existing shear reinforcement loses its efciency over time due to envi-
ronmental factors such as corrosion (Gao et al., 2021). Thus, addressing
these shortcomings and promptly improving their shear resistance is
imperative.
Several techniques have been developed and utilized to diminish
vulnerabilities associated with shear failure. One of the widespread
methods involves using externally bonded carbon ber-reinforced
polymer (EB-CFRP) sheets and laminates because of simple construc-
tion and strengthening efciency (Arslan et al., 2022; Al-Shalif et al.,
2022; Verbruggen et al., 2014a, 2014b; Joseph et al., 2023; Xu et al.,
2019; Gribniak et al., 2019; Samb et al., 2021). However, constructional
limitations in existing RC buildings hinder EB-CFRP wrapping and cause
anchorage problems (Arslan et al., 2022; Al-Shalif et al., 2022; Xu et al.,
2019; Samb et al., 2021; Gribniak et al., 2016). The vulnerability of
carbon ber-reinforced polymer (CFRP) materials to forces acting in
transverse to ber direction complicates the strengthening of non-at
surfaces (Dong et al., 2023; Samb et al., 2021; Gribniak et al., 2023).
Implementing EB-CFRP shear-strengthening systems for RC beams with
L- or T-shaped cross-sections, typical for RC beam-slab assemblies
(Wang et al., 2022), also presents a signicant challenge. Thus, the
specic retrotting approach involves the EB-CFRP installation on the
beam web’s three sides, forming a U-shaped arrangement (Wang et al.,
2022; Gebreyes and Mohammed, 2020; Nguyen et al., 2022; Samb et al.,
2022). This approach often results in brittle failures because of the CFRP
debonding (Mohammadi et al., 2017; Tatar et al., 2023; Aggelis et al.,
2015; Hanif et al., 2023; Tsangouri et al., 2021).
Various anchoring methods have been developed to delay or address
the premature debonding issue and improve the overall performance of
the EB-CFRP strengthening systems (Gribniak et al., 2017, 2023; Godat
et al., 2020; Aksoylu, 2021). Among these, spike anchors, typically CFRP
* Corresponding author. Department of Civil Engineering, School of Engineering, Democritus University of Thrace (DUTН), Xanthi, 67100, Greece.
E-mail address: chaliori@civil.duth.gr (C.E. Chalioris).
Contents lists available at ScienceDirect
Developments in the Built Environment
journal homepage: www.sciencedirect.com/journal/developments-in-the-built-environment
https://doi.org/10.1016/j.dibe.2024.100394
Received 4 November 2023; Received in revised form 23 February 2024; Accepted 3 March 2024
Developments in the Built Environment 18 (2024) 100394
2
ropes, gained traction as a practical solution, anchoring the EB-CFRP
sheets to the concrete substrate (Godat et al., 2020). The spike anchor
consists of the section embedded in the concrete (the dowel) and the part
afxed to the anchored sheet (the fan). In the latter component, the -
bers are gradually expanded (forming a fan) to improve adhesion to the
EB-CFRP sheets. The anchor and CFRP sheet are made from the same
material (material compatibility), which helps to avoid local damage to
the material to be anchored; these anchors are easily installable and
adaptable to space and geometry limitations (Del Rey Castillo et al.,
2018; Ilia and Mostonejad, 2019; Li et al., 2022; Abdulsalam et al.,
2021).
There are several potential failure modes associated with the anchor,
including the pullout of the anchor from the concrete, local fracture of
the anchor at the point of bend (occurring when there is a sudden change
in slope to facilitate the anchorage), local ber fracture in the fan sec-
tion, and debonding of the fan from the sheet to which it adheres (Godat
et al., 2020). A preferred failure mechanism involves rupturing the
anchored sheet, which completely utilizes the material strength. The
spike anchors’ efcacy has been studied by individually examining po-
tential failure mechanisms through experimental testing (Carozzi et al.,
2018; Del Rey Castillo et al., 2019a, 2019b; Llaurad´
o et al., 2017;
Mahrenholtz et al., 2015; Saeed et al., 2020; Sun et al., 2020; Zhang and
Smith, 2012; Zhang et al., 2012) or evaluating their performance in
beam structural members (Abdalla et al., 2023; Alotaibi et al., 2019;
Aryan et al., 2023; Kim et al., 2015; Mhanna et al., 2020, 2021a, 2021b;
Smith et al., 2013; Zaki et al., 2020). However, even today, a universally
accepted method for anchorage design has yet to be established. This is
evident from the limited number of methodologies available at the
research level (Shekarchi et al., 2020; Cortez Flores et al., 2020; Del Rey
Castillo et al., 2019c; Villanueva Llaurad´
o et al., 2017), some of which
rely on empirical data and the absence of specic design guidelines,
challenging engineers and hindering the widespread adoption of this
strengthening technique.
Furthermore, the experimental investigation (Del Rey Castillo and
Kanitkar, 2021) demonstrated that the installing technology affects the
anchoring efcacy even within a controlled laboratory environment
conducted by procient individuals. Insufcient ber impregnation,
inadequate embedding length in the concrete, and fan-forming errors
can result in large deviations, a signicant reduction in the anchorage’s
contribution, and premature failure of the strengthening system. Thus,
increasing the dowel ber content and fan length is necessary to realize
the CFRP sheet’s strength.
Strengthening method appropriate to be implemented by practice
engineers must have simple and clear design requirements, be easy to
apply, and maximize material consumption (resulting in lower nancial
costs). Considering the limitations and challenges associated with EB-
CFRP sheet anchorage using CFRP spike anchors and also recognizing
the adaptability and versatility of CFRP ropes, researchers have explored
their potential as a primary strengthening material by using it to create
closed form stirrups. Using CFRP rope to form a closed stirrup ensures
that failure modes related to the anchor, such as fan debonding, fan ber
breakage, and anchor pullout, will be eliminated. Further, the technique
is easier to apply because no fan formation is necessary, and it is a more
cost-effective option due to its simplicity. The design of a closed stirrup
conguration can also adhere to regulatory requirements in areas with
anti-seismic building standards (Shekarchi et al., 2020; Cortez Flores
et al., 2020).
Therefore, there has been a growing interest in using bers in the
form of rope for various strengthening applications. Experiments on
external beam-column joints (Karayannis et al., 2022a; Golias et al.,
2021a, 2021b) and RC beams with rectangular cross-sections (Papado-
poulos et al., 2023; Chalioris et al., 2018) demonstrated the CFRP rope
efciency. The literature results proved the applicability of this tech-
nique for enhancing resistance against exure (Kaya et al., 2017; Qaisi
et al., 2022; Murad and Abu-Al Haj, 2021; Zheng et al., 2020), shear
(Al-Khreisat et al., 2023; Obaidat et al., 2023; Saadah et al., 2021), and
torsion (Al-Bayati et al., 2018) in rectangular RC beams and against
shear in beam-column joints (Karayannis and Golias, 2022; Karayannis
et al., 2022b; Obaidat, 2022; Murad and Alseid, 2021; Karabini et al.,
2023; Ashteyat et al., 2023). However, current experimental research on
application of CFRP ropes for shear strengthening in RC beams focus on
rectangular cross-section specimens, disregarding the limitations and
effects from the potential monolithic connection of beams to slabs in real
structures. In addition, this system was efcient in conning RC columns
(Obaidat et al., 2020; Hussain et al., 2020; Fanaradelli and Rousakis,
2020) and strengthening RC slabs (Makhlouf et al., 2023).
The above literature review indicates that slabs or other geometric
restrictions impede wrapping the element, necessitating incomplete U-
shaped applications vulnerable to debonding failure. So far, the appli-
cation of CFRP spike anchors has been found to be an efcient solution
for bonding performance. However, uneven stress distribution in com-
posite sheets, causing premature failure, and complicated installation
impede this method. Thus, the application of innovative composite
building materials such as CFRP ropes as primary strengthening material
in the form of closed transverse stirrups can overcome these limitations.
Preliminary test results indicate that using composite ropes as shear
reinforcement yields positive outcomes, signicantly improving the
response of RC members. Further, the ease of implementation, minimal
disruption during installation, and cost-effectiveness contribute to the
advantages of this approach.
This study investigates the use of CFRP ropes as near-surface
mounted (NSM) reinforcement for shear-strengthening of RC T-beams.
The CFRP ropes are employed in the form of closed-type transverse shear
reinforcement, acting as external stirrups. The formation of CFRP ropes
as closed stirrups is implemented in two different congurations, either
by embedding the rope through the slab section or the beam’s web
section, and the efciency of these two types is compared and examined.
Further, this study also compares the proposed approach with a
strengthening alternative that employs the combination of ЕВ-CFRP
sheets and CFRP rope anchorage to form a closed-type of strengthening.
To the best of the authors’ knowledge, these innovative materials and
retrotting techniques have not been extended for the shear strength-
ening of RC T-shaped beams. The experimental program includes nine
full-scale beams featuring the different CFRP strengthening systems. In
addition to the aforementioned, this current research presents a
comprehensive, step-by-step methodology for applying the strength-
ening procedure to minimize construction-related defects.
2. Experimental program
2.1. Description of specimens
The experimental program encompassed nine RC beams. The beams’
length was 2.0 m with a 0.8 m shear span. The specimens had a T-shaped
cross-section with 300 ×150 mm web and 50 ×300 mm slab to imitate
the slab-to-beam joint. The beams were designed considering the labo-
ratory’s capabilities and the intention the tested beams to fail in shear.
The longitudinal steel reinforcement was the same for all the test spec-
imens, consisting of four 16 mm bars in the tensile zone and two 14 mm
bars at the web’s top. In addition, two 10 mm longitudinal bars were
placed in the slab area. The specimens had no transverse reinforcement
in the shear span aside from three beams (S1, S1-U, and S1-RF) with an
8 mm stirrup in the middle of the shear span. Fig. 1 depicts the beam’s
geometric characteristics and arrangement of the steel reinforcement.
The chosen shear span to effective depth ratio (2.92) has exceeded 2.5,
satisfying the slenderness criterion by Eurocode 2 (Gribniak et al.,
2021). The shear reinforcement was eliminated (or reduced to a single
stirrup) to realize the strengthening effect.
The specimens were named to characterize the shear reinforcement
peculiarities. Each specication begins with the letter S, followed by 0 or
1, denoting the steel stirrup’s lack or presence. In the case of strength-
ening with U-shaped CFRP sheets, the third character is the letter U. The
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
3
notation “U-AF” determines the anchorage with CFRP rope through the
slab; “U-AW” designates the anchorage through the web. For instance,
notation S0-U-AF describes a beam without a steel stirrup strengthened
with U-shaped CFRP sheets anchored through the slab. In the rope-
strengthening case, the letter “R” appears in the notation before “F” or
“W.”
Two beams (S0 and S1) were the reference specimens, while the
remaining beams were strengthened in the shear zone. Four beams (S0-
U, S1-U, S0-U-AW, and S0-U-AF) had EB-CFRP sheets in a “U” shape
(with or without CFRP rope anchorage), and three beams (S0-RF, S0-
RW, and S1-RF) were strengthened using NSM CFRP rope. The
strengthening percentage was approximately the same in all the beam
specimens for comparison purposes. Table 1 summarizes the reinforce-
ment characteristics of the beam specimens. In this table, the following
expression determines the tension (
ρ
), stirrup (
ρ
w
), and strengthening
(
ρ
f
) reinforcement percentages:
ρ
=As
b•d(1)
ρ
w=Aw
b•lw
(2)
ρ
f=Af
b•lw
.(3)
In the above equations, А
s
and А
w
are the total cross-section areas of the
tension reinforcement and a stirrup; b and d are the width and effective
depth of the beam; l
w
is the stirrup spacing (0.4 m in this case); А
f
is the
total cross-section area of dry bers in CFRP sheet or rope.
The web of beams S0-U and S1-U was externally bonded with uni-
directional CFRP sheets. The U-shaped sheets had a 0.331 mm equiva-
lent thickness and 160 mm width. As Fig. 2 shows, the clear space
between the sheets was equal to 160 mm. The identical conguration of
EB-CFRP sheets was used to strengthen beams S0-U-AW and S0-U-AF,
except for the CFRP rope anchorage system (Table 1). The EB-CFRP
sheets were anchored through holes in the web beneath the slab
(Fig. 2a), imitating an inaccessible slab situation in specimen S0-U-AW,
or holes in the slab (Fig. 2b), representing an accessible slab case in the
test specimen S0-U-AF.
Smith et al. (2013) recommended implementing several anchors to
avoid stress concentrations and reduce the fan size. Thus, every branch
of the U-shaped sheet was anchored with two CFRP rope anchors
(Fig. 3). The CFRP ropes in the anchors were long enough to create a
closed-form loop with a cross-section area exceeding the sheet’s ber
content following the literature recommendations (Mhanna et al.,
2020). Gradually widening the ber bundle formed the fan at the end of
each anchor to x the CFRP sheet. The fan’s angle was 27◦; its length and
width were 80 mm. Fig. 3 provides a detailed representation of the CFRP
rope conguration where the rope anchorage is installed through the
slab. A similar conguration was employed when the rope anchorage is
installed through the web section.
Unidirectional CFRP bundles (ropes) installed in vertical grooves in
each shear span at a spacing of 160 mm were used to strengthen spec-
imens S0-RF, S0-RW, and S1-RF. In specimens S0-RF and S1-RF, the
ropes were mounted in grooves drilled around the web and in the upper
side of the slab, with holes in the slab, allowing for a complete wrapping
of the cross-section (representing accessible slab, Fig. 4a). In contrast, in
beam S0-RW, the entire wrapping of the CFRP ropes was accomplished
by drilling a hole slightly below the slab through the beam web (i.e.,
inaccessible slab, Fig. 4b). To form the closed-loop stirrup and prevent
rope slippage, the rope at the top of the slab (specimens S0-RF and S1-
RF) or inside the hole of the beam’s web (specimen S0-RW) had a
100 mm overlap (10 times the nominal diameter of the rope, Fig. 5)
corresponding to the available groove length after the intervention.
All the drilled grooves had a square 15 ×15 mm cross-section except
for the tracks on the top of the slab of specimens S0-RF and S1-RFs,
Fig. 1. Geometry and reinforcement arrangement of the reference specimen S1.
Table 1
Reinforcement and strengthening characteristics of the beam specimens.
Beam Steel bars (mm) CFRP Reinf. ratio (%)
Longitudinal Stirrup Type
a
Anchor
b
ρ ρ
w
ρ
f
Bottom Top
S0 4 ×Ø16 2 ×∅14 – – – 1.96 – –
S0-U EB – 0.221
S0-U-AW +EB EtW 0.221
S0-U-AF EB EtF 0.221
S0-RW 2 ×∅10 Rope EtW 0.233
S0-RF Rope EtF 0.233
S1 4 ×Ø16 2 ×∅14 Ø8 – – 1.96 0.168 –
S1-U +EB – 0.221
S1-RF 2 ×∅10 Rope EtF 0.233
a
Strengthening type: EB =U-shaped EB-CFRP sheets with a 0.331 mm equivalent thickness and 160 mm width; Rope =CFRP rope with dry ber cross-section
exceeding 28 mm
2
according to the manufacturer’s specication.
b
Anchorage system: EtW =embedded through the web; EtF =embedded through the ange.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
4
Fig. 2. Geometry and arrangement of CFRP sheets with CFRP rope anchors installed: (a) through the slab (beam S0-U-AF); (b) through the web (beam S0-U-AW).
Fig. 3. Detail of the CFRP sheet with CFRP rope anchors installed through the slab (beam S0-U-AF).
Fig. 4. Geometry and arrangement of the NSM CFRP ropes installed: (a) through the slab (beam S0-RF); (b) through the web (beam S0-RW).
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
5
which had a 20 mm depth to allow the rope overlapping. All the holes
had a 16 mm diameter. All the groove edges (at the intersection and
inclination points) were smoothed with a 30 mm bend radius to avoid
stress concentration and local ber breakage.
2.2. Materials
Twelve ∅150 ×300 mm cylindrical specimens were cast together
with the beam specimens to determine the mechanical properties of
concrete. They were tested together with the beam specimens eight
months after concrete casting. The compressive strength of the concrete
was ascertained using six cylinders; the remaining cylinders determined
the splitting tensile strength. The mean compressive strength of the
concrete was found to be 44.2 MPa, along with a standard deviation of
1.9 MPa. The mean tensile strength was 3.1 MPa with a 0.3 MPa stan-
dard deviation. The longitudinal and transversal reinforcements were
either B500C class.
Unidirectional carbon ber (CF) sheets SikaWrap®-600C were used
in this study. The manufacturer specied the impregnated material’s
characteristics: the sheet’s equivalent thickness was equal 0.331 mm,
the average tensile strength was 3.00 GPa, the average modulus of
elasticity was 225 GPa, and the elongation at rupture was equal to
1.33%. The epoxy Sikadur®-300 was used to impregnate and attach the
sheet. The tensile strength, modulus of elasticity, and rupture elongation
of the resin, as provided by the manufacturer, were 45 MPa, 3.50 GPa,
and 1.5%, respectively.
Unidirectional CF ropes SikaWrap® FX-50C were employed to an-
chor the CFRP sheets and to strengthen the beams. The manufacturer
specied the impregnated material’s characteristics: the approximate
rope area is 78 mm
2
(dry ber cross-section >28 mm
2
), the tensile
strength is 2.00 GPa, the elastic modulus is 230 GPa, and the ultimate
strain is 0.87%. According to the manufacturer’s instructions, the epoxy
resin Sikadur®-52 Injection LP was used to impregnate the bers and
Sikadur®-330 to embed and anchor the CFRP reinforcement. According
to the manufacturer, the impregnation resin had a 34 MPa compressive
strength, a 24 MPa tensile strength, and a 1.10 GPa modulus of elas-
ticity. Whereas, the Sikadur®-330 had tensile strength, elastic modulus,
and rupture elongation values of 30 MPa, 4.5 GPa, and 0.9%,
respectively.
2.3. Strengthening procedure
The beams were classied into two groups based on the strength-
ening technique used. One group contained the beams strengthened
with EB-CFRP sheets in a U-shape (with or without CFRP rope
anchorage); the second group included those reinforced with NSM CFRP
rope.
In the beams belonging to the rst group, the sheet application areas
on the surface were pre-determined and marked before attaching CFRP.
Subsequently, an angle grinder machine was utilized to remove a thin
outer layer of concrete (≈2–3 mm) to reveal the coarse aggregate
beneath (Fig. 6a). The web’s edges were smoothed to mitigate the local
stresses in the CFRP sheets (Fig. 6b). In specimen S0-U-AF, the holes
were drilled in the slab to facilitate the anchoring of the sheets (Fig. 6c).
The holes were prepared in the web of beam S0-U-AW just beneath the
slab for the same reason.
The CFRP application zones were carefully cleaned using com-
pressed air (Fig. 6d) to remove dust and other debris. Following precise
measurements and careful cutting of the CFRP sheets and ropes to the
desired dimensions (Fig. 6e and f), the designated areas of the surface of
the beam were covered with epoxy resin (Fig. 6g). In the meantime, the
CFRP sheets were thoroughly saturated with the impregnation resin
(Fig. 6h), adhering to the instructions provided by the manufacturer.
Finally, with a particular roll, the sheets were attached and pressed onto
the beam’s surface to ensure the release of any entrapped air between
the sheet and the concrete (Fig. 6i). In the beams with anchored sheets
(S0-RF and S0-RW), after the installation of the sheets, the CFRP rope
was impregnated (Fig. 6j), and the grooves and holes were lled using
Sikadur®-330 resin. The CFRP rope was placed in the predened posi-
tion, and the anchor fan was formed to ensure the proper length,
opening angle, and consistent thickness (Fig. 6k).
Unidirectional carbon ber bundles strengthened beams S0-RW, S0-
RF, and S1-RF. The intervention areas were initially marked on the
surface of the beams. Notches were created into the web region of the
beam (and the slab area for specimens S0-RF and S1-RF) using a circular
saw (Fig. 7a). Grooves were formed using a drilling apparatus (Fig. 7b
and c), ensuring the constant 15 mm width and depth. For specimens S0-
RF and S1-RF, the holes were drilled in the slab and connected using the
drilling machine (Fig. 7d). In beam S0-RW, drilling holes were formed in
the web close to the slab. The bottom edges of the web (Fig. 7e) and all
intersection and inclination edges (e.g., near the drilled holes) were
curved with a 30 mm radius to prevent stress concentration and local
Fig. 5. Detailed view of the NSM CFRP rope installed through the slab.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
6
ber breakage. Compressed air was used to clean dust and debris from
the rope attachment areas.
The strengthening procedure was conducted in two phases to ensure
the ropes’ accurate installation and maximize the support effect. After
being cut to the required length and impregnated with the epoxy resin, a
small part of the rope free end was positioned in the groove at the top of
the beam (or in the hole in the upper part of the web) and lled with the
anchoring resin (Fig. 7f). The rope was left undisturbed for a day to
allow the impregnation resin to harden. This small part, once hardened,
serves as a xed point crucial for stabilizing the rope when it is stretched
during its application. The impregnation resin was then applied to the
remaining part of the rope using a roller. Simultaneously, the interior
surfaces of the holes and grooves were covered with the anchoring epoxy
(Fig. 7g). The impregnated rope was distributed in the grove with
particular care to minimize ber damage (Fig. 7h). The anchoring resin
was then used to ll all the remaining gaps (Fig. 7i). Lastly, a modest,
steady load was applied to the free end of the ropes to ensure that the
bers were aligned correctly and thoroughly stretched (Fig. 7j). The
weights were removed after 24 h as the resin had been hardened.
2.4. Experimental setup and instrumentation
The experimental setup of the beams involved the cylinder supports
positioned 100 mm from the element ends. The loading was imposed
using a servo-hydraulic actuator with a 500 kN load capacity, using a
displacement control mode at a rate of 0.01 mm/s. The displacement
was applied through two steel cylinders 100 mm apart from the beam’s
midpoint. Two linear variable differential transducers (LVDT) were
installed at the midspan. In addition, two LVDT devices were placed at
supports. Fig. 8 shows the loading scheme and test setup.
3. Results and discussion
This research introduces a novel application of NSM CFRP ropes in
closed-form stirrups for shear-strengthening T-beams. It also compares
the proposed approach with an alternative strengthening method which
employs both EB-CFRP sheets and CFRP rope anchorage to also form a
strengthening of closed-type. In addition, this work also experimentally
investigates the efciency of the stirrup congurations embedded
through the beam’s web and slab.
3.1. Experimental response of the beams
This section describes the beam specimens’ cracking behavior and
discusses the failure mechanisms. All the specimens exhibited an
equivalent behavior during the initial testing phase because of the
Fig. 6. Attaching U-shaped CFRP sheets with CFRP rope anchors: (a) removal of the outer layer of concrete; (b) smoothing the web’s edge; (c) drilling the grooves
and holes in the slab; (d) cleaning the dust and debris with compressed air; (e) and (f) cutting the CFRP sheet and rope in desired dimensions; (g) coating the concrete
surface with epoxy resin; (h) impregnation of CFRP sheets; (i) attaching the sheets with a particular roll; (j) impregnation of the anchor ropes, (k) placing of the rope
in the resin prelled groove and forming the fan anchor.
Fig. 7. Attaching NSM CFRP ropes: (a) creating notches; (b) and (c) ensuring the groove geometry by using a drilling machine; (d) drilling holes; (e) smoothing the
web edges; (f) impregnation of small part of the rope end and placing it in the grooves of the upper part of the beam (slab); (g) prelling the holes and grooves with
epoxy resin to attach and impregnate the remaining rope; (h) placing the rope; (i) lling the resin; (j) stretching the rope with small weights.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
7
similar cross-section and reinforcement details.
In beam S0, a reference beam lacking a shear stirrup, the rst crack
that appeared in a shear span (at a 131.9 kN load) steadily developed
and spread onto the slab region. Expanding the slab’s crack (at a 143.4
kN load) eventually destroyed the beam. On the contrary, in beam S1, a
reference beam containing the shear stirrups, an additional load increase
led to the formation of cracks within each shear span (156.8 kN and
158.9 kN). The crack opening observed in the shear spans and the
concrete’s severe damage in the slab area eventually led to the beam’s
failure (at a 167.0 kN load). Both specimens exhibited shear collapse,
characterized by a brittle behavior and the development of one or two
wide inclined cracks across the shear span.
In beams S0-U and S1-U strengthened with EB-CFRP sheets, the shear
cracks were observed between the sheets in the middle of the shear
spans (144.4 kN in S0-U and 123.7 kN in S1-U). Further shear cracks’
growth intersected the CFRP sheets with a minimal load increase,
causing debonding of the sheets in both specimens at the one shear span
(175.9 kN in S0-U and 238.8 kN in S1-U). The sheet debonding onset
resulted in a swift rise of the crack width, propagating into the slab area
and nally resulting in the beams’ failure. The presence of the
strengthening led to the development of a few additional shear cracks.
Nonetheless, the premature debonding of the EB-CFRP sheets still
resulted in a brittle shear-failure.
The specimen S0-U-AF strengthened with EB-CFRP sheets with CFRP
rope anchors embedded through the slab behaved similarly to specimen
S0-U until the sheet debonded, activating the anchors. Contrary to the
above-discussed specimens, the increasing load led to the formation of
new cracks instead of the crack width increase characteristic of the
previous specimens. Further load increase (to 303.6 kN) resulted in the
gradual appearance of a pronounced cracking pattern in the middle of
the beam, indicating the yielding of the tensile reinforcement. Individ-
ually, several exural cracks were formed, but an increase in the length
and width of already existing ones was predominant.
Specimen S0-U-AW, strengthened with CFRP sheets and CFRP rope
anchors embedded through the web, and specimen S0-RW, with the rope
embedded through the web, exhibited similar behavior. Horizontal
cracking emerged below the slab after the development of shear cracks
intersecting the anchored sheets (282.5 kN in S0-U-AW and 282.4 kN in
S0-RW). The crack started from the support, with an increased inclina-
tion up to the strengthening sheet, which was eventually driven under
the slab (joint point of web and slab). Then, it gradually extended par-
allel to the beam’s longitudinal axis, passing over the strengthened zone
and towards the loading point. The yielding of the tensile reinforcement
(284.9 kN in S0-U-AW and 297.4 kN in S0- RW) led by a modest increase
in load and the formation of exural cracks in the middle span. Growing
and widening the horizontal crack accompanied the spreading of the
exural cracks. Specimen S0-U-AW has been destroyed at that moment.
On the contrary, a second cracking branch emerged (312.1 kN) with an
increased slope with direction from the support to the horizontal line
after widening the exural cracks in specimen S0-RW.
Specimens S0-RF and S1-RF strengthened with CFRP ropes through
the slab demonstrated similar shear-resistance mechanisms. The initial
shear cracks (141.0 kN in S0-RF and 147.6 kN in S1-RF) were succeeded
by others that progressively intersected all the strengthening regions.
Still, the strengthening entirely limited the crack width increase. At the
same time, new exural cracks appeared while existing ones prolonged
(305.6 kN in S0-RF and 296.4 kN in S1-RF). As the load rose, the cracks
primarily concentrated in the pure bending zone expanded substan-
tially, and concrete crushing signs were observed at the top of the slab.
The shear failure of the reference specimens (S0 and S1) and those
strengthened with CFRP sheets (S0-U and S1-U) was brittle. It resulted
from creating one or two shear cracks. Implementing CFRP sheets with
CFRP rope anchors (specimens S0-U-AW and S0-U-AF) and NSM CFRP
ropes (specimens S0-RW, S0-RF, and S1-RF) altered the failure mecha-
nism from brittle shear to ductile exural. However, embedding CFRP
support through the web, either the sheets with CFRP rope anchors or
the closed-form stirrups, caused cracks to pass over most of the
strengthening zone and spread horizontally afterward. Even if the fail-
ure of the beams changed to exural, potentially higher stresses acting
on the strengthening led to premature failure of the specimens regarding
Fig. 8. Typical test setup and instrumentation (the reference specimen is illustrated).
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
8
the specimens where the rope was embedded through the slab. The
beams’ exhibited behavior aligns with the literature’s observations
(El-Saikaly et al., 2015; Bourget et al., 2017). The difference in response
between the drilling methods resulted from increasing the connement
area when the rope passes through the slab. This effect is typical for the
beams with a relatively small percentage of transversal reinforcement
(El-Saikaly et al., 2015; Bourget et al., 2017); simultaneously, the me-
chanical behavior tended to be that of the strengthening applied through
the slab with increasing the steel reinforcement percentage.
Table 2 species the failure mechanisms and the characteristic load
and displacement values. It shows the load values at the onset of the
shear crack (P
Vcr
), shear resistance maximum (P
Vu
), yielding of the steel
reinforcement (P
My
), and ultimate load-bearing capacity (P
Mu
); the table
also includes the corresponding vertical displacements at the midspan
(d
Vcr
, d
Vu
, d
My
, and d
Mu
). Beams S0, S0-U, S1, and S1-U exhibited brittle
shear failure with a critical diagonal crack formation. The retrotted
beams S0-U-AW, S0-U-AF, S0-RW, S0-RF, and S1-RF demonstrated
adequate exural ductility regarding vertical displacements. Specif-
ically, the displacement ratio at ultimate load and steel yielding (d
Mu
/
d
My
) are 3.4, 4.5, 2.0, 4.6, and 4.9, respectively. In contrast, the beams
S0-U, S1, and S1-U collapsed due to shear in a brittle and abrupt manner,
demonstrating the strengthening efciency.
The load values at the shear cracking onset (Table 2) are expected
because of the CFRP strengthening inefciency before the rst diagonal
crack. However, the strengthening effect on the load-bearing capacity
and especially the mechanical performance of the retrotted beams S0-
U-AW, S0-U-AF, S0-RW, S0-RF, and S1-RF is apparent by altering the
brittle shear failure (characteristic of the reference specimens S0 and S1)
to a exural one with displacement ductility ratio d
Mu
/d
My
exceeding 2.
The retrotted beams with EB-CFRP sheets without CFRP rope anchors
(i.e., specimens S0-U and S1-U) collapsed in shear (“S” failure type in
Table 2), presenting an increased capacity (20% and 43%) regarding
beams S0 and S1. Strengthening with the NSM CFRP ropes through the
ange (specimens S0-RF and S1-RF) ensured a ductile exural response
and failure of the concrete compressive zone after yielding the tension
steel reinforcement (“F” failure type in Table 2), enhancing load-bearing
capacity (2.24 and 1.96 times) and displacement at ultimate load (7.5
and 5.2 times) regarding the reference beam specimens S0 and S1.
Beam S0-U-AF with EB-CFRP sheets and CFRP rope anchors through
the slab presented a similar exural failure mechanism to the specimens
strengthened with NSM CFRP ropes installed through the slab, exhibit-
ing an enhanced load and displacement bearing capacity. The CFRP
sheets with CFRP rope anchors through the web (specimen S0-U-AW)
and the web-anchored ropes (beam S0-RW) also improved the exural
resistance after yielding the tension reinforcement. Still, a critical crack
localized at the web-to-slab connection above the CFRP strengthening
caused the collapse of those specimens (“FC” failure type in Table 2).
Notwithstanding the formation of this critical cracking, both beams S0-
U-AW and S0-RW demonstrated an improved load-bearing capacity
(2.13 and 2.14 times) and increased displacement at ultimate load (7.0
and 3.7 times) regarding the reference beam S0.
3.2. Effectiveness of U-shaped CFRP sheets with CFRP rope anchors
Fig. 9 shows the load versus mid-span displacement diagrams and the
representative cracking patterns of the beams S0 (reference), S0-U
(strengthened with U-shaped CFRP sheets), S0-U-AW (with U-shaped
CFRP sheets and CFRP rope anchors through the web), and S0-U-AF
(with U-shaped CFRP sheets anchored through the ange). The dia-
grams of beams S0 and S0-U show that the U-jacketing technique
slightly increases the shear resistance but does not affect the brittle
failure mechanism. This result was expected because of the insufcient
anchorage length of CFRP sheets.
On the contrary, the load-displacement diagrams of the U-jacketed
beams with anchors (S0-U-AW and S0-U-AF) reveal that the rope
installation as fan-type anchor can signicantly improve the shear per-
formance by avoiding the bond failure of the CFRP sheets. It is essential
that both beams S0-U-AW and S0-U-AF exhibited a ductile response with
tension steel reinforcement yielding and exural failure because of the
concrete crushing in the compression zone at high deformation capa-
bilities. The load-bearing capacity improvement of these beams is also
signicant (regarding the reference beam S0 that collapsed in shear)
since the well-predictable bending resistance of the RC beam cross-
section governs it.
Table 2
The load resistance characteristics of the test beams.
Beam Failure type
a
Load capacity (kN) Displacement (mm)
Shear Flexure Shear Flexure
P
Vcr
P
Vu
P
My
P
Mu
d
Vcr
d
Vu
d
My
d
Mu
S0 S 131.9 146.3 – – 2.8 5.0 – –
S0-U S 144.4 175.9 – – 3.1 5.2 – –
S0-U-AW FC 148.8 – 284.9 311.9 3.0 – 10.4 34.9
S0-U-AF F 154.8 – 303.6 324.2 3.0 – 8.7 39.0
S0-RW FC 148.9 – 297.4 313.5 3.0 – 9.5 18.6
S0-RF F 141.0 – 305.6 327.3 2.7 – 8.1 37.4
S1 S 123.7 167.0 – – 2.2 7.2 – –
S1-U S 143.6 238.8 – – 2.5 5.6 – –
S1-RF F 147.6 – 296.4 327.5 2.5 – 7.6 37.2
S =the brittle shear response and diagonal cracking failure.
F =the ductile exural response by crushing the concrete compressive zone after yielding the tension steel reinforcement.
FC =the ductile exural response is because of the horizontal crack separating the concrete compressive zone over the CFRP anchors after yielding the steel
reinforcement.
a
Failure mode and representative cracking patterns.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
9
3.3. Effectiveness of CFRP ropes and U-shaped sheets with CFRP rope
anchors
Applying CFRP ropes in existing RC beams as transverse NSM rein-
forcement is a relatively new shear-strengthening technique. The
scheme of the retrotted beams of this study with CFRP ropes involved
complete wrapping of the rope around (a) the web of the beam, passing
it through the web, beneath the slab area (specimen S0-RW), and (b) the
total cross-section of the beam passing it through the ange (specimen
S0-RF), representing the inaccessible and accessible slab situations.
Fig. 10 shows the load-displacement diagrams and nal cracking
patterns of beams S0-RW and S0-RF with NSM CFRP ropes, reference
specimen S0, and specimens S0-U-AW and S0-U-AF strengthened beams
with U-shaped CFRP sheets with anchorage through the web and the
ange. This gure shows that all the retrotted beams demonstrated an
almost identical exural failure mechanism with high ductility capac-
ities since the anchored sheets and ropes prevented shear failure. These
specimens sustained the developed shear stresses without the formation
of critical diagonal cracking due to the increased shear resistance of the
CFRP-wrapped region.
Nevertheless, in the case of the beams S0-U-AW and S0-RW with
anchorage and rope wrapping through the web, the critical crack
beneath the slab (above the CFRP connection) has been formed at high
loading levels. Although these cracks did not inuence the overall load-
displacement response, it could be a reason to prefer the retrotting
scheme through the slab.
Fig. 9. Load-vertical displacement diagrams and nal cracking patterns of specimens S0, S0-U, S0-U-AW, and S0-U-AF.
Fig. 10. Load-vertical displacement diagrams and nal cracking patterns of specimens S0-U, S0-U-AW, S0-U-AF, S0-RW, and S0-RF.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
10
3.4. The synergetic effect of steel stirrups and CFRP strengthening
Fig. 11 examines the steel stirrups’ effect on the shear performance of
the U-shaped CFRP sheets without anchors and NSM CFRP ropes
wrapped through the ange in the closed-form loop form. This gure
shows the load-displacement diagrams and the crack patterns of the
beams with steel stirrups: the reference beam S1, specimen S1-U
strengthened with U-shaped CFRP sheets, and specimen S1-RF
strengthened with CFRP ropes wrapped through the ange.
Comparing the S1 and S1-U results deduces that U-shaped sheets
without anchors provided a notable increase in the shear capacity but
did not alter the brittle shear failure since both specimens exhibited
critical diagonal cracks. The beam counterparts S0 and S0-U without
stirrups faced a similar shear collapse (Fig. 9), though the CFRP-
jacketing effect on the shear resistance is more signicant in the
beams with steel stirrups.
The load-displacement diagrams in Fig. 11 demonstrate that the NSM
CFRP rope retrotting technique can improve the shear performance
and alter the brittle shear failure to a more desirable exural-dominated
one. The results of specimen S1-RF support this conclusion.
Figs. 12 and 13 show the load-displacement diagrams and cracking
patterns of specimen counterparts without and with stirrups: the beams
S0-U and S1-U strengthened with U-shaped CFRP sheets, and specimens
S0-RF and S1-RF retrotted by the ropes installed through the ange. As
expected, the inuence of the steel stirrups in the shear strength is
evident in the case of the U-jacketed beams since both specimens
collapsed in shear, and beam S1-U exhibited 1.36 times higher load
capacity than beam S0-U. On the contrary, the exural failure is char-
acteristic of beams S0-RF and S1-RF retrotted with the CFRP ropes, and
concrete crushing in the compression zone controls the load-bearing
capacity. Thus, steel stirrups cannot contribute to mechanical resis-
tance. On the other hand, the NSM CFRP rope strengthening approach
demonstrated its efciency even in the absence of steel stirrups,
ensuring sufcient resistance of the shear zone.
3.5. Further research
Strengthening RC structures using NSM CFRP ropes is an innovative
and evolving eld of engineering and research. This method can be
easily applied with little intervention to the element. Due to the exi-
bility of the rope material, it can be adapted by satisfying severe struc-
tural limitations and adjusting to a complex shape. Examples of such
cases include the shear strengthening of the monolithic beam-to-slab
joints by forming closed-form loops (NSM stirrups) or cross-link sup-
porting systems in the area of a beam-column connection. The pre-
liminary experimental studies (Golias et al., 2021a, 2021b; Karayannis
and Golias, 2022) revealed a promising performance of the X-type CFRP
rope strengthening method in enhancing the mechanical performance of
beam-column joints. Still, further investigations are necessary to
examine this application’s effectiveness and explore the effects of
different variables, such as the reinforcement percentage and installa-
tion technology. Studies (Chalioris et al., 2018; Saadah et al., 2021;
Al-Bayati et al., 2018) also investigated the CFRP rope strengthening
performance in RC beams with rectangular cross-sections. Although
these results demonstrated substantial performance of the strengthening
technique, extensive research is still required to optimize the rein-
forcement arrangement and develop the design principles.
Experimental testing in which the strengthening reinforcement rea-
ches its strength limit and fails, especially in specimens, is also necessary
to optimize the strengthening technology, reaching the material per-
formance of CFRP ropes and ensuring the strengthening reliability. The
strength reduction of the CFRP material because of the local bending,
curve-linear arrangement, and lack of stretching of NSM ropes de-
termines the essential investigation object. The anchorage length, shape,
and technology also need clarication. Different loading conditions (e.
g., cyclic, dynamic, and long-term mechanical and temperature loads)
must also be considered to evaluate the strengthening efciency. The
inuence of the above mentioned should be expressed through analyt-
ical formulations. The development of such formulations will quantify
the contribution of the strengthening reinforcement and further
contribute to the applicability and practical implementation of the
proposed reinforcement technique in structural engineering
applications.
It is also essential to evaluate the cost-effectiveness of NSM CFRP
ropes regarding the existing strengthening alternatives. The research
parameters are determined by the material cost, installation expenses,
Fig. 11. Load-vertical displacement diagrams and nal cracking patterns of specimens S1, S1-U, and S1-RF.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
11
strengthening duration, service life extension, maintenance costs, and
long-term structural performance.
3.6. Conclusions
This study developed a new application of CFRP ropes as near-
surface mounted (NSM) reinforcement for shear-strengthening RC T-
beams in the form of closed stirrups. Moreover, the study compared the
efcacy of this strengthening method to an alternative one which in-
corporates the use of EB-CFRP sheets along with CFRP ropes and also
leads to the formation of a closed type strengthening. An experimental
investigation was conducted on nine large-scale T-beams subjected to
monotonic loading up to failure. Two reference specimens had no CFRP
strengthening, four specimens were retrotted using EB-CFRP sheets
(with and without anchors), and three beams were strengthened with
NSM CFRP rope. The application of the ropes (either as NSM or anchors)
was performed in two different arrangements to replicate scenarios
where the slab is either accessible (the rope embedded through the slab)
or inaccessible (the rope embedded through the web). Based on the
ndings of this study, the following conclusions are delivered:
Fig. 12. Load-vertical displacement diagrams and nal cracking patterns of specimens S0-U and S1-U.
Fig. 13. Load-vertical displacement diagrams and nal cracking patterns of specimens S0-U, S1-U, S0-RF, and S1-RF.
A.G. Zapris et al.
Developments in the Built Environment 18 (2024) 100394
12
•The proposed NSM CFRP rope closed-form stirrups are particularly
effective for T-beams’ shear-strengthening. Both investigated ar-
rangements, i.e., embedding the rope either through the slab or
through the web, resulted in a substantial increase in the shear
resistance of the beam. The strengthening altered the brittle shear
failure observed in the reference beam to a ductile exural collapse.
The overall load-bearing capacity increased by 1.8 to 2.2 times
regarding the reference specimens.
•The proposed strengthening technique is straightforward, fast, and
exible with minimal intervention in the structural member. It is
barely affected by geometric restrictions, such as a monolithic beam
connection with the slab. Moreover, the closed-loop application of
CFRP ropes is more reliable and faster regarding the CFRP rope an-
chors of CFRP sheets, diminishing the installation and avoiding fan
formation.
•As expected from the literature, forming the closed-form CFRP rope
stirrups through the slab outperformed the strengthening alternative
through the web. The specimens retrotted through the web
demonstrated a critical crack, separating the slab from the retrotted
web above the CFRP connections. The distinct resistance mecha-
nisms resulted from the more effective connement of the space in-
side the CFRP loop on the rst alternative. At the same time, both
strengthening setups succeeded in altering the brittle shear beam’s
failure to the acceptably ductile exural mechanism.
•The beams strengthened with CFRP sheets having CFRP rope anchors
and NSM CFRP ropes demonstrated similar shear resistance mecha-
nisms. The strengthening resulted in a similar increase in shear
strength, converting the brittle shear collapse to favorable ductile
exural failure. When the anchoring of the sheets was carried out
through the web, similar horizontal cracking was observed, as in the
corresponding beam strengthened with CFRP ropes. Until the alter-
ation of the failure mechanism to a exural, no indications of rupture
were noticed in the sheet anchors.
•The NSM CFRP rope is more effective regarding the well-spread EB-
CFRP sheet strengthening alternative until yielding the steel rein-
forcement. Dense cracks with a reduced width were observed, indi-
cating a more efcient incorporation of the strengthening into the
structural member. The CFRP rupture can change the structural
behavior, determining the object for further research. In addition,
research is required to determine the benets and limitations of these
strengthening methods and, ultimately, establish design formula-
tions for engineering practice.
Funding
Violetta K. Kytinou, Viktor Gribniak, and Constantin E. Chalioris
acknowledge the nancial support of this work from the European
Regional Development Fund (Project No 01.2.2-LMT-K-718-03-0010)
under a grant agreement with the Research Council of Lithuania
(LMTLT), till August 2023.
CRediT authorship contribution statement
Adamantis G. Zapris: Conceptualization, Data curation, Formal
analysis, Investigation, Methodology, Validation, Visualization, Writing
– original draft, Writing – review & editing. Violetta K. Kytinou: Data
curation, Formal analysis, Investigation, Methodology, Writing – orig-
inal draft, Writing – review & editing. Viktor Gribniak: Formal anal-
ysis, Funding acquisition, Validation, Visualization, Writing – review &
editing. Constantin E. Chalioris: Conceptualization, Methodology,
Project administration, Supervision, Visualization, Writing – review &
editing.
Declaration of competing interest
The authors declare the following nancial interests/personal
relationships which may be considered as potential competing interests:
Constantin Chalioris reports nancial support was provided by Euro-
pean Regional Development Fund (Project No 01.2.2-LMT-K-718-03-
0010) under a grant agreement with the Research Council of
Lithuania (LMTLT). Violetta K. Kytinou and Viktor Gribniak reports
nancial support was provided by European Regional Development
Fund (Project No 01.2.2-LMT-K-718-03-0010) under a grant agreement
with the Research Council of Lithuania (LMTLT). If there are other au-
thors, they declare that they have no known competing nancial in-
terests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
Adamantis G. Zapris gratefully acknowledges the nancial support
from the Eugenides Foundation toward doctoral studies.
The authors wish to express their sincere gratitude to Sika Hellas
ABEE for donating all the materials for the strengthening applications of
the test program (CFRP sheets, ropes, and epoxy resins).
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