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International Journal of Civil Engineering and Technology (IJCIET)
Volume 9, Issue 10, October 2018, pp. 427–440, Article ID: IJCIET_09_10_044
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
©IAEME Publication Scopus Indexed
REPAIR OF DAMAGED PRESTRESSED
CONCRETE BEAMS USING CFRP FABRIC AND
STITCHING TECHNIQUES
M. Y. Sabra
Civil Engineering Department, Beirut Arab University, Beirut, Lebanon
Y. A. Temsah
Civil Engineering Department, Beirut Arab University, Beirut, Lebanon
O. M . Baalbaki
Civil Engineering Department, Beirut Arab University, Beirut, Lebanon
Z. Abou. Saleh
Civil Engineering Department, Rafic Hariri University, Meshref, Lebanon
ABSTRACT
Since the early 1950’s, prestressed concrete has been used in the construction
concrete structures. Using prestressed concrete offers many advantages such as
larger span and thinner elements size compared to the conventional reinforced
concrete. Repairs of prestressed concrete structures are necessary when existing
tendons are damaged (e.g. corroded, cut, or broken...).
The objective of current paper is to focus on repairing of damaged prestressed
concrete beams using carbon fiber reinforced polymer (CFRP) fabric technique, and
to explore a new technique for the strengthening by stitching the structural elements
using external post-tension strands. This work was conducted in order to investigate
the suitability of such techniques through laboratory experiments by testing simply
supported prestressed beams subjected to bending load. The repaired beams are
compared with the control undamaged beam for evaluation purpose.
The outcome of this paper will lead to a set of guidelines for optimal repair
technique to be used for the repair of damaged presterssed beams.
Key words: Prestressed concrete, CFRP, Stitching, Concrete repair, Load Capacity.
Cite this Article: M. Y. Sabra, Y. A. Temsah, O. M . Baalbaki and Z. Abou. Saleh,
Repair of Damaged Prestressed Concrete Beams Using CFRP Fabric and Stitching
Techniques, International Journal of Civil Engineering and Technology (IJCIET)
9(10), 2018, pp. 427–440.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=10
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
http://www.iaeme.com/IJCIET/index.asp 428 [email protected]
1. INTRODUCTION
Many structural members have been in serious need of repair due to aging, poor quality of
materials, faulty construction practices, severe environmental and accidental influences (e.g.
overloads, vehicular impacts, strong earthquakes, fire) changes in use that increase service
loads (e.g. load enhancement beyond the original design values), and increased safety
requirements, T.Alkhrdaji and J. Thomas (2004) [1]. The structural changes of concrete
structures can be achieved using one of many different technique methods such as reduction
in span length, externally bonded steel, external composites (carbon fibers), external or
internal post-tensioning systems, Aravinthan T. and Heldt T, (2010) [2], section enlargement,
or a combination of these techniques. Repair systems must perform in a composite manner
with an existing structure to be effective and to share the applied loads, L. Krauses (2006) [3].
The objective of this study is to investigate a technique that can be applicable for repairing of
damaged prestressed concrete beams. External prestressing was first used in the late 1920’s
and has recently being used in bridges, A. F. Daly and W. Witarnawan (1997) [4], both for
new construction as well as repairing of existing structures. Stitching is a technique of
prestressing concrete structural elements to increase its capacity. However this technique is
not commonly used or well-known.
Strengthening of prestressed concrete using CFRP is a commonly used technique to
enhanced durability, serviceability, and to increase the flexural capacity of damaged girders,
control crack propagation if it is present, and reduce deflections under subsequently applied
load (Schiebel et al. 2001 [5]; Tumialan et al. 2001 [6]; Klaiber et al. 2003 [7]; Reed and
Peterman 2005[8]; Reed et al. 2007[9]). This conventional application of CFRP strips/fabric
is referred to as externally bonded CFRP.
2. EXPERIMENTAL PROGRAM
2.1. Structural Material Properties
A set of three beams were constructed using a ready mixed concrete with an average 28-days
compressive strength of 45 MPa based on standard cylinder. A prestressing low relaxation
ASTMA416 7 wire strands of 1860MPa tensile strength and 12.7mm of diameter was used in
reinforcing each beam. The beams were also reinforced with two upper and lower bars of
10mm diameter, and stirrups of 10mm diameter spaced at 200mm.
2.2. Carbon Fiber Reinforced Polymers (CFRP) Properties
CFRP fabric was adhesively bonded to the damaged prestressed concrete beam using high
strength epoxy to compensate the loss of strength due to damage. The CFRP used in this
research is the Tyfo SCH-41 composite, which consists of Tyfo S Epoxy and Tyfo SCH-41
reinforcing fabric. Tyfo SCH-41 is a custom, uni-directional carbon fabric orientated in the
zero degree direction parallel to the N.A of the beam. The Tyfo S Epoxy is a two-component
epoxy matrix. The material properties are presented in Table 1 and Table 2.
Table 1 Physical and mechanical CFRP properties
Typical dry carbon fiber properties
Property Typical test value
Tensile strength 3.79 GPa
Tensile Modulus 230 GPa
Ultimate Elongation 1.7%
Density 1.74 g/cm3
Minimum carbon fibers weight 644 g/cm2
Weight 710 g/m2 ± 35 g/m2
Repair of Damaged Prestressed Concrete Beams Using CFRP Fabric and Stitching Techniques
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Table 2 Epoxy material properties
Epoxy material properties
Properties Testing method Typical test value
Tg ASTM D4065/ EN12614 82o C
Tensile Strength ASTM D638 Type 1 72.4 MPa
Tensile Modulus ASTM D638 Type 1 3.18 GPa
Elongation Percent ASTM D638 Type 1 5.0%
Flexural Strength ASTM D790 123.4 MPa
Flexural Modulus ASTM D790 3.12 GPa
2.3. Testing Program
In this paper three prestressed beams measured 250mm wide x 300mm depth x 3200mm long
were constructed. The beam B1 represents the control beam, and the two others CFB1 and
SB1 are the CFRP and the stitching repaired techniques beams respectively. CFB1 and SB1
were subjected to damage by cutting the strand using core drill machine. The beams
dimensions and reinforcement details are shown in Figure 1.
Figure 1 Beam dimensions and reinforcements
Both beams were tested to failure and compared with the control beam. Figure 2 illustrates
the steps of beams preparation.
Figure 2 (a) Preparing of reinforcement, (b) pretention of the strand, (c) pouring, (d) prestressed beams
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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2.4. Damaging the specimens
Two beams were subjected to damage, which consisted of coring the beams in the mid span,
using coring cylinder of 2.5 inch diameter. The target of coring is to cut the strand and to
simulate sudden damage and cut in the prestressing steel. Figure 3 shows the site coring.
Figure 3 Coring of beams
2.5. Repairing the Specimens
To strengthen the damaged beams two techniques were implemented, the carbon fiber
reinforced polymers (CFRP) fabric and the stitching techniques.
2.5.1. CFRP fiber repair technique
The Tyfo System was designed to meet specific design criteria. The design is based on the
allowable strain for each type of application and the design modulus of the material.
The repair of beam CFB1 needed 2 sheets of CFRP fiber with 250mm wide x 2000mm
long x 1mm thick. Additional layers of CFRP reduce the debonding strain, and are therefore
proportionally less effective, Jarret L. Kasan et al. (2014) [10]. The following steps were
conducted to achieve the repair by CFRP:
Surface Preparation
In general, the surface was cleaned to be dry and free of protrusions or cavities, which may
cause voids behind the Tyfo composite. For discontinuous wrapping surface of beam was
grinded for bonding. Sharp and chamfered corners were rounded off by grinding see Figure 4.
Figure 4 Grinding the beam
Mixing with Epoxy
The CFRP fabric was cut into 2 pieces of 250mm wide x 2000mm long, and the epoxy was
prepared by pouring the hardener into the epoxy, and mixed thoroughly until the desired
viscosity was achieved, Figure 5.
Repair of Damaged Prestressed Concrete Beams Using CFRP Fabric and Stitching Techniques
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Figure 4 Fiber fabric sheet and epoxy
Application
The prime coat of epoxy was applied on the substrate by using a roller. An approved hand
method was used. Saturated and applied of subsequent layers of the fabric according to the
specifications and the design requirements. A final coat of thickened Epoxy was applied and
detailed all fabric edges, including splice, termination points and jacket edges. See CFRP
fabric repair in Figure6.
Figure 5 CFRP fabric repair
2.5.2. Stitching Repair Technique
Figure 6 stitching layout
The stitching technique consists of two external post tensioned strands of 1860MPa tensile
strength and 9.53mm of diameter (54.8 mm2 of area). Two strands were used to provide the
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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necessary prestressing effect, taking into consideration the loss of prestressing force by
friction. The strands inserted through the depth of the beam starting from the top by drilling
two opposite holes for each strand with inclined direction of 45o with the bottom. The strand
continues horizontally in the bottom of the beam to cover the damage cut zone, and then turn
back to the top in the opposite hole as shown in Figure 7.
The strand is gripped at both ends, tensioned from one or two edges and anchored to stress
the concrete. The stitching technique does not require professional staff for installer. Figure 8
illustrates the steps of stitching technique.
(a) (b)
(c) (d)
Figure 7 (a) Drilling, (b) anchoring,(c) post tensioning, (d) stitching technique
2.6. Test set up
Figure 9 shows the test set up, the three beams were simply supported with 3000mm span,
and subjected to flexure with two point loads located at 1/3 and 2/3 of span length from the
support. The load-deformation curve was determined using an automated testing machine
provided with data acquisition unit.
Figure 8 test setup
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0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80
Load
KN
Deflection mm
Load vs Deflection
Control
Inelastic elastic
On-line measurements of load and deflection were taken from transducers and transferred
to the computer through the data acquisition unit. Specialized software allowed the on-line
monitoring of the load deflection curve. The load was acting monotonically through a
displacement control method. The load produced single curvature bending in the beam.
3. EXPERIMENTAL RESULTS AND DISCUSSION
3.1. Control Beam B1
Figure10 illustrates the failure pattern and the load-deflection curve which was recorded
during testing. The experiment generated vertical cracks within the beam part, and between
the two point loads and the middle of span.
Figure 9 control beam B1 failure
The control beam B1 exhibited flexural cracks that located near the mid-span and
propagated between the two point loads. It was noticed that the failure was flexural type when
the beam reached the ultimate moment capacity. The load-deflection curve was bilinear and
divided into two parts, elastic part and inelastic part as shown in Figure 11, and it shows that
the control beam failed at load of 120 KN, with deflection of 68mm.
Figure 10 control beam B1 load-deflection curve
3.2. CFRP Fabric Beam CB1
For the CFRP repaired beam CFB1, the cracks started at the mid-span and propagate under
the fiber sheet until a brittle failure was experienced at the ultimate load that can be supported
by the beam; this failure appeared when the fiber sheet pulled away with the concrete from
the specimens as shown in Figure 12.
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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0
20
40
60
80
100
120
140
160
0 10 20 30 40 50
Load
kN
Deflection mm
Load vs Deflection
CFRPFabric
Inelastic elastic
Figure11 CFRP beam CB1 failure
The CFB1 beam failure was described first by yielding of the internal reinforcement, and
then the concrete crushed and separated from the specimen with CFRP fabric due to tension
failure.
The curve in Figure 13 illustrates the capacity of the CFRP repaired beam which is 140
KN, with 13.5mm deflection, this result indicate the brittle failure and quasi no ductile
behavior was observed.
Figure 12 CFRP beam CB1 load-deflection curve
3.3. Stitched Beam SB1
Cracks in beam SB1 began to form at the mid- span in the tension side during testing, a few
number of cracks appeared before failure occurred at the bottom side of the beam in contact
with the strands as shown in the Figure14.
Figure 13 stitching beam SB1 failure
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0
20
40
60
80
100
120
140
160
180
0 10 20 30 40 50 60 70
Load
kN
Deflection mm
Load vs Deflection
Stiching
Inelastic elastic
0
20
40
60
80
100
120
140
160
180
0 10 20 30 40 50 60 70
Load
kN
Deflection mm
Load vs Deflection
Control
CFRP Fabric
Stiching
Inelastic elastic
The load deflection curve in Figure 15 reflects the beam behavior, which is affected by the
length of the strand in contact with the bottom of the beam creating the uplift line load. The
load deflection curve is divided into two parts, the elastic part, and inelastic part which exhibit
stiffness reduction caused by the cracks. The damage occurred when the capacity of the
specimen reached 162 KN with 31.5mm deflection. It was noticed a reduction of deflection
due to camber resulting from prestressing. The load displacement curve showed no brittle
failure.
Figure 14 stitching beam SB1 load-deflection curve
3.4. Beams Comparison
Figure 16 exhibits the load-deflection curves of the 3 tested beams, control beam B1, CFRP
fabric beam CFB1, and stitching beam SB1.
Figure 15 Loads deflection curves
It is well noticed the similarities of the behavior in the elastic stage for the three beams,
whereas the behavior of the beams are different in the inelastic stage. Comparing the loads
capacity of specimens, beam CFB1 and SB1 exhibited an increase in load capacity by 16.66%
and 35% respectively from the control beam B1.Comparing to the control beam, it can be
seen in Figure 15 that the failure of beam CFB1 was quasi brittle failure and occurred at a
small deflection. On the other hand, beam SB1 showed better ductility; hence the failure
occurred at a deflection of 31.5mm. Since the ductility helps in preventing sudden
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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catastrophic failures K. Q. Walsh and Y. C. Kumara (2004) [11], therefore, SB1 beam may be
more suitable in the earthquake zones, L. Panian, M. Steyer & S. Tipping (2007) [12].
The ductility is provided by the reinforcement presented in tension and compression zone
of all the 3 beams, even though the amount of steel was the same, the control beam showed
more ductility than CFB1 and SB1, this is due to the continuity of the prestressing strand in
B1, therefore the repaired beams generated less ductility.
The failure of CFB1was quasi brittle and the CFRP didn’t compensate the loss of
ductility.
The percentage of forces regained using CFRP and stitching repair are shown in the Table 3
Table 3 Repair Type and Percentage of Loading Regained.
Specimen # Type of repair Ultimate
load, (KN)
Percentage of force
regained
B1 Control beam 120 -
CFB1 CFRP 140 116.66%
SB1 Stitching 162 135%
4. THEORETICAL INVESTIGATION
Conventional beam theory was used to predict the capacity of the beams tested in this paper
as followings:
The Control Beam
The theoretical capacity of the beams was calculated using the following criteria:
To calculate the stress in bonded strand at failure stage, Eq.1 (AC1 318-08) was used
(
[
( )]) (1)
= 1750 MPa.
Where fps is the stress at prestressed reinforcement at nominal strength, fpu is the tensile
strength of prestressing tendon, 1860 MPa, f’c is the 28 days of compressive strength of the
concrete,45MPa, is the reinforcement index of the prestressed reinforcement and equal to
, is the steel percentage of the strand, dp is the distance from the compression fibers
to the center of the prestressed reinforcement, d is the is the distance from the compression
fibers to the center of tension reinforcement, , is the reinforcement index for the
compression of steel reinforcement, β1, is the factor relating depth of equivalent compression
block to depth of neutral axis,0.8.
Figure 16 Equilibrium of Tensile and Compressive Couple Forces
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From equilibrium system Figure 17, the depth of equivalent a, was calculated as follow
according to ACI-318:
Tst+Ts=Cc+Cs
a=19mm
Where Tst & Ts are tensile forces of strands and bottom reinforcement respectively, Cs &
Cc are compressive forces of upper reinforcement and concrete respectively, b is the width of
the beam, fy is the yield strength of steel reinforcement, 420 MPa, d is depth of the main
reinforcement layer, As is the total cross section area of reinforcing bars, Aps is the cross
section area of the strand, 99mm2, and a is the depth of equivalent rectangular stress block.
The moment capacity will be as shown in Eq.2
M= (
) (
) (2)
M=59 KN.m
Figure 17 Loading setup
To calculate the load capacity, Eq.3 was used, see
Figure 17:
M=PL/3 (3)
P=59 KN load capacity= 2P=118 KN
The CFRP fabric Beam CFB1
Gross Laminate Properties of Tyfo Composite System according to ACI-440.2R-08 are as
follow:
Ultimate tensile strength ffu = 834 MPa, modulus of Elasticity: Ef = 82.0 GPa, ultimate
elongation:
εfu= 0.0085, thickness per layer: tf = 1.00 mm, tensile force produced by the Tyfo SCH-41
Composite System:
Ff = Αf · ff Af = n · tf · wf
where n = 2 layers wf,face = 200 mm < 250 mm
Af =400 mm²
ff = Efd · εfe with εfe ≤ εfd = 0.41 √(fc / n·Ef·tf) and εfd <0.90·εfu = 7.65‰ εfe =5 ‰
ff = Efd · εfe → ff = 410 MPa
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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Figure 18 Equilibrium of Tensile and Compressive Couple Forces
From equilibrium system
Figure 18, the depth of equivalent a, was calculated as follow according to ACI-318:
Tf+Ts=Cc+Cs
.
a=26.3mm.
The moment capacity will be as shown in Eq.4
M= (
) (
) (4)
M=65 KN.m
To calculate the load capacity, Eq.3 was used, see
Figure 17:
P=65 KN load capacity= 2P=130 KN
The Stitched Beam SB1
To determine the theoretical capacity of beam SB1, the stress in the strands at ultimate should
first be determined. To calculate the stress in member with unbonded strands and with a span-
to-depth ratio of 35 or less, Eq.5 (AC1 318-08) was used
(5)
= 1446 MPa.
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Figure 20 Equilibrium of Tensile and Compressive Couple Forces
Where fps is the stress at prestressed reinforcement at nominal strength, fpe is effective
stress in prestressing steel (after allowance for all prestress losses), 1068 MPa, is the steel
percentage of the strand, dp is the distance from the compression fibers to the center of the
prestressed reinforcement.
From equilibrium system Figure 20:
Tst+Ts=Cc+Cs
.
a=16.6mm
Where Tst is the tensile force of the external strands, Aps is the cross section area of the
strand, 54.8mm2.
The moment capacity will be as shown in Eq.6
M= (
) (
) (6)
M=64 KN.m
To calculate the load capacity, Eq.3 was used, see
Figure 17:
P=64 KN 2P=128 KN
5. THEORETICAL AND EXPERIMENTAL RESULTS
Table 4 shows the summary of the experimental and theoretical ultimate loads. The nominal
experimental capacity of control Specimen B1 was calculated to be 120 kN. Compared with
the theoretical ultimate load of 118kN, the ratio of the experimental to theoretical ultimate
load is 1.01, indicating that the theoretical prediction is in good agreement with the
experimental value. In CFB1 and SB1 the ratios of experimental to the theoretical ultimate
load are 1.07 and 1.28 respectively, which are appropriate for predicting the ultimate capacity.
Table 4 Experimental and theoretical ultimate loads
Specimens # Experimental
ultimate load,(kN)
theoretical ultimate
load , (kN)
Ratio of experimental to
analytical ultimate load
B1 120 118 1.01
CFB1 140 130 1.07
SB1 162 128 1.26
M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh
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6. CONCLUSIONS
In this research two prestessed beams CFB1 and SB1 were subjected to damage in the
prestressing steel in order to assess two repair techniques, the CFRP fabric and the stitching.
Laboratory tests were carried out on these beams CFB1, SB1, and B1, where B1 is the
control beam, to evaluate the structural performance of strengthening techniques by
investigate their flexural behavior.
It was concluded that both techniques can be considered as adequate for the intended
purpose, and they do not significantly increase the designed loads to the structural elements. It
was also concluded, based on the experiments in this research, that specimens CFB1 and SB1
were successfully repaired using CFRP fabric, and stitching technique, and these techniques
are able to compensate the damaged strand by increasing the ultimate load of 16.66% and
35% respectively compared with control beam.
However, the stitching technique had offered the following advantages compared to CFRP
fabric technique:
An improvement with ductile behavior when compared to the quasi brittle failure of the beam
repaired by CFRP fabrics. Therefore SB1 beam more suitable in the earthquake zones than
CFRP beam.
Stitching material is readily available at a lower cost.
Stitching is more traditional and can easily apply while CFRP fabrics require highly skilled
labor for proper installation.
The theoretical analysis can be used to properly predict the capacity of the repaired beams.
However, additional investigation should be performed several variables to stitching
technique (such as inclination angle of strand, strand length ….) in order to enlarge the
domain of application.
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[3] Larry Krauses, Repair Modifications and Strengthening with Post-Tensioning, PTI
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[9] Reed, C. E., Peterman, R. J., Rasheed, H., and Meggers, D. “Adhesive applications used
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[10] Jarret L. Kasan, A.M.ASCE; Kent A. Harries, M.ASCE; Richard Miller, M.ASCE; and
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[11] Keven Q. Walsh and Yahya C. Kumara, “Behavior of Un-bonded Post-Tensioning
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