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International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013 Available online at http://www.ijsrpub.com/ijsrk
ISSN: 2322-4541; ©2013 IJSRPUB
http://dx.doi.org/10.12983/ijsrk-2013-p123-139
123
Full Length Research Paper
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon
Fiber Reinforced Polymer (CFRP) Rebars With and Without Chopped Carbon Fiber
(CCF)
Omar Qarani Aziz1*, Bahman Omar Taha
2
1Assist Prof. in Department of Civil Engineering, University of Salahaddin-Hawler, Kurdistan Region, Iraq
2Ph.D Student in Department of Civil Engineering, University of Salahaddin-Hawler, Kurdistan Region, Iraq
*dr_omer_qarani@yahoo.com
Received 17 March 2013; Accepted 30 April 2013
Abstract. The flexure strength and behavior of high strength concrete beams reinforced with carbon fiber reinforcement
(CFRP) rebars with and without chopped carbon fiber (CCF) were investigated by conducting flexural testes on a total of 27
simply supported HSC beams under two symmetrical point loads. The main parameters considered in the study were the
reinforcement ratio ρ, compressive strength of the concrete f`c and volume fraction of chopped carbon fiber Vf. It can be seen
from the experimental results that the maximum carrying capacity increases as the reinforcement ratio, concrete compressive
strength and the volume fraction of copped carbon increases. Crack spacing of the fiber reinforced concrete (FRC) beams was
about 20% smaller than plain concrete beams at service load (30% of ultimate load). Addition of fibers significantly improves
the system’s ductility; nonetheless the ductility index depends on amount of reinforcement (higher reinforcement allows for
lower deformation, thus a lower ductility index) obtained.
Key words: high strength concrete (HSC), fiber reinforcement polymer (FRP) rebars, chopped carbon fiber (CCF), Volume of
fraction (Vf), flexure , behavior, beams, ductility, crack
1. INTRODUCTION
Steel reinforced concrete (RC) structures have been
used successfully in all types of infrastructure for
more than a century. Nonetheless, under aggressive
exposure conditions such as marine environments, the
steel reinforcement can corrode very rapidly,
corrosion can lead to costly repair and maintenance
operations, reduced service life of the structure and, in
severe cases, structural failure. Various measures and
procedures have been developed to mitigate corrosion.
However, none of these provides a comprehensive and
cost effective solution (Raed, 2006).
Recently, composite materials made of fibers
embedded in a polymeric resin, also known as fiber
reinforced polymers (FRP), have become an
alternative to steel reinforcement for concrete
structures. Because FRP materials are nonmagnetic
and noncorrosive, the problems of electromagnetic
interference and steel corrosion can be avoided with
FRP reinforcement. Additionally, FRP materials
exhibit several properties, such as high tensile
strength, that make them suitable for use as structural
reinforcement (ACI 440.1R, 2006).
ACI committee 363(ACI 363, 1996) defined high
strength concrete (HSC) as a concrete having cylinder
compressive strength exceeding 41 MPa and it
excludes concrete made using exotic materials or
exotic techniques. High performance concrete (HPC)
is defined as any concrete which satisfies certain
criteria proposed to overcome limitations of
conventional concrete, so high strength concrete
(HSC) is one type of (HPC) (Zia and Lemin, 1990). In
general, the economic advantages of high-strength
concrete are most readily realized when the concrete
is used in the columns of high-rise buildings, Parking
garages, bridge decks, and other installations requiring
improved density, lower permeability, and increased
resistance to freeze-thaw and corrosion have become
prime candidates for consideration of the use of high-
strength materials (ACI 363, 1997). High strength
concrete have the same components of ordinary
strength concrete with especial properties such as low
permeability, high strength and more durability. The
compressive strength curves illustrate important
differences compared with normal strength concrete,
including higher elastic modulus and an extended
range of linear elastic response: disadvantages include
brittle behavior and somewhat reduced ultimate strain
capacity (Nilson and Darwin, 2004).
One of the problems of a cement-based matrix is
inherently brittle type of failure which occurs under
tensile stress systems or impact loading and in the
construction industry; a major reason of growing
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
124
interest in the performance of fibers in cement based
materials is the desire to increase toughness or tensile
properties of the basic matrix (Hannant, 1978).
HSC is considered as a relatively brittle material
and the post-peak portion of its stress-strain diagram
almost vanishes and descends steeply with the
increase in compressive strength. This inverse relation
between strength and ductility is a serious drawback
in the use of high strength concrete, a compromise
between strength and ductility can be obtained by
using discontinuous fibers. Addition of fibers to
concrete makes it a homogeneous and isotropic
materials and converts brittleness into a ductile
behavior. When concrete cracks, the randomly
oriented fibers start functioning, arresting both the
randomly oriented micro-cracking and its propagation
and thus improving strength and ductility (Ashour and
Wafa, 1992).
Previous research findings clearly establish that
ductility of concrete structural members can be greatly
enhanced with the use of fibers. In addition, fibers
generally give favor improvements in first crack,
ultimate member strength, impact resistance and shear
resistance. If property designed, fibers can be added to
structural member especially when used together with
conventional steel main reinforcements (rebar)
(Victor, 2002). Carbon fiber has gained more
popularity in structural materials due to their high
strength, additional properties imbued by carbon fiber,
particularly electrical properties, have gained attention
for their possible applications to structural sensing and
electrical actuation (Christiana and Gangbing, 2011).
Carbon fibers are inert, medically safe and stronger
than steel fibers and more chemically stable than glass
fibers in an alkaline environment. Moreover, Carbon
fibers are low in density, especially compared to steel
fibers; their strength-to-density ratio is one of the
highest among all fiber types ( Zheng and Chung,
1989). Carbon fibers have much higher specific
strength and stiffness than metallic fibers and for this
reason their use for strengthening and stiffening
building materials such as plastics and concrete, are
attractive (Nilson and Darwin, 2004). Carbon fiber
cement-matrix composites are structural materials that
are gaining in importance quite rapidly due to the
decrease in carbon fiber cost and the increasing
demand of superior structural and functional
properties. The improved structural properties
rendered by carbon fiber addition pertain to the
increased tensile and flexural strengths, the increased
tensile ductility and flexural toughness, the enhanced
impact resistance, the reduced drying shrinkage and
the improved freeze-thaw durability (Omar and
Bahman, 2013).
2. EXPERIMENTAL WORK
2.1. Materials
The following materials were used for producing
concrete mixes:
(1). Ordinary Portland Cement (OPC -I 42.5 R),
according to ASTM C150.; (2). Silica Fume (CSF-
90), according to ASTM C1240.; (3). Normal Fluvial
Sand, according to ASTM C33.; (4). Coarse aggregate
(Gravel), crushed gravel with maximum size of
9.5mm, according to ASTM C33; (5). Super
plasticizer- Glenium ACE 30; (6). Water, normal
drinking water; (7). Chopped carbon fiber, with: l=20
mm, Ø =7-8 µm, fu = 2.84 GPa& E=235 GPa. (8).
Carbon fiber reinforcement polymer rebars, with
diameter =5 mm, ultimate tensile strength 2300 MPa,
modulus of elasticity 130GPa & ultimate deformation
1.8%
2.2. Beams description
A total of twenty seven specimens of actual
dimensions (Table 1), were cast and tested in the
laboratory; all the specimens tested in this program
were rectangular beams with 100*150 mm cross
section and had clear covers of 15 mm. The beams
were loaded at two points where arrangements were
made to avoid local failure at load points and supports
by means of steel plates the beams were designed to
fail in flexure with tensile or compressive modes. To
avoid shear failure, sufficient amounts of steel stirrups
were used, within the shear span. Two nominal 6mm
steel bars were used as top reinforcement within the
shear span to hold the stirrups. The total length, clear
span and shear spans of all beams were 2250, 2000
and 700 mm respectively. Layouts of the beams and
their geometric and reinforcement details are given in
Fig.1.
2.3. Beam identification
The test specimens were divided into three groups as
shown in Table 2havingsame cross sections and
lengths. The detail of the groups according to the
parameters (percentage of tension reinforcement,
compressive strength of the concrete, and percentage
of chopped carbon fiber), are shown below:
Group 1: Consists of nine specimens without chopped
carbon fiber CCF (non-fibers concrete).
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
125
Fig. 1: Details of the tested beam
The first three beams having f`c equal to 60 MPa
with three different percentage of tension
reinforcement ρ (ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb), the
second three beams having f`c equal to 80 MPa with
three different percentage of tension reinforcement ρ
(ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb) and the third three
beams having f`c equal to 100 MPa with three
different percentage of tension reinforcement ρ (ρ <ρb,
ρb<ρ<1.5ρb and ρ >1.5 ρb).
Group 2: Consists of nine specimens with volume
fraction of chopped carbon fiber CCF equal to 0.50%
(fibers concrete).
The first three beams having f`c equal to 60 MPa
with three different percentage of tension
reinforcement ρ (ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb), the
second three beams having f`c equal to 80 MPa with
three different percentage of tension reinforcement ρ
(ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb) and the third three
beams having f`c equal to 100 MPa with three
different percentage of tension reinforcement ρ (ρ <ρb,
ρb<ρ<1.5ρb and ρ >1.5 ρb).
Group 3: Consists of nine specimens with volume
fraction of chopped carbon fiber CCF equal to 0.50%
(fibers concrete).
The first three beams having f`c equal to 60 MPa
with three different percentage of tension
reinforcement ρ (ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb), the
second three beams having f`c equal to 80 MPa with
three different percentage of tension reinforcement ρ
(ρ <ρb, ρb<ρ<1.5ρb and ρ >1.5 ρb) and the third three
beams having f`c equal to 100 MPa with three
different percentage of tension reinforcement ρ (ρ <ρb,
ρb<ρ<1.5ρb and ρ >1.5 ρb).
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
126
Table 1: Details of cast specimens
No. Specimen
symbol
f`c
(MPa)
No. of CFRP rebar’s ρ ρb* ρ/ ρb CCF%
1 B1 62.77 1 Ø 5mm 0.00150 0.00215 0.70 0.00
2 B2 62.77 2 Ø 5mm 0.00300 0.00215 1.40 0.00
3 B3 62.77 3Ø 5mm 0.00451 0.00215 2.09 0.00
4 B4 84.55 1 Ø 5mm 0.00150 0.00290 0.52 0.00
5 B5 84.55 2 Ø 5mm 0.00300 0.00290 1.03 0.00
6 B6 84.55 3Ø 5mm 0.00451 0.00290 1.55 0.00
7 B7 97.96 2 Ø 5mm 0.00300 0.00336 0.89 0.00
8 B8 97.96 3Ø 5mm 0.00451 0.00336 1.34 0.00
9 B9 97.96 4Ø 5mm 0.00652 0.00336 1.94 0.00
10 B10 63.78 1 Ø 5mm 0.00150 0.00219 0.68 0.25
11 B11 63.78 2 Ø 5mm 0.00300 0.00219 1.37 0.25
12 B12 63.78 3Ø 5mm 0.00451 0.00219 2.05 0.25
13 B13 86.22 1 Ø 5mm 0.00150 0.00296 0.51 0.25
14 B14 86.22 2 Ø 5mm 0.00300 0.00296 1.01 0.25
15 B15 86.22 3Ø 5mm 0.00451 0.00296 1.52 0.25
16 B16 100.55 2 Ø 5mm 0.00300 0.00345 0.87 0.25
17 B17 100.55 3Ø 5mm 0.00451 0.00345 1.30 0.25
18 B18 100.55 4Ø 5mm 0.00652 0.00345 1.89 0.25
19 B19 64.10 1 Ø 5mm 0.00150 0.00220 0.68 0.50
20 B20 64.10 2 Ø 5mm 0.00300 0.00220 1.36 0.50
21 B21 64.10 3Ø 5mm 0.00451 0.00220 2.05 0.50
22 B22 86.70 1 Ø 5mm 0.00150 0.00298 0.50 0.50
23 B23 86.70 2 Ø 5mm 0.00300 0.00298 1.01 0.50
24 B24 86.70 3Ø 5mm 0.00451 0.00298 1.51 0.50
25 B25 100.83 2 Ø 5mm 0.00300 0.00346 0.87 0.50
26 B26 100.83 3Ø 5mm 0.00451 0.00346 1.30 0.50
27 B27 100.83 4Ø 5mm 0.00652 0.00346 1.88 0.50
*ρb : Balanced reinforcement ratio
3. RESULTS AND DISCUSSION
3.1. Crack pattern and modes of failure
The crack pattern at failure for all the beams were
shown in Fig. 2, the crack pattern and mode of failure
of all the test beams were not similar, due to
differences in reinforcement ratio, compressive
strength of the concrete, and volume fraction of
chopped carbon fiber.
Beams (B1, B4, B5, B7, B8, B10, B11, B13, B14,
B15, B16, B17, B19, B22, B23, B25, and B26) were
failed due to rupture of the FRP rebars. The cracking
started in the constant moment region with the cracks
originating from the bottom fibers which were
subjected to the maximum principal stresses. These
cracks were mainly vertical flexural cracks, which
were perpendicular to the longitudinal axis of the
beam. As the load is increased, additional cracks
developed in the mid span and new vertical cracks
formed in the shear span. More secondary cracks
developed at the bottom face of the beam and began to
be inclined towards the main cracks and often joined
them. Rupture of the rebars causes the crack to
penetrate through the entire section. Hence, the beam
is literally cut into two separate segments and
collapses.
Beams (B2, B6 and B9) were failed due to
crushing of concrete and followed, immediately,
rupture of the rebars. Similarly, cracking was initiated
when the applied moment reached the cracking
moment. The cracking consisted of vertical cracks
perpendicular to the direction of the principal tensile
stress induced by pure moment. As the load increased,
flexural cracks spread into the shear span, some
horizontal cracks appeared at mid span.
Beams (B3, B12, B18, B21, and B27) were failed
by crushing of concrete at top surface, of the pure
bending zone. Also, at early stages of the post-
cracking stage, flexural cracks were observed in the
beams throughout the mid span. As the load was
increased cracking outside the constant moment zone
started similarly to the flexural cracking, but at a
higher load level, some of these cracks gradually
increased in depth and began to be inclined towards
the applied loads.
Beams (B20 and B24) were failed by crushing of
concrete at top surface, out of the pure bending zone.
Also, the cracking stages are same as beams failed in
compression at pure bending zone.
As it is clear from modes of failure of the beams
when the concrete compressive strengths increases
(without CCF) from 60 MPa to 80 and 100 MPa the
amount of the balanced bar provided by ACI 400 is
not an exact criteria to determine the type of failure,
since beams failed by rupture of the rebars while ρ
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
127
between ρb and 1.5 ρb., it is applicable only in cases
where the ratio of bars are lower than the balanced
mode that ruptures occur in reinforcement area.
Generally the effect of CCF can explained as that
for beams having ρ between ρb and 1.5 ρb. By adding
the CCF with Vf=0.25%, modes of failure changed
from compression-tension to tension failure for
concrete compressive strengths (60 and 80 MPa),
while for beams with f`c=100 MPa modes of failure
changed from compression-tension to compression.
Adding the CCF by VF=0.50%, modes of failure
changed from compression-tension to compression
failure for concrete compressive strengths (60 and 80
MPa), and for beams with f`c=100 MPa modes of
failure changed from compression-tension to tension
failure.
Table 2: Distribution of specimens into groups according to the considered parameters
Group No. Specimen symbol f`c (MPa) ρ/ ρb CCF%
G1 B1 60 <ρb 0
G1 B2 60 ρb-1.5 ρb 0
G1 B3 60 >ρb 0
G1 B4 80 <ρb 0
G1 B5 80 ρb-1.5 ρb 0
G1 B6 80 >ρb 0
G1 B7 100 <ρb 0
G1 B8 100 ρb-1.5 ρb 0
G1 B9 100 >ρb 0
G2 B10 60 <ρb 0.25
G2 B11 60 ρb-1.5 ρb 0.25
G2 B12 60 >ρb 0.25
G2 B13 80 <ρb 0.25
G2 B14 80 ρb-1.5 ρb 0.25
G2 B15 80 >ρb 0.25
G2 B16 100 <ρb 0.25
G2 B17 100 ρb-1.5 ρb 0.25
G2 B18 100 >ρb 0.25
G3 B19 60 <ρb 0.50
G3 B20 60 ρb-1.5 ρb 0.50
G3 B21 60 >ρb 0.50
G3 B22 80 <ρb 0.50
G3 B23 80 ρb-1.5 ρb 0.50
G3 B24 80 >ρb 0.50
G3 B25 100 <ρb 0.50
G3 B26 100 ρb-1.5 ρb 0.50
G3 B27 100 >ρb 0.50
3.2. Load-deflection behavior
Figs.3 to Fig.5 show the load-deflection curves at the
mid-span of each beam group specimen. The load
deflection relationship for a beam is useful for
describing the behavior of beam under loads. In
general, two major stages in behavior are observed.
An initial linear branch with a steep slope,
corresponding to the un-cracked condition of the
beam is detected. When the cracking load is achieved,
a drop in the slope is observed, due to the progressive
cracking of the beam. Finally, the cracking process
stabilizes and an almost linear segment is observed
until failure.
The reinforcement ratio have an effect on the stiffness
of the beam specimens and, therefore, on their load-
deflection behavior. As expected, larger deformations
are obtained for lower reinforcement ratios, and vice
versa.
It is quite obvious from the load deflection plots
that the inclusion of CCF had marked effect on the
deflection capability of the beams generally a
relatively stiffer response at the post-cracking stage
and after cracking stage for all beam specimens
containing chopped carbon fiber can be observed.
This may be due to the high specific strength and
stiffness of carbon fiber.
By increasing concrete compressive strength the
deflection was decreased for corresponding load
levels the percentages of decreasing varied by
variation in reinforcement ratio and volume fraction
of chopped carbon fiber. As clear from the figures the
compressive strength have no effect on the deflection
at first stage of load deflection curve (before cracking)
while after cracking the compressive strength have
effect on deflection of beams till failure.
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
128
Fig. 2: Crack patterns for beams A1- A9 at ultimate loads
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
129
Fig. 2: Continued, Crack patterns for beams A10- A18 at ultimate loads
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
130
Fig. 2: Continued, Crack patterns for beams A10- A18 at ultimate loads
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
131
Fig. 3: Load deflection curves for G1
Fig. 4: Load deflection curves for G2
Fig.5: Load deflection curves for G3
3.3. First cracking and ultimate load of the beam
specimens
Table, 3 and Fig. 6 shows the experimental values of
first cracking load, Pcr, (first flexural cracking load,
i.e., cracking at the bottom of the beam between the
two point loads) and ultimate carry capacity load, Pu.
The following sections explain the effect of the
parameters included in this project on the first
cracking load, and ultimate load.
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
132
3.3.1. Effect of reinforcement ratio, ρ
It may be seen that, by increasing ρ from ρ<ρb to
ρb<ρ<1.5ρb and ρ>1.5ρb,Pcr and Pu were increased
when compared with the beams with ρ<ρb as follows:
Pcr increases by 33.33%, and 80.00 %,
respectively and the maximum carry capacity
increases by 101.94 and 167.10% respectively for
beams having f`c=60 MPa and Vf=0%.
Pcr increases by 20.00, and 32.00 %,
respectively and the maximum carry capacity
increases by 110.63 and 210.63% respectively for
beams with f`c=80 MPa and Vf=0%.
Pcr increases by 16.67, and 30.00 %,
respectively and the maximum carry capacity
increases by 59.88 and 96.00% respectively for
beams with f`c=100 MPa and Vf=0%.
Pcr increases by 40.00, and 60.00 %,
respectively and the maximum carry capacity
increases by 113.84 and 146.54% respectively for
beams with f`c=60 MPa and Vf=0.25%.
Pcr increases by 14.29, and 35.71 %,
respectively and the maximum carry capacity
increases by 120.75 and 237.74% respectively for
beams with f`c=80 MPa and Vf=0.25%.
Pcr increases by 10.64, and 23.40 %,
respectively and the maximum carry capacity
increases by 41.19 and 49.05% respectively for
beams with f`c=100 MPa and Vf=0.25%.
Pcr increases by 4.44, and 11.11 %,
respectively and the maximum carry capacity
increases by 46.93 and 70.39% respectively for
beams with f`c=60 MPa and Vf=0.50%
Pcr increases by 10.20, and 14.29 %,
respectively and the maximum carry capacity
increases by 106.75 and 146.01% respectively for
beams with f`c=80 MPa and Vf=0.50%.
Pcr increases by 1.81, and 14.81 %,
respectively and the maximum carry capacity
increases by 47.08 and 55.99% respectively for
beams with f`c=100 MPa and Vf=0.50%.
The above numbers indicate that the percentages of
increasing in fist cracking load decreases by
increasing f`c while the percentages of increasing in
ultimate load for beams having f`c=100 MPa lower
than beams with f`c equal to 60 and 80 MPa.
3.3.2. Effect of compressive strength of the
concrete, f`c
The first cracking, Pcr and ultimate, Pu, load increased
with increasing the compressive strength of the
concrete
Increasing the compressive strength from 60
MPa to 80 Mpa tends to increase first cracking load
by 66.67% and the ultimate load by 3.23% for beam
reinforced by one bar of CFRP and Vf=0% of CCF.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 50.99 and 50.00 %, respectively,
and the ultimate load by 7.67 and 5.11 %
respectively for beam reinforced by two bar of
CFRP and Vf=0% of CCF.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 22.22 and 29.63 %, respectively,
and the ultimate load by 20.05 and 27.05 %
respectively for beam reinforced by three bar of
CFRP and Vf=0% of CCF.
Increasing the compressive strength from 60
Mpa to 80 Mpa tends to increase first cracking load
by 40.00% and the ultimate load was not changed
for beam reinforced by one bar of CFRP and
Vf=0.25% of CCF.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 14.29 and 67.86 %, respectively,
and the ultimate load by 3.24 and 8.53 %
respectively for beam reinforced by two bar of
CFRP and Vf=0.25% of CCF.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 18.75 and 62.5 %, respectively, and
the ultimate load by 36.99 and 32.91 % respectively
for beam reinforced by three bar of CFRP and
Vf=0.25% of CCF.
Increasing the compressive strength from 60
Mpa to 80 Mpa tends to increase first cracking load
by 8.89% and the ultimate load by 2.52% for beam
reinforced by one bar of CFRP and Vf=0.50% of
CCF.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 14.89 and 25.00 %, respectively,
and the ultimate load was not increased for f`c=80
Mpa while the ultimate load increased by 5.59% for
beam reinforced by two bar of CFRP and Vf=0.50%
of CCF (.
Increasing the compressive strength from 60
Mpa, to 80 and 100 Mpa tends to increase first
cracking load by 12.00 and 10.00 %, respectively,
and the ultimate load by 2.30 and 34.69 %
respectively for beam reinforced by three bar of
CFRP and Vf=0.50% of CCF.
It can be shown that the compressive strength of
concrete has more effect on the cracking strength of
the specimen, and the effect on the maximum carry
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
133
capacity for the beams changed according
reinforcement ratio and volume fraction of CCF.
3.3.3. Effect chopped carbon volume fraction Vf
The effect of volume fraction on first cracking and
ultimate loads of tested beams was as follow
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
33.33and 200.00 % respectively and the ultimate
loads increased by 2.58 and 15.48 % respectively for
beam with f`c=60 MPa and ρ<ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
40.00and 135.00 % respectively and the ultimate
loads increased by 8.63 when Vf increased to 0.25%
while the ultimate load decreased by 15.97% when
Vf increased to 0.50% for beam with f`c=60 MPa
and ρb<ρ<1.5ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
18.52 and 85.19 % respectively and the ultimate
loads decreased by 5.31 and 26.33% respectively for
beam with f`c=60 MPa and ρ>ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
12.00 and 96.00% respectively and the ultimate
loads decreased by 0.62% when Vf added by 0.25%
while when Vf of CCF increased to 0.50% the
ultimate load increased by 1.88 % for beams with
f`c=80 MPa and ρ<ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
6.67 and 80.00 % respectively and the ultimate loads
increased by 4.15% when Vf increased to 0.25%
while the ultimate load did not effected when Vf
increased to 0.50% for beam with f`c=80 MPa and
ρb<ρ<1.5ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
15.15 and 69.70% respectively and the ultimate
loads increased by 8.05% when Vf increased to
0.25% while the ultimate load decreased by 19.32%
when Vf increased to 0.50% for beam with f`c=80
MPa and ρ>ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
56.67and 80.00 % respectively and the ultimate
loads increased by 12.16 and 9.12 % respectively for
beam with f`c=100 MPa and ρ<ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
48.57 and 57.14% respectively and the ultimate
loads decreased by 0.95% when Vf added by 0.25%
while when Vfof CCf increased to 0.50% the
ultimate load increased by 0.38 % for beams with
f`c=100 MPa and ρb<ρ<1.5ρb.
Increasing the Vf of CCF from 0 to 0.25 and
0.50%tends to increase the initial cracking load by
48.72 and 58.97% respectively and the ultimate
loads decreased by 1.08% when Vf added by 0.25%
while when Vf of CCf increased to 0.50% the
ultimate load increased by 0.72 % for beams with
f`c=100 MPa and ρ>ρb.
The results indicate that the addition of carbon
fibers causes a considerable increase in the first crack
load; the percentage increase for fiber inclusion is
between 33%-200%, while there is a slight increase in
ultimate load between 0%-16%percent relative to the
plain concrete beams.
Fig. 6: Fist crack load and ultimate carrying capacity of tested beams
0.00
10.00
20.00
30.00
40.00
50.00
60.00
B1
B2
B3
B4
B5
B6
B7
B8
B9
B1
0
B1
1
B1
2
B1
3
B1
4
B1
5
B1
6
B1
7
B1
8
B1
9
B2
0
B2
1
B2
2
B2
3
B2
4
B2
5
B2
6
B2
7
Load
carr
yin
g c
ap
aci
ty k
N
Beam disgnation
First crack load Ultimate load
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
134
Table 3: Test results of high strength beans beams
Specimen symbol First crack load (kN) Failure load (kN) Failure mode
B1 1.50 15.50 Tension
B2 2.00 31.30 Compression-Tension
B3 2.80 41.40 Compression
B4 2.50 16.00 Tension
B5 3.00 33.70 Tension
B6 3.30 49.70 Compression
B7 3.00 32.90 Tension
B8 3.50 52.60 Tension
B9 3.90 55.60 Compression-Tension
B10 2.00 15.90 Tension
B11 2.80 34.00 Tension
B12 3.20 39.20 Compression
B13 2.80 15.90 Tension
B14 3.20 35.10 Tension
B15 3.80 53.70 Tension
B16 4.70 36.90 Tension
B17 5.20 52.10 Tension
B18 5.8 55.00 Compression
B19 4.50 17.90 Tension
B20 5.60 26.30 Compression*
B21 6.20 30.50 Compression
B22 4.90 16.30 Tension
B23 5.40 33.70 Tension
B24 5.60 40.10 Compression*
B25 5.40 35.90 Tension
B26 5.50 52.80 Tension
B27 6.20 56.00 Compression
* Compression failure in shear spans (out of pure bending region)
3.4. Strains in CFRP tension reinforcement and
concrete top
Table 4 shows strains of concrete and CFRP rebars at
ultimate load. It can be seen that after cracking, the
strains in the reinforcement increased almost linearly
up to failure. For the beams failed in concrete
crushing rather than FRP reinforcement rupture, the
maximum measured strains in the reinforcement were
less than the ultimate tensile strains.
The measured ultimate concrete strains of plain
concrete beams, were 0.00358, 0.00414 and 0.00433
for concrete strengths 60, 80 and 100 MPa, while
adding 0.25% of CCF the measured ultimate concrete
strains were 0.00432, 0.00398 and 0.00402 for
concrete strengths 60, 80 and 100 Mpa, while adding
0.50% of CCF the measured ultimate concrete strains
were 0.00394 and 0.00393 for concrete strengths
60and 100, So that the Vf of chopped carbon fiber had
no effect on ultimate concrete strains.
The beams failed in FRP reinforcement rupture
rather than concrete crushing, all the maximum
measured strains in the reinforcement were equal or
greater to the FRP rebars ultimate tensile strains. The
measured concrete strains of plain concrete beams,
were 0.00121, 0.00270 and 0.00282 for concrete
strengths 60, 80 and 100 Mpa, while adding 0.25% of
CCF the measured ultimate concrete strains were
0.00242, 0.00131 and 0.00249 for concrete strengths
60, 80 and 100 Mpa, but adding 0.50% of CCF the
measured ultimate concrete strains were 0.00124,
0.00289 and 0.00239 for concrete strengths 60, 80and
100
Adding CCF by Vf=0.25% concrete strains at
failure of CFRP rebars was increased by 100.00% for
concrete strengths 60 Mpa while decreased by 92.30%
for concrete strengths 80 Mpa but the concrete strain
at failure of FRP rebars did not effected changes for
concrete strengths 100 Mpa.
It should be noted that with the increase of ultimate
concrete strain, the balanced reinforcing ratio, ρb will
increase accordingly. From this standpoint, in order to
take more reinforcements are required to achieve
failure by crushing of concrete
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
135
Table 4: Concrete and CFRP strains at failure load
No. Specimen symbol Concrete strain at failure FRP strain at failure
1 B1 -0.00121 0.02018
2 B2 -0.00422 0.01837
3 B3 -0.00358 0.01619
4 B4 -0.00270 0.01847
5 B5 -0.00378 0.0200
6 B6 -0.00414 0.01727
7 B7 -0.00282 0.01879
8 B8 -0.00457 0.02094
9 B9 -0.00433 0.02068
10 B10 -0.00242 0.01844
11 B11 -0.00459 0.02028
12 B12 -0.00432 0.01517
13 B13 -0.00131 0.01822
14 B14 -0.00235 0.01936
15 B15 -0.00398 0.02029
16 B16 -0.00249 0.02084
17 B17 -0.00353 0.02069
18 B18 -0.00402 0.01785
19 B19 -0.00124 0.02065
20 B20 -0.00197 0.01357
21 B21 -0.00394 0.01859
22 B22 -0.00289 0.01854
23 B23 -0.00285 0.02021
24 B24 -0.00273 0.01200
25 B25 -0.00239 0.02018
26 B26 -0.00324 0.02071
27 B27 -0.00393 0.01672
3.5. Cracks Spacing
Table 5 shows the average crack spacing at 30% the
flexural capacity and at ultimate. With the increase of
load, crack spacing slightly decreased. Interestingly,
by comparing the crack spacing between the plain
concrete beams and the FRC beams, the crack spacing
was virtually the same at the ultimate load for both
plain concrete and FRC beams, while the crack
spacing of the FRC beams was about 20% smaller
than that of plain concrete beams at service load (30%
of ultimate load).
Studies suggest that the flexural cracking can be
closely approximated by the behavior of a concrete
prism surrounding the main reinforcement and having
the same centroid. Cracks initiate when the tensile
stress in the concrete exceeds the tensile strength of
concrete. When this occurs, the force in the prism is
transferred to the rebar. Away from the crack, the
concrete stress is gradually built up through the bond
stress between the rebar and the concrete. When the
stresses in the concrete are large enough and exceed
the tensile strength of concrete, a new crack forms.
The above mechanism is demonstrated in Fig.7.
With the addition of fibers, the mechanism of
crack formation is slightly changed, as shown in Fig.
7(b). Some tensile loads can be transferred across the
cracks by the bridging of fibers. Thereby, the stress in
the concrete comes from not only the bond stress but
the bridging of fibers as well. With the contribution
from the fibers, less bond stress is needed to reach the
same cracking stress. Consequently, the spacing of
crack is smaller in the FRC beams than in the plain
concrete beams (S2 < S1 as shown in Fig.7.
At the high level of load, due to loss of bond
between the fibers and concrete, fibers are pulled out
and the contribution from the bridging of fibers is
diminished.
As shown in the Table 5 the crack spacing was
measured at 30 of ultimate load and the ultimate load.
It can be seen that the crack width decreases as the
reinforcement ratio increases.
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
136
Fig. 7: Mechanism of crack formation in plain concrete and fiber reinforced concrete
Table 5: Crack spacing of the beam specimens
No. Specimen symbol Crack spacing at 30% of ultimate load (mm) Crack spacing at ultimate load (mm)
1 B1 170 113 2 B2 142 91 3 B3 100 67 4 B4 167 104 5 B5 150 89 6 B6 130 83 7 B7 140 92 8 B8 90 61 9 B9 61 43
10 B10 133 115 11 B11 122 97 12 B12 87 69 13 B13 123 100 14 B14 104 85 15 B15 89 77 16 B16 111 94 17 B17 75 63 18 B18 55 49 19 B19 140 131 20 B20 127 118 21 B21 74 97 22 B22 140 127 23 B23 120 107 24 B24 108 97 25 B25 117 93 26 B26 78 67 27 B27 53 52
3.6. Ductility
Ductility is a structural design requirement in most
design codes. In steel reinforced concrete structures,
ductility is defined as the ratio of post yield
deformation to yield deformation which it usually
comes from steel. Due to the linear-strain-stress
relationship of FRP bars, the traditional definition of
ductility cannot be applied to structures reinforced
with FRP reinforcement. Several methods, such as the
energy based method and the deformation based
method have been proposed to calculate the ductility
index for FRP reinforced structures. As mentioned
previously, since the traditional definition of ductility
cannot be applied to the structures reinforced with
FRP reinforcement, there was a need for developing a
new approach and a set of ductility indices to both
quantitatively and qualitatively evaluate the FRP
reinforced members. Ductility index calculations
related to the FRP reinforced members have been
widely studied. One of the approaches has been in the
literature proposed to address this problem is energy
based approach. Based on the definition of the energy
based approach, ductility can be defined as the ratio
between the elastic energy and the total energy, as
shown in Fig.8 (Naaman and Jeong,, 1995) proposed
the following equation to compute the ductility index
International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
137
Where:-
DE: Ductility index; Et: is the total energy computed
as the area under the load deflection curve; Ee: is the
elastic energy.
The elastic energy can be computed as the area of
the triangle formed at failure load by the line having
the weighted average slope of the two initial straight
lines of the load deflection curve, as shown in Fig. 8.
although there are different ways to calculate the
ductility index, ductility can no doubt be defined as
the ability to absorb the inelastic energy without
losing its load capacity. Higher inelastic energy
absorption of the same system means higher ductility.
Obviously, from this standpoint, the addition of fibers
significantly improves the system’s ductility. The
ductility indices computed and the percentages of
increasing of ductility indices are shown in Table 6.
As can be seen in Table 6, the ductility index
depends on amount of reinforcement (higher
reinforcement allows for lower deformation, thus a
lower ductility index).
Fig. 8: Namman and jeong’s definition of ductility index
Table 6: Ductility of the beam specimens
Specimen symbol f`c
(MPa)
ρ Ductility index (DE) % of increasing in DE due to adding CCF
B1 62.77 0.00150 1.122
B2 62.77 0.00300 1.079
B3 62.77 0.00451 1.140
B4 84.55 0.00150 1.379
B5 84.55 0.00300 1.113
B6 84.55 0.00451 1.178
B7 97.96 0.00300 1.047
B8 97.96 0.00451 1.064
B9 97.96 0.00652 0.994
B10 63.78 0.00150 1.216 8.3
B11 63.78 0.00300 1.148 6.4
B12 63.78 0.00451 1.180 3.5
B13 86.22 0.00150 1.559 13.0
B14 86.22 0.00300 1.115 0.2
B15 86.22 0.00451 1.191 1.1
B16 100.55 0.00300 1.113 6.3
B17 100.55 0.00451 1.082 1.7
B18 100.55 0.00652 1.078 8.4
B19 64.10 0.00150 1.306 16.4
B20 64.10 0.00300 1.199 11.2
B21 64.10 0.00451 1.162 1.9
B22 86.70 0.00150 2.694 95.3
B23 86.70 0.00300 1.229 10.4
B24 86.70 0.00451 1.232 4.6
B25 100.83 0.00300 1.249 19.2
B26 100.83 0.00451 1.123 5.6
B27 100.83 0.00652 1.129 13.5
Aziz and Taha
Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP)
Rebars With and Without Chopped Carbon Fiber (CCF)
138
4. CONCLUSIONS
From the tests performed on the flexure strength of
HSC reinforced with CFRP contained different
volume fraction of CCF, the following conclusions
can be drawn:
1-Increasing the ρ from ρ<ρb to ρb<ρ<1.5ρb and
ρ>1.5ρb, leads to increases in the value of Pcr and Pu in
different percentages depending on amount of
reinforcement provided and the failure mode of the
beams.
2-The first cracking, Pcr and ultimate, Pu, load
increased with increasing the compressive strength of
the concrete.
3-Addition of chopped carbon fibers causes a
considerable increase in the first crack load 33%-
200%, while there is a slight increase in ultimate load
0%-16% relative to the plain concrete beams
4-The crack spacing was virtually the same at the
ultimate load for both plain concrete and FRC beams
5-Crack spacing of the FRC beams was about 20%
smaller than that of plain concrete beams at service
load (30% of ultimate load).
6-The ductility index depends on amount of
reinforcement (higher reinforcement allows for lower
deformation, thus a lower ductility index).
7-The addition of fibers significantly improves the
system’s ductility.
8-The ultimate concrete strain at failure was about
0.004, with the increase of ultimate concrete strain,
the balanced reinforcing ratio, ρb will increase
accordingly. The modes of failure defined by ACI 440
will not be correct, from this standpoint; in order to
take more reinforcements are required to achieve
failure by crushing of concrete.
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International Journal of Scientific Research in Knowledge (IJSRK), 1(6), pp. 123-139, 2013
139
Dr. Omar Qarani Aziz is an Assistant Professor in the Civil Engineering Department, College of
Engineering; University of Salahaddin-Erbil, Iraq. He received B.Sc. degree in Civil Engineering,
M.Sc. and PhD in structural engineering, Building and Construction Dept., University of Technology,
Baghdad-Iraq in 1993 and 1997. He has published over 40 refereed articles in professional journals,
supervised six M.Sc. and two PhD students, structural design and consulting of different type of
projects. He is editor and reviewer of several international journals. His area of specialization is
Structural Engineering, Shear in Deep Beams and Corbels, High Strength Concrete, Flat Slabs, Ultra
High Performance.
BahmanOmar Tahais a PhD candidate in Structural engineering, Civil Engineering Department,
College of Engineering; University of Salahaddin-Erbil, Iraq. He is a lecturer and researcher in the
Hawler Polytechnic University, Iraq. He received his B.Sc. degree in Civil Engineering and M.Sc. in
structural engineering, Civil Engineering Department, College of Engineering; University of
Salahaddin-Erbil, Iraq. He has published articles in professional journals. His area of specialization is
Structural Engineering, Shear in Ferro cement Beams, High Strength Concrete, Chopped carbon fiber,
and Fiber reinforcement Polymer rebars.