Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP) Rebars With and Without Chopped Carbon Fiber (CCF)

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

*[email protected]

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).










Group 3: Consists of nine specimens with volume

fraction of chopped carbon fiber CCF equal to 0.50%

(fibers concrete).










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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


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



128

Fig. 2: Crack patterns for beams A1- A9 at ultimate loads


129

Fig. 2: Continued, Crack patterns for beams A10- A18 at ultimate loads

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130

Fig. 2: Continued, Crack patterns for beams A10- A18 at ultimate loads


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



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 %,



beams with f`c=80 MPa and Vf=0%.




beams with f`c=100 MPa and Vf=0%.




beams with f`c=60 MPa and Vf=0.25%.












beams with f`c=60 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.


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.





respectively for beam reinforced by three bar of

CFRP and Vf=0% of CCF.


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.





respectively for beam reinforced by two bar of

CFRP and Vf=0.25% of CCF.



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.


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.




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 (.





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


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.




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.



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.



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.



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.




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.




loads increased by 12.16 and 9.12 % respectively for

beam with f`c=100 MPa and ρ<ρb.





while when Vfof CCf increased to 0.50% the


f`c=100 MPa and ρb<ρ<1.5ρb.





while when Vf of CCf increased to 0.50% the


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



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


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


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


B19 4.50 17.90 Tension

B20 5.60 26.30 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


* 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


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



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


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



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.

REFRENCES

ACI Committee 440 (2006). Guide for the Design and

Construction of Concrete Reinforced with FRP

Bars. ACI Manual, Part One.

ACI 363R-92 (1997). State of the Art Report on High-

Strength Concrete. Reapproved by ACI

Committee 363,

Al-Sunna RAS (2006). Deflection Behaviour of FRP

Reinforced Concrete Flexural Members. Ph.D.

thesis, Sheffield University 2006.

American Society for Testing and Material, ASTM

C33 (20001). Standard Specification for

Concrete Aggregates.

American Society for Testing and Material, ASTM

C1240 (2000). Standard Specification for Use

of Silica Fume as a Mineral Admixture in

Hydraulic-Cement Concrete, Mortar, and

Grout", July, 2000.

Ashour SA, Wafa FF (1992). Use of Steel Fiber as

Shear Reinforced in High Strength Concrete

Beams. Proceeding of the Fourth International

Symposium on Fiber Reinforced Cement and

Concrete, Sheffield, PP. 517-529.

Aziz OQ, Taha BO (2013). Mechanical Properties of

High Strength Concrete (HSC) With and

Without Chopped Carbon Fiber (CCF).

International Academy of Science Engineering

and Technology (IASE), International Journal

of Civil Engineering (IJCE), 2(1): 1-12.

Chang C, Song G (2011). Effects of Temperature and

Mixing on Electrical Resistivity of Carbon

Fiber Enhanced Concrete the 6th International

Workshop on Advanced Smart Materials and

Smart Structures Technology ANCRiSST.

Hannant DJ (1978). Fiber Cements and Fiber

Concretes. A Wiley-Interscience Publication.

Naaman AE, Jeong SM (1995). Structural Ductility of

Concrete Beams Prestressed with FRP

Tendons. Proc., 2nd

Int. RILEM Symp.

(FRPRXS-2), Non-Metric (FRP)

Reinforcement for Concrete Structures,

RILEM, Bagneux, France, pp. 379-386.

Nilson H, Darwin D (2004). Design of Concrete

Structures. International Edition, 13th Edition.

Victor CLI (2002). Large Volume, High-Performance

Applications of Fiber in Civil Engineering.

Journal of Applied Polymer Science, 83: 660-

686.

Zheng Q, Chung DDL (1989). Carbon Fiber

Reinforced Cement Composites Improved by

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pp.1-2.


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.

Documents

Flexure Behavior of High Strength Concrete (HSC) Beams Reinforced With Carbon Fiber Reinforced Polymer (CFRP) Rebars With and Without Chopped Carbon Fiber (CCF)