STRENGTHENING OF NORMAL AND HIGH STRENGTH CONCRETE CORBELS WITH HORIZONTAL AND INCLINED STRIPES OF CARBON FIBER

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 7, Issue 3, May–June 2016, pp. 271–282, Article ID: IJCIET_07_03_027
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ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
STRENGTHENING OF NORMAL AND HIGH
STRENGTH CONCRETE CORBELS WITH
HORIZONTAL AND INCLINED STRIPES OF
CARBON FIBER
Asst. Prof. Aamer Najim Abbas and Eng. Wahig Abrahim Abd Al-kareem
Al-Mustansiriya University, Iraq
ABSTRACT
In this study, there were two modes of applying the carbon fiber strips on
reinforced concrete corbels: the first one is application three horizontal strips
and its width is (50 mm) and the other is applying three inclined strips with
angle about (45°), the both modes applied on the two faces of concrete corbel
specimens. Two types of concrete were used in this study; normal strength (28
MPa) and high strength concrete (57 MPa). Each types strengthening with
two modes of carbon fiber stripes.
The ultimate and cracking capacity of tested specimens were improved as
a result of strengthening with carbon fiber strips, in addition to development
of energy absorption and stiffness characteristics.
Key words: Strengthening, Carbon Fiber, High Strength Concrete, Carrying
Capacity, Cracking Capacity, Energy Absorption, Deflection.
Cite this Article: Aamer Najim Abbas and Wahig Abrahim Abd Al-Kareem,
Strengthening of Normal and High Strength Concrete Corbels with Horizontal
and Inclined Stripes of Carbon Fiber. International Journal of Civil
Engineering and Technology, 7(3), 2016, pp.271–282.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3
INTRODUCTION
Sometimes cracks develop on the concrete surface in a structure because of weakness
designing, incorrect placement of bars of reinforcement, temperature variations,
change in use of building or due to any other unforeseen reason. In order to improve
the strengthening of structures, one of the most common strengthening methods is
wrapping by carbon fiber sheets. (1)
Carbon fibers could be described as high-performance materials. They could be
defined as fibers containing at least 92 % weight of carbon gained by the controlled
pyrolysis of convenient fibers. The carbon fibers have been intensively used because

Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem
they mostly have efficient tensile features, excellent creep resistance, good thermal
and electrical conductivities, high thermal and chemical stabilities, low densities and
the absence of oxidizing agents.(2)
Carbon fibers are used in composites with a lightweight matrix. Where strength,
stiffness, lower weight, and outstanding fatigue characteristics are critical
requirements, carbon fiber composites are perfectly appropriated to applications. Also
carbon fiber composites can be used in the occasion because of the importance of high
temperature, high damping and chemical inertness.(3)
The main target of using carbon fibers as reinforcement is their resistance to
corrosion and rusting. The advantages of using carbon fiber in reinforced concrete
depends on an important factors such as length, shape, cross section, bond
characteristics of carbon fibers and fiber content.(4)
Carbon fiber has been produced with higher flexural strength; higher shear
strength and higher modulus of elasticity, hence enhance the deflection of structural
members. (4)
The main benefits of (CFRP) could be described as increase the durability of
strengthening members against alkalis, aggressive materials and corrosion, achieve
very high strength with tensile strength greater than (2400MPa) and modulus of
elasticity greater than (165 GPa), enhance the fire resistance of structures, saving the
cost of maintains, decrease the construction period, it's light weight as a result its
easily handling and transportation in the form of rolls, CFRP meets the requirements
of using in all wanted lengths , decrease the mechanical fixing and excellent fatigue
resistance. But the disadvantages of CFRP could be described as it's susceptible to
mechanical impacts and the high cost. (5)
CORBELS DIMENSIONS AND REINFORCEMENT DETAILS
It had been casted and tested six concrete corbels, three of them is normal strength
concrete (NSC), and the other corbels is high strength concrete (HSC). All specimens
designed according to shear until failure, see Table (1).
The dimensions of column supporting the both opposite sides of corbels were (150
mm depth x 200 mm width x 650 mm height) , and the concerned column reinforced
at corners with (4ɸ10 mm) deformed longitudinal bars and (8 mm) diameter of closed
ties with spacing (150mm) center to center.
The dimensions of corbels are shown in Figure (1) below, Corbels were reinforced
with (3Ø12mm) of deformed steel bars as a main reinforcement.

Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined
Stripes of Carbon Fiber
Figure 1 Description of Tested Corbels
Table 1 Details of Concrete Corbels
Specimen
Number
Type of Concrete
Mode of Carbon Fiber
Arrangement
C1 NSC ---
C1T NSC Horizontal Strips
C1S NSC Inclined Strips
C4 HSC ---
C4T HSC Horizontal Strips
C4S HSC Inclined Strips
Concrete Compressive Strength
According to (BS1881:Part116)(6)
, the compressive strength test of concrete
( ) was performed using standard 150 mm3 concrete cubes and 150 mm ×
300 mm concrete cylinders respectively. Two types of concrete mixes are used (NSC)
of (28) MPa and (HSC) of (57) MPa compressive strength.
Splitting Tensile Strength (ft)
According to (BS1881: Part 117)(7)
the splitting tensile test had been carried out in
the laboratory by using (150mm*300mm) concrete cylinder, see Table (2).

Table 2 Tensile Strength of Concrete
Type of Concrete Tensile Strength (MPa)
Normal Concrete 1.41
High Strength Concrete 2.92
Flexural Strength (fr)
The flexural strength can be expressed by the modulus of rupture. It is carried out by
using (100*100*500) mm prism, loaded with two point loads hydraulic machine of
(50 kN ) capacity according to (ASTM C78-02)(8)
, see Table (3).
Table 3 Modulus of Rupture of Concrete
Type of Concrete Modulus of Rupture
(MPa)
Normal Concrete 5.16
High Strength Concrete 7.88
Static Modulus of Elasticity (Ec)
The modulus of elasticity is calculated by plotting the stress to strain diagram of
loaded axially cylinder. As recommended by (ASTM C469)(9)
the chord-modulus
method has been used, see Table (4).
Table 4 Static Modulus of Elasticity of Concrete
Type of Concrete Static Modulus of Elasticity (MPa)
Normal Concrete 33205
High Strength Concrete 129159
Testing of Steel Bars
According to ASTM A615 (10)
, the tensile strength test of steel bars was
performed, see Table (5).
Table 5 Properties of Steel Bars
Nominal Diameter
(mm)
Bar Type
fy
(MPa)
fu
(MPa)
Es
**
(GPa)
Elongation
%
8 Deformed 412 591 200 10.8
10 Deformed 404 566 200 10.3
12 Deformed 401 548 200 11.1
Cement
Tables (6) and (7) illustrate the physical and chemical properties of cement used in
this research. The test was performed according to American Specifications ASTM-
C150 (11)
.

Table 6 Physical Properties of Cement
Physical Properties Test Results
Fineness using Blain air permeability apparatus
(m2
/kg)
392
Initial setting time using Vicat's instruments (hr: min.) 1:48
Final setting time using Vicat's instruments (hr: min.) 4:51
Safety (soundness) using autoclave method (%) 0.01
Compressive strength for cement paste cube
(70.7mm) at : (3days) in (N/mm2
) or (MPa)
22.49
Compressive strength for cement paste cube
(70.7mm) at : (7days) in (N/mm2
) or (MPa)
26.6
Table 7 Chemical Composition of Cement
Compound Name
Compound Chemical
Composition
% (weight)
Silica SiO2 19.20
Alumina Al2O3 5.31
Iron Oxide Fe2O3 3.68
Lime CaO 63.77
Magnesia MgO 1.93
Sulfate SO3 2.21
Insoluble Residue I.R. 1.11
Loss On Ignition L.O.I 3.68
Tricalcium aluminates C3A 8.29 (From X.Ray
diffraction)
Lime Saturation Factor L.S.F 0.83
Fine Aggregate
Natural sand with fineness modulus of (2.69) is used. Fine aggregate characterized by
rounded particle shape and smooth textures. Only the passing sand from the sieve
(4.75mm) is requires achieving the requirement of mixing. The grading is shown in
Table (8).
Table 8 The Grading of Fine Aggregate (Sand)
No.
Sieve size
(mm)
Present work of fine
aggregate (% passing)
BS882:1992 limit (%
passing)(12)
1 10 100 100
2 5 92.48 89-100
3 2.36 81.83 65-100
4 1.18 53.46 45-100
5 0.6 62.43 25-80
6 0.3 41.03 5-48
7 0.15 8.22 0-15

Coarse Aggregate
The maximum size of crushed gravel is (14mm). The grading is shown in Table (9),
which confirms to the BS882:1992 Specification(12)
.
Table 9 The Grading of Coarse Aggregate (Gravel)
No.
Sieve size
(mm)
Present work of
coarse aggregate
(% passing)
BS882:1992 limit (%
passing)(12)
1 20 100 100
2 14 93.72 90-100
3 10 73.6 50-85
4 5 4.2 0-10
5 2.36 0 0
Admixtures
In order to produce high strength concrete mixes, super-plasticizer based on poly
carboxylic ether must be used. Also, it can be called (high range water reducing agent
HRWRA). Glenium51 is one of the new generation of polymer which mainly used in
designed super-plasticizer; the normal dosage for Glenium51 is ((0.5-0.8) L/100kg) of
cement. Table (10) illustrates the typical properties of super-plasticizer.
Table 10 Typical properties of Glenium 51
No. Main action Concrete super plasticizer
1 Color Light brown
2 pH. Value 6.6
3 Form Viscous liquid
4 Chlorides Free of chlorides
5 Relative density 1.08 – 1.15 gm/cm3
@ 25C
6 Viscosity 128  30 cps @ 20C
7 Transport Not classified as dangerous
8 Labeling No hazard label required
Carbon Fiber
The technical data information of CFRP sheets and epoxy that used in this work can
be clearly seen from Tables (11) and (12) respectively.
Table 11 SikaWrap Hex-230C (Carbon Fiber Fabric) Technical Data *
Property Results
Fiber type High strength carbon fibers
Fiber orientation The fabric equipped with special weft fibers which
prevent Loosening of the roving (heat set process).
Areal weight 225 g/m2
Fabric design thickness 0.13 mm (based on total area of Carbon fibers).
Tensile strength of fibers 3500 MPa
Tensile E – modulus of fibers 230 GPa
Elongation at break 1.5 %
Fabric length/roll ≥ 45.7 m
Fabric width 305/610 mm

Table 12 Sikadur-330 (Impregnating Resin) Technical Data*
Property Results
Appearance Comp. A: white
Comp. B: grey
Part A+B mixed: light grey
Density 1.31 kg/l (mixed)
Mixing ratio A : B = 4 : 1 by weight **
Open time 30 min (at + 35C◦)
Viscosity Pasty, not flowable
Application temperature + 15C◦ to + 35◦C (ambient and substrate)
Tensile strength 30 MPa (cured 7 days at +23◦C)
Flexural E-modulus 3800 MPa (cured 7 days at +23◦C)
Concrete Mix Proportions
According to ACI committee 211.1-91(13)
, two concrete mixtures were designed.
Anyway, to ensure that the required strengths (28 and 57 MPa) were achieved, many
trial mixes were made. The proportions of the suitable mix are as given in Table (13).
Table 13 Concrete Mixes
DISCUSSION AND RESULTS
General Behavior and Failure Patterns
At the early stages of load application, the specimens appear high stiffness and show
high resistance to loads until appearance of the first crack, the vertical displacement is
small and no cracks appear.
After appearance of the first crack, the stiffness begins to decrease and the vertical
displacement begins to increase. At this stage, the flexural cracks begin to appear at
the tension face of corbels near the column; the cracks are narrow and increase with
load increment.
At the advanced stages of loading, diagonal shear cracks start to develop near the
supports and propagate quickly towards the column face with an angle about 60°.
These cracks are wider than the flexural cracks. The failure is sudden and
uncontrolled except the strengthened specimens with carbon fiber strips and steel
fibers which the failure is more ductile than the un-strengthened specimens. The
plates (1) to plate (6) show the failure mode of tested specimens.
(MPa)
Cement
(Kg/m3
)
Sand
(Kg/m3
)
Gravel
(Kg/m3
)
W/C
Superplastizer
Liter/m3
28 418 542 1200 0.46 ‫ـــــــ‬
57 510 590 1000 0.32 4

Load – deflection Behavior
By studying the load- deflection curve in Figure (2) and Figure (3), it has been found
that all the specimens exhibit three stages as indicated below
1. The first linear stage
This stage started from the beginning of load application with relatively straight line
until the appearance of first crack, this stage represent the elastic behavior of concrete
and steel just because the stresses in concrete and steel virtually small as compared
with later stages, in other words the specimens returned to the original manner after
the load releasing because there is no local slipping between concrete and steel. At
this stage of load – deflection curves, the specimens have the highest value of flexural
rigidity.
2. The second linear stage
In this stage there was a change in the slope of load versus deflection curves in
comparison with pre cracking phase. This stage started from the appearance of first
crack up to the yield of tensile steel bars, this stage characterized by plastic behavior.
In other words, specimens did not return to the original manner after the load
releasing. At this stage, the specimens have less value of stiffness because extension
of the flexural cracks and the loose of bond characteristics between the concrete
Plate (1) Failure Mode of
Corbel (C1)
Corbel (C1T)
Corbel (C1S)
Corbel (C2)
Corbel (C2T)
Corbel (C2S)

matrix and steel bars. Thus, the less toughness of structural member means large
deformations.
3. The non-linear stage
This stage starts at the yield of tensile steel bars until the failure of specimens. At this
stage, the specimens have stiffness less than the previous stages because of the
increasing the number of cracks, width of cracks and loose of bond between steel and
concrete.
Figure 2 Load-deflection Curve of Normal Strength Concrete Corbels
Figure 3 Load-deflection Curve of High Strength Concrete Corbels
0
50
100
150
200
250
300
350
400
450
0 1 2 3 4 5
Corbel - C1-
Corbel -C1S-
Corbel -C1T-
0
100
200
300
400
500
600
700
0 1 2 3 4 5
Load-kN
Deflection-mm
Corbel -C2-
Corbel -C2S-
Corbel -C2T-

STRENGTHENING EFFICIENCY OF CARBON FIBER STRIPS
Effect of Carbon Fiber Strips on carrying capacity
The using of carbon fiber strips in strengthening the concrete corbels effectively
enhances the ultimate strength of the specimens. The improvement of strengthening
corbels with horizontal strips less than the other specimens with inclined strips
because the later are perpendicular on cracks propagation and this configuration play
a vital rule in delaying the extension of cracks such as the specimen C1S which have
an increasing ratio about 60% over the reference specimen C1, while the
improvement of specimen C1T reached to 16.445 in comparison with same reference
specimen. Accordingly, the increasing of the specimen strength of C2T is about
67.167% while the specimen C2S is about 69.667% in comparison with reference
specimen C2, see Table (14).
Table 14 Effect of Carbon Fiber Strips on Carrying Capacity
Specimen No.
Ultimate Loading
Capacity
Percentage of
Improvement (%)
C1 225 ----
C1T 262 16.445
C1S 360 60
C2 300 ----
C2T 501.5 67.167
C2S 509 69.667
Effect of Carbon Fiber Strips on Cracks Appearance
The strengthening mode does not effect on the appearance of first crack in normal
concrete corbels. The amount of improvement is about (63.636%) for specimens C1T
and C1S in comparison with reference specimen C1. But, the improvement of high
strength concrete corbels is clearly seen in specimens (C2T and C2S) which have the
ratio of improvement about 116% and 60% respectively in comparison with reference
specimen C4, see Table (15).
Table 15 Effect of Carbon Fiber Strips on Cracking Capacity
Specimen No.
Ultimate Loading
Capacity
Percentage of
Improvement (%)
C1 110 ----
C1T 180 63.636
C1S 180 63.636
C2 125 ----
C2T 270 116
C2S 200 60
Effect of Carbon Fiber Strips on Energy Absorption
The energy absorption of tested specimens can be calculated from the area under the
load-deflection curve.
The using of carbon fiber strips as a strengthening method improved the energy
absorption of tested corbels. The strengthened specimens of normal strength concrete
C1T and C1S increased about 68% and 237.21% respectively as compared with

reference specimen C1. Also, the high strength concrete corbels C2T and C2S have an
improvement about 13.886% and 36.795% respectively over reference specimen C2,
see Table (16).
The inclined strengthening configuration achieved good energy absorption in
comparison with horizontal configuration.
The amount of energy absorption gives an indication about its ductility; the
greater amount means high ductility. Therefore, the strengthened specimens have a
ductility more than the non-strengthened specimens, and the inclined configuration of
carbon fiber gives ductility greater than the horizontal one.
Table 16 Effect of Carbon Fiber Strips on Energy Absorption
Specimen No.
Ultimate Loading
Capacity
Percentage of
Improvement (%)
C1 934.585 ----
C1T 1570.116 68
C1S 3151.518 237.21
C2 3980.419 ----
C2T 4533.148 13.886
C2S 5445.033 36.795
Conclusions
1. The ultimate carrying capacity of tested corbels is affected positively by using carbon
fiber strips as a strengthening method.
2. The cracking capacity of normal strength concrete corbels does not affected by using
carbon fiber strips, while the high strength concrete corbels achieved good
improvement.
3. The amount of energy absorption increased as a result of using carbon fiber strips.
4. The using of carbon fiber strips does not exchange the failure type of tested corbels.
5. The strengthened corbels by carbon fiber strips have a good stiffness in comparison
with un-strengthened corbels.
REFERENCES
[1] Raghavendra R. H., Atul D. and M. G. Kamath, CARBON FIBERS,
http://www.engr.utk.edu, April, 2004.
[2] Norazman M. N., Mohd H. A. B. and Mohammed A. Y., Carbon Fiber
Reinforced Polymer (CFRP) as Reinforcement for Concrete Beam, International
Journal of Emerging Technology and Advanced Engineering, www.ijetae.com,
ISSN 2250-2459, ISO 9001:2008 Certified Journal, 3(2), February 2013,
[3] Alaa H. Kadhim AL-Musawi, Experimental Study of Reinforced Concrete
Columns Strengthened with CFRP under Eccentric Loading, MS.c. Thesis,
University of Al-Mustansiriya, July-2012. 120p.
[4] Bayda M. H., Stiffness Enhancement of Reinforced Concrete Beams
Strengthened with CFRP Sheeets, MS.c. Thesis, University of Al–Mustansiriya,
August- 2012. 93p.
[5] Committee Euro-International du Beton, 1993, Model Code for Concrete
Structures, Federation International Precontrainte (CEB – FIP) (MC-90), Thomas
Teleford, London, UK.

[6] British Standard Institute, Method for Determination of Compressive Strength of
Concrete Cubes, BS 1881: part116: 1983.
[7] BS 1881-117-1983, Method for Determination of Tensile Splitting Strength,
January 1983.
[8] ASTM C78 – 02, Standard Test Method for Flexural Strength of Concrete (Using
Simple Beam with Third-Point Loading, Annual Book of American Society for
Testing Concrete and Materials, Philadelphia, Pennsylvania, 2014.
[9] ASTM C 469, Standard Test Method for Static Modulus of Elasticity and
[10] Poisson’s Ratio of Concrete in Compression, Annual Book of American Society
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[11] ASTM A615. Standard Specification for Deformed and Plain Carbon-Steel Bars
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Concrete and Materials, Philadelphia, Pennsylvania, 2009.
[12] ASTM C-150, Standard Specification for Portland Cement, ASTM International,
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Engineering and Technology, 6(12), 2015, pp.1–7.
[14] Thallapaka Vishnu Vardhan Reddy, K. Rajasekhar and Seelanani Janardhana,
Study and Performance of High Strength Concrete Using With Nano Silica and
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[15] Aamer Najim Abbas Ali Sabah Ahmed and Saad Khalaf Mohaisen,
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Using Epoxy Injection Technique. International Journal of Civil Engineering and
Technology, 7(3), 2016, pp.31–42.
[16] BS 882-1992, Specification for Aggregate from Natural Source for Concrete,
December 1992.
[17] ACI 211.1-91, Standard Practice for Selecting Proportions for Normal,
Heavyweight, and Mass Concrete, American Concrete Institute, ACI 211.1–91,
Reapproved 2002, 38PP.

STRENGTHENING OF NORMAL AND HIGH STRENGTH CONCRETE CORBELS WITH HORIZONTAL AND INCLINED STRIPES OF CARBON FIBER

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