Ijciet 06 10_003

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 6, Issue 10, Oct 2015, pp. 25-35 Article ID: IJCIET_06_10_003
Available online at
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
ANALYSIS FOR BEHAVIOR AND
ULTIMATE STRENGTH OF CONCRETE
CORBELS WITH HYBRID
REINFORCEMENT
Prof. Dr. Ammar Yaser Ali and Ahmed Mohammed Mahdi
Civil Engineering Dept. Collage of Engineering
University of Babylon, Iraq
ABSTRACT
This research is devoted to investigate the behavior and ultimate strength
of concrete corbels with hybrid reinforcement (steel and CFRP) bars subject
to vertical distributed applied load only. Experimental investigation were
carried on eighteen specimens and divided in two groups according to
location of hybrid reinforcement. The experimental program variables include
ratio of hybridization, location of hybrid reinforcement, and shear span-to-
effective depth (a/d) ratio. All specimens had the same dimensions and tested
as upside down double-symmetrical concrete corbels.
The experimental program results obtained from the adopted hybridization
reinforcement technique showed that a significant improvement in the
behavior and carrying capacity of the tested corbels with an increase about
(5.6% - 44%), and (2.3% - 20%) in the ultimate load with (a/d) (0.75, and
0.33) respectively, for specimens with main tension hybrid reinforcement with
ratio (20, 40, 60, 80, and 100) %. For specimens hybrid in horizontal (closed
stirrup) reinforcement, when increase the hybridization ratio of secondary
reinforcement from (0 to 50) % and from (0 to 100) %, the ultimate shear
strength will increase by about (16%, 10%) and (6.4%, 15.5%) with shear
span-to-effective depth ratio (0.75, and 0.33) respectively. The increase in
strength and stiffness was realized at the expense of a loss in ductility due to
the brittle behavior of CFRP bars. The presence of CFRP bars increased the
deflection and crack width at service state, and increased the concrete strain
with increasing the hybridization ratio, this was expected due to lower the
modulus of elasticity and bond strength of CFRP bars compared with steel
bars.
Keywords: Corbel, Behavior and ultimate strength, Hybrid reinforcement and
Steel with CFRP bars.

Cite this Article: Prof. Dr. Ammar Yaser Ali and Ahmed Mohammed Mahdi.
Analysis for Behavior And ultimate Strength of Concrete Corbels with Hybrid
Reinforcement. International Journal of Civil Engineering and Technology,
6(10), 2015, pp. 25-35.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
1. INTRODUCTION
Corbels are short cantilevers with a shear span to depth ratio lower than unity which
tend to act as deep beams or simple trusses rather than flexural members [1], they are
generally built monolithically with the columns or walls. They are also used
particularly in precast structures where their principal function is the transfer of
vertical and horizontal forces to supporting principal members.
Corbels are principally designed to resist the ultimate shear force Vu applied to
them by the beam and they behave like short cantilevered deep beam so the common
behavior is governed by shear rather than flexural. The applied loads are transferred
predominately through shear, because of the usually low shear span to effective depth
ratio. The mechanical behavior of concrete corbels at failure may be either a flexural
failure or beam-shear failure after yielding of the reinforcement. The ultimate strength
of a corbel can be calculated by taking it to be lesser of (a) shear strength of the corbel
interface, which can be calculated using the shear friction theorem; and (b) vertical
load that cause to the development of the flexural ultimate strength of the corbel-
column interface [2].
FRP internal reinforcement is widely used in commercial applications as an
alternative to conventional steel reinforcement primarily to enhance the corrosion
resistance of reinforced concrete structures. Three important physical characteristics
of fiber reinforced polymer materials must be considered: high-tensile strength; low-
modulus of elasticity; and linear-elastic brittle behavior to failure [3]. CFRP bars
using in the present study with steel bars as hybrid reinforcement in primary or
secondary reinforcement.
2. EXPERIMENTAL PROGRAM
A total of eighteen corbels were tested under vertical distributed applied load. The
experimental study consisted of two test groups. Group (A) included main hybrid
reinforcement with percentage (20%, 40%, 60%, 80%, and 100%), while Group (B)
included horizontal (closed stirrup) hybrid reinforcement with percentage (50%, and
100%). The two (a/d) ratios evaluated were (0.75), and (0.33) used in each one of the
test groups (A and B).
As shown in Figure (1), the column supporting the two corbels cantilevering on
opposite side was 200mm by 180mm in cross section and 575mm long. Corbels had
cantilever projection length of 300mm, 180mm width, and total depth of 275mm at
face of column and 150mm at the free end, and the effective depth 240mm with shear
span 180 mm, or 80 mm. Columns were reinforced with four deformed bars having a
16mm diameter and stirrups having a 6mm diameter placed at pitch of 125mm. The
primary reinforcement (main bars) having diameter 6mm of steel and/or CFRP bars
with varying ratios of hybridization, placed at the top of the corbel with an effective
cover of 35mm. Main bars were welded with cross bar of 8mm diameter , near the end
of each corbels, to provide additional anchorage. The horizontal closed stirrup having
diameter 6mm of steel and/or CFRP bars with varying ratios of hybridization. These
closed stirrups were anchored by framing bars of diameter 6mm.

Analysis for Behavior And ultimate Strength of Concrete Corbels with Hybrid Reinforcement
Figure 1 Details of reinforcing and geometry of tested corbels
The hybridization processes are replaced part of the steel bars by CFRP bars with
an equal area to improve properties of reinforcement system and benefit from non-
metallic material in the present study field. Hybridization process used an equal area
of reinforcement and symmetry distribution to easy comparison, and explored the
effect of considered variable. Two pilot, two control corbels (homogenous) and
fourteen hybrid reinforcement corbels are tested with deferent (a/d) ratio (0.75, and
0.33). Twelve of them hybrid in main tension reinforcement and the other in
horizontal (closed stirrup) reinforcement. Figure (2) shows hybridization processes in
these groups.

a) Hybridization in Main Reinforcement.
b) Hybridization in Horizontal Reinforcement.
Figure 2 Hybridization processes
3. MATERIALS
In the experimental program, (Ø6mm) deformed, mild steel bars and (Ø6mm) of
CFRP bars are employed as tension reinforcement and closed stirrup. Because of the
need to weld and bend in the reinforcement details of corbels according to the
requirements of ACI-code 318-11, and the inability to do that with the CFRP bars, we

searched for a way that led us to do that, galvanized steel clamp was the acceptable
way for this purpose. Ordinary portland cement (Type I) from Iraq plant named
TASLUJA. Crushed gravel from Al-Nibaey region with maximum size of (14 mm).
Natural sand from AL-NAJAF city in Iraq with maximum size of (4.75 mm) and
fineness modulus of (2.46) [4].
Normal strength concrete was used to cast all specimens. Normal strength
concrete mix was designed in accordance with ACI-211 mix design with nominal
compressive strength of about (30MPa). In order to select the mix proportion for the
concrete used in preparing the reinforced concrete corbels, three trial mixes were
carried out in order to obtain cylinder strength of (30MPa) after age 28-days.The final
mix used was 1:1.8:2.3 by weight. The water cement ratio was equal to 0.53 and
cement content was 407 kg/m3.
4. TEST MEASUREMENT AND INSTRUMENTATION
The hydraulic universal testing machine has a capacity of (2000 kN) was used to test
the corbel specimen, as shown in Plate (1). The deflections were measured by means
of (0.01 mm) accuracy dial gauge. Strain of concrete measured used demic point and
dial gauge with accuracy of (0.001 mm).
5. TEST PROCEDURE
All corbels were painted with white color to observe the crack development and
marked, demic discs were fixed on marking location. At first, the specimens loaded
by 5 kN to seat the support and the loading system, then unloading to zero. The load
increment was 15 kN along the test. The deflection corresponding to the applied load
was measured at every load step at center of column. Also, recording the first crack
load, the ultimate load, and the concrete strain were measured, for each corbel. Finally
the maximum crack width was measured at the end the test by crack meter.
Plate 1 Testing Machine

6. EXPERIMENTAL RESULTS
The overall behavior and strength of sixteen corbels reinforced with steel bars and/or
CFRP bars will be investigated and discussed. During the experimental work, load
versus deflection, first cracking load and ultimate loads, cracking patterns, maximum
crack width, concrete strains and modes of failure were recorded for each tested
corbel specimen. Table (3) shows first crack, ultimate load, ultimate deflection, and
mode of failure.
Table 1 Cracking load, ultimate load and failure modes of the tested corbels
6.1. Deflections and Cracks Pattern for Specimens with Main Hybrid
Reinforcement
Load-deflection curves of the tested corbels and cracks pattern at all stages of loading
up to failure were constructed and shown in Figures(3) and (4), and Plate(2).

Figure 3 Load-deflection curve for specimens with main hybrid reinforcement and (a/d=0.75)
Figure 4 Load-Deflection Curve for Specimens with Main Hybrid Reinforcement and
(a/d=0.33)

Plate 2 Cracks pattern for specimens with main hybrid reinforcement
6.2. Deflections and Cracks Pattern for Specimens with Secondary Hybrid
Reinforcement
Load-deflection curves of the tested corbels and cracks pattern at all stages of loading
up to failure were constructed and shown in Figures(5) and (6), and Plate(3).
Figure 5 Load-deflection curve for specimens with secondary hybrid reinforcement and
(a/d=0.75)
Figure 6 Load-deflection curve for specimens with secondary hybrid reinforcement and
(a/d=0.33)

Plate 3 Cracks pattern for specimens with secondary hybrid reinforcement
7. CONCLUSIONS
Based on the experimental testing results and the theoretical analysis results obtained
by ANSYS Program version (14.5) for the models of reinforced concrete corbels with
hybrid reinforcement together with parametric study, the following conclusions can
be stated within the scope of this study:
The crowded corbel reinforcement according to the requirements of ACI- 318-11
code provisions and the small size of corbels when compared with other structural
elements make the use of hybrid reinforcement better solutions for this problem.
The problems associated with the brittle nature of CFRP bars can be overcome by
combining CFRP and steel bars to take advantage of features with collected together
in hybrid reinforcement technology.
Presence of CFRP bars as an alternative to steel bars led to increase the ultimate
shear strength of concrete corbels by about (5.6 - 44) %, and (2.3 - 20) % for
specimens with hybrid in main reinforcement only (group A) with (a/d) (0.75, and
0.33) respectively when an increase in the hybridization ratio (20-100)%.
When increase the hybridization ratio of secondary reinforcement from (0 to 50)%
and (0 to 100)%, the ultimate shear strength will increase by about (16% , 10%) and
(6.4% , 15.5%) for span of shear-to-effective depth ratio (0.75, and 0.33),
respectively.
The first cracking loads were increased by about (0 - 43.4)% and decreased by
about (0 - 18.6)% with increasing hybridization ratio (20 - 100)% for specimens

hybridization in main reinforcement only with (a/d) (0.75, and 0.33) respectively and
decreased by about (18.6%) and (13.9%) for specimens hybridization in closed stirrup
only with (a/d) (0.75) and a little effect in first crack load with (a/d) (0.33) when
increased the hybridization ratio (50% to 100%).
For concrete corbels with hybrid reinforcement in main tension and with (a/d)
(0.75), the mode of failure altered from tension to compression flexural failure
followed by diagonal splitting with increasing the hybridization ratio. While the
corbels with (a/d) (0.33), the mode of failure classified as shear-friction failure
without changing.
For concrete corbels with hybrid reinforcement in horizontal closed stirrup and
with (a/d) (0.75), the mode of failure altered from flexural tension failure to premature
diagonal splitting failure with increasing the hybridization ratio. While the corbels
with (a/d) (0.33), the mode of failure classified as shear-friction failure without
changing.
Deflection at service loads increase for all specimens with increase the
hybridization ratio, due to brittle behavior, lower modules of elasticity, and lower
bond strength for CFRP bars, in sup-group AI (main hybrid reinforcement with (a/d)
(0.75)) the deflection increasing (18.75% - 125%) and for sup-group AII (main hybrid
reinforcement with (a/d) 0.33)) the increasing (18.4% - 67.3%), for sup-group BI, and
BII (secondary hybrid reinforcement with (a/d)(0.75, and 0.33) the increasing (20% -
22.5%), (12.2% - 22.4%), respectively with increase the hybridization ratio (50% to
100%).
The ultimate shear strength predicted by the numerical analysis were close to that
measured during experimental testing with maximum difference (2.8%) as average.
The first cracking load obtained from numerical data showed results lower than
the experimental data recorded with difference about (8.75%) as average.
8. REFERENCES
[1] ACI Committee 318, 2011, Building Code Requirements for Structural Concrete
(ACI 318-11), American Concrete Institute, Farmington Hills, USA, pp. 190-
194.
[2] Mattock, A. H., Chen, K.C, and Soongswang, K., 1976, The Behavior of
Reinforced Concrete Corbels, Journal of PCI Journal, March, pp. 53-77.
[3] ACI Committee 440, 2006, Guide for the Design and Construction of Concrete
Reinforced with FRP Bars (ACI 440.XR), American Concrete Institute,
Farmington Hills, pp. 100-106.
[4] Javaid Ahmad and Dr. Javed Ahmad Bhat. Ductility of Timber Beams
Strengthened Using CFRP Plates. International Journal of Civil Engineering and
Technology, 4(5), 2013, pp. 42 – 54.
[5] Iraqi Specification No.45, 1984, Natural Sources for Gravel that is used in
concrete and construction, Baghdad, 1984.
[6] Iraqi Specification No.5, 1984, Portland cement, Baghdad, 1984.
[7] Yaman S.S. Al-Kamaki, Riadh Al-Mahaidi and Azad A. Mohammed. Behavior
of Concrete Damaged By High Temperature Exposure and Confined with CFRP
Fabrics. International Journal of Civil Engineering and Technology, 5(8), 2014,
pp. 148 - 162.

Ijciet 06 10_003

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