Influence of fabrics layers on strengthened reinforced concrete short corbels

International INTERNATIONAL Journal of Civil Engineering JOURNAL and OF Technology CIVIL (IJCIET), ENGINEERING ISSN 0976 – AND
6308 (Print),
ISSN 0976 – 6316(Online), Volume 5, Issue 11, November (2014), pp. 33-43 © IAEME
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 5, Issue 11, November (2014), pp. 33-43
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IJCIET
©IAEME
INFLUENCE OF FABRICS LAYERS ON STRENGTHENED
REINFORCED CONCRETE SHORT CORBELS
Ivelina IVANOVA1, Jules ASSIH2, Alex LI3, Yves DELMAS4
1, 2, 3, 4Laboratory of Civil Engineering, University of Reims Champagne-Ardenne, France
33
ABSTRACT
The present paper investigates the experimental study of strengthening reinforced concrete
short corbel by bonding composite carbon fiber fabrics, including the local and global strain
distribution, load-carrying and mode of failure. The influence of some parameters on mechanical
behavior of strengthened structure is examined, as such as, carbon fiber fabric effect with different
number of layer, type of strengthening effect by gluing directly carbon fiber fabrics on concrete
corbel or by wrapping. The ultimate strength and displacement of the specimen are compared to the
reference structure under monotonically increasing static bending load. The cracking and failure
mode are presented.
Keyword: Strengthening, Short Corbel, Reinforced Concrete, Composite Materials, Damage,
Mechanism of Crack and Break.
1. INTRODUCTION
Indeed, reinforced concrete short corbels often are used in civil engineering, in buildings or
structures. Corbel is one important element of steel or concrete structure to support the pre-cast
structural system such as pre-cast beam and pre-stressed beam. Many concrete structures no longer
meet the current safety standards or have excessive cracks. Steel corrosion may also cause
occurrence of high deflection or instability of structure itself. It is generally manifested by poor
performance under service loading in the form of excessive deflections or cracking. Sometimes, even
by inadequate ultimate strength.
The use of composite materials is alternative to carry out a program of strengthening [1]was
much more attractive. Maintenance of civil engineering works is to protect them by ensuring better
sealing or limiting corrosion, to repair them by trying to compensate for the loss of rigidity and
resistance to cracking due to strengthening and improve performance and durability of structures.

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
The problem of increasing concern since the cost of new structures is becoming higher and repair
conditions increasingly difficult [2].
In this paper, the structure is a reinforced concrete short corbel. The corbel is cast monolithic
with the column element or wall element. Corbel is reinforced concrete member [3], who is a short-haunched
cantilever used to support the reinforced concrete structural element. It is interesting to
study the mechanical behavior of this very short element of the structure (corbel) using carbon fiber
materials [4]. Most of the existing research [5],[6]and[7] discussed mechanical behavior of
strengthened reinforced concrete corbels.
34
2. EXPERIMENTAL RESEARCH
2.1 Geometric description of test specimens
The test specimen represents a column supporting two short trapezoidal corbels cantilevering
on either side was 150 by 300mm in cross section and 1000mm long. Corbels had cantilever
projection length of 200mm, with thicknesses of 150mm at both faces of column and free end.
Details and dimensions of corbels are shown in figure 1. All reinforced concrete corbel specimens
have the same dimensions and are reinforced alike. The specimens were tested using a single patch
load with a shear span to depth ratio a/d equal to 0.45 (a is distance of the embedding section at the
bearing point and d is the height of the corbel to steel tie rod point).
a) b)
Figure 1: Test Specimena) Steel reinforcement; b) Details of corbel geometry
2.2 Materials properties
The eight test specimens were made in Laboratory of Civil Engineering, URCA, France.
Normal strength concrete materials are rolled gravel dried sand and ordinary Portland cement. The
cement:sand:gravel proportions in the concrete mix were 1:1.73:2.93 by weight and the
water/cement ratio was 0.50. Portland cement type CEM II was used and the maximum size of the
aggregate was 12.5 mm.
The concrete cubes were cast and tested at 28 days of age by compressing test. The steel
specimens are characterized by simple testing tensile. The stress fc, fu, the modulus of elasticity Es
and Poisson ratio values are in Table 1.

Table 1: Properties of materials
35
Material
Young’s
modulus
(GPa)
Strength
(MPa)
Poisson ratio
Concrete 30±2 33,2±2 (fc) 0,25
Steel bar 200±1 610±10 (fu) 0,30
Adhesive 4,1±1 36±1 (fu) 0,41
CFRP fabric* 86±1 1035±63 (fu) 0,45
*CFRP fabric: Unidirectional carbon fiber reinforced polymer fabric.
The glue used for the CFRP fabric bonding technique are generally two part systems, a resin
and a hardener, and when mixed. Only unidirectional fibers is used. The experimental results
obtained for adhesive and carbon composite unidirectional is showed in table1. The CFRP fabric
have a linear elastic behavior up to failure.
2.3 Preparation of the surface
The surface preparation was of primary importance and calls for care (figure 2). Preparation
of the concrete surfaces must be carried out to remove any loose or weak material, oil, grease etc... In
this case grit blasting was being the good method. The four corners of the corbel are rounded to
reduce the decrease in strength and to prevent tearing of the composite material. Preparation of
surface should be carried out just prior to the bonding operation to prevent any contamination. After,
the contamination can be avoided by applying the glue to the concrete and carbon fiber fabrics is
applied with the brush. The concrete surface has already become roughened and then leveled before
sticking on the wraps using epoxy adhesive. Pressure must be applied to squeeze out excess glue and
held the plate in place until the glue has hardened. Resin and hardened glue is just mixed before the
gluing operation.
Figure 2: Surface of Corbel blasting
2.4 Strengthening configurations
From the large amount of obtained data from tests, only typical data are reported here. The
following classifications have been made according to the number of laid layers. Test specimens
were strengthened with externally bonded CFRP fabric in horizontal form (front and rear face)and in
wrapping form.
The control specimen without strengthening is denoted “C0”, the letter C means the
Corbel, zero 0 indicate without strengthening.

The name of strengthened test specimens is made as follows: The first letter C is, as
previously, Corbel, and the second letter represents thetype of strengthening (e.g.: P for Plate, B for
Bandage). Then, digit indicates the number of layers (e.g.: 1, 2, 3, 5) and finally, the small letter
indicates the type of composite material (e.g. u for unidirectional).
Test specimens were strengthened with externally bonded CFRP fabric to front and rear face
of the corbel (CP1u; CP2u;CP3u), with externally horizontal wrapping around entire corbel
(CB1u;CB2u; CB3u; CB5u).The widths were extended 150 mm cemented to the corbel face. A
specimen was strengthened with unidirectional CFRP fabric in which the fibers resist the applied
load.
36
2.5 Testing procedure
All the corbels are tested under tree-points load (figure 3a). At each test, concrete strain
distribution, strain of CFRP fabric and cracking are noted. Concrete, CFRP fabric and steel strain
were measured at embedding section (where the deformations are high) with strain gauges. The
specimen is submitted to a vertical load equivalent to the response of the bearing which is half of the
load jack. All tests are performed with a loading speed average of 0.2 kN / s. The registration system
system Vishay data is recorded every 0.1 seconds (fig.3b).
(a) (b)
Figure 3: Bending device for strengthened reinforced concrete corbel
a) Test Specimen. b) Vishay data acquisition chain
3. RESULTS AND DISCUSSIONS
From the large amount of data obtained from the tests, only typical data are reported here.
Comparable shear behavior, including the cracking pattern and failure mode were recorded for the
test specimens considering the strengthening adopted method.
3.1 Effect of Strengthening of reinforced concrete short corbel
Test specimens were strengthened with externally bonded CFRP fabricon front and rear face.
The wraps were extended 150 mm cemented bonded to front and rear face. The specimens were
tested to show the CFRP fabric effect on ultimate loads.
Eight concrete corbels were tested. Seven strengthened with carbon fiber fabrics and one
control specimen. Three strengthened by bonded to front and rear face at reinforced concrete corbel
and four were strengthened with different CFRP fabric layers by wrapping respectively, one, two,
three and five layers. The results show an increase in the load of 35 to 82% with a rupture of the
CFRP fabric for one layer.

3.2 Carbon fibers fabrics bonded to front and rear face
In this part of test, specimens were strengthened with externally bonded CFRP horizontal to
front and rear face to corbel specimens. The height of carbon fiber fabric was 150 mm. Different
layers of fabric are used. In Table 2, the ultimate loads of strengthened reinforced concrete corbel,
are noted. The results show an optimum value of carbon fiber fabric thickness to 2mm.
Beyond the second layer of carbon fabric thickness, there is a peeling of fabric. This
phenomenon is accentuated even more when the carbon fabric thickness increases, so the efficiency
of the strengthening is limited by two layers.
Table 2: Influence of number of carbon fiber fabric layers (n= number of layers)
Configuration of concrete corbels 2Fr(kN)
C0 without strengthening 357
CP1u strengthening by fabrics (n=1) 532
37
3.3 Carbon fibers fabrics bonded by wrapping
This group of test specimens was strengthened with externally bonded CFRP horizontal by
wrapping. The height of carbon fiber fabrics was 150 mm wrapping all-around to corbel specimens.
Different layers of fabri care used. The results show the optimum value from the third layer after
from which there is a slight decrease of the maximum tensile strength. The composite sheet 3mm
(three layers of fabric) provides the ultimate optimal loads of strengthening reinforced concrete short
corbels. In addition, the ultimate maximum load decreases as the thickness of the CFRP fabric
increases. When the thickness of the carbon fabric is high, the CFRP fabric becomes more rigid and
causes damage to prematurely concrete corbel. The stiffness of the increases as the fabric thickness
increases. The rigidity was significantly reduced about one-thirds.
Table 3: Influence of number of carbon fiber fabric layers
Configuration of concrete corbels 2Fr(kN)
C0 without strengthening 357
CB1u strengthening by bandage (n=1) 488
3.4 Mechanics behavior of strengthened reinforced concrete corbel
Local behavior investigation by using the strains gauges to measure deformations in the steel,
concrete and carbon fiber sheets is investigated. In this investigation, deformations, cracking and
ultimate failure are studied
The result of extensometer technique based on deformation gauges, strain versus applied load
used to study the local behavior of structure. Strains were measured by strain gauges of 120 W and
gauge factor 2.09. The strain gauges are attached to each of the experimental specimen. Precision
strain gauges length of 5 mm are used to measure the strain on the main steel reinforcement as well
as horizontal frame, steel tie rod placed along the tensile and the compression strut. The curves F = f
() describing the local behavior of the CFRP fabric, steel and concrete material is presented (F is the
applied load and the strain indicated by gauge located at x). The change of slope dF/d corresponds

to the appearance of micro cracks or a change of state. The change of sign of dF/d corresponds to
the initiation of rapid crack propagation around of ultimate failure of the structure.
It appears three main fields: the elastic field, field cracking and recovery loading by the steel
bar, and the last field of rapid crack propagation until the sudden rupture of the specimen. The results
show in figure 4 an increase more than 82% of the maximum load when the reinforced concrete
corbels were strengthened by gluing carbon fabrics. From these curves, there are three main
domains:
- Elastic Domain until the appearance of cracks
- Crack and Propagation Domain, absorbing load by steel tie rods and the carbon fiber
composite plate, marked especially by the appearance of an oblique crack
- Unstable Domain which causes the corbel collapse.
Figure 4: Load-strain curves by means of strain gauges bonded on steel tie rod of unstrengthening
and strengthening reinforced concrete corbels.
The rigidity is improved by strengthening. There are renewed strength as gauge indicates the
two stuck on the drawing from 360kN. The particular configuration of corbel indicates the
occurrence of the opening and the diagonal crack. The results showed that the bonding sheet to the
tensile face of the concrete beams has a greater effect on the first visible crack load. The load to
create the first crack for strengthened beams was about 1.5 times greater than the control specimen.
38
3.4.1 Specimen C0, without CFRP
This specimen was tested as « Control specimen ».Figure 4 shows the local deformation of
the steel rods. The first major crack appeared at 130 kN and she was a vertical crack appearing
approximately at the corbel face close to the column side. The other crack was a diagonal crack
almost at an angel of 45 degrees, this was at a load level 35% of the ultimate failure load diagonal
shear cracks formed at a load level of 240kN. As the load increased, this crack started to widen and
propagated leading to failure. The maximum applied load was 357kN.

3.4.2 Effect of carbon fiber fabric layer on strengthened corbel to front and rear face
Figure 5 shows strain curves of the tested concrete - CP1u, CP2u and CP3u and their
behavior during loading. The first cracks for specimen CP1u, CP2u and CP3u were respectively 140
kN, 145kN and 145kN, but they are very small compared with the first crack in the control corbel
and they are not seen.
Actually in these three reinforced corbels, it is observed only one major diagonal crack
appeared at a load level of 246 kN, 325 kN and 310 kN respectively for specimen CP1u, CP2u and
CP3u. Ultimate load increased to 532 kN, 552 kN and 380 kN.
Three specimens are shared similar failure patterns. Significant increase in the corbel
deflection was observed near failure as shear strain became dominant once the governing inclined
crack has being developed. The strain of flexural main steel markedly increased at this step of
loading. The ultimate loads of failure are significantly different because when CP3u observed
debonding of composite material, while CP1u observed splitting.
This strengthening method showed the ultimate load in the specimen CP2u. In section cracks
in figures 5 illustrate the load-strain curves of strain gauges of the composite plate and concrete of
strengthened reinforced concrete short corbel as CP1u, CP2u and CP3u.
Specimen CP2u similar failure patterns. Flexural cracks were observed first, at approximately a
load level 145 kN and then a few diagonal cracks were observed. With increasing load the flexural
cracks grew upward and became wider. The first major crack appeared at a load level of 325 kN and
ultimate load increased to 552kN. The figure 5 shown curves of load-strain relationship for corbels
CP1u, CP2u and CP3u.
Figure 5: Comparison of load-strain relationship for corbels C0, CP1u, CP2u and CP3u
3.4.3 Effect of carbon fiber fabrics layers on strengthened corbel by wrapping
The figure 6 shows the comparison of curves obtained by gauges “G1” (steel tie local
deformation) for four different corbels. These curves are compared to reference reinforced concrete
corbel without strengthening. The results show that curves are similar and ultimate load increases
with a 1/3 reduction of strains. The curve of the three layer (CB3u) presents less strain (2000
μm/m) and highestultimate load 651 kN.
39

Figure 6: Comparison of load-strain relationship for corbels CB1u, CB2u, CB3u and CB5u
40
4. MODES OF FAILURES
Specimen C0, without CFRP fabric – This specimen was tested as « Control specimen ». The
ultimate load failure was 357kN. Type of failure for this corbels is classic failure shear with two
major cracks, as depicted in figure 7. The first major crack appeared at 60 kN and it was a vertical
crack appearing approximately at the corbel face close to the column side. The other crack was a
diagonal crack almost at angel of 45 degrees with load level at 43% of the ultimate failure load. The
diagonal shear crack appeared at a load level of 235 kN. As the load increased, this crack started to
widen and propagated leading to failure (figure 7).
C0 - Splitting failure C0 - Detail
Figure 7: Mode of failure in corbel C0
Specimen CP1u – For to enhance the ultimate load capacity this specimen was strengthened with
externally bonded one horizontal strip of CFRP fabric. The first cracks are very small and are not
detected. Only one major diagonal crack started at the bearing plate, and propagated towards the
junction of the column and face of corbel. This crack appeared at a load of 246kN and causes failure
of corbel. The corbel failed at an ultimate load of 532 kN with an increase in strength of about

(49%) compared to unstrengthened corbel specimen C0 (control corbel). The results show a rupture
of carbon fiber fabrics at 532kN(figure 8).
Specimen CP2u - The ultimate load capacity this is specimen was strengthened with externally
bonded two horizontal strip of CFRP fabric. Only one major diagonal crack started at the bearing
plate, and propagated towards the junction of the column and face of the corbel. This crack appeared
at a load of 325kN and causes collapse of the corbel. The corbel failed at an ultimate load of 552 kN
with an increase in strength of about 55% compared to unstrengthened corbel specimen C0. The
results show a specific rupture by splitting and CFRP fabric peeling off, which explains the higher
value of the maximum capacity (figure 8).
Specimen CP3u – This specimen was strengthened with externally bonded tree horizontal CFRP
fabric. As with the above specimen CP1u and only one major diagonal crack started at the bearing
plate, and propagated towards the junction of the column and face of the corbel. This crack appeared
at a load of 310 kN and caused corbel failure. The corbel failed at an ultimate load of 380 kN. When
the number of CFRP fabric increases, the rupture of corbels is caused by peeling off the strips of
cloth. The accumulation of several strips of fabric on top of each other results for high thickness of
CFRP fabric, the peeling off easily cannot reach to the maximum load-bearing capacity of the
composite carbon fiber fabric. No apparent damage the fabric, just cracked concrete with major
diagonal shear crack beneath the fabric. The concrete crack induced by debonded fabric on the front
and rear face of the corbel (see Figure 8).
Carbon fibers fabrics bonded to front and rear faces
CP1u - Fabric failure
CP2u - Splitting and CFRP
fabric peeling off
41
CP3u - CFRP fabric peeling
off
Figure 8: Modes of failure in strengthened corbel CP1u; CP2u; CP3u

Specimens strengthened by wrapping-.Study the effect of strengthening reinforced corbel with
horizontal externally bonded carbon fiber reinforced polymer (CFRP) fabric wrapping several corbel
specimens CB1u, CB2u, CB3u and CB5u with one, two, three and five layers. The failure and crack
patterns of the group strengthening with externally bonded one horizontal strip of fabric shown in
figure 9.
In all four cases in this group the first micro cracks started at approximately the same load
between 130kN and 140kN. There is one main diagonal shear crack almost at angel 45 degrees and
this crack started at the bearing point. This crack caused failure of the corbel. The corbels failed at
ultimate loads were 488kN, 508kN, 651kN and 626 kN for CB1u, CB2u, CB3u and CB5u
respectively. Figure 9 show different failures of corbels.
Carbon fibers fabrics bonded by wrapping
42
CB1u- CFRP
fabric failure
CB2u -
Compression
failure
CB3u - Shearing and
Splitting failure
CB5u - Compression
failure with two
diagonal cracks
Figure 9: Modes of failure in stregthened corbel CB1u; CB2u CB3u; CP5u
5. CONCLUSION
The contribution of strengthening reinforced concrete short corbels is very significant and
interesting. The result showed an increase in failure tensile strength more than 82% by bonding
CFRP fabric.
This experimental study of the influence of CFRP fabric thickness showed that the plate of 2
layers of fabric bonded to front and rear face allowed to obtain an optimum value of the ultimate
load. The results of strengthening by wrapping, show the optimum value from the third layer after
from which there is a slight decrease of the maximum flexion strength. The thick CFRP fabric of
3mm (tree layers of fabrics) provides the ultimate optimal load of strengthened short reinforced
concrete corbels. It is the optimum value from the third layer from which the ultimate load is a slight
decreased. Result show that strengthening by wrapping of fabrics remains still the best configuration.
The effect of the containment by wrapping provides higher values than those at which the CFRP

fabrics are glued directly on the front rupture loads. The results show also three main domains:
Elastic domain, Crack and propagation domain and Unstable domain.
In conclusion, the results show 5 modes of failure. For the eight strengthened reinforced
concrete short corbel, mainly the following failures are observed:
- Rupture of sheet and a rupture of the concrete layer situated between the reinforced steel and
43
CFRP fabric.
- CFRP fabric peeling off.
- Splitting failure and Compression which often causes a tearing of CFRP fabric or the failure of
rupture of the composite material.
- Failure bending strength shear whit one splitting failure.
- Failure by compression with two diagonal cracks.
REFERENCES
1. Assih T., Li A., Delmas Y. (1997), “Strengthening of Reinforced concrete beams with carbon
fibre composite plate”, EUROMECH358.
2. Chris Burgoyne and Ioannis Balafas (2007), “Why is FRP not a financial success”, FRPRCS-
8 University of Patras, Greece, July 16-18.
3. Khadraoui A.(1998), “Analyse et Modélisation du comportement mécanique des consoles
courtes en béton armé”. Thèse, Université de Reims(France).
4. Ivanova I.(2013), “Mechanical behavior of short concrete corbels reinforced or repaired by
bonding of composite materials”, PhD thesis, URCA, France.
5. Anis A. Mohamad-Ali and Muhammad Abed Attiya (2012), “Experimental behavior of
reinforced concrete corbels strengthened with carbon fibre reinforced polymer strips”, Basrah
Journal for Engineering Science.
6. Rejane Martins Fernandes Mounir Khalil El Debs, (2005), Análise da capacidade
resistente de consoles de concreto aramado considerando a contribuição da armadura de
costura. Cadernos de Engenharia de Estruturas, São Carlos, v. 7, n. 25, p.103-128.
7. Erfan, A. M., (2010), Behavior of Reinforced Concrete Corbels Strengthened with CFRP
Fabrics, M.Sc. Thesis, Department of Civil Engineering, Benha University, Shoubra, Egypt.

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