Strengthening of Cutouts in Existing One-Way Spanning R. C. Flat Slabs Using CFRP Sheets
International Journal of Concrete Structures and Materials
Strengthening of Cutouts in Existing One-Way Spanning R. C. Flat Slabs Using CFRP Sheets
Hamdy K. Shehab 0
Ahmed S. Eisa 0
Kareem A. El-Awady 0
0 Structural Engineering Department, Faculty of Engineering, Zagazig University , Zagazig , Egypt
Openings in slabs are usually required for many different applications such as aeriation ducts and air conditioning. Opening in concrete slabs due to cutouts significantly decrease the member stiffness. There are different techniques to strengthen slabs with opening cutouts. This study presents experimental and numerical investigations on the use of Carbon Fiber Reinforced Polymers (CFRP) as strengthening material to strengthen and restore the load carrying capacity of R.C. slabs after having cutout in the hogging moment region. The experimental program consisted of testing five (oneway spanning R.C. flat slabs) with overhang. All slabs were prismatic, rectangular in cross-section and nominally 2000 mm long, 1000 mm width, and 100 mm thickness with a clear span (distance between supports) of 1200 mm and the overhang length is 700 mm. All slabs were loaded up to 30 kN (45% of ultimate load for reference slab, before yielding of the longitudinal reinforcement), then the load was kept constant during cutting concrete and steel bars (producing cut out). After that operation, slabs were loaded till failure. An analytical study using finite element analysis (FEA) is performed using the commercial software ANSYS. The FEA has been validated and calibrated using the experimental results. The FE model was found to be in a good agreement with the experimental results. The investigated key parameters were slab aspect ratio for the opening ratios of [1:1, 2:1], CFRP layers and the laminates widths, positions for cutouts and the CFRP configurations around cutouts.
cutouts; flat slab; reinforced concrete; strengthening; CFRP; bonding; line loads; numerical analysis
The ACI 318-14 code allows reinforced concrete slabs to
have openings with the condition of performing full
structural analysis to assure slab safety, strength, and
serviceability under different expected loads. Whereas the ACI
318-14 (ACI 318-14) Code gives procedures and limits for
opening location and size. If designer satisfies those
requirements the analysis could be abandoned, hence,
problem becomes more complex when openings are planned
to be made in existing slab, the most common way to
substitute additional steel reinforcement is to apply CFRP
strengthening before cutting a hole. The ACI 318-14 (ACI
318-14) recommends the size and location of openings in
two-way slab systems as shown in Fig. 1.
Today, the use of carbon fiber reinforced polymers (CFRP)
as external reinforcement to strengthen existing slabs due to
openings is becoming more popular, partly due to ease of
installation and partly due to space saving. In these situations,
CFRP sheets are applied to the slab before the opening is made
even though CFRP is used for strengthening of openings, very
few studies on the structural behavior of slabs with openings
have been carried out. Casadei et al. (2003) ANSYS (2011)
reported a series of tested one-way slabs with openings
strengthened with CFRP. The slabs had openings at the centers
and in areas close to the supports. Most of the reported work
was related to the strengthening of slab openings at the
positive moment regions. It was found that the presence of
openings in the negative moment areas usually increases the shear
stresses (Casadei et al. 2003). Tan and Zhao (2004) performed
a study incorporated strengthened slabs with symmetric and
asymmetric openings. The strengthened slabs showed equal or
higher capacity of the control slabs with openings. Most of the
slabs showed flexural failure mode whereas some of them
showed a different failure pattern where the cracks extended
from the opening corners. One of the Tan et al. (2004) findings
was related to the failure mode dependency on the opening
location. Slabs with openings placed in the maximum moment
region failed in flexural mode while openings located in the
shear region failed in shear mode.
Enochsson et al. (2007) tested two-way slabs strengthened
with CFRP and the results showed that the stiffness and
ultimate load of the slabs with large opening is higher than
the small opening with the same opening locations. Those
results could be attributed to the equivalency of larger slabs
to hidden beams (Enochsson et al. 2007). Mota and Kamara
(2006) presents a particularly detailed review of forming
cutouts in two-ways slab systems. A lower-bound analytical
model provided herein serves as an alternative form of
practical analysis of FRP-strengthened slabs with cutouts. In
addition, it considers the FRP effect and associated failure
modes. The analytical method is essentially a strip method of
analysis, and is based upon the ultimate moment of
resistance provided by the slab along critical crack lines in any
direction It considers longitudinal and transverse slab
behavior, and is found to correlate well with existing and
current test data provided certain assumptions are made for
the calculation of the sectional strength and position of the
Vasquez and Karbhari (2003) showed that the appropriate
design of the strengthening measure enables capacity
reduced by the presence of the cutout to be regained while
mitigating and retarding crack growth. Ultimate failure was
through a sequence of cracking and debonding of the FRP
composite reinforcing strips with a decrease in load capacity
after debonding to the response level of the unstrengthened
slab with a cutout after yield of the steel reinforcement. More
information about reinforced concrete slabs with cutouts
strengthening could be found in Mosallam and Mosalam
(2003), Ozgur et al. (2013).
Muhammed (2012), tested eight self-compacting concrete
slabs. Results showed that, the use of CFRP strips is more
effective than the steel fiber, use of steel fiber increased the
load capacity by 26.67 and 9.83% for small and large
opening respectively, while CFRP increased the load
capacity by 46.67 and 55.7% for small and large opening
respectively, CFRP and steel fiber reduced the cracks at the
inside faces of the opening while CFRP prevent it at the
inside corners of opening. Smith and Kim (2009) reported
the results of strengthened one-way slabs with FRP cutouts
at their centers. Four slabs with cutouts were tested in
addition to two slabs without cutouts. The effect of different
load application positions was investigated, in addition to
distribution of stresses around the cutout. All
FRPstrengthened slabs failed by de-bonding, however, the extent
of de-bonding and the ability of the slab to sustain load
postinitiation of de-bonding was dependent on the position of the
load. The slab in which the line load was located adjacent to
the cutout exhibited transverse bending action and as a result
was able to withstand more extensive de-bonding prior to
loss of load-carrying enhancement from the FRP.
Sorin-Codrut et al. (2015) studied two-way simply
supported reinforced concrete slabs subjected to a uniformly
distributed load. Slabs with strengthened and
non-strengthened openings have been investigated. CFRP sheets have
been used for the strengthening. Focusing on examining the
structural behavior of two-way RC slabs strengthened with
CFRP due to a sawn-up opening, test results clearly showed
that the investigated strengthening system can be used to
strengthen existing slabs with made openings, and even that
the load carrying capacity can be increased when compared
to the homogeneous slab, The slabs with the larger openings
have a noticeable higher load carrying capacity and a stiffer
load–deflection response than the slabs with the smaller
openings. In addition to that, it was stated that the ultimate
load of strengthened slabs with the cutouts increased by
The existence of openings in slabs can also degrade the
inplane capacity and stiffness of when they are subjected to
inplane/earthquake loads. Khajehdehia and Panahshahib
(2016) conclude that that presence of openings clearly
changed the in-plane behavior of RC slabs compared to
those of slabs without openings and that this
oversimplification in design and analysis of slabs by ignoring the
opening effects might lead to erroneous results, Song et al.
(2012). Tested three isolated interior flat slab-column
connections that include three types of shear reinforcement
details; stirrup, shear stud and shear band were tested under
reversed cyclic lateral loading to observe the capacity of
slab-column connections. The results were applied to the
eccentricity shear stress model presented in ACI 318-08. The
failure mode was defined by considering the upper limits for
punching shear and unbalanced moment. In addition, an
intensity factor was proposed for effective widths of slabs
that carry an unbalanced moment delivered by bending.
The main aim of this study is to investigate the behavior of
reinforced concrete one-way flat slabs with cutouts. The
cutouts were made during slab loading which represents
slabs under service conditions. The data generated in this
paper are mainly came out of testing five slabs
experimentally under various key parameters and by using finite
2. Experimental Program
2.1 Description of Tested Slabs
The experimental program consisted of testing five
(oneway spanning R.C. slabs) with an overhang. All slabs were
prismatic, rectangular in cross-section and nominally
2000 mm long 9 1000 mm width 9 100 mm thickness with
a clear span (distance between supports) of 1200 mm and a
cantilever 700 mm long. All slabs were reinforced with
10 mm diameter steel bars top and bottom spaced at 160 mm
Table 1 Description of tested specimens.
Cutout aspect ratio
Cutout size (mm)
Use of CFRP sheets
in both directions. Description and details of tested specimens
are listed and shown in Table 1 and Fig. 2, respectively.
In this study, locally produced materials are used in all
concrete mixtures, coarse and fine aggregates are composed
of harsh desert sand, free from impurities, crushed dolomite
from Ataka near Seuz Canal zone, ordinary Portland cement
locally produced, and tap drinking water. High-grade steel
locally produced was used as reinforcement; tests were
carried out to determine the properties of the used materials
according to the ASTM standard specifications. The
characteristics of the strengthening materials (CFRP sheets and
its impregnating resin) were taken from the manufacturing
company product data sheets as well as the instructions of
the installation process. Tables 2 and 3 show the mechanical
properties of steel bars and the CFRP used in this study.
Concrete cubes were taken from all mixtures to track the
concrete compressive strength. The results of the tested
control cubes for the concrete mix reached the required
compressive strengths = 31 N/mm2 at 28-days.
Fig. 2 Details of tested specimens.
Table 2 Mechanical properties of steel bars.
Yield load (kN)
Ultimate load (kN)
Yield stress (N/mm2)
Ultimate strain (%)
Areal weight (±10)
Adhesive strength on concrete
Tensile strength of fibers (nominal)
Tensile E-modulus of fibers (nominal)
Strain at break of fibers
Fabric design thickness
Tensile E-modulus in flexural
Table 3 Physical and mechanical properties of Sikawrap Hex -230C.
Sikawrap Hex -230C properties
CFRP unidirectional properties
230 ± 10 (g/m2)
Fig. 3 Measuring instruments (strain gages and LVDTs) on the sides of the tested slabs.
Fig. 4 Loading arrangement and test setup.
2.3 Sensors and Measurements
The specimens were instrumented to record the strains of
concrete, steel bars and CFRP sheets as well. The load and
deflection of all specimens were instrumented. The strain
gages data were collected using a data logger system. Three
electrical strain gages (S1, S2, and S3) were installed on top
layer steel bars and two on the bottom concrete surface;
strain gages (C1 and C2) as shown in Fig. 3. Four electrical
strain gages were installed on top surface of the CFRP
sheets, strain gages (F1, F2, F3, and F4), two in the
longitudinal direction and the other two on the transverse
direction. Deflection was measured using Linear Variable
Distance Transducers (LVDT). Two LVDTs were installed
on the lower side of each slab (LV1 and LV2).
Fig. 5 Cutting concrete and steel bars.
Table 4 Ultimate loads and failure modes of tested slabs.
Ultimate load (kN)
the designed sizes as shown in Fig. 5, after that, the load was
increased till failure.
3. Experimental Results and Dissusion
3.1 Flexural Failure Mode
The modes of failure of the tested slabs are listed in
Table 4, and the crack patterns are shown in Fig. 6. Two
modes of failure were observed during the tests; the control
slab had a cracks parallel to the support line and at the
maximum negative moment region and experienced flexural
failure. Slabs S1 and S2 showed initially flexural cracks
occurred at the maximum negative moment region, parallel
to the support line and then diagonal cracks originated from
each corner of the cutout. As the load was increased,
additional flexural cracks formed and became wider, especially
the main diagonal cracks from each corner of the cutout as
shown in Fig. 6.
3.2 Rupture of CFRP Sheets
Failure mode of the strengthened slabs S3 and S4 was
mainly due to the rupture in the CFRP sheets. However, the
rupture was preceded by steel yielding. The load carrying
capacity at failure was relatively larger for the CFRP
strengthened slabs. No significant differences in crack
patterns between strengthened and un-strengthened slabs was
Rupture of CFRP sheets
Rupture of CFRP sheets
2.4 Loading Arrangement and Test Procedure
All slabs were tested using a hydraulic machine of 25 ton
capacity under three-point bending, the load was applied
using a spreader I-beam as shown in Fig. 4. The spreader
beam distributes this load on two rigid steel Sects. (500 mm
long) as shown in Fig. 4. All slabs were loaded up to 30 kN
service load (45% of ultimate load for reference slab, and
before reaching the steel yielding stress) and that load was
kept constant. While the service load was applied constantly,
the opening were made at the predefined locations and with
Fig. 6 Crack patterns of tested slabs.
observed, the extent of cracking from the corners of the
cutout to the slab edges become not wider in strengthened
3.3 Behavior of Tested Slabs
Based on the experimental results, the behavior of the
tested slabs is discussed in terms of observed crack patterns,
ultimate load, measured deflection, and measured strains at
different locations along the reinforcing steel bars, concrete
surface, and CFRP sheets. The relationships between the
applied load, deflection, and the longitudinal strains for
concrete, steel, and CFRP of the tested slabs were typical for
all tested slabs. Linear behavior followed by a nonlinear
Trend and strain hardening and softening until failure. The
slope of the first part of the plotted curves (load vs strain) of
the tested slabs showed expected behavior until reaching 30
kN (where cutout was made), after that being sharper for
unstrengthened slabs, the slope decreases by strengthening the
cutouts using CFRP sheets. The reference slab (S0) where no
cutout was made showed higher point of initial cracking than
the rest of all tested slabs due to the presence of cutout that
reduces the slab stiffness.
The ultimate loads of slabs (S3) and (S4) increased by
about 10.7 and 9.7%, compared to slabs (S1) and (S2),
respectively. The strengthening of the cutouts using CFRP
sheets showed higher ultimate load due to the confinement
stresses provided by the CFRP sheets. The deflection of
slabs (S3) and (S4) decreased by about 23 and 17%
respectively compared to the deflection at failure loads for
slabs (S1) and (S2), respectively as shown in Fig. 7. CFRP
sheets reached the ultimate strain at failure load when the
steel bars reached the ultimate strain before failure load due
to the effect of strengthening application. The steel bars
close to the cutout had been strained significantly; the
strengthened slabs were strained less than the
un-strengthened slabs due to the effect of the encirclement by CFRP
laminates as shown in Fig. 8. Transverse CFRP laminates
were strained proportionally with loading because of the
diagonal cracks originated from each corner of the cutout as
shown in Fig. 9.
4. Finite Element Analysis
4.1 Element Type and Meshing
The finite element (FE) code ANSYS (2011) was used in
this study. The experimental results were used to calibrate
the FE models. The FE used to extend the parametric study
beyond the limited number of specimens performed
experimentally. Finite element models of reinforced concrete
structures have generally been based on mesh discretization
of a continuous domain into a set of discrete subdomains,
usually called elements representing the concrete and the
steel reinforcement. In this study, the discrete element
approach was used to simulate reinforcement, where the
reinforcement is modelled using beam elements connected to
the concrete at certain shared mesh nodes as shown in
Fig. 10. Also, since the reinforcement is superimposed in the
concrete mesh, concrete exists in the same regions occupied
by the reinforcement. The drawback of using the discrete
model is that the concrete mesh is restricted by the location
of the reinforcement. Full bond is generally assumed
between the reinforcement and the concrete.
Concrete and resin was modeled using 3D 8-node solid
elements (SOLID65). The main feature of this element is the
ability to account for material nonlinearity. This element is
capable of considering cracking in three perpendicular
directions, plastic deformation and crushing, and creep. The
element is defined by eight nodes having three translation
degrees of freedom at each node in the x, y and z directions
as shown in Fig. 11.
The SOLID185 element is used for modeling the steel
plates and the CFRP composite. This element is defined by
eight nodes having three degrees of freedom at each node;
translations in the nodal x, y, and z directions. The element is
capable of plasticity, hyper elasticity, stress stiffening, creep,
Fig. 7 Load-deflection curves for tested slabs.
Fig. 8 Load-strain curves for steel bars of tested slabs.
Fig. 9 Load-strain curves for CFRP sheets of strengthened slabs.
Fig. 10 Discrete model for reinforced concrete (ANSYS).
large deflection, and large strain capabilities. SOLID185
element uses enhanced strain formulation, simplified
enhanced strain formulation, or uniform reduced integration.
The SOLID185 in the form of homogeneous structural solid
is used in this study to model the Carbon fiber and the steel
plate as shown in Fig. 12.
A LINK180 element is used to model steel reinforcement.
The element is a uniaxial tension–compression element with
three degrees of freedom at each node: Translations in the
Fig. 11 SOLID 65 3D-reinforced concrete solid element,
nodal x, y, and z directions. This element is also capable of
plastic deformation. Figure 13 shows the geometry of
The properties of the FE elements depend on the element
type such as cross-sectional area of beam element is known
in ANSYS as real constants. Not all element types require
real constants to be defined, and different elements of the
same type may have different real constant values. In case of
Fig. 12 SOLID185 3D- Homogeneous Structural solid
element, ANSYS (2011).
Fig. 13 LINK180 element geometry, ANSYS (2011).
concrete, real constants defined only for SOLID65 element
and in the present study the concrete is modeled using
discrete reinforcement. Therefore, all real constants which
activate the smeared reinforcement are disabled by putting it
equal to zero. As there are no reinforcements through the
resin, then, the same real constants are specified to the
SOLID65 element for resin. In general, crushing stiffness
factor (CSTIF) for concrete is set to be 0.1. SOLID185 in
form of homogeneous Structural Solid or layered Structural
Solid does not require the definition of real constants.
LINK180 has real constants; cross sectional area, and added
mass (mass/length). Both tension and compression
capability is chosen.
In modern fracture mechanics concrete is considered as a
quasi-brittle material, that’s where the stress decreases
gradually after the peak stress, and the properties of concrete
in compression and tension are different from each other.
The tensile strength of concrete is typically 8–15% of the
compressive strength. Figure 14 shows a typical stress–
strain curve for normal weight concrete according to
Bangash (Mota and Kamara 2006).
As shown in Fig. 14, when concrete subjected to
compression load, the stress–strain starts linearly in an elastic
manner up to about 30 percent of the maximum compressive
strength rcu, then, the stress increases gradually up to the
maximum compressive strength, and then, the curve
descends into a softening region, and eventually crushing failure
occurs at an ultimate strain ecu. In tension zone, the stress
strain curve is approximately linearly elastic up to the
Fig. 14 Typical Uniaxial Compressive and Tensile Stress–
Strain Curve for Concrete, Bangash, (Mota and
maximum tensile strength. After this point, the concrete
cracks and the strength decreases gradually to zero.
Typical shear transfer coefficients range from (0.0 to 1.0),
with 0.0 representing a smooth crack (complete loss of shear
transfer) and 1.0 representing a rough crack (no loss of shear
transfer). This specification may be made for both the closed
and open crack. When the element is cracked or crushed, a
small amount of stiffness is added to the element for
numerical stability. The stiffness multiplier CSTF is used
across a cracked face or for a crushed element to be equal to
0.1, ANSYS 2011 (ANSYS 2011). A number of preliminary
analyses were attempted in this study with various values for
bt and bc within a range between (0.15 to 0.9) and (0.5 to
0.9) respectively. Where bt and bc are shear transfer
coefficient for open cracks (bt), and shear transfer coefficient for
closed cracks (bc), (ANSYS 2011). For this analysis bt and
bc were set to 0.2 and 0.8 respectively, achieving a good
converging problem. The uniaxial cracking strength is taken
to be equal to the modulus of rupture of concrete. Due to the
similarity of resin with concrete in its behavior toward the
tensile and compression stress, so SOLID65 solid element
with linear and nonlinear properties is used to represent the
resin in the present model.
4.3 FRP Composites
The FRP composites are anisotropic materials; where the
material properties are different in all directions. For the
unidirectional lamina, it has three mutually orthogonal
planes of material properties, (xy, xz, and yz planes). The
xyz coordinate axes are referred to as the principal material
coordinates where the x-direction is the same as the fiber
direction, and the y and z directions are perpendicular to the
x direction. It is a so-called especially orthotropic material.
The perpendicular plane of fiber direction can be considered
as isotropic material, that’s where; the properties in the
y-direction are the same as those in the z-direction. FRP
laminates have stress–strain relationships that are roughly
linear up to failure. In the nonlinear analysis of the full-scale
transverse slabs, no FRP elements show stresses higher than
their ultimate strengths. Consequently, in this study it is
assumed that the stress strain relationships for the FRP
laminates are linearly elastic.
4.4 Steel Reinforcement
The reinforcement element was assumed to be a bilinear
isotropic elastic-perfectly plastic material and identical in
tension and compression as shown in Fig. 15. Poisson’s ratio
of 0.3 was used for all types of steel reinforcement.
Fig. 15 Stress-strain curve for steel reinforcement.
4.5 Loads and Boundary Conditions
The bond between concrete and steel is assumed to be
perfect, and poisson’s ratio is assumed to be constant
throughout the loading steps. Time-dependent nonlinearities
such as creep, shrinkage, and temperature change are not
included in this study. Concrete damaged plasticity model in
ANSYS provides a general capability for modeling concrete.
It uses the concepts of isotropic damaged elasticity and
isotropic tensile and compressive plasticity to represent
concrete inelastic behavior. To ensure that the model acts the
same way as the experimental slabs, boundary conditions
were applied at two supports (steel plates have 20 mm
thickness, 50 mm width and 1000 mm long) which located
under slabs to prevent local cracking in concrete, The nodal
displacement load is used to model the boundary condition
in this ANSYS models, The hinged support was created by
putting the value of the displacement DOFs for X, Y and Z
directions to be equal zero, consequently and the roller
support was created by putting the value of the displacement
DOFs for Y and Z directions to be equal zero, consequently.
The load was applied as line load in Y direction.uniformly in
Z direction in two positions. All slabs were loaded up to (30
KN) load then restart analysis for producing cut out by
killing elements, after that operation the slabs were loaded
till failure. Figure 16 shows the FE models of two of the
specimens. The FE model of the studied slabs with and
without opening is shown in Fig. 16.
Fig. 16 FE model of the control slab, slab (1), and reinforcement mesh.
Table 5 Comparison between experimental and finite element results.
Experimental ultimate load Finite element ultimate load
pue (kN) Puf (kN)
Fig. 17 Load-displacement history of slabs S0, S1, and S3.
5. Comparison Between FE
and Experimental Results
The finite element results were compared to the
experimental results obtained previously. The results were found to
be in good agreement, therefore the finite element test
program is extended further beyond the experimental cases to
investigate the behavior of more slabs. Table 5 shows the
ultimate loads from the FE model and the experiments.
Figure 17 shows the load–displacement history of slabs S0,
S1, and S3, it can be shown that the differences in the results
are in very good agreement with a percentage of error of
±3%. The obtained results showed that the FE model could
be used for further slab cases (Fig. 18).
More cases were considered by studying the effect of
number of CFRP layers and laminates areas as shown in
Table 6. The CFRP layers width increased from 100 to
200 mm, where the CFRP area was increased from 13 to
130 mm2 as a function of the number of layers. As expected,
Increasing the cross sectional area of CFRP by 100%, led to
an increase in the ultimate load 0f 14.1% for one layer.
Figure 19 shows the effect of number of layers on the
ultimate slab load. In general, increasing the number of CFRP
layers, has a significant increase in the ultimate load.
Fig. 18 Load-strain history in the longitudinal direction (F1) of slabs S3 and S4.
Laminate width= 100 mm
Laminate width= 150 mm
Laminate width= 150 mm
Fig. 19 Effect of CFRP layers for slab groups 1 (left) and 2 (right).
Fig. 20 Position of cutouts in hogging region, quarter and mid span.
5.1 Calculations of CFRP Amount Required for Strengthening
According to the results listed in Table 6. The amount of
CFRP used to strengthen slabs was computed under the
premise that the loss of steel reinforcement caused by the
cutout would be replaced by an equivalent amount of CFRP
to restore the load carrying capacity of R.C. slabs after
having cut out according to the following simple
The amount of steel reinforcement lost is equivalent to:
where (N) is the number of steel bars which have been cut
we can compute the equivalent area of CFRP for each side
Since each layer has a width (b CFRP), the necessary
overall thickness of CFRP laminate is given by:
Table 7 Effect of changing cutouts positions on ultimate load.
S3 = 60.0
S4 = 45.8
S30 = 63.6
S40 = 59.4
S300 = 64.8
S400 = 64.2
Fig. 21 Strengthened slabs with CFRP sheets with 45 .
Table 8 Effect of changing CFRP sheets configuration around cutouts on ultimate load.
Ultimate load (kN)
% increase ultimate load
Parallel to cutout edges
Inclined 45 at cutout corners
Parallel to cutout edges
Inclined 45 at cutout corners
Fig. 22 Strengthened Slabs with CFRP sheets with 90 and 45 .
Given that thickness of one layer = 0.13 mm, the total
number of layers required is:
No:of layers ¼ 0:013 mm
For group 1, (N = 1) is the number of steel bars which
have been cut (for slab with opening 100 * 100 mm).
Substituting into Eq. (3) we can compute the equivalent area of
CFR = 33.3 mm2. For group 2 (N = 3) is the number of
steel bars which have been cut (for slab with opening
5.2 Effect of Cutouts Location
Further FEA study has been considered in this section, by
investigating the slab behavior based on the cutout location.
All slabs in this parametric study had the same properties,
dimensions, cutouts dimensions, reinforcement and
boundary conditions as slabs which tested experimentally. The
cutout location was measured from the intermediate support
toward the simply supported edge as shown in Fig. 20. The
cutout was considered at hogging moment region, 200 and
500 mm from the intermediate support incorporated by the
cutout sizes (200 9 200 and 200 9 400 mm).
It can be seen form Table 7 that locating the cutout at the
hogging zone (S2), increased the ultimate load by 9%
compared to the control slab. Whereas locating the cutout at
the mid span did not have any significant effect on the load
carrying capacity of the tested slabs.
5.3 Effect of Changing of CFRP Configurations Around Cutouts
The effect of changing the CFRP configurations has been
studied as well. The CFRP has been assumed at 45 around
the cutout for slabs S3 and S4 as shown in Fig. 21.
From Table 8, Slabs strengthened with CFRP sheets along
the cutout edges (90 ) give results higher than the cases
where CFRP sheets are inclined by 45 with respect to the
cutout corners. In case of strengthening with 90 and 45
will get the highest load carrying capacity due to the crack
propagation is hindered by three layers of CFRP sheets as
shown in Fig. 22.
1. The relationships between the applied load, deflection,
and the longitudinal strains for concrete, steel, and
CFRP of the tested slabs were typical for all tested slabs,
a linear increase behavior followed by a nonlinear
behavior until failure has been observed.
2. The slope of the first part of the axial load-strain curves
of the tested slabs showed the expected trend and
behavior until reaching 30 kN loading (producing cut
out), after that the slope became sharper for
unstrengthened slabs, the sharpness degree decreased by
strengthening the slabs by CFRP sheets.
3. The ultimate loads increased by about 10.7 and 9.7% for
slab groups 1 and 2, respectively when slabs
strengthened using CFRP sheets and that is due to the
confinement stress provided by the CFRP sheets. The
deflection decreased by about 23 and 17% for slab
groups 1 and 2, respectively when slabs strengthened
using CFRP sheets.
4. Reference and un-strengthened slabs had flexural mode failure, where the steel bars reached the ultimate strain at failure load. For strengthened slabs, the rupture of CFRP sheets was the control mode of failure, the CFRP sheets
reached the ultimate strain at failure load when the steel
bars reached the ultimate strain before failure load due
to strengthening technique.
5. The steel bars beside cutout directly had been strained
significantly; the strengthened slabs had strained less
than the un-strengthened slabs due to the effect of the
encirclement by CFRP laminates, whereas transverse
CFRP laminates strained proportionally with loading
although it was parallel to load line but the practical
reason was the diagonal cracks originated from each
corner of the cutout.
6. The finite element model results closely agreed with the
experimental results; the model overestimates the values
of the ultimate loads of the tested slabs by 2–3%.
7. The amount of CFRP used to strengthen slabs was
computed under the premise that the loss of steel
reinforcement caused by the cutout would be replaced
by an equivalent amount of CFRP to restore the load
carrying capacity of R.C. slabs after having cutout.
8. To select suitable cutout location in existing R.C.
slabs, the moment of resistance should be considered
for slab at cutout’s section. The position of the cutout
with CFRP strengthening doesn’t provide significant
change in load capacity, and finally slabs strengthened
with CFRP sheets along the cutout edges give results
higher than CFRP sheets at the cutout’s corners with
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