PUNCHING SHEAR STRENGTH OF THE FIBERREINFORCED
CONCRETE SLABS
1
1,2
I.Siva Kishore, 2Ch.Mallika Chowdary
Assistant Professors Department of Civil Engineering, K L University, Vaddeswaram, Guntur, A.P.
ABSTRACT: This is an investigation carried for the behavior and resistance of steel fiber reinforced concrete (SFRC) flat
slabs under punching shear force. In this investigation a total of twelve small-scale flat slabs of different dimensions that consists
of nine SFRC and three ordinary were tested. The resultants spectacle a significant increase of the punching shear capacity and
considerable improvement of cracking behavior as well as good integrity of column-slab connection of the slabs with fibers. The
slabs without fibers fails in very brittle manner, while the fiber reinforced ones collapsed in more ductile and observed that, the
size of the specimen increases and the failure shear stress of the specimen decreases.
.’
KEYWORDS: Punching shear, Circular flat slabs, Normal strength concrete, steel fibres
1. INTRODUCTION:
Heavy localized forces in Reinforced concrete are caused
mainly due to shear forces and this failure is named as
punching shear. An investigation of punching shear for
flat slabs structures is undertaken and for this the failure
occurs at the column support points. In flat slabs there
won’t be any beams, instead of beams drop panels are
used. Direct connection of slab and column will be
observed in flat slab only and this is the most critical
portion in flat slabs. The catastrophique failure between
the slab and the column is observed and this is due to high
bending moments and shear forces.
Fig.1 –punching failure surface of flat slabs
Punching Shear occurs when an area is subject to a high
state of stress relative to its immediate environs. Failure
can occur either by pure punching or by bending induced
punching where, the initial tension cracks will grow
tangentially to form the punching surface. The cracked
profile of the punched area indique the mode of failure.
1.1 IMPORTANCE OF PUNCHING SHEAR
I. Occurrence of punching shear:
Punching shear strength of flat slabs is the most important
limitation of this particular structural system
due to its fragile nature. This kind of failure must be
avoided by providing the slab-column connection of
efficient mechanisms of strength and ductility.
II. Advantages of Flat Slabs: As we know that Flat slab
systems are widely been used in construction. There are
romper. Advantages as architectural design, rapid
construction methods. New prestressing materials and
techniques, and special concretes such as the auto
compact age concrete and high-strength concrete.
1.2 FACTORS AFFECTING THE PUNCHING
SHEAR STRENGTH OF THE CONCRETE.
I. Thickness of Slab or Footing: Thickness of slab or
footing is directly proportional to the Punching Shear
strength of concrete.
II. Least lateral dimension of the column: Minimum
c/s dimension of the column is directly proportional to
the Punching Shear resistance of the concrete.
III. Shape of the column: The failure is more in case
of Rectangular or Square columns. Circular columns
have better resistance for the Punching Shear Failure.
IV. Column location: It depends on Interior column or
Exterior column. Interior columns are more prior to the
Punching Shear failure than Exterior column.
V. Grade of Concrete: Grade of concrete is directly
proportional to the Punching Shear resistance of the
concrete.
VI. Size of the Aggregate: Depends according to the
size of the aggregate to the size of the specimen taken. If
small size specimen is taken then the aggregate size
should be smaller than the normal size of the aggregate
generally taken (20mm).size of aggregate directly
proportional to the size of the specimen.
1.3 PUNCHING SHEAR FOR THE FLAT SLABS:
As slabs are directly supported by the columns without
beams and so this helps to reduce building height and
increase used space. The important issue existing in this
system is that punching shear failure of the slabs is due to
high concentration of stress in slab- column connections.
This failure type is very dangerous (i.e.) brittle nature.
Resistance of the structure is significantly reduced when
the punching shear failure occurs this causes separation
of the slab and column, hence this leads to collapse of the
whole structure.
MANY METHODS USED FOR INCREASING THE
PUNCHING SHEAR CAPACITY:
(1)Traditional shear reinforcing method using stirrups but
this method is inapplicable to slabs with shallow depth
less than 150 mm (ACI 318-2002)
(2) Headed-studs and take much time for construction
(Feretzakis 2005).
(3)Steel fibers improve the punching shear resistance and
cracking control of slab-column connections shown good
results
(Alexander
and
Simmonds
1992;
Theodorakopoulos and Swamy 1993; Harajli et al. 1995;
McHarg et al. 2000, Naaman et al. 2007; Cheng and
Montesinos 2010a). Steel fibers also indicate high
effectiveness in structures sustained lateral loads (i.e.)
seismic because of their ability to absorb energy
dissipation of the structures (Megally and Ghali 2000;
Cheng and Montesinos 2010b). Several formulas exist
which were proposed for estimation of punching shear
capacity of SFRC slabs (Shaaban and Gesund 1994;
Harajli et al. 1995; Choi et al. 2007). Formulas of Shaaban
and Gesund and of Harajli were pure-empirical based on
punching model of ACI 318. These formulas are simply
and easy to use, but due to their experimental nature, the
formulas cannot determine mechanisms of punching
shear transfer quantitatively that can lead to inaccurate
results in comparison with tests results. Simply as the
availability of the steel fibres is nearby, so here steel
fibres are been considered
2. EXPERIMENTAL PROGRAM:
The program consists of casting and testing of the
eighteen slab specimens of normal strength concrete. The
main parameters in this study were size of the slab, grade
of concrete and percentage of steel added in the concrete
mix. This program consists of slab- column specimens,
namely PN/dc/d/D/f ‘N’ represents Normal strength
concrete of M30 grade concrete, dc represents diameter of
the column, d represents thickness of the slab, D
represents the diameter of the slab and f represents
percentage of steel fibres in the slabs. In this
investigation, for each series there are three different sizes
of the slab column specimens were used i.e. 25mm
x25mm x100mm, 50mm x50mm x200 and 100mm
x100mmx400mm.
This program was to study the punching shear strength of
normal strength slab specimens with 0 %, 2 %, and 4%,
of steel fibres in concrete slabs.
3. MATERIAL DETAILS:
Ordinary Portland cement (OPC) OF M30 grade
conforming to IS 12269 – 1983 was used for the concrete
mix and Specific gravity was found to be 3.5 was used.
The fine aggregate (sand) that falls in zone –II was used.
The specific gravity was found to be 2.60. Crushed coarse
aggregate of size 20mm are used in this mix, having
specific gravity of coarse aggregate is 2.78. Potable water
supplied by the college is used in this work. Hooked steel
fibres of 0.1mm diameter, 50mm long and aspect ratio
500 were used in concrete mix. The fibres taken form
STEWOLS INDIA (P) LMT, at Nagpur. The details of
mix proportions are listed in Table1.
3.1 STEEL FIBERS:
The SFRC is a composite material made of cement, fine
and coarse aggregates and discontinuous discrete steel
fibers. They possess excellent dynamic performances
such as high resistance. SFRC plays major role for the life
of the structure. As these gives more strength and these
have good interlink connection with the concrete, so the
cracking may not be occurred within short period. The
mechanical properties of SFRC are influenced by the type
of fiber, aspect ratio, and volume fraction of fibers and the
size of the aggregates. Among the various types of fiber
currently available, steel fibers are most widely used.
(a)
PROPERTIES OF STEEL FIBER

Improve strength

Reduce steel reinforcement requirements

Improve ductility

Reduce crack widths and control the crack
widths tightly thus improve durability

Improve impact & abrasion resistance
 Improve freeze-thaw resistance
3.2 Characteristics of steel fibers:
I. Aspect Ratio (L/D):
Increase the length and decreasing the diameter leads an
increase in the aspect ratio.
II. Fiber Anchorage Details:
The SFRC to continue to carry load and deform
plastically after cracking has occurred. This means the
full capacity of the fiber is achieved over high
deformations giving high energy absorption and the
characteristic ductility required to prevent brittle failure.
In terms of fiber geometry the following is important
Small changes to the shape of a hooked end anchorage
can mean the difference between high and low toughness
continuously deformed fibres, enlarged end fibres and
mill cut fiber will bond over a short length of the fiber
rather than pull through and a more brittle failure will
result.
III. Physical Properties of Steel Fibers:
To maintain the ductility of SFRC, it is imperative
that the well-anchored fibres do not break. Breaking
fibres equates to a brittle failure mode (this is the main
problem with fibres continuously deformed). To prevent
breakage steel fibres should be manufactured with
sufficient tensile strength to ensure the ultimate failure
mode is pullout rather than breakage.
4. FIBRE PACKING:
High aspect ratio fibres are generally highly
efficient however, they tend to ball when mixed in
concrete unless packaged correctly. High aspect ratio
fibres when collated in this way can be added to the mix
almost like an extra aggregate and no balling will occur.
The aspect ratio of the bundle is less and the water-soluble
glue guarantees that the fibres are uniformly distributed
in the mix. Fibres are added Products for Concrete into
the truck in degradable bags for ease of handling. Low
aspect ratio, loose fibres tend to be packaged in boxes.
4.1 Hooked Ends Steel Fibre:
Steel fibre with hooked ends is made using highquality low-carbon steel wire. A kind of highperformance steel fiber, with the characteristics of the
high tensile strength, good toughness, low prices, etc. The
product is widely used in concrete strengthening. Hooked
ends steel fiber is made in accordance to the country
standard YB/T151-1999 Standard for Steel Fibers for
Concrete Uses, and the JG/T3064-1999 Standard of Steel
Fiber for Concrete Building Industry.
4.4 Moderate volume fraction: The presence of fibers at
this volume fraction increase the modulus of rupture,
fracture toughness, and impact resistance. These
composite are used in construction methods such as
shotcrete and in structures that require energy absorption
capability, improved capacity against delamination,
spalling, and fatigue.
4.5 High volume fraction: The fibers used at this level
lead to strain hardening of the composites. Because of this
improved behaviour, these composites are often referred
as high-performance fibre-reinforced composites
(HPFRC).
4.6 CONSIDERED CODES:

ACI 318-08

IS 456-2000

BS 8110-97

AS 3600-01
ACI: ACI 318-08 allows the use of shear reinforcement
for slabs and footings in the form of bars, as in the vertical
legs of stirrups. ACI 318 emphasizes the importance of
anchorage details and accurate placement of the shear
reinforcement, especially in thin slabs. American
concrete Institute 318 does not evaluate the impact of the
longitudinal reinforcement and the scale factor on the
punching shear strength. ACI 318 admits that the
maximum punching shear strengths in a slab 0.5d from
the column surface are of constant size and direct
distribution.
BS-8110 : Consequently, the margin of safety in these
structures against punching shear is generally lower than
for structures of more modern design (i.e) those designed
to BS8110-1997 include a substantially improved method
for punching shear design in normal in-suit flat slab
construction with regular spans, but give little guidance
on structures where spans are irregular.
•
4.2 Classification according to volume fraction:
•
•
•
Low volume fraction (<1%)
Moderate volume fraction (between 1 and 2%)
High volume fraction (greater than 2)
4.3 Low volume fraction: The fibers are used to reduce
shrinkage cracking. These fibers are used in slabs and
pavements that have large exposed surface leading to high
shrinkage crack.
•
For this reason , and also because of other
aspects on which the code does not give
guidance
It should be in mind, when using BS8110 as a
basis for the structural appraisal of column/slab
structures, that the design procedure’s might not
achieve level of safety against punching shear
comparable to those for bending caution is also
needed when using some structural analysis
methods to calculate the effective shear on the
punching perimeter. Effective shear may be
underestimated.
IS 456-2000: This describes in detail the various aspects
of the concrete design procedure that is used by SAFE
when the Indian Code IS 456-2000 [IS 2000] is selected.
For referencing to the pertinent sections of the Indian
code in this chapter, a prefix “IS” followed by the section
number is used. The design is based on user-specified
load combinations. The program provides a set of default
load combinations that should satisfy the requirements for
the design of most building type structures.
AS 3600-01: This describes in detail the various aspects
of the concrete design procedure that is used by SAFE
when the Australian code AS 3600-2001 [AS2001] is
selected.
•
The design is based on user-specified load
combinations.
• SAFE enforces the upper material strength limits
for flexure and shear design of beams and slabs
or for torsion design of beams. The input
material strengths are taken as the upper limits if
they are defined in the material properties as
being greater than the limits. The user is
responsible for ensuring that the minimum
strength is satisfied.
5. Casting:
The moulds taken were tightly fitted and all the
joints were sealed by plaster of Paris in order to prevent
leakage of cement slurry through the joints. The inner side
of the moulds was thoroughly oiled before going for
concreting. The mix proportions were put in miller and
thoroughly mixed. The prepared concrete was placed in
the moulds and is compacted using needle& plate
vibrators. The same process is adopted for all specimens.
After specimens were compacted the top surface is
levelled with a trowel. Here the moulds implies that slabs
and columns.
5.1 Test Setup and Testing Procedure:
All the specimens were tested on the universal
testing Machine (UTM) of 1000 KN capacity under
displacement control at a rate of 0.2mm/min for all large,
medium and small specimens. After 7days of curing the
samples were taken out from the curing tank and kept for
dry. After this the sample was coated with white wash.
One day later the sample was kept for testing. The slab
column specimen was kept at the centre of testing
machine and the specimen was placed on the specially
prepared perimeter support consisted of a smooth
continuous steel plate bent into a circular shape.
6. TEST RESULTS AND DISCUSSIONS:
The mechanical properties are been shown in table2.The
slab specimens were tested on the universal testing
machine (UTM) under load rate control. The load is
applied through the steel column. At every stage of
loading, displacement readings were recorded which
indicates the vertical movement of the column. The load
at which punching shear failure occurred is recorded. The
failure load and punching shear strength of slab column
specimens were presented in Table. The load
displacement diagrams of slab specimens were shown in
graphs 1
Nominal shear stress at failure due to punching,
may be expresses as
N 
P
bd
In which P is the maximum load at which
punching shear failure occurred, b is the punching
diameter at critical section and d is the slab thickness.
Here the above formula is used and the values are been
shown in table 2.
The punching shear values are been calculated for the
ACI code and IS code and then through the experimental
values the punching shear values are been calculated and
here the comparison between these are been shown below
in the tabular form. Here it states that there is a slight
variation between the ACI code and the experimental
value and for then the IS code value is half that of the
experimental value.
A graph is plotted between the ultimate load and the
nominal shear stress and here it is been observed that for
2% addition of steel fibres in the concrete there is an
increase in strength, but for 4% the strength is been
decreased. Hence from here it states that the steel fibre is
not interlinked with the concrete. The graph is been
shown as graph2.
7. CONCLUSIONS:
Analyzing the results obtained from this
investigation, the following conclusions are drawn

The slabs with higher fraction volume
of fiber sustain greater deflection with high
ultimate punching shear load.

The punching shear tests of
geometrically similar concrete slabs of different
sizes, carried out as part of the present
investigation indicate that the size effect exits
i.e. the nominal stress at failure decreases as the
size increases.

Moreover, steel fibers increase stiffness
of the slabs and improve concrete ductility and
integrity of vicinity of slab-column connections.
Since here there is no interlink connection
between fiber and the concrete so the strength is
decreased.

The results from the evaluation
indicated that the formulas gave inaccurate
results with a large scatter, in comparison with
the experimental results.

For 0% of steel fibres the nominal shear
percentage is of 27.2% and for 2% it is increased
to 50.0% and for 4% there is a decreased value
and that is of 41.9%. Hence there is a decrease
in the strength observed for the 4% of the steel
fibres. As this may depends on many factors like
cement, water and aggregate.as we are taking the
available college water (i.e.) if that is salt
contained or the bleaching powder may be added
so, this may also cause decrease in strength. But
while doing experiment we don’t think a drastic
change may occur due to water. Hence we have
to check and take the water.so water plays major
role. This may be silly reason but it’s true.

Here the selection of size of aggregate
is also important. If 10mm size of aggregate is
taken then the interlink between the concrete and
the aggregate is used to be good when placed in
the column, then failure of the column may not
be occurred.

But minimum size of aggregate mostly
considered is 20 mm, so the size of the circular
slabs have to be increased. If the size is increased
then the results will be good with improvement
of strength.
8. ACKNOWLEDGEMENT
We the authors are very thankful to the authorities of
bapatla engineering college, bapatla, for supporting and
providing the facilities for carrying the work.
9. REFERENCES:
1. ACI
Committee
318:
Building
Code
Requirements for Reinforced Concrete. Detroit.
American Concrete Institute, 1999.
2. Bazant Z.P., “Size effect law”.(Bazant
1984,bazant et al 1991;Bazant&Xi 1991,Bazant 7
chen 1997)
3. Bazant Z.P(1997) “Size effect in shear failure of
reinforced concrete”.Journal of engineering
mechanics 123(1997)1276-1288
4.
ACI Committee 318. (2001). ACI 318-02:
Building code requirements for structural
concrete. American Concrete Institute.
5.
BS 8110 (1985). Structural use of concrete. Part
1 Code of practice for design and construction.
British Standard Institution.
6.
Feretzakis (2005). using headed-studs
Table-1: Quantities of Materials.
S.NO
Type Cement
Fine
of
aggregate
(kg/m3)
mix
(kg/m3)
1
(1:1.57:3.63)
NSC
360
565.2
Coarse
aggregate
(Kg/m3)
Water
Cement
ratio
1306.8
0.464
Table-2: Mechanical properties of concrete.
Slab series
Cube Compressive
strength
Split tensile strength
(N/mm2)
2
(N/mm )
PN/0
22
1.845
PN/2
26.0711
2.3092
PN/4
22.646
2.0110
Table-3 Ultimate load, shear stress
Specimen
Ultimate Load
designation
(kN)
Nominal Shear stress(  N )
(N/mm2)
PN/25/25/100/0%
8.8
2.24096
PN/25/25/100/2%
20.6
5.24590
PN/25/25/100/4%
13.4
3.41238
PN/50/50/200/0%
34.2
2.17773
PN/50/50/200/2%
57.8
3.67977
PN/50/50/200/4%
40.22
2.56056
PN/100/100/400/0%
101.6
1.61706
PN/100/100/400/2%
165.6
2.63568
PN/100/100/400/4%
113.42
1.805188
Table. 3 Ultimate load, Punching shear strength of tested M30 slab specimens.
SPECIMEN
NOMINAL
SHEAR
STRESS
SLAB
THICKNESS
(d)
ACI (P)
IS(P)
PN/30/100/0
2.24096
PN/30/200/0
2.17730
PN/30/400/0
25
6.13647
4.60247
8.8
50
24.54651
18.4098
34.2
1.61706
100
98.18604
1476.64
101.6
PN/30/100/2
5.24590
25
6.680192
5.01014
20.6
PN/30/200/2
3.67977
50
26.72077
20.0405
57.8
PN/30/400/2
2.63583
100
106.8831
80.1623
165.6
PN/30/100/4
3.14123
25
6.225247
4.6689
13.4
PN/30/200/4
2.56056
50
24.90099
18.675
40.22
PN/30/400/0
1.80518
100
99.60396
74.702
113.42
Graph-1
For 200 mm diameter slab
Ultimate Load(P) from
experimental value(KN)
GRAPH-3
Ultimate load Vs shear stress
2%
MEDIUM
4% LARGE
DISPLACEMENT IN MM
NOMINAL SHEAR
STRESS(N/MM2)
LOAD IN KN
O% SMALL
ULTIMATE LOAD VS
NOMINAL SHEAR
STRESS
LOAD IN KN
0% SMALL
2%
MEDIUM
4% LARGE
DISPLACEMENT IN MM
2
4
ULTIMATE LOAD(KN)
Graph-2
For 400mm diameter slab
0
THE TOTAL SLABS SHOWN HERE AFTER
TESTING:
Figures of failure flat slabs:
100 MM DIAMETER SLAB: After applying load the
slab failure is in this pattern
Complete flat slab failure
200 MM DIAMETER SLAB: Here just a crack is
been formed, that is a light pattern of crack. This is
formed in cone shape
400MM DIAMETER SLAB: Crack is formed in the
form of a cone shape. But only one pattern will be
thicker and the other pattern formed will be clear in
nearby vision only