behaviour of simply supported steel reinforced sifcon...
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Indian Journal of Engineering & Materials Sciences
Vol. 15, August 2008, pp. 326-333
Behaviour of simply supported steel reinforced SIFCON two way
slabs in punching shear
H Sudarsana Raoa*, K Gnaneswar
a & N V Ramana
b
aCivil Engineering, Jawaharlal Nehru Technological University College of Engineering, Anantapur 515 002, India
bRayalaseema Thermal Power Project, Aandhrapradesh Power Generation Corporation, V V Reddy Nagar,
Kadapa, 516 312, India
Received 6 September 2007; accepted 11 June 2008
This paper reports the behaviour of slurry infiltrated fibrous concrete (SIFCON) two-way slabs in punching shear.
SIFCON slabs are cast with 8, 10 and 12% fibre volume fraction and for comparison, fibre reinforced concrete (FRC) with
2% fibre volume fraction and reinforced cement concrete (RCC) slabs are cast and tested. The results of the experimentation
show that the SIFCON slabs with 12% fibre volume fraction exhibits excellent performance in punching shear among other
slabs. The experimental results have been compared with the provisions of ACI and IS codes. A regression model has been
proposed for estimating the punching shear strength of reinforced SIFCON slabs.
Slurry infiltrated fibrous concrete (SIFCON) is one of
the new addition to the high performance concrete
family. SIFCON is the extension for conventional
FRC that differs in terms of fabrication and
composition. In FRC the fibre content varies from 1
to 3% by volume whereas, in SIFCON, the fibre
content may vary from 6 to 20%. SIFCON is prepared
by infiltrating cement slurry into a bed of preplaced
fibres. Even though, SIFCON is a recent construction
material, it has found applications in the areas of
pavements repairs, repair of bridge structures, safe
vaults and defense structures due to its excellent
energy absorption capacities1-3
. Due to its
extraordinary ductility characteristics, it has a lot of
potential for applications in structures subjected to
impact and dynamic loading. Lankard1 presented the
basic properties of SIFCON (prepared with 12.5% of
fibres) such as load-deflection curve, ultimate
compressive and flexural strengths, impact and
abrasion resistance. Bhupinder Singh et al.4 presented
the method of production, structural properties and
applications of SIFCON, compact reinforced
composites (CRC) and densified small particles
(DSP). Naman and Baccouche5 presented the shear
response of dowel reinforced SIFCON and observed
that the shear strength of SIFCON is 10 times higher
than that of the plain matrix. Yan et al.6 studied
mechanical properties and digital image analysis of
SIFCON with 4%, 6%, 8% and 10% fibre volume
content. Fractal dimension is used as a parameter to
characterize the crack pattern on the surface of
SIFCON and concluded that there exists a good
correlation between mechanical properties and fractal
dimension. Sudarsana Rao and Ramana7 tested the
SIFCON slab elements under flexure and compared
the results with FRC and plain cement concrete (PCC)
slabs and concluded that SIFCON slabs exhibit
superior performance in flexure when compared to
FRC and PCC slabs. Yazici et al.8 studied the effect
of incorporating high volume of class C fly ash on
mechanical properties of autoclaved SIFCON and
concluded that by increase in the fibre volume
remarkably increases flexural strength and toughness
of SIFCON. This behaviour was more pronounced at
10% fibre volume. Mansur et al.9
reported the
punching shear test on square ferro cement slabs.
This paper presents experimental results that
describe the behaviour of steel reinforced SIFCON
simply supported slabs in punching shear. In this
experimental study, SIFCON slabs are produced by
using black steel wire fibres. The investigation
envisages studying the strength and deflection
behaviour of steel reinforced SIFCON slabs under
punching shear. The results are compared with slabs
made of reinforced cement concrete (RCC) and fibre
reinforced concrete (FRC).
Experimental Procedure The experimental program comprise of casting and
testing of nine reinforced SIFCON slabs (8, 10 and _________
*For correspondence (E-mail : [email protected])
SUDARSANA RAO et al.: BEHAVIOUR OF SIMPLY SUPPORTED STEEL REINFORCED SIFCON
327
12% fibre volume), three fibre reinforced concrete
slabs (2% fibre) and three RCC slabs (M20 grade
concrete and Fe 415 steel) simply supported on all
four edges. The mix proportions of the various slabs
are presented in Table 1. All the slabs are square and
are of size 600 × 600 × 50 mm. The slabs are white
washed for easy identification of crack patterns and
placed over the platform for testing. To simulate the
simply supported edge condition, 16 mm mild steel
rods are placed in between the slab and platform.
Materials
Material properties of cement, fine aggregate and
water are presented in Table 2.
Cement
Ordinary Portland cement of 53 grade
manufactured by Birla Company confirming to IS
12269 was used. The specific gravity of the cement
was 3.01. The initial and final setting times were
found as 40 min and 340 min respectively. Fine aggregate
Locally available river sand passing through 4.75
mm IS sieve was used. The specific gravity of the
sand is found to be 2.62.
Coarse-aggregate
Crushed granite aggregate available from local
sources has been used. To obtain a reasonably good
grading, 50% of the aggregate passing through 20 mm
IS sieve and retained on 12.5 mm IS sieve and 50% of
the aggregate passing through 12.5 mm IS sieve and
retained on 10 mm IS sieve was used in preparation of
FRC and RCC slab specimens.
Reinforcement
All the slabs are reinforced with 8 mm diameter Fe
415 grade steel rods, placed at 150 mm spacing in
both directions. The yield strength of steel bars was
250 N/mm2.
Fibres
The present investigation aims at producing
SIFCON with locally available fibres. Accordingly,
black annealed steel wire fibres of 1.0 mm diameter
were used. The fibres were cut to the required
length of 50 mm by using shear cutting equipment
giving an aspect ratio of 50. The ultimate tensile
strength of fibre was 395 MPa. These black steel
wires are commercially available and are generally
used for binding the steel reinforcement in RCC
works.
Table 1 — Mix proportions
S.No Type of slab Mix proportion Number of
slab
specimens
Volume
fraction of
fibre
W/C ratio Dosage of
super
plasticizer
Mode of vibration
1 SIFCON (8%) Cement and sand
(1:1 by wt)
3 8% 0.45 1.5% Hand tamping
2 SIFCON (10%) Cement and sand
(1:1 by wt)
3 10% 0.45 1.5% Hand tamping
3 SIFCON (12%) Cement and sand
(1:1 by wt)
3 12% 0.45 1.5% Hand tamping
4 FRC (2%) Cement, sand and
coarse aggregate
(1:1.54:3.17)
3 2% 0.50 Nil Table vibration
5 RCC Cement, sand and
coarse aggregate
(1:1.54:3.17)
3 No fibres 0.50 Nil Table vibration
Table 2 — Material properties
Cement Fine aggregate Water
Fineness of cement(Blains specific surface area) – 320 m2/kg Specific gravity - 2.62 pH value – 7.0
Specific gravity – 3.01 Fineness modulus – 2.74 Turbidity (NT unit) – 3.0
Initial setting time – 40 minutes
Final setting time –340 minutes
Density
Loose state – 14.67 kN/m3
Dense state – 16.04 kN/m3
Hardness(mg/L) – 400
Compressive strengths
3 days – 28.2 N/mm2
7 days – 38.7 N/mm2
28 days – 54.3 N/mm2
Grading – zone II Sulphates (mg/L) – 300
Chlorides (mg/L) – 280
INDIAN J. ENG. MATER. SCI., AUGUST 2008
328
Water
Potable fresh water available from local sources
was used for mixing and curing of SIFCON, FRC and
RCC slabs.
Super plasticizer
To improve the workability in slurry and concrete,
CONPLAST-300 confirming to IS 9103-1999 and
ASTM C 494 type F high range water-reducing agent
has been used.
Casting of test specimens
Steel moulds were used to cast the slab specimens
of required size. Two L-shaped frames with a depth of
50 mm were connected to a flat plate at the bottom
using nuts and bolts. Cross-stiffeners were provided
to the flat plate at the bottom to prevent any possible
deflection while casting the specimens. The gaps were
effectively sealed by using thin card-boards and wax
to prevent any leakage of cement-sand slurry in
SIFCON slab specimens. The moulds are shown in
Fig. 1. Initially, the steel mould was coated with
waste oil so that the slab specimens can be removed
easily from the moulds. Then the mat of 8 mm steel
rods @ 150 mm c/c was kept, at the bottom of mould
over 10 mm thick layer of sprinkled fibres. Then the
steel fibres are placed randomly in the mould such
that they occupy the entire volume of the mould. In
the mean time, cement-sand slurry was prepared using
CONPLAST – 300 which was later poured into the
mould uniformly over the pre-placed mat and fibres.
The details of casting are shown in Fig 2. In case of
fibre reinforced concrete slabs, fibres are first mixed
in the dry mixture of cement and sand and then spread
over the heap of coarse aggregate. Hand mixing was
done after adding required quantity of water to
achieve uniform dispersion of fibres and to prevent
the segregation or balling of fibres during mixing. For
both FRC and RCC specimens, table vibration was
adopted. The test specimens were de-moulded after
24 h and were cured for 28 days in curing water
ponds. After removing the slab specimens from the
curing pond, they were allowed to dry under shade for
a while and then they were coated with white paint on
both sides, to achieve clear visibility of cracks during
testing. The loading position on the top and the dial
gauge position at the bottom of the slab were marked
with black paint.
Loading arrangement and testing
The set-up for loading the slab consists of a solid
plate of 100 × 100 × 20 mm placed at the center of the
top face of slab specimen. Over this solid plate, solid
circular rod of 50 mm diameter was kept to distribute
the load from hydraulic jack to the slab specimen. The
whole arrangement has been made to obtain the
punching shear effect on the slab specimen, as shown
in Fig. 3. The loading platform consists of four
welded steel beams of ISMB 200@254 N/m in square
shape. These steel beams were stiffened using small
size steel I-Sections (ISMB100@50N/m). This
loading platform has been supported by brick walls on
two sides and the other two sides are supported with
two steel rods. The load was applied through
hydraulic jack and was measured with a calibrated
proving ring of 500 kN capacity. The vertical
deflections were measured by using dial gauge with a
least count of 0.01 mm. The vertical deflections were
measured at the centre of the slab specimens.
The load has been applied incrementally. The load
increment was selected such that there will be as
many number of readings as possible. The load was
applied in increments of 833.3 N which corresponds
to one unit of proving ring. Deflections have been
recorded for each load increment. The load at the first
crack and the corresponding deflection at the bottom
Fig. 1 — Slab mould filled with fibre and steel reinforcement
Fig. 2 — Casting of steel reinforced SIFCON slabs
SUDARSANA RAO et al.: BEHAVIOUR OF SIMPLY SUPPORTED STEEL REINFORCED SIFCON
329
centre of the slab were recorded. The ultimate
punching shear load and corresponding deflection at
the centre were also observed and recorded.
The determine cube compressive strength of RCC
and FRC, six numbers of cubes (3-RCC, 3-FRC) of
150 × 150 × 150 mm size have been cast and tested,
and the average compressive strength of three cubes
for RCC and FRC is 20.1 N/mm2 and 32 N/mm
2
respectively. Similarly to determine cube compressive
strength of SIFCON, thirty number of cubes (10 –
SIFCON 8%, 10 – SIFCON 10% and 10 – SIFCON
12%) were cast and tested and the compressive
strengths are given in Table 3.
Results and Discussion
The results of the experimental investigation are
summarized in Table 4. The values presented here
represent the average of punching shear strengths,
load and deflection obtained for three slab specimens
in each series. The effect of percentage of fibres on
the ultimate punching shear load of the steel
reinforced SIFCON slabs is shown in Table 4. From
Table 4, it is observed that there is an increase in first
crack strength with the increase in volume fraction of
fibres in punching shear. The slab specimens
reinforced with higher volume fraction of fibres
behaved better than those containing lower volume
fractions of fibres. The maximum first crack load of
61.82 kN has been achieved for slabs reinforced with
12% volume fraction of fibres. FRC slab specimens
and RCC slab specimens have failed at significantly
lower loads. The percentage increase in first crack
load in steel reinforced SIFCON slab specimen when
compared to RCC and FRC slab specimens is in the
range of 1121-1755% and 876-1384% respectively
for different volume fractions of fibres. This confirms
the superior performance of steel reinforced SIFCON
specimens in punching shear.
From Table 4, it is observed that ultimate punching
shear strength increases with increase of volume
fraction of fibre in steel reinforced SIFCON slab
specimens. The maximum ultimate punching shear
load of 124 kN has been obtained for steel reinforced
SIFCON slabs with 12% volume fraction of fibres
which is 21% higher than that of 8% volume fraction
slab specimens. The ultimate punching shear strength
of steel reinforced SIFCON slab specimens is about
665-830% when compared to FRC slab specimens
with maximum values corresponding to 12% volume
fraction of fibres. The increment when compared to
RCC specimens is about 841-1045% .
The maximum central deflection values of various
slab specimens are presented in Table 4. From this
table, it is also observed that the maximum central
deflections and the ultimate punching shear load of
Fig. 3 — Testing of slabs for punching shear
Table 3 — 28 day cube compressive strength of SIFCON
Cube compressive strength (N/mm2) Nomenclature
1 2 3 4 5 6 7 8 9 10
SIFCON-8% 40.00 45.00 41.00 47.11 39.07 47.95 44.44 51.55 48.84 43.51
SIFCON-10% 51.06 51.50 53.28 55.05 48.84 51.04 51.06 53.28 48.84 51.84
SIFCON-12% 52.80 57.72 57.72 52.39 52.39 55.50 55.50 54.56 52.80 56.32
Table 4 — Details of test results
Nomenclature First crack
load (kN)
Ultimate
punching
shear load
(kN)
Maximum
Central
deflection in
punching
shear (mm)
RCC 3.332 10.829 10.25
FRC-2% 4.165 13.328 15.50
SIFCON-8% 40.690 102.000 20.50
SIFCON-10% 51.900 114.000 21.50
SIFCON-12% 61.820 124.000 22.50
INDIAN J. ENG. MATER. SCI., AUGUST 2008
330
reinforced SIFCON slab specimens are considerably
higher than RCC and FRC specimens indicating
higher energy absorption capacities of reinforced
SIFCON slab specimens. The steel reinforced
SIFCON slabs have not only carried higher loads, but
also sustained greater deflections till ultimate stage.
The crack patterns of different slab specimens are
depicted in Figs 4-8. From Figs 4-6, it is observed that
the crack pattern is almost similar in all SIFCON
slabs. In all slabs, the first crack originated at the
centre and propagates radially towards the corners. At
higher loads, already formed cracks get widened with
formation of new cracks. The new formations of
cracks are mainly concentrated at the point of
application of punching load. Cracks are mainly
localized up to a particular distance from the loading
point in circular area. This circular area increases with
increase in volume fraction of fibres in steel
reinforced SIFCON slab specimens. A few cracks are
identified on the top surface steel reinforced SIFCON
slab specimens. Similar type of failure pattern was
reported earlier9.
From Fig. 7, it is observed that the FRC slab
specimens show more number of cracks underneath
the slab. A few hair cracks are noticed on the top
surface. From Fig. 8, it is observed that the RCC slab
specimens show very much pealing of concrete on
bottom face. Regarding strength, load deflection and
crack pattern the steel reinforced SIFCON slab
specimens exhibit superior performance compared to
FRC and RCC slab specimens. This may be due to the
incorporation of higher volume fraction of fibres
which lead to crack arresting and crack bridge
mechanism in the matrix.
Comparisons of experimental results with codal provisions
There is hardly any code for SIFCON material in
punching shear is reported so far. However, in the
present analysis, the two major building codes ACI
318-2005 and IS 456-2000 have been considered for
comparison. In the strict sense, the above two
building code methods may not be applicable to
SIFCON material.
As per the ACI 318-2005 code, the ultimate
punching shear strength Pu is taken as the smallest
value given by the following
Pu = (0.166+(0.332/Bc))√fc u d ...(1)
Fig. 4 — Tested steel reinforced SIFCON – 8% slab
Fig. 5 — Tested steel reinforced SIFCON – 10% slab
Fig. 6 — Tested steel reinforced SIFCON – 12% slab
Fig. 7 — Tested FRC – 2% slab
SUDARSANA RAO et al.: BEHAVIOUR OF SIMPLY SUPPORTED STEEL REINFORCED SIFCON
331
Pu = (0.166+(0.083 α d/u))√fc u d …(2)
Pu = 0.332√fc u d … (3)
According to the Indian standard code IS: 456-
2000, the expression for calculating the punching
shears strength Pu by considering partial safety factor
for material as unity is given as
Pu=Ks cτ u d …(4)
Ks = (0.5+Bo) ≤ 1
cτ =0.25 √fck
The above two specified code provisions are used
to calculate the ultimate punching shear load and the
predicted values are given in Table 5. From this table,
it can be seen that the experimentally observed values
are higher than those calculated as per ACI and I.S
code procedures. The experimental loads for SIFCON
slabs are higher by 52-67% and 102-122% when
compared with ACI code and IS code respectively.
From the results it can be concluded that the IS code
is more conservative than the ACI code and there is a
need to define specific procedures for computation of
punching shear of SIFCON. Accordingly a regression
model is prepared for estimating the punching shear
strength of SIFCON.
Regression model for punching shear strength of steel
reinforced SIFCON slabs
A simple regression model has been developed
from the results of present investigation for predicting
the punching shear strength of steel reinforced
SIFCON slabs. To develop the punching shear
strength model, linear regression technique has been
adopted.
The proposed model for punching shear is given as:
Pu = τ us d ...(5)
A simple linear regression equation has been
developed between compressive strength (fc) and
shear stress (τ) of SIFCON by using Tables 3 and 6
and presented below.
τ = 0.311√fc …(6)
However, the compressive strength (fc) of SIFCON is
dependent on fibre volume fraction. In the present
work three fibre volume fraction, viz., 8,10 and 12%
have been used. Ten cubes of 150 × 150 × 150 mm
size for each percentage of fibre were cast and tested
for compressive strength. The various 28 day
compressive strength (fc) values of SIFCON obtained
from the experimentation are given in Table 3. A
regression model has been developed by using the
results of Table 3 for cube compressive (fc) strengths
of SIFCON in terms of fibre volume fraction (Fv), as
it will be convenient to estimate fc with fibre volume
fraction (Fv). The regression model is given as:
fc = 25.32 + 2.505 Fv ...(7)
Substituting the Eq. (7) in Eq. (6)
τ = 0.311√(25.32 + 2.505 Fv) …(8)
The critical perimeter us can be defined with reference
to Fig. 9 as:
Table 5 — Comparison of Experimental punching shear values with standard codes of practice
Nomenclature Ultimate Punching Shear loads (kN) EXP values/ EXP values/
Exp values ACI 318-2005 IS: 456-2000 ACI 318-2005 IS 456-2000
RCC 10.83 21.00 15.81 0.516 0.685
FRC-2% 13.33 28.17 21.22 0.473 0.628
SIFCON-8% 102.00 67.08 50.40 1.520 2.023
SIFCON-10% 114.00 70.69 53.10 1.612 2.146
SIFCON-12% 124.00 74.12 55.80 1.672 2.223
Fig. 8 — Tested RCC slab
INDIAN J. ENG. MATER. SCI., AUGUST 2008
332
us = 4[ b+ 2d tanθ] …(9)
From the present experimental work, it is observed
that the dispersion angle θ depends on fibre volume
fraction FV. Accordingly a simple linear regression
equation is developed to connect θ with FV from the
results of the present work and is presented as
Eq.(10).
Θ = 44.055+1.312(Fv) …(10)
Substituting Eq.(10) in Eq.(9)
us = 4[ b+ 2d tan(44.055+1.312(Fv))] …(11)
Substituting Eqs (8) and (11) in Eq.(5), a model for
predicting punching shear strength is obtained as:
Pu ={(0.311√(25.32 + 2.505 Fv))( 4[ b
+2d tan(44.055+1.312(Fv))]) d} …(12)
A comparison of the ultimate punching shear loads
predicted by the regression model (Eq.(12)) with
experimental values is given in Table 6. From Table
6, it can be observed that the proposed model
compared well with the experimental ultimate loads.
The ratio of experimental punching shear load to that
predicted by regression model is given in column 4 of
Table 6. It can be observed that the ratio varies from
1.012 to 0.985. Thus, the proposed regression
equation is able to predict the ultimate punching shear
loads of SIFCON slabs quite satisfactorily.
Conclusions Analyzing the results obtained from this
investigation, the following conclusions are drawn:
(i) The punching shear carrying capacity of the
steel reinforced SIFCON slabs is much higher than
the fibre reinforced concrete and reinforced cement
concrete slab specimens.
(ii) With increase of fibre volume, the punching
strength carrying capacity increases in steel reinforced
SIFCON slabs.
(iii) The zone of influence area (dispersion angle)
increases with increase of volume fraction of fibres.
(iv) The steel reinforced SIFCON slabs are intact
even after ultimate failure but this is not so in RCC
slab specimens.
(v) The steel reinforced SIFCON slabs with higher
volume fraction of fibre sustain greater deflection
with high ultimate load.
(vi) In steel reinforced SIFCON slabs the stiffness
increases with increase of fibre volume and the
stiffness of steel reinforced SIFCON slabs specimens
is very much higher than the FRC and RCC slab
specimens.
(vii) Existing codal provisions for punching shear
are not suitable for steel reinforced SIFCON slabs.
There is need to develop specific codal provisions for
punching shear of steel reinforced SIFCON slabs.
Nomenclature b = breadth of the patch load, mm
Bc = the ratio of long side to short side of the loaded area
Bo = ratio of short side to long side of column
d = effective depth of the slab, mm.
fc = specified compressive strength of concrete, N/mm2
fck = characteristic cube compressive strength of concret, N/mm2
Fv = fibre volume fraction, %
Pu = ultimate punching shear strength, N.
u = length of the critical perimeter (mm), taken at a distance of
d/2 from the column/pedestal (for ACI and IS codal
provision)
us = length of the critical perimeter (depending on angle of
dispersion for SIFCON slabs), mm
α = 40 for symmetric punching.
τc = shear stress in concrete, N/mm2
θ = dispersion angle
Table 6 — Performance of regression model
Nomenclature Exp Ultimate
load, (kN)
Ultimate Load
predicted by
Regression
model, (kN)
Exp value/
Predicted
value
SIFCON-8% 102.000 100.752 1.012
SIFCON-10% 114.000 112.566 1.012
SIFCON-12% 124.000 125.793 0.985
Fig. 9 — Dispersion angle for slabs under punching shear
SUDARSANA RAO et al.: BEHAVIOUR OF SIMPLY SUPPORTED STEEL REINFORCED SIFCON
333
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