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 : hanchate123@yahoo.co.in)

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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