STEEL FIBRE BASED CONCRETE IN COMPRESSION

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International Journal of Advanced Engineering Technology
E-ISSN 0976-3945


IJAET/Vol.II/ Issue I/January-March 2011/96-111

Research Article

STEEL FIBRE BASED CONCRETE IN COMPRESSION
1
M. A. Tantary;
2
A. Upadhyay;
3
J. Prasad

Address for Correspondence
1
Associate professor Dept. of Civil Engineering NIT Srinagar
2
Associate Professor Dept. of Civil Engineering IIT Roorkee
3
Associate professor Dept. of Civil Enggg. IIT Roorkee
E Mail manzoor3000@yahoo.com
ABSTRACT
Of all the strengths, compressive strength is the most important property in concrete because of the qualitative relation of
compressive strength with other types of strengths. Moreover, it is easy to determine and has got intrinsic importance in
construction. But the fact can not be denied that as yet no exact quantitative relationship between compressive strength
of cube and cylinder has been established nor is it likely to be. This is because of the variability in the constituents of
concrete from place to place and even from batch to batch in a given concrete mix. Approximate or stastical
relationships, in many cases, have been established and certainly provide very useful information. This approximation
gets magnified, when fibres are used in concrete and hence demands a comprehensive know-how of compression
behavior of the concrete being used in any construction project or research program. In this study, complete
compression behavior of fibre based concrete, including strength, toughness and failure modes, have been thoroughly
studied to provide an adequate know-how about the behavior of the material. Though the increase in compressive
strength of concrete was found marginal, yet the increase in toughness was found significantly large, resulting in the
failure of concrete in ductile mode, which otherwise is brittle in nature.
KEY WORDS: Fibre factor, Compressive strength, Toughness, Failure mode.
INTRODUCTION
Testing of hardened concrete (determination of
mechanical properties) plays an important role in
deciding, controlling and confirming the quality of
cement concrete works. Systematic testing of raw
materials, fresh concrete and hardened concrete
are inseparable part of any quality control program
for concrete, which helps to achieve higher
efficiency of the material used and grater
performance of the concrete in a structural system
with regard to both strength and durability.
Many high-rise reinforced concrete buildings,
bridges and other structures have employed using
concrete of high compressive strengths. It is well
known that high strength concrete is more brittle.
As such the applicability of high strength concrete
in practice is severely limited by its more brittle
behavior. However, this brittleness can be
overcome by adding randomly distributed fibres in
the concrete body. The addition of fibres has
shown to increase both ductility and strength of
concrete. During last 30 years, steel fibre concrete
is being increasingly used in structural
applications and the research is in progress on the
use of steel fibres to reinforce structural members
in combination with conventional reinforcement.
Chen and Carson designed an investigation to
measure the influence of the length of randomly
oriented wire fibres (38.2, 25.4, 12.7 mm) on the
mechanical properties of fibre based concrete and
reported an increase of 45 % in compressive and
tensile strength for 12.7 mm long fibres at 0.75 %
volume fraction
4
. The compressive strength tests
were carried out by Shah and Naaman on concrete
specimens reinforced with different lengths and
volume fractions of steel fibres
13
. No significant
increase in stresses and strains at first crack were
obtained. However, extensive micro cracking was
observed on the surface of the failed specimens
indicating a significant contribution of the matrix
even after initiation of cracking. Khan et al.
investigated the behavior of FBC under
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E-ISSN 0976-3945


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compression
8
. The strength was found to increase
linearly with increasing fibre factor, defined as the
product of fibre aspect ratio and volume fraction
f
f
f
d
l
V.
Parviz Soroushia also reported the results of an
investigation of fibre-type effects on the
performance of steel fibre based concrete
11
.
Hooked fibres were found to be more effective
than straight and crimped ones in enhancing the
post peak energy absorption capacity of concrete
under compression stresses. The effect of fibre
reinforcement on compressive strength was found
relatively small, and different fibre types seemed
to act similarly in this regard.
Ramakrishna reviewed the advancements in fibre
concrete composites
12
. It was concluded that the
increase in compressive strength due to addition of
fibres varied from 0 to 20 % only, though there
was a substantial increase in the ductility and
toughness. The compressive toughness index
increased up to 30 % due to addition of fibres.
Krishnamoorthy and Kumar

showed that effect of
steel fibres on the compressive strength of
concrete was just marginal and reported that the
workability was adversely effected with the
addition of fibre volume exceeding 2 %
9
. But the
presence of fibres significantly improved the post
cracking behavior.
Kaushik et al. presented the results of an
investigation to study the properties of fresh and
hardened FBC
7
. The basic parameters were fibre
volume fraction (0.5, 1.0, 1.25, 1.5 %), maximum
aggregate size (10 mm and 20 mm) and fibre types
(straight and hooked). The compressive strength of
concrete with straight fibres increased by 9 to 21
% when 10 mm coarse aggregates were used and
the same was 11 to 27.5 % when 20 mm coarse
aggregates were used. Nitrajan et al. in his study
on FBC reported an improvement in stress strain
behavior of concrete due to fibres
10
. On similar
lines Ghugal carried out an investigation on the
effects of crimped steel fibres on the properties of
fresh and hardened concrete
5
. The author reported
an increase of 9.56% and 12.69 % in compressive
strength using fibres with aspect ratio of 38 and 50
respectively at 1 % volume fraction. Above 1.5 %
volume fraction of fibres, a decrease in the
strength was observed. Yeol Choi et al. concluded
that no real workability problem were encountered
with mixes containing 1.5 % of steel fibres
14
.
Authors also reported an increase of 120.2 % in
split tensile strength of lightweight fibre concrete.
Talukdar et al. carried a comprehensive study on
the Compressive, Flexural, Tensile and Shear
strength of concrete with fibres of different origins
and reported in general an improvement in the
performance of concrete due to fibres
3
. The
authors also pointed out the efficiency and
advantages of steel fibres in structural use over
other types of fibres.
Therefore, the literature shows that there exists a
vast difference in opinions by different researchers
regarding the compression behavior of fibre
concrete. Some of the authors have shown that
compressive strength of concrete has a definite
increase with the addition of fibres, while others
have opposed by quoting that the compressive
strength have no or least increase due to fibres.
Some useful guide lines regarding the use of FBC
are included in ACI committee 363 report, which
are not sufficient to predict the behavior of FBC in
compression
1
. In Indian contest no such
information is available in code form for the
structural use of FBC. The ultimate strain in
compression taken as 0.0035 in the code would be
very conservative when fibre concrete is used.
RESEARCH SIGNIFICANCE
Controversies in the prediction of compression
behavior of fibre concrete by different
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investigators have lagged the rational and unified
opinion regarding the characteristics of the
material. Therefore, there is a need to carry out
thorough investigation on the compression
behavior of steel fibre based concrete, so as to
develop useful guidelines for the structural
applications of the material.
EXPERIMENTAL PROGRAM
An extensive experimental study has been carried
out to investigate the compression behavior of
Steel-Fibre-Based Concrete, here-after referred to
as FBC. This has resulted into the experimentation
of 200 specimens, comprising of standard cube
specimens of 150 x 150 x 150 mm in size and
cylindrical specimens of 150 x 300 mm in size.
MATERIALS AND MIXTURE
The Fibre geometry and the proportion of concrete
mixture ingredients as used in this study are
defined in Table 1. The shape of different fibre
types is reflected in Fig. 1.



(a):0.45 mm dia fibres (b):1.0 mm dia fibres







(c): Crimped Flat fibres

Fig. 1: Fibre Shapes


Table 1: Characteristics of Concrete Mixes and Steel Fibres
Concrete Mixtures Steel Fibres
Mix
ID
Cement
Kg/m
3

C: Fa: Ca
w/c
ratio
Fibre type
Length
(mm)
Diameter
(mm)
Aspect
ratio
C1 440 1.0: 1.71: 2.85 0.35 Crimped-flat 50
0.8 x 2 mm
X-sec
33.5
C2 400 1.0: 1.80: 3.10 0.40 Crimped-round 36 1.0 36
Crimped-round 45 1.0 45
C3 360 1.0: 2.00: 3.40 0.45
Crimped-round 36 0.45 80
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FABRICATION OF SPECIMENS
Firstly the mixture of cement, coarse and fine
aggregates was blended in the mixer for about 30
seconds. 80 % of the total quantity of ‘solution of
water and plasticizer’ was then added into the
mixer with mixing allowed to continue for another
one minute. The fibres were then added manually
but slowly by hand with mixer continuing to rotate
for about 3 minutes. The remaining 20 % of
solution of water and plasticizer was then added
and the mixer was given another 5 to 7 rotations
before discharging. In the case of over mixing, the
fibres plucked out from the mix and readily
clumped to form fibre balls in the mix and is
referred to as indigestion or vomiting of fibres.
This indigestion was also found to be increasing at
increased fibre content of the mix. Fig. 2 shows
the formation of fibre balls in the mix due to over-
mixing. From each batch of concrete, 2 cubes (150
x 150 x 150 mm) and 2 cylinders (150 mm
diameter and 300 mm height) were cast. Before
casting, the moulds were de-assembled, cleaned,
oiled with mineral oil and then fitted back by
means of tightening the screws.
Then the cylindrical moulds were filled with given
concrete material in three layers with each layer
followed by vibration for a period of 6 to 8
seconds using vibration table, vibrating at 50
cycles/sec, ensuring adequate compaction; while
as the cube specimens were cast only in two layers
and compacted with the help of vibrations as
followed for cylindrical elements. The top surfaces
of the specimens were then finished manually with
the help of a steel-planner. On the following day,
the specimens were de-molded and submerged in
clean water for curing purpose. After 28 days the
specimens were taken out from water. The
surfaces of the samples were then cleaned, wiped
out with cotton and then left out in lab
environment until testing.


(a): Fibre balled concrete mix (b): Fibre Balls

Fig. 2: Balling of Fibres


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TESTING PROCEDURE
In this research program to study the behavior in
compression, cube and cylinder specimens were
tested in displacement controlled, closed loop
type, servo based hydraulic testing machine of 250
ton capacity. The rate of displacement was
maintained constant at 1mm/minute for all the
tests.
Prior to testing, each cylindrical specimen was
capped with Sulphur at the top only. The capping
operation was performed using a capping mould
having 15 mm depth groove at the base with
slightly larger dia than that of the specimen. The
mould was placed over a smooth platform with
surface exactly horizontal in position. The sulphur
in the form of solid rods was heated to melt and
poured into the groove of the mould. Immediately
the cylindrical specimen (with required capping
surface downwards) was allowed to slip in
between the edges of the mould to rest over the
molten sulphur for a period of about 3 minutes till
the sulphur again solidified. The specimen was
then removed from the mould and was allowed to
rest for at least two hours in lab environment
before testing. The capping procedure was
followed to insure parallel loading faces of the test
cylinders and constant length for all specimens.
Very precise measurements of deformation
characteristics of cylinders under compression
were recorded automatically with the help of data
acquisition system attached with the testing
machine. To eliminate the platen effect of the
machine over the longitudinal strain of the
specimen, a displacement fixture containing two
direct current LVDTs (linear voltage differential
transformer) and a load cell, was mounted over the
middle two-third portion of the specimen in
between two rings, 200 mm apart and a parallel set
of readings were taken for each specimen using a
separate data acquisition system assembled to
LVDTs. The average of the two LVDT readings
was processed for prediction of results. The
readings taken over the full length of specimen
through the data processing system of the machine
were used as a back check on the readings of the
LVDT fixture. It was observed that maximum
strength (load) of a specimen recorded with the
help of LVDT fixture was within ± 0.13% of the
maximum load recorded with the help of servo
system of the testing machine. The maximum
strain recorded over the full length of specimens
was found less by 2 to 5 % than that recorded over
the middle third portion of the specimen. An
average of readings of two specimens for each
property corresponding to average of two LVDT
readings for each specimen were taken and
processed for predicting stress-strain behavior of
Steel fibre based concrete in compression. The
assembly of LVDT fixture with the testing
machine is explained here through Fig. 3
Machine Platen
Machine Platen
Sulphur
Cap
LVDT
Load Cell

Fig. 3: LVDT Fixture System
TEST RESULTS
Specimens corresponding to 3 non-fibrous and 45
fibrous concrete mixes were subjected to
destructive testing to evaluate the influence of
fibres on the properties of concrete, such as
compressive strength and toughness
characteristics. Cracking patterns and deformation
characteristics of all the specimens were precisely
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noted down to reflect the physical behavior of
Steel Fibre Based Concrete (FBC). Postmortem of
some selected specimens was also carried out to
describe the internal structure of FBC. Table 2
shows the results of tested specimens (average
value) in compression. The variation in test results
of individual specimens of same kind was found
within ± 2.5 %. Therefore, the confidence level
was successfully achieved up to 97.5% and hence
indicates a good quality control on the production
and re-production of concrete.
Table 2: Properties of FBC in Compression
Concrete
Series
Fibre
Content
( % )
Fibre Aspect
ratio

Fibre factor
Cube Comp.
Strength (MPa)
Cyl. Comp.
Strength (MPa)
C/S
ijk

f
V
l / d
d
l
V
f
M
c

'
c
f
C100 0.0 54.69 43.18
C200 0.0 46.82 36.71
C300
0 0
0.0 33.73 26.49
S111 33.5 0.167 55.89 43.60
S112 36.0 0.18 56.83 44.90
S113 45.0 0.225 57.20 45.24
S114
0.5
80.0 0.40 57.73 46.19
S121 33.5 0.335 58.23 46.19
S122 36.0 0.36 57.90 48.14
S123 45.0 0.45 59.00 48.78
S124
1.0
80.0 0.80 58.78 51.16
S131 33.5 0.502 59.90 47.92
S132 36.0 0.525 63.65 49.65
S133 45.0 0.675 63.78 50.51
S134
1.5
80.0 1.20 57.20 45.76
S141 33.5 0.67 57.75 44.47
S142 36.0 0.72 61.49 49.89
S143
2.0
45.0 0.90 62.79 51.18
S211 33.5 0.167 48.06 37.06
S212 36.0 0.18 49.23 38.16
S213 45.0 0.225 48.71 37.80
S214
0.5
80.0 0.40 48.83 38.49
S221 33.5 0.335 49.08 38.53
S222 36.0 0.36 52.55 41.10
S223 45.0 0.45 53.08 41.80
S224
1.0
80.0 0.80 53.08 42.47
Continue…….

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S231 33.5 0.502 50.50 40.00
S232 36.0 0.525 52.36 41.47
S233 45.0 0.675 52.75 42.20
S234
1.5
80.0 1.20 45.62 35.13
S241 33.5 0.67 47.31 36.67
S242 36.0 0.72 48.44 40.08
S243
2.0
45.0 0.90 49.97 41.51
S311 33.5 0.167 35.19 26.75
S312 36.0 0.18 35.13 26.88
S313 45.0 0.225 34.64 26.95
S314
0.5
80.0 0.40 35.42 27.28
S321 33.5 0.335 34.86 27.54
S322 36.0 0.36 35.03 27.86
S323 45.0 0.45 35.42 28.34
S324
1.0
80.0 0.80 36.52 28.38
S331 33.5 0.502 35.26 27.68
S332 36.0 0.525 35.13 28.34
S333 45.0 0.675 37.32 29.00
S334
1.5
80.0 1.20 32.72 25.69
S341 33.5 0.67 31.78 25.43
S342 36.0 0.72 36.71 28.82
S343
2.0
45.0 0.90 38.72 29.43

The tabular results demonstrate the interaction of
fibre characteristics (volume fraction, and aspect
ratio of fibres) with the matrix properties of
concrete. The increase in compressive strength
was found marginal, but a function of fibre factor
(
d
l
V
f
). It was found insignificant (around 3 %)
at low values of fibre factor (0.16 – 0.23 here)
corresponding to low volume fraction (0.5 % in
this study) of fibres in the mix. However, for the
same volume fraction of 0.5 % of fibres with
higher fibre factor i.e. 0.4 here (aspect ratio equal
to 80), the increase in compressive strength was
found more than 7 %. Though the fibre content i.e.
0.5 % was same in both the cases, yet the increase
in strength was found more for the smaller
diameter fibres with higher aspect ratio and hence
higher fibre factor. This can be attributed to the
availability of more number of fibres (because of
smaller dia) over a given cross section, referred to
as fibre concentration. This was noticed on
carrying out the postmortem of few cylindrical
specimens. The specimens were cut both
longitudinally and transversely using a
mechanized stone-cutter and the cut surfaces were
examined for fibre count. Good number of fibres
were found available over a cut surface even when
small volume fraction of small diameter fibres
(0.45 mm here) were used as against same volume
fraction of large diameter fibres (1.0 mm here).
More number of homogenously distributed fibres
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provided more resistance to deformation and
hence resulted in more increase in strength.
Again at higher fibre content (more than 1.5 %),
the rate of increase in compressive strength was
not found convincing. The strength was found to
increase from 15 % on an average at 1 % fibre
content to 20 % at fibre addition of 2 %. Even in
some lean mixes (Cement content - 360 kg/m
3

here), a decrease in strength was observed at
higher volume fraction of fibres and can be
attributed to the inadequate compaction of
concrete due to significant loss in workability at
higher fibre content, resulting in air entrapment
and hence formation of voids in concrete. Mostly
in such samples (see Fig. 4) internal fibre
pocketing and voids were noticed, especially with
the specimens containing smaller diameter (0.45
mm) fibres. Also the mixes loaded with 2.5% fibre
content produced weak and honeycombed end
products. For reference, a pictorial view of
honeycombed fibre based specimens at higher
fibre content (2.5 %) is shown if Fig. 5.
Therefore, both low and very high volume
percentage of fibres of the order of 2 % or more,
does not contribute much to the performance of
concrete. The performance of fibre based concrete
was found satisfactory around 1 – 1.5 % of fibre
content, both from workability, strength and
durability point of view.

Fig. 4: Cut surface of a cylindrical specimen at 2 % fibre content

Fig. 5: Honeycombed Specimens at 2.5 % Fibre content

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Fig. 6(a) through Fig. 6(c) illustrates the influence
of fibre content and fibre aspect ratio on the
compressive strength of concrete. A rich, Fatty
and cohesive concrete mix has got better
efficiency to disperse more amounts of fibres to
produce reasonably a good quality concrete with
better structural performance. On the other hand
lean mixes could not digest effectively the fibres,
especially with large aspect ratio, and the concrete
produced was found to contain both voids and
fibre pockets. This may serve as a base for
aggressive environmental agents to seep into the
concrete and would certainly result in durability
problems of concrete. Therefore, the contribution
of fibres could be effectively used in high strength
concrete that is rich enough to disperse the fibres
homogenously. A decrease in compressive
strength of fibre-based concrete was noticed in this
study in contract to conventional concrete when
the mix was lean and the fibre content was high
enough. As seen in Fig. 7(c), a significant decrease
in strength of fibre concrete was noticed beyond
1.5 % fibre addition in lean concrete mixes (Mix
series-C3), especially when long fibres with
smaller diameter are used. This can be attributed
to the scarcity in paste material required to
envelop fibres and hence resulting in voided and
fibre pocketed concrete, in addition to clumping
affinity of fibres of large aspect ratio. However at
small volume fractions (around 1 %), the
performance of fibres with reasonably large aspect
ratio was found quite remarkable. From the view
of performance in terms of dispersion of fibres,
workability, strength and toughness, the concrete
containing fibres with aspect ratio of 45 was found
quite excellent.
mix series-C1(1 : 1.71 : 2.85, w/c = 0.35 )
0
5
10
15
20
0 0.5 1 1.5 2 2.5
Vol. fraction of fibres (%)
% increase in Strength
l / d = 33.5
l / d = 36
l / d = 45
l / d = 80

(a): Mix series- C1





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(a): Mix series- C2

mix series-C3(1 : 2 : 3.4, w/c = 0.45 )
-4
-2
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5
Vol. fraction of fibres (%)
% increase in Strength
l / d = 33.5
l / d = 36
l / d = 45
l / d = 80

(a): Mix series- C3

Fig. 6: Influence of fibre content and aspect ratio of fibres on Compressive Strength of Concrete


mix series-C2(1 : 1.8 : 3.1, w/c = 0.4 )
-10
-5
0
5
10
15
20
0 0.5 1 1.5 2 2.5
Vol. fraction of fibres (%)
% increase in Strength
l / d = 33.5
l / d = 36
l / d = 45
l / d = 80
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Mode of failure
The mode of failure of specimens with and
without fibres was precisely observed during
testing under compression. Fig. 7 shows the
typical mode of failure of cylindrical specimens in
compression. Two distinct types of failure modes
were observed during testing. The first type of
failure corresponds to specimens without fibres.
The crack propagation was found nearly parallel to
the loading direction and the specimen failed with
splitting-out of concrete and lashes (pieces)
spreading out in the vicinity. This can be attributed
to the brittle nature of concrete. The second type
of failure, shown in the figure, pertains to fibre
based specimens. The mode of failure was gradual
enough and several fibres pulled out randomly
from the surface of specimens during testing. The
irregular failure surface was seen in the failed
concrete, which is in contrast to that of plain
concrete. During testing, the popping sound of
fibres, failing through pull out, was one of the
special phenomena associated with fibre based
concrete. The type of failure was marked by the
bulging of the specimen in the lateral direction
with cracking along the outer surface near the
middle zone. This demonstrates how the addition
of fibres can improve the ductility in a brittle
concrete.


Fig. 7: typical failure mode of fibre based concrete under compression



0 % Fibre

0.5 % Fibre

1.0 % Fibre

(a)

(b)
(c)

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y = 1.2546x
R
2
= 0.9812
0
10
20
30
40
50
60
70
20 30 40 50 60
Cylinder Comp. Strength (MPa)
Cube Comp. Strength (MPa)

Fig. 8: Cube compressive strength Vs Cylinder Compressive Strength
Cube and cylinder strength
Some of the national codes including IS-456
13

consider the cube compressive strength as the
design parameter
6
, while as other national codes
like ACI-318 etc take up the cylinder compressive
strength as the design parameter
2
. Therefore, it
becomes imperative to correlate the cube and
cylinder compressive strength of fibre concrete for
diversity in using different national codes.
The values of cube compressive strength against
cylinder strength are plotted here in Fig. 8 and the
correlation between the two have been established
after carrying out the regression analysis of
experimental the results using the square fit. The
predicted expression is mathematically expressed
as
c
M =1.254
'
c
f

Where,
c
M =cube compressive strtength of concrete in
MPa
'
c
f =cylinder compressive strength in MPa
This expression is found to be in agreement with
those of previously established standards. As per
Indian philosophy, for the normal range concrete
(20 MPa to 50 MPa), the cube compressive
strength, in general, is taken approximately equal
to 1.2 times the cylinder compressive strength.
This shows that the relation between cube and
cylinder strength of FBC is more or less similar to
that of conventional concrete.
TOUGHNESS AND STRESS STRAIN
BEHAVIOR
Average load deformation curves for different
strength concretes, strengthened with different
types of fibres in various volume fractions, were
obtained using the setup, described in Fig. 3. The
data was fed back to the processing system and
finally average stress-strain curves for different
specimens were obtained. Fig. 9 shows the
experimentally expedited stress-strain curves for
different steel fibre concretes.
The graphical results reveal that for all the four
types of fibres in all the three mixes, the slope of
decending portion of stress strain curves decrease
at increased volume fraction of fibres.
This causes an increase in the area under the
curves and hence resulting in an increase in
toughness [Fig. 10(a)]. The increase in toughness
was found also found increasing with increasing
aspect ratio of fibres, as shown in Fig. 10(b).
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Mix series-C1 (1 : 1.71 : 2.85, w/c = 0.35 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
2.0 % Fib.
0 % Fib
10-3 x Strain
Stress ( Mpa)

Mix series-C2 (1 : 1.8 : 3.1, w/c = 0.4 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0 % Fib.
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
2.0 % Fib.
10-
3
x Strain
Stress ( Mpa)

Mix series-C3 (1 : 2 : 3.4, w/c = 0.45 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0 % Fib.
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
10-3 x Strain
Stress ( Mpa)




Mix series-C1 ( 1 : 1.71 : 2.85, w/c = 0.35 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0 % Fib.
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
10-3 x Strain
Stress ( Mpa)
Mix series-C2 (1 : 1.8 : 3.1, w/c = 0.4 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0 % Fib.
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
10-3 x Strain
Stress ( Mpa)
Mix series-C3 ( 1 : 2 : 3.4, w/c = 0.45 )
0
10
20
30
40
50
60
0 10 20 30 40 50
0 % Fib.
0.5 % Fib.
1.0 % Fib.
1.5 % Fib.
10-3 x Strain
Stress ( Mpa)
Fig. 9: Stress Strain curves of FBC
(a): 0.8 x 2 mm x sec. fibres with
aspect ratio of 33.5
(b): 0.45 mm diameter fibres with
aspect ratio of 80.0
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IJAET/Vol.II/ Issue I/January-March 2011/96-111

Therefore both the volume fraction and aspect
ratio of fibres resulted in a significant increase in
toughness, though there was either marginal or no
increase in compressive strength of concrete.
Since both volume fraction and aspect ratio of
fibres lead to similar improvements in toughness,
therefore the combined effect of fibre content and
aspect ratio of fibres can be better quantified by
expressing the toughness as a function of fibre
factor (product of volume fraction and aspect
ratio). The valus of toughness verses fibre factor
are plotted in Fig. 10© , and correlations have
been formulated through regression analysis using
a square-fit. The relations are shown graphically in
Figs. 11(a) through 11(c), and mathematically
expressed as
T. I = 2.4853
f
f
f
d
l
V + 2.6188
T. I = 1.682
f
V + 1.754
T. I = 0.0461
f
f
d
l
+ 1.663
The ascending portion of stress strain curves of
fibre concrete was found nearly independent (little
effect) of fibre factor. It was also found that the
addition of steel fibres caused an increase in strain
at peak stress without producing any significant
change in peak stress.
Therefore, stress strain profile of fibre based
concrete, in addition to usual characteristics of
concrete, is a function of fibre factor,
f
f
f
d
l
V.
Higher the fibre factor higher is the toughness.
ACI method was implemented here to measure the
toughness of concrete in compression. According
to this method toughness is expressed as the total
area under the stress strain curve up to a strain of
1.5 %, which is 5 times the ultimate concrete
strain i.e. 0.3 % as per ACI-318 code. To quantify
the toughness of fibre concrete in compression, it
is measured as the ratio of area under stress strain
curve of fibre concrete up to a strain of 1.5 % to
the area under the stress strain curve up to the
strain at first crack.

y = 1.682x + 1.754
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5
Volume fraction of fibres (%)
Toughness Index



y = 0.0461x + 1.663
0
2
4
6
8
10
0 20 40 60 80 100
Fibre Aspect ratio
Toughness Index
Fig. 11(a): Toughness vs Vol. fraction of fibres
Fig. 11(b): Toughness vs Aspect ratio of fibres
International Journal of Advanced Engineering Technology
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IJAET/Vol.II/ Issue I/January-March 2011/96-111

y = 3.2266x + 2.3507
0
2
4
6
8
10
0 0.25 0.5 0.75 1
Fibre factor
Toughness Index



CONCLUSIONS
The results and discussions on the experimental
study carried out on cube and cylinder specimens
of FBC, out-crop in the following conclusions.
• The addition of fibres to concrete mixes
causes significant reduction in workability
and results in the formation of fibre balls
in the mix when used in excess quantity
(more than 2 % here) or when over mixed.
• The addition of steel fibres to concrete
causes an increase in strain at peak stress
without producing any significant change
in peak stress. Therefore the failure strain
corresponding to peak stress is increased.
• Fibres do not increase much the
compressive strength of concrete.The
compressive strength was found to
increase from 0 % to 20 % due to addition
of fibres of different aspect ratios in
various volume fractions. A decrease in
compressive and flexural strength was
noticed at higher volume fraction of
fibres, 2 % or more in this study. This
decrease in strength was found more with
fibres of higher aspect ratio (0.45 mm dia,
36 mm long with l/d = 80). Optimum
performance was found to be shown by 1
mm dia, 45 mm long fibres, fibre factor =
45, both in terms of fresh and hardened
properties concrete.
• The relation between cube and cylinder
strength of FBC in compression is
comparable with that of conventional
concrete and can be expressed as:
c
M = 1.254
'
c
f

• Addition of fibres to concrete increased
the toughness considerably. The increase
in toughness was found directly
proportional to fibre factor and was found
marginally higher for lower grade of
concrete compared to higher grade of
concrete. It was found as much as 600 %
at fibre factor of 0.7.
• Toughness of fibre concrete is directly
dependent on fibre factor and can be
conveniently calculated using the
following proposed expression.
T. I.=2.4853
f
f
f
d
l
V + 2.6188


Fig. 10(c): Toughness vs Fibre factor
International Journal of Advanced Engineering Technology
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IJAET/Vol.II/ Issue I/January-March 2011/96-111

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