08-10-2014, 02:09 PM
FLEXURAL CHARACTERISTICS OF STEEL FIBRE
REINFORCED SELF COMPACTING CONCRETE BEAMS
FLEXURAL CHARACTERISTICS.pdf (Size: 454.32 KB / Downloads: 21)
Abstract.
Fibre reinforced concrete with steel fibres attracted the attention of engineers and researchers during the
last five decades. In recent times, self compacting concrete has been accepted as a quality product and are widely
used. A large number of studies are available with respect to several parameters viz., flexural strength, load
deflection behaviour, toughness, ductility, crack control, effects of beam dimensions, concrete filling sequence,
flexural toughness parameters etc., of conventional fibre reinforced concrete. The present study aims to study the
flexural behaviour of scc beams with steel fibres. An experimental programme has been designed to cast and test
three plain scc beams and six scc beams with steel fibres. The experimental variables were the fibre content (0vf%,
0.5vf% and 1.0vf %) and the tensile steel ratio (0.99%, 1.77% and 2.51%). The type of fibre configuration used was
crimped fibre 30mm in length, an equivalent diameter of 0.5mm and with stirrups of 2l-8 mm dia bars 150mm c/c.
The scc design mix was proportioned to obtain a compressive strength of 70 mpa and yield strength of steel used was
595 MPa. The experimental constants were the geometry of the beams (2000x125x250)mm as well as the test set up.
A series of trials was made to obtain a suitable mix proportioning of SCC based on “Nan Su et al” method. The
cracking, deflection and ultimate failure behaviour were experimentally studied. The available theoretical formulae
proposed by Suji et al and Samir for FRC beams with conventional vibrated concrete beams were examined for the
case of SFRSCC beams with respect to load deflection behaviour, ultimate moment, deflection and width of crack at
service loads. A simplified effective moment of inertia function is proposed for the estimation of deflections of
SFRSCC beams at all stages of loading. The results are presented and discussed.
INTRODUCTION
Self-Compacting concrete offers many benefits to the
construction practices i.e., the elimination of the
compaction work results in reduced costs of placement,
shortening of the construction time and therefore
improved productivity. It also overcomes the congestion
of steel reinforcement in case of heavily reinforced
structures viz., seismic resistant structures. The technical
benefits of SCC are extended to crack bridging ability,
higher toughness and long-term durability with the use
of fibres. Addition of short discrete randomly oriented
steel fibres improves many of the engineering properties
of conventional concrete. Fibres bridge the cracks and
retard their propagation and also decrease the width of
cracks, thereby improving the tensile strength or the post
cracking behaviour. Steel fibres were incorporated in
SCC leading to the development of SFRSCC. The
applications include the pre-stressed sheet piles and steel
fibre reinforced tunnel segment.
Literature review
Sonebi et al [1] reported the structural performance of
full scale beams cast using ordinary concrete and SCC
with steel fibres. A total of eight beams of class C35 and
C60 were cast and tested. His investigation showed that
the ultimate moment capacity of SCC60 beam was
comparable with RC60 beams. The maximum deflection
of SCC60 beam was higher than that of RC beam.
Ganesan et al [2] reported an experimental investigation
consisted of casting and testing of eighteen SFRSCC
flexural elements. Their study showed that all the
theoretical models available in the literature were found
to underestimate the ultimate strength of SFRSCC
beams. They suggested that modifications are required in
these models to reduce the range of the predicability of
the ultimate moment of SFRSCC members.
EXPERIMENTAL WORK
Cement conforming to IS was used. Locally available
river-sand, free from silt and organic matters and passing
through 4.75mm sieve was used. The specific gravity
was 2.56, loose density was 1500kg/m3
, packed density
was 1651 kg/m3
and the fineness modulus was 2.43.
Locally available crushed granite aggregate passing
through 12.5mm and retaining on 4.75mm was used for
all of the mixes of SCC and CVC. The specific gravity
of CA used was 2.66, loose density was 1373 kg/m3
and
packed density was 1496 kg/m3
. Class F fly-ash from
Raichur Thermal Power Plant was used as cement
replacement material for SCC mixes. The specific
gravity of the fly-ash used was 2.4. Potable water was
used for both mixing and curing. Glenium B233,
carboxylic ether polymer was used and the dosage was
between 0.5 and 0.15 litres per 100 kg of cementitious
material. Viscosity Modifying Agent used was Glenium
Stream-2. The dosage recommended was between 0.5 to
1.0 litres per cubic meter of binder. The steel crimped
fibres used were low carbon drawn flat wires. The length
of fibre was 30mm. The aspect ratio was 60. The width
was 2 to 2.5mm and the tensile strength was 400 to 600
MPa. 8mm, 10mm, 12mm and 16mm diameter steel rods
were used as main reinforcement while 8mm diameter
rods were used as stirrups. The yield strength of
reinforcement was 569 MPa. Nan Su et al [5-6] method
has been used to arrive at the mixing proportion of SCC
and steel fibres were incorporated in the same SCC mix
for casting SFRSCC specimens with different volume
factors
. SUMMARY AND CONCLUSIONS
An experimental program has been designed to cast and
test nine Self Compacting Fibre Reinforced Concrete
beams with three steel ratios (0.98, 1.77 and 2.51) and
three different volume fractions of fibres (0%, 0.5% and
1.0%) under flexure.
The ultimate moment carrying capacity of the beams was
calculated using ‘modifying’ Suji et al’s method and
Samer’s method. It is noted that the former method over
estimates the Ultimate moments and the average ratio of
Mu)thr to Mu)exp was 1.32 with a CV 0.16 while in the
later method the respective values were 1.90 and 0.17
respectively.
An effective moment of inertia function has been
proposed and is able to predict the short term deflection
satisfactorily at all stages of loading.
The available formula for FRC beams given by RILEM
has been examined for SFRSCC beams and is found that
it requires modification and as such is unable to predict
the width of crack satisfactorily.