ce 04.pdf
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SELF COMPACTING CONCRETE
S.SARAVANAN (III YEAR CIVIL)
Mail id: [email protected]
S.RAJESHWARI (III YEAR CIVIL)
CONTACT: 9585620201
ABSTRACT:Self compacting concrete, also
referred to as Self consolidating concrete
(S.C.C) was developed first in 1980’s to
achieve durable concrete structure, with ease
in construction process and procedures., it has
proved beneficial economically because of a
number of factors like faster construction,
reduction in site manpower, better surface
finishes and grater freedom in design. As the
name indicates, we don’t need any extra
compacting and vibrating equipment in
placing S.C.C. Fresh S.C.C flows into place
and around obstructions under its own weight
to fill the formwork completely and self-
compact, without any segregation and
blocking, thereby need for vibration or
compacting can be eliminated resulting in
healthy working environment and better
quality concrete. For attaining the qualities
like flow ability, filling ability with minimum
segregation we need some extra ingredients
like Polycarboxylate based admixtures and
Viscosity modifying agents, in addition to the
ingredients used in normal day to day
concrete. Now a day’s the usage of S.C.C in
Japan is over 70% compared with that of
ordinary concrete, where as in Europe its
30%. In fact 300,000 m3 of S.C.C has been
used in the construction of Akashi-Kaikyo
Bridge, a suspension bridge with longest
central span in the world (2km).
1. INTRODUCTION
Self Compacting Concrete has left his early
stage of laboratory studies and has now
become an industrial product. Several building
applications, many in high volumes, show it
can be prescribed and used as a reliable
product in mass applications with confidence
in supply and casting. High strength self
compacting concrete has been used in many
projects and structures in a beneficial way,
which proved that self compacting concrete
can be adopted in day to day constructions.
Even though the initial cost of construction
may increase up to 30% due to usage of
admixtures like superpalsticizers, viscosity
modifying agents etc, the final bid of the
product is found to be about 12% lower
because of decreased labour, decreased
finishing cost and further more there is no need
for vibration and additional repair of the
surface (as the surface finish obtained to very
smooth)
Why Self compacting concreteCompacting procedures applied to the concrete
placed in a structure plays a major role in
gaining the initial as well as the ultimate
strength of the structure. It may not be always
possible to compact concrete exactly, where as
if vibration is not properly done; the strength
of structure obviously crushes to the
considerable extent. In some cases even though
extreme care is taken, if was found to be
difficult to make concrete spread evenly
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thought the structure and where a dense
reinforcement is unavoidable it literally a
tough task to make concrete spread
homogeneously. Casting monolithic structures
has been a though task using normal concretes,
whose flow ability is very less. So an urge for
development of a concrete which is flow able,
pump able, self levelling and homogeneous
when spread out rose. S.C.C is found to be
more users friendly and producing much
positive results in construction of Buildings
columns, foundations and walls, Pre cast
structures, Piers, Piles and Pile caps and all
kinds of repair works.
2. CHEMISTRY BEHIND
WORKABILITY
The most important principle for flowing and
unsegregable concretes like S.C.C is the usage
of superplasticizer combined with a relatively
high content of powder materials in terms of
Portland cement, mineral fillers, fine
aggregates etc.The admixtures like
superplasticizer, viscosity modifying agents,
air entraining agents are necessary to make
S.C.C. Super plasticizers or water (high range
water reducers) are low molecular weight,
negatively charged organic molecules that
primarily adsorb at solid water interface and
finally result in neutralising the opposite
surface charges on solid particles and cause all
particles to carry uniform charge of like sign (-
ve), which makes the concrete a workable mix
with a desired slump even at water cement
ratio’s below0.4.
Fig 1 Cement admixtures
These admixtures also result in variation of
strength (concretes of compressive strength >
50Mpa can be manufactured by using
admixtures and even tensile strength of self
compacting concrete at 7 days is found to be
grater than that of the normal concrete at 28
days) and other properties of concrete.
Therefore choice of good
admixtures and all other ingredients like
cement, sand, gravel etc is to be made
according to the purpose to achieve, required
slump and strength without segregation.
3. MIX DESIGN
There are certain basic thumb rules,
which should be followed during the mix
design of S.C.C
Maximum aggregate should never be
grater than 25 mm
Paste content should be about 400
litre/ M3
Water fines ratio (water/fines) should
be around 0.31 to 0.36
Fines content of about 500 to 600
kg/M3 (this could be any fine
material, which is puzzolonic or inert
in nature)
Coarse aggregate to sand ratio (coarse
aggregate/sand ratio) should be
roughly equal to one i.e., we have
almost equal volumes of coarse
aggregate and sand ratio in the equal
volumes, in contrast with the
conventional concrete, where we use
60% of the aggregate is coarse and
40% goes to fine aggregate
Superplasticizer can be used to obtain
fluidity
Viscosity modifier nay be used or
stability and robustness (strength)
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Flow chart mix design
Coming to the mix design procedures, there
are several methods that are actually followed
worldwide. One among them is as follows and
is the most accepted and followed.
A) OPTIMISATION OF THE PASTE
COMPOSITION
1. OPTIMUM SUPER
PLASTICIZER DOSE
2. OPTIMUM FILLER DOSE
B) OPTIMISATION OF AGGREGATE
SKELETON
A) OPTIMISATION OF THE PASTE
COMPOSITION:
Here paste refers to the system composed of
cement, water super plasticizer and filler
I.e. paste = cement + water + superplasticizer +
filler.
In self compacting concrete, we never use the
water cement ratio grater than 0.4
i.e. water/cement 0.4
Therefore to reach the paste composition we
need two things
How much of super plasticizer to be used
How much of fines to be used
3.1. OPTIMUM SUPER PLASTICIZER
DOSE:-
Hence first we need to calculate the amount of
superplasticizer to be added. The tests from
which the amount of super plasticizer to be
added can be found is known as MARSH
CONE TEST. Marsh cone is a hollow metal
cone of height about 30 centimetres with an
aperture of about 8mm at the bottom. Concrete
is poured in this cone and the fluidity or the
time taken for the concrete to flow through this
cone is measured. The test is simple. We just
pour some quantity of paste into the cone
(say500 ml) and the time taken for the paste to
flow is determined. For a certain super
plasticizer to cement ratio, let’s say if the super
plasticizer to cement ratio is very small, the
time taken for the paste to flow will obviously
be more. On other hand if we increase to super
plasticizer to cement ratio (i.e. increasing of
super plasticizer dosage) the time taken for the
paste to flow through the cone will be small or
shorter, but up to a certain point. Here what we
should notice is that the paste gets saturated at
a certain dosage of super plasticizer, what we
call as point of saturation of super plasticizer.
For easy understanding, I have represented it
in the graphical form as shown beside. And
that dose of super plasticizer is suited most for
the paste to be optimized.
Fig 2 range of super plasticizer
Now the second part of the paste optimization
is to calculate the dosage of filler.
3.2. OPTIMUM FILLER DOSE:
For the calculation of optimum dosage of
mineral filler, we use a test called MINI
SLUMP TEST. In this test we fill the mould
(as shown in the next page), with the paste of
known filler dose, and then the moulds is lifted
and how much the concrete spreads and the
time taken to reach so are noted down To have
a nice mix the diameter of spread of paste
should range between 170 mm to 190 mm
(here the basic idea is to prepare the paste,
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which is flowable but not too much, i.e. the
paste should be flow able but at the same time
it should be sticky and cohesive) and the time
taken should be about 2 to 4 seconds. The base
of mini slump cone is about 4 centimetres
Fig 3 slump test
Therefore by using mini slump cone test the
optimum filler dosage can be determined as
explained above and with this we obtain the
paste which is fluid and also cohesive.
The next step to optimize the aggregate
skeleton
B) OPTIMISATION OF AGGREGATE
SKELETON:
This test is also a simple one which needs a
quite simple apparatus. In this test we measure
the density (or) the unit weight of mix
containing sand and gravel. When we add
more sand, the density (or) unit weight of the
mix is increased. Here we vary sand content
from 0 to a certain value where the further
addition of the sand does not increase the
density of the mix, which indirectly results in
the minimum voids in the system. So a sand
gravel ratio that gives a highest compactness
can be obtained by this test.
Finally we determine the mix
proportion by varying the quantities of
optimised paste and aggregate skeleton to
obtain the self compacting concrete of high
strength and high workability.
4. FIELD CONSIDERATIONS
MIXING
There is no requirement for any specific mixer
type. Forced action mixers, including paddle
mixers, free fall mixers, including truck
mixers, and other types can all be used. The
mixing time necessary should be determined
by practical trials. Generally, mixing times
need to be longer than for conventional mixes.
PLACING
Though it is easier to place SCC than ordinary
concrete, the following rules are advised to
minimise the risk of segregation:
~>Limit the vertical free fall distance to 5 m
~>Limit the permissible distance of horizontal
flow from point of discharge to10 M.
FORMWORK
Generally the wooden formwork can be used
for S.C.C, but the external reinforcement needs
to be very close and wall height and length.
Form work of S.C.C needs no difference than a
good sealing formwork used for normal
concrete. But S.C.C will find way out of the
formwork if formwork is weak in sealing.
CURING
SCC tends to dry faster than conventional
concrete because there is little or no bleed
water at the surface. Initial curing should
therefore be commenced as soon as practicable
after placing in order to minimise the risk of
shrinkage cracking
APPLICATIONS
S.C.C can be used in any kind of
constructions, because of its unique properties
like flowability and also as it is very easy to
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obtain the high range of strength values. Some
of the pioneering applications are
Bridges (anchorage blocks and piers)
Liquefied gas storage tanks
Tunnel linings
Building components like columns,
foundations and walls
Sandwich structures (steel concrete
structures)
Repair works (girders, tunnel linings)
Prefabricated elements (panels,
furniture, beams etc)
Piles and pile caps
Monolithic structures
5. ADVANTAGES IN USING OF SELF
COMPACTING CONCRETE:
Reduction of time of construction.
Improved quality.
Improved working environment.
Over all cost reduction.
It is very economical as the final bid
of the structure reduces to 10% by use
of S.C.C
Waste materials like fly ash can be
used in a beneficial way to obtain a
better product.
A range of high quality of surface
finishes can be obtained.
Uniform spacing of aggregate on thesurface can be achieved without any
kind of vibration.
Some other materials like white
marble powder crushed black granite
aggregates can be used to produce an
aesthetic look.
Tensile strength of reinforced Self
compacting concrete is more than that
of normal concrete
6. CONCLUSION
Self compacting concrete can be used in any
type of construction as the placement of
concrete mix is not delayed and a smooth
aesthetic finish of surface can be obtained by
using no vibration techniques and is very good
when considered in environment point of view
as we are using fly ash, and other puzzolonic
material like blast furnace slag etc, which may
cause pollution when dumped or left out to
atmosphere (there are cases where 70% of the
cement in SCC is replaced with fly ash).
Moving one step forward, small pieces steel
fibres can be added to the self compacting
concrete mix, which acts as a concrete with
self reinforcement.
7. REFERENCES
[1]. Christopoulos, C., Tremblay, R., Kim, H. J. and
Lacerte, M., (2008). ‚Self -Centering Energy Dissipative
Bracing System for the Seismic Resistance of Structures:
Development and Validation,‛ ASCE Journal of Structural
Engineering, 134(1), 96 – 107.[2] Clifton, G.C., (2005). Semi-Rigid joints for moment
resisting steel framed seismic resisting systems. PhD
Thesis, Department of Civil and Environmental
Engineering, University of Auckland.
[3]. Marine structure engineering by Gregory P. Tsinker.
[4].Concrete technology by M.S. Shetty
[5].Corrosion of steel in concrete by John P. Broomfield
[6]. Google
[7].Wikipedia