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SELF COMPACTING CONCRETE

S.SARAVANAN (III YEAR CIVIL)

Mail id: saransenguttu@gmail.com

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