self compacting concrete new

8/4/2019 Self Compacting Concrete New

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Paper Presentation

On

SELF COMPACTING CONCRETE

Submitted by

S.SREEKANTHIV B.Tech. (Civil)

Email id: [email protected]

DEPARTMENT OF CIVIL ENGINEERING

SREEKALAHASTEESWARA INSTITUTE OF TECHNOLOGY SRIKALAHASTI - 517 640CHITTOOR DISTRICT, A.P.



Contents

• Abstract

• Introduction

• Chemistry behind workability

• Mix Design

• Tests on fresh concrete

• Field considerations

• Applications

• Advantages

• Conclusion

• Acknowledgements

• References



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).

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 concrete

Compacting 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

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.

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. A partial replacement of

Portland cement (sometimes up to 70% of cement) with inert materials like GGBS

(Ground Granulated Blast Furnace Slag) was found to be producing positive results in

terms of physical properties, resistance to segregation, strength level, particularly in mass

concrete structures exposed to restrained thermal stress produced by cement heat

hydration. 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.



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.

SPECIFICATIONS OF INGREDIENTS

Generally a good quality of cement like 53 grade cement is preferred but it may

vary according to the grade of S.C.C needed.

The maximum size of the aggregates depends on the particular application and is

usually limited to 20 mm. the particles smaller than 0.125 mm contribute to the

powder content. The moisture content in the aggregates should also be closely

monitored and must be taken into account.

Suitability is established for mixing water and for recycled water from concrete

production conforming that the minerals and other elements do not effect the

properties of concrete Admixtures used shall comply with EN 934-2:2000 (including annex A), where

appropriate superplasticizer is an essential component of S.C.C to provide the

necessary workability. Other types may be incorporated as necessary, such as

Viscosity modifying agents (VMA) for stability, air entraining admixtures,



retarders for control of setting etc. supperplasticizers like Glenium b 232, Structro

100 are found to be more effective.

Finely crushed lime stone, Dolomite (dolomite may pose a durability risk due to

alkali carbonate reaction and therefore it may not preferred always) or Granite

may be used to increase the amount of powder, the fractions less than 0.125 mm

will be of most benefit.

Fly ash is a fine inorganic material with pozzolanic properties, which can be

added to S.C.C to improve its properties and also as a substitute of cement to

certain extent.

Silica fume gives very good improvement of the rheological as well as the

mechanical and chemical properties. It also results in the improvement in the

durability of the concrete.

Ground Granulated Blast furnace Slag (GGBS) is a fine granular and mostly

latent hydraulic binding material, which can also be added to S.C.C to improve

the physical and other properties of the concrete.

Ground Glass Filler is usually obtained by finely grinding recycled glass. The

particle size should be less than 0.1 mm and the specific surface area should be

2500 cm2/g.

In addition to all the above ingredients, some other pigments and admixtures shall also be

used according to the working conditions and requirements.

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



Locally availablematerials like cement and

other ingredients

53 grade cement

Water cement ratioshould not exceed

0.4

Maximum aggregate sizeis not to be more than

25mm in general. But toachieve high strength mix

agg size is confined to15mm

Dosage of super

plasticizer Dosage of mineral

filler Sand to gravel ratio

Aggregate skeleton

Paste composition Composition of concrete obtained by

varying pastevolumes

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)

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.

We consider concrete asa composite i.e., a

composite consisting of

two phases namely paste

phase and aggregate

phase; an optimization

scheme is followed to

optimize paste phase

and aggregate skeleton

separately. In the

representation, the parts

that are shaded are to be determined, which we call asmix design.



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

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.



Now the second part of the paste optimization is to calculate the dosage of filler.

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, 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

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.

TESTS ON FRESH S.C.C

The properties that are examined for a mix of known paste and aggregate volumes are

1. Filling ability 2.passing ability 3.Segregation

Tests on filling ability Tests on passing ability Tests on segregation

Slump flow test L-box test Wet sieving test

V- Funnel test J-ring test Penetration test

Speaking practically, the testes on segregation are not generally performed because if the

mix segregates, it can be found out while performing the other tests like slump fellow test

etc. But if we need to calculate the segregation, the need to tests on segregation is

unavoidable.

After performing all the above tests on different optimised

paste and aggregate proportions, the proportion which gives the values that coincides

with the acceptable range of values (shown in next page) is conformed for the field use.



Acceptable range of values for different tests

And finally talking about the strength of dry and cured self compacting concrete, the

compressive strength of cylinder of standard specifications should be grater than 50 MPa

(7 days compressive strength) and the gravel in the concrete structure should be

uniformly distributed.

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.

METHOD

(TEST) UNIT

TYPICAL RANGE OF VALUES

MINIMUM MAXIMUM

Slump Flow (50 cm) Seconds 2 5

J-Ring mm 0 10

V- Funnel at t = 5 seconds 6 12

L – Box H2/H1 0.8 1.0

U - Box ( R 1-R 2 ) mm 0 30



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 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



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 the surface 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

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.

ACKNOWLEDGEMENT

We are exactly, indeed happy to express my selves for having guide ness from our

beloved principal Dr.K.JAYACHANDRA garu, Head of the Civil Engineering

Department Dr.K.JAYACHANDRA garu and all the staff of my civil engineering

department for giving the permission to submit this paper presentation. They have

extremely co-operated me in preparation and in motivating me. We also need to thank

Google website for acting as a single platform for every search word.

REFERANCES

1. A seminar on Self compacted concrete by Dr.Ravindra Gettu, IIT, Chennai, The

concrete website, several e-books and journals downloaded from various websites

2. A text book on properties of concrete by A.M.Neville

3. A text book on concrete technology by R.N.Swami,

4. Concrete technology test book by A.M.Neville and J.J.Brooks.

5. Information provided by several civil engineers in Concrete technology

community (Orkut)

self compacting concrete new

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