Transcript
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Paper Presentation
On
SELF COMPACTING CONCRETE
Submitted by
S.SREEKANTHIV B.Tech. (Civil)
Email id: sreekanth.civil@yahoo.co.in
DEPARTMENT OF CIVIL ENGINEERING
SREEKALAHASTEESWARA INSTITUTE OF TECHNOLOGY SRIKALAHASTI - 517 640CHITTOOR DISTRICT, A.P.
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Contents
• Abstract
• Introduction
• Chemistry behind workability
• Mix Design
• Tests on fresh concrete
• Field considerations
• Applications
• Advantages
• Conclusion
• Acknowledgements
• References
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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)
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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.
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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,
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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
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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.
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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.
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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
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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.
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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
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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
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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
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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)
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