When large quantity of heavy reinforcement is to be placed in a reinforced
concrete (RC) member, it is difficult to ensure that the formwork gets
completely filled with concrete, that is, fully compacted without voids or
honeycombs. Compaction by manual or by mechanical vibrators is very difficult
in this situation. The typical method of compaction, vibration, generates
delays and additional cost in the projects. Underwater concreting always
required fresh concrete, which could be placed without the need to compaction;
in such circumstances vibration had been simply impossible. This problem can
now be solved with self-compacting concrete.This type of concrete flows easily
around the reinforcement and into all corners of the formwork. Self-compacting
concrete (SCC) describes a concrete with the ability to compact itself only by
means of its own weight without the requirement of vibration. Self-compacting
concrete also known as Self-consolidating concrete or self levelling concrete.Fig.1 shows
the flow of Self-compacting concrete.
Self-compacting concrete is placed or poured in the same way as ordinary
concrete but without vibration. It is very fluid and can pass around obstructions
and fill all the nooks and corners without the risk of either mortar or other
ingredients of concrete separating out, at the same time there are no entrapped
air or rock pockets. This type of concrete mixture does not require any
compaction and is saves time, labour and energy. The surface finish produced by
self-compacting concrete is exceptionally good and patching will not be
necessary.
Self-compacting concrete has been successfully used in France, Denmark, the
Netherlands and UK, apart from Japan. It is gaining wide acceptability because
no vibration is needed and noise pollution is eliminated. The construction
process is safer and more productive. 
Fig.1-Flow of SCC
MATERIALS
The Materials used in SCC are the same as in conventional concrete except
that an excess of fine material and chemical admixtures are used. Also, a
viscosity-modifying agent (VMA) will be required because slight variations in
the amount of water or in the proportions of aggregate and sand will make the
SCC unstable, that is, water or slurry may separate from the remaining
material. The powdered materials are fly ash, silica fume, lime stone
powder, glass filler and quartzite filler. The use of pozzolanic materials
helps the SCC to flow better. The pozzolanic reaction in SCC, as well as in
Conventional Slump Concrete (CSC), provides more durable concrete to
permeability and chemical attacks.
To achieve a high
workability and avoid obstruction by closely spaced reinforcing, SCC is
designed with limits on the nominal maximum size (NMS) of the aggregate, the
amount of aggregate and aggregate grading. However, when the workability is
high, the potential for segregation and loss of entrained air voids increases. These
problems can be alleviated by designing a concrete with a high
fine-to-coarse-aggregate ratio, a low water–cementitious material ratio (w/cm),
good aggregate grading, and a high-range water-reducing admixture (HRWRA).
Following are bases which are commonly used as superplasticizers.
·
Modified
Lignosulfonates (MLS).
·
Sulfonated
Melamine Formaldehyde (SMF)
·
Sulfonated
Naphthalene Formaldehyde(SNF)
·
Acrylic Polymer
based(AP)
Copolymer of Carboxylic Acrylic
Acid with Acrylic Ester(CAE)
Cross Linked Acrylic Ploymer(CLAP)
Polycarboxylatethers(PCE)
Multicarboxylatethers(MCE)
Polyacrylates
·
Combination of
above
Different bases of New Generation super Plasticizers or High Water reducing
agents(HRWRA) have different water reduction capacities. The advantage of this
water reduction can be taken either to increase the strength as in high
strength concrete or to obtain a better flowability as in case of self
compacting concrete.
PRODUCTION OF SCC
Based on the original conception of Okamura and Ozawa, in general three
types of SCC can be distinguished:
a) Powder type self compacting concrete: This is proportion ed to give the required self
compatibility by reducing by reducing the water-powder (material < 0.1mm)
ratio and provide adequate segregation resistance. Superplasticizers and air
entraining admixtures give the required deformability.
b) Viscosity agent type self compacting concrete: This type is proportioned to provide self
compaction by the use of a viscosity modifying admixture to provide segregation
resistance.Superplasticizers and air entrainment admixtures are used for
obtaining the desired deformability.
c)combination type self compacting concrete: This type is proportioned so as to obtain
self compatibility mainly by reducing the water powder ratio, as in
the powder type ,and a viscosity modifying admixture is added to reduce the
quality of fluctuation of the fresh concrete due to the variation
of the surface moisture content of the aggregates and their gradations
during the production .This facilitates the production control of
the concrete.
Test Methods for Self Compatibility
Conventional workability tests, devised for normal
ranges of concrete mixtures are not adequate for self-compacting concrete, because
they are not sensitive enough to detect the tendency to segregation. For
example, a slump test may show collapse, ( a slump of say 280 mm) and yet in
one case the mixture may be stable and in other cases either the aggregate may
settle down or the slurry may tend to “run”. Therefore test equipment was
fabricated for judging the following characteristics.
(1) Self-compatibility: The U-tube test gives an indication of the
resistance of the mixture to flow round obstructions in a U-type mould, Fig
2. This test also detects the tendency of the coarse aggregate
particles to stay back or settle down, when the mixture flows through
closely-spaced reinforcements.
(2)Deformability: The slump flow test as specified by the Japan
Society of Civil Engineers (JSCE) judges the ability of concrete to deform
under its own weight against the friction of the base, Fig 3.
This test, however, cannot evaluate whether the concrete will pass through the
space between the reinforcement bars. This test is useful also as a
routine control test, to detect the tendency for slurry to separate from the
mixture.
(3)Viscosity: Viscosity of the mortar phase is obtained by a
V-funnel apparatus, Fig 4.This is useful for
adjusting the powder content, water content and admixture dosage.
(4)Filling ability
test: It is also
used to determine the ability of the concrete to deform readily through closely
spaced obstacles.(fig.5)
Many different methods have been developed to
characterise the properties of SCC. No single method has been found till
date which characterises all the relevant workability aspects and hence, each
mixed has been tested by more than one test method for the different
workability parameters.
Fig.3.slump flow apparatus
Fig.5.Filling ability test
apparatus
PROPERTIES OF SCC
Hardened properties of SCC
Development
of concrete strength with time: The compressive strength, as one of
the most important properties of hardened concrete, in general is the
characteristic material value for the classification of concrete in national
and international codes. For this reason, it is of interest whether the
differences in the mixture composition and positive dissimilarities in the
microstructure, as mentioned before, affect the short and long term
load-bearing behaviour. Accordingly, clarification is still necessary to
determine whether the hardening process and the ultimate strengths of SCC and
conven-tional concrete differ. After 28 days the reached compressive strength
of SCC and normal vibrated concrete of similar composition does not differ
significantly in the majority of the published test results. Isolated cases,
however, showed that at the same water cement ratios slightly higher
compressive strengths were reached for SCC. At the current time there is
insufficient research to result in generalized conclusions with this
fact. The comparison of hardening processes shows that the strength
development of SCC and conventional concrete is similar, Fig. [6]. Some of the
published test results show that an increase of the cement content and a
reduction of filler con-tent at the same time increases the initial concrete
strength and the ultimate concrete strength. For young SCC aged up to 7 days
the relative compressive strength spreads to a greater extend as given in the
CEB-FIB Model Code 90, whereas higher values as well as lower ones are reached.
Especially if limestone powder is used higher compressive strengths are
noticeable at the beginning of the hardening process.
Splitting tensile
strength :All parameters which influence the characteristics of the microstructure of
the cement matrix and of the interfacial transition zone (ITZ) are of decisive
impor-tance in respect of the tensile load bearing behaviour. By evaluating the
created database it could be shown, that most results of the measured splitting
tensile strength values are in the range of valid regulations for normal
vibrated concrete with the same compressive strength. However, in about 30% of
all data points a higher splitting tensile strength was stated, Fig. [7].
Fig.6 Development of concrete strength with time
acc. to CEB-FIB Model Code 90
Fig. 7: Splitting tensile strength at SCC in
comparison to CEB/FIB Model Code9
Hence it appears the
tendency of a higher splitting tensile strength of SCC. Likely as not, the
reason for this fact is given by the better microstructure, especially the
smaller total porosity and the more even pore size distribution within the
interfa-cial transition zone of SCC. Further on a denser cement matrix is
present due to the higher content of ultrafines. The time development of
tensile strength of SCC and normal vibrated concrete are subjected to a similar
dependence. Only few publications about SCC refer to a more rapidly increase of
the tensile strength opposite to the compressive strength.
Fig. 8: Modulus of elasticity of SCC in comparison to CEB-FIB Model Code 90
|
Modulus of elasticity:As it is known, the
modulus of elasticity of concrete depends on the proportion of the Young´s
moduli of the individual components and their percentages by vol-ume. Thus, the
modulus of elastisity of concrete increases for high contents of aggregates of
high rigidity, whereas it decreases with increasing hardened cement paste
content and increasing porosity. A relative small modulus of elasticity can be
expected, because of the high content of ultrafines and additives as dominating
factors and, accordingly, minor occurrence of coarse and stiff aggregates at
SCC. Indeed, it was shown by analysing the database that the modulus of elasticity
of SCC can be up to 20 % lower compared with normal vibrated concrete having
the same compressive strength and made of the same aggregates. Nevertheless, it
is mainly still in the range of the CEB-FIB Model Code 90, Fig.[8]
TECHNICAL ADVANTAGES OF
SELF-COMPACTING CONCRETE
Simple inclusion even in complicated formwork and tight reinforcement
Higher installation
performance since no compaction work is necessary which leads to reduced
construction times, especially at large construction sites.
Reduced noise pollution
since vibrators are not necessary.
Higher and more
homogenous concrete quality across the entire concrete cross-section,
especially around the reinforcement.
Improved concrete
surfaces (visible concrete quality).
Typically higher early
strength of the concrete so that formwork removal can be performed more
quickly.
APPLICATION
Current condition on application of self-compacting concrete in Japan:
After the development of the prototype of self-compacting concrete at the
University of Tokyo, intensive research was begun in many places, especially in
the research institutes of large construction companies. As a result,
self-compacting concrete has been used in many practical structures. The
first application of self-compacting concrete was in a building in June
1990. Self-compacting concrete was then used in the towers of a
prestressed concrete cable-stayed Shin-Kiba Ohashi bridge in 1991.
Lightweight self-compacting concrete was used in the main girder of a cable-stayed
bridge in 1992. Since then, the use of self-compacting concrete in actual
structures has gradually increased. Self-compacting concrete has been
successfully used in France, Denmark, the Netherlands,Germany,USA and UK, apart
from Japan.
A typical application
example of Self-compacting concrete is the two anchorages of Akashi-Kaikyo (Straits)
Bridge opened in April 1998, a suspension bridge with the longest span in the
world (1,991 meters) (Fig. 9). The volume of the cast concrete in the
two ahchorages amounted to 290,000 m3. A new construction system, which makes
full use of the performance of selfcompacting concrete, was introduced for
this. The concrete was mixed at the batcher plant beside the site, and was the
pumped out of the plant. It was transported 200 meters through pipes to the
casting site, where the pipes were arranged in rows 3 to 5 meters apart. The
concrete was cast from gate valves located at 5 meter intervals along the
pipes. These valves were automatically controlled so that a surface level of
the cast concrete could be maintained. In the final analysis, the use of
self-compacting concrete shortened the anchorage construction period by
20%, from 2.5 to 2 years.
Self-compacting concrete
was used for the wall of a large LNG tank belonging to the Osaka Gas Company,
whose concrete casting was completed in June 1998. (Fig.10) The volume
of the selfcompacting concrete used in the tank amounted to 12,000 m3. The
adoption of self-compacting concrete means that
(1) the number of lots
decreases from 14 to 10, as the height of one lot of concrete casting was
increased.
(2) the number of
concrete workers was reduced from 150 to 50.
(3) the construction
period of the structure decreased from 22 months to18 months.
Self-compacting concrete
is often employed in concrete products to eliminate the noise of vibration.
This improves the working environment at plants and makes it possible for
concrete product plants to be located in the urban area. The annual production of
concrete products using self-compacting concrete exceeded 200,000 tons in 1996
.
Application in under water construction
40000 m3 of concrete placed under water (Fig.
11-13) by using the Tremie method for the construction of a dry dock.
· Massive structures such as reinforced foundations
for skyscrapers (Fig.14-16)
·
Vertical Walls
& Columns Congested Re-Bar(Fig.17)
·
Stripped SCC
Wall (Fig.18)
·
SCC Pumped into Column(Fig.19)
·
SCC Used In
Block fill(Fig.20)
·
SCC Horizontal
Application(fig.21-22)
·
RMC application(fig.23)
·
Precast concrete element plants.(Fig.24)
 Fig.10 LNG Tank |
Fig.9 Anchorage 4A of AkashiKaikyo
|
||
concrete under water
|
removing sea water
SCCin a reinforced slab foundation of a skyscraper
(Commercial Center) in New York
|
||
|
Fig.15 View of the reinforced slab foundation
Placed under water without Vibration
|
||
 Fig.17 Vertical Walls & Columns Congested Re-Bar |
 Fig.18 Stripped SCC Wall |
||
Fig.20 SCC Used In Block fill
|
Fig.19 SCC Pumped into Column
|
||
Fig.21SCC Used in roof
|
Fig.23 Placement of SCC
|
Fig.24 Finished precast unit
CONCLUSIONS
·SCC is made from the ingredients, which are almost same
used in producing in conventional concrete. Thorough understanding of role
played by each of the ingredient of SCC is essential.
·Properties of
fresh and hardened SCC should be established in the laboratory before their use
in the field. Even though the initial cost of SCC is comparatively higher than
the conventional concrete. Considering the long service of the structure,
minimum maintenance, labour cost, cost due to the vibrators required, benefit
cost ratio is very much in favour in case of SCC.
·Self
Consolidating Concrete, as well as Conventional Slump Concrete, requires proper
mixt proportion to become a durable concrete.
·The uses of
pozzolanic materials, such as slag, fly ash, silica fume, etc., will help SCC
more durable, otherwise these are waste products demanding with no practical
applications and which are costly to dispose of.
· The use of
proper super plasticizing admixture in combination with proper air entraining
admixture is the absolute key to durable concrete due to freeze-thaw and
scaling resistance.
· Advantage with
respect to sound pollution.
· Considerable
improvements in exposed surface (Fair Faced Concrete)
Self compacting concrete
is ideal for concrete parts with complicated shapes and elements with high
quality visible concrete.
Vibrating
concrete in congested locations may cause some risk to labour in addition to
noise stress. There are always doubts about the strength and durability placed
in such locations. So it is worthwhile to eliminate vibration in practice, if
possible.
In countries
like Japan, Sweden, Thailand, U.K and U.S.A, etc., the knowledge of SCC has
moved from domain of research to application. But in India, this knowledge is
to be widespread.
