30-04-2014, 12:40 PM
A PROJECT REPORT ON STUDY OF MIX DESIGN OF SELF COMPACTING CONCRETE ON M30 GRADE.
STUDY OF MIX DESIGN.pdf (Size: 1.35 MB / Downloads: 193)
ABSTRACT
Concrete occupies unique position among the modern construction materials, Concrete is a
material used in building construction, consisting of a hard, chemically inert particulate
substance, known as a aggregate (usually made for different types of sand and gravel), that is
bond by cement and water.
Self – compacting concrete (SCC) is a high – performance concrete that can flow under its own
weight to completely fill the form work and self consolidates without any mechanical vibration.
Such concrete an accelerate the placement, reduce the labor requirements needed for
consolidation, finishing and eliminate environmental pollution. The so called first generation
SCC is used mainly for repair application and for casting concrete in restricted areas, including
sections that present limited access to vibrate. Such value added construction material has been
used in applications justifying the higher material and quality control cost when considering the
simplified placement and handling requirements of the concrete.
The successful production of self – compacting concrete (SCC) for use, is depended on arriving
at an appropriate balance between the yield stress and the viscosity of the paste. Specially
formulated high range water reducers are used to reduce the yield stress to point to allow the
designed free flowing characteristics of the concrete. However, this alone may result in
segregation if the viscosity of the paste is not sufficient to support the aggregate particles in
suspension.
GENERAL
Self – compacting concrete (SCC) is a fluid mixture, which is suitable for placing difficult
conditions and also in congested reinforcement, without vibration. In principle, a self –
compacting or self – consolidating concrete must:
Have a fluidity that allows self – compaction without external energy
Remain homogeneous in a form during and after the placing process and
Flow easily through reinforcement
Self – consolidating concrete has recently been used in the pre – cast industry and in some
commercial applications, however the relatively high material cost still hinders the wide spread
use of such specialty concrete in various segments of the construction industry, including
commercial and residential construction.
Compared with conventional concrete of similar mechanical properties, the material cost of SCC
is more due to the relatively high demand of Cementation materials and chemical admixtures
including high – range water reducing admixtures (HRWRA) and viscosity enhancing
admixtures (VEA). Typically, the content in Cementation materials can vary between 450 and
525 Kg/m3 for SCC targeted for the filling of highly restricted areas and for repair applications.
Such applications require low aggregate volume to facilitate flow among restricted spacing
without blockage and ensure the filling of the formwork without consolidation. The
incorporation of high volumes of finely ground powder materials is necessary to enhance
cohesiveness and increase the paste volume required for successful casting of SCC.
DEVELOPMENTS OF SELF – COMPACTING CONCRETE
For several years beginning in 1983, the problem of the durability of concrete structures was a
major topic of interest in Japan. The creation of durable concrete structures requires adequate
compaction by skilled workers. The designs of modern reinforced concrete structures become
more advanced, the designed shapes of structures are becoming increasingly complicated and
heavy reinforcing is no longer unusual. Furthermore, the gradual reduction in the number of
skilled workers in Japan‟s construction industry has led to a similar reduction in the quality of
construction work. One solution for the achievement of durable concrete structure independent
of the quality of construction work is the employment of self – compacting concrete, which can
be compacted into every corner of a form work, purely by means of its own weight and without
the need for vibrating compaction. Okamura proposed the necessity of this type of concrete in
1986. Studies to develop self – compacting concrete, including a fundamental study on the
workability of concrete, have been carried out by “Ozawa and Maekawa” at the university of
Tokyo.
The prototype of SCC was first completed in 1988 using materials already on the market. The
proto type performed satisfactorily with regard to drying and hardening shrinkage, heat of
hydration, denseness after hardening, and other properties.
NECESSITY FOR NEW STRUCTURAL DESIGN AND CONSTRUCTION
SYSTEMS
Self – compacting concrete saves the cost of vibrating compaction and ensures the compaction of
the concrete in the structure. However, total construction cost cannot always be reduced, except
in large – scale constructions. This is because conventional construction systems are essentially
designed based on the assumption that vibrating compaction of concrete is necessary.
Self – compacting concrete can greatly improve construction systems previously based on
conventional concrete that required vibrating compaction. This sort of compaction, which can
easily cause segregation, has been an obstacle to the rationalization of construction work. Once
this obstacle is eliminated, concrete construction can be rationalized and a new construction
system including form work, reinforcement, support and structural design, can be developed.
One example of this is the so called sandwich structure, where concrete is filled into a steel shell.
Such a structure has already been completed in Kobe, and could not have been achieved without
the development of self – compacting concrete (Shishido et al, 1999).
Micro Silica
Silica fume also referred to as micro silica or condensed silica fume, is another material that is
used as an artificial pozzolonic admixture. It is a product resulting from reduction of high purity
quartz with coal in an electric and furnace in the manufacture of silicon or ferrosilicon alloy.
Silica fume rises as oxidized vapors. It cools, condenses and is collected in cloth bags. It is
further processed to remove impurities and to control particle size. Condensed silica fume is
essentially silicon dioxide (more than 90%) in non crystalline form. Since it an airborne material
like fly ash, it has spherical shape. It is extremely fine with particle size less than I micron and
with an average diameter of about 0.1 micron, about 100 times smaller than average cement
particles. Silica fume has specific surface area of about 20,000m2/kg, as against 230 to 300
m2/kg that of cement.
Role of S.P in cement
We know that the main action of S.P is to fluidity the mix and improve the workability of
concrete. Port land cement, being in fine state of division will have a tendency to flocculate in
wet concrete. This flocculation‟s entraps certain amount of water used in the mix and there by all
the water is not freely available to fluidity the mix. When plasticizers are used, they get absorbed
on cement particles. The absorption of charged polymer on cement particle creates particle to
particle repulsive forces, which overcome the attractive forces. This repulsive force is called zeta
potential, which depends on the base, solid contents and quality of super plasticizer used. The
overall result is that the cement particles are deflocculated and the water trapped inside the flocks
gets released and now available to fluidity the mix.