SPECIAL CONCRETES
SPECIAL TYPES OF CONCRETE
LIGHT-WEIGHT CONCRETE
The light-weight concrete as we call is a
concrete whose density vanes from 300 to
1850 kg/m3.
One of the disadvantages of conventional
concrete
is the high self weight of concrete. Density of
the
normal concrete is in the order of 2200 to
2600 kg/m3.
This heavy self weight will make it to some
extent an
uneconomical structural material.
How to make concrete light weight?
Basically there is only one method for making
concrete light i.e.. by the inclusion of air in
concrete. This is achieved in actual practice by
three different ways.
(a) By replacing the usual mineral aggregate
by cellular porous or light-weight aggregate.
(b) By introducing gas or air bubbles in
mortar. This is known as aerated concrete.
(c) By omitting sand fraction from the
aggregate. This is called ‘no-fines’ concrete.
By using light weight-aggregates
Light weight-aggregates
Properties of light weight concretes
By using expanded perlite or vermiculite, a concrete of
density as low as 300 Kg/rn3 can be produced, and by the
use of expanded slag. sintered fly ash, bloated clay etc., a
concrete of density 1900 kg/rn3 can be obtained.
The strength of the light-weight concrete may also vary
from about 0.3 N/mm2 to
40 N/mm2.
A cement content of 200 kg/rn3 to about 500 kg/rn3 may
be used.
Strength of light-weight concrete depends on the density
of concrete.
Less porous aggregate which is heavier in weight produces
stronger concrete particularly with higher cement
content,
AERATED CONCRETE
Aerated concrete is made by introducing air
or gas into a slurry composed of Portland
cement or lime and finely crushed siliceous
filler so that when the mix sets and hardens,
a uniformly cellular structure is formed.
Aerated concrete is also referred to as gas
concrete, foam concrete, cellular concrete.
In India we have at present a few factories
manufacturing aerated concrete.
A common product of aerated concrete in
India is Siporex.
AERATED CONCRETE
There are several ways in which aerated
concrete can be manufactured.
(a) By the formation of gas by chemical
reaction within the mass during liquid or
plastic state.
(b) By mixing preformed stable foam with
the slurry.
(c) By using finely powdered metal
Properties and uses of aerated concrete
Use of foam concrete has gained popularity not only
because of the low density but also
because of other properties mainly the thermal
insulation property.
Aerated concrete is made in the density range from
300 kg/rn3 to about 800 kg/rn3.
Lower density grades are used for
insulation purposes.
Medium density grades are used for the manufacture
of building blocks or load bearing walls and
comparatively higher density grades are used in the
manufacture of prefabricated structural members in
conjunction with steel reinforcement.
No-fines Concrete
No-fines concrete as the term implies, is a
kind of concrete from which the tine
aggregate
fraction has been omitted.
The third method of producing light
concrete is to omit the fines from
conventional concrete.
This concrete is made up of only coarse
aggregate. cement and water.
Very often only single sized coarse aggregate.
of size passing through 20 mm retained on
10mm is used.
No-fines Concrete
No-fines concrete is becoming popular
because of some of the advantages it
possesses over the conventional
concrete.
The single sized aggregates make a good
no-fines concrete, which in addition to
having large voids and hence light in
weight, also offers architecturally
attractive look,
Application of No-fines Concrete
It is used in large scale for load
bearing cast in-situ external walls for
single storey and multistoreyed buildings.
This type of concrete has been used for
temporary structures because of low
initial cost and also for the
ease with which it can be broken and
reused as aggregate.
Architects consider this as an
attractive construction material
Application of No-fines Concrete
Because of rough texture, ¡t gives a good
base for plastering.
Even if the outside surface of the no-fines
concrete wall is subjected to rain
beating, the inside of the wall will be free
from dampness because of low capillary
action on account of large voids.
Where sand is not available, no-fines
concrete should become a
popular construction material.
High density concrete
To call the concrete, as high density concrete, it must
have unit weight ranging from about 3360 kg per
cubic meter to 3840 kg per cubic meter, which is about
50% higher than the unit weight of conventional
concrete .
Density of normal concrete is in the order of about
2400 kg. per cubic meter. The density of light-weight
concrete will be less than about density 1900 kg per
cubic meter.
They can, however be produced with the densities up to
about 5280kg per cubic meter using iron as both fine
and coarse aggregate.
High density concrete
The high density concrete
is used in the construction of radiation shields.
The advent of the nuclear energy industry
presents a considerable demand on the concrete
technologists. industrial radiography, and, X-ray,
gamma-ray therapy. require the need of shielding
material for the protection of
operating personnel against the biological hazards
of such radiation.
Concrete, both normal and high density are
effective and economic construction materials for
permanent shielding
purposes.
Sulphur-Infiltrated Concrete
New types of composites have been produced by the
recently developed techniques of impregnating
porous materials like concrete with sulphur.
Sulphur impregnation has shown
great improvement in strength.
Physical properties have been found to improve by
several
hundred per cent and large improvements ¡n water
impermeability and resistance to corrosion
have also been achieved.
In the past, some attempts have been made to use
sulphur as a binding material instead
of cement.
Application of Sulphur- infiltrated
Concrete
The sulphur-infiltration can be employed in the precast
industry.
This method of achieving high strength can be used in
the manufacture of pre-cast roofing elements, fencing
posts, sewer pipes. and railway sleepers,
Sulphur-infiltrated concrete should find considerable use
in industrial situations, where high corrosion resistant
concrete is required.
Preliminary studies have indicated that sulphur-
infiltrated precast concrete units is cheaper
than commercial concrete.
The techniques are simple. effective and inexpensive.
The tremendous strength gained in pressure application,
Fibre reinforced concrete
Fibre reinforced concrete can be defined as a
composite material consisting of mixtures of
cement, mortar or concrete and discontinuous,
discrete, uniformly dispersed suitable fibers.
It has been recognized that the addition of small,
closely spaced and uniformly dispersed
fibres to concrete would act as crack arrester
and would substantially improve its static and
dynamic properties.
Applications of
Fibre reinforced concrete
Fibre reinforced concrete is increasingly used on account of the
advantages of increased static and dynamic tensile strength, energy
absorbing characteristics and better fatigue strength.
The uniform dispersion of fibres throughout the concrete provides
isotropic properties not common to conventionally reinforced
concrete.
Fibre reinforced concrete has been tried on overlays of air-field,
road pavements, industrial floorings, bridge decks, canal lining.
explosive resistant structures, refractory linings etc.
The fibre reinforced concrete can also be
used for the fabrication of precast products like pipes, boats, beams,
stair case steps, wall panels. roof panels, manhole covers etc...
Fibre reinforced concrete in also being tried for the manufacture of
prefabricated formwork moulds of “U” shape for casting lintels and
small beams.
