1.POLYMER CONCRETE
We know that cement concrete is a porous material. The percentage of
pores reduces the strength and durability of concrete and it exhibits a
tendency to deteriorate when exposed to action of severe chemicals. As a result
of continued research it has been established that addition of polymers in
concrete brings about marked improvement in its compressive strength, fatigue
resistance, impact resistance, toughness and durability. Polymer concrete is
observed to be highly impermeable and resistance to attack by acids, alkalies
and other chemicals.
Polymer concrete can be classified in following categories:
1.Polymer Impregnated Concrete (PHC)
It is a hardened Portland cement concrete that has ben impregnated with a
monomer and subsequently polymerised in situ. In this case, the cement concrete
is cast and cured in the conventional manner. After the concrete product gets
hardened and dried, air from its voids is removed under partial vaccum a and a
low viscosity monomer is diffused through the pores of the concrete. The
concrete product is then finally subjected to polymerisation by radiation or by
heat treatment thereby converting the monomer filled in the voids into solid
plastic. Polymer impregnated concrete is used as kerb stone, pre cast slabs for
bridge decks, roads, in marine structure, food processing etc.
2. Polymer Cement Concrete (PCC)
This type of concrete is produced by incorporating as emulsion of a
polymer or a monomer in ordinary Portland cement concrete. The ingredient
comprising of cement, aggregates and monomer are mixed with water and monomer
in the concrete mix is polymerised after placing concrete in position. The
resultant concrete has improved workability, strength, adhesion, chemical
resistance, impact resistance, abrasion resistance besides having increased
impermeability and reduced absorption. Polymer cement concrete can be used with
advantage in marine work.
3. Polymer Concrete (PC)
In this type of concrete polymer/monomer is used to act as a binder in
place of cement. The monomer and the aggregate are mixed together and the
monomer is polymerised after placement of aggregate in position. It is however,
necessary to per heat the coarse and the fine aggregate while miing monomer.
Polymer concrete serves as a very good dielectric material. It can be used with
advantage for the construction of irrigation works.
SUPERPLASTICIZED CONCRETE
This is a type of concrete in which highly effective plasticisers known
as superplasticisers are incorporated as admixture to produce flowing concrete
which is more of less self compacting type. Addition of superplasticisers
increase the workability of concrete, permits use of less quantity of water,
thereby producing the high strength concrete without increasing cement. super
plasticized concrete can be adopted in the following three possible situation:
· When it is required to produce
concrete of required strength with reduced quantity of cement. We know that to
achieve concrete of desired strength, it is necessary to maintain, certain
fixed value of the water cement ratio. In case the cement content in the
concrete is reduced, it would be necessary to have corresponding reduction in
the quantity of mixing water to maintain the water cement ratio. Reduction of
water can render the concrete mix comparatively dry, difficult to be placed in
position and hence unworkable. Addition of super plasticisers in suitable doze,
compensates for the reduction of mixing water and makes it possible to maintain
the water cement ratio as well as workability of the concrete with reduced
quantity of cement.
· When it is required to produce high
strength concrete having a very low water cement ratio. The increase in
strength of concrete is generally proportional to the reduction in water cement
ratio. It has been established that by adding super plasticiser it is possible
to achieve water reduction upto 30% and water cement ratio as low as 0.2. Thus
to achieve high strength concrete, the water content of the mix is reduced,
maintaining the cement content same and the admixtures of superplasticiser
compensates the reduced workability due to use of less water without producing
any determental effect either in the plastic or in the hardened concrete.
· When it is required to produce
concrete having good flowability without any danger of bleeding. Segregation or
reduction in strength. By admixture of superplasticiser it is possible to
produce flowing concrete which is self compacting and self levelling type. In situations
where concrete is to be placed in heavily reinforced structural members,
superplasticisers can be added to the concrete with the sole aim of increasing
the slump. In this process it is possible to achieve increase in slump from 75
to 200 mm without producing adverse effect on the strength of the concrete.
The quantity of superplasticisers to be added in the concrete mix depends
upon the purpose for which it is being used; i.e. to achieve good flow ability
or to produce high strength concrete by using less water. In general,
superplasticiser, can be added in concrete in doses upto 0.7% by weight of
water.
FIBRE REINFORCED CONCRETE (FRC)
Because of its low tensile strength and impact resistance, plain cement
concrete is considered to be a brittle material. It has now been established
that by addition of small diameter, short length, randomly distributed fibers
it is possible to bring about marked improvements in the tensile strength and
impact resistance properties of concrete. The fibre could be of steel, glass of
asbestos. Concrete with fibre is termed as fibre reinforced concrete. The
extent of improvement in the properties of fibre reinforced concrete depends
upon various factors like material of fibre, their shape, size, pattern of
distribution and magnitude in the concrete mix.
Out of various possible types of fibre reinforced concrete the following
two types are mostly being recommended.
Steel Fibre Reinforced Concrete:
This type of concrete is formed by adding steel fibres in the ingredients
of concrete. Steel fibres are normally produced by cutting 10 to 60 mm length
of low carbon steel wires 0.25 mm to 0.75 mm in diameter. Besides round fibres
use of flat sheet steel fibres is also common. Flat sheet steel fibre are
produced by shearing 0.15 mm to 0.40 mm thick steel plates in widths ranging
from 0.25 mm to 0.90 mm and length 10 mm to 60 mm. steel fibres have a tendency
to cluster together which creates difficulties in ensuring their uniform random
distribution in the concrete. This difficulty is overcome by using fibre
bundles ( fibre loosely assembled in the form of bundles with the help of water
soluble glue). The steel fibre in the fibre bundles separate out during the
process of mixing concrete and get distributed in a random fashion in the
concrete mix. By addition of 2 to 3% of fibres it is possible to achieve two to
three times increase in the flexural strength of concrete and substantial
increase in explosion resistance, crack resistance and other properties of
concrete. Steel fibre reinforced concrete is considered suitable for the
construction of pavements, bridge decks pressure vessels, tunnels lining etc.
Glass Reinforced Concrete:
It is observed that strength of glass fibre increases as its diameter is
reduced. It is also seen that although small diameter glass fibres are
reasonably strong in tension, they are very brittle and cannot be used in long
lengths. Moreover glass fibres get corroded due to the effect of alkalies
present in Portland cement. Thus to utilise glass fibres as micro reinforcement
they are suitably treated to protect from alkalies attack. It is seen that
addition of 10% of glass fibre brings almost two folds increase in tensile
strength and substantial increase in impact resistance of concrete. Investigations,
are on to use glass fibre reinforced concrete in the manufacture of precast products
like spun pipes, wall cladding etc.
SULPHUR IMPREGNATED CONCRETE
This type of concrete consists of a mixture of sulphur, coarse aggregate and
fine aggregate. The concrete as formed by adding molten sulphur to the pre
heated aggregate in a mixer. The hot concrete mix is a poured in moulds to from
pre cast units. Sulphur impregnated concrete develops high compressive strength
at an early age. It needs no curing and the mould can be stripped soon after
the concrete mixed get solidified. The main advantage of this type of concrete
is that it can be re used and re moulded without any wastage. Sulphur impregnated
concrete has god chemical durability and high strength. This concrete is more
durable in acidic environments but is unstable in alkaline conditions. Due to
its brittleness and corrosive action of sulphur on reinforcement, this type of
concrete is not suitable for structural work. This concrete is used in Industrial
plant and for lining canals and tunnels.
ROLLER COMPACTED CONCRETE
This type of concrete can be defined as a dry concrete material which has
been consolidated by external vibrations using vibratory rollers. It differs
from normal concrete basically in its consistency. The roller compacted
concrete should be dry enough to support the load of vibratory rollers and at
the same time it should be wet enough to permit proper consolidation by
vibrations. This type of concrete is used with advantage in mass concrete work.
It can be laid in thin layers of about cm depth in continuous operation. This results
in substantial reduction in the heat of hydration and hence the need to perform
special concrete cooling operations in mass concrete work can be avoided. The use
of roller compacted concrete permits faster construction and saving in labour
cost.
ULTRA HIGH STRENGTH CONCRETE
We are conversent with use of concrete having compressive strength
varying from 100 to 300 kg/cm^2. With the introduction of pre stressed concrete
in the construction industry, constant research has been going on to develop
concrete having very high or ultra high strength. As a result thereof it has
now been established that by suitable seclection of material and by adopting
special method of production it is possible to make concrete having compressive
strength exceeding 1000 kgcm^2. Ultra high strength can be produced by improved
compaction and adhesion of cement matrix to aggregates or by adoption of cementitious
aggregates. Ultra high strength concrete can be easily produced by use of high
quality coarse aggregates, synthetic aluminous fine aggregates and cement. Use
of high temperature to increase the lime silica bond and use of spiral wrapping
of concrete to induce triaxial stress are the other techniques being tried to
develop ultra-high strength concrete. Polymer concrete and superplasticised
concrete also get covered in the category of ultra-strength concrete to some
extent.
By use of ultra high strength concrete, load bearing capacity of columns
can be increased considerably. Hence, by its use it is possible to adopt
slender columns in multi storied buildings which besides looking aesthetically
good also permits greater utilisation of useful floor space. On account of its
high compressive as well as tensile strength, this type of concrete finds wide
application in prestressed concrete.
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