UNIT 5

SPECIAL CONCRETE

5.1. Light-weight Concrete

 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.

 Attempts have been made to reduce the self-weight of concrete to increase the
efficiency of concrete as a structural material. The light-weight concrete means
whose density varies from 300 to 1850 kg/m3.

Advantages of light weight concrete

 There are many advantages of having low density. It helps in reduction of dead load
of building. The weight of a building on the foundation is an important factor in
design, particularly in the case of weak soil and tall structures.
 If floors and walls are made up of light-weight concrete it will result in considerable
economy.
 Another most important characteristic of light-weight concrete is the relatively low
thermal conductivity, a property which improves with decreasing density.
 The adoption of light-weight concrete gives an outlet for industrial wastes such as
clinker, fly ash, slag etc. which otherwise create problem for disposal.

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.
 By replacing the usual mineral aggregate by cellular porous or light-weight
aggregate.
 By introducing gas or air bubbles in mortar. This is known as aerated concrete.
 By omitting sand fraction from the aggregate. This is called no-fines concrete.

Light Weight Aggregates

 Light-weight aggregates can be classified into two categories namely natural light-
weight Aggregates and artificial light-weight aggregates
Natural Aggregates
 Natural light-weight aggregates are not found in many places and they are also not
of uniform quality. As such they are not used very widely in making light-weight
concrete.
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Natural light-weight aggregate Artificial light-weight aggregate

a) Pumice (A)Artificial cinders

(b) Diatomite (b) Coke breeze
(c) Scoria (c) Foamed slag
(d) Volcanic cinders (d) Bloated clay
(e) Sawdust (e) Expanded shales and slate
(f ) Rice husk (f ) Sintered fly ash
(g) Exfoliated vermiculite
(h) Expanded perlite
(i ) Thermocole beads.

Pumice


Pumice is a rocks of volcanic origin which occur in many parts of the world.
Pumice is one of the oldest kinds of light-weight aggregates which has been even
used in Roman structures.

They are light and strong enough to be used as light-weight aggregate. Their
lightness is due to the escaping of gas from the molten lava when erupted from
deep beneath the earth‘s crest.

Types of Bulk density of Dry density Compressive Drying Thermal

concrete aggeregate(kg/m3) (kg/m3) strength at shrinkage conductivity
28 days 10–6 Jm/m2 5°C
Pumice 500–800 1200 15 1200 0.14


Pumice is usually light coloured or nearly white and has a fairly even texture of
Interconnected cells. Pumice is mined, washed and then used. The density and
other properties of pumice concrete

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 Diatomite

 This is a hydrated amorphous silica derived from the remains of microscopic aquatic
plants called diatoms. The deposits of this aquatic plants are formed beneath the
deep ocean bed.

 Subsequently when the ocean bed is raised and becomes continent, the diatomaceous
earth become available on land. In pure form diatomite has an average weight of 450
kg/m3.
 But due to impurities, the naturally available diatomite may weight more than 450
kg/m3. It have good workability agent and also have good pozzolanic material.

Scoria

 Scoria is also light-weight aggregate of volcanic origin which is usually dark in

colour and contains larger and irregularly shaped cells unconnected with each other.
Therefore, it is slightly weaker than pumice.

Volcanic Cinders


These are also loose volcanic product resembling artificial cinder. Volcanic cinder
usually has thicker vesicle walls than pumice and produces a heavier but stronger
lightweight concrete with insulating qualities less than those of pumice concrete of
comparable strength and weight.

Saw Dust
 Saw dust made by soft wood and hard wood. It is used as a light-weight aggregate in
flooring and in the manufacture of precast products. A few difficulties have been
experienced for its wide-pread use. Saw dust affect adversely the setting and
hardening of Portland cement owing to the content of tannins and soluble
carbohydrates.
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  Saw dust manufactured from soft wood, the addition of lime to the mix in an amount
equal to about 1/3 to 1/2 the volume of cement will counteract this. But the above
method suitable only when the saw dust is made from some of the hard woods.
 Saw dust concrete has been used in the manufacture of precast concrete products,
joint less flooring ad roofing tiles. It is also used in concrete block for holding the
nail well. Wood aggregate also has been tried for making concrete.

Rice Husk
 Limited use of the rice husk, groundnut husk and bagasse have been used as
lightweight aggregate for the manufacture of light-weight concrete for special
purposes.

Artificial Aggregates
Brick Bats


Brick bat aggregates are made from slightly over burnt bricks, which will be hard
and absorb less water. Brick bats are one of the types of aggregates used in certain
places where natural aggregates are not available or costly.

The brick bat aggregates cannot be really brought under light-weight aggregates
because the concrete made with this aggregate will not come under the category of
light-weight concrete.

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Cinder, Clinker and Breeze aggregates


The term clinker, breeze and cinder are used to cover the material partly fused
particles arising from the combustion of coal. Cinder aggregates undergo high
drying shrinkage and moisture movement.

Cinder aggregates have been also used for making building blocks for partition
walls, for making screening over flat roofs and for plastering Purposes.

The unsoundness of clinker or cinder aggregates is often due to the presence of
excessive unburnt coal particles. Sometimes unburnt particles may be present as
much as 15 to 25%.

This high proportion of coal expand on wetting and contract on drying which is
responsible for the unsoundness of concrete made with such aggregate.

Foamed Slag


Foamed slag is one of the most important types of light-weight aggregates. It is
made by rapidly quenching blast furnace slag, a by-product of produced in the
manufacture of pig iron.

If the cooling of the slag is done with a large excess of water, granulated slag is
formed which is used in the manufacture of blast furnace slag cement. Such a
product is also called foamed slag or expanded slag.

The texture and strength of foamed slag depends upon the chemical composition and
the method of production. But in general, the structure is similar to that of natural
pumice.

The foamed slag must be (a) Free from contamination of heavy impurities (b) Free
from volatile impurities such as coke or coal. (c) Free from excess of sulphate.

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Bloated Clay
 When certain glass and shales are heated to the point of incipient fusion, they expand
 Their original volume on account of the formation of gas within the mass at the
fusion temperatures.
 The cellular structure so formed is retained on cooling and the product is used as
light-weight aggregate. Example: Haydite, Rocklite, Gravelite are some of the patent
names given to bloated clay or shale.

Sintered Fly Ash (Pulverised Fuel Ash)

 Fly ash is finely divided residue comprising of spherical glassy particles, resulting
from the combustion of powdered coal.
 By heat treatment these small particles can be made to forming porous pellets or
nodules which have considerable strength.
 The fly ash is mixed with limited amount of water and is first made into pellets and
then Heated at a temperature of 1000° to 1200°C.
 The fly ash may contain some unburnt coal which may vary from 2 to 15 per cent or
more depending upon the efficiency of burning

Exfoliated Vermiculite

 Raw vermiculite is a micaceous mineral and has a laminar structure. When heated
with certain percentage of water it expands by delamination in the same way as that
of slate or shale.
 This type of expansion is known as exfoliation. Due to exfoliation, the vermiculite
expands many times its original volume.
 The fully exfoliated vermiculite which may have expanded even as much as 30 times
will have a density of only 60 to 130 kg/m3. The concrete made with vermiculite as
aggregate, therefore, will have very low density and hence very low strength.
Expanded Perlite

 Perlite is one of the natural volcanic glasses like pumice. This when crushed and
heated to the point of incipient fusion at a temperature of about 900 to 1100°C it
expands to form a light cellular material with density of about 30 to 240 kg/m3. This
light material is crushed carefully to various sizes and used in concrete.
Light-weight Aggregate Concrete

 Light-weight concrete is made by the use of light weight aggregates. Naturally when
light weight aggregates is used, concrete of different densities are obtained. By using
expanded perlite or vermiculite, a concrete of density as low as 300 Kg/m 3 can be
produced.
 By the use of expanded slag, sintered fly ash, bloated clay etc., a concrete of density
1900 kg/m3 can be obtained.

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  The strength of the light-weight concrete may also vary from about 0.3 N/mm2 to 40
N/mm2. Less porous aggregate which is heavier in weight produces stronger
concrete particularly with higher cement content.
 The grading of aggregate, the water/cement ratio, the degree of compaction also
effect the strength of concrete.
 For increasing the strength, improving the workability and reducing the water
requirement, sometimes natural sand is used instead of crushed sand made out of
light-weight aggregate.
 Use of air-entrainment will greatly improve the workability, and the tendency for
bleeding in the light-weight concrete. But the use of air-entrainment will result in
further reduction in strength also.
 Most of the light-weight aggregates have a high and rapid absorption quality. This is
one of the important difficulties in applying the normal mix design procedure to the
light-weight concrete.

Design of Light -weight Aggregate Concrete Mix

 Mix design methods applying to normal weight concrete are generally difficult to
use with light weight aggregate concrete.
 The lack of accurate value of absorption, specific gravity, and the free moisture
content in the aggregate make it difficult to apply the water/cement ratio accurately
for mix proportioning.
 Light-weight concrete mix design is usually established by trial mixes. The
proportions of fine to coarse aggregate and the cement and water requirement are
estimated based on the previous experiences with particular aggregate.
 Various degree of water absorption by different light-weight aggregates is one of the
serious difficulties in the design of mix proportions.
 Sometimes the aggregate is saturated before mixing so that it does not take up the
water used for mixing. The quality of concrete does not get altered on account of
absorption by aggregate.
 It has been seen that the strength of the resulting concrete is about 5 to 10 % lower
than when dry aggregate is used for the same content and workability. This is due to
the fact that in the latter case some of the mixing water is absorbed prior to setting.
 This water having contributed to the workability at the time of placing gets absorbed
later, thus reduce the bad effect of excess of water.
 Moreover, the density of concrete made with saturated aggregate is higher and the
durability of such concrete, especially its resistance to frost is lower.
5.2. Foam Concrete -Materials, Properties, Advantages and Production Methods
 Foam concrete is a type of lightweight concrete that is manufactured from cement,
sand or fly ash, water, and the foam. Foam concrete is in the form of foamed grout
or foamed mortar.
 Foam concrete can be defined as a cementations material that consists of minimum
20 percent of foam, that is mechanically entrained into the plastic mortar. The dry
density of foamed concrete may vary from 300 to 1600 kg/m3. The compressive
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 strength of foam concrete determined at 28 days, ranges from 0.2 to 10N/mm 2 or can
go higher.
 Foam concrete is differentiated from air entrained concrete in terms of the volume of
air that is entrained. The air entrained concrete takes in the air of 3 to 8 percent. It
also differs from the retarded mortar and aerated concrete for the same reason of
percentage of air entrained.
 In the case of retarded mortar systems, it is 15 to 22 percent. The bubbles are
chemically formed in the case of an aerated concrete.

History of Foam Concrete

 The foamed concrete has a long history and it was first made into use in the year
1923. It was initially used as an insulating material. Improvements throughout the
past 20 years in the areas of production equipment and better quality foam making
agents make the use of foam concrete in large scale.
Production of Foam Concrete
 The production of foam concrete involves the dilution of surfactant in water, which
is passed through a foam generator that will produce foam of stable form. The foam
produced in mixed with the cementitious mortar or the grout, so that foamed
quantity of required density is produced.
 These surfactants are also used in the manufacture of low density fills. These are
also called as controlled low strength Material (CLSM). Here, to obtain an air
content of 15 to 25 percent, the foam is added directly to a mix of low cement
content and rich sand.
 It must be kept in mind that low density fillers are supplied as foamed concrete by
some manufacturers, so misleading must be taken care.
Two main methods are used for production of foamed concrete:

 Inline Method and

 Pre-foam Method

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Inline Method of Foam Concrete Production
 The base mix of cement and sand is added to a unit. In this unit, the mix is blended
with foam thoroughly. The process of mixing is carried out with proper control. This
will help in mixing of larger quantities. The inline method comprises two processes;

 Wet Method- Inline System

 Dry Method -Inline System
 Wet Method of Inline System: The materials used in the wet method will be wetter
in nature. With the help of a series of static inline mixers, the base material and the
foam are fed and mixed together. The continual on board density monitor is used to
check the blending of the whole mix.
 The output volume is dependent on the density of the foamed concrete and not on
the ready mixed truck. That is one 8m3 base material delivery would produce
35m3 of a foamed concrete of 500kg/m3 density.
 Dry Method of Inline System: Here the dry materials are used. They are taken into
the onboard silos. From here they are weighed properly and mixed with the help of
on- board mixers. The mixed base materials are then pumped to a mixing chamber.
 In wet method of foam concrete production, foam is added and mixed. This method
employs a large amount of water for mixing. 130 cubic meters of foamed concrete
can be produced from a single delivery of cement or fly ash blend.

Pre- Foam Method of Foam Concrete Production

 Here, the ready-mix truck brings the base material to the site. Through the other
end of the truck, the pre- formed foam is injected into the truck, while the mixer
is rotating.
 So, small quantities of foam concrete can be produced for small works, like for
grouting or trench fill works.
 This method would provide foam concrete with densities ranging from 300 to
1200 kg/m3. The foam input will be from 20 to 60 percentage air. The final
volume of the foam can be calculated by reducing the amount of other base
material. As this is carried out in the truck.
 Control of stable air and density is difficult for this method. So, a degree of
under and over yield must be specified and allowed.
 When the foam is formed, it is combined with a cement mortar mix having water
cement ratio of 0.4 to 0.6. If the mortar is wet, the foam becomes unstable. If it is
too dry, the pre-foam is difficult to blend.

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Materials for Foamed Concrete
Cement for Foam Concrete
 Ordinary Portland cement is commonly used, but rapid hardening cement can
also be used if necessary. Foam concrete can incorporate a wide range of cement
and other combination, for example, 30 percent of cement, 60 percent of fly ash
and limestone in 10 percent. The content of cement range from 300 to 400
kg/m3.
Sand for Foam Concrete
 The maximum size of sand used can be 5mm. Use of finer sands up to 2mm with
amount passing through 600 micron sieve range from 60 to 95%.
Pozzolanas
 The supplementary cementitious materials like fly ash and ground granulated
blast furnace slag have been used widely in the manufacture of foam concrete.
The amount of fly ash used ranges from 30 to 70 percent. White GGBFS range
from 10 to 50%. This reduces the amount of cement used and economical.
 Silica fume can be added to increase the strength; at an amount of 10 percentage
by mass.
Foam
 The hydrolyzed proteins or the synthetic surfactants are the most common forms
based on which foams are made. The synthetic based foam agents are easier to
handle and are cheap. They can be stored for a longer period.
 Lesser energy is required to produce these foams. The protein based foam are
costly but have high strength and performance. The foam can be of two types:
wet foam and dry foam.
 Wet foams with densities lesser than 100 kg/m3 are not recommended for the
manufacture of foam concrete. They have a very loosely place large bubble
structure. To a fine mesh, the agent and the water are being sprayed. This process
produces foam that has bubbles with size ranging from 2 to 5mm.
 Dry foam is highly stable in nature. A solution of water and the foaming agent is
forced by restrictions into a mixing chamber by compressor air. The produced
foam have bubble size which is smaller than the wet foam. That is less than
1mm. These give a structure of bubbles, which are evenly arranged.

BS 8443:2005 covers the foaming admixtures.

Other Materials and Aggregates for Foam Concrete
 The coarse aggregate or other replacement for coarse cannot be used. This is
because these materials would sink in the lightweight foam.
Mix Details of Foam Concrete
The foam concrete properties depend upon the following factors:

 The volume of the foam

 The cement content in the mix
 The filler material
 The age
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The influence of water cement ratio has a very little influence on the properties of the foam
concrete, unlike foam and the cement content.
Properties of Foam Concrete
 The foam concrete properties in its fresh and hardened state are explained below;
Visual Appearance of foam Concrete
 The exact comparison for the foam that is manufactured to produce foam
concrete resembles the shaving foam. When this is mixed with the mortar of
standard specification, the final mix will resemble the consistency of yogurt or in
the form of a milkshake.

Fresh Properties of Foam Concrete

 The workability of foamed concrete is very high and have a slump value of
150mm to collapse. These have a strong plasticizing effect. This property of
foam concrete makes it highly demanded in most of the applications.
 Once the flow of the mix has remained static for a longer period, it is very
difficult to restart its original state. Foam concrete in the fresh state is thixotropic
in nature.
 The chances of bleeding in foamed concrete are reduced due to high air content.
When the mix temperature increases, good filling, and contacts are carried out
due to the expansion of air.
 If the amount of sand used is higher or coarse aggregates is used other than the
standard specifications, there are chances for segregation. This can also lead to
the collapse of the bubble, which would reduce the total volume and the foam
structure.
 It is fine to carry out pumping of fresh foam concrete with care. Free fall of foam
concrete at the end with turbulence, may result in the collapse of the bubble
structure.

Hardened Properties of Foam Concrete

 The physical properties of the foam concrete are clearly related to the dry
density. The variation is seen in the tabulation given in the table below.

Table.1: Typical Properties of Foamed Concrete in its Hardened State

Dry Density Compressive Tensile Strength Water Absorption

Kg/m3 Strength N/mm2 N/mm2 Kg/m2

400 0.5 – 1 0.05-0.1 75

600 1-1.5 0.2-0.3 33

800 1.5 -2 0.3-0.4 15

1000 2.5 -3 0.4-0.6 7

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 1200 4.5-5.5 0.6-1.1 5

1400 6-8 0.8-1.2 5

16 00 7.5-10 1-1.6 5

 The thermal conductivity of foam concrete ranges from 0.1W/mk to 0.7W/mk.

The drying shrinkage ranges from 0.3 to 0.07% at 400 and 1600kg/m3
respectively.
 The foamed concrete does not possess an equivalent strength similar to an
autoclaved block with similar density. Under the action of load, there is internal
hydraulic pressure created within the structure, which would cause the
deformation of the foam concrete.
 The hardened foam concrete has good resistance against freezing and thawing. It
was observed that the application of foamed concrete in an area of temperature
ranging from -18 degree Celsius to +25 degree Celsius showed no signs of
damage. The density of foamed concrete employed here range from 400to
1400kg/m3.
Advantages of Foam Concrete
 The foam concrete mix does not settle. Hence it does not need any compaction
 The dead weight is reduced as it is light weight concrete
 The foamed concrete under its fresh state has freely flowing consistency. This
property will help in completely filling the voids.

 The foam concrete structure has excellent load spreading and distributing
capability
 Foamed Concrete Does not impose significant lateral loads
 The Water absorption property
 The foam concrete batches are easy to produce, so quality check and control are
easily done
 The foam concrete has higher resistance to freezing and thawing
 Non-hazardous and faster work completion
 Cost effective, less maintenance
Disadvantages of Foam Concrete
 Presence of water in the mixed material make the foam concrete very sensitive
 Difficulty in finishing
 Time of mixing longer
 With the increase in density, the compressive strength and flexural strength
decreases.

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5.3. Self compacting concrete

 The necessity of this type of concrete was proposed by Okamura 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 self-compacting concrete was first completed in 1988 using
materials already on the market .
 The prototype performed satisfactorily with regard to drying and hardening
shrinkage, heat of hydration, denseness after hardening, and other properties. This
concrete was named “High Performance Concrete” and was defined as follows at
the three stages of concrete:
o Fresh: self-compactable
o Early age: avoidance of initial defects
o After hardening: protection against external factors.
 At almost the same time, “High Performance Concrete” was defined as a concrete
with high durability due to a low water-cement ratio by Professor Aïtcin et al.
 Since then, the term high performance concrete has been used around the world
to refer to high durability concrete. Therefore, the authors have changed the term
for the proposed concrete to “Self-Compacting High Performance Concrete.”

Mechanism for achieving self-compactability

 The method for achieving self-compactability involves not only high
deformability of paste or mortar, but also resistance to segregation between coarse
aggregate and mortar when the concrete flows through the confined zone of
reinforcing bars. Okamura and Ozawa have employed the following methods to
achieve self- compactability (Fig. 3) (1995):
 (1) Limited aggregate content
(2) Low water-powder ratio (3)
Use of super plasticizer
 The frequency of collision and contact between aggregate particles can increase as
the relative distance between the particles decreases and then internal stress can
increase when concrete is deformed, particularly near obstacles. Research has found
that the energy required for flowing is consumed by the increased internal stress,
resulting in blockage of aggregate particles.
 Highly viscous paste is also required to avoid the blockage of coarse aggregate
when concrete flows through obstacles (Fig. 4). When concrete is deformed, paste
with a high viscosity also prevents localized increases in internal stress due to the
approach of coarse aggregate particles. High deformability can be achieved only by
the employment of a super- plasticizer, keeping the water-powder ratio to a very
low value.

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  The mix proportioning of self-compacting concrete is compared with those
of normal concrete and RCD (Roller Compacted concrete for Dams)
concrete. The aggregate content is smaller than conventional concrete that
requires vibrating compaction.
 The degree of packing of coarse aggregate in SCC is approximately 50% to
reduce the interaction between coarse aggregate particles when the concrete
deforms. In addition, the ratios of fine aggregate volume to solid volume
(S/Slim) in the mortar are shown in the same figure. The degree of packing of
fine aggregate in SCC mortar is approximately 60% so that shear
deformability when the concrete deforms may be limited. On the other hand, the
viscosity of the paste in SCC is the highest among the various types of concrete
due to its lowest water-powder ratio. This characteristic is effective in inhibiting
segregation.

Benefits & Advantages of Self Compacting Concrete

Self compacting concrete (SCC) can be classified as an advanced construction material.
The SCC as the name suggests, does not require to be vibrated to achieve full
compaction. This offers following benefits and advantages over conventional concrete.
 Improved quality of concrete and reduction of onsite repairs.
 Faster construction times.
 Lower overall costs.
 Facilitation of introduction of automation into concrete construction.
 Improvement of health and safety is also achieved through elimination of
handling of vibrators.
 Substantial reduction of environmental noise loading on and around a
site.

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  Possibilities for utilization of “dusts”, which are currently waste products
and which are costly to dispose of.
 Better surface finishes.
 Easier placing.
 Thinner concrete sections.
 Greater Freedom in Design.
 Improved durability, and reliability of concrete structures.
 Ease of placement results in cost savings through reduced equipment and
labor requirement.
 SCC makes the level of durability and reliability of the structure
independent from the existing on – site conditions relate to the quality of
labor, casting and compacting systems available.
 The high resistance to external segregation and the mixture self –
compacting ability allow the elimination of macro – defects, air bubbles,
and honey combs responsible for penalizing mechanical
performance and structure durability.

Materials for self-compacting concrete

Following are the materials for self compacting concrete: Cement for self compacting
concrete
 All types of cement conforming to EN 197 are suitable. Selection of the type of
cement will depend on the overall requirements for the concrete, such as strength,
durability etc., C3A content higher than 10% may cause problems of poor
workability retention.
3 3
 The typical content of cement is 350-450Kg/m more than 500Kg/m cement
3
can be dangerous and increase the shrinkage. Less than 350Kg/m may only
be suitable with the inclusion of other fine filler, such as fly ash, pozzolona, etc

Fig: Self Compacting Concrete

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Aggregates for self compacting concrete:
1. Sand
 All normal concreting sands are suitable for SCC. Either crushed or
rounded sands can be used. Siliceous or calcareous sands can be used.
 The amount of fines less than 0.125 mm is to be considered as powder and
is very important for the rheology of the SCC. A minimum amount of fines
(arising from the binders and the sand) must be achieved to avoid segregation.

2. Coarse aggregate
 All types of aggregates are suitable. The normal maximum size is generally 16
– 20 mm. however particle size up to 40 mm more have been used in SCC.
Consistency of grading is of vital importance.
 Regarding the characteristics of different types of aggregate, crushed
aggregates tend to improve the strength because of the interlocking of the
angular particles, whilst rounded aggregates improve the flow because of lower
internal friction.
 Gap graded aggregates are frequently better than those continuously graded,
which might experience greater internal friction and give reduced flow.

3. Admixtures for Self compacting concrete:

 The most important admixtures are the super plasticizers (high range
water reducers), used with a water reduction greater than 20%.
 The use of a Viscosity Modifying Agent (VMA) gives more possibilities of
controlling segregation when the amount of powder is limited. This admixture
helps to provide very good homogeneity and reduces the tendency to
segregation.

Test methods on SCC

 The methods presented here are devised specifically for SCC. Existing
rheological test procedure have not considered here, though the relationship
between the results of these tests & the rheological characteristics of the
concrete is likely to figure highly in future work, including standardization
work. In considering these tests there are number of points which should be
taken into account:
o There is no clear relation between test results & performance on site
o There is little presice data, therefore no clear guidance on compliance
limits

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 A concrete mix can only be classified as SCC if the requirements for all the following three
workability properties are fulfilled.
1. Filling ability,
2. Passing ability, &
3. Segregation resistance.

 Filling ability: It is the ability of SCC to flow into all spaces within the formwork
under its own weight. Tests, such as slump flow, V-funnel etc, are used to
determine the filling ability of fresh concrete.
 Passing ability: It is the ability of SCC to flow through tight openings, such as
spaces between steel reinforcing bars, under its own weight. Passing ability
can be determined by using U-box, L-box, Fill-box, and J- ring test methods.
 Segregation resistance: The SCC must meet the filling ability and passing
ability with uniform composition throughout the process of transport and placing.

Test methods to determine workability of SCC are:

1. Slump flow test
2. V Funnel Test
3. L Box Test
4. U Box Test
5. Fill Box Test

5.4. Vacuum Concrete

 Vacuum concrete is the one from which water is removed by vacuum pressure after
placement of concrete structural member. Vacuum concrete has high strength and
durability than normal concrete.
 Water-cement ratio is detrimental for concrete. We always try to restrict the water-
cement ratio in order to achieve higher strength.
 The chemical reaction of cement with water requires a water-cement ratio of less than
0.38, whereas the adopted water-cement ratio is much more than that mainly because of
the requirement of workability. Workability is also important for concrete, so it can be
placed in the formwork easily without honeycombing.
 After the requirement of workability is over, this excess water will eventually evaporate
leaving capillary pores in the concrete. These pores result into high permeability and less
strength in the concrete. Therefore, workability and high strength don’t go together as
their requirements are contradictory to each other.
 Vacuum concrete is the effective technique used to overcome this contradiction of
opposite requirements of workability and high strength. With this technique both these
are possible at the same time.

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  In this technique, the excess water after placement and compaction of concrete is sucked
out with the help of vacuum pumps. This technique is effectively used in industrial floors,
parking lots and deck slabs of bridges etc. The magnitude of applied vacuum is usually
about 0.08 MPa and the water content is reduced by upto 20-25%. The reduction is
effective upto a depth of about 100 to 150 mm only.

Technique and Equipments for Vacuum Concrete:

 The main aim of the technique is to extract extra water from concrete surface
using vacuum dewatering.
 As a result of dewatering, there is a marked reduction in effective water-cement ratio and
the performance of concrete improves drastically. The improvement is more on the
surface where it is required the most.

Mainly, four components are required in vacuum dewatering of concrete, which are given below:

1. Vacuum pump
2. Water separator
3. Filtering pad
4. Screed board vibrator
 Vacuum pump is a small but strong pump of 5 to 10 HP. Water is extracted by vacuum
and stored in the water separator. The mats are placed over fine filter pads, which prevent
the removal of cement with water.
 Proper control on the magnitude of the water removed is equal to the contraction in total
volume of concrete. About 3% reduction in concrete layer depth takes place. Filtering
pad consists of rigid backing sheet, expanded metal, wire gauze or muslin cloth sheet.
 A rubber seal is also fitted around the filtering pad as shown in fig.1. Filtering pad should
have minimum dimension of 90cm x 60cm.

Fig. 1: Vacuum dewatering of concrete

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Advantages of vacuum concreting:

 Due to dewatering through vacuum, both workability and high strength are achieved
simultaneously.
 Reduction in water-cement ratio may increase the compressive strength by 10 to 50% and
lowers the permeability.
 It enhances the wear resistance of concrete surface.
 The surface obtained after vacuum dewatering is plain and smooth due to reduced
shrinkage.
 The formwork can be removed early and surface can be put to use early.
 The advantages of dewatering are more prominent on the top layer as compared to
bottom layer. The effect beyond a depth of 150mm is negligible.

5.5. High strength concrete

 High strength concrete (HSC) may be defined as concrete with a specified characteristic
cube strength between 60 and 100 N/mm2. Concrete is defined as ―high-strength
concrete‖ on the basis of its compressive strength measured at a given age.
 High-strength concrete where the matrix is extremely dense. In high-strength concrete,
the aggregate plays an important role on the strength of concrete.
 Aggregate must be selected carefully for high strength mixes, as weaker aggregates may
not be strong enough to resist the loads imposed on the concrete and cause failure to start
in the aggregate.
 High strength concrete is made by lowering the water cement (W/C) ratio to 0.35 or
lower. Often silica fume is added to prevent the formation of free calcium hydroxide
crystals in the cement, which might reduce the strength at the cement aggregate bond.
 Low w/c ratios and the use of silica fume make concrete mixes significantly less
workable, which is particularly likely to be a problem in high-strength concrete.
 To compensate for the reduced workability in the high strength concrete mix,
superplasticizers are commonly added to high-strength mixtures.

111
Use
 high-strength concrete leads to a reduction of the size of the crystalline compounds.
There is a reduction of the thickness of the interfacial transition zone in high-strength
concrete.
 The densification of the interfacial transition zone allows for efficient load transfer
between the cement mortar and the coarse aggregate, contributing to the strength of the
concrete.

High performance concrete

 High performance concrete (HPC) for concrete mixtures possessing high workability,
high durability and high ultimate strength.

5.6. 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
fibres. Continuous meshes, woven fabrics and long wires or rods are not considered to be
discrete fibres.
 Plain concrete possesses a very low tensile strength, limited ductility and little resistance
to cracking.
 Internal microcracks are inherently present in the concrete and its poor tensile strength is
due to the propagation of such microcracks, eventually leading to brittle fracture of the
concrete.

 It has been recognised 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. This type of concrete is known as Fibre Reinforced Concrete.

Fibres Used
 The fibres that could be used are steel fibres, polypropylene, nylons, asbestos, coir, glass
and carbon fiber.
 Fibre is a small piece of reinforcing material possessing certain characteristic properties.
They can be circular or flat.

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  The fibre is often described by a convenient parameter called ―aspect ratio. The aspect
ratio of the fibre is the ratio of its length to its diameter. Typical aspect ratio ranges from
30 to 150.
 Steel fibre is one of the most commonly used fibre. Generally, round fibres are used. The
diameter may vary from 0.25 to 0.75 mm. The steel fibre is likely to get rusted and lose
some of its strengths. But investigations have shown that the rusting of the fibres takes
place only at the surface. Use of steel fibre makes significant improvements in flexural,
impact and fatigue strength of concrete, It has been extensively used in various types of
structures, particularly for overlays of roads, airfield pavements and bridge decks. Thin
shells and plates have also been constructed using steel fibres.
 Polypropylene and nylon fibres are found to be suitable to increase the impact strength.
They possess very high tensile strength, but their low modulus of elasticity and higher
elongation do not contribute to the flexural strength. Glass fibre is a recent introduction in
making fibre concrete. It has very high tensile strength 1020 to 4080 N/mm2.
 Glass fibre which is originally used in conjunction with cement was found to be effected
by alkaline condition of cement. Therefore, alkali-resistant glass fibre by trade name
―CEM-FIL‖ has been developed and used. The alkali resistant fibre reinforced concrete
shows considerable improvement in durability when compared to the conventional E-
glass fibre.
 Carbon fibres posses very high tensile strength 2110 to 2815 N/mm2 and Young‘s
modulus. It has been reported that cement composite made with carbon fibre as
reinforcement will have very high modulus of elasticity and flexural strength. The limited
studies have shown good durability. The use of carbon fibres for structures like clading,
panels and shells will have promising future.

Factors Effecting Properties of Fibre Reinforced Concrete

 Type of fibre, fibre geometry, fibre content, orientation and distribution of the fibres,
mixing and compaction techniques of concrete, and size and shape of the aggregate

Relative Fibre Matrix Stiffness

 The modulus of elasticity of matrix must be much lower than that of fibre for efficient
stress transfer.
 Low modulus of fibers such as nylons and polypropylene is unlikely to give strength
improvement, but they help in the absorption of large energy and, therefore, impart
greater degree of toughness and resistance to impact.
 High modulus fibres such as steel, glass and carbon impart strength and stiffness to the
composite. Interfacial bond between the matrix and the fibers also determine the
effectiveness of stress transfer, from the matrix to the fiber.
 A good bond is essential for improving tensile strength of the composite. The interfacial
bond could be improved by larger area of contact, improving the frictional properties and
degree of gripping and by treating the steel fibres with sodium hydroxide or acetone.

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Volume of Fibres
 The strength of the composite largely depends on the quantity of fibres used in it. Fig
show the effect of volume on the toughness and strength. It can be seen from that the
increase in the volume of fibres, increase approximately linearly, the tensile strength and
toughness of the composite is 12.7. Use of higher percentage of fibre is likely to cause
segregation and harshness of concrete and mortar

Aspect Ratio of the Fibre

 Another important factor which influences the properties and behavior of the composite
is the aspect ratio of the fibre. It has been reported that upto aspect ratio of 75, increase in
the aspect ratio increases the ultimate strength of the concrete linearly. Beyond 75,
relative strength and toughness is reduced.

Orientation of Fibres

One of the differences between conventional reinforcement and fibre reinforcement is
that in conventional reinforcement, bars are oriented in the direction desired while fibres
are randomly oriented.

To see the effect of randomness, mortar specimens reinforced with 0.5 per cent volume of
fibres were tested.

In one set specimens, fibres were aligned in the direction of the load, in another in the
direction perpendicular to that of the load, and in the third randomly distributed.

It was observed that the fibres aligned parallel to the applied load offered more tensile
strength and toughness than randomly distributed or perpendicular fibres.

Workability and Compaction of Concrete

 Incorporation of steel fibre decreases the workability considerably. This situation
adversely affects the consolidation of fresh mix. Even prolonged external vibration fails
to compact the concrete.
 The fibre volume at which this situation is reached depends on the length and diameter of
the fibre. Another consequence of poor workability is non-uniform distribution of the
fibres.
 Generally, the workability and compaction standard of the mix is improved through
increased water/cement ratio or by the use of some kind of water reducing admixtures.

Size of Coarse Aggregate


Several investigators recommended that the maximum size of the coarse aggregate

should be restricted to 10 mm, to avoid appreciable reduction in strength of the
composite. Fibres also in effect, act as aggregate.

Although they have a simple geometry, their influence on the properties of fresh concrete
is complex. The inter-particle friction between fibres, and between fibres and aggregates
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 controls the orientation and distribution of the fibres and consequently the properties of
the composite.


Friction reducing admixtures and admixtures that improve the cohesiveness of the mix
can significantly improve the mix.

Mixing

Cement content : 325 to 550 kg/m3 Fiber content

W/C Ratio : 0.4 to 0.6 0.5 to 2.5 per cent by volume of mix
% of sand to total aggregate : 50 to 100 % Steel —1 per cent 78 kg/m3
Maximum Aggregate
Size : 10 mm Glass —1 per cent 25 kg/m3
Air-content : 6 to 9 % Nylon —1 per cent 11 kg/m3

 Mixing of fibre reinforced concrete needs careful conditions to avoid balling of fibres,
segregation, and in general the difficulty of mixing the materials uniformly. Increase in
the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify
the difficulties and balling tendencies. Steel fibre content in excess of 2 per cent by
volume and an aspect ratio of more than 100 are difficult to mix.

Applications
 Fibre reinforced concrete is increased static and dynamic tensile strength, energy
absorbing characteristics and better fatigue strength.
 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.

5.8. Glass Fibre Reinforced Cement (GRC)

 Glass reinforced cement consists of 4 to 4.5 per cent by volume of glass fibre mixed
into cement or cement sand mortar. This glass reinforced cement mortar is used for
fabricating concrete products having a sections of 3 to 12 mm in thickness.
 Glass reinforced cement (GRC) has been used for clading of buildings, permanent and
temporary formwork, pressure pipes, doors and doors frames, decorative grills, sun
breakers, bus shelters and park benches. This will find its use in many application as
building components.

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Current Development in FRC
The following are the three new developments taking place in FRC.

 High fibre volume micro-fibre systems.

 Slurry infiltrated fibre Concrete (SIFCON).
 Compact reinforced composites.

Slurry infiltrated fibre Concrete (SIFCON).

 Slurry infiltrated fibre concrete (SIFCON) was invented by Lankard in 1979. Steel fibre
bed is prepared and cement slurry is infiltrated. With these techniques, macro-fibre
contents up to about 20% by volume can be achieved, with a consequent enormous
increase in both flexural load carrying capacity and toughness. With such high fibre
volume, a very high compressive strength is also achieved. SIFCON can be used for blast
resistant structures and burglar proof safe vaults in banks and residential buildings.

5.8. Polymer concrete

 Polymer concrete is part of group of concretes that use polymers to supplement or replace
cement as a binder. Polymer concrete (PC) is a mixture of aggregates with a polymer as
the sole binder. To minimize the amount of the expensive binder, it is very important to
achieve the maximum possible dry packed density of the aggregate.

Type of Polymer Concrete

Four types of polymer concrete materials are being developed presently. They are:
 Polymer Impregnated Concrete (PIC).
 Polymer Cement Concrete (PCC).
 Polymer Concrete (PC).
 Partially Impregnated and surface coated polymer concrete.

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Polymer Impregnated Concrete (PIC)
 Polymer impregnated concrete is one of the widely used polymer composite. It is nothing
but a precast conventional concrete, cured and dried in oven, or by dielectric heating from
which the air in the open cell is removed by vacuum.

 Then a low viscosity monomer is diffused through the open cell and polymerised by
using radiation, application of heat or by chemical initiation. Mainly the following types
of monomer are used:
(a) Methylmethacrylate
(b) Styrene
(c) Acrylonitrile
(d) t-butyl styrene,

Polymer Cement Concrete (PCC)

Polymer cement concrete is made by mixing cement, aggregates, water and monomer. Such
plastic mixture is cast in moulds, cured, dried and polymerised. The monomers that are used in
PCC are:
(a) Polyster-styrene.
(b) Epoxy-styrene.
(c) Furans.
(d) Vinylidene Chloride.

Polymer Concrete (PC)

 Polymer concrete is an aggregate bound with a polymer binder instead of Portland
cement as in conventional concrete. The main technique in producing PC is to minimise
void volume in the aggregate mass so as to reduce the quantity of polymer needed for
binding the aggregates.
 This is achieved by properly grading and mixing the aggregates to attain the maximum
density and minimum void volume. The graded aggregates are prepacked and vibrated in
a mould. Monomer is then diffused up through the aggregates and polymerisation is
initiated by radiation or chemical means.
 A silane coupling agent is added to the monomer to improve the bond strength between
the polymer and the aggregate. In case polyester resins are used no polymerisation is
required. An important reason for the development of this material is the advantage it
offers over conventional concrete where the alkaline Portland cement on curing, forms
internal voids.
 Water can be entrapped in these voids which on freezing can readily crack the concrete.
Also the alkaline Portland cement is easily attacked by chemically aggressive materials
which results in rapid deterioration, whereas polymers can be made compact with
minimum voids and are hydrophobic and resistant to chemical attack.

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Advantages Disadvantage

Good adhesion to most surfaces

Product hard to manipulate with
Good long-term durability with respect to
conventional tools such as drills and
freeze and thaw cycles presse du density.
s e to its Recommend
Low permeability to water and aggressive
getting pre-modified product from the
solutions
manufacturer
Good chemical resistance
Small boxes are more costly when
Good resistance against corrosion precas
Lighter weight (only somewhat less compared to its t counterpart
dense however pre cast concretes induction
than traditional concrete, depending on of
the stacking or steel covers quickly bridge
the
resin content of the mix)
gap.
May be vibrated to fill voids in forms
Allows use of regular form-release agents
(in some applications)

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 5.9. Gunite or Shotcrete


Gunite can be defined as mortar conveyed through a hose and pneumatically projected at a
high velocity on to a surface.Recently the method has been further developed by the
introduction of small sized coarse aggregate into the mix deposited to obtain considerably
greater thickness in one operation and also to make the process economical by reducing the
cement content.

Normally fresh material with zero slump can support itself without sagging or peeling off.
The force of the jet impacting on the surface compact the material. Sometimes use of set
accelerators to assist overhead placing is practised.

The newly developed ―Redi-set cement‖ can also be used for shotcreting process. There is
not much difference between guniting and shotcreting. shotcrete is a recent development on
the similar principle of guniting for achieving greater thickness with small coarse
aggregates. There are two different processes in use, namely the ―Wet-mix‖ process and
the ―dry-mix‖ process. The dry mix process is more successful and generally used.

Dry-mix Process

The dry mix process consists of a number of stages and calls for some specialised plant The stages
involved in the dry mix process is given below:
 Cement and sand are thoroughly mixed.

The cement/sand mixture is fed into a special air-pressurised mechanical feeder termed as
gun.

The mixture is metered into the delivery hose by a feed wheel or distributor with in the gun.

This material is carried by compressed air through the delivery hose to a special nozzle.

The nozzle is fitted inside with a perforated manifold through which water is sprayed under
pressure and intimately mixed with the sand/cement jet.

The wet mortar is jetted from the nozzle at high velocity onto the surface to be gunited.

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Wet-mix Process
 In the wet-mix process the concrete is mixed with water as for ordinary concrete before
conveying through the delivery pipe line to the nozzle, at which point it is jetted by
compressed air, onto the work in the same way, as that of dry mix process.
 The wet-mix process has been generally discarded in favour of the dry-mix process, owing
to the greater success of the latter. The dry-mix methods makes use of high velocity or low
velocity system.
 The high velocity gunite is produced by using a small nozzle and a high air pressure to
produce a high nozzle velocity of about 90 to 120 metres per second. This results in
exceptional good compaction. The lower velocity gunite is produced using large diameter
hose for large output. The compaction will not be very high.

Advantages and Disadvantages

Shotcrete concrete layers are very It has high cost. Its lining is less durable than
strong. ordinary
concrete lining of the same thickness.

These types of concrete do not

need Uses of shotcrete. Shell or folded roofs.
construction or expansion joints.

It can be evenly applied on Thin and lightly reinforced sections likes curtain walls
uneven etc.
surfaces and can be applied
from a
distance.

Lining of tunnels, canals etc, protective covering for

soft
dock

5.10. Ferrocement
 Ferro-cement is a relatively new construction material consists of wire meshes and cement
mortar. It was developed by P.L.Nervi, an Italian architect in 1940. Ferro cement is widely
used due to the low self weight, lack of skilled workers, no need of framework etc.Quality
of ferro-cement works are assured because the components are manufactured on
machinery set up and execution time at work site is less. Maintenance cost of ferro-cement
is low. Ferro-cement construction has come into widespread use only in the last two
decades.

120
  It consists of closely spaced wire meshes which are impregnated with rich cement
mortar mix. The wire mesh is usually of 0.5 to 1.0 mm dia wire at 5 mm to 10 mm
spacing and cement mortar is of cement sand ratio of 1 : 2 or 1 : 3 with water/cement
ratio of 0.4 to 0.45. The ferrocement elements are usually of the order of 2 to 3 cm.

 In thickness with 2 to 3 mm external cover to the reinforcement. The steel content varies
between 300 kg to 500 kg per cubic metre of mortar. The basic idea behind this material
is that concrete can undergo large strains in the neighbourhood of the reinforcement and
the magnitude of strains depends on the distribution and subdivision of reinforcement
throughout of the mass of concrete.

Ferro cement
 Highly versatile form of reinforced concrete. Its a type of thin reinforced concrete
construction, in which large amount of small diameter wire meshes uniformly through
out the cross section.
 Mesh may be metal or suitable material. Instead of concrete Portland cement mortar is
used.
 Strength depends on two factors quality of sand/cement mortar mix and quantity of
reinforcing materials used.

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Constituent Materials Cement, Fine Aggregate ,Water, Admixture ,Mortar, Mix Reinforcing
mesh , Skeletal Steel, Coating

Application
 Housing ,Marine ,Agricultural ,Rural Energy ,Anticorrosive Membrane
Treatment.Ferrocement has been successfully used for casting domestic over-head water
tanks. The tank being light and flexible can be transported. The ferrocement tank will
help in preserving the grain uneffected by moisture and rodents.

Similar ferrocement container can be used as gas holder unit in ‗Gobbar gas‖ plants With
a few modifications, ferrocement tanks can also be used as septic tank units. The
properties of ferrocement make it an ideal material for boat building.

It has been reported that ferrocement boats 14 m long weighs only 10 % more than
wooden boats and the same is 300 % cheaper than fibre reinforced concrete boats, 200 %
cheaper than steel boats and 35 % cheaper than timber boats.

Ferrocement manhole cover is becoming very popular to replace cast iron manhole cover
over sewers around domestic building where they are not subjected to heavy vehicular
traffic.

Owing to the reason that the cost of ferrocement manhole cover is only about 1/10 of the
cost of cast iron manhole cover, and that it can be manufactured readily, it makes a good
substitute for cast iron manhole cover. Ferrocement is becoming a popular material for
prefabricated roof units. Ferrocement folded plate being light, could be used
advantageously as prefabricated roof units for garages and storage sheds.

Ferrocement is a suitable material for pressure pipes. It will be much lighter compared to
normal reinforced concrete pipes.

Ferrocement is found to be a suitable material for casting curved benches for parks,
garden and open-air cinema theatre.

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 Cost effectiveness of ferro cement structure Recent application
Economic Residential and Public Buildings.
Relative cost of labor.  Industrial Structures.
Capital and local tradition of
construction
Agricultural structures.
procedure.
Doesn‘t need heavy plant or machinery. Transportation Structures.
Low cost of construction materials.

Advantages of Ferro-Cement: Disadvantages of Ferro-Cement:

Basic raw materials are readily available in Structures made of it can be punctured by
most collision
countries. with pointed objects.

Corrosion of the reinforcing materials due

Fabricated into any desired shape. to the
incomplete coverage of metal by mortar.

It is difficult to fasten to Ferro-cement with

Low labour skill required. bolts,
screws, welding and nail etc.

Ease of construction, low weight and long Large no of labors required. Cost of semi-
lifetime. skilled and
unskilled labors is high. Tying rods and
mesh
together is especially tedious and time
consuming.

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 5.11. Geopolymer concrete

Geopolymer concrete is considered to be an innovative material that is a viable alternative to

traditional Portland concrete or cement used in transportation infrastructures, certain
constructions and offshore builds. It is very resistant to several of the durability issues that can
cause traditional concretes to crack and crumble. This is accomplished by using minimally
processed natural materials or industrial byproducts. Geopolymer concretes will cure more
quickly than Portland concretes. They have obtained most of their strength within 24 hours.
Some major pros and cons of geopolymer concrete will be discussed in this article. Importance
of Using Geopolymer Concrete This type of concrete is starting to revolutionize concrete. It is
being used more and more in highway construction projects and offshore applications. It is still
a little too pricey for the do-it-yourself projects that abound, but contractors are starting to use it
more and more in other construction projects.

Advantages of Geopolymer Concrete

It is a newer product that is making traditional concrete look not so spectacular. Here are some of
the top advantages of geopolymer concrete.
1. High Strength – it has a high compressive strength that showed higher compressive strength
than that of ordinary concrete. It also has rapid strength gain and cures very quickly, making it an
excellent option for quick builds. Geopolymer concrete has high tensile strength. It is less brittle
than Portland concreteand can withstand more movement. It is not completely earthquake proof,
but does withstand the earth moving better than traditional concrete.

2. Very Low Creep and Shrinkage – shrinkage can cause severe and even dangerous cracks in the
concrete from the drying and heating of the concrete or even the evaporation of water from the
concrete. Geopolymer concrete does not hydrate; it is not as permeable and will not experience
significant shrinkage.

3.Resistant to Heat and Cold – It has the ability to stay stable even at temperatures of more than
2200 degrees Fahrenheit. Excessive heat can reduce the stability of concrete causing it to spall or
have layers break off. Geopolymer concrete does not experience spalling unless it reaches over
2200 degrees Fahrenheit. As for cold temperatures, it is resistant to freezing. The pores are very
small but water can still enter cured concrete. When temperatures dip to below freezing that
water freezes and then expands this will cause cracks to form. Geopolymer concrete will not
freeze.

4. Chemical Resistance – it has a very strong chemical resistance. Acids, toxic waste and salt
water will not have an effect on geopolymer concrete. Corrosion is not likely to occur with this
concrete as it is with traditional Portland concrete.

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Disadvantages of Geopolymer Concrete

While geopolymer concrete appears to be the super concrete to take the place of traditional
Portland concrete, there are some disadvantages such as:
 Difficult to Create – geopolymer concrete requires special handling needs and is
extremely difficult to create. It requires the use of chemicals, such as sodium hydroxide,
that can be harmful to humans.
 Pre-Mix Only – geopolymer concrete is sold only as a pre-cast or pre-mix material due to
the dangers associated with creating it. Geopolymerization Process is Sensitive – this
field of study has been proven inconclusive and extremely volatile. Uniformity is lacking.

5.12. Ready-mix concrete

 Ready mix concrete is concrete that is manufactured in a factory or batching plant,
according to a set recipe, and then delivered to a work site, by truck mounted in–transit
mixers.
 Ready-mix concrete is also referred as the customized concrete products for commercial
purpose. Ready-mix concrete, or RMC as it is popularly called, refers to concrete that is
specifically manufactured for delivery to the customer's construction site in a freshly
mixed and plastic or unhardened state.
 Concrete itself is a mixture of Portland cement, water and aggregates comprising sand
and gravel or crushed stone. In traditional work sites, each of these materials is procured
separately and mixed in specified proportions at site to make concrete. Read-mix
concrete is bought and sold by volume - usually expressed in cubic meters

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Disadvantages of ready-mix concrete

The materials are batched at a central plant, and the mixing begins at that plant, so the traveling
time from the plant to the site is critical over longer distances. Some sites are just too far away,
though this is usually a commercial rather than a technical issue.
 Generation of additional road traffic. Furthermore, access roads and site access have to be
able to carry the greater weight of the ready-mix truck plus load. (Green concrete is
approx. 2.5 tonne per m³.) This problem can be overcome by utilizing so-called 'mini
mix' companies which use smaller 4m³ capacity mixers able to reach more-restricted
sites.
 Concrete's limited time span between mixing and curing means that ready-mix should be
placed within 210 minutes of batching at the plant. Modern admixtures can modify that
time span precisely, however, so the amount and type of admixture added to the mix is
very important.

126