AGGREGATES
There are two types of aggregates
a.
Coarse Aggregates
b.
Fine Aggregates
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Fine and coarse aggregates make up the bulk of a concrete mixture.
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Sand, natural gravel and crushed stone are mainly used for this purpose.
2.
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 COURSE AGGREGATES
 It is the aggregates whose particles completely pass through 7.5cm mesh
sieve and which are entirely retained on 4.75mm sieve.
 It is either measured by volume or weight
 FINE AGGREGATES
 Grading limits of fine aggregates has been specified by grading zones,
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namely zone I, II, III and IV
Zone I Fine aggregate is coarse
Zone II Fine aggregate is finer than zone I
Zone III Sand is finer than zone II sand
Zone Iv is the finest.
 GRADING OF AGGREGATES
 Grading of aggregates means particle size distribution of the
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aggregates.
Principle of grading is that the smaller size particle fill up the voids left
in larger size particles.
Properly graded aggregate produce dense concrete.
The grading of aggregate is expressed In terms of percentages by
weight retained on a series of sieves.
Grading detemines the workability of the mix, which controls
segregation, bleeding, water-cement ratio, handling, placing and
compacting.
 SIGNIFICANCE OF BULKING OF SAND
 Bulking of sand means increase in its volume.
 Fine aggregates or sands, increase in volume when they possess some
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moisture.
Bulking is due to the formation of a thin film of water around the fine
aggregate or sand particles.
Thickness of water film goes on increasing with more and more
moisture and consequently, increase in volume continues.
But after certain percentage of water, volume of sand starts decreasing
with increasing amount of water.
At certain percentage of water, increase in volume completely vanishes
and volume of water occupied by sand becomes equal to the volume of
dry sand.
See the next slide for the graphical representation of Bulking of Sand.
 WATER
 It is added to cement concrete to bring about the hydration of the
cement and lubricate the aggregate particles.
 Water which is suitable for drinking is usually considered safe for
concreting purpose.
 Water is measured by volume, litres per bag of cement.
 WATER TO CEMENT RATIO (W/C RATIO)
 It is the ratio of water to cement in a concrete mixture
 The strength of concrete increases with decreasing water cement ratio.
 Or in another words, lesser the volume of water in the concrete, the
greater will be the strength of concrete.
 See graphical representation on next slide.
 MIX DESIGN/SPECIFICATIONS FOR CONCRETE/ CONCRETE
MIX PROPORTIONING
 Mix design is the process of proportioning the ingredients of concrete
with the specified properties like strength, durability and cost.
 Concrete when used on construction works is specified in one of the
following ways
a)
Prescribed Mix/ Nominal Mix.
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The constituents of the mix should be in the ratio of 1:2:4 concrete,
means one part of cement, two parts of fine aggregates and four parts
of coarse aggregates.
For non-structural works e.g. boundary walls, building carrying low
loads.
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b) Designed Mix
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Designed mix is used for structural works in terms of the grade.
The grade denotes the 28days cube stength in N/mm2
The strength of concrete is determined by casting it in 15cm cube
moulds, curing it and testing it after 28 days.
 GRADING OF CONCRETE
 In the designation of concrete “M”stands for mix design and numerical
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figure stands for characteristic compressive strength of 150mm cube at
28 days in N/mm2.
Minimum grade of concrete for reinforced concrete is M20.
Ordinary Concrete – Grades M10, M15, M20 are grouped as Ordinary
Concrete
Standard Concrete – Grade M25, M30, M35, M40 and M50 are
grouped as Standard Concrete.
High Strength Concrete – Grade M60, M65, M70 and M80 are
grouped as High Strength Conrete.
 ADDITIVES OR CHEMICAL ADMIXTURES
 Chemical admixtures are materials in the form of powder or fluids that
are added to the concrete to give it certain characteristics not
obtainable with plain concrete mixes.
 They are added at the time of batching/mixing
 Common admixtures are listed below.
i.
Accelerators – They speed up the hydration of the cement e.g CaCl2
NaCl.
ii.
Retarders – They slow the hydration of concrete.
iii. Air Entraining Agent – Tiny air bubbles are added in concrete
iv. Plasticizers/ Superplasticizers – Used to reduce the water content of
a concrete thereby increases the workability and they have been
called water reducers.
v.
Pigments – Can be used to change the colour of concrete for
aesthetics.