31-Dec-20

Concrete
By Badal Soni (Ex IES)
Faculty Civil Engineering
badalsoni@madeeasy.in
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Introduction
• Concrete is a composite man made material and is most widely used
building material in the construction industry.
• It is a mixture of binding material such as lime or cement, well
graded coarse and fine aggregate, water and sometimes admixtures.
• Basic requirement of good concrete is that it should be satisfactory
in hardened state and also in fresh state.
• Fresh state consistency of mix should be such that it can be
compacted by the desired means without excessive effort and mix
should be cohesive enough for the methods of transportation and
placing, used so as not to cause segregation.
• In hardened state satisfactory compressive strength and durability
is required.
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Classification of Concrete
1. Based on binding material :
a) Mud concrete :
• Made by kneading good quality clay and water and mixing coarse
aggregate in it.
• Properties are due to interlocking of irregular aggregate particles
and filling voids by mud.
• It has poor impermeability, durability, strength and easily affected
by moisture.
b) Lime concrete :
• Mixing coarse aggregate with hydraulic lime as a binding material.
• It has fairly good durability and strength characteristics and
flexibility.
• Good water proofing property and prevents sub soil dampness in
floor and walls.
• It does not harden in water and gains strength slowly. 3

• Made easily and cheaper

c) Portland cement concrete :

• Mixing of portland cement, sand, aggregate and water.
2. Based on perspective specification :
• Cement concrete is specified by proportions (by weight) of different
ingredients. Eg 1 (cement) : 1.5 (fine aggregate) : 3 (coarse
aggregate).
• This type of concrete mix is known as Nominal Mix.
• It is presumed that by adhering to such perspective specifications
satisfactory performance may be achieved.
• Quantity of aggregate and cement is fixed irrespective of water
cement ratio and maximum size of aggregate to be used.
Mix proportions of cement concrete
Grade of concrete M5 M 7.5 M 10 M 15 M 20 M 25
Mix proportion 1:5:10 1:4:8 1:3:6 1:2:4 1:1.5:3 1:1:2
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• Here M refers to mix.
• IS 456 restricts use of nominal mix upto M 20 grade only.

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3. Based on performance oriented specification:

• When the concrete properties such as strength, water-cement ratio,
compaction factor, slump, etc. are specified then concrete can be
classified as designed mix concrete.
• In a design mix concrete the mix is designed to produce the grade of
concrete having the required workability and a characteristic
strength not less than the specified values.
4. Based on grade of cement concrete:
• On the basis of strength (N/mm2) of concrete cubes (150 mm side)
at 28 days, concrete is classified as,

Ordinary Concrete ≤ 20 MPa

Standard Concrete 25 – 60 MPa
High Strength Concrete 65 – 100 MPa

5. Based on bulk density :

Extra light weight < 500 kg/m3
Light weight 500 – 1800 kg/m3
Dense weight 1800 – 2500 kg/m3
Super heavy weight > 2500 kg/m3
6. Based on place of casting :
• In-situ concrete: When concrete is placed in position at the site, it is
known as cast in-situ concrete.
• Pre-cast concrete: When concrete is used for making prefabricated
units in a factory is called as precast concrete.

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Aggregates
• Aggregates are used as filler with binding material in production of
concrete.
• They form body of concrete occupying 70% to 80% of volume of
concrete.
• They exert considerable impact on the characteristics and properties
of concrete.
• They should be clean, hard, strong, durable and graded in size to
achieve utmost economy from the paste.
• They are derived from igneous, sedimentary and metamorphic rocks
or manufactured from blast furnace slag etc.
• Aggregates used in concrete are of two sizes
i. Coarse aggregate – forming main matrix of concrete.
ii. Fine aggregate – forming filler matrix between coarse aggregate.
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Note: Earlier aggregates were considered to be chemically inert but the
latest research has revealed that some of them are chemically active.

Classification of aggregates
On the basis of geological origin:
Natural aggregates Artificial aggregates
Sand, gravel, crushed rock such Broken brick, Blast furnace
as Granite, Basalt, Sandstone, slag, Sintered fly ash,
Quartzite, etc. Bloated clay etc.
Note:
• Crushed rock aggregate have irregular shape, hence it provide good
interlocking bond therefore high compressive strength can be
achieved
• Broken bricks (brick bats) are suitable for mass concreting
(foundations), but not used for RCC.
• Blast furnace slag aggregate is obtained from slow cooling of the
slag followed by crushing. These dense and strong aggregates has
good fire resisting properties but are responsible for corrosion of
reinforcement due to sulphur content of slag.
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• Synthetic aggregates are produced by thermally processed materials
such as expanded clay and shale used for making light weight
concrete.

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On the basis of size:

a) Coarse aggregate:
• 4.75 mm to 80 mm
• Main matrix of concrete.
• Obtained from natural disintegration or artificial crushing of rocks.
b) Fine aggregate:
• 4.75 mm to 75 micron
• Filler matrix in between aggregate.
• Natural sand deposited by rivers, crushed stone or crushed gravel
sand.
On the basis of shape:
a) Round aggregates (void = 32%)
• Usually obtained from river or seashore.
• These have minimum ratio of surface area to volume, thus cement
and water required is minimum to achieve desired workability. 9
• Poor interlocking bond between particles makes it unsuitable for
high strength concrete.

b) Irregular aggregates (voids= 36%)

• They require more cement paste as compared to round aggregates.
• Due to irregularity in shape they develop good bond and are suitable
for making ordinary concrete.
c) Angular aggregates (voids = 40%)
• Sharp, angular and rough particles.
• Provide very good bond than earlier two, most suitable for high
strength concrete.
• Requirement of cement paste is relatively more than previous two.
d) Flaky and elongated aggregates
• Both influence the concrete properties adversely.
i. Flaky aggregate
• Least lateral dimension (thickness) < 0.6 times mean dimension.
• Not applicable to sizes < 6.3 mm
• For example, the mean sieve size for an aggregate piece passing
through 50mm and retained on 40 mm sieve is (50 + 40)/2 = 45mm.
If the least lateral dimension is less than 0.6 x 45 = 27.0 mm, the 10
aggregate is classified as flaky.

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ii. Elongated aggregate

• Length (greatest dimension) > 1.8 times mean dimension.
• Not applicable to sizes < 6.3 mm.
Note:
• Very sharp and rough aggregates or falky and elongated particles
require more fine material to produce a workable concrete.
Accordingly water content and the cement content increases.

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On the basis of unit weight:

Aggregate Unit weight (KN/m3) Example

Light weight 12 Dolomite, Pumice,

Cinder, etc.
Normal weight 23-26 Sand, Gravel, Granite,
Limestone etc.
Heavy weight 25-29 Magnetite, Baryte,
Scrap iron, etc.

Characteristics of aggregates
1. Strength
• Strength should be at least equal to that of concrete.
• Natural aggregates are generally stronger than concrete, but they are
still required to be tested for production of high strength and ultra
high strength concrete.
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Stress strain curve for aggregate

Test Significance
Crushing test Compressive strength
10% fines test Compressive strength
Impact value Toughness
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Abrasion test Hardness

2. Stiffness
• Modulus of elasticity of concrete = weighted average of the moduli of
cement paste and aggregate.
• Modulus of coarse aggregate has an important influence on stiffness
of concrete.
• A high value reduces the dimensional changes due to creep and
shrinkage of cement paste.
3. Bond Strength
• Strength of bond between aggregate and cement paste has important
influence on strength of concrete.
• Due to different coefficients of thermal expansion of paste and
aggregate, also due to shrinkage of cement paste during hardening,
concrete is in a state of internal stress even if no external forces are
present.
• Stresses are likely to be greatest at paste – aggregate where minute
cracks exist, even in concrete that has never been loaded.
• There is no standard test for bond strength for bond but it is known
that the rougher the surface texture of the particles, the better is the 14
bond strength.

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4. Shape and Texture

• Shape and texture influences the properties of fresh concrete more
than hardened concrete.
• Rounded aggregate are highly workable but yield low strength
concrete. Same is the case with irregular shaped aggregate.
• Flaky aggregate require more cement, produce maximum voids and
are not desirable.
• Angular aggregate is the best.
• Rough textured aggregate require more water to produce workable
concrete than smooth aggregate. Hence cement content must also be
increased to maintain the water cement ratio.
5. Specific Gravity
• For most of the natural aggregates it lies between 2.6-2.7.
• Specific gravity is indicative of its quality.
• Low specific gravity indicate high porosity and therefore poor
durability and low strength. 15
• Concrete density will greatly depend on specific gravity of aggregates.

6. Water absorption and moisture content

• Water absorption of aggregate is determined by measuring the

increase in weight of an oven dry sample when immersed in water for
24 hours.
• In mix design calculation weight of aggregates are based on condition
that aggregates are saturated and surface dry (SSD).
• But in practice, aggregates in such ideal condition is rarely met with.
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• Aggregates are either dry and absorptive (exposed to the sun for a
long time) or they have surface moisture (exposed to rain)

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• Both these conditions are harmful for the quality of concrete.

• If aggregates are dry they absorb moisture from mixing water and
affect workability.
• If aggregates contains surface moisture they contribute extra water to
the mix and thereby increase the water/cement ratio.
• Corrective measures should be taken both for absorption and for free
moisture so that the water/cement ratio is kept exactly as per design.
7. Bulking of fine aggregate

Thin water film 17

• Bulking is increase in the volume of given mass of sand caused by the

films of water pushing the sand particles apart due to surface tension.
• Extent of bulking depends upon percentage of moisture in the sand
and fineness.
• It increases up to moisture content of (4-6%), reaches maximum,
thereafter the film of water on sand surface breaks, and then it starts
decreasing.
• If sand is measured by volume and no allowance is made for bulking,
the moist sand will occupy considerably larger volume than that
prepared by the dry sand consequently the mix will be richer.
• For example, if bulking of sand is 20% and mix ratio is 1:2:4 and if
bulking correction is not applied to sand then dry sand in concrete will
be 2/1.2= 1.667 instead of 2 per unit volume of cement. Mix
proportion will 1:1.667:4 in stead of 1:2:4 which indicates less
production of concrete.
• To counter the effect of bulking volume of sand used in the mix should
be equal to 1.2 x 2 = 2.4 instead of 2 per unit volume of cement.
Note: 18
In weight batching, bulking effect of sand is not accounted

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8. Fineness modulus (FM)

• FM = (sum of cumulative percentage retained on the sieves of
standard sizes)/100
• It gives some idea about the mean size of the particles.
• Standard set of sieves – 80mm, 40mm, 20mm, 10mm, 4.75mm,
2.36mm, 1.18mm, 600 mm, 300 mm, and 150 mm.
• FM varies as 2 to 3.5 for fine aggregates, 5.5 to 8.0 for coarse
aggregates, and 3.5 to 7.5 for all in aggregates.
• Purpose of finding FM is to grade the given aggregate for the strength
and workability of concrete mix with minimum cement.
• Higher FM aggregates result in harsh concrete mixes and lower FM
result in uneconomical concrete mixes.
• FM for mix aggregates is calculated as,

% of coarse aggregate ×FM of coarse aggregate + % of fine aggregate ×FM of fine aggregate
100 19

Example of fineness modulus calculation for coarse aggregate

Let dry weight of coarse aggregate = 5000gm
Sieve wt. retained Cumulative wt. retained Cumulative %
size (gm) (gm) retained

80 mm 0 0 0
40 mm 250 250 5
20 mm 1750 2000 40
10 mm 1600 3600 72
4.75 mm 1400 5000 100
2.36 mm 0 5000 100
1.18 mm 0 5000 100
0.6 mm 0 5000 100
0.3 mm 0 5000 100
0.15 mm 0 5000 100
Sum= 717 20

• FM = (sum of cumulative % retained/100) = 717/100 = 7.17

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• Fineness modulus of 7.17 means, the average size of particles of given

coarse aggregate is in between 7th and 8th sieves, that is between
10mm and 20mm.

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Example of fineness modulus calculation for fine aggregate

Let dry weight of fine aggregate = 1000gm
Sieve wt. retained Cumulative wt. retained Cumulative %
size (gm) (gm) retained
4.75 mm 0 0 0
2.36 mm 100 100 10
1.18 mm 250 350 35
0.6 mm 350 700 70
0.3 mm 200 900 90
0.15 mm 100 1000 100
Sum= 275
• FM = (sum of cumulative % retained/100) = 275/100 = 2.75
• Fineness modulus of 2.75 means, the average size of particles of given
fine aggregate is in between 2nd sieve and 3rd sieve, that is between
0.3 mm and 0.6mm.
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9. Soundness
• Soundness is defined as the ability of aggregate to resist changes in
volume.
• Physical conditions responsible for unsoundness are freezing and
thawing, thermal changes, alternate wetting and drying etc.
• Porous and weak aggregates containing undesirable extraneous
matter undergo excessive volume changes under favorable
conditions.
• Freeze and thaw resistance of aggregate is related to its porosity,
absorption and pore structure. This may cause surface cracking which
may lead to structural failure.
10. Alkali aggregate reaction
• It is reaction between siliceous mineral in aggregate and alkaline
hydroxides in pore water derived from the alkalis (Na2O and K2O) in
cement.
• As a result, an alkali silicate gel is formed, either in planes of 23
weakness or pores in the aggregate (where reactive silica is present)
or on the surface of the aggregate particles.

• The gel destroys the bond between aggregate and surrounding

hydrated cement paste.
• Gel is ‘unlimited swelling’ type it imbibes water and increase in
volume, leading to expansion, cracking and disruption of the hydrated
cement paste.
11. Thermal properties
• Thermal properties of coarse aggregate are specific heat, thermal
conductivity and coefficient of expansion.
• First two are detrimental in case of mass concrete and light weight
concrete used for thermal insulation purpose.
• Third one affects the concrete in general because the coefficient of
thermal expansion of concrete increases with that of coarse
aggregate.
• Any appreciable difference in the coefficients of coarse aggregate and
cement paste may break the bond between the two.
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Grading requirements of aggregate

• Grading is the particle size distribution of aggregate.
• It is shown by grading curve, in which cumulative percentage passing
is plotted against the standard IS sieve size.
• Grading of aggregates indicates whether it is too coarse or too fine
or deficient in particular size.
• Grading (type and size) of aggregate influences workability and
finishing characteristics of fresh concrete and compaction of
hardened concrete.
• Incomplete compaction results in voids, lowers density, compressive
strength, and adversely affects the impermeability and durability
characteristics of concrete.
• Well graded aggregate, has all size of particle (fine & coarse), helps
concrete mix to acquire minimum voids and will require minimum
paste to fill the voids.
• Minimum paste means less quantity of cement and water which 25
results in increased economy, higher strength, lower shrinkage and
greater durability.

• Poorly or uniformly graded aggregates (almost same size particles),

are not effectively packed, concrete will be more porous, unless a lot
of cement paste is used.
• Gap graded aggregate (one or more size of aggregates are missing).
It can make good concrete, if workability required is relatively low.
• Because of lower surface area, (w.r.t well graded aggregate) lower
water-cement ratio is demanded. If used in high workability mixes
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segregation occurs.

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Water
• Water is most important ingredient for production of concrete and
less costly.
• Purpose of using water is to cause hydration of cement.
• Quality of water is to be carefully controlled during manufacture of
concrete.
• Water is also used for washing aggregates and curing.
• Water in excess of that required for hydration act as a
a) Lubricant between coarse and fine aggregates produces workable and
economical concrete.
b) Cement along with water comes to surface by capillarity and forms a
thin layer on surface known as laitance. It weakens bond between the
successive lifts.
c) Excess water leak through form work, causing honeycombed concrete
and on evaporation makes concrete porous.
• Whereas lesser water reduces workability (ease and homogeneity
of concrete in mixing, placing, compacting and finishing) which 27
results in low strength.

Quality of water
• It is perceived that natural potable water that has no pronounced
taste or odour is acceptable for concrete, but this statement is not
true in all conditions.
• Excessive impurities affects setting time, strength, durability
efflorescence, and corrosion of steel.
• Effects of impurities in water are mainly expressed in terms of initial
setting time of OPC with impure water and distilled water.
1. Inorganic salts
• Manganese, tin, zinc, copper and lead in water causes reduction in
strength of concrete.
• Na2S – detrimental to concrete.
• CaCl2 accelerates setting and hardening. Chlorides also causes
corrosion of reinforcing steel.
2. Acid and Alkalies
• Water containing acids or alkalies (industrial waste water) is
unsuitable for making concrete. 28
• PH 6-8 should only be used

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3. Sugar
• Sugar as an admixture (retarding agent) is used for delaying the
setting time of concrete without detrimental effect on the ultimate
strength.
0.05% by weight of water Harmless
0.15% by weight of cement Reduces early strength and increases 28 day strength
0.2%by weight of cement Rapid setting with reduced 28 days strength

• Skimmed milk powder (casein) has a retarding effect mainly due to

sugar content.
4. Oil contamination
• Mineral oils not mixed with animal or vegetable oils, have no adverse
effects on the strength of concrete.
• Vegetable and animal oils have bad effect on strength of concrete at
later stages.
5. Algae
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• Algae may be present in mixing water or on the surface of aggregate
particles.

• Combines with cement and reduces bond between aggregate and

cement.
• Water containing algae has the effect of entraining large quantities of
air in concrete and thus lowering the strength of concrete.
Effect of mixing water from different source
1. Ground water:
• Ground water seldom contain more than 20 to 30 ppm of iron, but
acid mine water may carry large quantities of iron.
• Iron salts in concentration up to 40,000 ppm do not usually affect
mortar strength adversely.
2. Sea water:
• Sea water generally contains 3.5 % of dissolved salts, of which 75% is
NaCl and 15 % is chloride and sulphate of magnesium.
• As per IS 456 sea water shall not be used for mixing and curing of
RCC and pre-stressed work.
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• But it permits for PCC in unavoidable condition.
• Sea water shall not be used for PCC if aggregates are alkali reactive.

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• Salts in sea water may cause efflorescence and persistent dampness.

Hence it should be avoided when appearance of concrete is
important. It is also not advisable for plastering purpose.
• Sea water slightly accelerates the setting time of cement but it
reduces the 28 days strength by about 10-20%.
Water for curing
• Water fit for concrete can be used for curing.
• Water containing more than 0.08 ppm of iron is not recommended
for curing, if appearance of concrete is important.

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Stages of concrete production

1. Batching or measurement of materials
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Finishing
7. Curing
Batching
• Batching refers to controlling quantity of each material required for
making concrete.
• There are two prevalent methods of batching materials
a) Volume batching
b) Weight batching 32

• Tolerance limit for ingredient in weight batching (IS 4925 : 2004)

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Type of material (wt/wt) Tolerance

Cement and cementitious material ±1%
Water ±1%
Aggregates ±2%
Admixture ±3%

a) Volume batching
• Gauge box (standard box) is used to measure loose volume of solid
ingredients.
• Recommended for small job only
• Correction for bulking of sand is done, if volume batching is adopted
b) Weight batching
• For important works weight batching is recommended.
• Bulking effect of sand is not taken into account.
• Weight of surface water of wet aggregates must be accounted.
• In weight batching water is not added by graduated buckets as it
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may spill over during its addition. A horizontal or vertical tank is
fitted to the mixer.

Mixing
• Objective is to obtain homogeneous, uniform color and consistent
concrete of desired strength.
• If mixing time is increased up to 2 minutes , compressive strength of
concrete produced is enhanced and beyond this time the
improvement in compressive strength is insignificant.
• Prolonged mixing may cause segregation, water may get absorbed
by the aggregates or evaporate resulting in loss of workability and
strength.
• Mixing is done either by hand or by machine mixing.
Note: For optimum quality, materials should be first mixed dry and
then water is added.
a) Hand mixing:
• Hand mixing is adopted for small jobs where the quantity of
concrete involved is small.
• Mixing time should be approximately 2 minutes and should never 34
exceed 3 minutes.

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b) Machine mixing:
• When a large quantity of concrete of the desired quality is to be
produced, the machine mixing becomes imperative as concrete can
be produced at a faster rate with better quality.
• Mixers can be broadly classified as
1. Batch mixer
2. Continuous mixer
Batch mixer
• It produces concrete batch by batch with time interval.
• All ingredients are loaded in to the mixer and mixed until a
homogeneous material is produced and discharged from the mixer
in a single lot.
• The output of a batch mixer is measured in Kg/batch.
• They are used for small or medium size works.
• Batch mixers are of two types
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i. Drum type
ii. Pan type

Drum type mixer

• Drum type concrete mixer is of three types
a) Tilting (T) : 85T, 100T, 140T, 200T
b) Non-tilting (NT) : 200NT, 280NT, 340NT, 400NT, 800NT
c) Reversing (R) : 200R, 280R, 340 R, 400 R
(number representing nominal mixed batch capacity in litres)
a) Tilting mixer
• Blades are fixed inside the drum.
• The revolving drum with the mixing blade
gives lifting and free fall to the mix and
agitates it.
• The mixed concrete is discharged from
the open top of the drum by tilting it
downwards.
• It gives better result even with mixes of
low workability and containing large size 36
aggregates.

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Non-tilting mixer
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b) Non-tilting mixer
• It consists of a non-tilting cylindrical drum with blades inside and
two circular openings at the two ends.
• The ingredients are fed from one opening and the mix is discharged
form the other opening
• Blades help in mixing and discharge of concrete.
• Segregation occurs due to slow rate of discharge.

c) Reversing drum mixer

• It consists of a horizontal non-tilting type drum with two sets of
blades.
• One set of blades mix while drum in rotating in one direction while
the second set of blades discharges the mix when the drum is
reversed.

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Pan-type mixer (Stirring mixers)

• It consist of a circular pan in which concrete is mixed by blades

which are arranged in star shape inside the pan.
• Because of design of blades cross flow is created which forces the
concrete towards the center causing turbulence and produces a
perfect homogeneous mix.
• Very efficient in working especially with stiff mixes.
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• Apart from mixing the ingredients, kneading and crushing action is
also performed.

Continuous mixers
• Loading, mixing and discharging of mix is continuously done.
• Ingredients are continuously charged into the mixer in accordance
with the formulation.
• The mixing takes place as the material travels from the charging
point to the discharge nozzle, from where it is continuously
discharged.
• The output of a continuous mixer is measured in kg/hr.
• It is used for large scale projects such as dams, bridges,
construction of high rise buildings etc.

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