Concrete Manufacturing process

1. Unit no. 2
Fresh Concrete
Mr. Kiran R. Patil
Assistant Professor,
Department of Civil Engineering,
D. Y. Patil College of Engineering & Technology, Kolhapur

2. • Introduction:
• Fresh concrete or plastic concrete is a freshly mixed material which can be moulded into
any shape. The relative quantities of cement, aggregates and water mixed together, control
the properties of concrete in the wet state as well as in the hardened state.
• Properties of Fresh Concrete
• The properties of concrete in plastic state are,
1. Workability
2. Segregation
3. Bleeding
1. Workability
• Workability is the ease with which the fresh concrete can be placed, compacted and
finished. Workable concrete is the one which overcomes the frictional resistance offered by
the formwork surface or reinforcement with the help of compaction.
• Knowledge of workability is required to design a concrete mix. A mix designer should
specify workability in the mix design process with full understanding of the type of work,
distance of transport, method of placing, method of compaction, etc.
• The factors affecting workability are,
1. Water content
2. Mix proportions – aggregate/cement ratio

3. 4. Size of aggregates
5. Shape of aggregates
6. Surface texture of aggregates
7. Grading of aggregates
8. Use of admixtures
1. Water Content:
• Water content has great influence on the workability. The higher the water content, the
higher will be the workability. For controlled concrete, water should not be arbitrarily
increased. It will result in reduced strength. If more water is to be added, then
correspondingly extra cement should be added to maintain the water-cement ratio; so that
the strength remains the same.
2. Mix Proportions:
• Aggregate/cement ratio is an important factor influencing the workability. The higher the
aggregate/cement ratio, the leaner is the concrete. in lean concrete, less quantity of cement
paste is available for providing lubrication and hence the workability will be less. In case of
rich concrete with lower aggregate/cement ratio, more paste will be available to make the
mix more workable.
3. Size of Aggregates:
• The bigger the size of the aggregate, the less is the surface area and hence less amount of
water is required for wetting the surface. Also, less cement paste is required for lubricating
the surface. Thus, bigger size of aggregates will give more workability.

4. 4. Shape of Aggregates:
• Rounded aggregates will give more workability than angular aggregates.
5. Surface Texture of Aggregates:
• Smooth or glassy aggregates will give better workability than rough textured aggregates.
6. Grading of Aggregates:
• A well graded aggregate has minimum voids. Thus, excess paste will be available to give
better lubrication and thus better workability.
7. Use of Admixtures:
• Plasticizers and superplasticizers improve the workability. Use of air-entraining admixtures
produces small air-bubbles which act as ball bearings between the particles and give
mobility to them.
 Measurement of Workability
The tests used to measure workability are,
1. Slump test
2. Compacting Factor test
3. Flow table test
4. Vee Bee Consistometer test

5. 1. Slump cone test:

6. • Slump test is the most commonly used method of measuring consistency of concrete which can
be employed either in laboratory or at site of work. It is not a suitable method for very wet or very
dry concrete.
• It does not measure all factors contributing to workability, nor is it always representative of the
ability of placing of the concrete. However, it is used conveniently as a control test and gives an
indication of the uniformity of concrete from batch to batch.
• Repeated batches of the same mix, brought to the same slump, will have the same water content
and water cement ratio, provided the weights of aggregate, cement and admixtures are uniform
and aggregate grading is within acceptable limits.
• Additional information on workability and quality of concrete can be obtained by observing the
manner in which concrete slumps.
• The apparatus for conducting the slump test essentially consists of a metallic mould in the form
of a frustum of a cone having the internal dimensions as under:
• Bottom diameter : 20 cm
• Top diameter : 10 cm
• Height : 30 cm
• The internal surface of the mould is thoroughly cleaned. The mould is placed on a smooth,
horizontal, rigid and non-absorbent surface.
• The mould is then filled in three layers. Each layer is tamped for 25 times by a tamping rod. After
the top layer has been tamped, the concrete is leveled with a trowel by removing excess concrete.
• The mould is immediately lifted slowly and carefully in vertical direction. This allows the
concrete to subside. The subsidence is called slump of concrete.
• The difference in level between height of the mould and that of the highest point of the subsided
concrete is measured. This difference in height in mm is taken as slump of the concrete.

7. Degree of
workability
Slump in mm Use for which concrete is suitable
Low 25–75
Roads vibrated by hand-operated machines. At the
more workable end of this group, concrete may be
manually compacted in roads using aggregate of
rounded or irregular shape.
Mass concrete foundations without vibration or
lightly reinforced sections with vibration.
Medium 50–100
At the less workable end of this group, manually
compacted flat slabs using crushed aggregates.
Normal reinforced concrete manually compacted
and heavily reinforced sections with vibration
High 100–150
For sections with congested reinforcement.
Not normally suitable for vibration. For pumping
and placing

8. 2. Compacting Factor Test:

9. • The compacting factor test is designed primarily for use in the laboratory but it can also be
used in the field. It is more precise and sensitive than the slump test and is particularly
useful for concrete mixes of very low workability as are normally used when concrete is to
be compacted by vibration.
• This test works on the principle of determining the degree of compaction achieved by a
standard amount of work done by allowing the concrete to fall through a standard height.
The degree of compaction, called the compacting factor is measured by the density ratio i.e.,
the ratio of the density actually achieved in the test (Partially compacted concrete) to
density of same concrete fully compacted.
• The essential dimensions of the hoppers and mould and the distance between them are
shown in Table
Upper Hopper, A Dimension cm Lower Hopper, B Dimension cm
Top internal
diameter
25.4 Top internal diameter 22.9
Bottom internal
diameter
12.7 Bottom internal diameter 12.7
Internal height 27.9 Internal height 22.9
Cylinder, C
Internal diameter 15.2 Distance between bottom of upper hopper and
top of lower hopper
20.3
Internal height 30.5 Distance between bottom of lower hopper and
top of cylinder
20.3

10. • The sample of concrete to be tested is placed in the upper hopper up to the brim. The trap-
door is opened so that the concrete falls into the lower hopper. Then the trap-door of the
lower hopper is opened and the concrete is allowed to fall into the cylinder.
• In the case of a dry-mix, it is likely that the concrete may not fall on opening the trap-door.
In such a case, a slight poking by a rod may be required to set the concrete in motion.
• The excess concrete remaining above the top level of the cylinder is then cut off with the
help of plane blades supplied with the apparatus. The outside of the cylinder is wiped
clean.
• The concrete is filled up exactly upto the top level of the cylinder. It is weighed to the
nearest 10 grams. This weight is known as ―Weight of partially compacted concrete‖.
• The cylinder is emptied and then refilled with the concrete from the same sample in layers
approximately 5 cm deep. The layers are heavily rammed or preferably vibrated so as to
obtain full compaction.
• The top surface of the fully compacted concrete is then carefully struck off level with the
top of the cylinder and weighed to the nearest 10 gm. This weight is known as ―Weight of
fully compacted concrete‖.
• The Compacting Factor = Weight of partially compacted concrete/ Weight of fully
compacted concrete

11. 3. Flow table test:

12. • This is a laboratory test, which gives an indication of the quality of concrete with respect
to consistency, cohesiveness and the proneness to segregation.
• In this test, a standard mass of concrete is subjected to jolting. The spread or the flow of
the concrete is measured and this flow is related to workability.
• It can be seen that the apparatus consists of flow table, about 76 cm. in diameter over
which concentric circles are marked.
• A mould made from smooth metal casting in the form of a cone is used with the following
internal dimensions. The base is 25 cm. in diameter, upper surface 17 cm. in diameter, and
height of the cone is 12 cm.
• The table top is cleaned of all gritty material and is wetted. The mould is kept on the
centre of the table, firmly held and is filled in two layers. Each layer is rodded 25 times
with a tamping rod 1.6 cm in diameter and 61 cm long rounded at the lower tamping end.
After the top layer is rodded evenly, the excess of concrete which has overflowed the
mould is removed. The mould is lifted vertically upward and the concrete stands on its
own without support.
• The table is then raised and dropped 15 times in about 15 seconds. The diameter of the
spread concrete is measured in about 6 directions to the nearest 5 mm and the average
spread is noted.
• The flow of concrete is the percentage increase in the average diameter of the spread
concrete over the base diameter of the mould.
• Flow, per cent = Spread diameter in cm -25 x 100
25
• The value could range anything from 0 to 150 per cent.

13. 4. Vebe Test/ Vee Bee Consistometer Test:

14. • This is a good laboratory test to measure indirectly the workability of concrete. This test
consists of a vibrating table, a metal pot, a sheet metal cone, a standard iron rod.
• Slump test as described earlier is performed, placing the slump cone inside the sheet metal
cylindrical pot of the consistometer.
• The glass disc attached to the swivel arm is turned and placed on the top of the concrete in
the pot.
• The electrical vibrator is then switched on and simultaneously a stop watch started. The
vibration is continued till such a time as the conical shape of the concrete disappears and
the concrete assumes a cylindrical shape.
• This can be judged by observing the glass disc from the top for disappearance of
transparency.
• Immediately when the concrete fully assumes a cylindrical shape, the stop watch is
switched off.
• The time required for the shape of concrete to change from slump cone shape to
cylindrical shape in seconds is known as Vee Bee Degree.
• This method is very suitable for very dry concrete whose slump value cannot be measured
by Slump Test, but the vibration is too vigorous for concrete with a slump greater than
about 50 mm.

15. 2. Segregation :
• Segregation can be defined as the separation of the constituent materials of concrete. A
good concrete is one in which all the ingredients are properly distributed to make a
homogeneous mixture.
• If a sample of concrete shows a tendency for separation of coarse aggregate from the rest
of the ingredients, then, that sample is said to be showing the tendency for segregation.
• Such concrete is not only going to be weak; lack of homogeneity is also going to induce
all undesirable properties in the hardened concrete.
• There are considerable differences in the sizes and specific gravities of the constituent
ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall
separately.
• Segregation may be of three types —
• firstly, the coarse aggregate separating out or settling down from the rest of the matrix.
• secondly, the paste or matrix separating away from coarse aggregate
• thirdly, water separating out from the rest of the material being a material of lowest
specific gravity.
• A well made concrete, taking into consideration various parameters such as grading, size,
shape and surface texture of aggregate with optimum quantity of waters makes a cohesive
mix. Such concrete will not show any tendency for segregation.
• The conditions favorable for segregation are, the badly proportioned mix where sufficient
matrix is not there to bind and contain the aggregates.
• Insufficiently mixed concrete with excess water content shows a higher tendency for
segregation.

16. • Dropping of concrete from heights as in the case of placing concrete in column concreting
will result in segregation.
• When concrete is discharged from a badly designed mixer, or from a mixer with worn out
blades, concrete shows a tendency for segregation.
• Conveyance of concrete by conveyor belts, wheel barrow, long lift by skip and hoist are
the other situations promoting segregation of concrete.
• Vibration of concrete is one of the important methods of compaction. It should be
remembered that only comparatively dry mix should be vibrated. It too wet a mix is
excessively vibrated, it is likely that the concrete gets segregated.
• While finishing concrete floors or pavement, with a view to achieve a smooth surface,
masons are likely to work too much with the trowel, float or tamping rule immediately on
placing concrete. This immediate working on the concrete on placing, without any time
interval, is likely to press the coarse aggregate down, which results in the movement of
excess of paste to the surface.
• The tendency for segregation can be resolved by correctly proportioning the mix, by
proper handling, transporting, placing, compacting and finishing.
• At any stage, if segregation is observed, remixing for a short time would make the
concrete again homogeneous.
• A cohesive mix would reduce the tendency for segregation. For this reason, use of certain
workability agents and pozzolanic materials greatly help in reducing segregation.
• The use of air-entraining agent appreciably reduces segregation. Segregation is difficult to
measure quantitatively, but it can be easily observed at the time of concreting operation.

17. 3. Bleeding :
• Bleeding is sometimes referred as water gain. It is a particular form of segregation, in
which some of the water from the concrete comes out to the surface of the concrete, being
of the lowest specific gravity among all the ingredients of concrete.
• Bleeding is mainly observed in a highly wet mix, badly proportioned and insufficiently
mixed concrete.
• In thin members like roof slab or road slabs and when concrete is placed in sunny weather
show excessive bleeding. Due to bleeding, water comes up and accumulates at the surface.
• Sometimes, along with this water, certain quantity of cement also comes to the surface.
When the surface is worked up with the trowel and floats, the aggregate goes down and
the cement and water come up to the top surface. This formation of cement paste at the
surface is known as ―Laitance‖.
• In such a case, the top surface of slabs and pavements will not have good wearing quality.
This laitance formed on roads produces dust in summer and mud in rainy season.
• Due to the fact that the top surface has a higher content of water and is also without of
aggregate matter; it also develops higher shrinkage cracks.
• If laitance is formed on a particular lift, a plane of weakness would form and the bond
with the next lift would be poor. This could be avoided by removing the laitance fully
before the next lift is poured.
• Water while traversing from bottom to top, makes continuous path & that path will remain
continuous even after concrete drys. This continuous bleeding channels are often
responsible for causing permeability of the concrete structures.

18. • Bleeding is an inherent phenomenon in concrete. All the same, it can be reduced by proper
proportioning and uniform and complete mixing.
• Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for
the water to traverse. The use of air-entraining agent is very effective in reducing the
bleeding.
• It is also reported that the bleeding can be reduced by the use of finer cement or cement
with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.

19. • Steps of Manufacture of Concrete :
1. Batching
2. Mixing
3. Transporting
4. Placing
5. Compacting and finishing
6. Curing.
1. Batching:
The measurement of materials for making concrete is known as batching. There are two
methods of batching:
(i) Volume batching (ii) Weigh batching
a) Volume batching:
• Volume batching is not a good method for proportioning the material because of the
difficulty it offers to measure granular material in terms of volume. Volume of moist sand
in a loose condition weighs much less than the same volume of dry compacted sand. The
amount of solid granular material in a cubic metre is an indefinite quantity. Because of
this, for quality concrete material have to be measured by weight only. However, for
unimportant concrete or for any small job, concrete may be batched by volume.
• Cement is always measured by weight. It is never measured in volume. Generally, for each
batch mix, one bag of cement is used. The volume of one bag of cement is taken as thirty
five (35) litres. Gauge boxes are used for measuring the fine and coarse aggregates.

20. • The typical sketch of a guage box is shown in above fig. The volume of the box is made
equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. Correction to
the effect of bulking should be made to provide for bulking of fine aggregate, when the
fine aggregate is moist and volume batching is adopted.
• Gauge boxes are generally called farmas. They can be made of timber or steel plates.
Often in India volume batching is adopted even for large concreting operations. In a major
site it is recommended to have the following gauge boxes at site to provide for change in
Mix Design or bulking of sand. The volume of each gauge box is clearly marked with
paint on the external surface.

21. (b) Weigh batching :
• In this system, all the ingredients are measured by weight. Manual or automatic weigh
batchers are used for this purpose. This system facilitates accuracy, flexibility and
simplicity. This system is used for all important works where quality concrete is
essential. In weigh batching, the weight of surface water (free moisture) carried by the
wet aggregates must be taken into account.

22. b) Mixing:
• The ingredients should be thoroughly mixed in order to produce uniform and
homogeneous concrete.
• There are two methods of mixing –
• Hand mixing:
• This method is generally adopted for small and unimportant concrete works. Usually 10 %
more cement is added to compensate for manual errors. Hand mixing is done over a water-
tight surface, preferably on a metal tray.
• Spread out the measured quantity of coarse aggregate and fine aggregate in alternate
layers. Pour the cement on the top of it, and mix them dry by shovel, turning the mixture
over and over again until uniformity of colour is achieved.
• This uniform mixture is spread out in thickness of about 20 cm. Water is taken with a rose-
head and sprinkled over the mixture and simultaneously turned over.
• This operation is continued till such time a good uniform, homogeneous concrete is
obtained. It is of particular importance to see that the water is not poured but it is only
sprinkled.
• Water in small quantity should be added towards the end of the mixing to get the just
required consistency. At that stage, even a small quantity of water makes difference.

23. (b) Machine Mixing:
• Mixing concrete ingredients by machine (mixer) is more efficient. It produces concrete of
better quality at a faster rate. Machine mixing is, therefore, recommended even for small
works.
• Mixers used for concrete are classified as Batch mixers and Continuous mixers. Batch
mixers produce concrete batch by batch with time interval whereas, continuous mixers
produce concrete continuously without stoppage. Continuous mixers are used on large
projects like dams, roads, etc.

24. • For normal works, batch mixers are used. Batch mixers are usually of drum type; which
are classified as tilting (T), non-tilting (NT) or reversing (R) type. The standard sizes of
mixer are,
• Tilting (T) – 85 T, 100 T, 140 T, 200 T
• Non-tilting – 200 NT, 280 NT, 340 NT, 800 NT
• Reversing – 200 R, 280 R, 340 R, 400 R
• The number indicates the capacity of mixer in litres.
• Normally, a batch of concrete is made with ingredients corresponding to 50 kg bag of
cement. Thus, the capacity of mixer should be such that it holds all the materials for one
bag of cement.
• About half the quantity of coarse aggregate is placed in the skip and about half the
quantity of fine aggregate is poured on it. On that, one full bag of cement is poured and
then remaining portion of coarse aggregate and fine aggregate is poured on it.
• Before loading the drum, about 25 % of the total water quantity is introduced into it to wet
the drum and to prevent sticking of cement to the blades or bottom of the drum. Then the
materials from the skip are discharged into the drum. The remaining 75 % of water is then
immediately poured into the drum.
• Mixing Time: Concrete mixers are generally designed to run at a speed of 15 to 20
revolutions per minute. About 25 to 30 revolutions are required for proper mixing. The
mixing time generally varies from 1½ to 2½ minutes.

25. c) Transporting:
• Concrete can be transported by a variety of methods and equipment's. The precaution to be
taken while transporting concrete is that the homogeneity obtained at the time of mixing
should be maintained while being transported to the final place of deposition.
• The methods adopted for transportation of concrete are:
1. Hand Pans:
• This is common method but wasteful and expensive. Pans should be wetted before use.
The method can be adopted for concreting at the ground level, below the ground level or
above the ground level.
2. Wheel Barrows:
• These are normally used for concreting at same level. The capacity of a wheel barrow is
35 litres (80 kg). These are not suitable for longer distances. There are chances of
segregation if the wheel barrows are used for longer hauls and on rough surfaces.
3. Troughs:
• When concrete is to be placed much below the ground level, as in basement slabs,
foundations, etc., a wooden or steel trough may be used for transporting concrete into
place.
4. Dumpers:
• Dumpers can be used for hauls up to about 5 km. these are suitable only for dry mixes;
otherwise there are chances of segregation. The concrete has to be covered with tarpaulins
to prevent evaporation. Capacity of a dumper is about 3 cu.m.

26. 5. Transit Mixers (T.M.):
• These are commonly used for transporting ready mixed concrete (R.M.C.) over a long
distance. These mixers are truck mounted having capacity of 4 to 7 m³. The mixer rotates
at a speed of 4 to 16 revolutions per minute when in transit. This reduces the chances of
segregation. Sometimes a concrete pump is also mounted on the truck. This pump places
the concrete at desired position.
6. Pumps and Pipelines:
• Pumping of concrete through steel pipelines is latest method of transporting concrete. This
system is commonly used in construction of high-rise buildings, tunnels, bridges and
dams. The pump capacity ranges from 20m³/hour to 150m³/hour. The normal distance to
which concrete can be pumped is about 2000 m horizontally and 500 m vertically.
Generally, pipes of 125 mm diameter are used for normal pumping.
7. Crane and Buckets:
• This combination can be used for transporting concrete above ground level. This system is
useful for high rise construction. Cranes can move concrete horizontally or vertically
along the boom and allows the placement of concrete at the exact location. Cranes carry
skips having discharge door at the bottom or buckets. Buckets are tilted for discharging.
8. Ropeway and Buckets:
• This system is adopted where other system is not feasible. e.g. concrete works in a valley
or dam or pier in the river. The mixing of concrete is done on the bank or abutment and
the bucket is conveyed with the help of a pulley. It is filled up and then taken away to the
desired location on the ropeway.

27. 9. Skip and Hoist:
• This method is very commonly used for transporting concrete up for multistory building
construction. The mixer feeds the skip at ground level and the skip travels up over rails up
to the level where concreting is to be done.
10. Rails and Trolley System:
• The rails are provided such that the trolley can directly discharge the concrete in the
formwork. This system is used when the surface is not suitable for normal wheeled traffic.
Troughs Transit Mixer

28. Pumps & Pumping

29. d) Placing:
• It is not enough that a concrete mix correctly designed, batched, mixed and transported, it
is of greatest importance that the concrete must be placed in systematic manner to yield
optimum results.
• Preparations to be made before placing of concrete are,
1. The forms must be examined for correct alignment and adequate rigidity to withstand the
weight of reinforcement, concrete, workers and impact loads during construction.
2. The forms must be checked for water-tightness to avoid loss of mortar which may result in
honeycombing.
3. The forms should be cleaned and treated with demoulding chemical.
4. The reinforcement should be checked for correctness, tightness, cover and clean surface.
• Precautions to be taken while placing the concrete are,
1. Concrete should be placed as closely as possible to its final position.
2. Concrete should not be dropped from more than 1 m height to avoid segregation.
3. When fresh concrete is to be placed on a previously placed concrete, the laitance should
be removed before pouring fresh concrete.
4. While concreting in walls, footings and other thin sections of more height, the concrete
should be placed in layers not less than 150 mm in depth.
5. The number of construction joints should be kept to a minimum as they are major sources
of weakness.
6. Delay in placing should be avoided.

30. d) Placing:
• It is not enough that a concrete mix correctly designed, batched, mixed and transported, it
is of utmost importance that the concrete must be placed in systematic manner to yield
optimum results.
• Preparations to be made before placing of concrete are,
1. The forms must be examined for correct alignment and adequate rigidity to withstand the
weight of reinforcement, concrete, workers and impact loads during construction.
2. The forms must be checked for water-tightness to avoid loss of mortar which may result in
honeycombing.
3. The forms should be cleaned and treated with demoulding chemical.
4. The reinforcement should be checked for correctness, tightness, cover and clean surface.
• Precautions to be taken while placing the concrete are,
1. Concrete should be placed as closely as possible to its final position.
2. Concrete should not be dropped from more than 1 m height to avoid segregation.
3. When fresh concrete is to be placed on a previously placed concrete, the laitance should
be removed before pouring fresh concrete.
4. While concreting in walls, footings and other thin sections of more height, the concrete
should be placed in layers not less than 150 mm in depth.
5. The number of construction joints should be kept to a minimum as they are major sources
of weakness.
6. Delay in placing should be avoided.

31. e) Compaction:
• Compaction is the process of removing the entrapped air from the concrete. Air is likely to
get entrapped during mixing, transporting and placing. Stiff concrete mix has high
percentage of entrapped air than high workable mix. If this air is not removed fully, the
concrete loses strength considerably. 5 % voids reduce the strength of concrete by about 30
% and 10 % voids reduce the strength by 50 %. Thus, 100 % compaction is a must in order
to obtain full strength.
• Inadequately compacted concrete leads to honeycombing and increased permeability of
concrete which results in rusting of reinforcement and reduction of durability of concrete. It
also reduces the strength of concrete.
• The methods of compaction are,
A) Hand Compaction
1. Rodding
2. Ramming
3. Tamping
B) Compaction by Vibration
1. Internal vibrator ( Needle vibrator)
2. External vibrator ( Formwork vibrator )
3. Table vibrator
4. Platform vibrator
5. Surface vibrator ( Screed vibrator)
C) Compaction by Spinning
D) Compaction by Pressure and Jolting

32. A) Hand Compaction:
1. Rodding:
• Rodding means poking the concrete with about 2m long and 16mm diameter rod around the
reinforcement and corners.
• The rodding action is effective for a depth of concrete equal to five times the maximum size
of aggregate. The rod should penetrate to the full depth of the concrete layer and into
underlying layer.
• The compaction should continue until the cement mortar appears on the surface of the
concrete. This method cannot assure dense and well compacted concrete.
2. Ramming:
• Ramming can be done in plain cement concrete. It is done manually with a heavy flat faced
tool.
3. Tamping:
• Tamping is adopted in compacting roof or floor slabs or road pavements. Tamping consists
of beating the top surface by wooden cross beams of section 10 cm X 10 cm.
B) Compaction by Vibration:
• Vibration is the commonly used method of compaction of concrete. It reduces the internal
friction between the ingredients of concrete by giving them motion. Thus, concrete becomes
dense and compact. The vibrations are given by means of vibrators which are operated with
the help of diesel/petrol engine, electric motor or pneumatic pressure. The vibrations are
caused by the rotation of an eccentrically loaded shaft at high speed.
• The various types of vibrators used are,

33. 1. Needle vibrators/ Internal vibrators/ Immersion vibrators
• It consists of a flexible shaft, a needle and a power unit. The vibrations are caused by
eccentric weights attached to the shaft of the motor of the vibrating unit. The needle
diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm. the
frequency of vibration varies from 6000 to 12000 cycles per minute.
2. External vibrator/ Formwork vibrator
• These are used while concreting columns, thin walls and precast units. The form vibrators
are clamped to the formwork at the predefined points so that both the form and concrete are
vibrated simultaneously. They consume more power than internal vibrators. These vibrators
can compact concrete up to 450 mm from the face. The shuttering must be strong and rigid
to take the vibrations safely.
3. Vibrating Table
• It consists of a rigidly built steel platform mounted on flexible springs and is driven by an
electric motor. Vibrating tables are used for compacting concrete cubes and small precast
members.
4. Platform Vibrator
• This is a large size table vibrator used to compact large precast members such as poles,
railway sleepers, etc.
5. Surface or Screed Vibrators
• These vibrators are placed directly on the concrete mass. These are best suited for the
compaction of shallow concrete members (thickness less than 150 mm) like road
pavements, P.C.C., floors, etc.

34. c) Compaction by Spinning
• This method is adopted for the compaction of reinforced concrete pipes. The plastic
concrete when rotated at very high speed gets compacted by centrifugal force.
d) Compaction by Pressure and Jolting
• This method is used for compacting very dry concrete. The stiff concrete is vibrated,
pressed and also given jolts. This method is commonly adopted for compacting solid or
hollow concrete blocks.
 Revibration of Concrete
• It is delayed vibration of concrete that has already been placed and compacted. It may occur
while placing successive layers of concrete when vibrations in the upper layer of fresh
concrete are transmitted to the underlying layer which has partially hardened.
• Revibration is possible only when concrete is sufficiently plastic. Revibration results in
improved compressive strength, bond strength and reduction in honeycombing. Revibration
rearranges the aggregate particles and eliminates entrapped water from under aggregates
and steel. This develops full contact between steel and mortar or mortar and coarse
aggregate. Thus, it produces more stronger and watertight concrete. Plastic shrinkage cracks
get closed by revibration.
 Retempering of Concrete
• During long hauls of concrete from central mixing plant, there are chances of loss of workability,
stiffening and segregation of concrete due rough roads. Partially set and stiffened concrete should be
rejected. The process of remixing concrete if necessary with an addition of small quantity of water is
known as retempering of concrete. The quantity of water shall not exceed the designed water-cement
ratio. Sometimes, a small quantity of cement is also added while retempering.

35. f) Curing:
• Curing is the process of keeping the concrete moist enough, so that the hydration of cement
can continue until the desired properties are developed.
• The quick surface drying of concrete results in the movement of moisture from the interior
to the surface. This steep moisture gradient cause high internal stresses which are
responsible for internal micro cracks. Thus, to avoid such cracking, curing is quite essential.
• Concrete releases high heat of hydration. This heat affects the volume stability. These
adverse effects can be minimized by adequate curing.
• The various methods of curing are,
A) Water curing
1. Immersion
2. Ponding
3. Spraying
4. Wet covering
B) Membrane curing
C) Application of heat
1. Steam curing
2. Curing by infra-red radiation
3. Electrical curing

36. A) Water Curing
1. Immersion Curing: Precast members are immersed in curing tank for a certain period.
2. Ponding: This method is commonly adopted for curing of concrete slabs or pavements.
This method consists of storing water to a depth of 50 mm on the surface by constructing
small ponds.
3. Spraying: Vertical members like columns, walls, plastered surfaces, etc. are cured by
spraying water.
4. Wet covering: The vertical members are wrapped by wet gunny bags or hessian cloth for
keeping them wet. For horizontal surfaces saw dust, earth or sand are spread and kept wet
by sprinkling water.
B) Membrane Curing
• The process of applying a membrane forming compound on concrete surface is called
membrane curing. The membrane serves as a physical barrier to prevent loss of moisture
from the concrete. The method is used in places where there is acute shortage of water and
concrete is placed in inaccessible or difficult places.
• The various membrane forming compounds are,
i. Bituminous and asphaltic emulsions
ii. Rubber latex emulsions
iii. Emulsions of resins, varnishes, waxes
iv. Water-repellent chemicals such as silicone
v. Emulsions of paraffin

37. C) Application of Heat
1. Steam Curing: This system is used for precast concrete members. The members are heated
up by steam either at low pressure or high pressure.
2. Curing by infra-red radiations: This system is adopted in very cold climatic regions.
3. Electrical Curing: Concrete can be cured by passing alternating current through it between
two electrodes.
• Curing Period
• IS: 456 – 2000 recommends minimum 7days moist curing.
• IS: 7861 (Part –I) – 1975 recommends a minimum 10 days moist curing under hot weather
conditions.