CHAPTER 5
PRODUCTION OF CONCRETE
Production of concrete involves two distinct
activities. One is related to ‘material’ and the other to
‘processes’. The material part is generally taken care by
every body, but the involved processes in the production of
concrete are often neglected. Therefore no wonder that it is
the ‘process’ which is responsible for good or bad quality of
concrete. If we take care of processes, the quality of
concrete will be improved automatically without incurring
any extra expenditure as the major expenditure has already
been made in procurement of material. In order to ensure
the quality, it is very important to have a knowledge of each
and every process.
Process for Concrete Production
The various process involved in concrete production
are as given below :
1) Proportioning
2) Mixing
3) Transportation
4) Placement
5) Compaction
6) Curing
1) Proportioning
It is the relative quantity of each ingredient to make
the desired concrete. It is decided based upon the
calculations of mix-design. The proportioning should be such
that the resultant mass should be compact with minimum
voids and the required strength should be achieved.
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2) Mixing
The purpose of proper mixing is to ensure that
mass should become homogeneous, uniform in colour and
uniform in consistency.
There are two types of mixing that are adopted in
the field i.e. Hand Mixing and Machine Mixing
(a) Hand Mixing
It is done manually and thus inferior to machine
mixing as it can’t give homogeneous and uniform concrete.
(b) Machine Mixing
It is an efficient and economical way of mixing in
which homogeneous and uniform mixing can be ensured. It
can further be divided into two categories as given below :
(i) Batch Mixing – The mixing of concrete is done in
batches.
(ii) Continuous Mixing – The mixing is done
continuously till the plant is working. All the
ingredients are fed through screw feeder in a
continuous manner. It is used in large works such
as dams and bridges.
Type of Batch Mixers
(1) Pan type mixers
(2) Drum type mixers
1. Pan Type Mixers:
It consists of a circular pan rotating about a vertical
axis with one or two stars scraper blades rotating about a
vertical axis which is not matching with the axis of the pan.
Scraper blades prevent sticking of mortar to the sides of
pan. The pan mixers are particularly efficient for stiff and
cohesive mixes.
These are mainly used in central batching plant on
large projects or at pre-cast manufacturing units. A smaller
version is generally available in laboratories too.
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2. Drum Type Mixers:
There are three types of drum mixers as given
below :
(a) Tilting – The rotating drum is able to tilt. This
type of mixer is normally used in the field.
(b) Non-Tilting - The rotating drum is not able to
tilt, therefore separate entry and exit for
concrete is provided.
(c) Reversing or Forced Action - It is similar to
non-tilting but entry and exit of concrete is
provided on the same side.
This mixers are shown in Fig-1.
Reversing Tilting Non-Tilting
Fig-1 Drum Mixers
As per IS:1791-1963, the mixers are designated by
capacity as given below :
Tilting : 100T, 140T, 200T
Non-Tilting : 200NT, 280NT, 375NT, 500NT and 1000NT
Reversing : 200R, 280R, 375R, 500R and 1000R
Here Figure represents capacity in litre and words
represent like: T for Tilting, NT for Non-tilting & R for
Reversing.
Sequence of charging ingredients
Based upon the field experience, it has been found
that the following sequence of charging should be adopted
in the field for achieving homogeneous and uniform mixing.
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(i) Half the quantity of CA
(ii) Half the quantity of FA
(iii) Full quantity of cement
(iv) Remaining quantity of CA
(v) Remaining quantity of FA
(vi) 25% of total water is introduced before loaded
skip is discharged into the drum. This will
prevent sticking of cement on blades or bottom
of drum. The balance 75% of water is added
after loaded skip is discharged into the drum.
Mixing time
It is the time required to mix the ingredients so as to
obtain a homogeneous and uniform consistency mass of
concrete. The time is reckoned from the moment all the
ingredients including water has been added. Generally 25
to 30 revolutions of drum mixer are sifficient for proper
mixing. The drum mixer rotate @ 15-20 rpm, therefore it will
take approximately 2 minutes for proper mixing.
Mixing time also depends upon the grade of
concrete. Richer mix requires more time. Similarly mixing
time will depend upon type of aggregate also and more time
will be taken for crushed aggregates as compared to
smooth aggregates. In case of doubt it is always better to
mix for a longer time. More mixing may sometimes result
into higher strength also probably due to:
(i) Reduced w/c due to evaporation and absorption by
aggregates
(ii) Increased workability due to abrasion action and
rounding of coarse aggregates.
3) Transportation
Transportation of concrete is an important activity in
the production of concrete. The time taken in transit should
be a design parameter as it depends on the initial setting
time as well as the requirement of workability at the
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destination. The method of transportation adopted at site
should be decided in advance so that suitable admixtures
can be decided. The various prevalent methods of
transportation are as given below :
(a) Mortar pan
It is a labour intensive method and generally used
for small works. There are no chances of segregation of
concrete. In hot weather, there is a substantial loss of water
due to more exposure of concrete to environment.
(b) Wheel barrow or hand cart
It is normally used on ground level i.e. road
construction and other similar structures. Segregation can
occur if transportation is done on rough roads however this
problem can be minimized if pneumatic tyres are used.
(c) Bucket and ropeway
It is suitable for works in valley, over high piers and
long dam sites. Excessive free fall of concrete should be
avoided to avoid segregation.
(d) Truck mixer and dumper
It is an improved and better method for long lead
concreting. The concrete is covered with tarpaulin if it is
transported in open trucks. If long distance is involved,
agitators should be used.
(e) Belt conveyor
It has limited application due to chances of
segregation on steep slopes, roller points and change in
direction of belt. It also involves over-exposure of concrete
to environment.
(f) Chute
It is generally used for concreting in deep locations.
Care should be taken that slope should not be flatter than
1V : 2.5H, otherwise concrete will not slide-down. But
workability should not be changed to suit the delivery by
chute. Technically it is not a very good method but it is
extensively used in the field.
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(g) Skip and hoist
It is a widely used method for high rise structures.
Concrete is fed into the skip which travels vertically on rails
like a lift. After discharging, it is better to turn over the
concrete before use to avoid segregation.
(h) Pump and Pipe-line method
It is the most sofisticated method particularly
suitable for limited space or when a large quantity of
concrete is to be poured without cold joints. Pumping of
concrete can be done @ 8 to 70 cum per hour up to a
horizontal distance of 300 m and vertical distance of 90 m.
Pipe dia is generally 8-20 cm and it is made of steel, plastic
or aluminium. The workability for pumped concrete should
have a minimum of 40-100 mm of slump or 0.90-0.95 CF.
At delivery point, the workability may be reduced by 25%
due to compaction and this factor should be kept in mind
while designing the mix.
At the end of day’s work, the pipe should be
cleaned by passing a special ball called ‘go devil’ forcing
through the pipe by air pressure.
The various parts of the system are as shown in
Fig-2.
Hopper
Inlet valve
Outlet valve Piston
Fig - 2 Concrete Pump
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4) Compaction
Compaction is a process of expelling the entrapped
air. If we don’t expel this air, it will result into honey-
combing and reduced strength. It has been found from the
experimental studies that 1% air in the concrete
approximately reduces the strength by 6%.
There are two methods of compaction adopted in
the field as given below:
(a) Hand compaction
(b) Mechanical compaction
(a) Hand Compaction
Hand compaction is used for ordinary and
unimportant structures. Workability should be decided in
such a way that the chances of honeycombing should be
minimum. The various methods of hand compaction are as
given below:
(i) Rodding – It is a method of poking with 2 m long, 16 mm
dia rod at sharp corners and edges. The thickness of layers
for rodding should be 15-20 cm.
(ii) Ramming – It is generally used for compaction on
ground in plain concrete. It is not used either in RCC or on
upper floors.
(iii) Tamping – It is a method in which the top surface is
beaten by wooden cross beam of cross section 10 cm x 10
cm. Both compaction and levelling are achieved
simultaneously. It is mainly used for roof slabs and road
pavements.
(b) Mechanical Compaction
Vibration is imparted to the concrete by mechanical
means. It causes temporary liquefaction so that air bubbles
come on to the top and expelled ultimately. Mechanical
vibration can be of various types as given under :
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(i) Internal vibration
It is most commonly used technique of concrete
vibration. Vibration is achieved due to eccentric weights
attached to the shaft. The needle diameter varies from 20
mm to 75 mm and its length varies from 25 cm to 90 cm.
The frequency range adopted is normally 3500 to 5000 rpm.
The correct and incorrected methods of vibration using
internal vibration needles are shown in Fig-3(a), 3(b), and
3(c).
(ii) External vibration
This is adopted where internal vibration can’t be
used due to either thin sections or heavy reinforcement.
No vibration Form work
Radius of influence
Incorrect Incorrect Correct
(very small needle) (small needle) (exact size needle)
Fig-3 (a)
Radius of
vibration New layer
Correct Incorrect
Old layer
method method
Fig-3 (b)
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Incorrect Correct
Void Void
App
300 mm
head
Poorly
compacted
concrete
Fig-3 (c)
External vibration is less effective and it consumes more
power as compared to the internal vibration. The formwork
also has to be made extra strong when external vibration is
used.
(iii) Table vibration
It is mainly used for laboratories where concrete
is put on the table.
(iv) Platform vibration
It is similar to table vibrators but these are
generally used on a very large scale.
(v) Surface vibration
These are also called screed board vibrators. The
action is similar to that of tamping. The vibrator is placed on
screed board and vibration is given on the surface. It is
mainly used for roof slabs, road pavements etc. but it is not
effective beyond 15 cm depth.
5) Curing
Curing is a procedure of promoting the hydration of
cement for development of concrete strength and controlling
the temperature. As a result of curing, we can achieve
higher strength and reduced permeability which is very vital
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for the long term strength or durability. The effect of curing
has been depicted in Fig-4.
Full time curing
7 days curing
→
3 days curing
Comp. strength
No curing
Time (days) →
Fig-4 Effect of curing
The curing is required for full development of
strength. Initially the entire concrete has sufficient quantity of
water for hydration. But over the passage of time, the water
is lost due to evaporation or it is consumed due to reaction
of hydration. The relative humidity, thus, falls below 80%
level and the hydration process eventually stops.
Methods of curing
Various methods of curing are adopted in the field
as given below :
(a) Replenishing the lost water
(i) By immersion in water
(ii) By ponding
(iii) By sprinkling
(iv) Using saturated coverings, e.g. jute bags etc.
(b) Preventing loss of moisture
(i) Using curing compounds
(ii) Using impermeable membrane coverings
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Out of above methods, immersion may be the ideal
method of curing but it is not practicable always. It is
restricted to laboratories or pre-cast units where small units
like cubes or PSC sleepers etc. can be immersed in water.
Ponding is suitable only for flat surfaces but not for vertical
or overhead surfaces. Sprinkling is the most commonly used
method in the field but it requires a large quantity of water.
Saturated covering is a better method which requires less
quantity of water. Curing compounds are effective for
concrete with high initial w/c ratio. Impermeable membrane
is also having the same effect as of curing compounds. Out
of these methods, the most widely used method in the field
is saturated coverings using jute bags.
Important observations regarding curing
(1) Curing should be started earliest as possible.
(2) For the portion of concrete which is covered
with formwork, the curing should be started as
soon as the formwork is removed.
(3) On exposed surface, it should be started when
concrete has sufficiently hardened such that it
doesn’t get disturbed by curing.
(4) Ensure uninterrupted curing. If it is discontinued
for any reason, the reaction of hydration will be
stopped permanently. The partial hydration
makes the capillary pores discontinuous and
water can’t enter the concrete even if the curing
is started again.
(5) High strength concrete should be cured at an
early age.
(6) There is a widespread belief that humid climate
is sufficient and curing is not required in rainy
season.
(7) The person generally entrusted for curing is the
most unskilled person. He doesn’t appreciate
the importance of curing. In fact he believes
that curing is a process of wastage of water
time and money.
(8) It can’t be made a measurable item in the
contract.
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Therefore the best and practical method to ensure
proper curing is the education of the person who is
responsible for curing. Once he understands the importance
of curing, he would certainly ensure it.
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