L#8 batching,mixing

Batching, Mixing Transportation andBatching, Mixing Transportation and
Placing of ConcretePlacing of Concrete

BatchingBatching
Batching is the process of measurement of cement, coarse
aggregates, fine aggregates and water for each operation of
concrete making.
METHODS OF BATCHING
(i) By Volume (Volume Batching
(ii) By Weight (Weigh Batching)
(Volume Batching)
(a)Batching of cement.
Cement is always measured by weight.
 Mostly it is used in terms of bags. One bag of cement weighs 50 kg and has
a volume of 1.25 cft (0.035 m3).
Cement should not be batched by volume because its weight per unit volume
varies according to the way the container is filled.

BatchingBatching
(Volume Batching)
(b)Batching of aggregates
A gauge box is used for batching of fine and coarse aggregates by volume is
shown in the fig. The box should not be too shallow, It should be completely
filled with the aggregates. The top of the materials should be struck off level
with a straight edge. The ratio of materials of concrete decides the capacity of
the box.
Generally capacity is equal to the volume of one bag of cement.

BatchingBatching
(Volume Batching)
(b)Batching of aggregates
The moisture present in aggregates and bulking of sand, must be accounted for
while volume batching is adopted
Convenient sizes of gauge boxes to suit different proportions are given in the
table below .
Capacity in m3
Inside Measurement in cm
Length Breadth Depth
0.025 25 25 40
0.030 25 25 48
0.035 27 27 48
0.040 29 29 48
0.045 30 30 50
0.050 31 31 52

BatchingBatching
(Weigh Batching)
In weigh batching the ingredients of concrete are measured by weight.
Generally weigh hatching is in practice for constructions where high quality
concrete is required.
Weigh batching is much more accurate than volume batching.
Different types of WEIGH BATCHERS are available, The particular type to be
used depends upon the nature of the job. Large weigh batching plants have
automatic weighing equipment.
Batches of concrete are based either on one bag of cement or its multiples unless
bulk cement, is used and weighed separately. In this type of batching no
correction is needed to allow for the bulking of sand, but an allowance should be
made for the weight of water contained by the wet aggregates. For comparatively
smaller works weigh batching can be done by :
(i) Simple spring balances.
(ii) Platform weighing machines.
(iii) Automatic weighing machines

BatchingBatching
BATCHING OF WATER
The water is usually measured by volume since this can be done quite
accurately. This is done by using a water tank on the mixer. The tanks are
designed to discharge a predetermined volume of water into the mixer.
Proper control of mixing water is more important than even accurate
measurement of cement and aggregates.
 The whole purpose of designing a concrete mix by the water cement ratio
law will be lost if the necessary precautions are not taken to maintain the
accurate measurement of water throughout the duration of a job.
The prevailing method of measuring the water by means of buckets and tin
cans direct into the mixer is inaccurate. It results in variable strength of
concrete.
Some of the mixers are equipped with a calibrated water tank attached
permanently to the mixer for greater accuracy. For the mixers not provided
with water tank a suitable arrangement may be constructed and fitted on them

MIXING OFCONCRETEMIXING OFCONCRETE
 The basic requirement of mixing is that it is thorough in order to produce
concrete of uniform consistency from beginning to end. Mixing involves a
working or strong rubbing of the cement paste (Cement and water) on to
the surface of the aggregate particles as well as a general blending of all
the ingredients.
 To accomplish this the mixing procedure adopted must be appropriate so
that the mixing is proper and with in the reasonable time.
METHODS OF MIXING
There are two basic methods of mixing concrete i.e.
(i) HAND MIXING
(ii) MACHINE MIXING

(i) HAND MIXING
Hand mixing is adopted for small works and where the quantity of concrete used
is small. The following procedure is followed
(a) Construct a platform of bricks, lean concrete or iron sheets of a size as per
requirement of the volume of concrete to be mixed.
(b) Spread out the-measured quantity of sand uniformly on the mixing surface.
Dump the cement on the sand and distribute it uniformly, Mix the sand and
cement with the shovels, turning the mixture over and over again until it is of
uniform colour throughout.

(i) HAND MIXING
(c) Spread out the sand-cement mixture once again uniformly on the mixing
surface. On top of this spread the measured quantity of coarse aggregate.
Don't dump at one place, otherwise the bigger particles will tend to roll out.
Mix again at least three times by shoveling from the centre to the side, then
back to the center and again to the side.
(d) Make a hollow in the middle of the mixed pile and pour slowly into it half to
three-quarters of the total quantity of water required. While the material is
turned in towards the centre with shovels, add the remainder of the water
slowly, turning the mixture over and again until the colour and consistency
are uniform throughout the pile. This will indicate that all the ingredients are
thoroughly combined. The water should be poured slowly into the mix. This
is best done by means of a gardener's water-can fitted with a rose-head.
Throwing water from a bucket all at a time will result in its running away
and taking cement with it as shown below.

(ii) MACHINE MIXING
In cases where a large quantity of concrete is to be produced, hand mixing
becomes costly even. "if the labour is cheap. Hence machine mixing becomes
essential and the concrete can thus be produced at a faster rate at a lesser cost
and of better quality.
MIXERS AND THEIR TYPES
The concrete mixers are either batch type or continuous type.
Batch mixers mix and discharge each load of materials separately whereas
continuous mixers produce a steady stream of concrete so long as it is in
operation.
Continuous type mixers are not very much in use on construction sites.
Batch mixer of drum type (conical/cylindrical) are preferred which are either.
TILTING TYPE
NON TILTING TYPE (ROTARY TYPE)
PAN TYPE

MACHINE MIXING
TILTING TYPE
In a tilting mixer the drum is conical and revolves about an inclined axis.
The drum can be tiled into different positions for charging, mixing and
discharging. Blades are provided inside the drum so that the materials are
continuously moved about as the drum rotates.
The materials are charged into the drum by a loading skip . In small capacity
mixers the materials may be charged into the drum.
The mixed concrete is discharged on a platform by tilting the drum.
Fig. below shows a tilting mixer with cable operated skip.

MACHINE MIXING
TILTING TYPE

MACHINE MIXING
Non Tilting Type
In this type the mixing drum is cylindrical and revolves about a horizontal axis. It
has an opening on each side.
The concrete materials are charged into the drum though one opening and
discharged through the other opening. They are provided with loading skips which
raised by wire ropes.
When the skip is raised the materials fall into the rotating drum. A number of
blades are fixed inside the drum. As the drum rotates the 'blades pick up the
concrete and when they reach their highest point concrete drops to the bottom.
The mixed concrete is discharged through a chute which may behind operated.

MACHINE MIXING
Non Tilting Type

MACHINE MIXING
Pan Type
Small drum mixers are not well suited to laboratory use, because of:
(i) Difficulty in obtaining proper mixing action
(ii) Too large a percentage of cement mortar in small batches sticks to drum
and blades. Because of this either Hand mixing is preferred or a small' mixer with a
flat bottom pan rotating clockwise on rollers with mixing blades mounted on a
vertical counter clockwise is being extensively used in the Laboratories and is
known as PAN TYPE MIXER.
The smaller unit has no discharge opening. The entire pan can be lifted to
discharge the entire batch. Comparatively larger units have a gate opening in the
bottom of the pan to discharge the batch.

MACHINE MIXING
Pan Type

Transportation of ConcreteTransportation of Concrete
 The process of hydration starts some time after water is added to cement a concrete
mix. The cement-water paste begins to solidify and the concrete becomes stiffer.
The concrete should therefore be transported as quickly as possible to the form work
and placed, ready to be compacted.
 The operation should be carefully planned. The specifications state that the process
of mixing, transporting placing and compacting should not take more than 30
minutes in normal circumstances.
 Depending upon the types of work and equipment available various methods are
employed to transport concrete from the mixing platform or mixer, to the forms.
 What so ever method or equipment is used it should fulfill the following
requirements:
 Uniform concrete of desired consistency is delivered at the forms:
 No segregation or excessive drying or stiffening takes place while transporting.

Equipment and methods of Transporting Concrete
(I) Pans
The concrete is transported in iron pans when its quantity is very small and access
to work is restricted.
This method is deadly slow and costly.
For raising concrete over some height so as to place it on the top floors of a
building ladders are erected and men stand at various levels. The same pan is
transferred from one person to the other quickly.
This method of transporting concrete is only suitable for small and medium size
buildings as it works to be the, cheapest method with such type of construction.

(ii) Wheel Barrows
For moderate distances and medium quantities, steel wheel barrows with tyres
may be used.
The capacity of a wheel barrow may be about 70 cm3
.

(iii) Truck Mixers.
Truck mixers are used when the place of deposit of concrete is at a very long
distance from the mixer such that the concrete cannot be transported and placed
in the forms within 30 minutes.
This happens in the case of ready mixed concrete.
This does not allow it to become stiff. Segregation is also prevented.
In summer when the temperature is high ice cold water is used to keep the
temperature Low to prevent its setting.

(iii) Truck Mixers.

(iv) Belt Conveyors.
Belt conveyors are used when the concrete is to be transported continuously and to a
higher level. The belt conveyor may be a single unit or in series.
These are installed in an inclined position.
The concrete should be of stiff consistency having a slump of not more than 50 mm.
A common width of belt conveyor is 60 cm and speed is about 80 m/min.
Due to high wind and temperature the moisture from the concrete evaporates. the
water content increases during that season.
During rain to maintain continuous working of the conveyors, they are therefore
covered with light sheet metal covers.
During discharge of concrete from conveyors it should not be allowed to fall through a
great height.

(iv) Belt Conveyors.

(v) Chutes:-
When concrete is to be placed below ground level the mixer may be placed
on an upper level and concrete discharged to the lower level through a chute
of corrugated iron or timber.
It leads to wheel barrows, carts or other transport facility or directly to the
final position where it has to be deposited.

(v) Chutes:-

(vi) Hoists:-
For tall structures hoists may be used either to lift the wheel barrow to the
required height or to discharge the concrete into the hopper from which it is carried
further. In confined areas the hoist must be erected within the building
and built up in sections.
The topmost will rise slightly above the working level.
The guide rails are held up against each floor level by studs. A vertical hoist can
frequently be arranged in multistoried buildings within the space allowed for
stairs or lift wells. Otherwise temporary openings, have to be left in the floor
panels to allow for the hoist. Cranes may be used with hoists mainly to lift skips
of concrete from the site of the mixer to the building where the concrete is to be

(vi) Hoists

(vii) Pumps:-
Pumps are used when largo quantity of concrete is to be transported continuously to
congested sites where mixing plant cannot be installed.
The concrete should be of medium consistency and should not have a tendency to
segregate.
The type of job for which pumping is used must be such that a steady flow of concrete
can be handled properly.
Concrete can be pumped for a distance of 300 m along a straight level pipe line
or up to about 40 m directly upward. The dia of pipe varies from 10 cm to 20 cm
depending upon max size of Coarse aggregate used.
Pumping should be started with mortar mix having the same proportions of cement
and sand as the concrete. Pumps and pipe lines should be cleaned after pumping.

(vii) Pumps:-

PLACING OF CONCRETEPLACING OF CONCRETE
 The concrete should be placed and compacted before setting commences.
 It should not be subsequently disturbed. The method of placing should be such
as to prevent segregation.
 It should not be dropped from a height of more than 1 meter.
 The first few batches of concrete may become leaner as some mortar sticks to
the mixer blades and dries as the moisture is absorbed by the forms. It is a good
practice to use richer and wetter mix till the work is well started.
 In slabs, sometimes, a layer of mortar alone is used to surround (embeded) the
bars. It ensures smooth ceiling surface

FORMWORK FOR CONCRETE
Formwork is a complete system of temporary structure built to contain fresh
concrete so as to form it to the required shape and dimensions and to support it
until it hardens sufficiently to become self supporting.
Formwork includes the surface in contact with concrete and all necessary
supporting structure.
Formwork is also known as SHU'ITERING.

FORMWORK FOR CONCRETE

Requirements of forms work
form work must conform to the following requirements
Be true to design grade and alignment
Be braced against displacement
Be strong enough and firm to resist all vertical loads, (pressure of fresh concrete,
weight of workers and equipment etc.)
Have tight joints to prevent leakage of mortar during placement of the concrete.
Be easy to handle, align, assemble and reassemble.
FORMWORK MATERIAL
 Timber
 Plywood
 Hard board
 Metal forms

PREPARATION OF FORMWORK BEFORE PLACING CONCRETE
Preparation of forms before concrete placing includes.
(a)Checking of forms
(b) Cleaning of forms
(c)Wetting and oiling of forms.
(a) Checking of forms
Before concreting, it is ensured that the form work will provide for the required shape and
dimension of the building unit.
The dimension of the forms as well as the proper position of vertical and horizontal form
work faces are checked.
All the supports and particularly all joints are checked to ensure that the form work is
stable and leak proof.
The area, below all the supports resting on soil should be increased to distribute the load
over larger area and thus avoid settlement.

PREPARATION OF FORMWORK BEFORE PLACING CONCRETE
(b) Cleaning of forms
All the surfaces on which concrete is poured must be cleaned as otherwise foreign
matter such as wood chips etc. will be embedded in the concrete and thus reduce
the strength and disfigure concrete surface.
(c) Wetting and oiling of forms
The faces of forms which will come in contact with concrete must be wetted
before placement of concrete.
The faces of forms may also be oiled with a special form oil, preventing the
penetration of concrete water in the wood.
In case steel forms are used, the oil shall consist of refined oil suitably
compounded with approved ingredients.

Precautions to be taken while placing concrete
(i) Concrete should be placed in horizontal layers not thicker than 30 to 45 cm for mass
concrete and 15 to 30 cm for R.C.C.
(ii) It should be placed as near its final position as practicable.
(iii) Concrete should not be placed in heavy rains unless suitable shelter is provided.
(iv) To avoid segregation concrete should not be dropped from a height of more than 1m.
(v) While placing concrete in RC members the alignment of form work and
reinforcement should not be disturbed.
(vi) Walking on freshly laid concrete should be avoided.
(vii) Concrete should be laid continuously to avoid irregular and unsightly lines.
(viii) Oiling should be done on the internal surface of forms so that concrete may not
stick to it.
(ix) No nails, bolt etc., should project beyond the surface of forms which is in contact
with concrete.

L#8 batching,mixing

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