g PRUThe volume of a bag of cement (50 kg) is 0.035 m^3. The volume of one gauge box is made
equal to 0.035m^3. The gauge boxes are made comparatively deeper with narrow top surface to
facilitate easy estimation of top level. Correction to the effect of bulking of fine aggregate
should be made if volume batching is adopted since the density of water is in kg/L, water is
measured either in kg or litres.
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For all important works weigh batching is used. Different types of weigh batchers are used. For
smaller works manual batching is done. All the operations of weighing and batching of the
ingredients are done manually. The weighing may also be done by ordinary platform weighing
machines. For large size works weigh bucket equipments are used. The weigh buckets are fed
from hoppers and these discharge the ingredients by gravity, straight into the mixer.
Automatic batching plants ranging from small to large capacity and manually or electrically
operated are available. However, they operate on same principle. By pressing a button the
weighing of all the different materials is put into motion and the flow of each is cut-off when
the correct weight is reached. Some of the recent batchers record graphically the weight of each
material delivered to each batch.
In weigh batching water is not added by graduated buckets as the water may spill over
during its addition. A horizontal or vertical tank is fitted to the mixer. The filling is so designed
to have a control to admit any desired quantity of water.
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The object of mixing is to make the concrete mass homogeneous and uniform in colour and
consistency. All the aggregate particles should have a coat of cement paste and all the ingredients
of the concrete should blend into a uniform mass. The mixing is done either by hand or by
machine called mixer.
r2w "
It is used for small jobs. Hand mixing is done over an impervious floor. Measured quantities
of coarse aggregate and fine aggregate are spread over the floor in alternate layers. Then
cement is poured over it and the ingredients are mixed dry with shovel until uniformity in
colour is achieved. This mix is spread out in thickness of 200 mm and water is sprinkled. The
mix is kept on turning over till a uniform colour is achieved. As the hand mixing cannot be
thorough, it is desirable to add some more cement (10%) to cater for the possible inferior
concrete produced by this method.
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For quality works mixing is carried out by mixer. Mixers can be broadly classified as batch mixers
and continuous mixers. The batch mixers produce concrete batch by batch with time interval,
whereas continuous mixers produce concrete continuously till plant is working. Batch mixers
are used for small and medium size works. Continuous mixers are used for large size works,
e.g., dams. A typical vertical batching mixing plant for large size work is shown in Fig. 10.2.
Batch mixer may be of pan type or drum type (Fig. 10.3). The drum type may be further
classified as tilting (T), non-tilting (NT), or reversing (R) type, based on the technique of