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Friday, 2 February 2018

PLAIN CONCRETE

Major ingredients of concrete are:
1. Binding material (like cement, lime, polymer)
2. Fine aggregate (sand)
3. Coarse aggregates (crushed stone, jelly)
4. Water.
A small quantity of admixtures like air entraining agents, water proofing agents, workability
agents etc. may also be added to impart special properties to the plain concrete mixture.
Depending upon the proportion of ingredient, strength of concrete varies. It is possible to
determine the proportion of the ingredients for a particular strength by mix design procedure. In the
absence of mix design the ingredients are proportioned as 1:1:2, 1:1(1/2):3
, 1:2:4, 1:3:6 and 1:4:8,
Which is the ratio of weights of cement to sand to coarse aggregate.
In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand
and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various



works are shown in Table 3.1.


Preparing and Placing of Concrete
The following steps are involved in the concreting:
1. Batching
2. Mixing
3. Transporting and placing and
4. Compacting.
1. Batching: The measurement of materials for making concrete is known as batching. The
following two methods of batching is practiced:
(a) Volume batching
(b) Weight batching.
(a) Volume Batching: In this method cement, sand and concrete are batched by volume. A gauge
box is made with wooden plates, its volume being equal to that of one bag of cement. One bag of cement
has volume of 35 litres. The required amount of sand and coarse aggregate is added by measuring on to
the gauge box. The quantity of water required for making concrete is found after deciding water cement
ratio. For example, if water cement ratio is 0.5, for one bag of cement (50 kg), water required is 0.5 × 50
= 25 kg, which is equal to 25 litres. Suitable measure is used to select required quantity of water.
Volume batching is not ideal method of batching. Wet sand has higher volume for the same
weight of dry sand. It is called bulking of sand. Hence it upsets the calculated volume required.
(b) Weight Batching: This is the recommended method of batching. A weighing platform is
used in the field to pick up correct proportion of sand and coarse aggregates. Large weigh batching
plants have automatic weighing equipments.
2. Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand and
coarse aggregate, first in dry condition and then in wet condition after adding water.
The following methods are practiced:
(a) Hand Mixing
(b) Machine Mixing.
(a) Hand Mixing: Required amount of coarse aggregate for a batch is weighed and is spread on
an impervious platform. Then the sand required for the batch is spread over coarse aggregate. They are
mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is
spread over the dry mix and mixed by shovels. After uniform texture is observed water is added gradually
and mixing is continued. Full amount of water is added and mixing is completed when uniform colour
and consistancy is observed. The process of mixing is completed in 6–8 minutes of adding water. This
method of mixing is not very good but for small works it is commonly adopted.
(b) Machine Mixing: In large and important works machine mixing is preferred. Figure 3.2
shows a typical concrete mixer. Required quantities if sand and coarse aggregates are placed in the
drum of the mixer. 4 to 5 rotations are made for dry mixing and then required quantity of cement is
added and dry mixing is made with another 4 to 5 rotations. Water is gradually added and drum is
rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this stage uniform and
homogeneous mix is obtained.

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