Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 14: Plan Cement Concrete

Mixing Of Concrete

The operation of manufacture of concrete is called 'mixing'. The mixing can be done by two methods:

1. Hand mixing
2. Mixing by concrete mixers.

1. Hand mixing

   In this method, mixing is done manually on a steel plate, or on a clean hard surface. This method is resorted to when the quantity of concrete needed for the work is small. Sand and cement in appropriate proportions are mixed first in a dry state. The coarse aggregate is then added and the whole mixture is mixed thoroughly with the help of shovels. The predetermined amount of water is then sprinkled over the mix. The mass is then' turned till the colour of concrete obtained is homogeneous and workable mix is obtained.

2. Machine mixing

   Concrete should normally be mixed in a mechanical mixer. The main part of mixer is a drum in which the ingredients are mixed thoroughly by mechanically rotating the drum. The drum is made of steel plates, with a number of blades put in inclined position in the drum. As the drum rotates, the materials encounter resistance to rotation from the blades and this disturbing effect helps in a good mixing of the ingredients. The mixers are either operated electrically or else are driven by oil engines attached to them. Coarse aggregate should be fed first, then sand and lastly cement. In the revolving state, when the three get thoroughly mixed, water' should be added either with the help of a can or automatically through the pipe attached to the mixer. Mixing should be continued until there is a uniform distribution of the materials and the mass is uniform in colour and consistency, but in no case shall the mixing be done for less than two minutes.

Concrete mixers may be of two types:

Batch type mixers are employed for work of relatively small magnitude. Batch type mixers can either be of titling drum type or closed drum type. In the tilting drum type, drum rotates about a trunnion axis and is so arranged that it is quite easy to rotate and tilt it when it is empty as well as when full. In the close drum type, the drum remains rotating in one direction and is emptied by means of the hopper which tilts to receive the discharge. Continuous mixers are used in mass concreting work where a large and continuous flow of concrete is required. In these mixers, processes of feeding, mixing and emptying go on continuously without break.