Mixing Of Concrete:
After proportioning all concrete ingredients their mixing is done. The mixing process should ensure homogeneous mass uniform color. Segregation should not take place during the mixing operation.
Mixing is done by two following methods:
1. Hand Mixing
2. Machine Mixing
1. Hand mixing:
This method is adopted where the quantity of concrete is very small or where mixing machine is not available.
Cement and sand are first of all mixed dry on a clean hard impermeable platform. Dry mixing is continued until the mixture attains uniform color. Now, this mix is spread on the measured stock of coarse aggregate in required amount and these are mixed again to have uniform color. Shovels are used for this mixing purpose.
Make hollow in the middle of the mixed pile and add 75% of the required amount of water. Mixing is done and the remaining quantity of water is added to acquire the uniform workability. In this method of mixing, about 10% more cement is used to make good the cement lost due to possible water flowing out of the mix and also to make good strength characteristic to inferior result of hand mixing.
2. Machine Mixing:
On large works where concreting work is quite large, machine mixing of concrete proves economical. Concrete produced by machine mixing is more homogeneous and can be prepared with comparatively less w/c ratio.
The concrete mixers may either be batch type or continuous type. Batch mixers mix and discharge each load of materials separately, whereas continuous mixers produce steady stream of concrete so long as it is in operation. Latter type mixers are not in common use. Batch type mixers are mostly adopted. These may be rotary o non-tiling type or tilting type.
a) Rotary Or Non-Tilting Type Mixer:
This mixer has a cylindrical drum fitted with a number of inclined blades in it. It revolves about horizontal axis and has two openings. Charge is admitted in the drum through one opening and mixed charge is discharged through the second one. This mixer is available in following capacities. (Concrete produced in a batch in liters)
140 NT, 200 NT, 280 NT, 400 NT and 800NT. (NT indicates Non-tilting).
b) Tilting Type Mixer:
This mixer has conical-shaped drum which revolves about inclined axis. The drum is tiltable in different positions of charging, mixing, and discharging. Blades are fitted inside the drum to affect through mixing of materials. The materials are charged into the drum by loading skip. These are available in the following capacities:
100 T, 140 T, and 200 T.
In capacity figures 100, 140 200 etc. indicate capacity in liters of one batch. The most common size of mixer is 200 T. It is a convenient size and permits a full bag batch of concrete of 1:2:4 ratio. Speed of revolution/minute of mixers is 15 to 20.