After various ingredients of concrete are proportioned, the next operation is their mixing, the mixing process should ensure homogeneous mass uniform in colour, and segregation should not take place during mixing operation. There are two methods of mixing the concrete:
Hand mixing: Hand mixing is adopted where quantity of concrete is very small. It can also be adopted where machine for mixing is not available or where noise is not desirable. Cement and sand are first of all mixed dry on a clean, hard, impermeable platform. Dry mixing is continued until the mix attains uniform colour. Now this mix is spread on the measured stock of coarse aggregate in required quantity and these are mixed dry again to have uniform colour. Shovels are used for this mixing purpose. Make hollow in the middle of the mixed pile and add about 75% of the required quantity 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 characteristics due to inferior results of hand mixing.
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 or non-tilting type or tilting type:
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...
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