Mixing of Concrete

Purpose of Mixing:

The purpose of mixing is to coat the surface of all aggregate particles with cement paste and to blend all the materials of concrete into a uniform mass. In this process cement, water, fine and coarse aggregate and possible admixtures of uniform consistency are mixed intimately. Mixing can be continued for a considerable tune without adverse effect.

Concrete Mixer:
Several types of mixer is used in different construction purposes. Such as –

Nominal Size of Mixer:

This is described by the volume of concrete after compaction ( BS 1305 : 1974), which may be as low as one- half of the volume of the unmixed ingredients in a loose state. Mixer are made in a variety of sizes from 0.04 cum. If the quantity of mixed represents less than one-third of the nominal capacity of the mixer, the resulting mix may not be uniform, and the operation would, of course, be uneconomical. Overload not exceeding 10% is generally harmless.

Two-Stage Mixing:

The pre-mixing of cement and water allows better subsequent hydration and, when used for concrete, leads to a higher strength at a given water/cement ratio than conventional mixing. For instance, at w/c ratios of 0.45 to 0.5, a gain in strength of 10% has been observed. But, a large amount of heat is generated at very low w/c ratios. Moreover, two stage mixing undoubtedly represents a higher cost and is likely to be justifiable only in special cases.

Uniformity of Mixing:

Sufficient interchange of materials between different parts of the chamber is expected in any mixture to have a uniform concrete mix. The efficiency of mixer can be measured by the variability of the mix discharged into a number of receptacles without interrupting the flow of concrete.

Assessment of Performance of Mixers:

ASTM C 94-94