MIXING
•Done in order to ensure each particle of aggregate in fresh concrete will be coated with the cement paste
•Also to give a homogenous even coloured product.
•First stage of mixing is called “BATCHING”.
•Batching is process where it involves weighing out or measuring out all of the ingredients for a batch of concrete.
•The materials that are mixed at any one time constitute a batch.
•Quality of concrete depends on the accuracy of the batching operation.
•However, thorough mixing is essential for complete blending of the materials that are required for the production of homogeneous, uniform concrete.
•Inadequate mixing result in lower strength.
•There are 2 method for concrete mixing that is:
a) hand mixing
b) machine mixing
HAND MIXING
•Uniformity more difficulty to be achieved
•Aggregate is spread in a uniform layer on a hard, clean & non porous basic.
•Then cement is spread over the aggregate and dry material are mixed by turning over from one end of the heap to another & cutting with a shovel till the mix appear uniform.
•Water is gradually added to the trough formed by the uniform dry mix.
•It involves usage of more labours compared concrete mixed using machine.
•The quality of mix is difficult to be controlled.
MACHINE MIXING
•Ensure the concrete mixer is clean and have no left over concrete before start mixing new one.
•There are many types of machine for concrete mixing such as Tilting Type Mixer, Non-Tilting Type Mixer and Pan Mixer.
•Machine mixing is efficient & economical compared to hand mixing.
•MIXING TIME
•Optimum length of mixing time depends on
a) type of mixer
b) size / condition of mixer
c) Speed of rotation
d) Quality of blending materials
•Uniformity of mix increase together in length of mixing time.
•Long mixing times may cause some breakdown of the aggregate.
•Lean, dry or harsh mixes require longer mixing times
•Concrete made with angular aggregates need more mixing than those made with rounded gravels.
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