Concrete Production - Weight (Weigh) Batching

This is a more accurate method of measuring materials for concrete than volume batching since it reduces considerably the risk of variation between different batches. The weight of sand is affected very little by its dampness which in turn leads to greater accuracy in proportioning materials. When loading a weighing hopper the materials should be loaded in a specific order:

1. Coarse aggregates - tends to push other materials out and leaves the hopper clean.
2. Cement - this is sandwiched between the other materials ,since some of the fine cement particles could be blown away if cement is put in last.
3. Sand or fine Aggregates - put in last to stabilise the fine lightweight particles of cement powder.

Typical Densities: cement - 1440 kg/m3 sand - 1600 kg/m3 coarse aggregate - 1440 kg/m3

Water/Cement Ratio: water in concrete has two functions

1. Start the chemical reaction which causes the mixture to set into a solid mass.
2. Give the mix workability so that it can be placed, tamped or
vibrated into the required position.

Very little water is required to set concrete (approximately 0.2 w/c ratio) the surplus evaporates leaving minute voids therefore the more water added to the mix to increase its workability the weaker mis the resultant concrete. Generally w/c ratios of 0.4 to 0.5 are madequate for most purposes.