Concrete for long have been specified by nominal mixes (mixes of fixed proportions which ensure adequate strength) and were arbitrarily applied with little regard to the required strength of concrete with water added to ensure workability. But for mixes of desired strength the three variable factors to be considered are:

  1. Water cement ratio and consistency of fresh concrete
  2. Cement paste or cement aggregate ratio
  3. Gradation of aggregate

These above three inter-related factors should not be manipulated. Generally some of these factors are specified and the others are adjusted to give maximum workability and economy.

1. Water Cement Ratio & Consistency of Fresh Concrete

Water cement ratio gives the diluting of paste and the cement content varies directly with the amount of paste of given dilution. The consistency is established by requirements of placing and not subjected to manipulation in proportioning. It is given in terms of slump and it is recommended that the mixes of stiffest consistency that can be placed effectively, be adopted.

2. Cement Paste

The effort should be made to use minimum amount of paste in concrete. The amount of paste shall ensure lubrication of the mass of concrete when fresh and bind aggregates and fill voids in them when hardened. Any excess of paste shall result in:

  • Enhanced cost
  • Greater drying shrinkage
  • Greater chances of percolation
  • Attack by weather action

3. Gradation of Aggregates

In order that the paste used is minimum, the gradation of aggregates used, be such that voids between them are relatively small. Although ideal gradation are not possible but effort should be made to combine fine and coarse aggregates in such proportions which shall require minimum paste.

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