concrete mix design - basic consideration

Concrete Mix Design – Basic Considerations

The proportioning of concrete mixes consists of determination of quantities of different concrete-making materials necessary to produce concrete having the desired workability and 28-days compressive strength of concrete for a particular grade of concrete and durability requirements. Emphasis is laid on making the most economical use of available materials so as to produce concrete of required attributes at the minimum cost.

Concrete has to be satisfactory both in fresh and hardened states. The proportioning of concrete mixes is accomplished by the use of certain established relationships from experimental data which provides reasonably accurate guidance for selecting the best combination of ingredients so as to achieve the desired properties of the fresh and hardened concrete. Out of all the physical characteristics of concrete compressive strength is often taken as an index. Therefore, the mix design is generally carried out for a particular compressive strength of concrete coupled with adequate workability so that the fresh concrete can be properly placed and compacted. In addition the mix proportions are also checked against the requirement of adequate durability for the type of exposure condition anticipated in service. The following basic assumptions are made in design of concrete mixes of medium strength:

  1. For given aggregate characteristics, the workability of concrete is dependent on its water content.
  2. The compressive strength of concrete is related to its water-cement ratio.

For high strength concrete mixes, considerable interaction occurs between these two criteria and validity of such assumptions may become limited. Moreover, there are various other factors which affect the properties of concrete e.g. the quality and quantity of cement, water, aggregates and admixtures (if used); procedures of batching, mixing, placing, compaction and curing etc. Therefore, the specific relationships that are used in proportioning concrete mixes, should be considered only as a basis for trial mixes

Concrete mix design on the basis of recommended guidelines is really a process of making an initial guess at the optimum combination of ingredients and final mix proportions are arrived at, only on the basis of further trial mixes.

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