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Conventional nominal mix proportioning often specified by rule of thumb such as 1:1:2; 1:2:4;
1:3:6, etc. have limited significance as these are with little consideration for the materials that
will be used. Since the quantity of fine aggregate is fixed irrespective of the cement and the
maximum size of aggregate, considerable variations in quality are obtained by this method.
Therefore, nominal mix proportions, arbitrarily fixed, have little use in concrete mix design.
Another reason not in favour of nominal mix concept is that the water-cement ratio, a most
important factor governing concrete quality, is not specified for nominal mixes, and has to be
kept as low as possible provided that the mix is fully compacted concrete. A more realistic
approach is to specify a minimum strength for the concrete and to choose the proportions of
cement, fine aggregate, coarse aggregate and water-cement ratio. There is trend away from the
specification of nominal mixes on the strength basis.
Rational proportioning of the ingredients of concrete is the essence of concrete mix design. The
proportions are controlled by factors governing the plastic state and the hardened state. If the
condition of the plastic concrete is not satisfactory, it cannot be properly compacted and the
performance of the hardened concrete will be considerably impaired, affecting the performance
of the structure. Satisfactory compaction can be obtained only if the concrete is sufficiently
workable for the method of compaction used. The ease of placement is governed by the
workability, while the durability is related primarily to the water-cement ratio, both these
properties being the pivotal points of proportioning.
The purpose of concrete mix design is to ensure the most optimum proportions of the
constituent materials to meet the requirements of the structure being built. Mix design should
ensure that the concrete:
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