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(i) complies with the specifications of structural strength laid down, which is usually stated
in terms of the compressive strength of standard test specimens,
(ii) complies with the durability requirements to resist the environment in which the structure
will serve its functional life,
(iii) be capable of being mixed, transported, compacted as efficiently as possible without
undue labour,
(iv) and lastly, but not least, be as economical as possible.
Thus concrete mix design may be defined as the art of obtaining a concrete of the required
properties, at the lowest cost, by a suitable choice and proportions of available materials.
Needless to say, a properly designed concrete mix for the specified strength requirements
should have the minimum cement content to make the mix economical. It should, however, be
stressed that the precise relationship falls between the properties of concrete and the specific
characteristics of the mix such as water-cement ratio, aggregate-cement ratio and grading,
apart from such elusive qualities as aggregate-particle shape and texture. Hence, concrete mix
design cannot be mechanically done and is likely to remain an art, rather than a science, for
some time to come.
The data used in selecting mix proportion should be expected merely to serve as a guide;
they should be backed up by personal experience and a knowledge of the basic principles of
concrete mix design. Nevertheless, their use is only the first state in designing a mix, and
should be followed, invariably, by the making of trial mixes. Based upon the observations
made on trial mixes the mix proportions have to be adjusted and the refinements carried to the
stage where the optimum proportions have been attained.
To sum up, concrete mix design is still very much a problem of trial-and-error, and any
calculations based on design data are really only a means of providing, at best, a starting point
so that the first tests can be conducted. Of course, the hardened concrete should ultimately
have the required strength, durability and surface finish.
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Concrete mix proportions can be expressed in terms of volume or weight. The present trend
is towards specifying mixes in terms of minimum strength requirements (usually with a
specified minimum cement content), rather than by volume proportions. For example, M 20