Mix design - Concrete Mix Design, Quality Control and Specification - page 192

concrete to prescribe mixes is definitely past. There may be some materials
or combination of materials that the (relatively rare) technically advanced
specifier knows to be unsatisfactory or particularly beneficial, but, this
excepted, it is undesirable that the prospective purchaser should inhibit
the process of optimisation or the selection of material suppliers by the
concrete producer.
A major advantage of this proposal is that a producer would develop a
standard range of mixes for all purchasers with the same requirements.
This would be of substantial assistance in achieving good quality control
and enable the producer to economically provide a more complete range
of tests. It would also enable the producer to reach cooperative arrange-
ments with suppliers of materials for them to carry out control testing on
their materials and give the concrete producer advance warning of change.
Specifiers and purchasers should also be aware that the field of such materi-
als is in a period of rapid change and that national standards and the like
may well not be up to date on all possibilities.
The first step in designing a range of mixes is to select the materials
to be used, but these will depend on the available production facilities.
There may be only provision for a single coarse aggregate, in which case
it is likely to be a graded 20 mm material, or there may also be pro-
vision for a second aggregate of 14 mm or less maximum size. Since
coarse aggregate will constitute the largest part of the concrete, price
will be a consideration, but minimising the requirement for the mortar
fraction will be even more important, since that material will be dis-
tinctly more expensive. The coarse aggregate will need to be sufficiently
strong for the highest strength concrete required, and to have satisfactory
bond characteristics and not exhibit moisture movement. It will also be
important that the production facilities for the material are such as to
ensure a consistent quality, including grading, particle shape, and bulk
density. Rounded gravel will probably provide the highest bulk density
(by volume), but its bond characteristic will be important and should
be checked by the indirect tensile (splitting) test, especially where high
strength is required.
If there are two coarse aggregates they will be proportioned to give
the minimum percentage voids. It will be some benefit that their relative
proportions can be adjusted if there are changes in the grading of either
material. The smaller aggregate alone may be preferable for very high
strength concrete (over 200 MPa). If there is a choice of fine aggregate,
this will probably be made on the basis of a flow test (see Chapter 3),
which is influenced by the grading and particle shape and surface texture.
If a satisfactory natural sand is not economically available, a crushed
material can be considered and, where an available natural sand has been
discarded as too fine (or perhaps too expensive) a combination of the
two may be worth consideration. Crushed sands tend to have what is