Mix design - Concrete Mix Design, Quality Control and Specification - page 207

Usually in plain concrete, the cost of the final blend of materials is an
important consideration because typically in developed markets, the profit-
ability or margins in producing and selling concrete are extremely low. This
being the case, and aggregates typically comprising 15% to 30% of the
total cost of producing a volume of concrete, one major consideration as to
the blend will be related to the actual cost of the materials.
However, the cost of the aggregate is not the deciding factor on the
total cost of the all of the materials that go into making that volume of
concrete. For example, in general terms, you could afford to pay 5% more
for a ton of sand if the more expensive sand was to allow you to use 1%
less cement.
8.5 VOLUMETRICS
A lot has been written about volumetric mix design, particle packing, and
minimum voids. When you have a collection of granular materials such
as cements, sands, and aggregates, there is only one combination of all
of these materials that will give you a maximum packing density. There
are many aggregate blending programs and methodologies that will either
allow you to calculate the combinations of materials to give you the maxi-
mum or indeed a targeted density; or, in reverse, a percentage of voids in
the mix. The .45 power curve that is commonly used in the United States
is one good example
It is the authors experience, however, that the maximum density, or
minimum voids content of all of the materials, or even just the sands and
aggregates will probably not give you a concrete that is workable or easy
to finish, and it probably won't pump very well, if at all! And therefore it
would not be commercially viable. The maximum density concrete made by
having the lowest voids content or highest aggregate packing density will in
many cases give a high compressive strength—in the laboratory.
Is it really an advantage to have the maximum packing density in our
aggregates? We think not. The “best” concrete has aggregate packing den-
sities that are lower than the maximum. It is the packing density of all of
the materials in a concrete mix design that is important. A given packing
density, or voids content can be achieved by many different combinations
of the materials.
If you look at the ISO packing pyramid used by de Larrard and others,
you will notice that the isobars allow quite a wide range in the combina-
tions of the materials to be blended to yield the same result.
There are many characteristics that influence a material and how it packs,
or that particular material's void content. These characteristics have differ-
ing impacts depending on the size and shape of the material in question.
Factors that influence aggregate and sand packing efficiency are