CONCRETE WORKS

It was previously the general custom in the United
Kingdom to specify concrete by a system of proportions
or ratios, eg 1:2:4 (being the proportions of cement:fine
aggregate:coarse aggregate) either by mass or by volume,
or as cement/aggregate ratio, water/cement ratio and
fine aggregate/coarse aggregate ratio, usually by mass.
Such systems have certain merits in terms of simplicity
of expression. However, they are not so convenient when
discussing the effect of mix parameters on the
characteristics of the concrete, nor do they adequately
describe the quantity of cement required to cast a given
volume of concrete.
The most fundamental way to specify mix parameters
is in terms of the absolute volumes of the different
materials required in a concrete mix. A more practical
method, based on similar principles, which has been
adopted in this publication is to refer to the mass of
materials in a unit volume of fully compacted concrete.
This method of referring to concrete mix proportions has
been in use for a long period in Europe and in the United
States of America, and is becoming the general practice in
the United Kingdom.
In order to use this approach, knowledge is required of
the expected density of the fresh concrete. This depends
primarily on the relative density* of the aggregate and the
water content of the mix. The effect of changes in the
cement content produces at the most about a 2% change
in the expected density and thus, for the purpose of this
publication, is ignored. The small quantity of air normally
entrapped in compacted concrete is also ignored. Data
are given from which an estimate of the wet density of the
fresh concrete can be made.
The method of mix design given in this publication
results in the mix being specified in terms of the mass in
kilograms of the different materials required to produce
one cubic meter of finished concrete. 

(Design of normal concrete mixes Second edition)