Concrere Mix

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)