British Standard 5328: Specifying concrete, including ready-mixed
concrete, gives a range of mixes. One range of concrete mixes in the Standard, ordinary prescribed mixes, is suited to general building work such as foundations and floors. These prescribed mixes should be used in place of the traditional nominal volume mixes such as 1:3:6 cement, fine and coarse aggregate by volume, that have been used in the past. The prescribed mixes, specified by dry weight of aggregate, used with 100 kg of cement, provide a more accurate method of measuring the proportion of cement to aggregate and as they are measured against the dry weight of aggregate, allow for close control of the water content and therefore the strength of the concrete.
The prescribed mixes are designated by letters and numbers as C7.5P, C10P, C15P, C20P, C25P and C30P. The letter C stands for ‘compressive’, the letter P for ‘prescribed’ and the number indicates the 28-day characteristic cube crushing strength in newtons per square millimetre (N/mm2) which the concrete is expected to attain. The prescribed mix specifies the proportions of the mix to give an indication of the strength of the concrete sufficient for most building purposes, other than designed reinforced concrete work.