These mixes are fixed cement aggregate ratio which ensure adequate strength to the concrete. In the fig. M5, M7.5, M10, M15, M20 and M25 are nominal mixes. Due to variability in mix, they differs in workability and compressive strength. The ratio of mix is required when we estimation of concrete.
In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. In the fig. M30 and above are design mixes.