Types of Concrete Mixes
In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified.However,due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength.
Nominal Mix Concrete
Traditional way of mix proportion specified in terms of fixed ratios of Cement : Sand : coarse aggregate (In general by volume)
- Useful for small works
- Useful for routine concrete construction
- Limited up to M20 grade
- Requires high cement content
The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications.These mixes are termed standard mixes.
IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm 2 . The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.
The proportions generally adopted are as given below
(Cement: FA : CA)
In these mixes the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down, specify the performance of the concrete. This is most rational approach to the selection of mix proportions with specific materials in mind possessing unique characteristics.
Several methods of mix design evolved over the years in different countries
Ex: ACI practice, British practice
Indian Standard recommendations. etc.
Recommendations For Concrete Mix Design Are Given In
I.S.10262-1982 & I.S.10262-2009
Designed mix should meet the design requirements in the fresh and hardened states