Composition of Concrete

Concrete is a composite material prepared by mixing binder, aggregate, water and admixtures in specified portions to gain a particular strength. There are two different methods through which composition percentages are decided, Nominal mix design and design mix design. These methods are framed after multiple laboratory experiments and theoretical studies. Basis for calculation in both the methods remains mostly same as strength of concrete. These parameters are fixed in the beginning and proportions are calculated considering through suitable methods. Concretes are divided into multiple types based on their Compressive strengths and denoted as M25, M30 etc. M 25 represents concrete type with 25 MPA of compressive strength.

In the nominal mix concrete, all the ingredients and their proportions are prescribed in the standard specifications. The mix proportions like 1:1.5:3, 1:2:4, 1:3:6 etc. (cement: fine aggregate: coarse aggregate) are adopted in nominal mix of concrete without any scientific base, only on the basis on past empirical studies. Thus, it is adopted for ordinary concrete, or you can say, the nominal mix is preferred for simpler concrete works. This method considers volumetric batching theory. It has a limited usability up to concrete strength of 25 MPA i.e., M25.

In design mix concrete, proportions of the ingredients are properly determined with their relative ratio to achieve the concrete of desired strength. Not only the desired strength but also according to the properties of fresh concrete like workability or performance of concrete with the certain specifications are taken in detail consideration. Mix design is the most intellectual and rational approach for the selection of specific materials with their proportion and for the unique characteristics. Design mix contains weight batching. It not only makes concrete of desired strength but also it is cost efficient. In the design mix method, you must check every property of the blended materials and after that, you can start design construction. Properties of materials which needs to be checked are grade of the cement, density of the aggregate, consistency of the cement, Fineness modulus, grading of the aggregate etc. A design mix concrete is preferred over a nominal mix concrete especially where the concrete is above M20 grade and where the requirement is of the different workability and risk of deviation. It is used in the large projects to get asserted of the assurance of strength, durability, and workability.

Reference: https://gharpedia.com/blog/nominal-mix-and-design-mix-of-concrete/