WHAT IS CONCRETE MIX DESIGN? TYPES OF MIX DESIGN.


Concrete Mix Design

Concrete is a composite material which is a mixture of Cement, Sand, Aggregate, Water and Admixture. Concrete mix design is the procedure for finding the right quantities of these materials to achieve the desired characteristic strength. Accurate concrete mix design makes concrete construction economical.  Infrastructures such as Dams, Bridges, Airports, Highways requires huge amount of concrete, using the right quantity of constituents make the structure economical. In order to calculate or find the right amount of cement, sand and aggregate required in 1m3 of concrete; you need to know about different grades of concrete.

As per IS456:2000, Different grades of concrete are classified into  M5, M7.5, M10, M15 etc., whereas M stands for Mix and the number behind M stands for Characteristic Compressive strength(fck) of the concrete in N/mm2 @28 days when checked with 15cm×15cm×15cm cube in direct compression test.

The Concrete mix ratio for M20 grade of concrete is 1:1.5:3 that mean 1 part of cement, 1.5 part of sand (fine aggregate) and 3 parts of aggregate (crushed stone) in volume and then batched for mixing.

Different grades of concrete has different Concrete Mix proportions which means different concrete mix ratios. As Per IS456:2000,  the Concrete mix design has the following concrete mix proportions :-



Types of  Concrete mixes:
Concrete Mixes are primarily divided into the two different types :

Nominal Mix: 
Nominal Mix is generally adopted for small scale constructions. In this type of mix, the mix ratios and concrete constituent proportions are prefixed and specified. Eg: M20(1:1.5:3);  the quantity of cement, sand and aggregate is batched in volume as per the fixed ratio 1:1.5:3. From the above table till M25 grade, the concrete proportions are called as Nominal mix concrete.

Design Mix: 
Design mix concrete is adopted for high rise constructions. In this type of mix, the mix ratios are decided by an Engineer after analysing the properties of individual ingredients of concrete. Like, cement is tested for Fineness modulus and Specific gravity of cement in the lab while deciding the Design mix ratio. There is No Pre-fixed ratio, and ingredients are are batched in weight. From the above table, concrete grades more than M25 falls in Design mix.

In Simple, Design Mix refers to the ratios which are decided by the designer.

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