12 Difference between Design mix and Nominal mix of Concrete

Nominal mix and design mix are the methods of preparation of concrete mix.

Concrete is the most significant material used to achieve desired properties of the structure, which is composed of cement, sand, coarse and fine aggregates mixed with water.

Nominal Mix

In the nominal mix concrete, cement, sand, and aggregates are used in fixed proportion or specification. For required strength to achieve, these proportions are specified in the ratio of cement to aggregate.

In a nominal mix, a portion of all the ingredients of concrete-like cement, aggregates are specified or fixed.

The ratio of water to cement in the nominal mix is considered by assumption or experience. So, if the w/c ratio is not taken care then it may lead to segregation or bleeding of concrete which ultimately affects the strength and durability of the concrete.

Nominal mix concrete is generally used for concrete of M20 grade or lower grade like M15, M10, M5, etc. They are generally used or adopted for lower grade concrete or if we want to build structures of relatively less importance or have a very low load.

The ratio of various grades of cement is shown below:

M20  1:1.5:3

M15  1:2:4

M10  1:3:6

M7.5 1:4:8

M5    1:5:10

In the above grades of concrete Mix, the ‘M’ refers to Mix whereas the attached number with M like:20,15,10 represents the compressive strength of the concrete after 28 days of curing in N/mm^2.

For M20, the ratio of ingredients is:1(cement):1.5(fine aggregate or sand):3(coarse aggregate).

Design Mix

Design mix is the method of preparation of concrete mix in which concrete of desired strength is obtained by determining the proportion of ingredients properly with their relative ratio.

Before obtaining a design mix of concrete, testing of all the ingredients like cement, fine and coarse aggregate is mandatory.

For example, We want to achieve a concrete grade M30, then we will make a minimum of 4 trial mix of different ratios of all the ingredients in the lab-based on our calculation or theoretical knowledge.

After 28 days, all the mix are observed and their strength, a grade is determined. The closest result is considered and the same ratio of ingredients is used to make concrete mix throughout the whole project.

In design mix, slump and strength can be related i.e. for the strength we can use different slump by changing the water-cement ratio with or without admixtures to suit concreting of different elements.

For example: for thin walls like chajja, a larger slump is required i.e. higher water-cement ratio. For structures like footing, it may have less slump i.e. lower water-cement ratio.

There are various grades of design mix concrete like M25, M30, M35, M40, etc. They are used to prepare high-grade concrete.

M40  1:1.74:2.75

M35  1:1.53:2.88

M30  1:1.5:2.8

M25  1:1:2

In the above grades of concrete Mix, the ‘M’ refers to Mix whereas the attached number with M like:40,35310 represents the compressive strength of the concrete after 28 days of curing in N/mm^2.

Difference between Design mix and Nominal mix

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