Nominal Mix Concrete Vs Design Mix Concrete

Concrete

Concrete is made up of mixing cement, sand, aggregate, admixtures and water. The strength, workability and durability of concrete depend on mixing proportions of its ingredients. 

Normally there are two types of methods that are used for proportioning different ingredients 

• Nominal Mix
• Design Mix 

The selection of the type of concrete mix depends on the requirement of strength and nature of work. So in this article, we will discuss the differences between nominal mix and design mix of concrete.

So let’s start

What is Nominal Mix Concrete?

The nominal mix is the process in which all the ingredients are prescribed as per specifications and their proportions are specified in the ratio of cement to aggregates for a certain strength achievement. 

The proportions of ingredients in the nominal mix are the same for all the projects. To simplify this statement let us take an example of M 20 grade concrete. 

Its nominal mix proportion is 1:1.5:3 it means we have to weigh them to 1 kg of cement 1.5 kg of fine aggregate that is sand and 3 kg of coarse aggregates. 

But we normally take them on a volume basis and multiply them with 1.5 by constant to get the bulk volume of the material so to conclude this we can say nominal mix generally contains volumetric batching.

What is Design Mix Concrete?

Concrete shall be called design mix concrete when the proportion of its ingredients are decided after trial and error of various options in the laboratory. 

After carrying out tests on individual ingredients with the object of producing concrete of certain minimum strength and durability as economically as possible. The proportions of ingredients in the design mix differ from project to project.

Designation	Concrete grade	Cement Kg/m3	C.A. Kg/m3	F.A. Kg/m3	Fly-ash Kg/m3	Glass Fibre Kg/m3	Water Litre	S.P. Litre	VMA Litre
SSC1		365	781	964	156	0	182	13.00	0.46
SSC2		365	781	964	162	0.25	182	13.80	0.46
SSC3	M40	365	781	964	169	0.50	182	14.70	0.47
SSC4		365	781	964	175	0.75	182	15.50	0.47
SSC5		365	781	964	182	1.00	182	16.40	0.48
SSC6		365	781	964	188	1.20	182	17.20	0.48

For example, this is the mix design for M 40 grade concrete here you can see the quantity of cement, coarse aggregate, fine aggregate and water is constant but the quantity of fly ash and two admixtures (S.P. & VMA) has changed in each proportion. 

Please note it is not necessary that the quantity of cement, sand aggregate and water should be constant it can change in each means. 

I am showing this picture just for your reference so accordingly the strength of the concrete cube in 7 days, 14 days and 28 days changes. 

Other concrete properties like durability, workability, surface finish etc also change so as per strength and project requirements the correct design mix shall be selected also, you can see here all the ingredients are on a weight basis.

Difference Between Nominal Mix and Design Mix Concrete

The difference Between Nominal Mix and Design mix are as follows:

Mixing

The nominal mix is site-level mixing small concrete mixer can be used to mix the ingredients. 

Whereas the design mix is designed from ready mix concrete plant that is RMC plant.

Tests

For a nominal mix, no laboratory tests are conducted.  

whereas the design mix is based on the laboratory trial-and-error experiment method.

Attributes of Ingredients

The nominal mix does not consider and check the specific properties of individual ingredients that are the properties of cement sand and aggregates. 

In the nominal mix, we don’t actually find the specific gravity of cement, silt content of sand and weight proportions etc.

Whereas in the case of design mix we need to check every property of ingredients and after that, the mix design shall be started. 

Properties of ingredients such as in case of cement, cement grade, consistency, initial setting time, final setting time, specific gravity etc. 

In the case of sand and aggregates density, bulking of sand, specific gravity, grading of aggregate, fineness modulus, particle size, silt content, water absorption, unit weight etc. 

Based on the above properties of ingredients, we will establish the Mix design according to our needs and then check them with the trail mix. 

Once the trial mix has been tested and passed on 7th and 28days for compressive strength then it will be approved for batching and execution at the site level.

Time

The nominal mix is easy to make at the construction site. It does not take more time to decide the proportion because proportions are already given in IS456:2000.

Whereas the design mix is very difficult and its process takes more than 30 days, time to find out the most suitable mix for the project for design mix IS-20262 shall be followed.

Skilled persons

For a nominal mix, there is no need for skilled persons.

Whereas for design mix skilled person shall be required to prepare the mix design.

Basis

The nominal mix is based on experience and has no scientific approach.

Whereas the design mix is more scientific than the nominal mix.

Strength

The nominal mix may or may not give exactly design strength unless all other factors like compaction, water-cement ratio curing of concrete are strictly followed.

Whereas design mix not only makes concrete of design strength but also makes it economical too.

Accuracy

In the case of nominal mix water-cement ratio is considered by the assumption so if we do not take care of it sometimes it leads to bleeding segregation and may make concrete of poor strength and durability.

Whereas in the case of design mix, there is no such problem slump and strength can be related that is for the strength. We can use different slump by changing the water-cement ratio with or without admixtures. 

To suit concreting of different elements that are for footing, it may have less slump that is less water-cement ratio but for thin elements like chajja or thin walls, it may need a larger slump that is the higher water-cement ratio or more quantity of admixtures this is not possible in the nominal mix.

Use

The nominal mix is preferred for simpler and small concrete works for mild exposure, in other words, the nominal mixes are adopted for ordinary concrete work. The nominal mix may be used for concrete of M 20 grade or lower grade.

Whereas design mixed is used for more extensive and important concrete works. Design mix can be used for all the grades but it is generally used for M 25 and above grades. 

So, friends, I hope I have covered all the differences between nominal mix concrete and design mix concrete in this article. 

If you learn something please be sure to share it with someone who might benefit from it. If you want to add any information which has been missed in this article you can mention it in the comment section.

Finally, thanks for reading this article.

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