Normally there are two types of methods which are used for proportioning different ingredients
- Nominal Mix
- Design Mix
Selection of 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?
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 the 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 weight 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 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|
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
Difference Between Nominal Mix and Design mix are as follows:
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.
For a nominal mix, no laboratory tests are conducted.
whereas the design mix is based on 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.
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 trial 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.
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.
For a nominal mix, there is no need of skilled persons.
Whereas for design mix skilled person shall be required to prepare the mix design.
The nominal mix is based on past experience and has no scientific approach.
Whereas the design mix is more scientific than the nominal mix.
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.
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.
So as 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.
The nominal mix is preferred for simpler relatively unimportant and small concrete works, in other words, the nominal mixes adopted for ordinary concrete work 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 missed in this article you can mention it in the comment section.
Finally, thanks for reading this article.