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.
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:
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 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.
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
|S.No||Nominal mix||Design mix|
|1.||For the nominal mix, specific properties of individual ingredients are not considered or checked.||For design mix, every property of ingredients should be known and checked.|
|2.||All the ingredients of concrete are mixed based on volume.||All the ingredients of concrete are mixed based on weight.|
|3.||It is generally used for simpler concrete works or unimportant areas.||They are used for fixed or specific areas depending upon the desire.|
|4.||They are usually used for concrete of M-20 grade or lower than M-20 grade||They are usually used for concrete of above M-25 grade like M-40, M-35, M-30, M-25, etc.|
|5.||They are site-level mixing. A small concrete mixture can be used to mix the ingredients.||They are designed from a ready mix concrete plant.|
|6.||They are not scientific, as they are based on past experiences and past empirical studies.||They are more scientific than nominal mix.|
|7.||For the nominal mix, no laboratory tests are conducted.||Laboratory tests are conducted as they are based on the laboratory trial and error experiment method.|
|8.||They are easy to make and is not time-consuming.||They are difficult to make and takes more than 30 days to find out the most suitable mix for the project.|
|9.||The ratio of water to cement is considered by assumptions based on experience.||Water is used in the proper amount according to the strength required for the project or the grade of concrete desired.|
|10.||A skilled person is not required to prepare a nominal mix.||A skilled person is necessary to prepare a design mix.|
|11.||They are generally used in small quantities or where the load is not very high.||They are used in heavy structures or where the load is very high.|
|12.||They have less strength and are not durable.||They assure high durability and strength.|
I hope this article on “Difference between Design mix and Nominal mix” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Mohan Khadka