Concrete Mix

 


                Concrete is a mix of cement sand and aggregate. The proportion of the cement sand and aggregate is according to the grade of concrete required. There are two types of concrete mix: -

  •  Nominal Mix  
  • Design Mix

 

a)       Nominal Mix: -

It is the mix or the proportion of the constituents of concrete which is specified by the code itself. For example, if we need to achieve M20 grade of concrete we need to mix the constituents in the ratio of 1:1.5:3 parts by volume. This ratio is given by the code. It means if we take 1m3 of cement then we need 1.5m3 of sand and 3m3 of aggregates.

We have different grades such as M10, M15, M20, M25. This grade means the characteristics strength of the concrete. For example, M20 grade concrete means this concrete has the characteristics strength of 20N/mm2. The following are grade of concrete and their constituent proportions according to IS code.

 

Grade

Cement

Sand

Aggregates

M10

1

3

6

M15

1

2

4

M20

1

1.5

3

M25

1

1

2

 

From the above table, we can analyze

1)      The sand and aggregates are always double in proportion.

2)      As the grade of concrete increases the quantity of sand decreases.

Note: -

One of the facts that engineers should know is that the Nominal mix can be uneconomical so it is better to do the calculation and prepare a design mix to use on-site.

b)      Design Mix: -

In the case of the nominal mix, the ratio of the material is given to us but the properties of the material are not discussed.  An example is discussed below: -

 

                Let us suppose from source A we brought cement sand and aggregate and we mix and we get a certain grade of concrete. Then we brought cement, sand, and aggregates from source B then we won’t get the same grade of concrete until and unless the properties of the material are exactly the same. If we are using cement from a brand in source A and in source B another brand then those two cements will have a different property. Similarly, sand from source A quarry and sand from source B quarry have different properties. The same thing goes for the aggregates. Since there is variation in material properties in source A and source B the concrete prepared will have different grades. This makes nominal mix since it doesn’t depend upon the properties of the material. Suppose the source from where you are using materials are of good quality. In this case in order to achieve M20 grade of concrete you might need a lower ratio. Here if we follow the nominal mix we obtain undesirable strength which also is uneconomical.

                Hence in order to overcome this, we have got a design mix. This mix entirely depends upon the properties of the constituent’s material. The following properties are considered while designing the mix: -

I)                    Characteristics Strength of concrete.

II)                  Specific gravity of coarse aggregate and fine aggregate.

III)                 Maximum and minimum size of coarse aggregate.

IV)                Type of cement (grade of cement).

Since this method depends upon the properties of the material so it is economical as well as this is the method that should be followed at the site to obtain accurate strength of concrete.


Comments and suggestions are warmly welcomed.                                                    


Prepared by: - Er. Biplov Sapkota

Reference: - Er. Gokul Saud

 

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