# Concrete Mix Design:-

Concrete is a composite mixture which consists of Cement, Sand and Aggregate. Concrete mix design is the procedure for finding the right quantities of these materials to achieve the desired strength. Accurate concrete mix design makes concrete construction economical.  Large constructions such as Bridges, dams requires huge amount of concrete, using the right quantity of constituents make the structure economical. In order to calculate or find the right amount of cement, sand and aggregate required in 1m3 of concrete; you need to know about different grades of concrete.

As per IS456:2000, Different grades of concrete are classified into  M5, M7.5, M10, M15 etc., whereas M stands for Mix and the number behind M stands for Characteristic Compressive strength(fck) of the concrete in N/mm2 @28 days when checked with 15cm×15cm×15cm cube in direct compression test.

Concrete Mix design of   M5, M7.5, M10, M15, M20, M25, M30 and higher grades of concrete is calculated as below:

The procedure for finding the different grades of concrete mixes are same. The below mentioned method can be applied to any grade of concrete by changing the values. To make ease in understanding we are finding the Concrete mix design of M20 grade concrete.

The Concrete mix ratio for M20 grade of concrete is 1:1.5:3 that mean 1 part of cement, 1.5 part of sand (fine aggregate) and 3 parts of aggregate (crushed stone) in volume and then batched for mixing. To know the Concrete Mix Design follow below:-

Mix Design (M) = Cement: Sand :Aggregate As per  BS 8500-2 British/European standards The grade of concrete is  denoted as C10, C15, C20,C25, etc., ‘C’ which means ‘Concrete Strength class’ and number behind C refers to characteristic Compressive strength of Concrete in N/mm2 @ 28 days when tested with the 15cm dia & 30cm height cylinder in direct compression test

The grade of concrete is also denoted as C16/20, C20/25, C25/30, etc., which means Concrete Strength Class (C) the number behind C refers to Compressive strength of Concrete in N/mm2 when tested with Cylinder / Cube. Remember : 1MPA = N/mm2

## Different grades of concrete :

Different grades of concrete has different Concrete Mix proportions which means different concrete mix ratios. As Per IS456:2000,  the Concrete mix design has the following concrete mix proportions :-

Compressive strength of
Concrete
@28Days in N/mm2
Ordinary concreteM51:5:105 N/mm2
M7.51:4:87.5 N/mm2
M101:3:610 N/mm2
M151:2:415 N/mm2
M201:1.5:320 N/mm2
Standard ConcreteM251:1:225 N/mm2
M30Design Mix30 N/mm2
M35Design Mix35 N/mm2
M40Design Mix40 N/mm2
M45Design Mix45 N/mm2
M50Design Mix50 N/mm2
High Strength
Concrete
M55Design Mix55 N/mm2
M60Design Mix60 N/mm2
M65Design Mix65 N/mm2
M70Design Mix70 N/mm2

## Different types of  Concrete mixes:

Concrete Mixes are primarily divided into the two different types :

### Nominal Mix:

Nominal Mix is generally adopted for small scale constructions. In this type of mix, the mix ratios and concrete constituent proportions are prefixed and specified. Eg: M20(1:1.5:3);  the quantity of cement, sand and aggregate is batched in volume as per the fixed ratio 1:1.5:3. From the above table till M25 grade, the concrete proportions are called as Nominal mix concrete.

### Design Mix:

Design mix concrete is adopted for high rise constructions. In this type of mix, the mix ratios are decided by an Engineer after analysing the properties of individual ingredients of concrete. Like, cement is tested for Fineness modulus and Specific gravity of cement in the lab while deciding the Design mix ratio. There is No Pre-fixed ratio, and ingredients are are batched in weight. From the above table, concrete grades more than M25 falls in Design mix.
In Simple, Design Mix refers to the ratios which are decided by the designer.

## Calculation of Quantities of Cement, Sand and Aggregate in 1m3 of concrete:-

To calculate the individual quantities of cement, sand and aggregate required for 1m3  assume a nominal mix grade of concrete such as M20. You can use same method to calculate any grade of concrete.

### Concrete Mix Design for M20 Grade Concrete:-

Now let us consider M20 grade concrete;

As per IS456:2000, M20 Grade concrete proportion is  = 1 : 1.5 : 3

We have to add all the volume to know the total volume = 1 + 1.5 + 3 = 5.5

As we know that  during concreting when we place wet concrete, it gets harden after certain standard time. Considering the same it had be decided upon by Civil design Engineers to take a factor of safety ranging from 1.54 to 1.57 to counter that shrinkage.

i.e volume of dry Concrete = 1.54 to 1.57 times Volume of wet concrete

I am Assuming 1.57 as factor of safety

So total volume of concrete required is 1.57 m3

#### Calculation of Volume of Cement in 1m3 of Concrete:-

To calculate the Quantity of Cement in Concrete the following formula is used.

Volume of Cement

=   CementCement+Sand+Aggregate    x 1.57 For 1m3 of Concrete = 403.2Kgs of Cement is required.

No. of Cement Bags required for 1m3 of Concrete  = 403/50 = 8.06 ~ 8 bags

Hence we require 8 bags of cement for 1m3 of concrete.

Detailed explanation for No. of bags required for 1 cum of cement

#### Calculation of Volume of Sand (Fine Aggregate) in 1m3 of Concrete:-

The following steps are followed to find the quantity of sand in  1m3 of concrete.

Volume of Sand

=   SandCement+Sand+Aggregate    x 1.57 Sand usually consists of moisture content. It increases the volume of sand (bulking of sand).

For accurate calculation use dry sand or include the effect of bulking as mentioned above in calculation.

Hence we require 672kg of sand  for 1 cu.m of concrete.

#### Calculation of Volume of Coarse Aggregate in 1m3 of Concrete:-

The following formula is used to  find the quantity of coarse aggregate in  1m3 of concrete.

Volume of Aggregate

=   AggregateCement+Sand+Aggregate    x 1.57 Density of aggregate may go higher if void spaces decreases. 5mm aggregate has more density when compared with 20mm size of aggregate

Hence We require 1326Kg of 20mm aggregate for 1mof Concrete.

#### Amount of Water Required for 1m3 of Concrete:-

For making concrete, required amount of water is added. Water quantity is depends upon the climatic factors and workability required. We generally maintain 0.35-0.50 of water cement ratio. Remember, above values  may change according to the moisture content present in sand and aggregate.

Cement Required for 1mof Concrete  = 400Kgs
Assume W/C ratio = 0.50 (50% of cement)
Water required for 1mof Concrete. = 400 x 0.50
Amount of Water Required for 1mof Concrete = 200kgs = 200 litres

Hence 200 Litres of Water is required for 1mof M20 Concrete

Summary:

8 bags of cement required for 1 cu.m of M20 grade concrete.

672 kg of Sand required for 1 cu.m of M20 grade concrete.

1326 kg of 20mm Aggregate is required for 1 cu.m of M20 grade concrete.

200 litres of water is required for 1 cu.m of M20 grade concrete

# Concrete Calculator (Concrete Mix Design Automatic Calculator):

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