 ## Introduction:

Grades of concrete are defined by the compressive strength and composition of the concrete, and its overall strength is measured in Mpa or Psi. Grades of concrete is a mixer of three materials along with water i.e cement, sand and coarse aggregates and if iron/steel bar is used in the same mixer then that grade is called reinforced cement concrete or RCC.

## Content:

Cement, sand & coarse aggregates are mixed in a proportion to form the particular mixer or grade of concrete and it’s denoted as M5, M10, M15 etc. In the above sentence ‘ M ’ represents Mix design 5, 10, 15, 20, etc. represents characteristic compressive strength of concrete after 28 days of curing. There is a procedure to find out the volume of cement (in Bags), Sand & coarse aggregates are required for a particular mixer/ grade of concrete. We shall discuss one by one as follows:

## Examples:

How to calculate the cement, sand and coarse quantity from the mixer/ grade of concrete M 20 ?

Solution:-

As we know that  during concreting when we place wet concrete, and it takes a certain time to get harder. By considering the same it had been 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

Here i take 1.57 as factor of safety.

We know that as per concrete grade the ratio for M20 is 1:1.5:3,

So volume of cement needed=

Cement ratio                                             X                    Factor of safety               ∑(ratios of Cement, sand and coarse aggregate)

= (1/ (1+1.5+3)) X 1.57 = 0.273 cum

So volume of sand needed=

sand ratio                                                    X                  Factor of safety               ∑(ratios of Cement, sand and coarse aggregate)

= (1.5/ (1+1.5+3)) X 1.57 = 0.409 cum

So volume of coarse aggregate needed=

coarse aggregate ratio                              X                      Factor of safety             ∑(ratios of Cement, sand and coarse aggregate)

= (3/ (1+1.5+3)) X 1.57 = 0.818 cum

The Standard density of 1 bag cement  of 50 k.g is 1440 K.g/m3

So the volume of 1 Cum of cement required for M20 grade mixer is

=    (0.273 X 1440)/ 50= 393.12/ 50 = 7.8624 bags

Hence 7.8624 bags of cement required for 1 cum of concrete in M20 grade / mixer, and the sand quantity & coarse aggregate quantity can be calculated respectively.

## Standard Chart:

The following table gives the idea about the how much water needed for 50k.g or 1 bag of cement for different grade of concrete/mixer.