Blog

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:

Grades of Concrete:

Concrete Grade	Mix Ratio	Compressive Strength
MPa (N/mm2)	psi
Normal Grade of Concrete
M5	1 : 5 : 10	5 MPa	725 psi
M7.5	1 : 4 : 8	7.5 MPa	1087 psi
M10	1 : 3 : 6	10 MPa	1450 psi
M15	1 : 2 : 4	15 MPa	2175 psi
M20	1 : 1.5 : 3	20 MPa	2900 psi
Standard Grade of Concrete
M25	1 : 1 : 2	25 MPa	3625 psi
M30	Design Mix	30 MPa	4350 psi
M35	Design Mix	35 MPa	5075 psi
M40	Design Mix	40 MPa	5800 psi
M45	Design Mix	45 MPa	6525 psi
High Strength Concrete Grades
M50	Design Mix	50 MPa	7250 psi
M55	Design Mix	55 MPa	7975 psi
M60	Design Mix	60 MPa	8700 psi
M65	Design Mix	65 MPa	9425 psi
M70	Design Mix	70 MPa	10150 psi

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.

Sr. No.	Grade	Mixer Ratio	Water/bag (50K.G)
1	M5	1:5:10	60 Ltr.
2	M7.5	1:4:8	45 Ltr.
3	M10	1:3:6	34 Ltr.
4	M15	1:2:4	32 Ltr.
5	M20	1:1.5:3	30 Ltr.
6	M25	1:1:2	28 Ltr.