## 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/mm^{2}) | 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. |