The grade of concrete changes with the changing proportion of concrete ingredients. The grade of concrete has been specified by IS 456 : 2000. But the limitation of IS code is that it specifies the concrete mix ratio up to a certain grade, which is M25, above which no certain mix proportion has been approved in industrial and infrastructure projects.

As per IS 456 : 2000, Nominal mix of concrete is given below.

Sr No | Grade of Concrete | Proportion (Cement : Sand : Coarse Aggregate) |

1 | M5 | 1:5:15 |

2 | M7.5 | 1:4:8 |

3 | M10 | 1:3:6 |

4 | M15 | 1:2:4 |

5 | M20 | 1:1.5:3 |

6 | M25 | 1:1:2 |

In industrial and infrastructure development, to serve the requirement for high strength concrete and concrete properties improvements, different high strength concrete has been initiated, taking it way greater than M100.

Because of limitations of IS 456:2000, a new design procedure has been adopted in India called Mix Design of Concrete which follows principles of IS:10262:2009.

In this article, I would like to mention some useful points on concrete mix design as well as factors affecting concrete mix design.

**Factors Affecting Concrete Mix Design:**

Various factors affecting concrete mix design as described below :

- Grade of cement
- Grading Zone of Aggregate
- Shape and Size of Aggregate
- Water absorption by aggregate
- Types of Admixture used
- Concrete slump value
- The specific gravity of the material
- Water cement ratio of concrete
- Method of transportation
- Degree of supervision
- Method of concreting

**Concrete Mix Design Procedure**

Total eight steps involved in concrete mix design procedure.

- Target Mean Strength
- Water Cement Ratio
- Determining Amount of Water
- Cement Content Determination
- Determining the amount of coarse and fine aggregates
- Find out total volume of all materials
- Find out the ratio of ingredients
- Preparation of trial mixes

**Target Mean Strength**

Before designing a concrete mix, we should decide the grade of concrete. For example, in the M45 grade of concrete, the characteristic strength of concrete is 45 Mpa. From characteristic strength of concrete, the target mean strength is carried out, from below given formula, for that specific concrete.

Target Mean Strength = Characteristic Strength + t x s

Where,

t = tolerance factor which equal to 1.65

s = standard deviation whose value can be found out based on grade of concrete from standard deviation chart from any concrete book.

**Water Cement (w/c) Ratio**

After Target Mean Strength determination, the water cement ratio is found, either using the chart (specified by IS:456:2000) on the basis of exposure conditions and grade of concrete.

**Determining Amount of Water**

After deciding the w/c ratio, the amount of water for a specific grade of concrete may be determined through a chart, which is based on the size of aggregates used in concrete.

- 10 mm aggregates : 208 kg
- 20 mm aggregates : 186 kg
- 40 mm aggregates : 165 kg

The above value is based on angular shape aggregates and slump range of 25 to 50 mm. For different shapes of aggregates and slumps ranges, the amount of water can be adjusted parallelly.

The quantity of water can be decreased by 10 kg for sub-angular aggregates, 20 kg for gravel and 25 kg for rounded shape aggregates. The amount of water can also be adjusted depending upon the value of slump. For every 25mm increased slump, the amount of water can be raised by an amount equal to 3%.

**Cement Content Determination**

After determining the amount of water, then from w/c ratio and amount of water, the cement content can easily be found out. Formula of w/c ratio is given below.

**W/C Ratio = Water Content/Cement Content**

**Determining the amount of coarse and fine aggregates**

The weight of the coarse aggregate depends upon grading zone and size of aggregates. Grading zones such as zone 1, 2, 3 and 4.

The value can be determined from the respective chart in the “Design Mix” topic of any concrete technology text book. The value got from the chart is the volume of the coarse aggregate, per unit volume of total aggregates volume. Then, volume of fine aggregates is determined by deducting the volume of coarse aggregates from total volume.

**Find out total volume of all materials **

The volume of all materials can be determined from the following equation.

**Volume = (Weight obtained / specific gravity) x (1 / 1000)**

If admixture is added, then the volume of admixture is determined by the following equation.

**Volume = (dosage of admixture) x (cement content) x (1/1000)**

Total volume of concrete considered as 1 cu.mt, the volume of cement, water and admixture are found out. The remaining value is the volume of all aggregates.

**Find out ratio of ingredients**

Prepare the ratio of all ingredients of concrete by considering the value of cement as 1.

**Preparation of Trial Mix **

Normally, four trial mixes are prepared to check the desired properties and strength.

**Conclusion **

These are eight steps included in calculation of concrete mix design. I would suggest seeing example of concrete mix design from any concrete technology text book, as an example would be helpful in remembering these points and properly understanding them.

**FAQs on Concrete Mix Design as per IS Code**

#### What is the IS code for concrete mix design?

Because of limitations of IS 456:2000, a new design procedure has been adopted in India called Mix Design of Concrete which follows principles of IS:10262:2009.

#### What is M20 concrete mix design?

The mixing ratio of M20 grade concrete is 1:1.5:3 (cement : sand : aggregate)

#### Is Code 456 slab thickness?

As per IS 456 : 2000, concrete slab thickness is kept 100 mm to 150 mm (4 to 6 inches) for residential building.

#### What is M10 concrete ratio?

M10 concrete mix ratio is 1 : 3 : 6 (Cement : Sand : Aggregates).

#### What is slump test used for?

Slump test of concrete is use to measure workability of concrete.

**Refference :**

- Concrete Technology: Theory and Practice – Book by M. S. Shetty

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