Today we will discuss regarding mix design of concrete as per IS Code ( IS- 10262 (2009)- guidelines for concrete mix design proportioning).

**Concrete mix design as per IS code**

**What is concrete mix design**

Concrete mix design is the proportion of ingredients which are used in concrete to achieve the targeted/desired strength of the concrete.

So, we can say that concrete mix design is depend on properties of ingredients of concrete.

Basically, A structure engineer/designer provide the concrete mix design for your structure.

## Why concrete mix design is necessary?

As we all know concrete is strong in compression and steel is strong in tension. Combination of concrete and steel makes a reinforced concrete structure.

We have all the calculated properties of steel. Or we can say that we don't make steel in site. It is pre-cast material, we just need to cut and placing the steel. But in the case of concrete we need to do so many critical things with it. So, we need proper guidance or specifications of making concrete to make our RCC structure safe and durable.

Before, dive in to concrete mix design, we should learn the basic things on ingredient of concrete.

For doing concrete mix design calculation we need some test results of concrete ingredients as concrete mix design depends on the properties of concrete ingredients.

In concrete, there are basic ingredients like cement, sand, aggregate and water.

Sand is isolated from the river. So, every sand in the earth might not same. There are different properties of sand with changing in the locations. So we need to do test on sand for achieving properties. Based on that test results we do the concrete mix design.

Also, we need properties of water, aggregate and cement which are different. So, for doing concrete mix design calculation, we need test results of various concrete ingredients.

#### Code for concrete mix design

IS 10262 -2009 is the code for concrete mix design. you can use it for your reference.

#### Factors that affect the concrete mix design

- Size of aggregate ( Fine and coarse)
- W/C ratio
- Slump value
- Water absorption of materials
- Specific gravity of materials
- Admixture if used, etc.

## Concrete mix design procedure

Now, let's discuss about concrete mix design steps.

### Step-. 1 - target mean strength

The proportion of concrete mix is to design for higher target mean characteristic strength. Target mean strength can be calculated by given formula.

Target mean strength = fck + 1.65 * s

fck = characteristic compressive strength at 28 days in Mpa

s= standard deviation in Mpa

; Standard deviation can be calculated of different grades of concrete from Indian standard code which is IS:10262 (2009) page- 2, table no-1.

### Step-2 - selection of water-cement ratio

Water-cement ratio is one of the major affecting factor for concrete mix design. Workability of concrete is depends upon the water cement ratio. Hence, the slump test result affect.

If you choose lower water cement ratio, than workability of concrete decreased and may lead to segregation of concrete and it might require a lot more effort of compaction.

If you increase the water cement ratio than workability of concrete will be increased.

So, now what water cement ration should you adopt? The answer is, You can choose water cement ratio from 0.4 to 0.6.

### Step- 3- estimation of air content

We can consider 2 % of air content, for maximum size of coarse aggregate of 20 mm.

### Step- 4 - selection of water content

Water content can be influence by various content like, aggregate shape, size, texture, cement, admixture, water-cement ratio, environment condition, etc.

Water demand can be reduced in concrete mix design if we increase aggregate size or use rounded aggregate.

1- 10 mm aggregate - 208 kg/cum water content required

2- 20 mm aggregate - 186 kg/cum water content required

3- 40 mm aggregate - 165 kg/cum water content required

In IS:10262(2009)- page- 3, table -2 is the table which shows the water content required as per aggregate size.

### Step- 5- determination of cement content

Selection of cement content can be done with water cement ratio, for achieving desired durability. Cement content should be in accordance with IS- 456:2000.

### Step- 6 - determination of fine and coarse aggregate

Coarse aggregate can be selected by the use of concrete mix, for example if you have to pump the concrete then you should select a lightweight and do proper mix design calculation for that.

Also, the weight of aggregate is depend on the size of aggregate.

From the table- 3 - page no - 3 of IS- 10262-2009, volume of coarse aggregate per unit volume of total aggregate for different zone of fine aggregate can be determined these value is approximate.

The content of fine and coarse aggregate can be determined by divided by their specific gravity and then multiplying that with 1/1000.

I will discuss an example of mix design so you can understand it clearly.

### Step- 7 - mix proportion

The volume of all the mix can be calculated with simple formula. weight of material divide by specific gravity multiply by 1/1000.

### Step- 8 - trial mix

Four trail mix can be done for mix design. necessity of trial mix is to check the proportion of mix design.

You can follow is code for concrete mix design for your reference.

We will discuss example of concrete mix design with different grades for better understanding of this topic.

Any doubts are welcome in the comment section.

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