DESIGN MIX CONCRETE METHOD - CIVIL ENGINEER VIJAY TECHNICAL

Concrete Mix Design As Per Is Code 

Concrete is a heterogeneous and hardened mass obtained from a mixture of cement, sand, coarse aggregate and water, in a certain ratio.

The grade of concrete varies considerably in accordance with the varying proportion of materials from which it is made. The proportion and ratio in which the materials should be mixed to obtain a particular type of concrete is already specified in IS 456:2000.

But the limitation of this code is that it specifies material proportions up to a certain grade, which is M25, above which no certain proportion is industrially approved.

S.No Grade Proportion (cement: sand: coarse aggregate)

1 M5    1:5:10

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

These are called Nominal Mix.

In the era of industrial development and infrastructure development, various high-strength concretes that exceed M100 have been introduced to meet the need for high-strength concrete and improve the overall properties of concrete.

Due to the limitation of IS:456, a new procedure has been adopted all over India which is to properly design the mix from scratch and thus find the required ratio for that particular concrete.

This is known as Mix Design of Concrete which follows certain principles suggested by IS:10262,2009. In this article I would like to mention some useful notes on mix design as well as factors dependent on mix design because a clear understanding of mix design processes is what a civil engineering firm expects from a fresher.

Factors dependent on mix design:

When designing a concrete mix, there are a number of factors involved in the entire process that contribute greatly to the final product, which is concrete and all of its properties. So considering all these factors and carefully maintaining them at the desired level would add significant value to the quality and strength of the concrete. Some of these factors were mentioned as follows:

Cement grade

Size and shape of aggregates

Aggregate sorting zone

Absorption of water by aggregates

Specific gravity of materials

Type of additive added

Concrete slump value

W/C ratio of concrete

Degree of supervision ensured

Method of transporting concrete

Concreting methods (such as Tremie or underwater concreting). 

Concrete mix design procedure:

The concrete mix is ​​designed with the above factors in mind. After the correct design of the structure by the structural engineer, the concrete technologists determine a certain minimum concrete strength that should be used when casting the structural elements. 

The structural engineer then prepares the concrete on site, while carefully monitoring the parameters, which overall contributes to the long-term durability and strength of the concrete. A properly designed mix is ​​therefore very necessary for the structure to safely withstand all the loads that will be applied to it during its lifetime.

1. Target average performance:

Before designing a concrete mix, the type of concrete for which the design will be made should always be known in advance. This is known as the characteristic strength of the concrete (for M45 concrete, the characteristic concrete strength is 45 MPa). From the characteristic strength, the target average strength for the given specific concrete is determined from the formulas

                     Target mean force = characteristic force + t x s

Where t is a tolerance factor equal to 1.65   , 5% of test results are expected to fall below this factor And s is the standard deviation, the value of which can be found from a separate class of concrete from the "table of standard deviations" from any particular book.

2. Water-cement ratio:

After determining the target mean strength, the water-cement ratio is determined, either using the table specified in IS:456,2000 based on the condition of exposure and the quality of the concrete, or using experience applied to a similar recently completed structure. The W/C ratio is one of the most important factors of concrete that controls the overall quality and has a direct or indirect effect on various properties of concrete.

3. Determination of water content:

Once the W/C ratio is determined, the amount of water needed to mix a particular grade of concrete can be found using a table based on the size of aggregate used to make that concrete. For 10 mm aggregates: 208 kg 20 mm aggregates: 186 kg 40 mm units: 165 kg the above value applies to aggregates with a square layer and a range of slumps of 25-50 mm. For different aggregate shapes as well as fall ranges, the water content can be proportionally adjusted.

The amount of water can be reduced by 10 kg for square aggregate, 20 kg for gravel and 25 kg for round gravel. The water content can also be adjusted depending on the slump value. For every 25 mm of increased slump, the water content can be increased by a value equal to 3 percent.

4. Determination of cement content:

After determining the required water content, the cement content can be easily determined from the water-cement ratio and the water content obtained.W/C ratio = water content/cement content

5. Determining the amount of course and fine aggregate:

Coarse and fine aggregates are then determined. The weight of the coarse aggregate depends on the size of the aggregate used and the aggregate grain size zones, specifically the four aggregate grain size zones, I, II, III and IV. 

The value can be found in the appropriate tables in the "Design Mix" chapter of any particular book. The value obtained from the graph is the volume of coarse aggregate per unit volume of total aggregate. Then the volume of fine aggregate is found by subtracting the volume of aggregate from number 1.

6. Determination of the total volume of all materials:

The volume of all materials can be obtained from the following formulas:

          Volume = weight obtained/specific gravity × 1/1000

If an Admixture is used, the volume of the Admixture is also determined. It can be determined by the following formulas – Volume = additive dosage × cement content × 1/1000

Considering the total volume of produced concrete as 1 cubic meter, the volume of cement, water and additives is determined. The remaining value is then the volume of total aggregates. By multiplying the volume of total aggregate by the volume of coarse and fine aggregate per unit volume of aggregate, the specific gravity of individual aggregates and the value of 1000.

7. Withdrawal from the relationship:

After determining the volume and weight of all materials, the ratio of individual materials is determined. This is generally done by setting the cement value to 1 and dividing the total element weight by the cement weight.

8. Preparation of test mixtures:

Generally, four test mixes are prepared to check the required strength and all other properties. A second test mix is ​​prepared with a constant w/c ratio of the first mix and varying water/admixture content. Test mix 3 and 4 are made by keeping the water content at a preselected value and varying the W/C ratio by 10 percent.

These are the eight steps involved in the Mix Design calculation. I would recommend to see a Design Mix example from any particular book as an example would help a lot to remember these points and also understand them properly. A properly designed concrete mix offers improvements in various properties that not only withstand all the loads expected on concrete, but also offer greater stability throughout its lifetime and beyond.