How to decide Grade of Concrete for a Building

Updated: Sep 5, 2021

What is mix of concrete?

Concrete is a mix of four or five major components in different proportions. The components as well as its proportions has to be right in order to achieve the desired properties for the concrete viz; durability and strength.

The major four constituent of a concrete mix are cement, water, coarse aggregates and fine aggregates. Additional component is air and many times, chemical admixtures are also added to achieve certain desirable properties as needed.


Why do we need right mix of concrete?

Right mix or proportion of these components are necessary in order to achieve the required desirable strength and durability for the concrete and thereby to the structure.


Code provisions for Grade of concrete

IS 456 and most code links the grade of concrete to its strength, i.e., a cube strength of 20 N/mm2 is called M20 concrete which indicates its strength. This has resulted in misunderstanding among young engineers that strength requirement alone drives the selection of concrete mix. This is not correct. Read on for more information. Additionally note that IS 13920 which is the ductile detailing code mandates M25 as minimum grade of concrete for seismic zones where ductile detailing is mandatory.

It is important to note that IS 456 mandates that the minimum grade of concrete has to be M20 for RCC structures and buildings. Though some of the lower grades and its strength can be achieved by volume batching, it has to be designed. There is a specific method to be adopted for mix design as per IS 10262:2009. Table 5 in IS 456 can be referred to know the various criteria including minimum cement content, maximum allowed water content and minimum grade specified. It is mostly related to durability aspects that decides this minimum grade specification.

It is important to realise these 2 aspects carefully that decides the mix of concrete in any project. I have summarised it below. The below principles let you know the mix selection criteria more clearly. It is better to understand this point about selection of concrete mix rather than blindly adopting a mix.


Table 5 - IS 456

Exposure

Reinforced Concrete

Reinforced Concrete

Reinforced Concrete

Min Cement Content

Max Water - Cement Ratio

Min Grade of Concrete

Mild

300

0.55

M20

Moderate

300

0.5

M25

Severe

320

0.45

M30

Very Severe

340

0.45

M35

Extreme

360

0.4

M40


What are the criteria for deciding concrete mix?

In any structure the grade of concrete is decided mostly by 2