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 criteria!
1) Durability
2) Strength
The minimum grade to be adopted as per durability criteria is M20 as per IS-456 Table 5. This is for Mild exposure of structure. Table 3-IS-456 covers what are the different exposures. A normal s building therefore may not need a mix more than M20
When will the strength criteria come to play?
It is important to understand that increasing compression strength is useful only if there is excessive compression in the member. To understand this, see the simply supported beam in the SKETCH
The tension is entirely taken by rebars. So, increasing compressive strength is of no help at tension zone. This tension and compressive zone are a very basic principle of Reinforced concrete design including filler slabs and I need not explain this more.
At the same time, it is beneficial in compressive zone if the compression is excessive.
What is excessive compression?
As long as the section depth is adequate to take the compression, M20 concrete (20N/mm2) is sufficient. If the spans of beams are short generally the depth will be sufficient. What I mean is, the client or architect will not have an architectural and functional challenge of having 350 to 450 mm beam depth in a regular building. If the spans are more, we may need 750 or more depth, strength point of view. Then starts all architectural and functional clashes! So, if there is a depth restriction, then to achieve compressive strength we have 3 options
Reduce the spans/forces in beams. Make the beam doubly reinforced (Introduce rebars at compressive zone) to take compression. Increase compression strength of concrete by increasing the grade from M20 to higher.
One among the three is adopted whichever is possible. If many or all beams has depth restrictions, may be increasing the grade of concrete will be the right solution.
In a vertical member like columns, the forces may not be high if the number of floors is lesser. In tall buildings the size of column may be high if we go with lower grades of concrete. Therefore often, lower level of buildings where compression forces are more, you may have to specify higher grades of concrete.
In some cases where the column percentage of steel is high at lower levels and very less at upper levels, engineers may be forced to reduce the size only at upper levels. This is not desirable as the stiffness variation makes the structural irregular some times. A better way to deal is to have higher grade of concrete at lower levels and the reduce the overall size of the column throughout its height. You can use M20 concrete at upper levels there by optimising the size and steel provided for the column.
Note: IS-13920.Clause 5.2 - The ductile detailing code specifies that the mix has to be M25 if the height of the building is more than 15m in seismic Zone 3 and above
Summary
When you are constructing a building, you will have to decide on the grade of concrete to be used. This will depend on the purpose of the building and the environment in which it’s situated as well as the strength requirement. We hope this blog post has helped you decide the grade of concrete for a building. If you have any other questions or concerns about the grade of concrete for a building, please contact me anytime here. Thank you for reading, I am always excited when one of my posts is able to provide useful information.