How to decide Grade of Concrete for a Building

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. 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 functonal 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 compressionIncrease compression strength of concrete by increasing the grade from M20 to higher.

One among the 3 is adopted which ever is possible. If many or all beams has depth restrictions may be increasing the grade will be the solution.

In a vertical member like columns, the forces may not be high if the number of floors are 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.

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