What is the purpose of providing M35 grade in a structure and how we can calculate its quantities in a shorter and quick manner?

M35 is a common type of concrete grade. It is used in concrete pavements because they are designed for flexural  strength. So generally we prefer higher grades in concrete pavements. The reason for flexural design is bending of slab under axle load. And depending upon temperature, seasonal variations there are stresses induced and they should be within permissible limits. It is a fatigue design criteria. We calculate the fatigue life of a pavement. M35 means compressive strength of concrete after 28 days of curing is 35 MPA. The strength can be checked by cube test. Approximately 80 percent strength is achieved after 28 days. For flexural the formula is 0.7√fck. Fck is compressive strength of concrete ie desired grade

Dear Civil/Structural Engineers,

For structures, minimum specified grade of concrete is M as per Bureau of Indian Standards and; M grade as per Buildings and Construction Authority of Singapore, like wise different countries specify different minimum grade of concrete for structural purpose. This is fixed considering the financial burden over the country's economy/spending capacity of the public. Many developed countries fixed at M grade of concrete as minimum structural concrete to be used in RCC /PSC works.

Further to add that the minimum grade of structural concrete is based on the conutry's economy and performance of the structural concrete, such as durability, exposure, firefighting requirements, etc. When a structural engineer is using higher than the minimum specified grade for the structural purpose, he has to substantiate with his calculations and convince the owner that using higher grade is ECONOMICAL.

Using higher grade than specifed minimum is upto the designer's choice. As we increase the concrete grade, we have to use higher grade steel, to minimize the cost and to achieve economy. 

For coulumns we may need to use very high grade concrete upto M. SCC (Self Compacting Concrete is also gaining popularity becase of ease of placing concrte upto any height at one go after complete form work is done.

We use M or higher grade in PSC members which is now becoming popular in buildings (Commercial and residential) in India. 

For buildings with moderate spans upto4.5 meters, the quantity requred for residential buildings is about1. CFT per SFT of building area, this includes foundation footings, columns, beams. For commercial it is slightly more. For the buildings without beams, i.e  flat slabs, it would go upto2. cft . This is for quick calculations only and not for exact.