Strength of concrete is designated by its grade. As per Indian Standards, concrete grades are denoted like M20, M25, M30, M35 etc. Here, M20 grade concrete is having 20 Newton per mm2 strength. And so does for others. These strengths are also known as the characteristic compressive strength of concrete. This is measured at 28 days after casting the concrete.
The necessity and selection of different grade of concrete depends on many factors. Few factors include exposure condition of the concrete, required strength of the concrete, purpose of the structure etc.
M20 means Mix 20, whereas M30 means Mix 30. Strength of concrete is primarily influenced by its mix proportions.
Different Mix Proportions For Different Grade Of Concrete
As per Is:456-2012 following mix proportions are suggested for nominal mix concrete.
The ratio of Cement : Sand : Aggregate
Mix Ratio For M10 is 1 : 3 : 6
Mix Ratio For M15 is 1 : 2 : 4
Mix Ratio For M20 is 1 : 1.5 : 3
Mix Ratio For M25 is 1 : 1 : 2
The ratio is often denoted as CSA ratio, also known as cement, sand and aggregate ratio.
As per Indian Standards, M10, M15 and M20 are recognized as ‘Ordinary Concrete’
M25, M25 and up to M60 grades of concrete are recognized as ‘Standard Concrete’
From M65 to M100 grades of concrete are known as high performance concrete as per IS 456-2000.
Other Interesting Facts About Concrete Grade And Ratio
In 4th Amendment of IS 456-2000 the M60 grade concrete has been shifted to standard concrete from high strength concrete.
It is advisable to use design mix concrete for M25 or higher grade.
Download this Concrete Mix Design Indian Standard IS:10262-2019 (PDF)
Download IS 456-2000 Design Of Plain And Reinforced Concrete
Important Websites For Concrete Engineers
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M20 is the minimum concrete grade for RCC works as per IS 456-2000.
It is the permanent strain in concrete members (particularly RCC member) under constant stress.
The minimum grade for extreme exposure conditions is M40.