Concrete is graded or designated on the basis of its compressive strength. As per IS456:2000 concrete is graded into fifteen types as given in the table (Table 2, IS456:2000)
| Group | Designation | Compressive Strength (N/mm2) |
| Ordinary Concrete | M10
M15 M20 |
10
15 20 |
| Standard Concrete | M25
M30 M35 M40 M45 M50 M55 |
25
30 35 40 45 50 55 |
| High Strength Concrete | M60
M65 M70 M75 M80 |
60
65 70 75 80 |
Concrete grades are expressed by letter M followed by a number. The letter ‘M’ refers to the mix and the number represents the characteristic compressive strength of concrete in N/mm2.
The specified characteristic strength is determined for 150 mm size cube at 28 days.
The characteristic compressive strength of concrete is defined as that strength below which not more than 5 percent of the test results are expected to fall. The various grades of concrete as per their use are listed below:
- For R.C.C. work – not lower than M20
- For post tensioning works – M35 and above
- For pretensioned and prestressed concrete – M40 and above
Concrete of grades lower than M20 may be used for plain concrete works, lean concrete, simple foundations, masonry walls and other simple constructions works.
