Grades of concrete are based on characteristic strength and the composition of the concrete, and the minimum strength that the concrete should have after 28 days of initial construction. Concrete quality is included in MPa measurements, where M stands for mix and MPa for overall strength. Concrete mixes are defined by an increasing number of 5, from 10, and show the compressive strength of concrete after 28 days. For example, C10 has a resistance of 10 Newtons, C15 has a resistance of 15 Newtons, C20 has a resistance of 20 Newtons, and so on.
| M10, M15, and M20 | ordinary grade of concrete |
| M25 to M55 | the standard grade of concrete |
| M60 to M90 | high strength concrete |
- IS 456:2000 is not applicable to grade above M60.
- A minimum grade of concrete to use for RCC is M20 under mild conditions, M30 for severe conditions,s and M40 for Extreme conditions.
- Concrete use in sea water or coastal area should be at least M20 for plain concrete and M30 for reinforce concrete.
Increase in strength of concrete with age
The age of concrete is counted from the date of its casting. Concrete is design by characteristic strength at 28 days but as cement hydrates, it gains strength over a long period. Hence, in order to estimate the increased strength at the time when full design loading is expected on the concrete, its characteristic strength should be multiplied by an age factor as follows
| Age when full design load is expected (months) | Age Factor |
| 1 | 1.0 |
| 3 | 1.1 |
| 6 | 1.15 |
| 12 | 1.2 |
Grade of Concrete
Also, Read————————– Limit State Method 0f Design | Reinforced Cement Concrete |
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