GRADE OF CONCRETE AS PER IS CODE
The clause states ''the concrete is normally a gain of strength beyound 28 days the quantum of increase depend upon the grade and type of cemnt ,curing etc .the design should be based on 28 days characterstic strength of concrete unless there is an evidence to justify a higher strength for a particular structure due to age .
in simple language ,the strength of the concrete ,the minimum strength the concrete should have 28 days of initial construction .
M stands for Mix
15,20 stands for number compressive strength of concrete in N/mm2
Group
grade of concrete are divide in three parts Ordinary concrete,Standard concrete , High strength concrete.
Many research workers have attempted to estimate the strength of concrete at 1 , 3 or 7 days and correlate it to 28 days strength .