The concrete develops strength with continued hydration. Rate of gain of strength is faster to start with and the rate gets reduced with age. It is customary to assume 28 days strength as the full strength of concrete. Actually concrete develops strength beyond 28 days also but codes have not been permitting to consider this for design purposes. Increased in strength beyond 28 days used to get immersed with factor of safety.
With better understanding of the material progressive designer have been trying to reduce FOS and make structure more economical. In this direction, the increase in strength beyond 28 days is taken into consideration in design of structures.
One of the main factor that affect rate of gain of strength is the fineness of cement. Particle of cement over 40u in size contribute to the compressive strength of concrete only over longer periods, while those particle smaller than 20 to 25 u contribute to 7 days strength and particle smaller than 5 to 7u contribute to I or 2 days strength.
