Target strength of concrete

How to calculate target strength of concrete?

           TARGET STRENGTH OF CONCRETE

 Formula;
     f'ck = fck + 1.65s 
Where;
       S = standard deviation

Grade of concrete            assumed 'S' N/mm²

   M10 to M15                                     3.5
   M20 to M25                                     4.0
   M30 to M55                                     5.0
From  IS 10262:2009

Ex: 
    Concrete grade = M25 - fck
                          fck = 25 N/mm² at 28 days
    That's why,  fck = 17 N/mm² at 7 days
   (M25 gain strength 17 N/mm² at 7 days)

Solution: 1
  Target strength of concrete at 7 days
f'ck = fck + 1.65s
        = 17 + (1.65×4)
        = 23.6 N/mm²

Solution: 2
   Target strength of concrete at 28 days
f'ck = fck + 1.65s
        = 25 + (1.65×4)
        = 31.6 N/mm²

For Proper understanding watch this video
https://youtu.be/NyFxv0jU-Ac

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