Concrete develops strength with continued hydration. Rate of gain of strength is faster to start with and rate get reduced with age. It is customary to assume the 28 days strength as full strength of concrete. But actually, concrete develops strength beyond 28 days also.
Percentage strength of concrete at various ages
The strength of concrete increases with age. Table shows the strength of concrete different ages in comparison with the strength at 28 days.
Age Strength per cent
1 day 16%
3 days 40%
7 days 65%
14 days 90%
28 days 99%
Concrete mix design is required to achieve target strength in structures.
Also Learn,
Why concrete cube test is carried out? & Why do we test concrete compressive strength after 28 days?
we civil engineers 
How to calculate strength of concrete in between 7 to 14 days,14 to 21days etc.
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You can calculate it by two methods.
1. You can test the specimen on that particular day, to do so you need extra concrete cube.
2. You interpolate the values (but this will give only approx. Strength).
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How to calculate %development strength of concrete
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where (in which code or Book) the concrete strength gain vs age table is given
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