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The28-day strength is often used to designate the concrete according to its (compressive) strength class.
For example, in Eurocode2 [C16/20] means: [Concrete] -- [characteristic cylinder strength = 16 MPa] -- [characteristic cube strength = 20 MPa].
depending on the climate a normal concrete is completely done in28 days. In cold weather its different.
Mister Atanas Todorov have reason. But If you used adjuvent for increase the time for drying or taking, you improve this data. There are a lot of parameter who can play on the result also the quality of element for your concrete, the time to mixed, ...... So We can give you a answer generaly on the middle data (generaly had). Your laboraory could you give an idea on result.
it depends on cement type used in concrete but generally has to gain90% of total anticipated strengh