Compressive strength of concrete shall be 24.1 MPa (3500 psi) at 28 days cylinder strength (minimum).

Прочность на сжатие бетона должна составлять 24,1 МПа (3500 фунтов на кв. дюйм) ...

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What does 28 -day strength mean?
Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.

Ссылка: http://www.cement.org/basics/concretebasics_faqs.asp