what does the 28 day strength mean ?

COncrete hardens and gain strength as it hydrates . THe hydration process conrinuess 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 od 28 days was selected by specification writing authorities as the age that all concrete shuld be tested . At this age , a substantal percentage of the hydration has taken place .