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.
Courtesy of Portland Cement Association,
http://www.cement.org/basics/concretebasics_faqs.asp