7 Day concrete strength

[charlierock]

Is there a formula for estimating the 28 day compressive strength on concrete based on the 7 day strength?

 

[Allangator]

A 7-days test is usually just one cylinder and is not the basis for acceptance or rejection, unless this is the specified age for acceptance. However, it does provide an indication of what strength may be expected at 28 days. Cements produced during the 1990s are different from those produced in the 1950s. Nowadays, cements are ground finer and have a slightly different chemical composition in order to produce a higher 7-day compressive strength when compared to the 28-day strength. For Type I Portland cement, the average 7-day strength is now about 78% of the 28-day strength on the average, instead of 66% forty years ago.

You can get more information at the ACPA web site:

http://www.pavement.com/PavTech/Tech/FATQ/fatq-strengthtests.html