Predicting 28 day strength from 7 day tests

[mikeh0le]

I did a concrete design mix and tested 3- 4"dia. x 8" lg. samples after 7 days. How could I predict what the 28 day strength of that concrete might of been? Is there a formula I could use if I have the applied load or psi of my tested concrete samples.

 

[JConc]

Typical 7-day strengths are 70-75% of 28 days. Percentage goes up if using mainly portland cement and decreases when using higher [20-25%] flyash or slag replacement.