With the caveats that all mixes are different, some are intentionally designed to achieve strength slower or faster than typical, and that the temperature can have a noticable impact on the relationship-
For a typical "run of the mill" mix we start to get concerned if we see much less than 60% of specified 28 day strength at 7 days. I am accustomed to seeing the 7 day strengths at about 2/3 of the 28 day test as a rule of thumb.
Here is a typical formula for calculating strength based on time for "normal" concrete. You have to know the design or expected f'c. t is time in days since the concrete was cast.
f'ci= f'c*(t/(4.00+0.85t))
If you don't know the f'c, back into it using the 7-day break value. Then use that f'c value to calculate your 28-day strength. My real world experience is that it is a decent estimate.
Many spec's set a "specimen age correction factor" to predict the 28-days compressive strength from the results obtained at an older or younger age.
Provided that the conditions of curing and testing are kept constant. Interpolation could be made for other ages but this will increase the deviation from the true strength.
I’ve used these correction factors in my day-to-day business and found it to be kind of reliable especially for type I Portland cement. Note that this is only an estimate; nothing will give you the exact 28-days strength as there are too many variables. I’ve seen concrete that failed at 28-days though predicted to pass, there's a reason why it's called MEAN compressive strength.
Also this gives no indication of the true compressive strength at the site and generally will yield a higher strength than the true one (depending on temperature, curing and type of cement)
charlierock...the estimates provided are good for Type I portland cement. With all the blends and mix contortions the ready mix suppliers use to save a few bucks, these correlations can sometimes be off a bit.
If your mix has any proportion of ground blast furnace slag cement in it (NEWCEM or similar), then you can expect the 7-day strength gain to be significantly lower than predicted or expected by correlations for plain Type I. The 28-day strengths will not be significantly affected, but the strength gain is slower than with plain Type I cement.
Fly ash causes a similar slowing of the strength gain, though less pronounced because it is usually used in smaller proportions.
Ask the ready-mix supplier for a strength gain curve for their mixes. This should get you where you want to be in your predictability.
If the concrete was supplied from a batch plant, the thing you should ask for is the "age-strength correlation" not the strength gain curves, as the former will give a better estimate of the predicted strength based on the true 28days strength using different Q/C systems such as the CUSUM which most of the concrete suppliers use to predict the 28-days strengths based on the 7-days strengths and this correlation is checked and updated daily as the actual 28-days strengths becomes available by comparing them to the predicted ones.
BUT....I won't totally trust data provided by the concrete supplier .... remember it's their concrete you are testing!
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