Riz...under most core evaluation criteria (ACI included), if the core strength achieves 85 percent of the expected strength, it is considered acceptable. This generally applies to 28 day and later strengths, but could be extrapolated, at least in part, to include earlier strength parameters.
As an example, you cored at 12 days with, I am assuming, a measured maturity of 67 percent. If your cores achieved 85 percent of that value (0.67 x 25 N/m^2 X (0.85)=14.2 N/m^2)then you would consider that acceptable. However, early strength gain is affected by lots of "stuff". The fineness of the cement, the type and quantity of admixtures, whether the cement was modified ( using fly ash, ground blast furnace slag cement, or materials), curing conditions, aggregate influence, w/c ratio, air content, temperature, and probably a few I have left out. This is a danger of coring/evaluating too early. The time between placement and about 15 to 18 days is the steepest part of the strength gain curve, so minor mix issue can significantly affect strength gain. These tend to moderate with time (20 to 60 days.
Your next step might be to get a petrographic examination of a core to evaluate a few of the hardened concrete characteristics that might lead you to a conclusion.
Good luck.
Ron