It is known that; many of specification stipulates that samples compressive strength of concrete must be at least "fck+delta (MPa)" at 28 days.
But;
1) In-situ construction of cure condition is different from the samples.
2) Hydration improves strength continuously at very long period and It is possible that even if strength is low, it may reach value of 28 days in the long run.
So, how can I interpret compressive strength of concrete at 28 days?