What you are saying it for normal concrete strength which is 17 to 28 mpa, higher strength concrete can take longer depending on the design mix and without the specifications on the building which would give the concrete design mix its anybodys guess, and thats where the money is saved and pocketed a "normal mix" ratio is 1 part cement, 2 parts sand, and 4 parts aggregate, less cement more sand more profit you still get the building but !! the safety and quality is gone