For a product that runs like mud and dumped out of a truck, Concrete is actually a very precision product. There are many other ingredients that can be added to the mix to add air, quick strength, early hardening and several other properties.
Without a substantial vapor barrier you will always have moisture in the concrete especially if it is on the ground.
28 days is considered the time of obtaining max strength, but the concrete can be worked on and used prior to that time depending on what you are doing. A safe will have little effect on the slab. It just isn't going to be heavy enough to damage the slab. Moisture will be your biggest problem.
