Can I pour concrete over existing concrete?

Adding a layer of concrete over existing concrete doesn’t join the two because the cement in the concrete doesn’t have a natural bonding agent. That means the concrete sits as a separate layer on top of the existing one. Thus, it doesn’t produce a robust and serviceable floor.
Additionally, when you bond a block of new concrete to an old one, cracks in the old slab transfer up to the new slab within a day or two.