You could possibly get the concrete topping to last for a while, but your probably only compounding the problem. The issue is settling or poor subgrade, & adding more weight will inevitably cause the slab to sink further in the future. It may seem strange to some, but in the thousands of yards of concrete i've torn out through the years, the thickest concrete seems to have performed the worst overall. Heavier concrete is no match for "Mother Nature!"I'm thinking now, rent jackhammer, build forms, put in rebar, buy many bags of cement.
It will be driven on and there is no easy way to provide a bed for this elevated slab.
Next question: do these elec. jack hammers have enough power to crack 6" of concrete?
The last one I rented could barely do sandstone.
Or, I could abrade or roughen or remove the top surface of the existing slab so that new concrete poured on top would be 2" to 4" thick?