Has anyone had any luck pouring concrete over existing, structurally sound concrete? This is outdoors and in any situation.

Also very important, if you have had luck how thick was the new (top) concrete?

Bush hammer, pressure wash, wet til saturated-surface dry, bonding agent, new concrete, prayers.

Thanks! In my current scenario I am hoping for a thin top layer, a couple inches. Basically, skim coating concrete. I cannot raise the grade more than 3 inches, preferably only 2 inches.

Don, how big will the new slab be?

Since you only want to increase the height 2-3", you might consider using some building wrap like Tyvek as a slip sheet instead of sand or rigid insulation. Just thoroughly clean the old slab, saturate the old slab with water, blow off any standing water with a leaf blower, lay down the house wrap & smooth with a broom and pour the new concrete. Definitely make sure you have them mix some chopped fibers to the mix. If you're not familiar with using chopped fiber do a Google search.

Depending upon what type of finish look you're going for, I might be inclined to use either PT lumber, 5/4 PVC boards or synthetic decking to make the forms. You could excavate down the sides of the existing slab & permanently attach the new forms to old slab.