You can pour more concrete over an existing slab if you do some preparation to the original surface. The problem with adding wet concrete to cured concrete is there will be no adhesion between the two layers. Even though the top layer will be very heavy, it can shift over time and separate from the lower later if there is any settling. Adding what could be hundreds of kilos or even tonnes of additional concrete to the existing slab will cause some settling. Just how much depends on the total weight and the subsoil under the slabs.
Adding more concrete
One reason to add concrete to an existing slab is to level or raise the height of the slab. Concrete patios and decks are usually just a poured section of concrete sometimes over a gravel base but usually just onto the bare soil. Over time, the slab can settle further in one corner than elsewhere. Settling in the middle can cause the slab to crack. Adding more concrete is a cheaper and more labour-efficient solution than sledging or jack hammering away the existing slab. The preparation before pouring more concrete involves creating a way to fasten the additional concrete to the existing slab.
Drilling holes in the existing slab is easier than removing the slab entirely. Large 3.7 to 5 cm (1.5 to 2 inch) holes spaced regularly around the surface of the slab will allow new concrete to flow into the whole and link the two layers together. This will prevent the top layer from laterally shifting on the existing layer.
When drilling the holes, it is best to pass through the slab and a few centimetres into the subsurface. This will allow the concrete to pass through the existing layer and have a footing underneath. This footing will create a strong bond.
If you want, you can add rebar reinforcement to the hole to add greater adhesion between the two layers. If you are adding 5 cm (2 inches) of new concrete, have the rebar post rise 3.7 to 4.3 cm (1.5 to 1.75 inches) from the surface of the existing layer. This will allow new concrete to flow into the hole, provide a secure reinforced footing and hide the rebar from the surface with 6 mm to 1.2 cm (1/4 to 1/2 inch) of concrete.
You can allow the rebar to rise above the surface of the new concrete. You can then cut it off flush with the surface. This procedure, however, will leave small round divots in your new concrete surface. Doing a little math and accurate height adjustment of the rebar post will leave a perfectly smooth unblemished new concrete surface.