You can put the concrete topping on the existing slab. In addition, you should keep its thickness at least 2 inches. Sometimes when new concrete is attached to old concrete, cracks are likely to form in the concrete and the two sections show a description of separation. To prevent this condition they should use ribs to work the joint together.Rebar reinforcing is used to support and strengthen the bond between two slabs. The size of the rebar depends on the size of the concrete and the need of the workplace. Heavy rebar is used for heavy loads. Similarly for small concrete, only half an inch of rebar is used. Such as sidewalk