Reinforcing bars are not always necessary for concrete projects. The natural strength of concrete is often enough to withstand the pressures it will be subjected to, and the floor can help hold it together. For a 4-inch above grade concrete slab, no rebar is required. A 4-inch thick cast slab on the ground and in permanent contact with it will float and no rebar is needed.
However, it is recommended to use reinforcing bars in concrete 5 to 6 inches thick. The purpose of the steel bars is to provide the concrete with greater tensile strength, which helps to reduce cracking. Concrete slabs poured into the ground with a properly prepared and compacted base, and which are not expected to bear heavy loads, do not require reinforcing bars. Rust marks on the surface of the concrete, unless it has been left by something placed on the concrete, often indicate oxidation of the reinforcing steel.
Depending on the type of rebar used, how well the concrete is sealed, and how much moisture is introduced, reinforced concrete can last less than a hundred years. The American Concrete Institute (ACI) lists the factors that influence the thickness of the facing concrete to support the rebar. Curing concrete isolates it from oxygen, which is what causes steel to oxidize. That's why thicker concrete slabs are better suited to steel reinforcement.
This thin level of concrete cannot be achieved when exposed to the elements or in contact with the ground. Generally speaking, if you are making flat concrete slabs in the ground and no more than four to five inches thick, you don't need rebar. Rebar concrete is a (relatively) new approach with completely new structural strengths than just poured and dried concrete. In general, when laying flat sheets of concrete, you should make sure that the concrete slab does not float when it rains because it acts like a boat in the mud.
At the same time, it prevents cracks from traveling along a concrete slab, helping concrete withstand pressures it would have failed before. Reinforcing steel is used to reinforce concrete and reduce the chances of it cracking and breaking. When the concrete dries, this tension is released, further compressing the concrete, which helps to apply the tensile strength of the reinforcing bar to the concrete itself. If your project requires heavy machinery or vehicles, a hot tub or spa, or a pizza oven made out of concrete, then it is recommended to use rebar. For cast-in-place concrete that will be in permanent contact with the ground, 3 inches of cover concrete is recommended.
Contact with the ground or outside climate can increase this depth.