A concrete slab has to support the weight of the structure above it plus anything that may be placed on top of it after construction. Concrete is simply a mixture of cement, sand and stone that is mixed together and allowed to harden. Rebar -- or steel bars -- arranged correctly inside a concrete slab helps to reinforce the concrete and hold it together to prevent it from cracking and crumbling once it is dry. Rebar can be laid in different ways, but the grid system is a proven method. Rebar comes in various diameters and the thicker the rebar, the greater the strength of the reinforced concrete.
Lay the rebar out loosely in a grid pattern in the excavated, level pad inside the forms. For a driveway, start 15 inches in and make a grid of 18-by-18-inch squares over the entire area to be concreted. For a patio, which will not bear so much weight, it is acceptable to make the grid 24-by-24 inches. Start 6 inches in for footings and size the grid squares according to the width of the footings.
Tie the rebar pieces together at every place where they cross one another or join. Rebar tying wire and a pair of pliers are needed for this job. Alternatively, clip the rebar rods together with plastic rebar links.
Lift the joined rebar grid structure onto bricks or blocks loosely placed so that they raise and support the entire rebar network. The rebar should be in the middle of the slab. If the slab is to be 6 inches thick, lift the rebar to a height of 3 inches off the ground. Plastic rebar supports also can be used for raising the rebar.
Pour the concrete and allow it to dry. Be aware that concrete shrinks as it dries, which causes cracks to form. Sawing a grid line in the concrete while it is still not completely cured, known as green cutting, is the standard procedure to help prevent cracking in large slabs.