Steel adds enormous strength to concrete. Concrete is strong in compression (squeezing) but weak in tension (pulling apart). You can pull concrete apart quite easily. For example, let's pour a slab on fill dirt. The fill settles and a hollow spot develops under the slab. Now let's drive my truck across the slab. The concrete stretches (pulls apart) to fill this hollow spot.

Put steel into the slab and it may take a tank to create the same effect. The reason is simple. The typical steel you buy at the supply house is almost always grade 40 steel. This means that it takes 40,000 pounds per square inch (PSI) of pulling pressure to tear the steel apart. Your slab, on the other hand, has only 1/10th the strength in tension as compression. Typical residential outdoor concrete is supposed to be 4,000 PSI in compression. This means it only takes 400 PSI to crack or pull it apart. Adding enough reinforcing steel can make the concrete 100 times stronger than without the steel!

I always had fantastic results by constructing a grid of steel in my slabs. By placing the steel bars two feet on center in both directions, I created a cage of steel. This steel must be surrounded by concrete. If you simply place it on the dirt, gravel or soil it will be worthless. The tension forces in a slab are concentrated in the lower third of the slab, so you should place the steel at the top of this boundary.

If you want to get fancy, you can purchase little supports that suspend the steel above your fill material. I usually used small broken pieces of brick or concrete block to support the steel. If your slab is larger than the length of the steel bars, be sure to overlap the steel at least two feet and tie the steel together with thin wire.