Concrete alone is capable of supporting a great deal of weight, but there are limits to how much a slab of concrete can take before it begins to crack or crumble under the load. You can raise the limit by adding a support structure to the concrete. A grid of rebar is a series of criss-crossing steel rods, placed into the centre of the concrete to hold the material together and distribute weight evenly across the slab's surface. To use, lay the rebar in place before pouring the concrete, then pour the concrete over and around the bars. When the concrete hardens, the steel support structure sits within, ready to provide the support your slab needs as the weight on top of the slab grows.
Measure the area of the concrete slab as well as the depth to determine the amount of steel rebar and support squares you'll have to buy. Purchase enough rebar to cover the area with a grid of the bars spaced 60 cm (2 feet) apart throughout the slab. Purchase enough concrete support stones to place one stone every 1.5 metres (5 feet) along the base of the foundation. Buy stone that's half the height of your planned slab.
Place the concrete support stones into the foundation hole atop any support materials such as a bed of gravel or a vapour barrier. Space the stones in rows and columns, all 1.5 metres (5 feet) apart and extending half the height of the foundation hole.
Lay the steel rebar into the foundation in a grid pattern. Use the concrete stones to hold the rebar at a consistent height that will place the bars in the centre of the slab. Arrange the bars in a crossing grid pattern spaced 60 cm (2 feet) apart, with the ends of the bar about 12.5 cm (5 inches) from the edge of the hole. Lay all of the bars in the hole running one direction, and then place the bars crossing those above the first layer.
Connect the bars where the two layers intersect with metal ties. Wrap the ties around the bars to hold them in place and then tighten the ties with pliers. When tied together, the bars will form a single grid across the top of the support stones.
Quickly cut through rebar when needed with a hydraulic bolt cutter.