Concrete foundations must be poured with enough strength to the support the entire weight of the structure built on them. Adding rebar to a concrete slab is a way to create the strength required. Rebar is a reinforcing bar, placed in a grid-like arrangement throughout the slab. The placement of a rebar grid holds the slab together and transmits the load of the supported structure evenly along the slab's surface. This even distribution of a structure's weight ensures that small sections of the slab aren't required to hold the weight of an entire wall alone.
Arrange a series of concrete support stones along the bottom of the foundation excavation. Concrete support stones are square concrete stones of a uniform height that support the rebar at the necessary height while the concrete is poured. Choose stones that are half the height of the planned slab. Place a stone every five feet in the excavation.
Place the rebar rods into the foundation atop the support stones. The grid should be placed with a bar spaded every two feet, starting two to five inches from the edge of the planned slab. Place a single layer of parallel bars, and then place the second layer of bars perpendicular to the first.
Place metal ties around the crossing points of the two layers of rebar to create a single grid. Twist the metal ties around the crossing bars using a pair of pliers.