Tornadoes occur in every state in the United States and on every continent, although none have been reported in Antarctica. An aboveground concrete storm shelter incorporated into a barn, the garage or placed next to the house provides safe, temporary shelter from storms. The structure must be sturdy enough to withstand high winds, so building on a concrete slab provides a firm base for anchoring the walls. Aboveground shelters are typically immune from damage from other hazards created by high winds, such as flooding and destruction from windblown debris.
Push a dowel or stake into the ground at each corner of the future slab; run a length of twine between them, pulling it taut and creating a string line. The tensile strength of the twine is unimportant because it's only used to mark the sides of the excavation. Use a shovel to dig out the area inside the string line to a depth of four inches and tamp the bottom smooth. Tampers, flat pieces of metal with wooden handles, are available at home improvement stores and tool rental outlets.
Build a form for the concrete slab from two-by-four inch lumber. Measure the inside edges of the excavation. Cut the lumber to match that measurement and nail the lumber together with the longer edge of the wood perpendicular to the ground. Place the form in the excavation.
Use a string level to make sure the form is level. Stretch the string from corner to corner and side to side. Adjust for height variations by building up or digging out the earth under the form until it is perfectly level. Backfill behind the form if it doesn't rest securely in place.
Prepare the wire reinforcing mesh. Measure the inside of the form and then subtract 3 inches from each dimension. Use bolt cutters to cut the mesh to fit inside the form. When the mesh is laid out, it should be 3 inches from the form on each side. If you need to cut the mesh into pieces to make it fit, use wire to join the pieces. Space bricks evenly across the bottom of the excavation and lay the mesh on top of them.
Dampen the soil at the bottom of the excavation. Begin pouring concrete. For a large pad, order a cement mixer and a load of concrete. Use a concrete calculator to determine how much you'll need. Typically, multiply the width, length and depth of the area inside the form; divide by 27 to determine how many cubic yards of concrete are needed.
Pour the concrete into the mold. Use a bow rake to spread the concrete into the corners; bang on the form with a hammer from time to time to create vibrations that smooth the concrete. Once the form is filled, drag a flat board across the surface to smooth it.
Allow the concrete to dry. How long it takes depends on the weather, but expect to wait at least 24 hours for it to dry and several days for it to cure. Once the concrete dries, remove the form and backfill around the edges to make a smooth transition from the slab to the ground.