An above-ground concrete storm shelter, despite its weight, must be attached to the ground. A simple solution is to use a precast shelter and attach it to a poured concrete pad. The manufacturer will deliver the shelter in a single piece or two halves with the hardware and adhesives your need to attach the shelter to the base.
Mark a 10-foot square on the grass where you want to construct the concrete pad. For this example a 10-foot pad is for and 8-foot square shelter.
Excavate the square to a depth of about 3 inches
Set 10-foot-long 1-by-4s on edge along each side of the square. Drive wooden stakes outside the square to keep the frame boards vertical. Find the lowest edge of the frame and use a level to level the edges of all the other frame boards. Nail the frame boards to the wooden stakes to hold them in place.
Lay the rebar pieces along the edges and parallel to each other across the framed pad. As you pour concrete around the rebar, lift the steel rods slightly to allow concrete to flow under the rebar and suspend it within the slab.
Pour the concrete starting at one corner and working across the slab. If possible, have the concrete truck pour directly into the frame. Otherwise you have to move it from the truck to the frame with a wheelbarrow.
With a partner holding the other end, use a 12-foot-long 2-by-6 to level the surface of the wet concrete by setting it on top of the wooden frame and sliding it back and forth while pulling to spread the concrete evenly in the frame.
Once the concrete is poured, let it set for 10 or 15 minutes. Then rub the surface in circular motions with a wooden float tool. If there are parts you can't reach, crawl out on a piece of plywood and clean any marks after you move on.
Wait till the slab begins to harden and, using the flat trowel, swipe the surface of the concrete in circular motions to flatten the surface. When almost dry finish by lightly brushing the slab with a broom.
Installing the Shelter
Measure the base of the precast storm shelter. Mark the outline of the base on the concrete slab.
Apply a thick application of epoxy to the area where the base of the shelter will sit.
Using a boom truck, have the operator lower the precast concrete shell onto the epoxy. Once lined up properly, let the epoxy cure for 24 hours. The epoxy seal will withstand more than 1 million pounds of lateral pressure when set.
Hang the steel shelter door according to the precast shelter manufacturer's instructions. Once set up, the shelter will withstand impacts of several tons.