If you didn't pour concrete yet, you may want to plan for the building. Pour in some 3/8" threaded rod with the end in the concrete bent at 90degrees. Locate the bend so it is about an inch above the sand or gravel base. Put a nut on the rod about an inch down so you can clean the threads after pouring. Use a length of threaded rod long enough to leave enough threads above the concrete to attach a footer for your building (6inches is a safe bet). Also seal it well with epoxy if you don't want it to attract moisture.