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Subgrade Preparation and Grading Remove the soil to such a depth that the final surface elevation of the slab will be slightly above the surrounding ground level, including any backfill you will add. Remove all topsoil, vegetation, wood, large rocks, and all soft and yielding material that will not compact readily. Level off and compact the loose soil with a tamper or heavy roller to a uniform density. Back fill with granular fills of sand, gravel, crushed stone, or slag then level it to a depth of 4 inches (or whatever thickness of concrete you are pouring) below the top of your designed grade. To keep your project from settling unevenly, you will need to compact the subgrade. For a job of any size, like a driveway or patio, you should rent a vibrating compactor. To determine how much backfill you will need to prepare the subgrade, use our aggregate calculator. FormingIt is important to set the correct grade of the concrete. Start by setting stakes in the corners and, using a string line and level, set the string at the desired height of the surface. Be sure to include a slight slope of 1/8" per lineal foot for proper drainage. Side forms are commonly made with 2x4-in.boards. Place them to outline the shape you have chosen. Now drive 2x4-in stakes into the ground at intervals of 4 ft, with the faces squared to your string lines. Stakes should also back up every joint in the form lumber. A stake can be used to lap the joints in the form to keep both ends in alignment. Now raise the forming lumber to the height of the string line and nail securely from the stake to the lumber. Use two-headed forming nails so that you can easily remove the forms. For short radius horizontal curves, use ¼" plywood making sure the grain is vertical. For longer radius curves, use 1 inch lumber or saw closely spaced cuts one-half to two-thirds of the way through the framing board. Wetting the lumber can make it easier to bend. Stakes should be closer together on curves to hold the form at proper grade and curvature, usually every 1 to 2 feet. On vertical curves, where the driveway changes slope, use short lengths of 2x4-in. board for forming. Use a string line tied to temporary stakes to outline the shape of the curve. Then lay forms to this line to give a smoother, more uniform grade change. Construction joints are used when the delivery of concrete will be interrupted. The construction joints should be located in the same position as control joints would be located. Isolation joints between existing structures should also be placed. When forms are set, check the subgrade to make sure it will accommodate the
desired thickness of slab. Pull a template, riding on top of the forms, across
the subgrade to get a smooth, uniform depth.
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