How to Design a Concrete Slab on Grade

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A foundation is an extremely critical part of the building; it is the first part to be built, and everything that comes afterward depends on it. Foundations come in many types, and each one has its strengths and weaknesses. Concrete slabs on grade are widely used for residential, single-family structures where the foundation is not subjected to heavy loading and is in warm climates. Sometimes these slabs are adapted to cold climates by insulating their edges. Slab on grade is a misnomer, since these types of foundations invariably include turned-down edges similar to, but not as deep as, a stem wall on a foundation for a cold climate.

Find out the soil type where the slab will be constructed. Check with people who have slab foundations in the area to find out what soils they encountered. In many cases, it is a good idea to hire a geologic engineer to assess the soils and make recommendations regarding the correct foundation to use. Soils with high clay concentrations need to have the water retention tendency of the clay mitigated with a layer of gravel between the clay and the slab.

Check the soil conditions. Note any surface drainage that may cause undercutting to the slab so you can plan on either locating the slab out of the path of the water or rerouting the flow to avoid the slab. Stony soils may limit the depth that the edges of the slab can be turned down, and that may affect the location of the slab if surface drainage poses a threat of undercutting the slab. Soils with high water content will require drainage along the perimeter of the slab.

Find out the local building code requirements for foundations. The codes will specify the maximum depth of the turndowns on slabs, the insulation that should be used if adopting the slab to a cold climate, and sometimes even the soil preparation necessary for slab installation. These codes will also specify any special requirements for seismic and wind loads in your area.

Determine the building loads by consulting the building plans. Wood-framed, single-family homes of two stories or less will generally be well suited to 4-inch-thick slab on grade foundations. Buildings that will house heavy equipment will require thicker slabs and may also require stronger concrete.

Set the excavation depth for the turned-down edges based upon the soil type, soil conditions, building code requirements and building requirements. Use the same information sources to also determine the type, size and amount of gravel for below the slab and the turned-down edges.

Determine the slab reinforcement. Use three or four continuous #4 rebar all around the perimeter of the slab turn-downed edge at a minimum, but final determination will depend upon local building codes. For slabs with light loads, as those for homes, include wire mesh midway up the thickness of the slab to reduce cracking. For heavier loads consider using #4 rebar laid out in an 18-inch by 18-inch grid midway in the slab.

Determine the concrete strength for the loads anticipated based upon seismic, wind and building loads. Concrete of 3000 psi is widely used for residential.

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