Concrete is a composite material of cement, aggregate, and water that hardens over time into a durable building material. Foundations are the structural base that transfers building loads to the soil. The choice of foundation type depends on soil conditions, frost depth, building size, and climate. Proper foundation design and construction are critical for building longevity, as foundation failures are expensive and difficult to repair.
Slab-on-grade foundations are a single pour of concrete, typically 10 to 15 centimeters (4 to 6 inches) thick, reinforced with steel mesh or rebar. They are suitable for warm climates, garages, sheds, and greenhouses. A gravel base of 10 to 15 centimeters (4 to 6 inches) is required for Drainage. Slab foundations are the most economical option.
Crawl space foundations use a concrete perimeter wall 0.6 to 1.2 meters (2 to 4 feet) high, with the interior floor supported by piers or beams. The crawl space provides access to plumbing and electrical systems and is vented or sealed depending on local code. This foundation type is common in areas with moderate frost depth.
Full basement foundations have concrete walls extending below the frost line, typically 2.4 meters (8 feet) or deeper. Basements provide additional living or storage space and are common in cold climates. The walls are reinforced with steel rebar, and waterproofing is applied to the exterior. Drainage tile at the base of the wall carries water to a sump pump or daylight drain.
The standard concrete mix for foundations is 1 part Portland cement, 2 parts sand, and 3 parts coarse aggregate by volume, with water added at a ratio of 0.45 to 0.50 parts water to cement by weight. The mix is poured continuously to avoid cold joints, and air is removed by vibrating the concrete. The concrete is cured for a minimum of 7 days, kept moist to achieve full strength. Cold weather pouring requires heated water and insulation blankets.
Steel reinforcement is required in all structural concrete. Rebar sizes are specified by diameter in eighths of an inch: a number 4 rebar is 1.27 centimeters (0.5 inches) in diameter. Rebar is placed in the lower third of the slab for bending resistance and in the center of walls for structural support. The spacing is typically 30 to 45 centimeters (12 to 18 inches) in each direction. Welded wire mesh is used for slabs on grade and light-duty applications.