The foundation of a house is the most expensive and critical element of the home's entire structure. The design and construction of a concrete foundation should only be done by a professional mason or a general contractor, who may also have to consult a licensed engineer. But the average homeowner can easily understand what goes into building a solid foundation.
Slab on Grade Foundation
A slab on grade is used in warmer climates where it is not necessary for the foundation walls to extend down below the frost line. It consists of a concrete slab (typically four inches thick) and a grade beam around the entire perimeter and under the load-bearing walls.
Full foundations consist of two primary components: the foundation wall and the footing on which it rests. The depth of a footing is the same dimension as the width of the foundation wall, and the width of the footing is twice that dimension. If a full basement is constructed, the slab acts as the basement floor and will have square footings or “pads” under columns, which in turn support girders. Concrete foundations are sometimes reinforced with metal rods or rebar. Footings will have reinforcement running horizontally, foundation walls will have vertical rods penetrating the footing, and slabs will be reinforced with welded wire mesh. All reinforcing is set in place after the forms have been built and before the concrete is poured.
Excavate to a depth of the bottom of the footing and dig trenches around the perimeter the size of the footings. Dig for column footings and grade beams also. Footings are sometimes poured directly into the trench without forms. A layer of crushed stone will provide a level base for the slab.
Build the Forms
The foundation forms can be built with framing lumber and later used in the construction of the house frame. The foundation wall for a typical residence can be 10 inches thick or greater, but an eight-inch thickness will usually meet the requirements. If the house exterior is to be finished with brick, the foundation wall thickness must be extended four inches to provide a brick ledge. All forms should be properly joined and securely braced and should provide openings for sleeves and pipes entering the basement. The length of horizontal form boards should be multiples of two feet, or plywood sheathing can be installed in sheets four feet wide. 2x4 vertical bracing is installed at every two feet along the wall, which is then braced diagonally. Wall forms are supported with spreader blocks and tie wires to ensure a uniform thickness once the concrete is poured. Anchor bolts are suspended in place along the top of the foundation wall as well.
Pour the Concrete
Concrete should be poured in whole sections and it cannot be dropped or poured from a height greater than three or four feet, or segregation will occur. Concrete should be spread evenly throughout the forms and should be rodded manually or vibrated mechanically to remove any trapped air. The slab is poured to a depth of four to six inches over a plastic moisture barrier sheet, and a fibrous expansion joint is installed where the edge of the slab meets the foundation wall. It is important to level the top of the foundation wall at the proper height.
Let it Cure
During the curing process, concrete should not be allowed to become too hot (above 80 degrees) or too cold (below freezing) and should be kept damp or moistened or it can crack. The concrete will attain its final set in about a week, and reach its designed strength in 28 days. After a week the forms can be stripped from the foundation, and the boards and studs can be cleaned and reused if they are removed undamaged.