Whether you’re an unskilled novice or an experienced professional, there are several important things to remember when building a solid, durable concrete slab for a home foundation. A house foundation is the most expensive and most critical part of the entire house structure.
There are two basic types of foundations: slab on grade and foundation walls on footings surrounding a full basement. 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 4 inches thick) and a grade beam around the entire perimeter and under the load-bearing walls. Here are some helpful tips to ensure that each step of the job is done properly.
Step 1 - Prepare the Base
A 4 inch deep layer of crushed stone about ¾ inches in diameter laid over the undisturbed subsoil will provide a level base and will drain water away from the slab. Sheets of polyethylene plastic spread over the crushed stone will provide a moisture barrier.
Step 2 - Build the Forms
An edge form made from framing lumber is constructed around the perimeter of the slab and leveled at a height above the crushed stone equal to the thickness of the slab. The forms should be coated with a form-release compound (ordinary motor oil will suffice) so that the forms are easily removed when the concrete has set.
Step 3 - Install Reinforcing
A typical residential concrete slab is reinforced with welded wire fabric 6 x 6 – W1.4 x W1.4, which refers to the spacing in inches in each direction and the wire size number respectively. The reinforcing is laid onto the moisture barrier covering the crushed stone before the concrete is poured. As soon as the concrete is poured, the welded wire fabric is raised to the midpoint of the slab thickness.
A thickened slab or a grade beam is installed around the perimeter of the slab to support load-bearing walls. A grade beam may require horizontal steel reinforcing rods called rebar along its entire length. If necessary, consult an engineer to determine the size, spacing and the location of the rebar within the grade beam.
Step 4 - Install Expansion Joints
If the slab is larger than 100 feet in either direction, control or expansion joints made of fiber boards are recommended to prevent or minimize cracking.
Step 5 - Pour the Concrete
Concrete is either poured directly from a truck chute or spread manually with a wheel barrel and a shovel. If controls joints are installed, the concrete should be poured in complete sections and should not be dropped or poured from a height greater than 3 or 4 feet, or segregation will occur. The concrete is distributed evenly throughout the forms and should be rodded manually or vibrated mechanically to remove trapped air.
Step 6 - Finish the Slab
The first step in finishing the concrete is to draw a straightedge (usually a long plank) across the edge of the form-work in a back and forth motion to produce a level but rough surface. If a smoother finish is preferred, the slab is then floated with a flat wooden float for a finer, textured surface.
The next level of concrete finishing is called troweling, which is done several hours after the concrete is poured and gives a dense, solid, almost polished finish. Both floating and troweling can be done either manually or with rotary power tools.
Step 7 - Allow Time to 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. The concrete will attain its final set in about a week, at which point the forms can be removed. It will reach its design strength in 28 days, after which it will harden at a much slower rate.