Concrete slab foundations are common in mild-weather regions of the United States, such as southern California. They also are used as basement slabs in harsher climates. Flooring material such carpet, wood and tile are often laid directly on concrete floors, but concrete works as flooring material either as is or with additional treatment and coloring. In all cases, concrete floor must be in accordance with building regulations that guarantee durability and safety.
The United States contains thousands of local and state building authorities, each serving its own population. Nearly all the jurisdictions base their building codes on the International Residential Code (IRC) or International Building Code (IBC) managed by the International Code Council. The nonprofit organization was founded in 1994 to consolidate duplicate and conflicting regulations from three regulatory models that existed at the time. Because of the IRC and IBC, building professionals and homeowners can rely on the same methods and standards for construction no matter where they are in the country, instead of having to learn a new set of rules at every location.
The IRC defines the necessary strength of concrete floor slabs in pounds per square inch (psi), which is the amount of force that weight exerts on every square inch of floor. Allowable minimums depend on the location of the concrete as well as the type of weather. For example, concrete floors used as basement slabs or in interiors need a minimum of 2,500 psi. Weather is irrelevant in those uses of concrete because the floor is protected. For concrete floors used in porches, the minimum is 2,500 psi only in mild-weather climates. Minimums rise to 3,000 psi in moderate-weather areas and to 3,500 psi in severe-climate regions.
Both the IBC and IRC set the minimum thickness of concrete floor slabs laid directly on the ground at 3.5 inches. The IBC also requires a 0.006-inch polyethylene vapor retarder, or barrier, between the subgrade or base course and the slab. This barrier needs joints that overlap a minimum of 6 inches. If other approved equivalent methods can prevent vapor from seeping through the slab, they can substitute for the barrier. The vapor barrier is not required in garages, utility buildings, storage spaces or other unheated attached structures meant for three or more families. It also is not required in areas where moisture penetration into the concrete does not harm the building's occupants.
The IRC states that the fill material for the site on which the concrete floor is laid must not have vegetation and foreign material. The depth of the fill material must no more than 8 inches for earth fill and no more than 24 inches for clean sand or gravel fill. If the slab is below grade, the prepared subgrade requires a 4-inch-thick course. This course must be made of clean graded sand, gravel, crushed stone or crushed blast-furnace slag that was filtered through a 2-inch sieve. As the concrete is laid, reinforcing supports are needed from the center to the upper one-third of the slab.
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