What is a "Concrete Slab" foundation?

Shallow foundations such as slab-on-grade are also know as slab-on-grade, spread footings and mat-slab foundations.

Concrete slab foundations are usually built in areas where freezing soils are not common.  A slab foundation is a concrete and steel structure placed over a moisture barrier and the house's plumbing systems and allowed to mold to the compacted ground below.  Footings are required to help stabilize the structure over time and fight against the ongoing and changing pressure of the ground below as moisture changes take place in the soil. Footings for a slab should be numerous and be dug to a depth where load-bearing materials are found.  Forms, usually made of wood are constructed to hold in the concrete as its poured to mold the foundation.  Once the foundation begins to cure and is considered "Green", the forms are removed.  Later, a plaster coat is placed over the edges of the foundation to cover up the impressions left by the forms and make the edges of the slab more attractive.

Since plumbing is installed under the foundation, there is substantially more difficulty in accessing these items in case of a repair.  In addition, the copper plumbing supply lines must be protected by sleeves to prevent the corrosive components of the concrete from making contact.  Over time, if steel or concrete are in contact with copper, corrosion or dielectric reactions occur causing deterioration and eventual leaking may occur. Another problem related to slab foundations is the difficulty to repair or level slabs once excessive movement occurs or failing sections break away. 

A slab foundation has the advantage of warmer floors in the winter and a good choice when expansive clay soils are not an issue on the construction site.  However, even in Texas and California where expansive clay soils are prevalent, the cost to performance ratios still show the benefit of using lower cost concrete slab foundations in theses areas.

There are two basic types of concrete foundations used today.  The mild steel foundation is usually used on custom or higher quality homes.  It involves engineered drawings of specific placement of steel components and other criteria.  Post tension cable foundations are generally used on track homes and rely on steel cables in plastic conduits.  After the concrete is place in the forms, tens of thousands of pounds of pressure is used to tension the cables and hold sections of the foundation from failing if cracking takes place.

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