What Makes Up a Solid Concrete Foundation?

When it comes to constructing a house, the foundation is one of the most important elements. The most common type of foundation is a concrete slab, which is typically four to eight inches deep and reinforced with steel bars known as reinforcing bars. Basement foundations, on the other hand, have underground concrete walls that support the structure above the ground. For Tom Silva, a renowned builder, “good” foundations are those made of poured concrete and reinforced with steel.

In comparison, all the laboriously assembled foundations of stone, brick and mortar that have been used for centuries, as well as the concrete block walls that were popular when This Old House was launched 25 years ago, are now considered outdated and prone to cracks and leaks. The footings are an integral part of building foundations. They are usually made of concrete with reinforcement from reinforcing bars that have been poured into an excavated trench. The purpose of the footings is to support the base and prevent it from settling.

Footings are especially important in areas with problem soils. For foundations in areas with certain types of clay soils, caissons (or deep holes filled with concrete) are often drilled into the bedrock. Then the base is placed on them instead of footings. This allows for expansion and contraction associated with certain soils.

Poured and block bases sit on concrete bases or poured pads that serve as the basis for walls. The footings are built in trenches dug below the basement floor level. These trenches are wider and longer than the walls they support and function as feet to distribute the weight of the wall and structure above it. The footings provide a firm surface to resist sinking or shifting to the ground or substrate.

A standing trench ranges from six inches to three feet deep, depending on the size of the building and the characteristics of the soil. Concrete is a mixture of crushed rock (aggregate), cement, water and air. The addition of steel bars (“reinforcing bars”) makes it reinforced concrete. Concrete resists compression (thrust forces) and steel resists tension (tensile forces), so the combination of concrete with steel bars makes the perfect joint to cope with the compressive and tensile forces of the weight of a building.

Unfortunately, some old houses may have foundations made only of concrete, without reinforcement. Even worse are foundations made only of bricks, without steel or concrete. Both types of foundations will suffer disproportionate damage from an earthquake tremor. Like full basement foundations, basement walls are usually made with poured concrete or mortar concrete blocks. When Tom Silva is building a house, he wants foundation walls that are plumb and level and free from discolorations that are signs of weak concrete.

Concrete walls must be created as a continuous pouring to ensure a good bond and prevent cracking at seams where a first layer of concrete has already set. You may need a concrete base for a fountain, patio furniture or even an air conditioning unit. Building a good foundation requires much more than digging a hole and pouring some concrete into forms. They are supported by a concrete base; both are reinforced with steel rods and the concrete blocks are filled with grout. A notable disadvantage of slab foundations is that water supply and drainage pipes are lined in concrete.

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