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Precast Concrete Foundations

Precast Concrete Foundations

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    All Photos: SUPERIOR WALLS

    Panels are factory-cast under controlled conditions.

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    The gravel base must be well-compacted prior to panel placement.

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    The first panel is positioned and braced.

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    Corner joints are mitered while longitudinal joints are butt joints.

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    Panels can include window openings.

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    Panels are leveled and plumbed, resting on the gravel base.

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    Above-grade precast insulated panels can be manufactured in any configuration.

While most concrete contractors would prefer to cast concrete foundation walls in place, precast concrete walls have some attributes that make this technique appealing to building owners and builders. For concrete contractors, either erecting or actually casting precast panels could be a lucrative addition to what you can offer your customers.

The most popular precast foundation walls come as nearly finished panels, incorporating insulation and interior studs ready to accept drywall. On site, precast panels go up much more quickly than cast walls, including those constructed with insulating concrete forms (ICF). And since there’s no concrete placed on site, construction can proceed in virtually any weather without concern about freezing or wet conditions. Precast wall erection is also simpler than cast-in-place or ICF construction and can be done by low-skilled workers with only one trained lead. “We feel it is the future of residential foundations,” says Jim Costello of Superior Walls.

Insulated panels come in various configurations, depending on the owner’s requirements. The exterior face of the monolithic panel is 2-inch-thick concrete with integral 6 ¾-inch-deep concrete studs and reinforced bond beams, top and bottom. Standard wall panels have an insulation value of R-5 from 1 inch of rigid insulation attached to the concrete between the studs. Thicker insulation can increase that to R-21. Manufactured in the controlled environment of a precast plant, the panels are typically 8 to 10 feet tall and as long as 20 feet. The panels can incorporate window or door openings. There are no stock panels; walls are cast specifically for each project and each panel is cast with its neighbor to match exactly.

This system results in crack-free and waterproof walls. The panels are made with 5000-psi concrete with low permeability. Since the concrete is cast in a factory setting, all shrinkage has taken place prior to erection of the walls so panels virtually never crack in service. A broom finish is standard for the exterior of the panels, although other finishes are available, including simulated brick or flagstone textures.

No footer

The base for precast foundation panels is crushed, compacted gravel—no footing is poured. After excavation, 4 inches of ½-inch crushed stone is placed and compacted with a drainage system beneath. This base is adequate for most soils and loading conditions. “It can be designed to handle heavier loads if needed,” says Costello.

The panels are shipped from the factory on an A-frame trailer. Panels for a typical home can be shipped using two or three trailers. Erection is handled with a crew of three or four workers, including at least one lead worker who is certified in the process by the manufacturer. Erection requires a 40- to 50-ton truck crane, based on reach, that is located such that most panels can be placed without moving the crane. Panels are lifted with a specialized sling and strongback.

The first panel is placed and braced, then leveled and plumbed with a laser. Each panel has an 8-inch-wide concrete base and a similar 8-inch top for attachment of the sill plate for the above-grade timber walls and floors. The second panel is placed next to make a corner which eliminates the need for bracing. Corner joints are mitered, while longitudinal joints are butted. A triple bead of caulk is placed on the joints and the panels are bolted together with cast-in connectors at the top and bottom of the panel. The panels for most homes can easily be placed in a single day.

Once all panels are in place, a vapor retarder is placed and a 4-inch basement slab is poured. Prior to backfilling, both the slab and the floor joists or trusses must be in place. Some precasters also provide hollow-core concrete planks for floors. Waterproofing, drainage panels, or both is applied to the outside face of the panels where needed.

Above grade

Precasters also provide panels for above-grade walls. The Thermomass system is a sandwich panel that has interior and exterior concrete wythes with a rigid insulation interior, all attached with fiber-composite connectors.

Dukane Precast, Naperville, Ill., produces what they call double-wall panels intended to be used for an entire house—walls, floors, and even roof deck. Interior and exterior 2 3/8-inch-thick concrete wythes are cast, held together by wire trusses. Rough electrical and plumbing are installed in the cavity and the void is filled with foam insulation. Panels are connected with an infill of high-strength grout. Dukane claims that a medium-sized home can be erected in about five days, although that is preceded by several weeks of planning.

Superior Walls’ above-grade panels are virtually the same as the foundation wall panels. They have been used to construct up to three-story structures (a foundation wall and two levels above grade). Panels can be designed for any architectural feature desired.

The base cost of precast foundation walls is higher than a standard poured wall, but they come with all of the insulation and interior studs in place, not accounting for the other advantages of quick erection and low labor costs. Considering everything, “the walls are competitive or even ahead of the game,” says Costello. Superior Walls are manufactured in defined territories in the eastern half of the United States by licensed operators.