Precast concrete foundations gain ground in the residential housing market.
By Kip Heekin
Kip Heekin is president of Tri-State Superior Walls, Lebanon, Ohio.
Precast concrete has continually proven its value and cost effectiveness over the years in a wide variety of applications. We have grown accustomed to seeing precast concrete wall panels, box culverts, septic tanks, pipe, engineered slabs and a multitude of other concrete products. One exciting application that is gaining momentum is precast concrete foundations – using precast concrete panels below ground.
In what has traditionally been a block or cast-in-place concrete market, precast concrete foundation systems are becoming more common as architects and engineers realize the many benefits of precast concrete.
Most precast concrete foundation systems are based on variations of thin-wall systems. These are newer precast concrete wall systems made up of a stud and cavity wall designed to handle the loads of residential construction. The panels tend to be lighter and more cost effective to transport than a solid concrete wall and can use up to 60 percent less concrete than a traditional cast-in-place concrete foundation. These precast concrete foundation systems get their strength from a minimum 5,000 psi, fiber-reinforced concrete mix, rebar-reinforced concrete or metal studs, and foundation connections to the floor deck designed to transfer the lateral loads from the backfill pressure through the first floor deck.
In addition to superior strength, the concrete mixes used in precast foundations have the added benefit of being less permeable to water, and many systems qualify as an alternate acceptable method of damp proofing to satisfy the residential building code. In some cases, no additional damp proofing spray is required.
Another benefit of precast concrete foundations is that most if not all of the systems are insulated or provide an easy way to add additional insulation if desired. This energy efficiency is a great feature as oil and natural gas prices continue to soar. Precast concrete foundation systems may include glass board already installed or studs ready for hanging drywall, another source of time and material savings for a builder or owner who wishes to finish the basement.
As pre-engineered systems, each precast concrete foundation system has its specific design criteria and rated capacities for a maximum uniform load on the top of the wall. The systems have different means of accommodating concentrated point loads from trusses or steel beams that exceed the maximum uniform loads.
The systems can accommodate a wide range of design options, as each job is custom-made based on the project’s plans. Precast concrete foundation systems can be used on virtually any foundation layout, from a simple four-corner box to the most complicated radius bay configuration. The systems support a variety of standard wall heights and nearly any length to accommodate everything from a crawl space to an entry-level house to a high-end custom home. Window and door openings, beam pockets and other features can be built into the panels where specified, allowing virtually unlimited design flexibility.
While the manufacturing process for precast concrete foundation systems is similar to other precast concrete operations, it is obviously quite a departure from traditional cast-in-place foundations. With precast foundations, the wall panels are laid out in horizontal forms to the required specifications, and the forms can be calibrated on a regular basis to ensure the panels are level and plumb.
A plant’s quality assurance program ensures panel measurements are within tolerances, reinforcing steel is in its correct locations, concrete mixes meet specifications consistently, curing is performed correctly and documentation maintained. Many precast concrete foundations are made in certified plants, which also have third-party inspections to verify compliance with the plant’s quality assurance program.
The controlled factory environment provides an excellent curing environment for the concrete that helps ensure a consistent, high-quality product and reduces cold joints, honeycombing, shrinkage cracks and other problems common in the field.
Precast concrete foundation systems may be placed on crushed stone or traditional concrete footings. The hole is excavated and inspected by the local building department if footer inspections are required. Then 4-inch perforated drain tile is run around the perimeter of the hole and at least 4 inches of clean, 1/ 2-inch crushed stone is installed throughout the hole. At this point the site is ready for installation. The walls are delivered on trailers to the job site and usually installed with a crane by an installation crew. The panels are bolted together and the joints are sealed with a polyurethane sealant. Because of the controlled conditions in which they are cast, installation of the wall panels on site can typically be done to tighter tolerances than cast-in-place foundations.
While precast concrete foundations increase the speed of construction throughout the year, winter time is when the products really shine. Walls can be poured during even the coldest days of winter, and as long as the subgrade and footer are protected from freezing, the wall panels can be installed in temperatures well below acceptable levels for placing concrete on site. With precast concrete foundations, construction becomes a year-round business in even the coldest climates.
The wave is building
At one point, nearly every component of a new house was built on site: foundation, framed walls, roof, kitchen cabinets, etc. However, contractors and specifiers have come to recognize that centralizing production in a controlled factory environment removes a number of costly variables present in field manufacturing. Roof trusses and panelized walls came about as a result of this realization. The foundation is the last component of a new home that has not yet completed the evolution to off-site production.
However, that is changing as the residential construction market begins to recognize the many benefits of precast concrete foundations. In the past 25 years, more than 70,000 homes have been built in the United States with precast concrete foundations. The last two years alone account for more than 20,000 of that total. Builders are recognizing that precast concrete foundations help them shave days, even weeks from their construction schedules and provide damp proofing and insulation benefits not available with a poured-in-place foundation. It appears precast concrete foundations are well-positioned to continue gaining market share and completing the evolution to off-site production in new home construction.