Precast foundation walls simplify basement construction, save time and money, and result in warmer, drier basements. Precast foundation walls are comprised of individual concrete panels that are manufactured off-site using high-strength concrete. Each panel contains solid concrete studs for load-bearing support and a built-in concrete footing. Some panels include rigid insulation to keep the basement warmer, and rebar and polypropylene fibers for strength. Walls can be customized by project to allow for door and window openings, steel beam pockets, and brick ledges. Manufacturers offer other convenience features including built-in access for wiring and small plumbing and wood or steel nailers for easy drywall installation. Panels range in size from 2-12' wide by 8-12' high to accommodate a variety of basement configurations.
Additional features and benefits of using precast foundation walls include:
- Time and labor savings. A typical precast concrete foundation can be erected in 4-5 hours; about one-sixth the time needed for a formed concrete wall. The panels can be erected in cold temperatures, which means weather delays are not an issue. And, there is no on-site curing time—the walls can be backfilled as soon as the slab and first-floor joists are in place. This quicker, easier assembly not only saves time, it also reduces labor costs.
- Stronger, lighter, safer walls. Precast concrete panels are manufactured in temperature-controlled environments, which enables manufacturers to produce concrete mixes that harden to 5,000 psi. This is stronger than the 2,500-psi concrete in standard poured walls and stronger than concrete block, yet the precast panels are thinner and lighter weight. Precast panel systems also are more resistant to fire, termites, dry rot, and tornado and hurricane damage than other foundation types.
- Better indoor comfort. Because many precast concrete panels are formed with integral rigid insulation, little or no additional insulation is needed to achieve a warm basement. Precast panels also are low maintenance and have better sound-proofing characteristics than other foundation types.
- Water-resistant construction. The high-strength, low-water concrete mix used for precast panels is denser than typical concrete, making it more water resistant. This helps keep basements dry without additional waterproofing. In regions with high rainfall, however, local building codes may require additional waterproofing.
Cost Considerations
Precast concrete systems generally cost between $55 and $65 per linear foot of wall, making them competitive with other foundation walls. These systems can be installed rapidly; however, installers who are not familiar with precast concrete panel assembly will need additional training. Some manufacturers only allow certified installers to deliver and erect their systems. Finally, a precast concrete foundation is best used to construct a full basement with consistent sill height. Different wall heights slow the process, minimizing the cost benefit.
Installation Basics
Constructing a basement using precast foundation walls includes preparing the gravel base, placing and connecting the panels, installing a capillary break, installing the flooring, and backfilling.
After the site is excavated and perimeter drain pipe is laid, a bed of clean, compacted gravel is prepared over the drain pipe. The gravel facilitates sub-slab drainage; water drains down the panels, through the gravel bed, and into the perimeter drain system. The gravel also transfers the load from the foundation wall to the surrounding soil. The gravel layer will be 4-12" thick, as determined by a soils engineer, to meet the foundation's load-bearing needs.
Next, a crane is used to place the first concrete panel. It's braced, and a second panel is placed to form a corner. The joint is sealed with a urethane sealant, and the panels are bolted together. Additional panels are placed in the same fashion, usually without needing any bracing. All corner joints are mitered and all other joints butt together. Once all panels are placed, sealed, and bolted, any code-required waterproofing is applied.
Third, a 4-6" layer of gravel is spread over the slab footprint. A 10-mil vapor barrier is installed on top of the gravel layer, covering the entire footprint of the foundation. Together, these form a capillary break between moisture in the soil and the underside of the basement slab.
Finally, the basement slab is poured and floor joists are installed above the foundation. Both of these provide bracing for the foundation walls before backfilling begins—the concrete slab anchors the bottom and the floor framing locks the top. The foundation is then backfilled and can be considered complete.
To learn more about Precast Foundation Walls, visit:
www.toolbase.org/Technology-Inventory/Foundations/precast-concrete-panels
www.superiorwalls.com/
www.superiorwalls.com/pdf/Energy_Smart_Foundation_in_Two_Days.pdf
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