The first two-unit (duplex) precast concrete home in the United States was built in 2003. Despite the novelty, the system is cost-competitive with traditional wood framing. Perhaps more importantly, the public is more apt to accept such a design for home building.
Research at the University of Nebraska-Lincoln (UN) that led to the development of the Nebraska University Concrete House began in 1990, with sponsorship from the U.S. Department of Energy, the Precast/Prestressed Concrete Institute, and other organizations. The incentive for the project was based on the advantages that precast concrete structures offered over traditional wood frame homes. These include material quality control, rapid erection, resistance to fire, high winds, and insect damage, and superior thermal efficiency.
Three new elements were the focus of the research efforts: a fully-insulated exterior sandwich wall panel system (patented by the University of Nebraska), a precast concrete floor joist panel system, and a roof under-girder framing system. Enterprise Precast Concrete of Omaha produced all of the precast elements.
Several plant-manufactured precast concrete panel housing systems had already been attempted. The system used in this house is unique because it utilizes a highly energy efficient 10-inch-thick sandwich wall panel. Special fiberglass bar connectors are installed between the two 2-inch wythes, allowing the panel to be fully engaged as a 10-inch-thick structural wall.
Six-inch-thick rigid board insulation establishes an excellent wall R-rating of over 30 because the meticulously designed structural wall connections prevent thermal bridging and loss of thermal efficiency.
The NU Concrete House offers significant competitive advantages over traditional wood construction systems for residential homes. These include:
Sandwich wall panel
- Material durability. Precast concrete housing provides exceptional long-term durability and requires little or no conventional home maintenance, such as painting, staining, weatherproofing, or aluminum cladding. Resistance to fire and natural disasters such as tornados, earthquakes, and flooding are benefits.
- Variety in architectural finish. Numerous architectural treatments are possible with precast concrete. Many high-quality surface textures, colors, and finishes are available with precast plant production methods. Permanent brick and “stucco” exterior finishes offer even more options for architectural designs.
- Sustainability and environmental friendliness. Precast concrete is an environmentally friendly or “green” material for construction. The thermal mass of concrete and the high insulation efficiency of precast concrete save non-renewable energy resources through lower fuel and electric costs. Also, a precast concrete envelope maintains steady indoor comfort .
- Open, flexible design. Concrete home framing establishes long open spans with loadbearing external wall panels, eliminating the interior support walls and columns of conventional wood construction and creating unobstructed interior spaces. This allows for maximum flexibility in interior design options and offers homeowners creative room layout potential, with minimal structural restrictions on future remodeling. It is possible to achieve ceiling heights of 10 feet or more at virtually no additional cost.
- Lower long-term maintenance costs. Because precast concrete exteriors maintain their just-built look with little maintenance and their operating costs are much lower than those of a conventional wood house, the concrete building retains high resale value over time.
The NU sandwich wall panel is both a totally composite and a fully insulated system. The NU Concrete House incorporates a sandwich wall panel consisting of two relatively thin concrete wythes and an in-between layer of insulation. During its manufacture, the insulation is placed to produce full structural composite action as well as establishing an uninterrupted thermal barrier between the wythes.
Concrete wythes in the NU sandwich panel are connected through the insulation layer with fiber reinforced plastic (FRP) bars that are specially formed. The wall wythe connectors are C-shaped FRP bent bars in the form of truss diagonals that run along the height of the wall panel. Lines of FRP connectors are placed at 2-foot spacing within the panel to transfer shear forces from one wythe to the other, thereby taking advantage of the full structural thickness of the panel.
In each concrete wythe, a welded wire reinforcement (WWR) sheet, designated as 3x3-D10xD10 with a steel grade of 550 MPa (80 ksi), is placed at mid-thickness. The panel is strong enough to resist winds exceeding 160 mph with no additional reinforcement.Floor joist panel system
The main components of the precast floor joist panel system are the thin longitudinal prestressed concrete joists held together with cross-ribs to form an open grid joist panel.