Three short years from now, concrete building systems will account for 25% of above-grade structural walls in single-family homes in the United States, according to predictions by the Portland Cement Association (PCA). That figure represents a doubling of market share in the decade beginning in 2000.
"It's been a little slow getting off the ground, but it's a market with a lot of momentum and a ton of potential," says Jim Niehoff, residential promotion manager for PCA. The concrete masonry (CMU) segment of the market remains steady at about 10%. The fastest-growing category, insulating concrete forms (ICFs), is expected to overtake CMUs by 2010, thanks largely to aggressive marketing by PCA and other industry organizations.
Meanwhile, attention turns toward the dark horse in this race: cast-in-place concrete homes. "Foundation contractors are looking at the success of ICFs and saying, 'why can't we go above-ground as well?' " says Niehoff. Contractors have been using removable handset forming systems for decades to construct residential basements in the United States and above-grade walls in many other countries.
"The rest of world builds houses with concrete or masonry, not lumber," says Charles Engelken, president of Wall-Ties & Forms Inc., manufacturer of reusable aluminum forming systems. Visitors from other countries may wonder why the U.S. housing market prefers wood-frame construction, he notes. "But cast-in-place housing means a new paradigm for us. It requires a lot of equipment and alliances between [concrete contractors] and builders. It's not something you can try once or twice-it's very capital intensive and there's a learning curve." The up side? "Instead of selling 50 or 100 yards in driveways and foundations, contractors can sell 300 or 400 yards," Engelken says.
A few contractors have built thousands of cast-in-place homes in the Gulf Coast region, particularly Florida, where the demand is driven by the need for structures resistant to hurricanes, termites, and mold. Because the initial investment in removable or reusable forming systems typically costs tens of thousands of dollars, cast-in-place construction is most cost-effective in repetitive applications such as townhouses, multi-family dwellings, and single-family developments using similar floor plans.
The advantage of this method lies with speed. A small crew can set and pour a unit or building level in one day. On the following day, the reinforced concrete is sufficiently cured and workers can remove the forms and proceed to the next unit or level.
Many forming manufacturers provide support to assist contractors with the considerable investment. "We look at the number of structures the contractor is going to build, do the form takeoffs, and fit him with a workable system specifically for his needs," says Jim Carty, vice president of distribution for Precise Forms Inc. "As we develop custom designs such as doorways, windows, and radius walls, we provide CAD drawings and training. We also work with an independent leasing company to offer financing."
Handset systems consist of modular aluminum panels on aluminum frames, 8 to 10 feet tall and 2 to 3 feet wide. The lightweight forms require only a few tools to assemble. Their simple application allows for fast-paced construction of exterior and interior walls, decks, stairs, columns, or the entire concrete shell in one monolithic pour. Shut-off plates can be used to create openings for doors and windows, eliminating the need for wood blockouts.
A typical forming system includes ledger forms, wall panels, deck panels, jack posts, beams that function as supports and a form surface, door and window bucks, deck beam forms, and stair forms. Installed at the top of the inside wall panels, the ledger form connects the wall and deck forms. Ledgers are available with a square corner detail, crown-molding pattern, or pitched versions for pitched-roof homes. Deck forms extend from the ledger to the beam forms, which have jack posts underneath for support. Walers at the bottom of the wall panels provide bracing after the forms are set.
Different forming systems use proprietary connection hardware for wall ties, window bucks, and electrical boxes. Some systems use attached, self-locking hardware that reduces accessory cost, setting time, stripping time, and labor cost. Each system also includes blockouts for HVAC and devices for easy attachment of insulation panels, rough electrical, and window and door frames.
The forming crew installs electrical boxes, conduit, plumbing, and mechanical sleeves as they set the wall forms. This method differs from that used with ICFs, in which crews cut openings for these elements after placing the forms.
The concrete walls can be insulated by positioning polystyrene insulation board onto one or both faces of the concrete or by positioning the insulation along the middle of the wall section and casting concrete on both sides. A 2-inch-thick layer of expanded polystyrene (EPS) or extruded polystyrene (XPS) is standard in warm climates, with up to 4 inches used in cold climates.
According to Western Forms, which markets systems accommodating each insulation approach, adding 2 inches of EPS foam on one side can increase the R-value from 2.5 to 11.5. Insulating both sides of the concrete produces an R-value of 20.5.
In systems with insulation on one face of the concrete, there is space within each form to insert insulation pieces as the formwork is assembled. Standard polystyrene sheets are cut to fit the form panels and notched along the edges to fit around the ties. Some systems use plastic connectors with flanges that hold the foam in place and become attachment points for installing window and door frames afterward. Another approach is to rely on the natural adhesion of the foam to the concrete. Wallboard, siding, or fixtures can be fastened to the wall on the foam side.
With the sandwich method, the foam sections are drilled at regular points. Some systems include foam with predrilled holes. Composite plastic pins inserted into each hole connect the two concrete layers mechanically when the forms are removed. The pinned foam sections usually are positioned along the centerline of the form cavity, but may be offset to create a thicker structural concrete section on one side.
The placement of the insulation varies according to the building's engineering requirements, climate conditions, and the contractor's choice. Each configuration has its pros and cons.Insulation on the exterior surface regulates and stores the thermal energy of the concrete. Insulation on both the interior and exterior surfaces maintains the thermal mass of the concrete wall longer, slowing the energy transfer and yielding a more consistent temperature. Hardware that secures the insulation panels to the concrete also can be used to attach exterior cladding.
Placing insulation panels only on the interior wall surface allows the use of formliners or architectural finishes on the exterior concrete surface. Having foam on the interior walls also simplifies placement of vent pipes and electrical boxes. Surfaces are finished typically with a skim coat of plaster followed by paint.
The center "sandwich" insulation arrangement allows for finished concrete wall surfaces on both sides. Steel trusses, which also provide horizontal reinforcement, hold the foam in place permitting both sides of the concrete wall to be cast together. However, the steel conducts heat and cold through the insulation.
When the forms are stripped, the exterior walls are ready for painting. Unlike typical foundation forms, wall forms leave very little seam lines between panels, producing walls that are exceptionally flat and smooth. Textured aluminum forms can be combined with smooth forms to add creative design elements to homes and apartments. Some forming systems accommodate direct attachment of finishing materials, eliminating the need for wood furring or other drywall preparation.