Above: Placing panels of expanded polystyrene foam with reinforcing mesh on each side is easy and fast. Low velocity concrete is sprayed on both sides of the panel to construct walls that can handle both seismic and high wind forces. Middle: The worker shown here is melting EPS foam to provide a bond beam for a deck. Bottom: Setting panels for a ceiling.
Walls of any shape or size can be built with sprayed concrete techniques. Engineered walls can be built to resist hurricanes, tornadoes, earthquakes, fires, insect infestations, and, like all other concrete home building systems, sprayed walls require little energy to heat and cool. One wall system that makes it easy to build walls with low velocity spraying techniques was invented by EVG in Austria in the early 1970s. In the United States it is sold as the Tridipanel System.
Until recently both low velocity and high velocity sprayed concrete were called shotcrete. The American Shotcrete Association, Farmington Hills, Mich., however, recently decided that low velocity sprayed concrete didn't fit the definition of shotcrete. The primary difference is that the concrete is more compacted and densified with high velocity application. The system described in this article uses low velocity sprayed concrete.
ELEMENTS OF THE SYSTEM
Falling under the classification of sandwich panels (the insulation is sandwiched between concrete on either side), Tridipanel panels start with either a 1- or 2-pound density expanded polystyrene (EPS) core. The EPS can be ordered in panels between 2 and 5 inches thick. The R-value of 2-inch-thick EPS is approximately 10, with each additional inch in thickness adding an R-value of 5. So, for example, a 5-inch-thick panel of EPS would have an R-value of 25.
On each side of the EPS panel there is a 2-inch grid of welded wire mesh. There are also choices regarding the wire in the mesh. It can be galvanized or smooth steel and fabricated with 11, 12.5, or 14 gauge wire. Connecting the mesh on either side of the EPS are 9 gauge wires, which pass diagonally through the insulation. The wires hold the mesh 5/8 inches away from the EPS. Rod Hadrian, president of Hadrian Construction, Vista, Calif. explains that the diagonal wires provide a truss-like effect between the layers of mesh.
Typical concrete for the system is a mixture of portland cement and sand. Hadrian recommends using 3.5 parts sand to 1 part cement to produce a minimum strength of 2500 psi.
SETTING EXTERIOR WALLS
The process starts with concrete footings that have rebar dowels protruding on 2-foot centers. Hadrian tries to position the panels, which typically weigh about 1½ pounds per square foot, so that the bars are between the wire mesh and the EPS. “Sometimes I melt a little of the foam away in order to do this,” he says. “When concrete is sprayed it must completely surround the rebar.” With one panel in position and braced vertically, the adjacent panel is set. Next, workers tie a piece of 12-inch-wide wire mesh across the joint between panels to secure them to each other. At inside and outside corners the 12-inch wide mesh is bent around the corners and secured in position. Hadrian says that he uses a “hog ring” tool to tie the wire, which makes the process go much faster.
With the exception of positioning window and door bucks and setting steel for bond beams, walls can be set up very rapidly. Standard wood bucks are inserted for windows and doors and when a bond beam is needed some of the foam at the center of the panel is removed and rebar is positioned in the void. If an above-grade concrete deck or floor is supported by walls, workers remove some of the foam at the intersection of the wall and floor and add rebar as specified by the engineer.
With low velocity, concrete is sprayed on a wall surface. Workers then float it, strike it off to achieve a flat surface, and trowel the surface to achieve a smooth finish if it is required.
Finding contractors to spray the concrete, who are focused less on volume work and more on precision placement is important, says Hadrian. He prefers to hire plastering and stucco contractors because they are usually more willing to go slowly, making sure that concrete is placed at an even thickness of 1½ inches. After the concrete is sprayed on, workers strike off the surface to make it flat. A stucco finish is often used on outside surfaces and a slurry coat of silica sand and plastic cement provides a good inside wall finished surface. He adds that the concrete, shotcrete, mortar mix, or gunite mix can be applied in different thickness, the minimum being 1½ inches.
An all-concrete home is currently being constructed on an island in Mahopac Lake in New York that was designed by Frank Lloyd Wright in 1950 (see March 2006 issue of RESIDENTIAL CONCRETE). The builder, Lidia Wusatowska, used Tridipanels in unusual ways for some of the exterior walls and as deck forms in other locations. “In one room of the house, the plans called for a wood panel ceiling,” she said. “We pushed wood pieces between the mesh and the foam on one side of the Tridipanels and then supported them with shores, using them as forms for the concrete deck. The deck was reinforced with rebar and post-tensioning reinforcement was placed in several inches of concrete. After the curing period we attached the finished wood ceiling to the wood pieces under the mesh.” The same technique was used for some walls, except that the outside face of the panels was sprayed with concrete. When spraying concrete she preferred to use a concrete mix that included 3/8-inch pea gravel. Pea gravel mixes can make it more difficult to install smooth, straight walls and they are difficult to use for spraying ceilings.
As you can see in this photo, all kinds of shapes are possible.
Like all systems for building concrete walls, the final cost has a lot to do with the skill and experience of the work force installing it. Wusatowska says she considers the panels to be inexpensive and the installed cost is reasonable if workers learn to be more productive. Smoothing the sprayed concrete wall surfaces was where they experienced a learning curve.
Hadrian says that costs in his area average $3 to $4 per square foot for the panels, depending on the thickness of the foam and the gauge of the wire ordered. He estimates that the finished cost for walls is $14 to $20 per square foot, depending on the surface finishes chosen; ceilings and roofs are more.
INCLUDING CONCRETE WALLS
The values and benefits of concrete walls far exceed other material selections. Wusatowska says that it was easy to feel the difference in temperature between the Tridipanel walls and other wall materials. Owners notice significant savings in their energy bills. Hadrian adds that 40% of the homes they build also include concrete interior walls for sound control. And all concrete wall systems can be engineered for wind and seismic resistance.
For more information about Tridipanel systems, visit www.tridipanel.com.