In the scenic country between Springfield and Branson, Mo., a 72,000-square-foot concrete house, currently under construction, will be one of the largest single-family homes in the United States once completed. The owner and designer, Steven Huff, says Pensmore will be a second home for his family and a place for charitable organizations and universities to gather for conferences and meetings.
However, the house also is being constructed to prove the efficacy of a new forming system for concrete walls. Kirk Brown, CEO of TF Forming Systems, Green Bay, Wis., says he and Huff bought the company because they thought this new concept for forming concrete walls had merit. After making several changes to the system, they cast test panels two years ago and on the strength of those results decided to cast all the structural walls of the house using the system, continuing to make refinements to form elements as they went along.
A hybrid ICF system
One reason that concrete buildings are energy efficient is the thermal mass of concrete walls. Energy, or the lack of energy, can be stored in walls and floors to play a part in heating and cooling, reducing overall energy consumption while moderating temperatures. This means concrete walls must be isolated from outside ambient conditions but exposed to the inside environment. To accomplish this, Gerald Spude, the founder of TF Forming Systems, converted Huff and Brown’s ideas for the formwork into a practical, working system.
The TF forming system is different than a typical ICF system where the foam insulation thickness is equal on both the inside and outside of a wall, and is strong enough to resist form pressures during concrete placement. The TF (or TransFrom) System has 4- or 5-inch-thick rigid foam insulation on the outside of the wall, serving as the form panel during construction and insulation for the wall afterward. But the inside wall surface is covered by only a 3/4-inch-thick rigid foam insulation panel. This thin panel dampens thermal transfer while still making it possible to use the concrete as a heat sink for the interior heating and cooling of rooms. Brown contends that walls are a better location for radiant heating and cooling than floors, so for the Pensmore project, they are installing PEX tubing at the center of all exterior concrete walls.
To achieve a net-zero energy balance, fluid circulating through panels mounted on the roof will be heated by the sun during the winter, sending it through the PEX tubing in the walls to heat the house. During the summer, fluid circulated through underground piping will be cooled by the earth to 55º F and circulated in the walls to provide cooling.
Structural form elements
At the heart of the TF System is polyvinyl chloride (PVC) recycled plastic studs, or rails, with built-in channels. Workers set the studs in position on 16-inch centers just as they would if they were erecting a wood wall. There is a stud wall support system for the inside and another for the outside. Both support systems become a permanent part of the wall after concrete is placed.
When the exterior studs are in place, workers insert the thick foam insulation panels into channels on the studs that hold them in position. They become the form panel for the concrete and the exterior flange of the studs becomes the point of attachment for the exterior finish system.
The inside wall-forming system is more complicated. The PVC studs feature two sets of channels: one to hold the 3/4-inch-thick insulation panels in place and the other to position PVC panels that strengthen the insulation sheets and resist the forces of freshly placed concrete. The PVC removable form panels are supported by strong-backs to keep the wall surfaces flat and straight, and also serve as the inside attachment for wall ties, scaffolding, wales, and braces. When the PVC panels and strong-backs are removed afterward, a 3 1/2-inch void is created, providing a chase for pipes, electrical boxes, conduit, and wiring for phones, televisions, computers, and other electronics. The inside flange is used to secure drywall and other interior finishes.
The forming system depends on form ties connected to the inside and outside forms. Brown says the ties can be any length in order to accommodate the required or desired wall thickness. He adds that inside and outside forms don’t need to be installed at the same time, making it easy to install reinforcement, PEX tubing, and other inserts.
Concrete for the project includes steel fibers in the mix for added strength and the addition of a superplasticizer to make it easier to pump.
Workers at Pensmore place concrete for walls in 16-foot vertical lifts, using internal vibrators for consolidation. The robust forming system eliminated blowouts and deformed wall surfaces—the same result one would expect from any good form panel system.
The concrete floors (decks) at Pensmore are constructed with a standard steel joist system with pan deck forms that are easily connecting to the walls. Workers place concrete for each floor deck after the walls are completed. This allows them to do all the bracing for the next level of wall forms on the inside of the structure.
Workers have completed 70 vertical feet of walls with no deformations. Concrete work on the building will be complete this spring and the roof will be in place by the end of 2012. Brown believes this building system will result in 50% to 60% more energy efficiency than timber frame wall construction, enabling them to achieve 100% efficiency with their roof-mounted hot-water system. The goal is to have a very green and healthy living environment—using all recycled plastic, reusable forming hardware, almost no wood in the structure (a food source for mold), and producing little construction waste.