Arro Design, Waitsfield, Vermont, is a design-build construction company specializing in concrete structures. Sandy Lawton, Principal, has extensive experience using concrete, "I guide clients towards concrete. It's such a creative medium to work with."
This tree-house in the woods, is a perfect example of using concrete to enhance a design. "Our client wanted to retreat with nature, trees and light, and so we needed height without a bulky structure. Fast-Tube™ columns are strong, simple and in color harmonics with indigenous rocks", said Sandy.
The tree-house also includes a suspended concrete walkway running to the main house.
The structural frame is made up of four 12" diameter concrete columns 29' tall, running from the foundation to the roofline.
"We wanted continuous full length columns without the spiral lines of cardboard tubes", confirmed Sandy. "As Fast-Tube™ comes on a 120' rolls, the four 29' columns can be cut from of a single roll."
Scaffolding was set up around each of the four columns, with laminated veneer lumber (LVL) placed horizontally at 10', 20' and 30' of height to accurately position the forms. Muffler hanger clamps were used to attach the LVLs to the scaffolding. A 30' length of LVL was run vertically from the foundation to the top of the column, and screwed against the horizontal LVLs. A 16' and 14' 2x4 was screwed to the vertical LVL to hold the fabric tab in place.
The structural engineer specified five #5 vertical rebar spaced radially inside 9" compression rings 10" on center. The reinforcing was installed in position and held in place temporarily using the vertical 2x4 and tie wire.
"On our first column, we used 2" circular spacers attached to the vertical steel members", said Sandy, "but we found the spacers constrained the flow of concrete down the tube."
"On our next column, we used 1-1/2" spacers, and placed them on the compression rings so that they presented a smaller area to the concrete flow. This worked perfectly."
A roll of Fast-Tube™ was taken to the top of the scaffolding and carefully slid over the steel reinforcing, taking care not to rip the fabric. Fast-Tube™ was cut to length, and the alignment tab stapled to the single 2x4. A second 2x4 was then screwed to the first, sandwiching the alignment tab.
"Our main concern was the potential for aggregate segregation with the 30' drop", said Sandy. "To prevent this, we went with a high range concrete (super plasticized) to make it sticky."
"On our first column, we used high range concrete with 3/4" aggregate and a 4" slump. However we found we were getting voids under the 2" horizontal spacers."
"On the next column, we used 1-1/2" spacers aligned vertically, a 5" slump, and 3/8" aggregate - with perfect results."
"To vibrate the concrete, we tapped each side of the column with our hands as we were filling it. Fast-Tube™ shakes like Jell-O and consolidation is easily achieved."
"Everybody was very skeptical from the beginning", said Sandy, "even my site superintendent couldn't understand how 5 pounds of fabric could support a 30' column of concrete weighing 3,500 pounds. But we proved the skeptics wrong."
"To ensure the limits of the fabric weren't exceeded, we poured 11' of height, waited 30 minutes; then 10', waited 15 minutes; and then the balance."
"Fast-Tube™ being made of polyethylene is perfect for curing. Polyethylene doesn't stick to concrete so it's very easy to strip."
"Fast-Tube™ has amazing potential for concrete forming", enthused Sandy. "It eliminates the limitations of fixed length cardboard forms, and allows more creative architectural solutions."
An award for "Excellence in Architecture" was given out at the Annual meeting and Design Awards Presentations of the American Institute of Architects, Vermont Chapter in the Landscape Category for this project. For more information on this artistic architectural use of fabric formwork, contact Sandy Lawton, ArroDesign, (802)496-3234.