Regardless of the building system or materials used, there is no faster way to build a house than with tunnel form construction. With the smallest labor force, walls and decks (ceilings) require only one forming operation and a single concrete placement. Tunnel form construction can be ideal for high-rise buildings, multifamily and single-family residences, hotels, town houses, military housing, prisons, and some warehouse applications. But to take full advantage of the system, the decision to use tunnel form construction must be made before the design process starts. There must be a crane on the jobsite, and there must be the opportunity for repetitive use of the tunnel forms.
Cranes “fly” each half of a tunnel form into position.
Tunnel formed housing is most prevalent in Florida. To provide hurricane, termite, and mold resistance, most housing there is either cast-in-place concrete or masonry construction. Tunnel forms are the result of the great need for affordable housing after World War II. Demand for affordable single family residences and apartments caused Guy Blonde, technical director for Outinord, a small start-up manufacturer based in France, to come up with the idea of tunnel forms in the early 1950s. The system saved money and reduced construction time because workers formed both walls and decks in one operation.
With today's refinements, tunnel forming systems are ideal for projects that require repetitive forming—the more that forming operations can be reduced to repetitive steps, the greater the benefits of the system. One very important limitation, however, is that there must be one open wall for removing the form after concrete placement.
Robert Fallowfield, sales manager for Outinord, says that repetitive forming cycles reduce labor requirements and make very accurate construction possible. The trick is to plan for the maximum number of repetitive steps possible in the positioning of a form and the placing of concrete to increase productivity and reduce the possibility of errors. The most productive projects also maximize the number of times the form is used.
WHAT ARE TUNNEL FORMS?
Construction companies usually rent tunnel forms for a project; they seldom buy them. Companies like Outinord, Miami, discuss their use with owners and engineers at the start of a project. The supplier provides tunnels of the size needed and adjusts them properly for the application. The forms are simply two upside-down steel “L” shaped pieces that join together at the top to form a long tunnel. Tunnels can be as long as 40 feet and from 12 to 22 feet wide. With the addition of a “table,” the deck width can extend to 26 feet. A diagonal brace is positioned from the bottom of the wall extending to the end of the deck. It supports and braces the deck form. Adjusting the brace is a technical procedure, and the bracing shouldn't be changed during construction. The form provider makes the adjustment so that as the formwork deflects under the load of the concrete, the ceiling ends up level. The same adjustment must be made to the other half of the tunnel form so that a locking device that joins the two tunnel deck sections comes together properly.
At the bottom of the wall side of the form are jack-screws and wheels. Workers adjust the jack-screws to establish the right deck elevation, lifting the wheels off the floor in the process. Elevating the form also makes it possible to remove it afterwards. A wall form as well as collapsible door and window bucks can be a dded to the back of a tunnel.
Workers set one half of a tunnel form into position. It can take as little as five minutes to move the halves of a tunnel form together and secure them.
PLANNING FOR TUNNEL FORMING
To realize the greatest benefits for using tunnel forms, an owner first decides to build with tunnel forms and then instructs his design team to do their design work and engineering around the forming system. Two principles guide construction: shells must be the same height and width, and there should be many uses for the form. It also helps if door and window bucks are in the same location for each shell. The length of a shell, however, can change.
Given the requirements to make tunnel forming operations the most productive, the challenge for architects is to design houses that have individual character. The object is to design around the use of tunnel forms, taking advantage of all their benefits. Contractors must think carefully about the daily flow of construction. They will want their labor force to perform the same tasks each day. Other trades must organize to keep up with the pace set by the concrete contractor. There's no benefit to the owner when other trades fall too far behind. And with low-rise or home construction, projects get in trouble if shell work proceeds too far ahead of finish work.