Using one set of tunnel forms, you can build the shell for a house in one day.

Source: RESIDENTIAL CONCRETE MAGAZINE
Publication date: 2006-01-01

By Joe Nasvik

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.

Outinord Universal 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.

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.

Joe Nasvik 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.

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.

The first step is construction of the floor slab. Workers also place a 4-inch-high concrete curb either at the time of slab placement or afterwards but before setting the forms. The curb is as wide as the wall thickness and serves to position the bottom of the forms and stop the flow of concrete when the tunnel form is jacked up a couple inches to its final position.

Workers then place steel reinforcement for walls and rough electrical before the tunnel forms are positioned, while there is room to work. Then each half of the tunnel form is lifted by crane and set in place. The two halves of the deck are then locked together. Door and window bucks are mounted on one wall of the form, making their installation very easy. A trick that workers often use is to establish the final elevation for the forms by using a laser level to mark a chalk line 2 inches above the floor on the curb. Then, when the form is being set, they can jack the tunnel form into position so that the bottom of the form is on the chalk line. This allows the elevation of the form to be quickly established, time after time. Taper ties are placed on 4-foot centers horizontally and in two or three rows vertically, depending on ceiling heights.

With the forms in place, setting steel on the deck is the last step before placing concrete. Contractors usually place concrete that gains strength quickly so that the following morning the forms can be removed and the cycle can be repeated.

To remove forms, workers unlock the two halves at the deck, remove the taper ties, and lower the jacks so that the built-in wheels touch the floor. Each form is then rolled out through the open wall, one third of its length, exposing a “lifting hole” located on the top of the form. A crane lifts the form and moves it to the next location, starting the cycle over again. The cycle can be repeated to construct another shell either on top of or alongside the work just completed.

Joe Nasvik Shown here is a completed tunnel form, the curbs they rest against, and the pre-tied wall reinforcement with rough electrical (the blue tubes) in position.

When the tunnel forms are removed, two or three walls and the deck are complete. What remains is the one open exterior wall and any interior walls. These can be constructed with steel studs or unit masonry. Sometimes these walls are cast in place using conventional wall forming systems. With this approach, a block-out placed in the deck during the tunnel form pour, over the position of the wall forms, makes it possible to fill the forms with concrete.

In the Florida housing market typical costs range from $12 to $16 per square foot. Construction costs should at least be equal to wood-frame construction. Although, as one industry member commented, “Why would you choose to compare a high-quality home building system to a low-quality system?”

There are other hidden cost benefits. Within a couple days the shell and the primary interior walls, the rough electrical, the insulation, and block-outs for plumbing and HVAC are complete. This means that other trades don't waste time cutting holes or preparing to do the work they are hired to do. Construction cycles are therefore much shorter than with other building systems so the duration of construction loans is shorter, and homes can be turned over to their owners more quickly, enhancing cash flow.

The blessing and the curse of tunnel form construction is to be found in the details and planning required to fully take advantage of the system. With the opportunity to strip, set forms, and place concrete every day, shell contractors can easily produce more than other trades can keep up with, so they too must be involved in the planning.