How Formwork Works

Forms, or the molds used to build formwork, have countless permutations. They can be derived from timber, plywood, steel, plastic, fiberglass and a variety of other materials. The way they're erected on a job site can vary, too. Sometimes formwork can arrive as a series of panels, along with hardware like ties, wedges, clamps, braces and brackets, which workers assemble by hand. On other occasions, prefabricated forms designed and built in a factory can be shipped to a job site, hoisted to the right position with a crane, and connected with simple locking mechanisms. Collaboration between the project's architect-engineer and the concrete contractor often determines the best formwork setup. Maybe plywood better accommodates crucial details of the concrete structure; maybe it would prove more economical to reuse forms that the contractor has stockpiled from a previous job.

All formwork must account for two key factors: the rate of pour and lateral pressure. The rate of pour is literally the speed at which concrete is poured into the vacancy of the form. Wet concrete is heavy, weighing roughly 150 pounds (68 kilograms) per cubic foot. As the vertical height of the poured concrete increases, the substance exerts lateral pressure as it pushes against the interior faces of the form. If too much concrete is poured at once, and the connecting devices aren't strong enough to contain the pressure, the wet concrete can burst through the form.

The concrete poured at the bottom needs time to set and gain adequate strength before additional concrete is poured on top. Stamaty says that a rate of pour of 4 to 5 feet (1.2 to 1.5 meters) per hour is a good guideline to follow for virtually any formwork system, meaning a 12-foot-tall (3.7-meter) wall would take up to three hours for the full pour [source: Stamaty].

How exactly is formwork put into action on a construction job? Turn the page to find out.

Formwork accounts for roughly 40 to 60 percent of the concrete budget for a given project [source: Lab]. Materials actually comprise a small part of the expense of formwork: Most of the money is spent on the labor to assemble and dismantle forms. Here are the basics of how formwork is used on the job site.

After the contractor and architect-engineer have looked at the structural and architectural drawings of a project and considered the form materials available to them, they'll select a formwork system, which, as mentioned earlier, either arrives in component pieces or already assembled. Once the formwork has been erected, workers must treat the interior of the molds with a release agent -- a compound made from wax, oil or plastic that prevents the form from sticking to the concrete -- because untreated forms can cause the surface of the concrete structure to become discolored or misshapen, which just requires more work to fix it.

Once the form is in place and the concrete mixture is approved, workers begin pouring the concrete into the form's vacant space. (Representatives from the company supplying the concrete are usually on-site during larger commercial projects.) The concrete is distributed by hand or through a hose connected to a concrete truck, with a pump helping the concrete reach portions of the project where the truck can't travel. After the pour, workers often use industrial vibrating devices, which can either attach to the exterior of the form or be placed in the thick of the concrete. Vibration helps consolidate the elements of the concrete and eliminate air pockets. If necessary, the construction works can treat the concrete with water or steam.

After the concrete has dried, the forms are disassembled and removed, or "stripped" in construction parlance. The form can be repositioned in another area of the job site, stockpiled for later use or, if necessary, destroyed. These days, most forms are designed and built to be reused dozens of times.

Effective formwork has to balance several competing interests. Click ahead to find out what they are.

A common sentiment in the concrete industry is that formwork should balance quality, cost and safety. In terms of quality, forms should accurately create concrete structures to the architect-engineer's specifications, without causing bulges or other defects to the surface of the concrete. In terms of cost, we already mentioned that formwork comprises a good chunk of the project's concrete budget. Failure to finish formwork in a timely fashion can also delay the rest of the construction schedule and lead to additional labor expenses, giving concrete workers an incentive to work efficiently.

But safety trumps all. About 25 percent of all construction failures result from collapses and failure of concrete structures, and formwork failures account for more than half of the figure [source: Hanna]. In December 2011, a slab collapsed during the pouring of concrete during a casino construction project in Cincinnati, Ohio, resulting in several injuries to workers on the job site. The following month, another slab at different casino construction site collapsed during the concrete pour in Cleveland, Ohio [source: Feran].

Formwork has also caused hundreds of deaths on construction sites [source: Nemati]. In Bailey's Crossroads, Va., in 1972, workers removed the shores -- the supports that hold up horizontal slabs while the concrete sets -- from the 24th floor of an apartment building too early, triggering the collapse of the entire building, killing 14 workers and maiming many others [source: Hurd]. Forms can collapse from overloading with concrete, inadequate bracing, inadequate shoring, insufficient strength in the concrete before removal of the formwork, improper stripping and a host of other reasons.

Organizations like the Occupational Safety and Health Administration issue guidelines for designing and working with formwork. Having supervisors on site during the erection of the forms and the pouring of concrete can help minimize the hazards of using formwork. If the formwork design needs to be altered, contractors should consult the form designer ahead of time. Formwork might be a temporary structure, but its consequences to the structure and those who build it can be permanent.

For lots more information on the construction process, see the links on the next page.