Types of Formwork
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.