Formwork is a large part of any cast-in-place concrete job, and projects with elevated slabs require a lot of formwork. Fortunately, selecting the most appropriate system can help minimize formwork costs. To do that, the contractor must understand the different forming systems available today and must weigh a number of factors as they apply to the particular job.
Labor and material costs are two important and interrelated factors. Project scheduling requirements are another. But total project scope is the biggest consideration with elevated slabs, where repetition leads to greater efficiency with high-production systems.
Forming systems for elevated slabs fall into two general groups, depending on how they are erected. The larger, heavier assemblies, such as flying forms and tunnel forms, require a crane to move them from one location to the next. Handset forms break down into smaller pieces that can be moved with less mechanical assistance. There are several hand-set systems, from single use wood forms to sophisticated assemblies with few workers needed to set them up.
Additional variations include pan forms, which are used to create beams or a waffle grid as a part of the slab, and tunnel forms, where the form is for both walls and slab. Selecting the best system for any given project depends on many things, including the project's design, scope, and scheduling requirements.
To get an idea of what manufacturers use what system, click here to peruse our chart.
Contractors select flying forms (also referred to as “table forms”) for projects where a rapid cycle time is required and there will be more than eight or 10 repetitive floors. They consist of pre-assembled large form table surfaces with engineered trusses to support them.
The component pieces are delivered to the jobsite, assembled, and lifted into position by crane. After each floor section is cast and hardened, the assembly is lowered, rolled out from under the completed slab, raised, and swung into position at the next location for the next placement. The system works best on projects where the columns are fairly well aligned.
These forms come with two different support systems. They can be supported by shore legs attached to the trusses that extend to the floor below. Or the load can be supported by the surrounding columns or walls by special hardware. Column-supported systems do not require the floor below to carry the weight of the fresh concrete placement above and offer the advantage of keeping floors clear.
Table forms work best for “flat plate” construction, meaning that floors are of uniform thickness. But they also can easily be used for floors designed with pan forms.