When constructing a suspended concrete slab that is not directly supported by the ground, slab formwork is necessary to create the structure. Formwork is a term that refers to the panels and falsework that creates a mold in which concrete can be poured for setting. Slab formwork can include formwork panels, joists, stringers, and other supports that allow concrete to be poured and suspended above the ground. Such formwork often combines a variety of materials, including metal, plywood, and in some cases even plastic components used to give concrete its shape and strength.
When a slab is suspended, it must be supported while it cures. This is done by creating slab formwork that will essentially support the weight of the concrete while it sets and is positioned on permanent supports. Bases known as sills can be made from wood or metal; these sills will support vertical stringers that will in turn support horizontal joists. The joists will create a flat surface on which flat pieces of plywood can be secured. In many cases, the slab formwork may use aluminum or steel sheets instead of plywood. A sheath of some sort is usually placed over the plywood, creating the flat surface on which the concrete can be poured.
The formwork itself does not necessarily need to create a horizontal surface, either. It can be positioned in such a way that a diagonal concrete structure, or even a vertical one, can be created. The orientation of the slab formwork components will generally dictate what types of concrete structures will be built. Since formwork tends to be highly modular and therefore customizable, seemingly endless shapes and designs are possible. The falsework, or components that secure the forms in place, will help add to the modular nature of the formwork.
The materials used to create the slab formwork can vary depending on the size of the project. Steel forms are commonly used for larger projects, while plywood and aluminum are suitable for smaller projects. In most cases, the forms are reusable, so they must be cleaned after use to ensure the faces of the panels stay straight and no concrete builds up on them. Plywood forms usually feature a laminate that protects the wood from moisture damage, as the concrete can transfer moisture directly into the wood otherwise. Plywood forms tend to be the least expensive option, though aluminum formwork is not far behind.