Concrete Formwork

In home construction, three types of foundations are used, basement, crawlspace and slab on grade; all three involve formwork. Formwork is the term used to denote the molds that concrete is poured into to form walls, footings and slabs. Some footings are poured directly into excavated holes, but most formwork consists of lumber and stakes.

Footings are poured and formed first. These are the perimeter slabs of concrete which distribute and carry the weight of the house, and anchor it to the undisturbed soil. Footings need to reach at least one foot below the frost line. Reinforcing bars and wall tie-in anchors, known as J-bolts, are usually set into the concrete of the footing as well. After this step the formwork for floor slabs, beams or walls which will support the house framing is constructed.

Making the formwork typically takes around two days from the start time until they are ready for concrete to be poured. To a person not familiar with construction, formwork will look haphazard and ragged. These temporary structures are typically built out of old lumber and are reused from project to project.

Once concrete has started to set, the formwork is removed from the foundation. The process of removal leaves gouges and dents in the lumber as nails and bolts are pulled out and ties and bracing and taken away. Although formwork may be damaged on the outer surface, it is still usable as long as the inner surface remains straight and unwarped. They are inspected regularly to ensure the quality of the resulting foundation.

Formwork Systems

For slab and grade beams (the poured concrete members which reinforce and connect foundation piers), formwork is made of 2 x 6 lumber usually. Staked in position and strapped along the top surface, it forms a trellis-like structure. Slab foundation forms have the 2 x 6 boards set on one side only, to form the edges of the slab.

For basement foundations, the formwork for the walls is made from pre-built panels of about 4 x 8 feet, which are reused equipment. One form is set for each side of the wall, forming a gap the width of the wall into which the concrete is poured. Braces, spreaders and wire ties are added to maintain a constant wall thickness. Any interior ties between the forms are cut off flush with the wall after the forms are removed.