Formwork for Foundation

Its smoothness affects the appearance of the visible surface. However, for foundations, this is not particularly important; what matters is that the panel's strength can withstand the pressure of the concrete mix.

It is precisely this characteristic (strength) that determines which type of formwork to use in a specific case, and how many supports and fasteners are needed.

Materials for Formwork

Formwork comes in several types, depending on the panel material: wooden, metal (steel, aluminum), and plastic.

In private construction, wooden formwork is most commonly used because it is cheaper and easier to shape into any structure. For formwork panels, boards, moisture-resistant plywood, or OSB panels with a thickness of at least 20 mm are used. Additionally, wood can later be reused for other purposes (subfloor, roof battens) — nearly zero-waste construction.

This type of formwork is the most widespread, as it has a multi-century history confirming its reliability and strength, is relatively inexpensive, and accessible to all.

For foundations, boards 20–35 mm thick are used, depending on the panel size and the calculated foundation thickness. Wood moisture content should be no less than 25% to prevent absorption of cement milk. If the wood is too dry, it must be soaked in water. For foundations, wood of grades 3–4, both coniferous and deciduous, is suitable.

Steel formwork is mainly used in industrial-scale projects. It is more expensive than wooden formwork but has a higher number of reuse cycles. Although some construction companies occasionally use profiled steel sheets for private home foundations.

Plastic formwork is a relatively new and gaining popularity type. It features high strength, the highest number of pour cycles, and high cost, so it is primarily used by specialized construction firms.

Structure and Installation of Wooden Formwork

If wooden boards are chosen instead of a prefabricated modular system, proper assembly is crucial. Use planed, dimensioned boards. The inner side of the panel is always flat, so all connecting elements are placed on the outer side.

For foundation construction, low panels are typically used. Boards in a panel are fastened with nails and vertical connecting strips spaced about 1 meter apart (more is not recommended). For added strength, horizontal strips may be added on top of the vertical ones. Panels are connected using brackets or additional strips.

Joint connections at corners. Two parallel panels are made longer than needed (overlapping), and support strips are nailed to their inner side to serve as a stop for the perpendicular panel (lapped). There should be no gaps larger than 3 mm in the panels or between them. If gaps exist, they must be sealed with packing or filled with plywood.

The inner side of the panel is covered with plastic film or treated with hydrophobic coating. This reduces adhesion between concrete and wood, making separation easier. Additionally, the film prevents wood from getting dirty, allowing reuse for other construction purposes.

To maintain shape and prevent buckling, external supports or stakes driven into the ground are used. To prevent inward tilting, panels are secured from above with braces and from inside with hollow concrete beams or wire. It's best to install the rebar cage as the formwork is assembled, which simplifies reinforcement work.

Non-removable Formwork

Non-removable formwork includes polystyrene and concrete blocks. EPS is used to ease installation and as thermal insulation for foundations. It also serves as a moisture barrier, though drainage must still be installed. Modules are joined in grooves. Holes are provided for rebar and utility conduits.

Reinforced concrete panels are used when the calculated wall thickness exceeds 0.5 m. Reinforced concrete formwork panels are 5–8 cm thick, manufactured at concrete plants, and installed using specialized equipment. The same concrete grade used for foundation pouring is used in their production. The panel surface is textured for better adhesion. An internal rebar cage is installed.

Conclusions

Which type of formwork to use in a specific case should be advised by professionals or determined through structural calculations. Often, the choice is based on budget and architectural design. The main purpose of formwork is to create a strong, properly shaped foundation. Therefore, high-quality materials and accurate calculations are essential.

If possible, formwork can be rented. In large cities, there are always companies that rent construction materials. Pre-assembled panels are connected with special fasteners, simplifying the process. However, these companies may have worn-out systems, so it’s always advisable to inspect the material before making a decision.