Formwork

Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering molds.

Use / Function

Its practical purpose is to hold wet concrete in place until it sets and hardens, defining the shape and surface texture of the final structure.

• Primary use: Casting Concrete walls, columns, slabs, and foundations.
• Secondary uses: Creating decorative textures on concrete surfaces, supporting arches during construction.
• Scale: Ranges from small DIY garden projects to massive industrial dams and skyscrapers.

Operating principle

Hydrostatic Pressure Containment: Wet concrete acts like a heavy fluid (approx. 2400 kg/m³). The formwork must resist the outward pressure exerted by the liquid concrete without bursting or deforming. As the concrete hardens (hydrates), it becomes self-supporting, and the formwork can be removed (stripped).

How to create it

1. Design: Determine the shape and dimensions. Calculate the pressure to ensure the form won’t burst (blowout).
2. Fabrication: Cut panels (sheathing) to size.
3. Assembly:
   • Sheathing: The face that touches the concrete (e.g., plywood, steel plate).
   • Studs/Joists: Vertical or horizontal members that support the sheathing.
   • Walers: Horizontal members that support the studs and align the form.
   • Ties: Metal rods that hold opposing sides of the form together against the bursting pressure.
   • Bracing: Diagonal supports to keep the form plumb and stable against wind or impact.
4. Preparation: Apply a release agent (Vegetable Oil or similar) to the inner face so the concrete doesn’t stick.
5. Pouring: Fill with concrete.
6. Stripping: Remove the formwork once the concrete has gained sufficient strength.

Materials needed

• Essential materials:
  • Wood: Plywood for sheathing, timber for studs and bracing (traditional).
  • Steel: For heavy-duty frames, ties, and reusable panels.
  • Nail / Screws: To assemble wooden forms.
• Tools: Saw, Hammer, Drill, Level, Tape Measure.
• Possible substitutes:
  • Earth: For footings, the ground itself can act as formwork (trench pouring).
  • Brick / Block: Permanent formwork that stays in place.
  • Fabric: Flexible formwork for complex organic shapes.

Variants and improvements

• Traditional Timber: Built on-site using lumber and plywood. Flexible but labor-intensive and slow.
• Engineered Systems: Prefabricated modules (Steel/Aluminum frames with plywood/Plastic faces). Fast to assemble/disassemble and highly reusable.
• Insulating Concrete Forms (ICF): Polystyrene foam blocks that stay in place to provide insulation.
• Tunnel Forms: Cast walls and slabs in one operation (for repetitive cellular structures like hotels).
• Climbing Forms: Forms that rise up the structure as it is built (for tall towers).

Limits and risks

• Blowouts: If the form is too weak or the concrete is poured too fast, the pressure can burst the form, spilling tons of wet concrete.
• Leakage: Gaps in the formwork allow “grout loss” (honeycombing), weakening the concrete and leaving ugly surfaces.
• Safety: Formwork collapse is a major construction hazard. It must support not just the concrete, but also workers and equipment.
• Cost: Formwork can account for 40-60% of the cost of a concrete structure.