SlabCalc LogoSlabCalc Concrete Technical Division

Formwork

Temporary molds or structures that hold concrete in shape while it sets and cures

Formwork (or forms) consists of temporary molds or structures that hold concrete in the desired shape while it sets and cures. Forms define edges, establish elevation, and contain fluid concrete until it gains enough strength to be self-supporting.

Why It Matters

Quality formwork makes the difference between professional-looking concrete and amateur work. Forms that bulge, shift, or leak produce wavy edges, uneven surfaces, and wasted concrete. Strong, well-braced forms hold true under concrete's weight and pressure (up to 150 pounds per cubic foot).

For DIY flatwork, form construction is often underestimated. Proper forms require straight lumber, secure stakes, accurate leveling, and adequate bracing. Shortcuts here cause problems that can't be fixed without tearing out hardened concrete.

Technical Details

Common formwork materials:

Wood forms: Standard for residential flatwork. Use 2x4s or 2x6s for most slabs, 2x8s or 2x10s for walls/footings. Lumber should be straight, with smooth face toward concrete. Reusable several times if carefully removed.

Steel forms: Permanent or reusable metal forms for repetitive work. More expensive but create cleaner edges and stay straight under pressure.

Plastic/fiberglass forms: Flexible forms for curves and decorative edges. Less common in DIY work.

Tube forms (Sonotube): Fiber tubes for round columns and piers. Remain in place permanently.

Form construction requirements:

  • Strength: Must resist concrete pressure without bulging (use 2" nominal lumber minimum)
  • Rigidity: Stake every 3-4 feet; brace against lateral movement
  • Elevation: Set top edge to final slab grade; verify with level
  • Alignment: Straight edges, square corners (use string lines and diagonals)
  • Release: Coat forms with release agent or oil for easier removal
  • Joints: Seal gaps that leak concrete paste

Stripping (form removal):

  • Vertical forms: 12-24 hours (concrete must support itself)
  • Slab edge forms: 1-3 days (won't damage edges)
  • Structural forms: 7-28 days depending on loading and weather
  • Forms - Synonym for formwork
  • Framing - Setting up and bracing formwork
  • Stripping - Removing formwork after concrete cures

Learn More

← Browse all glossary terms