Concrete formwork and placement methods

One-line orientation

Concrete formwork gives the pour its shape, placement methods move the mix into position, and field tests check it. A slump test checks fresh-workability; a cylinder test checks cured compressive strength.

Key points

• Formwork systems:
  • Form release compound — wax/oil/plastic on the forms so concrete strips off cleanly.
  • Insulating concrete form (ICF) — forms double as permanent insulation (combines forming + insulation in one).
  • Slip forming — continuous pour into a moving form; suits tall or repetitive structures (e.g., cores, silos); smooth, non-stop finish but demands steady concrete supply.
  • Flying / gang formwork — large reusable panel assemblies moved by crane or jacks floor to floor; fast and labor-saving but high initial cost and careful planning.
  • Lift-slab system — slabs cast stacked on the ground, then hydraulically jacked up into final position; safer and faster but needs precise design/coordination.
  • Conventional (job-built) formwork — site-assembled wood/plywood or modular panel forms.
• Placement methods:
  • Chute — gravity flow off the truck for nearby, at-grade pours.
  • Pump — concrete pumped through a hose/boom to reach height or distance.
  • Tremie — a pipe kept buried in the fresh concrete to place underwater from the bottom up without washing out cement.
• Consolidation: after placing, concrete is consolidated (vibrated) to remove trapped air and honeycombing and seat it around the steel; self-consolidating concrete is fluid enough to skip mechanical vibration.
• Field tests (the headline pair):
  • Slump test — measures consistency / workability of the fresh mix.
  • Cylinder (break) test — cast cylinders are lab-cured and crushed to measure compressive strength.
• Slab-on-grade preparation (bottom up):
  1. Remove topsoil to reach stable subsoil.
  2. Add about 4 inches of crushed stone as a capillary break.
  3. Place the plastic-sheet moisture barrier.
  4. Set level formwork.
  5. Place welded-wire reinforcement (WWR/WWF) or rebar on chairs.
  6. Pour, screed, and float.
  7. Keep the slab damp for about one week.
  8. Cut control joints about every 15 feet.

Confusions / comparison

Field test	Measures	When / where
Slump test	Consistency / workability of fresh mix	At the truck/site, before placing
Cylinder break test	Compressive strength	Lab-cured cylinders crushed at a set age

Formwork / method	What it is	Best for
ICF	Forms double as insulation	Insulated walls, one-step forming
Slip forming	Pour into a moving form	Tall/repetitive (cores, silos)
Flying / gang form	Large reusable craned panels	Repetitive floor slabs/beams
Lift-slab	Slabs cast low, jacked up	Multistory flat slabs
Chute / pump / tremie	Gravity / pumped / underwater placement	At-grade / height & reach / submerged

Related

→ Properties & curing (this topic): the w/c ratio behind workability, and why a higher slump can mean a wetter, weaker mix · Slab systems (this topic): the floor systems these forms and pours build · Reinforcing & prestress (this topic): the WWR and rebar set into slab-on-grade.