What is permanent formwork?
Permanent formwork, also called lost formwork or stay-in-place formwork, is formwork for concrete that is not removed after pouring but remains in the structure. In Dutch it is called verloren bekisting. Ordinary formwork is stripped after the concrete has hardened and reused; permanent formwork becomes part of the structure or of the finish.
When do you choose permanent formwork?
- Stripping is difficult or impossible, for example under a ground beam in a crawl space or with a floor poured onto an existing structure.
- The formwork has a function of its own, such as insulating, carrying loads or serving as a ceiling finish.
- Saving time and labour: no stripping round is needed and the site stays tidier.
Examples
- EPS foundation forms: polystyrene forms for ground beams and strip footings. They insulate the foundation straight away and limit thermal bridges between foundation, floor and wall.
- Expanded metal formwork: metal mesh with film or polystyrene, often used for construction joints and foundations.
- Precast plank floor (lattice girder slab): a thin precast concrete slab with reinforcement that acts as formwork for the topping poured onto it.
- Composite steel deck floor: profiled steel sheeting that acts as formwork and as (part of) the reinforcement.
- U-shaped blocks in masonry, in which a lintel or ring beam is poured.
- Coffered floors and precast twin-wall panels.
In the past, corrugated asbestos cement sheets were also used as permanent formwork. If you come across them during alterations, have an asbestos survey carried out first.
Tips for EPS foundation forms
- Backfill with sand before pouring: secure the forms on both sides with sand, so the EPS does not bulge or crack under the pressure of the wet concrete.
- Openings in advance: have pile holes and sleeves for pipes made at the factory or before pouring. Drilling afterwards risks the wrong position and leaking joints.
- Pour in layers and do not vibrate too close to the EPS.
- Follow the supplier: the permitted pouring rate and height differ per system.
Related terms
- Formwork
- Expanded metal formwork
- Precast plank floor
- Composite steel deck floor
- U-shaped block
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.