Slip form concrete
Formwork is a temporary mould into which concrete is poured and formed. Traditional formwork is fabricated using timber, but it can also be constructed from steel, glass fibre reinforced plastics and other materials.
Slip form is a method of construction in which concrete is poured into the top of a continuously moving formwork. As the concrete is poured, the formwork is raised vertically at a speed which allows the concrete to harden before it is free from the formwork at the bottom.. Slip form is most economical for structures over 7 storeys high such as bridges and towers, as it is the fastest method of construction for vertical reinforced concrete structures, but it can also be used for horizontal structures such as roadways.
As long as there is a regular shape or core, formwork can be used for a smooth, continuous pour that requires no joints. The formwork supports itself on the core rather than relying on other parts of the building or permanent works, and rises at a rate of about 300 mm per hour.
The concrete to be used needs to be workable enough to be placed into the form and consolidated by vibration, yet quick-setting enough to emerge from the form with strength. The concrete must be constantly monitored for its setting rate to ensure the forms are not being raised too fast.
Usually, the formwork has three platforms:
- Upper platform: This acts as a storage and distribution area.
- Middle platform: This is the main working platform and sits at the top of the poured concrete level.
- Lower platform: This provides access for concrete finishing.
It is essential that the middle supporting platform is rigid so that all parts of the structure move simultaneously. The shuttering can drag if there is inconsistency in the rate at which the platform is moving which can cause difficulties.
- Service cores for commercial buildings.
- Lift and stair shafts.
- Concrete gravity structures such as oil platforms.
- Bridge pylons and piers.
- Mine headgear towers.
- Shaft linings.
- Surge shafts.
- Liquid containment vessels.
 Advantages of slip forming
- Slip forming can achieve high production rates, however, once continuous concreting has begun there is little flexibility for change and so very careful planning is required.
- Crane use is minimised.
- Only minimal scaffolding and temporary works are required allowing the construction site to be less congested, and so safer.
- The exposed concrete can be finished at the bottom of the rising formwork.
- Slip form systems require a small but skilled workforce on site.
- There is flexibility in that tapering structures with wall reductions (either gradual, over a short distance, or stepped) can be achieved.
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