Slipform

Slipform is similar in nature and application to jumpform, but the formwork is raised vertically in a continuous process. It is a method of vertically extruding a reinforced concrete section and is suitable for construction of core walls in high-rise structures – lift shafts, stair shafts, towers, etc. It is a self-contained formwork system and can require little crane-time during construction.

This is a formwork system which can be used to form any regular shape or core. The formwork rises continuously, at a rate of about 300mm per hour, supporting itself on the core and not relying on support or access from other parts of the building or permanent works. 

Commonly, the formwork has three platforms. The upper platform acts as a storage and distribution area while the middle platform, which is the main working platform, is at the top of the poured concrete level. The lower platform provides access for concrete finishing.

Benefits

• Careful planning of construction processes can achieve high production rates.
• Slipform does not require the crane to move upwards, minimising crane use.
• Since the formwork operates independently, formation of the core in advance of the rest of the structure takes it off the critical path – enhancing main structure stability.
• Availability of the different working platforms in the formwork system allows the exposed concrete at the bottom of the rising formwork to be finished, making it an integral part of the construction process.
• Certain formwork systems permit construction of tapered cores and towers.
• Slipform systems require a small but highly skilled workforce on site.

Safety

• Working platforms, guard rails, ladders and wind shields are normally built into the completed system.
• Less congested construction site due to minimal scaffolding and temporary works.
• Completed formwork assembly is robust.
• Strength of concrete in the wall below must be closely controlled to achieve stability during operation.
• Site operatives can quickly become familiar with health and safety aspects of their job.
• High levels of planning and control mean that health and safety are normally addressed from the beginning of the work.

Other considerations

• This formwork is more economical for buildings more than seven storeys high.
• Little flexibility for change once continuous concreting has begun therefore extensive planning and special detailing are needed.
• Setting rate of the concrete had to be constantly monitored to ensure that it is matched with the speed at which the forms are raised.
• The structure being slipformed should have significant dimensions in both major axes to ensure stability of the system. 
• Standby plant and equipment should be available though cold jointing may occasionally be necessary.

More information on formwork can be sourced from Formwork for Modern Efficient Concrete Construction, published by BRE.