Slipform construction is a construction method in which concrete is poured into a continuously moving form. It is a method for vertically extruding a reinforced concrete section and is suitable for the construction of core walls in high-rise structures: lift shafts, stair shafts, towers, etc. Requiring little crane-time during construction, Slipform is a self-contained formwork system that can also apply to horizontal structures such as central reservations in road construction.
Building a Platform
It is similar in nature and application to jumpform where the formwork is moved intermittently and 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.
Slipforming is a quick and efficient construction technique. Because the height of the formwork is usually only around 1.20 metres and the hourly production rate is normally between 15 to 30 cm, the concrete underneath is between four and eight- hours-old and must be stiff enough to bear its own weight However, it must not have set enough for some of it to stick to the rising formwork (‘plucking’).The main requirement for slipforming without problems is concreting all areas at the same level at the same time, and then allowing the concrete to set consistently to allow the lifting of the formwork. Therefore, not only is consistent concrete required in terms of consistence and performance but also the temperature of both the concrete and the environment are vitally important in understanding how the concrete will behave.
Concrete for slipforming
Modern concrete mixes for slipform construction vary considerably with a typical range of strengths between C40/50 and C76/95; a range of consistence values between S4 and self-compacting concrete and a range of controlled setting times to suit the particular construction project. The use of micro silica is becoming more prevalent with self-compacting mixes. Both consistency and flexibility are required for the successful supply of concrete to slipform structures. Sika has both the knowledge and expertise to optimise the use of Viscoflow superplasticisers, with Sika retarders and stabilisers to optimise performance of the mixes and to ensure smooth operations on site.
Points to consider for slipforming
- Careful planning of construction processes can achieve high production rates.
- 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.
Correctly installed, slipform concrete can help our buildings stand tall for our long-term benefit.