Slip Formwork is a construction method in which formwork continuously rises vertically as a work process. It is a method of vertical construction of a reinforced concrete section.
It is generally used to construct the core wall in building construction and also the lift shafts, stair shafts, towers, etc. Slip Forming is a self-supported formwork system that may need little help from a crane during construction.
Slip Formwork
These types of formwork systems can be used to construct any regular shape or core. In this method of concrete construction slip forms are raised or lifted upward at a rate of about 300mm per hour, supporting themselves on the core and not relying on support or any external support from the building.
Generally, the slip form technique has three platforms. The top part of the formwork acts as the storage and distribution area while the middle platform is located at the top of the main concrete pouring area. The bottom platform of slip forming provides ease for finishing the work.
Slip Formwork Construction
The following is the slip formwork construction method,
Slip forming is constructed approximately 1.0 to 1.2 meters high at the base of the structure. Slip forms consist of various belts of forms, one for each surface, 1 to 1.5 meters wide usually about 1.2m (4ft) made of timber or steel.
Slip formwork is generally placed on the internal and external surfaces of a wall, chimney, cooling tower, etc. As fresh concrete is poured in forms, the form is slowly and continuously raised by jack screws, hydraulic jacks, or pneumatic jacks.
As the form is raised, there is a facility to correct any error and the thickness of the wall as needed. The rate of raising the formwork is around 5 to 30 cm/hour as per requirements.
This process helps in achieving a construction rate between 1.2 to 7.2 m/day, which cannot be attained by any other construction method.
Components of Slip Formwork
Following are some of the Important Components of Slip Form-work,
- Sheathing or vertical forms
- Wales
- Yokes
- Jacks
- Jack rods
1. Sheathing or Vertical Forms
Sheathing forms also known as vertical forms, can be made up of wood staves, metal, plywood, glass-fiber-reinforced plastic, or a combination of these materials.
Every material defines its specific characteristics when used individually or combined. One of the important objectives of sheathing or vertical forms is to shape the concrete.
2. Wales
The function of Wales is to hold vertical form in place till the hardening. Hence it is considered to be an important component of vertical slip.
It is also trusted for supporting various platforms and scaffolding, as it can transmit lifting force from the yoke to other components and elements of form.
3. Yokes
Yokes are used to attach and support whales at regular intervals with legs. It is the component of vertical slip formwork.
Yokes are capable of transmitting lifting forces from the jacks used during the removal of formwork, to wales. It is also liable to resist the lateral force of plastic concrete within the form.
4. Jacks
Jacks are mounted on a jack rod which in turn is installed on the yokes beam. It acts as a lifter of the slip enabling the necessary force to lift the entire slip form system.
These all components like decks and scaffolds complete this entire system of slip form. They provide sufficient space for concreting, reinforcing steel, and embedding.
5. Jack Rods
The components discussed above are climbed using Jacks. Jacks are themselves mounted on Jack Rods.
Types of Slip Form Systems
Types of Slip Form Systems are,
- Vertical slip form system
- Cantilever Jump Form Method
- Egg shape slip form system
- Conical formwork system
1. Vertical Slip Form System
Quick Setting Property of concrete serves the dependency of the Vertical Slip Form System needed in-between initial strength gain and also the ability to work.
Concrete placing into form-work is a practice done to develop strong initial strength of concrete in order to that the form slips freshly onto concrete avoiding the ability to work.
Various types of hydraulic lifters are available to efficiently lift or extend formwork for the whole horizontal projection. That is where the Work platform comes into the picture.
2. Cantilever Jump Form Method
In Cantilever Jump Form Method, the work of lifting devices in order to lift and expand the formwork is done by the electric motors.
The cantilever Jump Form Method minimizes the use of cranes as it is attached to the large area of formwork with respect to story height.
3. Egg Shape Slip Form System
Being the different type of Slip Form System, however, its principle is based on jump form which is adapted to any of the geometric shapes.
The change in the circumference of the system results in the extension or removal of the cantilever plate. Adjustment of curvature can be done by simply adjusting the circumferential slope and axis.
Egg Shape Slip Form System is flexible to accommodate adjustment on each side of the wall as well as products of optimal working condition.
4. Conical Form-work System
Conical Form-work System is having conical formwork which is made of cantilever plates as well as overlapping plates which are attached through steel yoke frames. Cantilevers, as well as overlapping plates, have the mechanism of automatically adjusting the geometry.
Structures of varying wall thickness including tapering walls can be constructed through this system. However, the structures whose formwork changes geometrically from angular to circular are also constructed through it.
It provides integrated scaffolding systems which consist of horizontal work platforms provided at an optimal and safe working area for labor, engineers, and supervisors communities.
Uses of Slip Form
- The primary use of Slip formwork has been done for constructing
- tall towers and big bridges.
- Silos and grain elevators are constructed using the Slip Form technique.
- This technique has also been introduced in the construction of roads and highways.
History
- The slip-form construction method was first invented in the USA in 1910 for building silos, grain elevators, and cooling towers.
- The first impressive and effective use of slip forming is done in Skylon Tower near Niagara Falls, Ontario, which was completed in 1965.
- Another notable construction using the slip-form technique Sheraton Waikiki Hotel in, Hawaii, in 1969.
- In the 1990s in the U.K. also adopted the slip-formwork technique for paving roadways, bicycle paths, and curbs with the introduction of slip-form paving equipment. And this method is further widely used in the paving of airport aprons, taxiways, and runways.
Slip Form Technique
The slip-form technique is a special technique for placing concrete. This technique is generally adopted for all tall structures like chimneys and silos. It is also adopted for pavement construction. This technique is useful for vertical construction as well as horizontal construction.
In the case of vertical construction like chimneys and silos, using slip forming concrete is continuously placed, compacted, and formwork is pulled up by the formwork jacks, giving a reaction against main reinforcement.
The rate of slipping the formwork depends upon the temperature and strength development of concrete to withstand without the support of formwork. This technique is suitable for uniform-shaped structures with certain precautions.
Read More: Shotcrete – Its Methods & Applications
Advantages
The advantages of Slip Formwork Construction are as given below,
- Provision of a jointless structure.
- A saving of shuttering material initially and also saving the cost.
- Scaffolding is not required.
- Very rapid concreting. It is at least
- four times faster.
- Better finishing of concrete.
- Reduced labor cost.
- Slip-form construction does not require a crane, minimizing crane use.
- No plastering is required.
- Accuracy is more than regular formwork.
- Strength is more than regular formwork.
- Save formwork material.
- Economical for the structure above a certain size.
Disadvantages
The disadvantages of the Slip Formwork are listed below,
- Its installation required a longer time.
- Experienced and skilled supervision need for uniform movement of the slip form system.
- When material stoking is difficult on site.
- Good coordination and site organization are required.
- Large quantities of equipment (e.g. Generators, lighting systems, and hoists) are needed.
- A skilled labor force is required for the operation of equipment and methods.
- The operation must be continued in any weather
- High initial expense.
- Need 24-hour service facilities (e.g. Canteen, material supply, maintenance team, primary clinic).
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