Essay: Slip forms

During the process of construction, the slip-forms are raised while the concrete is in a plastic state. These are used for the construction of tall structures, e.g., chimney.
Slip-forms may be classified as :
(i)Straight slip-forms
(ii)Tapering slip-forms
(iii)Sand slip-forms for special applications.
Straight slip-forms are used for silos, straight-chimneys, water towers, etc. Tapering chimneys and ventilators are constructed using tapering slip-forms.
Once a set of slip-forms is completely assembled at the bottom of the structure, the forms are filled slowly with concrete. Concrete is allowed to gain sufficient rigidity.
Once attaining sufficient rigidity, the upward movement of the forms is started. It is continued at a speed that is controlled by the rate at which the concrete sets.
The raising of the slip-forms is done with the help of screw jacks or hydraulic jacks. Among these two jacks, hydraulic jacks are superior to screw jacks and they assure uniform upward movement.
Rate of lifting of form will depend on the temperature of concrete which controls the rate of set. Concrete will stick to the forms if the lifting is slow and it will be difficult for lifting the forms.
On the other hand, if lifting is fast the concrete will bulge at the bottom of forms. Such bulge will affect the stability of the entire structure.
Advantages of Slip-forms
1.As the forms are lifted continuously, there is no possibility of any joints.
2.Because of the non-formation of construction joints, there will be perfect water tightness.
3.As the construction work is continuously done, time lost in removing and re-setting of forms is saved.
Difficulties faced in the Construction Using Slip-forms
1.Good site organization and coordination are needed.
2.There is a need for large number of equipment.
3.Because of continuous operation day and night shifts are to be arranged.
2.18 Scaffoldings
In the routine construction of work, sometimes it is necessary to have some temporary structure or support to continue the work.
When the height of construction exceeds about 1.5m a temporary structure, usually of timber, is erected close to the work so as to provide a safe working platform for the workers and to provide adequate space to keep the working materials.
This temporary structure is known as scaffolding or simply a scaffold. Such temporary structures are used in construction, demolition, maintenance or repair work.
Parts of Scaffolding
One or all of the parts mentioned below are used in the construction of a scaffolding for a specific work.
1.Standards
These are the vertical members of the scaffold which are either supported on the ground or embedded in to the ground or on sand filled drums.
2.Ledgers
Horizontal members of the scaffold.
3.Putlogs
Transverse pieces which are placed on ledgers and perpendicular and supported on the wall.
4.Transoms
Putlogs whose both ends are supported on the ledgers.
5.Bridges
These are used to bridge an opening in a wall and support one end of the putlog at the opening.
6.Braces
Cross or diagonal pieces fixed on the standards.
7. Guard Rail
Horizontal member provided like a ledger at the working level.
8.Toe Board
Placed parallel to the ledgers and supported between the putlogs.
Made as a protective measure to work on the working platform.
9.Raker
Is an inclined support.
All these members are kept in position securely by means of devices such as nails, bolts, ropes, etc.
Requirements of Scaffolding
1.The method of erection should be easy with less wastage of material.
2.It should possess adequate strength and stability during the entire period of usage as men have to use at all heights.
3.The material required for scaffolding should be available easily in all sizes and lengths.
4.It should be possible to interchange the material for other works with less wastage.
5.The initial cost should be comparatively less and should have high scrap value.
Types of Scaffolding
Different types of scaffolding are explained below
1.Single scaffolding or Bricklayer’s scaffolding
2.Double scaffolding or mason’s scaffolding
3.Cantilever or needle scaffolding
4.Suspended scaffolding
5.Trestle scaffolding
6.Steel scaffolding
7.Patented scaffolding
1.Single Scaffolding or Brick layer’s Scaffolding
Particularly used in the construction of brickwork. It consists of a single row of standards placed at a distance of about 1.20 m from the wall. Spacing between the standards is about 2 to 2.5 m. Ledgers are fixed at a vertical distance of 1.20 to 1.80 m on the standards.
Details of single scaffolding
The putlogs are placed at a horizontal spacing of 1.20 to 1.80 m. Also called as putlog scaffolding.
2.Double Scaffolding or Mason’s Scaffolding
This is stronger than the single scaffolding. Similar to that of simple scaffolding except two rows of standards are used, out of which one is close to the wall and the other is 1.2 to 1.5 m away from the face of the wall.
No holes are made on the wall for putlogs and the putlogs are supported at both ends on ledgers. Sometimes diagonal bracing and inclined supports called racking shores are provided. It is used for stone-masonry construction.
Details of double scaffolding
3. Cantilever or Needle Scaffolding
General frame work may be of single or double type of scaffolding. Standards are supported by needles or ties.
These ties are projected out at floor levels or through openings or through holes provided in the masonry. Two types of cantilever scaffolding are shown in Fig.
Cantiliver scaffoldings
4.Suspended Scaffolding
It is a light type of scaffolding used only for maintenance works like pointing, white washing, etc. Working platform is suspended from the roofs.
Special arrangements are made with pulleys, ropes, etc. to suspend the platform from the roof and to raise or lower based on the need.
This type of arrangement is preferred as it does not create any obstruction on the ground and only a minimum space is required.
5. Trestle Scaffolding
Here the working platforms are supported on tripods, ladders, etc. which are mounted on boggies, wheels or lorries. It is suitable for minor repairs or painting work within a height of 5 m.
6.Steel Scaffolding
Here steel tubes are effectively used in the place of timber. 40 to 50 mm tables of 5 mm thick are used.
Tubes are commercially available in suitable lengths with special couplings and set-screws. There is no height restriction. It is strong and more durable.
Details of tubular steel scaffolding
Here Erection and dismantling are very easy. Material of tubes is resistant to fire and high scrap value.
7.Patented Scaffolding
It is also called as ladder scaffolding. It is a modified form of double scaffolding but of steel. These are patented scaffolds readily available in market with special types of couplings and frames.
Working platform is supported on a bracket which can be adjusted to any suitable height. It is used for light works like painting or decoration.
Details of patented scaffolding
2.19 De-shuttering forms
As a general rule the concrete may be left in place for as long as possible up to a maximum of 28 days in normal weather conditions.
Period to which the concrete should be left in place depends on the temperature of air, the shape and position of structural member, the load condition and the type of cement.
In case use of rapid-hardening cement, lighter-temperatures, low water-cement ratio and higher loads, the curing period may be reduced and early removal of forms may be permitted.
Hardness of concrete can be checked by striking the concrete and if a metallic sound is heard it can be presumed that the concrete is hardened and the forms may be removed.
For other reasons such as re-use of forms, early use of the structure and need to cool the concrete in massive concrete the forms may be removed subjected to the condition that the concrete is adequately hardened.
In normal conditions with the atmospheric temperature is above 20°C, normal OPC is used. Type of structure, the forms may be removed after a curing period as given in Table.
The forms should be slackened gradually in order to prevent the sudden imposition of load on the structure.