What is Formwork (Shuttering)?
There are different types of formwork according to the structure and materials used. Fresh concrete is in a plastic state when it is placed for construction purposes.
So, it becomes necessary to provide some temporary structure to confine and support the concrete unless it gains enough strength for support itself.
This temporary structure is known as formwork or shuttering. Formworks are classified as wooden, plywood, steel, combined wood and steel, reinforced concrete, and plain concrete.
Each type of form has its own character and of course some drawbacks. However, if forms are required to use for only one or two construction wooden forms are the best alternatives.
But, if it is to be used for repeated constructional work steel forms are economical.
Principal Requirements of formwork (Shuttering)
Good formwork should satisfy the following requirements
- The material of the formwork should be cheap and should be suitable for reuse several times.
- It should be practically water resistance so that it does not absorb water from the concrete structure. Also, it’s shrinkage and swelling should be minimal.
- It should be stiff so that deflection is minimum.
- It should be light as far as possible.
- It should be strengthened enough to carry all types of dead, live and seismic loads.
- There should not be leakage of cement grout through the joint of formwork. i.e (Bounded together with no gap.)
Types of formwork according to structure
Formwork for Column
Figure shows the formwork details for a rectangular or square column in the plan.
Following features of the formwork for these RCC columns should be noted Column box consists of two ends (see figure plank fixed with wedges) and two sides (with yokes), each built as a panel of planks sheeting.
At top cuts are made if the column is to receive girder or beam as shown. The height of each panel is equal to the story height minus the slab thickness and the floor sheeting.
The width between two opposite panels is kept equal to the actual dimension of the column plus twice the thickness of sheeting Yokes are equally spaced at both sides and ends.
They project at both ends about 25cm. A hole is usually provided at the bottom of the formwork to receive debris etc. before placing the concrete.
The cleanout hole is covered before starting the concreting work Formwork components for any column are required to be designed properly to resist high pressure resulting from quick filling of concrete.
Deshuttering time of column
For column the deshuttering time is 24 hours but in cold weather it may take 48 hours. This is deshuttering time of vertical structure.
Formwork for RCC Beam, Slab, Floor
In figure, the details of formwork for RCC beam and slab monolithic construction is shown. The decking or sheeting consists of plywood or timber planks.
The decking is supported on wooden joist (called bearer) which is properly spaced depending upon the loads to be borne within the permissible detection limits.
The maximum spacing of vertical post or shores depends upon the strength of stringers (wooden beams) or the capacity of shores to bear the loads.
The vertical posts or shores, lateral supports, shore braces, steel base plates, and wooden wedge provide a strong centering to the beam and the slab floor to bear all the loads due to them.
Deshuttering time of RCC Beam, Slab, Floor
The Deshuttering time for beam is about 14 days to 16 days and The Deshuttering time for slab is 20 days to 24 days.
Types of formwork Materials (Shuttering)
Timber formwork
The formwork constructed with the help of timber is known as timber formwork. The first way of formwork was timber formwork. But now a days it has been replaced by metal formwork.
Selection of timber formwork
It should be light weight
It should be well seasoned otherwise it may bent the shape of structure.
It should be free from any types defect of timber like knots, and decayed portion.
It should be thin as far as possible.
Advantages of timber formwork
It is lighter than metal.
It can be cut and make into desired shape easily.
It can be fixed at any shape with the help of nails.
It is available everywhere.
It can be reused for firewood if it is unworkable.
Disadvantages of Timber formwork
Timber doesn’t found in totally plane and smooth surface.
Timber are mostly have more weight so it is hard to carry.
Plywood formwork
This formwork is constructed with the help of plywood. It is very cheap in cost but less durable.
Advantages of plywood formwork
It have very smooth and plane surface.
It is very light in weight to carry one place to another.
it has low cost than timber.
It is more efficient for handling.
Disadvantages of plywood formwork
It damage easily while de-shuttering concrete structure.
It is less durable.
It may damage easily while it get moisturized.
Steel formwork
The shuttering done with steel is known as steel formwork. There are also other types of metal formwork like aluminium formwork.
Advantages of steel formwork
It is strong and more durable
It does not observe moisture to effect concrete.
It give smooth and plain surface to the concrete structure.
It is easy to shuttering and dismantling the formwork.
Disadvantages of steel formwork
It is expensive.
It is hard to to handle.
It cannot be cut and fit into required shape like timber.
It required skilled human resource to set.
Formwork safety precautions
The formwork should be strong enough to support the weight of concrete plus the additional load of nearly 30% to account for labor, storage of material, equipment etc.
Slab formwork may be supported by two arrangements namely Decking is supported bymeans of wooden centering on the floor below Planks decking is made to rest on the top of the beam and girder sides.
The formwork for the floor should be given the desired slope.
To avoid the construction of formwork or scaffolding, the beam formwork is fabricated on the ground and then hoisted and placed in position with the slab above.
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