What is formwork?
A formwork is a temporary casing that is designed and constructed to hold the fresh concrete in its place during the process of placing, compacting, and curing till the concrete attains sufficient strength to support its own weight.
In addition to the weight of concrete, formwork carries live loads due to the movement of materials, equipment, and workmen.
Establishing formwork is one of the important processes of building construction because it determines the geometry, shape, size, and finish of the concrete structure.
Requirements of good quality formwork
1) It should be cheap and reusable.
2) It shall be strong enough to withstand all external loads besides the concrete weight.
3) It should be waterproof so that it should not absorb water from the concrete.
4) It should be light in weight so that it is easy to transfer.
5) Swelling and shrinkage should be minimum.
6) Deflection of formwork should be minimum.
7) Surface should be smooth and it should afford easy stripping.
Major objectives of formwork
There are three main objectives,
- Safety and
Forms must be designed and built with sufficient stiffness and accuracy so that the size, shape, and position of the cast concrete are maintained.
Formwork should be built with a sufficient factor of safety so that they possess the necessary strength to support all the loads subjected to them.
The economy is a major concern since formwork costs constitute up to 60 percent of the total cost of concrete work in a project.
In designing and building formwork, the contractor should aim for maximum economy without sacrificing quality or safety.
Three stages of establishing formwork
There are three processes involved in establishing formwork. They are,
- Assembly & Erection
- Concrete placement
- Stripping & Dismantling
1) Assembly & Erection
This process includes the transportation of formwork from storage units to the construction site and erecting those formworks with accurate precision as mentioned in the plan.
Because a small change in dimensions in formwork position causes the entire concrete structure to disorient and results in heavy losses.
2) Concrete placement
Before concrete placement, the site supervisor has to check the strength of the formwork established. It should be strong enough to carry the concrete weight and the weight of machinery, and the men involved during the concrete process.
Also, make sure that the concrete should be compacted and leveled accurately according to the requirements.
3) Stripping & Dismantling
The process of removal of formwork is known as deshuttering of formwork (or) stripping of formwork (or) striking of formwork. It is to be done only after the poured concrete develops sufficient strength to support its own load and any other live loads experienced during construction.
The time from which the concrete is placed in formwork till the time the formwork is removed is called as supporting period.
Read more about Deshuttering time here.
Types / Classification of formwork
The formwork should be classified according to the,
- Brand name
- Location of use etc.
1) According to the size
In practice, formworks are classified into two types according to their size. They are Small sized and Large sized formwork.
- Small-sized formwork – Any size which is designed for operation by workers manually is small-sized.
- Large-sized formwork – These forms can be designed as large as practicable to reduce the number of joints. Cranes are used to lift these forms since they are heavy.
2) According to the material
Formwork is made of different types of materials. Each of them has its own advantages as well as disadvantages. They are,
- Timber Formwork
- Steel Formwork
- Aluminum Formwork
- Plywood Formwork
- Fabric Formwork
- Plastic Formwork
3) According to the brand name
Several patented or branded formwork systems have successfully entered the local construction market in the past decade.
Each of these firms offers its own specialized products, while some can even provide a very wide range of services including design support or tender estimating advice.
For example, Mivan shuttering – Mivan is a company name that manufactures aluminium shuttering.
4) Types of formwork according to the location of use
a) Foundation formwork
The formwork size and shape depend upon the size and shape of the footing. It is the first stage of concreting in any construction process.
The formwork consists of side support and side planks.
b) Column formwork
Proper care must be followed while establishing column formwork because while concreting it is subjected to lateral pressure. The formwork must have sufficient stiffness to withstand the lateral force and control deflection.
It consists of side & end planks, a yoke, Nuts & bolts.
c) Wall formwork
The shuttering shall be fixed at the required distance equal to the required wall thickness.
After establishing one side of the formwork, reinforcement is provided and then the other side of the formwork is established.
It consists of
- Timber sheeting
- Vertical posts
- Horizontal member
d) Beam formwork & Slab formwork
Beam formwork rests on the head tree and the slab formwork rests on battens and joists.
If the props exceed the height of 8 feet, provide horizontal bracing.
Formwork failure and its causes
Formwork failures are the cause of many accidents and failures that occur during concrete construction which usually happen when fresh concrete is being placed.
Generally, some unexpected event causes one member to fail, then others become overloaded or misaligned and the entire formwork structure collapses.
The main causes of formwork failure are,
- Inadequate supports to withstand both concrete and working loads
- Early stripping of formwork
- Early removal of props
- Inadequate bracing
- Lack of attention to formwork details
- Too much movement of workers and equipment on the formwork
Let’s see some of the major formwork fails.
Oh, man! How do you explain this to the clients?
Maintenance of formwork
As soon as the formwork is removed, all the chippings of concrete and sawdust must be removed from the interior surface of the formwork and the formwork should be piled up and covered with canvas.
In the case of steel formworks, the interior surface of the formwork should be treated with any releasing agents such as oil to prevent it from getting rusty.
Wood formwork has a limited number of reuses compared to other materials of formwork. Hence make sure, whether it is eligible for any reuse purposes.
For more information refer to IS 14687: 1999.
Hope you understand everything you need to know about Formwork & its types. If you have any queries (or) if you find this article helpful, let us know in the comment section.