What is Formwork in Concrete Construction? Formwork (shuttering) in concrete construction is a mold used to pour fresh concrete, which then hardens to form the desired structure. The types of formwork vary depending on the formwork material and the type of structural element being constructed.
Types of Formwork Formworks are classified based on the type of structural member being built, such as slab formwork for use in a slab, beam formwork, column formwork for use in beams and columns, respectively, etc.
Cost and Construction of Formwork The construction of formwork typically takes up to 20 to 25% of the total cost of the structure. The design of these temporary structures is made to be economical. The process of removing the formwork is called stripping, and the formwork itself can either be reusable (panel forms) or non-usable (stationary forms).
Common Materials Used for Formwork Timber is the most common material used for formwork. However, it is prone to warping, swelling, and shrinking, which can be mitigated by applying water-impermeable coatings to the surface of the wood.
Requirements of Good Formwork
Requirement for Structural Support : For formwork to be effective, it must be capable of withstanding all types of dead and live loads. It must be rigidly constructed and braced both horizontally and vertically to maintain its shape and prevent any damage to the concrete.
Preventing Leakage: The joints of the formwork must be tightly sealed to prevent leakage of cement grout. This will ensure the structural integrity of the concrete and the formwork itself.
Cost-Effective and Reusable: The materials used in formwork should be cheap, readily available and reusable. This will ensure that the formwork can be used multiple times, thus reducing costs and increasing its efficiency.
Accurate Alignment: Formwork should be accurately set to the desired line and levels and should have a plane surface. This will ensure that the concrete is properly formed and that it meets the required specifications.
Lightweight Formwork:
Formwork should be as light as possible, as this will make it easier to move and handle. At the same time, the material used should not warp or get distorted when exposed to the elements.
Firm Base: Finally, formwork should rest on a firm base to ensure that it is secure and that it does not move when the concrete is being poured. This will ensure that the concrete is formed correctly and that it will not crack or break under pressure.
Economy in Formwork
1. Planning for Economy in Formwork: When planning a building, minimizing variations in room size, floor area, and other features is an effective way to reduce formwork costs by enabling reuse of the formwork repeatedly. Careful design of building elements is also key to ensuring the formwork is used most economically, using the most slender sections possible.
2. Variety of Formwork Materials: Formwork can be constructed using timber, plywood, steel, precast concrete, or fiberglass, either separately or in combination. Steel forms are often used when a large number of re-uses are necessary, while timber formwork is more suitable for small works. Fiberglass, precast concrete, and aluminium can be used for cast-in-situ construction such as slabs or members with curved surfaces.
3. Minimizing Waste: Care should be taken to minimize sawing and cutting of wooden pieces in order to maximize reusability of the material. The quality of the formwork surface finish is also dependent on the quantity of surface finishing used.
Types of Formwork (Shuttering)
1. Requirements for Timber Formwork
Timber formwork must be well-seasoned, lightweight, and easy to work with nails without splitting. Additionally, timber used for shuttering for exposed concrete work should have a smooth surface on all faces that come in contact with concrete. This is to ensure a seamless finish and to prevent any damage caused by uneven surfaces. Furthermore, the timber should be free from loose knots, as these can weaken the structure and cause damage to the concrete.
Selecting Timber for Formwork:
When selecting timber for formwork, it is essential to consider the quality and durability of the timber. It is important to choose timber that is well-seasoned and light in weight, as this will ensure that the formwork is easy to work with and will not cause any damage to the concrete. Additionally, the timber should be free from loose knots, as these can weaken the structure and cause damage to the concrete. Furthermore, it is important to ensure that the timber has a smooth and even surface on all faces that come in contact with concrete, as this will allow for a seamless finish and prevent any damage caused by uneven surfaces.
Table 1: Normal sizes of members for timber formwork
| Sheeting for slabs, beam, column side, and beam bottom | 25 mm to 40 mm thick |
| Joints, ledges | 50 x 70 mm to 50 x 150 mm |
| Posts | 75 x 100mm to 100 x 100 mm |
2. Plywood Formwork
Advantages of Plywood Formwork: Plywood formwork has several advantages over timber shuttering, making it a cost-effective choice for construction projects. Firstly, a smooth finish can be achieved which eliminates the need for additional surface finishing costs. Secondly, large-size panels can be used to reduce labour costs associated with fixing and dismantling. Finally, the number of reuses of plywood formwork is greater than that of timber shuttering, with an estimated 20-25 reuses.
Cost Savings of Plywood Formwork: Using plywood formwork as opposed to timber shuttering can result in significant cost savings. By achieving a smooth finish and making use of large-size panels, labour costs associated with fixing and dismantling are reduced. Additionally, the number of reuses is higher, which further contributes to the cost savings. All these factors make plywood formwork a cost-effective option for construction projects.
3. Steel Formwork
Overview of Steel Formwork: Steel formwork is a type of formwork made of thin steel plates that are stiffened along the edges with small steel angles. It can be held together with clamps or bolts and nuts and is used in large projects or when a large number of reuses of the shuttering is possible. It is especially suitable for circular or curved structures.
Benefits of Using Steel Formwork: Steel formwork is a highly efficient and cost-effective solution for many construction projects. It can be fabricated in large numbers in any desired modular shape or size, which makes it ideal for a wide variety of structures. Additionally, it is easy to assemble and disassemble and can be reused multiple times, resulting in lower costs and improved efficiency.
Advantages of Steel Formwork: Steel formwork is a strong and durable option that offers a number of advantages. It is extremely resistant to heat, water, and corrosion, making it suitable for a variety of environments. Additionally, it is relatively light in weight, making it easier to transport and handle. Finally, it is easy to assemble and disassemble, and can be reused multiple times, making it a cost-effective solution.
Comparison between Steel and Timber Formwork
| Property | Steel Formwork | Timber Formwork |
|---|---|---|
| Strength and Durability | Stronger and more durable | Weaker and less durable |
| Installation and Dismantling | Easier and faster | More difficult and slower |
| Exposed Concrete Surfaces | Good quality, no extra treatment required | Lower quality, extra treatment required |
| Moisture Absorption and Shrinkage | Resistant to absorption and shrinkage | Prone to absorption and shrinkage |
Construction of Concrete Formwork
This normally involves the following operations:
1. Propping and Centering for Concrete Formwork: The first step in constructing concrete formwork is propping and centering. This involves the installation of support structures to ensure the formwork is stable and properly aligned. This may involve the use of steel or timber props, as well as adjustable metal and wooden wedges. The centering is done by using laser levels, spirit levels and other measuring tools to ensure the formwork is level and correctly aligned.
2. Shuttering for Concrete Formwork: The next step of constructing concrete formwork is shutter installation. This is the process of installing shutters, which are panels used to form the walls and other surfaces of the formwork. The shutters come in a variety of materials and sizes and must be properly installed to provide a secure and stable formwork.
3. Providing Camber for Concrete Formwork: Camber is an important feature of concrete formwork, which helps ensure the finished concrete structure is strong and even. The camber is the slight bend that is applied to the formwork when it is being constructed. This is done by using adjustable wedges and other tools to ensure the formwork is correctly curved.
4. Cleaning and Surface Treatment for Concrete Formwork: Once the formwork has been constructed, it must be cleaned and treated. This involves removing any dirt and debris, as well as applying a sealant or protective coating to the formwork. This helps ensure the formwork is durable and long-lasting. This process also helps to prevent water and other liquids from entering the formwork and causing damage.
Order and Method of Formwork Removal
Sequence for Formwork Removal: The formwork removal should follow a specific sequence for the best results. Firstly, the formwork that forms the vertical faces of walls, beams and column sides should be removed. This is because these sections bear no load and only retain the concrete. Secondly, the shuttering that forms the soffit of slabs should be removed. Finally, the shuttering that forms the soffit of beams, girders, or other heavily loaded structures should be removed.
Early Removal of Formwork: Under certain conditions, the formwork can be removed earlier than the recommended time. These conditions include the use of rapid hardening cement, warm weather conditions, and light loading. However, the formwork should not be removed until the concrete has reached a strength of at least twice the stresses to which the concrete may be subjected at the time of removal of formwork.
Careful Removal of Formwork: When removing the formwork, it is important to ensure that the load is not suddenly transferred to the concrete. This can be done by easing the formwork gradually and carefully.
Figure 1 to 6 shows formwork for different types of members in civil engineering construction.
Table 2: Period of Removal of Formwork
| S. No. | Description of structural member | Time Period |
| 1 | Walls, columns and vertical sides of beams | 1 to 2 days |
| 2 | Slabs (props left under) | 3 days |
| 3 | Beam soffits (props left under) | 7 days |
| 4 | Removal of props to slabs | |
| (a) For slabs spanning upto 4.5 m | 7 days | |
| (b) For slabs spanning over 4.5 m | 14 days | |
| 5 | Removal of props to beams and arches | |
| (a) Spanning upto 6 m | 14 days | |
| (b) spanning over 6 m | 21 days |
FAQs on types of formworks and their properties
1. What is timber formwork?
Answer: Timber formwork is a type of formwork that utilizes wooden boards and beams to create a temporary mold and support structure for a concrete pour. It is typically made from timber, plywood, and metal frames.
2. What is plywood formwork?
Answer: Plywood formwork is a type of formwork that uses thin sheets of plywood to form a temporary structure for a concrete pour. Plywood formwork is often used for curved walls and columns, as the thin plywood sheets are able to conform to any shape.
3. What is steel formwork?
Answer: Steel formwork is a type of formwork that utilizes steel panels, frames, and posts to create a temporary mold and support structure for a concrete pour. Steel formwork is extremely durable and can be used for complex shapes and forms.
What is formwork in building constructions?
Formwork is used to describe the process of building temporary molds in to which fresh concrete is poured to construct designated structural concrete elements and achieve its shape.
Formwork is an essential part of the construction process, as it acts as a base support system for the concrete while it is drying and setting. It can also be used to create a variety of shapes, such as columns, walls, beams and floors.
What are the requirements of formworks?
1. Adequately strong to withstand all types of loads. 2. Rigid enough to retain its shape. 3. Joints in the formwork tight against leakage of cement grout. 4. Formwork construction should permit the removal of various parts in desired sequences without damage to the concrete. 5. The formwork material should be cheap, readily available, and reusable. 6. The formwork should be set accurately to the desired line, and levels should have a plane surface. 7. It should be as light as possible. 8. It should rest on a firm base. 9. It should be easy to assemble and dismantle.
What are the advantages of formworks?
1. Formwork can be used to create complex shapes and forms. 2. Formwork systems can be used multiple times, reducing the amount of material waste. 3. It can reduce labor costs as the formwork can be pre-assembled and erected quickly. 4. Formworks can provide a high level of safety, as there is a reduced risk of falls and injuries due to the sturdy nature of the formwork. 5. Formwork systems can be easily adapted to suit a variety of projects.
How much does a formwork cost in comparison to the total cost of the structure?
What is the approximate cost of formwork in comparison to the total cost of the structure? Answer: The cost of formwork typically accounts for 20 to 25% of the total cost of the structure, or even more.
What are the characteristics of timber formwork?
Answer: Timber formwork should be well-seasoned, light in weight, easily workable with nails without splitting, and free from loose knots.
What are some examples of formwork systems?
Answer: Examples of formwork systems include timber formwork, plywood formwork, and steel formwork.