Formwork refers to the temporary or permanent molds into which concrete is poured to shape structural elements such as beams, columns, slabs, and walls. After the concrete gains sufficient strength, the formwork is removed.
Functions of Formwork
- Shape and Support: Formwork provides the necessary shape and support to fresh concrete until it hardens.
- Load-bearing: It must withstand the pressure exerted by the fresh concrete, which can be quite heavy.
- Smooth Surface Finish: Properly designed formwork helps in achieving a smooth surface finish and dimensional accuracy.
Types of Formwork
- Timber Formwork:
- Materials: Made from timber or plywood.
- Advantages: Easy to assemble and dismantle, lightweight, economical for small projects.
- Disadvantages: Not reusable for large projects, and can absorb water, leading to warping.
- Steel Formwork:
- Materials: Made from steel plates and angles.
- Advantages: Durable, reusable, provides smooth surface finishes, and has better rigidity.
- Disadvantages: Heavy and expensive compared to timber formwork.
- Aluminum Formwork:
- Advantages: Lightweight, reusable, and easy to handle, widely used in repetitive construction like mass housing projects.
- Disadvantages: Higher initial cost and less flexible for adjustments on-site.
- Plastic Formwork:
- Materials: Made from modular plastic panels.
- Advantages: Lightweight, reusable, and resistant to corrosion. Suitable for low-cost housing and mass housing projects.
- Disadvantages: Limited to simpler shapes and not very durable for large-scale projects.
- Plywood Formwork:
- Materials: Plywood sheets supported by timber or steel frames.
- Advantages: Provides a smooth surface finish and is relatively inexpensive.
- Disadvantages: Absorbs moisture, requiring waterproofing treatments.
- Fabric Formwork:
- Materials: Flexible fabric sheets.
- Advantages: Lightweight and flexible, used in creating complex and free-form shapes.
- Disadvantages: Requires specialized knowledge and careful handling.
Design Requirements for Formwork
- Strength: Must withstand the loads imposed by wet concrete, workers, and equipment.
- Rigidity: Must be rigid enough to prevent deflection or deformation.
- Ease of Handling: Formwork should be easy to erect, dismantle, and transport.
- Surface Finish: Should ensure smooth surfaces without any leakage of concrete.
- Economy: Should be reusable and cost-effective in terms of material and labor.
Formwork Removal (Stripping Time)
The formwork should not be removed until the concrete has gained sufficient strength to support itself. Stripping times are influenced by:
- Concrete Strength: Generally, formwork is removed when the concrete reaches about 70% of its design strength.
- Element Type: Beams, slabs, columns, and walls require different stripping times. Vertical formwork is removed earlier than horizontal formwork.