Concrete is one of the most widely used construction materials, thanks to its exceptional properties. However, to create building elements with concrete, it must be poured into a specially designed mold. This is known as formwork or shuttering.
Form-work can use temporary or permanent molds, which hold the poured concrete in shape until it hardens and achieves enough strength to support itself. Form-work/Shuttering can be classified in many ways:
- Type of material used
- By the concrete element supported
- Removable or permanent
Formwork has a fundamental role in concrete construction. It must have enough strength to bear all the loads present during casting operations, and must then hold its shape while concrete hardens.
Which Are the Requirements for Good Formwork?
Although there are many form-work materials, the following are general performance features to meet the needs of concrete construction:
- Capable of withstanding dead and live loads.
- Retaining its shape with adequate props and braces.
- Joints must be leak-proof.
- If form-work is removable, the process must not damage the concrete.
- Reusable material.
- As lightweight as possible.
- The form-work material should not warp or distort.
When selecting formwork it is important to consider the type of concrete and the pouring temperature, since both affect the pressure exerted. Also, formwork must be capable of resisting the loads of wet and dry concrete.
Formwork requires structures such as poles and stabilisers to avoid movement during construction procedures, and these are called falsework. To ensure high quality when working with concrete, a qualified workforce and adequate supervision are necessary.
The following sections provide an overview of some common formwork materials.
Timber Formwork
Timber form-work was one the first types used in construction industry. It is assembled on site and is the most flexible type, bringing the following advantages:
- Easy to produce and remove
- Lightweight, especially when compared with metallic formwork
- Workable, allowing any shape, size and height
- Economical in small projects
- Allows the use of local timber
However, before using timber its condition must be checked carefully, making sure it is free of termites. Timber form-work also has two limitations that must be considered: it has a short life span and is time consuming in large projects. In general, timber formwork is recommended when labor costs are low, or when complex concrete sections require flexible formwork.
Plywood Formwork
Plywood is often used used along with timber. It is a manufactured wooden material, which is available in different sizes and thicknesses. In formwork applications, it is mainly used for sheathing, decking and form linings.
Plywood formwork has similar properties as timber form-work, including strength, durability and being lightweight.
Metallic Formwork: Steel and Aluminum
Steel formwork and steel hardware is becoming more popular due to its long service life and multiple reuses. Although it is costly, steel formwork is useful for multiple projects, and it is a viable option when many opportunities for reuse are expected.
The following are some of the main features of steel form-work:
- Strong and durable, with a long lifespan
- Creates a smooth finish on concrete surfaces
- Waterproof
- Reduces honeycombing effect in concrete
- Easily installed and dismantled
- Suitable for curved structures
Aluminum formwork is very similar to steel form-work. The main difference is that aluminum has a lower density than steel, which makes form-work lighter. Aluminum also has a lower strength than steel, and this must be considered before using it.
Plastic Formwork
This type of formwork is assembled from interlocking panels or modular systems, made of lightweight and robust plastic. Plastic form-work works best in small projects consisting on repetitive tasks, such as low-cost housing estates.
Plastic form-work is light and can be cleaned with water, while being suitable for large sections and multiple reuses. Its main drawback is having less flexibility than timber, since many components are prefabricated.
Fabric Formwork
Fabric form-work is also known as flexible form-work. This system uses lightweight and high-strength sheets of fabric, designed to adjust to the fluidity of concrete and create interesting architectural forms.
This form-work type uses less concrete than rigid systems, which yields savings. It is an emerging technology in the shuttering industry, especially suited for constructions of irregular and complex shapes.
Stay-In-Place Formwork
This form-work is designed to remain fixed after the concrete has set, acting as axial and shear reinforcement. This form-work is made on-site from prefabricated and fibre-reinforced plastic forms. It is mainly used in piers and columns, and also provides resistance against corrosion and other types of environmental damage.
Another type of stay in place form-work is called coffer, which can be used in any type of building:
- It is composed of two filtering grids, reinforced by stiffeners and linked with articulated connectors.
- Thanks to its construction, it can be easily transported from a factory to the point of use.
Permanent Insulated Formwork
This is one of the most advanced form-work systems, offering permanent insulation. It may also include thermal, acoustic, fire-resistance and rodent-resistance properties. Insulating concrete forms (ICF) are the most common type of permanent insulated form-work, where concrete structures are insulated with polystyrene boards that stay in place after concrete has cured.
Permanent insulated form-work offers energy efficiency and sustainability, contributing to a lower environmental impact from the building sector.
Classifying Formwork Based on Structural Components
In addition to being classified by material, form-work can also be classified according to the building elements supported:
- Wall form-work
- Beam form-work
- Foundation form-work
- Column form-work
All form-work types are designed according to the structure they support, and the corresponding construction plans specify the materials and required thickness. It is important to note that form-work construction takes time, and it can represent between 20 and 25% of structural costs. To mitigate the cost of form-work, consider the following recommendations:
- Building plans should reuse building elements and geometries as much as possible to allow form-work reusing.
- When working with timber form-work, it should be cut into pieces that are large enough to be reused.
Concrete structures vary in design and purpose. Like in most project decisions, no option is better than the rest for all applications; the most suitable form-work for your project varies depending on building design.