Effective Concrete Formwork (Shuttering) | 3 Types, & Essential Requirements

Concrete Formwork

Concrete when mixed is a fluid and therefore to form any concrete member the wet concrete must be placed in a suitable mould, known as concrete formwork, to retain its shape, size and position as it sets. It is possible with some forms of concrete foundations to use the sides of the excavation as the mould but in most cases when casting concrete members a mould will have to be constructed on site. It is important to appreciate that the actual formwork is the reverse shape of the concrete member which is to be cast.

Falsework ~ The temporary structure which supports the concrete formwork.

Formwork sides ~ limits width and shape of wet concrete and has to resist the hydrostatic pressure of wet concrete which diminish to zero within a matter of hours depending on setting and curing rate.

Formwork base or soffit ~ limits depth and shape of wet concrete and has to resist the initial dead load of wet concrete and later the dead load of dry set concrete until it has gained sufficient strength to support its own dead weight which is usually several days after casting depending on curing rate.

Requirements of Good Formwork

• Formwork must be structurally strong and stable to withstand the pressure exerted by fresh concrete during pouring and the weight of the concrete itself.
• Formwork materials should be durable enough to withstand multiple uses and the effects of weather, chemicals, and other environmental factors.
• Formwork should provide a smooth and even surface finish to the concrete. This is especially important for architectural concrete where a high-quality finish is desired.
• Formwork must maintain dimensional accuracy to ensure that the final structure conforms to design specifications.
• Good formwork should be easy to handle, transport, and install. This helps in saving time and labor costs during the construction process.
• Formwork materials should be resistant to water absorption to prevent swelling, warping, or degradation. This is especially important for formwork used in wet or humid conditions.
• Formwork should be adaptable to various shapes and sizes, allowing for the construction of different architectural elements without the need for extensive modifications.

Erecting Concrete Formwork

1. Props positioned and leveled through
2. Soffit placed, levelled and position checked.
3. Side forms placed, their position checked before being fixed.
4. Strutting position and fixed.
5. Final check before casting.

Striking or Removing Formwork

1. Side forms removed as soon as practicable usually within hours of casting this allows drying air movements to take place around the setting concrete.
2. Soffit formwork removed as soon as practicable usually within days but as a precaution some props are left in position until concrete member is self-supporting.

Beam Formwork

This is basically a three-sided box supported and propped in the correct position and to the desired level. The beam formwork sides have to retain the wet concrete in the required shape and be able to withstand the initial hydrostatic pressure of the wet concrete whereas the formwork soffit apart from retaining the concrete has to support the initial load of the wet concrete and finally the set concrete until it has gained sufficient strength to be self-supporting.

It is essential that all joints in the concrete formwork are constructed to prevent the escape of grout which could result in honeycombing and/or feather edging in the cast beam. The removal time for the formwork will vary with air temperature, humidity and consequent curing rate.

Column Formwork

This consists of a vertical mould of the desired shape and size which has to retain the wet concrete and resist the initial hydrostatic pressure caused by the wet concrete. To keep the thickness of the formwork material to a minimum horizontal clamps or yokes are used at equal centers for batch filling and at varying centers for complete filling in one pour.

The head of the column formwork can be used to support the incoming beam formwork which gives good top lateral restraint but results in complex formwork. Alternatively the column can be cast to the underside of the beams and at a later stage a collar of formwork can be clamped around the cast column to complete casting and support the incoming beam formwork. Column forms are located at the bottom around a 75 to 100 mm high concrete plinth or kicker which has the dual function of location and preventing grout loss from the bottom of the column formwork.

Wall Forms

Conventionally made up of plywood sheeting that may be steel, plastic or wood faced for specific concrete finishes. Stability is provided by vertical studs and horizontal waling’s retained in place by adjustable props. Base location is by a kicker of 50 of 75mm height of width to suit the wall thickness.

Formwork sides to concrete walls of modest height and load can be positioned with long bolts or threaded dowel bars inserted through the waling’s on opposing sides. To keep the wall forms apart, tube spacers are placed over the bolts between the forms. For greater load applications, variations include purpose-made high tensile steel bolts or dowels. These too are sleeved with plastic tubes and have removable spacer cones inside the forms. Surface voids from the spacers can be made good with strong mortar. Some examples are shown below with the alternative coil tie system.

Types of Concrete Formwork

Concrete formwork comes in various types, each serving specific construction needs. Here are some common types of concrete formwork based on material:

1. Wooden Formwork

Plywood ~ Plywood is a popular choice due to its flexibility and ease of use. It is relatively inexpensive and suitable for curved or irregular shapes.

Timber ~ Timber is often used for formwork frames and is easy to work with. However, it may not be as durable as other materials and may require more maintenance.

2. Steel Formwork

Steel formwork is robust, durable, and can be reused many times. It provides a smooth finish to the concrete surface and is suitable for large and complex structures. However, it can be heavy and may require special equipment for handling.

3. Plastic Formwork

Made from plastic materials such as high-density polyethylene (HDPE). It is lightweight, durable, and can be easily handled and transported. It is resistant to corrosion and can be used for multiple cycles. It is suitable for smaller and simpler projects.