Formwork is a complex technique involved in construction which requires specialist contractors for proper use. It’s the term given to specific moulds or receptacles into which concrete is poured to make slabs for construction or actual components of buildings. Once the concrete has hardened to an adequate level, the forms are removed. This is known as ‘stripping’.
There are many different types of formwork which are used in building, for many different purposes. There are three main materials for creating formwork – traditional timber formwork, engineered formwork (usually a metal frame) and stay-in-place formwork systems (usually pre-made concrete systems). Using these types of formwork, there are a variety of different ways to create slabs and structures from formwork.
How It Works
Roman engineers began using slab formwork to complete major constructions which used concrete. The engineers built their constructions using arches and domes because concrete does not have a strong resistance to stress. Concrete only became a popular material with which to build once reinforced concrete was invented.
Different Types of Formwork
Traditional slab formwork uses timber, masonry and carpentry to complete construction. The method works through tree trunks or other lumber supporting rows of stringers which are placed three to six feet or one to two meters apart with joints placed between the stringers.
Metal beam slab formwork is much the same as the traditional slab formwork method, the only difference being that steel is used instead of timber and metal props are used instead of supports. This system is reusable and more methodical than the traditional method. The finish of the concrete is smoother and the formwork is easier to remove after the cement has cured.
Modular slab formwork is created from pre-made timber modules or modules made from steel or aluminium. These are usually produced in a factory offsite and added to construction once completed.
The Table or Flying Form System
Another type of formwork is table or flying form systems. These consist of slab formwork tables which are reusable. These tables do not have to be dismantled and can be used in high buildings where cranes or elevators are used to lift the tables. Once the table is positioned, the space between the wall and table is filled. Tables vary in size from eight square meters to 150 square meters. This type of formwork is a huge saver of both labour and time and is a favourite of construction engineers and architects. However, table formwork is best used in the construction of large, but simple structures.
Because the system is easily dismantled into single parts, it is transportable. The system is built in the same way as beam formwork, apart from these single parts. Joists and stringers are screwed, bolted or welded to become a deck.
Formwork should be placed at the correct height so that there is sufficient space to remove them once the concrete has set or cured. Due to this reason, the support systems of table formwork need to be height adjustable. Adjustable metal props can be used to support the systems. Some use steel or aluminium to insert stringers and supports into the systems, while others use metal frame shoring towers to attach the decks to. Others attach the decks to walls or columns that have been pre-cast which means that contractors do not need to use vertical props, simply support shoes bolted through holes.
Crane Lifting for Table Formwork
Tables produced which are five to seven meters wide with a potential length of over 100 feet are lifted by crane. The decks and formwork are moved to the edge of the building once the concrete has been cured and the crane lifts the protruding edge upwards, the rest of the formwork follows.
The advantages to crane-handled flying formwork include lowered labour costs and a more methodological way of functioning. However, lifting of this nature requires extremely advanced cranes to function, which can be expensive.
Elevator or Crane Fork Lifting for Table Formwork
When smaller tables are produced, such as those that are two to three meters in width and four to seven meters long, these are lifted by crane transport fork or material platform elevators which are attached to the side of the building.
Shifting trolleys are used to transport the tables horizontally to the elevator or crane platform. Crane fork flying formwork is used mostly in the US and Europe and other countries where labour costs are high as this technique is labour reducing, therefore cost reducing. Smaller tables can be customised to suit buildings of a more complex design. The disadvantage of this type of formworking is the cost of raw materials and cranes.
Formwork should always be undertaken by a professional contractor who is experienced and certified in their knowledge and services. Because formwork involves concrete which is heavily affected by a number of stresses, the incorrect use can be dangerous as the concrete may collapse. Formwork has however allowed people to build structures never before dreamed possible and paves the way for future construction.
About the Author:
Cape Formwork Contractors (CFC) are approved formwork contractors in Cape Town who use up-to-date formwork techniques and systems to keep their services at an outstanding, yet cost effective level.