Timber Formwork in Construction
By Angella Naluwenda, a quantity surveyor,
Formwork is a mold or open-box-like container into which fresh concrete is poured and compacted. When the concrete is set, the formwork is removed and a solid mass is produced in the shape of the inner face of the formwork. The top of the formwork is normally left open. False-work is the necessary support system that holds the formwork in the correct position. Formwork materials can be classified as: timber, metals, plastics.
The timber formwork is one of the most used in the construction industry, fabricated on site using timber. Timber shuttering is the most flexible type as it can be used for any shape and size. Timber formwork takes the form of a structure of boards surrounding an open cavity, and offers several advantages over other formwork types as shown below.
Advantages of using timber formwork
It is easy to construct for any shape, size, and height.
It is economical for small projects.
It can easily be made into any shape or size.
It can be constructed using locally available timber.
It is lightweight when compared to metal shuttering.
It is easy to disassemble.
Disadvantages of using timber forms
Cannot be used for long.
Timber forms have limited usage. So, they cannot be useful for more times.
Wet concrete will shrink round timber which has high moisture content (more than 20% moisture content) and cup. This leads to open joints and leakage of the grout.
Dry timber may absorb water from wet concrete which results in the reduction of strength in the concrete structure.
Types of formwork based on a structural member
Wall formwork: Wall formwork used for concreting of shear or RCC wall in dams, wing walls, basement RCC walls, etc. Wall shuttering made of vertically arranged upright timbers (bearers) to which plywood sheeting boards are nailed at the inner side. The upright timbers are diagonally braced with the help of boards on both sides.
Beam formwork: Beam is the most important member in an RCC-framed structure. Beam formwork has prefabricated formwork includes sheeting bottom and side sheeting panels. The individual parts of formwork are manufactured based on the beam size. For prefabrication of the sheeting parts, a table for fabrication must be manufactured on site.
Slab formwork: Floors require a large area of formwork to be provided usually from beam to beam. Timber floor formwork consists of timber boards or plywood sheets supported on a framework and resting on a series of timber joists. Again timber props can be used for vertical supports. Adjustable props are needed for leveling purposes.
Column formwork: Formwork arrangement for columns may differ on the basis of column outline like rectangular, circular, hexagonal or any other shape. The sheeting of column shuttering is constructed according to the column dimensions. The panels are placed in a foot rim, anchored in soil with the help of bolts.
A good formwork should satisfy the following requirements
It should be strong enough to withstand all types of dead and live loads.
It should be rigidly constructed and efficiently propped and braced both horizontally and vertically so as to retain its shape without undue deformation.
The joints in the formwork should be tight against leakage of cement grout.
Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.
The material of the formwork should be cheap, easily available, and suitable for reuse.
The formwork should be set accurately to the desired line and levels should have plane surface.
It should be as light as possible.
The material of the formwork should not warp or get distorted when exposed to the elements.
It should rest on a firm base.
Economy in formwork
The following points are to be kept in view to effect economy in the cost of formwork for concrete structures:
Design the formwork to provide adequate but not excessive strength and rigidity.
Fabricate the forms into modular sizes to provide more reuses without refabricating when practical.
Prepare working drawings prior to fabricating the forms.
Prefabricate form sections on the ground rather than on the scaffolding.
The timber used for shuttering for exposed concrete work should have a smooth and even surface on all faces which come in contact with concrete.
Order and method of removing the formwork
The sequence of orders and methods of removal of formwork are as follows:
The shuttering forming the vertical faces of walls, beams and column sides should be removed first as they bear no load but only retain the concrete.
Shuttering forming soffit of slabs should be removed next.
Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.
Rapid-hardening cement, warm weather, and light-loading conditions allow early removal of formwork.
Table: Period of Removal of Formwork
|No.||Description of a 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||11 days|
|5||Removal of props to beams and arches||15 days|
A safe practice for formwork during the design of formwork and construction at the site is important for the safety of workmen. Improper erection of formwork can cause damage to structural element as well as pose a threat to the safety of workmen. A formwork expert is highly recommended.