Formwork is used as a temporary structure to support the fresh (i.e., uncured) concrete until it is strong enough to support itself and the applied loads. Formwork is a very significant portion of the project, as much as 60% of the final cost of the finished concrete project.
A number of fundamental principles apply to all formwork.
Consideration must be given to:
- Acceptable tolerances permitted
- Use of appropriate materials
- Standards of workmanship
- Construction for ease of erection and stripping
- Care and maintenance of the formwork, so that the maximum number of re‐uses can be achieved.
Formwork is a temporary construction; however care must be taken to prevent damage to permanent work. Three general principles govern formwork design and construction:
- Quality accuracy of the concrete shape and the final finished surface quality.
- Safety strength of the formwork structure. Personal safety of people, both carpenters and the public.
- Economy: The structural frame is usually the most significant cost component, a dominant and critical factor in the time of construction.
DESIGN REQUIREMENTS OF FORMWORK
When designing formwork, consider the following:
- Strength: Forms and shutters have to be designed to support dead weight, live load and hydrostatic pressure. Sheathing must be rigid enough to resist bulging.
Formwork for vertical concrete elements i.e. columns and walls are subject to pressures on the form face. This is caused by the fluid action of the fresh concrete. The pressure of the fluid concrete on the vertical faces increases proportionately with the depth of concrete. The maximum pressure being at the bottom of the form. This maximum pressure for the full depth fluid concrete is the hydrostatic pressure for concrete and usually occurs when the concrete is placed very quickly.
It should not be possible for the bracing to be dislodged by impact, or wind, acting from any direction. Refer to illustration below.
- Speedy erection and dismantling: The formwork design and the methods of assembly must be as simple as possible to reduce time spent in erection and dismantling. The formwork should be Simple to remove without causing damage to the concrete.
- Tightness of joints: The liquid retaining properties of the formwork must be adequate to prevent leakage of cement and fine aggregate from the concrete.
- Rigidity: Brace formwork and support to ensure no movement may take place under wind pressure, or when the concrete is being placed and vibrated. The shutters must be rigid enough to keep the concrete member within the allowable tolerances.
- Reuse: Design for unit construction, if possible, so that you can strike and reuse as soon as possible. Use clamps, wedges, and similar devices to hold sections of formwork in place. Avoid nailing as much as possible, nail holes and bruising of the timber will spoil formwork for further use. The formwork material must be durable and capable of producing a good surface finish.
- Ease of handling: Forms and shutters must be of a size and weight that can be handled by the labour and plant available on site.
- Adjustment: Arrange all props, shores, and struts so that they can be properly adjusted. They must bear on sole plates, so that the load is safely distributed on to the structure below.
- Removal of debris: Provide all formwork with special cleanout holes to allow for the removal of sawdust, shavings, and other debris from the bottom of the formwork before the pour begins.
Factors Relating to Formwork
To design and construct satisfactory formwork an appreciation of the types of loads and combinations of loads that act on the formwork is required.
The heaviest load on forms usually occurs when the concrete is being placed. As well as the weight of the concrete itself, (i.e. dead load), allowance for the concrete gang and their gear must be considered, (i.e. live load). Depositing wet concrete in heaps creates uneven pressure on the formwork. It is important to evenly place the concrete in layers, distributing loads equally on the formwork. In walls, wet concrete exerts horizontal bursting pressures on the vertical soffit. The pressure is reduced as the semi‐liquid concrete hardens and becomes self-supporting. Formwork construction must be able to withstand the loads imposed.
Economy in Formwork
The following points are to be kept in view to effect economy in the cost of formwork:
- The plan of the building should imply a minimum number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly.
- Design should be perfect to use slender sections only in a most economical way.
- Minimum sawing and cutting of wooden pieces should be made to enable reuse of the material a number of times. The quantity of surface finish depends on the quality of the formwork.
Formwork can be made out of timber, plywood, steel, precast concrete or fiberglass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of precast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.
Types of formwork (shuttering) for concrete construction:
- Timber Formwork
Timber for formwork should satisfy the following requirement: It should be
- Light in weight
- Easily workable with nails without splitting
- Free from loose knots
Timber used for shuttering for exposed concrete work should have smooth and even surface on all faces which come in contact with concrete.
- Plywood Formwork
Resin bonded plywood sheets are attached to timber frames to make up panels of required sizes. The cost of plywood formwork compares favourably with that of timber shuttering and it may even prove cheaper in certain cases in view of the following considerations:
- It is possible to have smooth finish in which case on cost in surface finishing is there.
- By use of large size panels it is possible to effect saving in the labour cost of fixing and dismantling.
- Number of reuses is more as compared with timber shuttering. For estimation purpose, number of reuses can be taken as 20 to 25.
- Steel Formwork
This consists of panels fabricated out of thin steel plates stiffened along the edges by small steel angles. The panel units can be held together through the use of suitable clamps or bolts and nuts. The panels can be fabricated in large numbers in any desired modular shape or size. Steel forms are largely used in large projects or in situations where large number reuses of the shuttering is possible. This type of shuttering is considered most suitable for circular or curved structures.
Steel forms compared with timber formwork:
- Steel forms are stronger, durable and have longer life than timber formwork and their reuses are more in number.
- Steel forms can be installed and dismantled with greater ease and speed.
- The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment.
- Steel formwork does not absorb moisture from concrete.
- Steel formwork does not shrink or warp.
Construction of Concrete Formwork
This normally involves the following operations:
- Propping and centring
- Provision of camber
- Cleaning and surface treatment
Order and Method of Removing Formwork
The sequence of orders and method of removal of formwork are as follows:
- 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. The formwork should under no circumstances be allowed to be removed until all the concrete reaches strength of at least twice the stresses to which the concrete may be subjected at the time of removal of formwork. All formworks should be eased gradually and carefully in order to prevent the load being suddenly transferred to concrete.
Table: Period of Removal of Formwork
Advantages and Disadvantages of Different Formworks
Productivity rate is very important in estimating cost and duration for construction projects, and any labour productivity loss would affect the whole project in time and cost.
To prevent productivity loss, factors that affect productivity mentioned earlier rates should be examined as possible.