WHAT IS FORMWORK? TYPES OF FORMWORK.

FORMWORK

 When concrete is placed, it is in plastic state. It requires to be supported by temporary supports for casting in desired shape till it becomes sufficiently strong to carry its own weight.This temporary casing is known as formwork or forms or shuttering. The term moulds is sometimes used to indicate formwork of relatively small units such as lintels, cornices etc.

        Definition of formwork

“Forms or moulds or shutters are the receptacles in which concrete is placed, so that it will have desired shape or outline when hardened. Once concrete develops the adequate strength to support its own weight they can be taken out”.
“Formwork is the term given to either temporary or permanent moulds into which concrete or similar materials are poured”.

         Requirements of a good formwork :

The essential requirements of formwork or shuttering are: -
a) It should be strong enough to take the dead and live loads during construction.
b) The joints in the formwork should be rigid so that the bulging, twisting, or sagging due to dead and live load is as small as possible. Excessive deformation may disfigure the surface of concrete.
c) The construction lines in the formwork should be true and the surface plane so that the cost finishing the surface of concrete on removing the shuttering is the least.
d) The formwork should be easily removable without damage to itself so that it could be used repeatedly.

       Types of Formwork :

The material most commonly being used to date is timber. However, due to the depleting forest reserves and increasing cost of timber the use of alternate materials such as plywood and steel has become prominent. More recently, materials such as plastics and fiberglass are also being used for pre-fabricating formwork. The type of material to be used depends on the nature of construction as well as availability and cost of material. The constraints on the project such as overall cost, time of completion also play a major role in the use of a Particular material for formwork.

1. TIMBER FORMWORK

Timber is required for practically all jobs of formwork. The timber bring used for formwork must satisfy the following requirements. It should be durable and treatable. It should have sufficient strength characteristics. It should be light weight and well seasoned without warping. It should hold nails well. It is economical for small construction jobs. It is design flexible and easy to erect. It has good thermal insulation which makes it useful to be used in colder Regions. It can easily be made into any shape or size. And it is easy for transporting purpose for in between sites.

2. STEEL FORMWORK

Mostly used in large construction projects or in situations where large number of re-uses of the same shuttering is possible. It is Suitable for circular or curved shaped structures such as tanks, columns, chimneys etc. and also used for structures like sewer tunnel and retaining wall. Strong, durable & have longer life. Reuses can be assumed to vary from 100 to 120 wares timber varies from 10 to 12.Steel can be installed & dismantled with greater ease & speed resulting in saving in labour cost. Excellent quality of exposed concrete surface obtained. No danger of formwork absorbing water from the concrete and minimizing honeycombing.

3. PLASTICS FORMWORK
These forms have become increasingly popular for casting unique shapes and patterns being designed in concrete because of the excellent finish obtained requiring minimum or no surface treatment and repairs. Different types of plastic forms are available like glass reinforced plastic, fiber reinforced plastic and thermoplastics etc. The material allows greater freedom of design. Unusual textures and designs can be molded into the form. It allows the contractor to pour structural and finished concrete simultaneously. Because sections can be joined on the job site in such a way so as to eliminate joints, there is no size limitation. If carefully handled, a number of reuses are possible making it highly& Economical. It is lightweight and easily stripped. The disadvantage of using plastic forms is that it does not lend itself to field fabrication hence, the design and planning of this form must be carefully carried out. Also care must take not to damage the plastic by the heat applied for accelerated curing of the concrete. Trough and waffle units in fiberglass are used in construction of large floor areas and multistoried office buildings.

4. ALUMINIUM FORMWORK

Forms made from aluminum are in many respects similar to those made of steel. However, because of their lower density, aluminum forms are lighter than steel forms, and this is their primary advantage when compared to steel. As the strength of aluminum in handling, tension and compression is less than the strength of steel, it is necessary to use large sections. The formwork turns out to be economical if large numbers of reuses are made in construction. The major disadvantage of aluminum forms is that no changes can be made once the formwork is fabricated.


         Loads acting on Formwork :

In Construction, the formwork has to bear, besides its own weight, the weight of wet
concrete, the live load due to labor, and the impact due to pouring concrete and workmen on it. The vibration caused due to vibrators used to compact the concrete should also be taken care off. Thus, the design of the formwork is an essential part during the construction of the building.
For the design of planks and joists in bending & shear, a live load including the impact may be taken as 370kg/m². It is however, usual to work with a small factor of safety in the design of formwork. The surfaces of formwork should be dressed in such a manner that after deflection due to weight of concrete and reinforcement, the surface remains horizontal, or as desired by the designer. The sheathing with full live load of 370 kg/m² should not deflect more than 0.25 cm and the joists with 200kg/m² of live load should not deflect more than 0.25cm.
In the design of formwork for columns or walls, the hydrostatic pressure of the concrete should be taken into account. This pressure depends upon the quantity of water in the concrete, rate of pouring and the temperature.
The hydrostatic pressure of the concrete increases with the following cases:-

  •  Increase in quantity of water in the mix.
  •  The smaller size of the aggregate
  •  The lower temperature.
  •  The higher rate of pouring concrete

If the concrete is poured in layers at an interval such that concrete has time to set, there will be very little chance of bulging. Aluminium as usual is not a very strong material. So the basic elements of the formwork system are the panel which is a framework of extruded aluminium sections welded to an aluminium sheet. It consists of high strength special aluminium components. This produces a light weight panel with an excellent stiffness-to-weight ratio, yielding minimal deflections when subjected to the load of weight concrete. The panels are manufactured in standard sizes with nonstandard elements produced to the required size and size to suit the project requirements.

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