Formwork

1.1:-Introduction

               Formwork is also called centering or shuttering. Formwork is a temporary construction provided for laying out-in-situ concrete to the required shape. The construction of formwork involves the considerable expenditure of time and cost in building work, and even higher in bridges.

Forms are classified as plywood, steel, wooden,  combined wood-steel, reinforced concrete, and plain concrete. The most common material used for formwork is timber. Possibility of warping, swelling, and shrinkage of the timber are the disadvantage of wooden formwork.

                                                  OR,

(Formwork is used for the process of creating a temporary mold into which concrete is poured and formed.  Types of concrete formwork construction depend on formwork material and type of structural element.

Timber is the commonly used material for formwork. The disadvantage of timber formwork is that it will buckle, swell and shrink. Application of water-impermeable cost to the surface of wood reduces these defects.)

1.2:-Characteristics of formwork 

a)They must be strong enough to carry the weight of wet concrete, resist the impact due to consolidation, and carry the load of the workmen, etc.

b)They should be braced rigidly to prevent slippage, sagging, etc.

c)They should be such that erection and dismantling are easily accomplished.

d)The formwork should be watertight to prevent loss of fine materials, especially cement slurry, through gaps, which if found, should be closed with clay/ plaster of paris, but which in practice are simply covered by plastic/ polythene sheets.

e)The formwork should be clean before pouring.

1.3:-Requirements of formworks

                      Good formwork should satisfy the following requirements:-

1)Formwork material

a)The formwork material should be cheap and it should be reusable several times.

b)It should be strong enough to withstand all types of dead and live loads imposed on it. Such loads may be self-weight, the weight of reinforcement, weight of wet concrete, loads due to the workman, construction equipment, other incidental loads and forces caused by placement and consolidation of concrete during and after casting of concrete.

c)The formwork should be practically waterproof so that it does not absorb water from concrete and prevention of cement grout leakage.

d)The formwork should be as light as possible.

e)The material for formwork should be easily available.

2)Construction

a)The formwork should be rigidly constructed.

b)The formwork should be constructed in such a way that it may permit the removal of the various part in the desired sequence without damaging the concrete.

3)Support

The formwork should rest on the firm base.

  REQUIREMENTS OF GOOD FORMWORK

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  1. It should be strong good enough to withstand all types of dead and live loads.
  2. It should be strictly constructed and efficiently propped and support both horizontally and vertically, so as to keep its shape.
  3. The joints in the formwork should be tight enough against leakage of cement outside formwork.
  4. Construction of formwork should allow the removal of different parts in desired sequences without providing damage to the concrete.
  5. The material of the formwork should be less costly, easily available and should be good for reuse.
  6. The formwork should be position accurately to the desired line and levels should have enough plane surface.
  7. It should be as light as much as possible.
  8. The material of the formwork should not buckle or get twist when exposed to the elements.
  9. It should rest on a firm base.)

1.4:-Economy of formwork

            The cost of formwork can be split into the following components.

a)Material cost

b)Labour cost (For fabrication, erection, and removal of formwork)

           -In the case of building the cost of formwork may range 30 to 40 % of the cost of construction.

           -In the case of bridges, tall chimneys, dams, etc the cost of formwork will be 50-100% of the total cost of the work.

           -To minimize the cost of formwork in a building there should be minimum variation in the size of the rooms, floor area, etc so as to permit reuse of the formwork mold repeatedly.

1.4:-Types of formwork

           Formwork is classified based upon the material from which it is constructed. They are:-

a)Wooden

b)Plywood

c)Steel

d)Combined wood steel

e)RCC

f)PCC

Plywood Formwork:

Resin-bonded plywood sheets are connected to timber frames to construct up panels of needed sizes. The price of plywood formwork compares favorably with that of timber shuttering and it may even prove low costly in certain work in the following considerations:

  1. It is possible to have a smooth finish in which work on cost in surface finishing is there.
  2. By the use of large-size panels, it is possible to affect saving in the labor cost of fixing and disassembling.
  3. A number of reuses are more as compared with timber shuttering.

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 grip together through the use of good clamps or bolts and nuts. The panels can be falsified in a large number in the desired modular shape or size. Steel forms are largely used in large projects or in a situation where large number reuses of the shuttering is possible. This type of shutter is considered most suitable for circular or curved structures.

Steel formwork:

  1. Steel forms are stronger, durable and have a longer life than timber formwork and their reuses are more in number.
  2. Steel forms can be fitted and disassembled with more ease and speed.
  3. The quality of opened concrete surface by utilizing steel forms is suitable and such surfaces need no more treatment.
  4. Steel formwork does not absorb moisture from concrete.
  5. Steel formwork does not shrink or warp.

Construction of Concrete formwork:

This normally involves the following operations:

  1. Propping and centering
  2. Shuttering
  3. Provision of camber
  4. Cleaning and surface treatment

1.5:-Precaution to timbering

1)Possibility of the slip of earth

2)Safety helmet

3)Testing of timbering elements

4)Supervision

5)Safety of adjacent structure

6)First aid box

7)Traffic in the surrounding

8)Fencing and light

9)Provision of ladders

1.6:-Process of removal of formwork

         (Under normal circumstances with the atmospheric temperature of above 20-degree celsius)

-Damage to the concrete should not occur due to the vibration or shocks during removing.

-The gradual process of slacking off the wedges eases the props.

-Before taking out the formwork, the concrete is hammered softly to look for the metallic sound.

-In slab beam construction, removal of formwork has following sequential order:-

  a)The sides of the beams

  b)The underside of the slab and

  c)Finally the underside of the beam

   For rapid hardening cement 3/7th of the duration except for vertical sides.

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