TYPES OF FORMWORK (SHUTTERING)


Formwork is an ancillary construction, used as a mould for a structure. Into this mould, fresh concrete is placed only to harden subsequently. The construction of formwork takes time and involves expenditure upto 20 to 25% of the cost of the structure or even more. Design of these temporary structures are made to economic expenditure. The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable forms are known as panel forms and non-usable are called stationary forms.

Timber is the most common material used for formwork. The disadvantage with timber formwork is that it will warp, swell and shrink. Application of water impermeable cost to the surface of wood mitigates these defects.

A good formwork should satisfy the following requirements:

  1. It should be strong enough to withstand all types of dead and live loads.
  2. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.
  3. The joints in the formwork should be tight against leakage of cement grout.
  4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.
  5. The material of the formwork should be cheap, easily available and should be suitable for reuse.
  6. The formwork should be set accurately to the desired line and levels should have plane surface.
  7. It should be as light as possible.
  8. The material of the formwork should not warp or get distorted when exposed to the elements.
  9. It should rest on firm base.

Economy in Formwork

The following points are to be kept in view to effect economy in the cost of formwork:

  1. The plan of the building should imply minimum number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly.
  2. Design should be perfect to use slender sections only in a most economical way.
  3. 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 fibre glass 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 pre-cast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.

Timber Formwork:

Timber for formwork should satisfy the following requirement:

It should be

  1. well seasoned
  2. light in weight
  3. easily workable with nails without splitting
  4. 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.

Normal sizes of members for timber formwork:

Sheeting for slabs, beam, column side and beam bottom

25 mm to 40mm thick

Joints, ledges

50 x 70 mm to 50 x 150 mm

Posts

75 x 100mm to 100 x 100 mm

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:

  1. It is possible to have smooth finish in which case on cost in surface finishing is there.
  2. By use of large size panels it is possible to effect saving in the labour cost of fixing and dismantling.
  3. Number of reuses are more as compared with timber shuttering. For estimation purpose, number of reuses can be taken as 20 to 25.

Steel Formwork

This consist 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 number in any desired modular shape or size. Steel forms are largely used in large projects or in situation 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:

  1. Steel forms are stronger, durable and have longer life than timber formwork and their reuses are more in number.
  2. Steel forms can be installed and dismantled with greater ease and speed.
  3. The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment.
  4. Steel formwork does not absorb moisture from concrete.
  5. Steel formwork does not shrink or warp.

Construction of formwork:

This normally involves the following operations:

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

Order and method of removing formwork:

The sequence of orders and method of removal of formwork are as follows:

  1. 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.
  2. Shuttering forming soffit of slabs should be removed next.
  3. 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 atleast 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.

Figure 1 to 6 shows formwork for different types of members in civil engineering construction.

Details of timber formwork for RCC beam and slab floor

Figure 1(a): Details of timber formwork for RCC beam and slab floor

Details of timber formwork for RCC beam and slab floor

Figure 1(b): Details at section (A) shown in above figure

Details of timber formwork for circular RCC column

Figure 2(a): Elevation

Details of timber formwork for circular RCC column

Figure 2(b): Details of timber formwork for circular RCC column

Details of timber formwork for square or rectangular RCC column

Figure 3(a): 150 3D View

Details of timber formwork for square or rectangular RCC column

Figure 3(b): Details of timber formwork for square or rectangular RCC column

Sectional plan showing details of timber formwork for an octagonal column

Figure 4: Sectional plan showing details of timber formwork for an octagonal column

Details of formwork for stair

Figure 5: Details of formwork for stair

Timber formwork for RCC wall

Figure 6: Timber formwork for RCC wall

Table: Period of removal of formwork

S. No.

Description of structural member

Period of time

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

 

 

(a) For slabs spanning upto 4.5 m

7 days

 

(b) For slabs spanning over 4.5 m

14 days

5

Removal of props to beams and arches

 

 

(a) Spanning upto 6 m

14 days

 

(b) spanning over 6 m

21 days


110 COMMENTS

  1. I would like to learn all types of form work methods and designs kindly provide the information about the new methods & demos to improve my knowledge in my construction proffession.Regards,Rajendra.

  2. Dear Mr.Gopal, I am Rajesh from Sri Raam Traders,Tamil Nadu.I am one of the leading Building Construction MatConterial supplier in my city.And also young growing builder.Basically i have a 3 years experience in Construction field.Last May i was started a New Residance contract work.i have a lot of suggestion in this work.

    I wil send my plan to ur mail.so plsend me email id to my mail.i will send all the details to u.

    Thanks & Regards,

    Rajesh.

  3. Arianne,

    If you want to take an innovative study, please consider the following fields.
    1. Foundations – Our foundation cost on any structure is very high. Take a study and compare with one another and deliver a paper that the one which is economical.

    2. We are spending more on Form works. Take a study on different form works being used. Aluminum and Plastic form works are out in the market for commercial use. I understand from my experience, the plastic forms are efficient in handling, increase the out put and the number of turns is from 40 to 50 times, whereas wood is very costly. Your technical paper will get appreciation.

    Many more are there on the list.

    Try these two first and see your out put.

    Regards,
    Janardhanan
    Qatar

  4. The above information is very good but valuable to the beginners. If you can take the details to the depth, our community will take the benefit.
    If you can discuss the details of Plastic and Aluminum form work, the economy in the number of turns would benefit the construction field as a whole.

  5. 1:1.5:3 is the nominal mix for M20 concrete. We all know that 7 day strength of a standard cube (as per IS) is 2/3 rd of the 28 day strength. So, in this case the minimum 7 day strength for M20 concrete is 0.67 x 20 = 13.4 MPa. Maximum strength at 7 day is not estimated in field as the only maximum strength desired to be achieved after 28 days is the target characteristic strength. :)

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