Materials for Formwork, Types of Formwork , Advantages of steel for over timber form-work

Materials for Formwork
formwork can be made out timber, plywood,steel precast concrete or fibre, glass, used separately or in combinations.
the type of material to be used for formwork depends upon the nature of construction as well as the availability and cost of material.
for small works involving less number of re-uses, timber formwork proves economical.
fibre-glass is used mainly for making moulds for repetitive castings of precast concrete products.
moulds made up of precast concrete, fibre glass and aluminium are used in cast-in-situ construction such as waffle slabs or members involving curved surfaces.

Types of Formwork
1.Timber Formwork
it is used for the formwork should satisfy the following requirements.
(i) it should be well seasoned.
(ii) it should be light in weight .
(iii) It should be easily workable with nails without splitting.
(iv) It should be free from knots.

• the sizes of timber sections for different components of form-work depend upon the span of the slab or beam, floor to floor height and the centre spacing of centering supports.
• The size of timber planks or joists that can be adopted for different components of formwork for shuttering of 4.5 m span and 3.5 m height.
• for normal construction work where repetitive use of shuttering is possible , the quantity of timber shuttering can be worked out on the assumption that one set of shuttering can be used 10 to 12 times.

                         Table.1 Sizes of members for timber formwork

S.No.     Components of formwork              Size                                                                                                                (Varying according to the spacing of            centering props from 1 metre to 1.2 metres.)

(i.) Sheeting for slabs, beam and           25 mm to 40 mm--- thick

column side and beam bottom

(ii.)Joists, ledges                                50 x 75 mm to---cross section 50 x 150 mm

(iii.) Posts                                             75 x 100 mm. to 100 x 100 mm. .....

(iv.) Ballies                 Not less than 100 mm at mid length and 80 mm at thin end

2. Plywood Formwork

Use of plywood instead of timber planks is getting popular these days.

In this case resin bonded plywood sheets are attached to timber frames to make up panels of required sizes.

the panels thus formed can be easily assembled by bolting in the form of shuttering.

this types of shuttering ensures quality surface finish and is specially recommended in works where large exposed areas of concrete are to be constructed such as floor slabs faces of retaining walls etc.

it may prove to be cheaper in certain cases .

(i) By use of large size panel it is possible to effect saving in the labour cost of fixing and dismantling.

(ii) No of re-uses are more as compared with timber shuttering. for estimation purposes, number of re-uses can be assumed as 20 to 25 .

(iii) it is possible to have perfectly plain and smooth surface (without joint marks) by use of plywood shuttering. thus expenditure on surface finishing can be saved.

3.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 by two or more clamps pr bolts provided along each edge and the shuttering can be assembled and kept in alignment by use of horizontal or vertical centring of timber or steel.
• The panels can be fabricated in large number in any desired modular shape or size.
• The usual size for wall or slab panel varies from 60 cm x 60 cm to 60 cm x 120 cm. 
• this types of shuttering is considered most suitable for circular or curved shaped structures such as tanks, Columns , chimney etc.and for structures like large sewer, tunnels,and retaining walls.

Advantages of steel for over timer form-works 

 (i) Steel forms are stronger, more durable and have longer life as compared with timber forms. 

(ii) They can be put to sufficiently larger number of te-uses. For estimation purposes the number of reuses can be assumed to vary from 100 to 120. 

(ii) Steel forms can be installed and dismantled with greater ease and speed which results in saving in labour cost for this item of work. 

(iv) The quality of exposed concrete surface obtained by use of steel forms is excellent and it needs no further treatment. 

On the other hand construction carried out by use of timber formwork invariably requires plastering to obtain desired finish of the concrete surface. Thus there is saving in the cost of finishing the surface by use of steel forms.

 (v) There is no danger of the formwork absorbing water from the concrete and hence the chances of honeycombing are minimised.

 (6) They are not liable to shrink or distort and hence it is possible to achieve better workmanship and higher degree of accuracy by use of steel forms.