FORMWORK• It is a temporary construction used as a mould for the structure in

which concrete is placed

• Cost of formwork – 20 to 25% of the cost of structure – building

work

• Operation of removal of formwork commonly known as – stripping.

• Forms whose components can be reused several times – panel

forms.

• Some forms cannot be stripped & forms part of structure itself –

Stationary forms.

Cont..

• Forms may be

Wooden, plywood or steel.

• Timber – most common material for formwork.

• Disadvantage – warping, swelling, and shrinkage of timber.

• Overcome by applying water impermeable coating.

• This also prevents shuttering from adhering to concrete – so

makes stripping easier.

• Steel work is used for major works – but comparatively costly.

Advantages of Steel formwork

• Can be put to higher number of uses.

• Provides ease of stripping.

• Ensures an even & smooth concrete surface.

• Possess greater rigidity.

• Not liable to shrinkage or distortion.

Requirements of good formwork

• Easy Removal :

Should be removed easily with less number of hammering

which will prevent injury to concrete.

It can be made fit for reuse with less expenditure

• Economy:

It does not contribute to strength so cost to be brought down

– with enough safety

Reduction in number of irregular forms, standardization of rooms, reusing

formwork etc.

• Less Leakage

Should be arranged such that minimum leakage

through the joints

Tight joints between adjacent sections.

• Quality

Forms should be designed & built accurately –

desired shape size & finish of concrete is attained.

• Rigidity :

Should be rigid enough to retain the shape without

deformation.

Deflection limited to – visible surface – 1/300 of span ; hidden

surface 1/150 of the span.

Only a rigid formwork will allow repeated use.

• Smooth surface :

Inside surface should be smooth for a good concrete surface.

Achieved by applying crude oil or soft soap

Also makes removal of formwork easy.

• Strength : Should be strong enough to bear the dead load of wet concrete

as well as labour, equipments etc.

So careful design is necessary.

Over estimation – expensive formwork & underestimation –

failure of formwork.

Loads on vertical forms should be assessed

• Supports :

Formwork should rest on sound, hard & non-yielding supports.

Loads on Formwork:

• Live load due to labour etc.

• Dead weight of concrete

• Hydrostatic pressure of the wet concrete.

• Impact effect at the time of pouring concrete.

• Vibrations transmitted to formwork during pouring

concrete.

Guiding points in the design of Formwork.

1. For design purpose – temporary live load due to labour & equipment including impact may be taken from 3700N/m2 to 4000N/m2

2. Planks or boards used to form vertical facing of columns or

walls – is known as sheathing & those used for floor slabs –

decking.

3. Hydrostatic pressure due to fluidity of concrete in initial stages

of pouring depends on – amount of water, size of aggregate,

rate of pouring etc.

• Hydrostatic pressure decreases as concrete starts setting &

hardening.

Contd..

• Hydrostatic pressure mainly depends on the depth of concrete

poured before it starts setting.

• Upto 1.5m height of concrete equivalent weight of concrete

may be assumed as 23000N/m3.

• For value greater than 1.5m eq. fluid weight is decreased.

• If height of concrete poured in one hour is 6m eq. fluid weight

is 12000N/m3

4. Deflection of the sheathing & joists should be limited to a

max. value of 2.5mm.

Timber Formwork

• When formwork required for small works with less repetition

– timber preferred to steel.

• Timber formwork is cheap in initial cost.

• Timber used should be :

Well seasoned

Free from loose knots

Light in weight

Easily workable with nails without splitting

• Following points to be noted :

Timber formwork should be neither too dry nor too wet.

Moisture content of 20% is appropriate for timber formwork.

Dimension of components of timber formwork will depend on

load to be carried.

Minimum nails should be used & it should be kept projecting

to facilitate easy removal.

Timber formwork is economical – building with minimum

variation in dimension of rooms.

Properties of Timber used in Formwork

Name of

Timber

Density

in kg/m2

Modulus

of

Elasticity

in t/cm2

Range of

Ending

&

Tensile

Stress

Permissible stresses in kg/cm2

Shear

Across

grains

Shear

Along

grains

Comp

parallel

to grains

Comp

perpendi

cular to

grains

Fir 440 94 56-78 6 8 42-60 10-16

Deodar 545 95 70-102 7 10 56-78 17-36

Kail 515 68 50-66 6 8 38-52 10-17

Chair 575 98 60-84 6 9 46-64 14-22

Column Formwork

Column Formwork

• Sheathing : vertical timber planks provided all along the

perimeter of the column – 25mm thick – generally nailed to

yokes.

• Side Yokes & End Yokes : consists of two numbers each

placed along the length of the formwork. Side yokes are of

larger section than end yokes.

• Wedges : used to tighten various elements of formwork.

• Formwork should be designed to resist high pressure resulting

from quick filling of concrete .

• A cleanout or washout hole is provided at the bottom to

remove debris.

• A wash of water is given to the inside of formwork before

laying of concrete.

• Nails are kept projecting – for easy removal.

• Formwork for circular column is made of narrow vertical

boards known as staves. Staves are fixed to the yokes which

are also suitably curved.

Formwork for Beams & Floors

• Slab is supported on 2.5cm thick sheathing laid parallel to

main beams.

• Sheathing is supported on wooden battens – laid between

beams at suitable spacing.

• To reduce deflection battens are propped at middle of the span

through joists.

• Side forms of sheathing of beam are generally 3cm thick.

• Bottom sheathing of beam form may vary from 5 to 7cm thick.

• The ends of batten are supported on the ledger which is fixed on

cleats throughout the length.

• Cleats are fixed to the sides of beams – size 10 x 2cm or

10 x 3cm.

• The whole of beam form is supported on head tree.

• Head tree – it is a horizontal beam connected at top of the

vertical post or shore, through inclined cleats.

• At the bottom of the vertical post, a pair of wedges are provided

over sole pieces.