Formwork

Presented
By
K.Lavanya
M.Tech
1
FORMWORK

DEFINITION:
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 Its is an artificial support provided below and around the precast
or cast insitu concrete work.
 Formwork is commonly made of
 Steel
 wood
 Formwork construction & casting is of prime importance in
concrete industry. It share a significant amount of concrete cost.
 Formwork is designed according to The ACI document SP-4.

Qualities of formwork:
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It should be water tight
It should be strong
It can be reusable
Its contact surface should be uniform
It should be according to the size of member.

Uses:
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 Formwork molds the concrete to the desired
shape and size, and controls its position and
alignment.
 It is used to transfer the temporary external
loads.

Major objectives considered in formwork:
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 Quality
 Safety
 Economy

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 Quality:
Forms must be designed and built with sufficient stiffness and
accuracy so that the size, shape, position, and finish of the cast concrete
are maintained.
 Safety:
Forms must be built sufficient strength and factor of safety so that
they have the capable of all supporting loads.
 Economy:
Forms must be built efficiently, minimizing time and cost..

Requirements of formwork:
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 Material should be cheap and re usable,
 It should be practically water proof, so that it should not absorb water
from concrete,
 Swelling and shrinkage should be minimum,
 Strong enough to with stand all external loads,
 Deflection should be minimum,
 Surface should be smooth, and afford easy striping,
 Light in weight, so that easy to transfer,
 Joints should be stiff, so that lateral deformation and leak is minimum .

THREE STAGES:
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a) Assembly and erection
b) Concrete placement
c) Stripping and dismantling

TYPES OF FORMWORK:
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Temporary Structure
Temporary structure required to safely support
concrete until it reaches adequate strength
Permanent Structure

Formwork detail for different structural
members
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In concrete construction formwork is commonly
provided for the following structural members.
o Wall
o Column
o Slabs & Beams
o Stairs
o Chimneys
o Water tanks
o Cooling Towers

Formwork for Wall
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 It consists of
• Timber sheeting
• Vertical posts
• Horizontal members
• Rackers
• Stakes
• Wedges
 After completing one
side of formwork
reinforcement is
provided at the place
then the second side
formwork is provided.

Wall forms:
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 Wall forms principally resist the lateral pressure generated by fresh
concrete as a liquid or semi liquid material.
 The pressure can be quite large, certainly many times the magnitude
of live loads on permanent floors.
 So design often involves closely spaced and well-supported members.
 Snap ties, flat ties, loop ties are single use ties usually relatively low
capacity 680 kg to 1450 kg.
 Coil ties, he bolts, she bolts are left embedded in concrete , or it can
be reused.
 The tension capacity of heavy ties around 27,230 kg.

Formwork for Column
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It consists of the
following
Side & End Planks
Yoke
Nut & Bolts
Two end & two side
planks are joined by the
yokes and bolts.

Column forms:
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 Column-forms materials tend to vary with the column shape.
 Wood or steel is often used with square or rectangular column.
 Round column forms more typically pre manufactured in a range of
standard diameters, are available in steel, paper board, and fiber
reinforced plastic.
 Round column are more structural efficient compared to square, since
the internal pressures can be resisted by the hoop membrane.
 Round steel forms are generally used for larger columns and bridge
piers and come in diameters about 0.36m to 3mts..

Formwork for Slabs & beams:
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 It consists of
Sole plates
Wedges
Props
Head tree
Planks
Batten
Ledgers
 Beam formwork
rests on head tree
 Slab form work rests
on battens and joists
 If prop height are
more than 8’ provide
horizontal braces.

FORMWORK FOR STAIRS:
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It consists of
Vertical & inclined
posts
Inclined members
Wooden Planks or
sheeting
Stringer
Riser Planks

FORMWORK FOR STAIRS:
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 The sheathing or decking for deck slabs is carried on cross-
joists which are in turn supported on raking ledgers
 The ledgers are generally of 7.5cm x 10cm size .
 The cross- joists may be 5cm x 10cm size
 The riser planks are 4-5 cm thick and equal to the height
of riser
 The riser planks are placed after the reinforcement is
placed in position

FORMWORK FOR CHIMNEYS:
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For tall chimneys two types of forms techniques are
in generally use in our country
a) Jump form
b) Slip form

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Jump Form:
In this type jacking bars are either cast in concrete or else
are carried in tubes which are cast in concrete.
After casting a lift, concrete is allowed to set and then
the forms are raised by jacks and the next lift is cast.
Jacks are usually placed about 2m apart and are designed.

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Slip Form:
The slip form method of concrete construction is used
for forming both horizontal and vertical concrete
structures.
It often used for forming highway pavements as
continuous operation.
Slip form methods is also been used for forming various
types of vertical concrete structures.

Removal of formwork:
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 Time of formwork removal depends on the following factors
1. Type of Cement
1. Rapid hardening cements require lesser time as compared to OPC
(Ordinary Portland Cement)
2. Ratio of concrete mix
1. Rich ratio concrete gain strength earlier as compared to weak ratio
concrete.
3. Weather condition
1. Hydration process accelerates in hot weather conditions as compared to
cold and humid weather conditions.

Time of Removal of formwork:
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Sr
.
N
o
Structural Member
OPC
(Ordinary
Portland
Cement)
Rapid
Hardeni
ng
Cement
1 Beam sides, walls &
Columns
2-3 Days 2 Days
2 Slab (Vertical Supports remains
intact)
4 Days 3 Days
3 Slab (Complete Formwork
removal)
10 Days 5 Days
4 Beams (Removal of Sheeting,
Props remains intact)
8 Days 5 Days
5 Beams & Arches (Complete
formwork removal) (up to 6 m span)
14 Days 5-8 Days
6 Beams & Arches (Complete
formwork removal) (more than 6 m
span)
21 Days 8-10 Days

Maintenance of formwork:
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Due to continuous use wooden planks & steel plates
surfaces become uneven and require maintenance.
For wooden formwork use cardboard or plastic fiber
board. Bolt hole places must also be repaired.
For steel formwork plates must be leveled by mallet
and loose corners must be welded.

Cost of formwork
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 For normal works cost of formwork is about 30%-40% of the
concrete cost.
 For special works cost of formwork is about 50%-60% of the
concrete cost.
 Formwork cost is controlled by the following factors
• Formwork Material cost
• Formwork erecting cost
• Formwork removal cost
• Formwork jointing cost (Nails and Cables)
• Labor charges.

Advantages of steel form work:
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 It can be used for a no. of times.
 It is non absorbent.
 Smooth finish surface obtained.
 No shrinkage of formwork occurs.
 Easy to use.
 Its volume is less
 Its strength is more.

LOADS ON FORMWORK:
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Formwork is subjected to following loads:
a) Dead weight of concrete
b) Hydrostatic pressure of the concrete
c) Live load due to working laborers
d) Impact effect at the time of pouring concrete into the
formwork
e) Vibrations transmitted to formwork during pouring
concrete

FORMWORK DESIGN
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Guiding points in the Design of formwork
 For design purpose, temporary live load due to labor and
equipment including impact may be taken from 3700N/m2
to
4000N/m2
 Dead weight of wet concrete (26000 N/m3
)
 Planks or boards used to form the vertical facing of the columns
or walls is known as sheathing whereas those used for floor slabs
are called decking

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 The hydrostatic pressure due to fluidity of concrete in the initial stages
of pouring depends upon several factors such as amount of water, size of
aggregates, rate of pouring etc.,
 The hydrostatic pressure is maximum at the time of pouring, but after
some time it starts decreasing, as concrete starts setting and hardening.
This pressure is mainly dependent on the depth of concrete poured
before it starts setting
 Deflection of the sheathing and joists should be limited to a maximum
value of 2.5mm

Contd...
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 The minimum wind design pressure q not less than 15 psf and
bracing should be designed for at least
w =46 kg/ft- length applied to the top
 The minimum lateral load w, for design of bracing system would
be greater than,
q .(h/2) or 46 kg/ft

Contd...
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For design practice the maximum pressure is given by:
p = wh
Where,
p => lateral pressure of concrete,
w => unit weight of concrete,
h => depth of fluid or plastic concrete.

Contd...
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For horizontal pressure of concrete is given by:
For walls constructing R less than 2.1m/hr;
p =150+9000(R/T)
For walls constructing R varies from 2.1m/hr to 3m/hr;
p = 150+(43,400/T)+2800(R/T)
For columns 3m/hr;
p = 150+9000(R/T)
Where, R => rate of vertical placement,
T => time for placement.

DESIGN OF SLAB FORMWORK:
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Design of slab forms can be summarized in the following design steps:
 Step 1: Estimate design loads
 Step 2: Determine sheathing thickness and spacing of its supports (joist
spacing)
 Step 3: Determine joist size and spacing of supports (stringer spacing)
 Step 4: Determine stringer size and span (shore spacing)
 Step 5: Perform shore design to support stringers
 Step 6: Check bearing stresses
 Step 7: Design lateral bracing

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