Formwork



Definition
A structure, usually temporary (but
sometimes wholly or partly permanent),
used to contain poured concrete to mould it
to required DIMENSIONS, FINISHES and
SUPPORT it until it is able to support itself.
Formwork consists of contact face material
and the bearers which support the face
material.
1
Code of Practice for Formwork
Design


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
Concrete Pressure Computation:
Concrete Pressures on Formwork,
CIRIA Report 108 (Ref.)
Design Practice:
Formwork – a Guide to Good Practice,
published by Concrete Society and
IStructE (Ref.)
2
Common Types of Formwork

Wall Form
3
Common Types of Formwork
Wall Form
 ..\Teaching_Notes\TEMP_
WK\multimedia\Wall
form001.jpg

4
Common Types of Formwork
Wall
Form
5
Common Types of Formwork
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Wall Form
..\Teaching_Notes\TEMP_
WK\multimedia\Wall
form003.jpg
6
Common Types of Formwork
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Wall Form
..\Teaching_Notes\TEMP_
WK\multimedia\Wall
form004.jpg
7
Common Types of Formwork
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Column Form

..\Teaching_Notes\TEMP_WK\multimedia\Columnfo
rm1.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnfo
rm1001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnfo
rm1002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnfo
rm1003.jpg



8
Common Types of Formwork

Soffit Form

..\Teaching_Notes\TEMP_WK\multimedia\Sof
fit Form Frame.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Sof
fit Form Frame001.jpg

9
Common Types of Formwork
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Beam Form
..\Teaching_Notes\TEMP_WK\multimedia\Beam Form-Edge004.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Beam Form005.jpg
Stair Form
Permanent Formwork
10
Combined formwork and
falsework
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
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Formwork and falsework can be combined into a single
handleable unit
Time could be saved in erection and stripping and the
labor content of the operation thus significantly reduced
Large repetition is a must for this method to be effective.
Common examples include:
Table Form..\Teaching_Notes\TEMP_WK\multimedia\Tableform2.jpg,
Flying
Form..\Teaching_Notes\TEMP_WK\multimedia\FlyingForm2.jpg,

Travelling Form..\Teaching_Notes\TEMP_WK\multimedia\Travelform.jpg

Usually they are available from specialist suppliers
11
Formwork
Three general principles govern
formwork design and
construction:
 QUALITY
 SAFETY
 ECONOMY
12
Quality of Formwork
Relating to the formed faces of the
permanent concrete structure and
refers to the following two aspects:
Accuracy of Concrete Shape
 Quality of Concrete Surface

13
Quality of Formwork
Accuracy is controlled by the
deviations permitted
(Tolerances) in the :
 Formwork deflection
 Materials
 Components
 Workmanship
14
Quality of Formwork


To minimise deviations, all formwork
must have adequate means of:
Alignment and adjustment both at
construction joints and throughout the
formwork
e.g. Simple wedges, screw adjustments
on supports, camber adjustments
15
Quality of Formwork
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
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Failures in achieving surface quality are
often caused by:
Lack of formwork stiffness to resist the
movement coming from concrete vibrators
during concrete placement and the
subsequent grout loss at the joints (honeycombing, harder stripping)
Concrete shape, disposition of steel bars
Efficiency of conc. placement, forms
stripping
16
Quality of Formwork
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Classes of Finishes
Formed Finishes
F1, F2, F3, F4, F5 (CED G.S. Table 14.1) –
Class F5 the best
Unformed Finishes
U1, U2, U3, U4, U5 (CED G.S. Table 14.2) –
Class U5 the best
Treated Finishes
T1, T2, T3, T4, T5, T6 (CED G.S. Table 14.3)
17
Quality of Formwork


Different concrete structures buried
underground or exposed to naked eyes
require different class finishes specified
by the contract document
The formwork design requires
appropriate method of
construction/treatment, different surface
irregularities requirements and types of
contact face sheeting materials
18
Quality of Formwork

Tolerances
19
Safety of Formwork
Two major aspects must be
covered:
 Personal Safety of both
formworkers and the public
 Safety of the formwork
structure
20
Safety of Formwork
I.



Construction phase
As a general practice, the erected formwork
shall be :
Structurally safe
Having secured and effective guard rails,
toeboards, access ladders and stairs around its
periphery
Formworkers equipped with safety helmets and
boots
For high work, safety harnesses and security
screen are to be installed
21
Safety of Formwork
II. Design phase
a) Evaluation of possible Loading
Combinations occurred in the
following stages:
 Stage 1: before conc. Placement
 Stage 2: during conc. Placement
 Stage 3: after conc. Placement
22
Safety of Formwork
b) Structural requirements
Three important aspects to be met:
 STRENGTH (material strength and force
equilibrium)
 STABILITY (Sliding, overturning, uplift and
sidesway)
 STIFFNESS (accuracy and permissible
tolerances)
23
Economy of Formwork


Where formwork is highly repetitive
activity, a small reduction in
fabrication/stripping could result a
significant overall cost saving.
Cost involves formwork materials and
labor, erection (including hoisting),
stripping, repairing and cleaning of
formwork after concrete pour.
24
Economy of Formwork
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
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Increase in no. of reuses lower the unit cost
of the formwork
More sophisticated design could be
economically justifiable esp. when less
maintenance will be required.
Good formwork, apart from meeting the
technical specifications requirements,
means it is easy to fabricate, erect, adjust,
dismantle, and within the available
crane/man handling.
25
Care of Formwork


For maximum formwork life, efficient
stripping; care in formwork handling,
storage, and maintenance are essential
After stripping, the formwork units for
reuse should be moved away from the
work area (workers of other trades
may damage it as an item of
obstruction left there).
26
Care of Formwork
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
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They should be stored and protected
from weather to avoid surface damage
Large formwork panels should be
stacked in racks, facing away from direct
sunlight
Hardened slurry, dirt left from previous
pour should be removed from plywood
sheeting surfaces before they are getting
harder
27
Care of Formwork
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
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Mechanisms, locking devices and fixings
should be cleaned and oiled.
Face sheeting to be given a coat of release
agent
Care to be taken not to damage the presealed
surface of the plywood sheeting (Use a soft
brush in dust removal and a softwood wedge
to remove larger concrete particles)
28
Formwork Striking Procedures
-RC Slab Form

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Ease all supports by 1-2 turns for each prop
Starting from mid-span, remove the props
towards columns or walls
This will ensure no negative hogging bending
moment induced in the concrete slab if the last
few supports were left at the mid-span as
intended in the original design.
Cracking due to reverse bending will occur
otherwise
29
Formwork Striking Procedures
-RC Slab on Beam Form
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Strike the slab soffit form first
Then strike the beam soffit starting at mid-span towards
the columns/walls
This will ensure that all the imposed vertical load will be
supported by the slab which is in turn supported by the
beam form plus its falsework. By removing propping to
the beam form at the mid-span, the beam could then
span across the column/wall at both end as intended by
the original structural design.
Cracking due to reverse bending will occur otherwise.
30
Formwork Striking Procedures
-RC Cantilever Slab Form

Start the propping striking from the tip
of the cantilever and work towards the
column/wall/beam.

\Particular care should be observed that
any effect of overloading on the adjacent
spans temporarily when the propping to
cantilever is not properly striken.
31
Formwork
– Minimum Period before Striking (BS 8110)
Formwork
Column, wall, large beam
form
Surface conc.
temperature
>16 deg C
12 h
Surface conc.
temperature
0 – 10 deg C
300
h
t  10
Slab form
4 days
Beam & props to slab
form
10 days
100
h
t  10
250
h
t  10
Prop to beam
14 days
360
h
t  10
32
Backpropping

The speed of construction
vertically will dictate a number
of slab levels acting together to
support the total construction
loads imposed from the topmost
concrete slab casting operation
33
Backpropping

To avoid excessive loads building
cumulatively in the backprops,
backprops are loosened after
striking a slab formwork and then
repositioned and retightened. The
cast floor is thus allowed to take its
deflected shape hence the load
imposed from the floor(s) above
34
Formwork Materials
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Face Contact Materials (features left*)
Sawn board formwork (sawmarks*)
Plywood (fine & smooth surface*)
Steel form (variable color*)
Aluminium form (react with cement*)
Glass fibre reinforced plactics (highest quality
for sculptured profiles*)
* features/drawback
35
Formwork Materials
Waling/Bearer, Soldier/Props Materials
 Solid timber
 Steel in lattice configurations or cold
formed sections (standard/proprietary)

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Aluminium (Al Alloys) sections
36
Formwork Materials
Expanded metal (stopends)
 ..\Teaching_Notes\TEMP_WK\multi
media\expanded metal formwork.tif

37
Proprietary Formwork System

Basic Panel system – generally no walings, but
complete with corner panels, access brackets,
stabilisers
Table forms

..\Teaching_Notes\TEMP_WK\multimedia\Tableform2.jpg
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Flying forms
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..\Teaching_Notes\TEMP_WK\multimedia\FlyingForm2.jpg
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Slab support system
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..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Modular Frame .jpg
..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Modular Frame.jpg

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Waffle and trough moulds
Composite Floors with Steel Decking
38
Form Ties
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Recoverable and non-recoverable types
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..\Teaching_Notes\TEMP_WK\multimedia\Wall Tie001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie003.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie004.jpg

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Tie rods are the most critical parts in the formwork
design which will lead to the complete formwork
failure if any of them fails
39
Form Ties
Factor of safety in tension capacity
1) 1.5 for HT cold worked steel
2) 2.5 for mild steel
 Bearing plates/washers to be checked if
bearing stresses acting on the timber
waling/soldiers are exceeded or not
 Limitation on spacing of form ties is also a
concern for aesthetic reasons

40
Formwork Design –
Concrete pressure
Factors
Increase
Cement Types/
Mix Composition (C2)
Retarder,
PFA
Pressure
Changes
Rate of Pour (R)
Size & Shape of Formwork (C1)
Column,
Wall
Height of Form (H)
Concrete Temperature at Placing (T)
Unit Weight of Conc. (D)
41
Concrete pressure distribution
along a vertical wall form
(Trapezoidal)
Concrete is still hydrostatic
Concrete has been hardening
Pmax
42
Concrete Pressure Formula
(CIRIA Report 108)
Pmax  D 1.5 R  0.45 K H  1.5 R 
or Dh


in kPa whichever is THE SMALLER
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C2 is coefficient depending on mix ingredients
D is unit weight of concrete, kN/m3
C1 is coefficient depending on form size & shape
2
H is vertical form height, m
 36 
K
K is temperature coefficient
T  16

R is rate at which conc. rises vertically, m/h
 36 
K 
 T  16
2
43
Formwork Design - Loading
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Self-weight of Formwork
Imposed Loads (permanent work loads and
construction operation loads)
Concrete Pressure (for wall/column forms)
Environmental Loads (Wind loads, Snow/Ice
Loads)
Horizontal Loads (Imposed plant loads, skip
impact loads,.. And minimum horizontal stability
force i.e. 10% of the total form self-weight)
44
Formwork Design -
1)
2)
3)
Design procedures follow the design of
permanent works but:
Permissible stress design method are often
used;
Higher working material stresses
Formwork system will include
manufacturing and assembly tolerances
45
Formwork Design –
Soffit Form Stability
Stability of soffit formwork
erected at the top of falsework is
covered in BS5975 (formwork is
considered as an integral part of
the falsework)
46
Formwork Design –
Wall Form Stability
1)
2)
Stability of freestanding wall forms
must be checked for the following
three scenarios:
Maximum Wind and Nominal access
Loads on any working platform (W/P)
Working Wind and Full Construction
Operation Loads on any W/P
47
Formwork Design –
Wall Form Stability
3) Minimum Stability Force and Full
Construction Operation Loads on any
W/P
 Note: Working wind means the upper
wind speed limit below which operations
could still be allowed
48
Formwork Design – Design
Concepts
Single Face Formwork
 Cantilevered Formwork
 Discontinuity in Face Sheeting and
Soffit
 Impact against a fixed face/joint during
concreting
 Inclined Soffits
 Cantilevered Soffits

49
Special Formwork

Slipforms


..\Teaching_Notes\TEMP_WK\multimedia\Slipform.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Slipform001.jpg

Climbforms.

.\Teaching_Notes\TEMP_WK\multimedia\ClimbForm.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Climbform3.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Climbform4.jpg

Travellers
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..\Teaching_Notes\TEMP_WK\multimedia\Travelform.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Travelform1.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Travelform2.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Travelform3.jpg
50
Special Formwork
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Tilt-up Moulds
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..\Teaching_Notes\TEMP_WK\multimedia\Tilt-up Mould.tif

Gang Forms
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..\Teaching_Notes\TEMP_WK\multimedia\Gangform1.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Gangform2.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Gangform3.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Gangform4.jpg
Moulds for Prestressed Concrete
 Tunnel Form


..\Teaching_Notes\TEMP_WK\multimedia\Tunnel Form.tif
51
Special Formwork Details

Kickers for wall form
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..\Teaching_Notes\TEMP_WK\multimedia\Kicker.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Kicker001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Kicker002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Kicker003.jpg

Kickers for column form

..\Teaching_Notes\TEMP_WK\multimedia\Column Kicker.tif

Stopend forms for wall form

..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form.tif
..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form003.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Stopend Form004.jpg

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52
Special Formwork Details

Wall Ties
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..\Teaching_Notes\TEMP_WK\multimedia\Wall Tie.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall Tie001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie003.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Wall tie004.jpg

Props
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
..\Teaching_Notes\TEMP_WK\multimedia\Prop.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Prop001.jpg

U-head

..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form U-head.jpg

Sloping soffit form

..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Sloping Details.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Sloping Details001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Sloping Details002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Sloping Details003.jpg

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53
Special Formwork Details

Bracing for soffit form

..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Frame Bracing.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Soffit Form Bracing Connector.jpg

Bracing for beam form

..\Teaching_Notes\TEMP_WK\multimedia\Beam-Soffit Form.tif

Bracing for column form


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..\Teaching_Notes\TEMP_WK\multimedia\Columnform Bracing.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnform Bracing001.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnform Bracing002.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Columnform Bracing003.jpg
54
Slipforms – features

The system is intended for continuous
concreting work for walls of constant section.

The shutter rises from 150 to 300 mm per
hour depending on the rate of concrete
hardening.
55
Slipforms – features


The form is about 900-1200 mm deep,
fixed to and held apart by timber or steel
frame or yokes as shown in the figure on
the right.
On top of each yoke is fixed a hydraulic
jack which climbs along a steel jacking rod,
about 25 mm in diameter, which is cast
into the wall.
56
Slipforms – Operation
Procedures
(i) Fix steel reinforcement above the
shutter
 (ii) Pour concrete into the shutter and
vibrate it in final position
 (iii) Operate the jack to work against
the lower jaws to raise the yoke and the
form with it

57
Slipforms – Operation Procedures
(iv) When the oil pressure is released,
the upper jaws grip under the action of
a spring
 (v) The jack is operated in cycles (iii) +
(iv) and each cycle gives a rise of about
25 mm

58
Slipform – Uses and Limitations
Uses
 The process is used for constructing
concrete chimneys, silos, shaft linings,
towers and building cores and bridge
piers
 Possible to vary wall thickness and
layout over the height (despite overall
cost will be increased)
59
Slipform – Uses and Limitations
Limitations
 Steady and continuous concreting must
be maintained throughout the process
 Longer duration and sophisticated
equipment layout in the initial setup
 High capital cost in the plant &
equipment
60
Slipform – Uses and Limitations
Limitations
 Labors working in shifts and hence
higher labor costs
 No construction joints
 Working platform to be protected from
adverse weather e.g. high wind, heavy
pours
61
Slipform – Uses and Limitations




Limitations
Standby plant and operatives are needed
Openings formed by timber/polystyrene
within a film of concrete on each face could
avoid displacement of the former during the
vertical sliding process
It requires specialist working and
supervision operatives
62
Tilt-up Moulds
The unit mould is cast horizontally and may
subsequently be tilted through a designed
angle
 The vertical position of the cast unit on
completion of demoulding could facilitate
easy and early formwork removal, and the
subsequent vertical stacking
 Horizontal processing could allow easy tiling
works, or composite sandwich/hollow units
manufacture
63
Moulds for Prestressed Concrete
Sideforms which cannot be removed
before prestressing operations
commence should be designed to
allow vertical and horizontal
movement of the cast member during
prestressing
 Safety rules to all personnel
concerned must be adhered to during
the tendon stressing operation

64
Formwork Checklist
Design concept in relation to
expected concreting rate
 Material strength, stiffness,
condition and dimensions
 Access – adequate space, guardrails,
toeboards, ladder/stairs

65
Formwork Checklist




Assembly - layout dimensions, plumb &
stability, inserts fixed, ties tightened and
thread conditions checked, sealed against
kicker, panel joints sealed, waterstop
installed, filling over nail holes
Release agent applied
Formwork cleaned out
Stripping – nails bent/removed, resuable
materials separated
66