Formwork in civil engineering

Civil Engineering Drawing &
Graphics
What is formwork?
When concrete is fresh and in its liquid state it must
be restrained within a mould in order for it to set in its
required shape. Formwork is the term used to
describe this mould.
For most in-situ pours, the formwork is made of
wood. A smooth outer surface provides the main
support for the concrete as it sets. This is normally
made of plywood.
Concrete is approximately 2.4 times as dense as
water, and in its liquid state, it imposes considerable
forces on the formwork containing it. Consequently it
is necessary to reinforce the plywood with horizontal
wooden beams or wailings.

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If constructed properly, this mechanism of a
plywood skin supported by timber wailings
provides ample support for the setting
concrete. The mechanism of transferring these
loads to a solid support can be performed by
numerous methods which are collectively
known as falsework.

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GOOD FORMWORK:
There are two factors that decide how well the
concrete will look in the final result.. one is how
the concrete is placed and compacted, and the
other is the quality of the formwork.
Formwork is made from expensive materials, and
requires great skill and experience in its
manufactures. Its importance is evident when you
consider that the cost of fabrication, erecting and
striking the formwork, often exceeds the cost of
the concrete it is designed to shape and support.

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 Formwork usually needs to be used many
times for it to be cost efficient. This can only be
done if it is carefully handled, cleaned and
stored, regardless of what material it is made
from.
 For anything other than most smallest of
concreting jobs e.g. walls over 1m in height, a
drawing should be provided, showing what
formwork is required. This need not be an
intricate design drawing, a simple sketch is
usually sufficient.

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The below picture shows part of a concrete
structure soon after the formwork has been
struck. Not the poor finish (circled). This is an
imprint off the painted plywood formwork.
Some of the paint has run, and this has been
transposed to the concrete. It is important to
remember that the face of the concrete is
effectively a negative of the formwork face.
Always inspect the formwork prior to pouring.

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Good formwork should fulfill the following
criteria:
 It is strong enough to support the weight
of fresh concrete during placing and
compacting, and any other loads it may be
required to take.
 The face of the formwork is of sufficient quality
for its use.
 It can be easily erected and struck.
 It is sufficiently stable in all weathers.

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 It can be handled safely and easily using the
equipment available.
 It provides suitable access for placing and
compacting the concrete.
 It conforms with appropriate safety regulations.
 The joints between members are sound
enough to prevent grout leakage.

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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 up to 20
to 25% of the cost of the structure or even
more. Design of these temporary structures
are made to economic expenditure.

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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.

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A good formwork should satisfy the following
requirements:
 It should be strong enough to withstand all types
of dead and live loads.
 It should be rigidly constructed and efficiently
propped and braced both horizontally and
vertically, so as to retain its shape.
 The joints in the formwork should be tight against
leakage of cement grout.
 Construction of formwork should permit removal
of various parts in desired sequences without
damage to the concrete.

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 The material of the formwork should be cheap,
easily available and should be suitable for
reuse.
 The formwork should be set accurately to the
desired line and levels should have plane
surface.
 It should be as light as possible.
 The material of the formwork should not warp
or get distorted when exposed to the elements.
 It should rest on firm base.

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Economy in Formwork:
 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.
 Design should be perfect to use slender sections
only in a most economical way.
 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.

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Formwork can be made out of timber,
plywood, steel, precast concrete or fiber glass
used separately or in combination. Steel forms
are used in situation where large numbers of
re-use of the same forms are necessary.

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For small works, timber formwork proves
useful. Fiber glass made of pre-cast concrete
and aluminum are used in cast-in-situ
construction such as slabs or members
involving curved surfaces.

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Following are the materials which are used for formwork:
 Plywood:
This is by far the most common material used for the
facing panel. It is easily cut to shape on site, and if
handled and stored carefully, it can be used many
times.
Note the different thickness: A standard plywood
thickness on site is 18mm. this is usually sufficient for
most pours. However, if the formwork is curved, a
thinner plywood is used to facilitate bending. Thicker
plywood may be used when the weight of concrete
causes a standard thickness plywood to bow out,
distorting the concrete face.

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 Timber:
This is the most common material used for
bracing members to the form face. Like
plywood, it can be easily cut to size on site.
Formwork made from timber is called
traditional formwork. The construction methods
using timber formwork have been used on site
for years, and all well understood by trained
operators.

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Ply wood Timber

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Steel:
Steel is also used in pre-fabricated formwork.
Purpose made steel forms are fabricated when
dimensional tolerances are critical, or when
the forms are planned to be re-used. Steel
forms become cost-efficient after about a
dozen uses, although they can be used up to
100 pours if they are carefully cleaned and
stored.

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Aluminum:
Often used in pre-fabricated formwork, that is
put together on site. Aluminum is strong and
light, and consequently fewer supports and
ties are required. The lighter sections will
deflect more, but this can be avoided by simply
following the manufacturers recommendations.

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Plastics:
Glass reinforced plastics (GRP) and vacuum
formed plastics are used when complicated
concrete shapes are required (e.g. waffle
floors). Although vacuum formed plastics will
always need support, GRP can be fabricated
with integral bearers making it self supporting.
Like steel, plastic formwork can be re-used
many times, as long as care is taken not to
scour the surface whilst vibrating the concrete.

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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.

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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

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Plywood Formwork:
Resin bonded plywood sheets are attached to
timber frames to make up panels of required
sizes. The cost of plywood formwork compares
favorably with that of timber shuttering and it
may even prove cheaper in certain cases in
view of the following considerations:

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

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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.

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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.

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

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Construction of formwork:
This normally involves the following operations:
1. Propping and centering
2. Shuttering
3. Provision of camber
4. Cleaning and surface treatment

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Order and method of removing formwork:
The sequence of orders and method of removal
of formwork are as follows:
 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.
 Shuttering forming soffit of slabs should be
removed next.
 Shuttering forming soffit of beams, girders or other
heavily loaded shuttering should be removed in
the end.

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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 at least
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.

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Details of timber formwork for
RCC beam and slab floor Details at section shown in figure

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Elevation Details of timber formwork for
circular RCC column

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3D View
Details of timber formwork for
square or rectangular RCC
column

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Sectional plan showing details of
timber formwork for an octagonal
column
Details of formwork for stair

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Period of removal of formwork:
Walls, columns and vertical sides of beams
1 to 2 days
Slabs (props left under)
3 days
Beam soffits (props left under)
7 days

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Removal of props to slabs
(a) For slabs spanning up to 4.5 m 7 days
(b) For slabs spanning over 4.5 m 14 days
Removal of props to beams and arches
(a) Spanning up to 6 m 14 days
(b) spanning over 6 m 21 days

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PRE-CONCRETE CHECKS FOR FORMWORK:
Before the concrete is poured into the
formwork, it must be checked by someone
who has been trained to inspect formwork.
Depending on how big or complicated the pour
is, the inspection may just take few minutes or
it could take hours. Only when the formwork
has been approved, may the pour take place.

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Formwork pressures are function of height
(including the height from which concrete is
dropped into the forms) and are affected by
concrete workability, rate of stiffening and rate
of placing. One task of the temporary works
coordinator is to consider such factors as
ambient temperatures and concrete
composition, when calculating maximum
permissible rate of concrete placing.

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Exceeding this limit may lead to unacceptable
formwork deflections, loss of grout / concrete
at joints, or even collapse. The cost of
remedial work due to formwork deflection will
usually exceed the original cost of doing the
job properly.

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Below are the checks that should be verified
before pouring begins:
 Is the formwork erected in accordance with the
approved drawings?
 Is the formwork restrained against movement in all
directions?
 Is it correctly aligned and leveled?
 Are all the props plum, and at the right spacing?
 Are bolts and wedges secure against any possible
loosening?
 Has the correct number of ties been used? Are they in
the right places and properly tightened?

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 Are all inserts and cast-in fixings in the right
position and secure?
 Have all stop ends been properly secured?
 Have all the joints been sealed to stop grout
loss (especially where the formwork is against
the kicker)?
 Can the formwork be struck without damaging
the concrete?

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 Are the forms clean and free from rubbish such as
tie wire cuttings, and odd bits of timber or metal?
 Has the release agents been applied, and is it the
correct one?
 Are all projecting bars straight and correctly
positioned?
 Is there proper access for placing the concrete
and compacting?
 Have all the toe-boards and guard rails been
provided?

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RELEASE AGENTS FOR FORMWORK:
Formwork needs to be treated with a release
agent so that it can be removed easily after the
concrete has set. Failure to use a release
agent can result in the formwork sticking to the
concrete, which may lead to damage of the
concrete surface.

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A single application of release agent is all that
is required when forms are then used. Care
must be taken to cover all the surface that will
come in contact with the surface of concrete.
However, if there is an excess of release
agent, it may cause staining or retardation of
the concrete.

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There are different release agents depending
on what material is used for the formwork.
The three (03) most common release agents
for formwork are:
 Neat oils with surfactants: used mainly on
steel surfaces, but also suitable for timber and
plywood.

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 Mould cream emulsions: good general
purpose release agents for use on timber and
plywood.
 Chemical release agents: recommended for
high quality work, applied by spray to all types
of form face.

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ASSIGNMENT (A REPORT)
MiNi-Project
In M.S Word, do make a brief report of at least
20 pages having in it “Pictures” from original
sites covering 18 chapters of the referred text
book (Preferably 01 page for each chapter).
Try to make report understandable by
providing sequential pictures of the
phenomenon and a minimum of 4 to 6 lines in
each page and relevant topic.

Formwork in civil engineering

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