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INTRODUCTION
Formwork is a mould used to shape the concrete and support the concrete until it
attains sufficient strength to carry its own weight. It can be visualized as a baking tin in
which a cake is baked, the tin supports the wet dough till it is baked fully. Here the
concrete can be compared to wet dough and formwork as the baking tin.
ECONOMY IN FORMWORK
The total cost of concrete construction includes the cost of formwork.
Construction of formwork involves considerable expenditure in terms of cost of material,
cost of labour for fabrication, erection and removal of formwork and time element. In
case of buildings, the cost of formwork may range between 30 to 40 % of the cost of the
concrete work. In case of special structures like bridges, tall chimneys, dams etc; the cost
of formwork may range between 50 to 60% of the cost of the concrete work. A careful
watch should be kept on the cost of formwork and all attempts should be made to bring
down the cost of the formwork to a minimum so as to achieve overall economy in the
concrete work.
The quality of finished concrete surface depends to a great extent upon the
quality of formwork used in the work. In case the formwork is made of rough timber and
is not constructed properly, the resultant concrete surface will be irregular and full of
defects. The expenditure involved in rectification of defects in such works is invariably
more than the saving made in the cost of formwork. Thus to effect economy, the
formwork should be made with good quality material and should be constructed properly.
REQUIREMENTS OF FORMWORK
1) The formwork should be constructed in such a manner that it may permit the
removal of various parts in desired sequence without jarring or damaging the
concrete.
2) The material of the formwork should be cheap, easily available and should be
suitable for re-use several times.
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3) It should be strong enough to withstand all types of dead and live loads such as
selfweight, weight of reinforcement, weight of wet concrete, loads due to workmen,
construction equipment, other incidental loads and forces caused by placement and
consolidation of the concrete, imposed upon it during and after casting of concrete.
4) The inside surface of formwork should be smooth so as to turn out a good concrete
surface. This is achieved by applying crude oil or soft soap solution to the inside
surface of formwork. This also makes the removal of formwork easy. The material of
the formwork should be as light as possible and should not warp or get distorted when
exposed to sun, rain or water during concreting.
5) It should be rigidly constructed and efficiently propped and braced (both
horizontally and vertically) so as to retain its shape without undue deflection.
6) The joints in the formwork should be tight against leakage of cement grout.
7) The formwork should rest on firm base.
CHOICE OF FORMWORK SYSTEMS
Formwork can be divided into three approaches. They are
1) Traditional approach.
2) Systematic approach.
3) Stay in place approach.
Traditional approach: This approach is not much in use now days. The formwork is
built on the site out of timber. It makes use of wooden posts called ballies as supports
for shuttering and wooden planks as sheathing. This practice is labour intensive and time
consuming especially for tall structures. It is unsafe and many a times unreliable also.
The quality of concrete surface obtained is not good and hence plastering and rendering
becomes a necessity. It has a very low re-use value, as the materials are not properly
seasoned to be weatherproof. Hence it can rarely be used more than once. The following
photo shows the Traditional Formwork approach.
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very organized system of forming concrete. There are many components that combine to
form this formwork system. These components are specifically designed to take dead
loads and live loads. It has a very sturdy supporting system. This formwork system is
designed keeping in mind the safety of the workmen. It might be labor intensive at times
depending upon the structure type. The finish obtained is of high quality and needs no
rendering or plastering unless asked for. System facilitates fixing and removal at each
location without the use of any sophisticated tools and tackles.
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Collapsibletype props (CTprops)
A network of wooden H-beams to support thesheathing.
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Stay in place approach:In this approach the formwork stays in place after the concretehas cured to provide thermal and acoustic insulation, space to run utilities, and backing
for finishes. It is assembled on site usually out of prefabricated, Insulating Concrete
Forms. This formwork becomes an integral part of the concrete and is not removed.
Hence it gets its name stay in place formwork.
TYPES OF SYSTEM FORMWORKS
In the early days the carpenter with available timber and nails carried out the job
of formwork. The formwork techniques have also developed side by side along with the
growth in the development of concrete construction. With the technological advancement
and introduction of new materials of formwork, more rational approach is being made in
the design of formwork. The different types of system formworks are as follows.
1) Column/Wall formwork
2) Flex system
3) Beam forming supports
4) Heavy duty tower
5) Climbing formwork
6) Slip formwork7) Tunnel formwork
8) Aluminium formwork
COLUMN / WALL FORMWORK SYSTEM: This consists of a vertical mould of the
desired shape and size, which has to retain the wet concrete and resist the initial
hydrostatic pressure caused by the wet concrete. The column formwork consists of a box
prepared from four separate sides. Column forms are located at the bottom around a 75 to
100mm high concrete plinth, which has the dual function of location and preventing grout
loss from the bottom of the column formwork. They are used for large size-system for
housing and business buildings, industrial construction, civil engineering works and tank
construction. It has high strength tie system to bear large concrete pressures and to avoid
load-bearing Struts. It hasprovision for fixing scaffolding platform and alignment units
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built in. Careful planning is done to re-use the panels so as to eliminate reassembling
them to suit different plan dimensions. These panels can be used along with climbing
formwork systems for tall structures. Small panels are handled manually where as large
panels are handled using cranes.
A large column formwork with supports Wall formwork
FLEX SYSTEM: This formwork system is used for slab and beam formwork up to floor
heights of 4.4m. It provides the option of choosing conventional system or Beam forming
system for beam formwork. It provides flexibility in spacing between props and
flexibility related to heights of individual props. The components are light enough for
erecting and dismantling the formwork. This system avoids use of many skilled laborers
at site. It enables re-propping and frequent reuse of materials. Side forms may be
removed as soon as practicable usually within hours of casting. This allows drying air
movements to take place around the setting concrete. Soffit formwork may be removed
usually within days but as a precaution some props are left in position until concrete
member is self-supporting.
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CONSTRUCTION OF RC FLOOR USING FLEX SYSTEM
BEAM FORMING SUPPORT SYSTEM: This is a three-sided box supported and
propped in the correct position and to the desired level. The beam formwork sides have to
retain the wet concrete in the required shape and be able to withstand the initial
hydrostatic pressure of the wet concrete. The formwork soffit apart from retaining the
concrete has to support the initial load of the wet concrete and finally the set concrete
until it has gained sufficient strength to be self-supporting. It is essential that all joints in
the formwork be constructed to prevent the escape of grout, which could result in
honeycombing in the cast beam. The removal time for the formwork will vary with air
temperature, humidity and consequent curing rate. It uses standard components avoiding
skilled laborers making them at site. Beam is provided with sheathing at the bottom
which should be of plywood of at least 19mm or of some thicker or stiffer material. The
arrangement on sides remains independent of beam width. This system can be easily
assembled, aligned, fixed and dismantled.
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Beam Forming Support
HEAVY DUTY TOWER SYSTEM: This system is usedfor slab and beam formwork
with tall staging heights and heavier loading. Additional frames can be attached to the
tower. Height adjustment of 800mm is possible using both top and bottom tower
spindles. Inclined decking is also possible in this system. The Individual components are
light enough to be managed manually otherwise tower, as a whole requires a crane to
handle it. This load-bearing tower system comprises only a few individual parts and is
extremely versatile. Speedy assembly is possible even when the towers are high. This
system is used whenever heavy loads are to be carried. Heavy-duty tower formwork
system is used mainly for construction of bridges.
CLIMBING FORMWORK SYSTEM: Climbing here means that the formwork of
large area can be shifted in a single operation along with the decks both horizontally and
vertically. The hydraulic jacks provide the basic climbing energy for the whole climbing
forming. With the aid of automatic self-climbing systems, wall formwork elements are
suspended from the platform. This makes it possible to concrete the entire core, slabs,
and facade walls in a single pour, saving valuable time and manpower. The typical cycle
of climbing formwork consists of stripping the formwork, lifting it to next level, laying
rebar, setting up formwork, and then pouring concrete. Climbing formwork makes it
possible to use the technical advantages of a large area formwork at any height with the
same degree of safety as on the ground.
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CLIMBING FORMWORK
Features of climbing formwork:
1) High adaptability: The formwork assembly may be integrally shifted for 600mm,
facilitating form cleaning and bar wrapping. Flexible operation is resulted from the
formwork assembly which may be adjusted to the left and right or up and down in
relation to the support assembly. The crane-lifted climbing formwork can be used for
structures of any shape and any angle of inclination.
2) Short shifting times: It permits quick installation, reduces crane time because the
entire unit repositions quickly. It offers speedy precision adjustment of the formwork in
all directions.
3) Safe suspension system: The platforms allow for safe and efficient work and are
designed to carry high loads for the storage of equipment and reinforcing steel. The
climbing formwork provides superb workplace safety.
4) Sturdy design: They are designed to take up large loads. The embedded parts bear the
complete lateral concrete pressure, eliminating the need for bracings and resulting in
smooth concrete surface.
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The climbing formwork is ideal for all industrial high-rise projects, bridge piers, silos,
radio and television towers.
SLIPFORM: Slipform means a continuously moving form, moving at such a speed that
the concrete when exposed, has already achieved enough strength to support the vertical
pressure from concrete still in the form as well as to withstand nominal lateral forces
caused by jack rod, wind etc. This system is more suitable for structures with uniform
cross-sections or uniformly varying cross-sections.
Slipform can be classified as straight Slipform, tapering Slipform and Slipform
for special applications. Slip Forms have been used to construct Single- cell silos, Multi
cell silos, Piers, Towers, Water reservoirs, Chimneys, Buildings, Light House.
Features of Slipform:
1) It is suitable for tall structures with limited variation of shape and size in plan
with level.
2) Rapid construction is achieved by making use of Slipform.
3) Slipform uses hydraulic jacks instead of cranes for lifting.
4) Since Slipform is a continuously moving formwork; Continuous casting is done
which avoids construction joints.
5) It avoids expensive consumables and works out economical.
TUNNEL FORMWORK: With the help of tunnel forms, room size walls and slabs can
be cast together like a portal in a single pour of concrete. To facilitate removal of the
form, block work or pre-cast walls are provided for the facade. Generally these forms are
made of steel components and result in high productivity by repeated usage. This is
ideally used for construction of mass system housing and tunnels. It ensures rapid
construction. It is suitable for large repetitive projects where economy is derived from
material and labor. Tunnel formwork requires crane for handling of formwork.
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ALUMINIUM FORMWORK SYSTEM: Aluminium formwork system is to discharge
heavy loads coming from great heights. It is the latest in technology, which is an all
aluminium formwork system. All components except for some supporting members are
made up of aluminium alloy.
Features of Aluminium formwork system:
1) It is used for concrete wall and slab type of construction.
2) It requires not more than a hammer for its assembly and removal.
3) It is completely self- supporting and interlocking.
4) High speed of construction is achieved.
5) It is Very light in weight and hence easy to handle.
6) It is highly earth quake resistant.
7) This system does not rely on expensive machinery or skilled workmen.
8) The overall cost of construction is less when used for repetitive structures like
mass housing etc.
Limitations of Aluminium formwork system:
1) No changes in the formwork setup can be done once it is fabricated not even
adding conduits for services.
2) It can be used only for superstructures; foundation needs to be done in the
conventional way.
3) Its initial cost is higher.
4) It is economic only when number of repetitions of the formwork is obtained.
FAILURE OF FORMWORK
The safety must be the first place in the design, construction, erection and stripping of
formwork systems. Most failures of large formwork are progressive; vertically through
several floors or horizontally as each successive line of shoring collapses like a house of
cards. Generally some unexpected event causes one member to fail, then others become
overloaded or misaligned and the entire formwork structure collapses.
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Failure may result in a collapse of part or all of the forms. Failure of the formwork
causes distortion and movement of the forms, which will affect the stability of the
structure. Such failures lead to minor or fatal accidents. Formwork collapse causes
injuries, loss of life, property damage, and construction delays. Rectifying cost of failure
includes removal and replacement cost of formwork, removal cost of fresh or failed
concrete, removal of reinforcement, loss of time for rework and cost of injuries.
To eliminate all possibility of a large costly failure, the overall formwork system
should be reviewed before construction. Guidelines given in relevant Indian standards
should be followed regarding the period of removal of formwork.
CAUSES OF FAILURE OF FORMWORK
1) If the formwork is removed too soon before the concrete has developed sufficient
strength to support itself as well as the loads acting on it; it leads to the failure of
formwork.
2) Improper contact of the base and props and improper verticality of props cause
failure.
3) If inadequate shoring and bracing is provided it leads to failure of formwork.
4) All loads are not considered during design.
5) If the strength of shoring materials is insufficient to take up the loads.
DESIGN OF FORMWORK
Several failures of formwork have focused attention on the need for guidelines in
the design and construction of formwork so as to achieve safety, reliability and economy
in concrete construction. The importance of formwork at times is not appreciated well.
Formwork shall be designed to meet the requirement of the permanent structure using
relevant Indian Standards for materials selected for formwork. The design should take
into account the conditions of materials to be actually used for the formwork,
environment and site consideration. Before proceeding to the design, all the relevant
design information should be obtained from the relevant sources. The design information
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includes the site investigation report, expected loading scheme of load transfer, sequence
of erection and releasing, procedure of concreting and time frame. When the material for
formwork have been chosen, and the anticipated loading estimated, a form should be
designed strong enough to carry the anticipated loads safely, and stiff enough to hold its
shape under full load. Under-designing can result in failure of the forms and Over-
designing will result in an excessive cost.
Loads on Formwork and Combination of Loads
Formwork shall be designed to resist the expected dead load, imposed load, construction
load and environmental load. Loads on formwork are any combinations of the following:
1) Dead loads,
2) Imposed loads,
3) Environmental loads,
4) Incidental loads during erection and operation,
5) Lateral pressure.
DEAD LOAD: Dead loads shall include:
Formwork structure, self weight of formwork and any ancillary temporary work
connected or supported by formwork, and weight of freshly placed concrete for the
permanent structure directly supported by the formwork. The unit weight of wet concrete
including reinforcement shall be taken as 26 kN/m. Actual load of formwork shall be
evaluated for use in design. However, in absence of the data, load may be assumed as 500
N/m2 for the purpose of initial calculations.
IMPOSED LOAD: Loads during constructional operation shall constitute the imposed
loads for formwork design. Such loads may occur due to construction personal, plant and
equipments, vibration and impact of machine delivered concrete, lateral pressure of fresh
concrete, unsymmetrical placement of concrete, concentrated load and storage of
construction materials. The concrete should not be dropped from a free height greater
than 11 m if it be necessary to exceed these limitations, allowances for the additional
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loading should be made in design. Where allowance has only to be made for access and
inspection purposes, a loading of750N/m2 should be adequate.
ENVIRONMENTAL LOADS: These loads include:
1) Wind or seismic loads,
2) Earth pressure,
3) Water pressure,
4) Snow loads or ice loads, and
5) Thermal load, etc.
LATERAL PRESSURE (due to fresh concrete): The lateral pressure due to fresh
concrete depends on the temperature of concrete as placed, the rate of placing of concrete
and the concrete mix proportion. A set of curves giving typical values of pressurePmax
for unit height, on formwork are given in Fig 3 for guidance. The pressure distribution
varies along the height of formwork. For normal concrete, the maximum pressure may
occur at a height h,below the top as given by the following formula:
hm = Pmax/d
Where
hm, is in m,Pmax is in kN/m2, and
dis density of fresh concrete in kN/m3.
STABILTY: The formwork shall be designed to check against overturning and sliding.
A factor of safety of 1.5 may be used in design against overturning and sliding.
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Deflection limit
The formwork shall be designed so as to remain sufficiently rigid during placing and
compaction of concrete. The total calculated deflection of formwork including the initial
imperfection in the members should not exceed the following:
For beam span < 3 000 mm,
< 3mm
For beam length > 3 000 m
is the least of 30 mm OR L/l 000
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CONCLUSION
Formwork is base for any concrete structure.No desirable structure can be built withoutthe use of formworks.
Unfortunately very little attention is being paid to formwork design or its usage in India
even though formwork accounts for a large part of the cost of concrete work in a project.
Formwork can increase or decrease the cost of the project as under design & over design
will increase the cost of the project to considerable limits. Under-designing can result in
failure of the forms and Over-designing will result in an excessive cost. The formwork
design requires the same skill and attention to details as the design of permanent
structures. Formwork collapse causes injuries, loss of life, property damage, and
construction delays.
There has been a considerable advancement in Formwork recently worldwide. There are
lot of software tools for fast work preparation, planning and material allocation and
design. Summarizing, the goal of this study was to provide a comprehensible discussion
of the concepts and importance of formwork
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