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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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