Formwork Design-Introduction.pdf

Sanjivani Rural Education Society's
Sanjivani College of Engineering, Kopargaon 423603.
-Department of Strucutral Engineering-
By
Mr. Sumit S. Kolapkar (Assistant Professor)
Mail Id- kolapkarsumitst@sanjivani.org.in

Ø Introduction- Formwork as a Temporary
Strucutres
RC construction primarily consists of three components- formwork,
reinforcement and concrete.
1. Formwork accounts about 35 to 50 % of total construction cost.
2. Formwork operations consumes about 50 to 75 % of the total
time of construction.
3. Formwork components are highly loaded for a few hours during
concrete placement. Hence we can design these components by
allowing higher permissible stresses as compared to the stresses
taken for the design of permanent structures.

Ø Introduction- Temporary Strucutres
• Concrete Formwork (Shuttering)
• Scaffolding
• Falsework/Shoring
• Cofferdams
• Underpinning
• Diaphragm/Slurry Walls
• Earth retaining structures
• Construction dewatering

Ø Temporary Strucutres-
• Cofferdams- Watertight enclosure from which water is
pumped to expose the bed of a body of water in order
to permit the construction of a pier or other hydraulic
work below the waterline.

• Underpinning- Reinforcing of an existing building
foundation. It is required when the original foundation
is no longer strong enough to support the structure.
• Is provided underneath of an existing foundation to
maintain its stability. Used to repair, strengthen
foundation of existing structure.

• Diaphragm/Slurry Walls- An underground concrete
wall constructed panel-by-panel each interlocked to
ensure structural stability and water tightness.
• Uses: Earth retention (resisitng lateral load), load bearing
foundation, water proofing, groundwater barriers etc.

• Earth retaining structures-
• Construction dewatering-

• Concrete formwork-
• Purpose- To support its own weight and that of freshly
placed concrete as well as the construction live loads
including materials, equipment and workmen.
• It is a complete system of temporary structure built
to contain fresh concrete so as to form it into the
required shape and dimensions and to support it until
it hardens sufficiently to become self-supporting.

• Parts of Concrete formwork-
1. Sheeting (Sheathing)- That part of formwork, which is in contact with the
concrete.
2. Form (Shutter)- (a) That part of formwork, which consists of the sheeting and
its immediate supporting or stiffening members.
(b) A structure or mould in support of vertical strucutral member of concrete
while it is setting and gaining sufficient strength to be self-supporting.Ex:
Column, retaining wall, footings etc.
3. Falsework- (a) Falsework is the temporary structure erected to support the
work in the process of construction. It is composed of shores, formwork for
beams or slabs (or both) and lateral bracing. To hold the items in place.
(b) It supports the forms, usually for a long structure, such as a bridge.
4. Centering- It is a horizontal structural member supporting a structure.
Exd: soffit, slab etc. It is the formwork used in the construction of arches, shells
space structure where the entire falsework is struck or decentered as a unit, to
avoid introducing injurious stress in any part of the structure.

• Parts of Concrete formwork-
5. Mould- A frame for casting, precast concrete units.
6. Scaffold (Scaffolding)- A temporary structure for gaining access to higher
levels of the permanent structure during construction. For workers to stand on
while working on a building

• Requirements- Quality, safety and economy
• The formwork must be built and erected in such a way that the required shape, size, position,
quantity, and finish of the concrete are obtained.
• The formwork must be strong enough to take the pressure or weight of the fresh concrete, so
that the finish of the concrete is obtained. The material should not warp or get distorted.
• The formwork should be designed and constructed such that it can be easily and quickly
erected and struck with minimum skilled workforce leading to savings in both time and money. It
should be able to be set accurately to the desired line and levels.
• The formwork must be able to be struck without damage to the concrete or to the formwork itself.
• The formwork must be able to be handled using the available equipment or to be manually
handled, if necessary.
• The formwork arrangement must provide safe access for the handling of concrete and its
placing.
• The formwork must have all the necessary safety arrangement relating to working areas and
platforms.
• The formwork arrangement must be flexible enough to get the desired finish. That is, it should
be able to accommodate different types of sheathing material, e.g., plywood, steel etc.
• The formwork should also be able to accommodate any architectural features such as grooves,
surface grains etc. It should be possible to a ain tight joints so that there is no leakage of cement
grout.

• Requirements- Quality, safety and economy

• Selection-
1. the design of a building,
2. site constraints,
3. available resources,
4. the contractor’s experience with different systems, and their availability
5. The type of building element to be formed,
6. the type of sheathing material,
7. safety and serviceability of the structural frame,
8. economics
9. building design,
10. job specification,
11. local conditions, and
12. supporting organisation

• Types-

• Types- Traditional classification
I) Conventional System of Formwork:
• In this system, timber and plywood are used predominantly.
• The various formwork components are connected with the help of nails.
• Disadvantages:
• limited reuse value,
• higher cost,
• can be fabricated, installed, and removed only by skilled workmen,
• since the skill of workmen and the quality of formwork material vary from site
to site, the quality of concrete structures obtained is not consistent,
• in large projects, it is difficult to ensure an efficient utilisation of the timber
available on site.
Note- All these factors adversely affect the cost of formwork.

I) Conventional System of Formwork:

II) Proprietary/Patented System of Formwork:
• Most of the disadvantages observed in conventional systems can be
overcome by adopting good proprietary systems, which make use of
standard factory made components
• Very less formwork are made at site, therefore saves labor cost
• with uniform quality standard products, it is possible to achieve uniformly
good workmanship, improved quality of concrete surface and high reuse
value
• Can be assembled and dismantled with unskilled/semi-skilled labor

II) Proprietary/Patented System of Formwork:

III) Modular System of Formwork:
• the formwork modules are manufactured in a factory set up, and delivered to
the site in a pre-fabricated form.
• can be assembled very quickly at the project locations.
• Advantages:
1. as it uses standardized modules, and their installation process is simple, it
requires less skilled workers.
2. considerable reduction in erection time at the site.
3. large number of repetitions (reuse) result in the cost of formwork being
considerably on a lower side.
4. The safety of workers and materials is ensured, as it has high strength of the
form.
5. The quality of the concrete surface obtained is extremely good which
reduces the need for further finishing work such as plastering, after removal of
the form.

• Advantages:
6. it automate the formwork operation and improve the productivity as well as
the cost-effectiveness in a construction project.
7. for successful use of a modular formwork system, proper planning in the
architectural design stage as well as in the construction phase of a project is
necessary.
8. the factors such as the form reuse scheme, the allocation of modular form
sets, cranes, workers, etc., and the construction sequence need to be carefully
planned out to get the best out of the modular formwork.

• Types- Classification based on Hanna (1999)
I) Horizontal System of Formwork:

I) Horizontal System of Formwork:
• In Hand set formwork, the commonly used formwork systems are: (a)
conventional wood system (also known as stick form) and (b) conventional
metal system (also known as improved stick form). The conventional wood
system is available for laying the foundation, beams and slabs, and columns
etc.
• In Crane set formwork systems, we have the flying formwork system, column
mounted shoring system, and tunnel forming system.
• In Special horizontal formwork systems, we have the joist slab forming
system, and dome forming system.
• Generally consist of a series of interconnected falsework bays, independent
props or system scaffolds and supporting a number of panels.

II) Vertical System of Formwork:
Under vertical formwork system, we essentially have two subsystems: crane
dependent system and crane independent system.
• In crane dependent system, we have the conventional wall column framing
system, Ganged framing system, and Jump form.
• In crane independent system, the commonly used formwork systems are: (a)
slipform and (b) self raising formwork system.

II) Vertical System of Formwork:

• Types- Classification based on DSR Formwork Items
In India, often the formwork items are presented in a specific form in the bill
of quantities. These items are mentioned in the Delhi Schedule of Rates
(DSR 2007). From that clue the third system of classification is referred to as
“classification based on Formwork items as used in DSR 2007”.

• Different Formwork Materials-
The selection of material to be used for formwork and shoring shall take into
account its strength, rigidity, durability, workability, finished quality of concreted surface,
effect on the fresh concrete placed, and the economy.
I) Timber: Requirements
i. locally available and cost effective
ii. reasonably seasoned to avoid warping
iii. should hold nails well
iv. use hard wood in cases of heavy structures imposing large loads on the
formwork
v. timber for formwork should be softwood of partially seasoned stock to avoid
swelling or warping as per IS: 883–1994.

I) Timber: Characteristics
1. It should be easy to work with by hand or machine and should not split when nailed.
2. It should be hard enough to withstand damage on the contact surface under normal
conditions of erecting and stripping of the formwork, fixing steel reinforcement bars and
placing concrete.
3. It should be light enough for the workers to handle and carry to their respective work
areas.
4. It should be stiff so as to avoid undue deflection when loaded, and strong enough to
safely carry considerable loads and pressures that may be applied during concreting.
5. It should be reasonably stable and not unduly liable to cast or warp when exposed to
sun and rain or when unevenly we ed during rainy days or the monsoon seasons.
6. It should have the correct amount of moisture so that it will not warp and swell after
concrete is placed.
7. It should not have excessive formation of hemicelluloses (wood sugars) on the first
use after exposure to sunlight, as it will retard the curing of concrete.

I) Timber: Commonly Used Timber Sections for Formwork and Their Properties

I) Timber: Specifications of Timber for Formwork Applications
The specification for timber to be used in a formwork application must
conform to IS: 3629–1986
Grades of Timber: Based on prohibited and permissible defects of cut timber-
Grade I-The estimated effect in reduction of the strength of timber is not more than 12.5 percent.
Grade II-The estimated effect in reduction of the strength of timber is not more than 25 percent.
Grade III-The estimated effect in reduction of the strength of timber is not more than 37.5 percent.

I) Timber: Defects in Timber for Formwork Applications
Defects Prohibited - Loose grains, splits,heart wood rot, sap rot, warp,
worm holes made by powder post beetles and pitch pockets shall not be permitted.

I) Timber: Permissible Stresses for Timber
IS: 883–1994 classifies timber for constructional purposes in three groups
based on their strength properties, namely, modulus of elasticity (E) and extreme fiber
stress in bending and tension (fb ).

I) Timber: Permissible Stresses for Timber

I) Timber: Modification Factors
The modification factors to allow for change in the slope of grain (represented by K1) are
provided in Table 2.8. The K1 values are provided for both beams and ties and posts or columns.
The modification factors for change in the duration of loading are provided in Table 2.9.
This modification factor is represented by K2. It may be noted that with the decrease in loading
duration, the K2 values increase. It is maximum for instantaneous or impact loading conditions.

I) Timber: Grains

I) Timber: Application of Timber in Formwork
Timber is used for formwork sheathing, joists, beam bottoms, batten
supporting floor boards, battens supporting vertical sheeting of vertical
formwork, and for timber posts (vertical shores).
Requirement for timber board as sheathing member:

Requirement for timber board as sheathing member:
1) The material for the sheathing should not deteriorate extensively on the absorption of
moisture.
2) The repeated cycles of wetting and drying may cause some timber boards to crack,
spall, or become brittle. This may cause a considerable reduction in the strength. In
order to avoid the deterioration and consequently the reduction in strength, the
timber material should be selected and stored carefully.
3) The timber for formwork sheathing is usually made plain on the side in contact with
the concrete.
4) The timber should be rigid to avoid local bending and denting.
5) The block boards should not be used as sheathing member.
6) The recommended minimum thicknesses of timber board to act as a sheathing
member are given below for different applications-
i. For wall, vertical sides of beams : 25 mm
ii. For floors of normal loads : 30 mm
iii. For floors where heavier constructional loads are possible : 37 mm

Requirement for timber joists, beam bottoms, batten supporting floor boards,
and battens supporting vertical sheeting of vertical formwork:

Requirement for timber joists, beam bottoms, batten supporting floor boards,
and battens supporting vertical sheeting of vertical formwork:
The depth (or thickness) of battens or joists will be based on strength and deflection
requirements-
Note- Timber members used as joists, beam bottoms, battens supporting floor boards,
battens supporting vertical sheeting of walls act primarily in bending.
The maximum permissible unstiffened length is 50 times the width of the timber
batten (joist) for such applications.
For example, if the timber batten is 50 mm wide, the maximum spacing between
stiffeners (horizontal or vertical) or cross members shall be 50 x 50 = 2,500 mm = 2.5 m.

Requirement for timber posts (Vertical Shores):
The l/d ratio is one of the
where l = unsupported length of a member and
d = width of the face of the member under consideration, is one of the important design
parameters for timber shore design.
Note-
1. The floor to floor height in a normal building is of the order of 3 to 4 m.
2. The shores are provided with horizontal braces at a distance of about 1.80 m from
the floor level.
3. The 1.8 m distance is ensured, so that a crew member of average height can easily
work under the braces.
4. The effective length of the shores is about 2 times the height between the braces
(about 1.80 m as explained above).
5. The l/d ratio in case of long columns is recommended to be 50.
6. Thus, in order to avoid buckling and to avoid the shores against the collapse, the
minimum recommended cross section of timber shores/posts should be taken as 100
mm x 100 mm.
7. The free standing shores (without horizontal braces) of very large heights should be
avoided.

II) Plywood: Introduction
• Is the most commonly used material for formwork because of the ease with which it
can be cut and assembled on the site therefore reduces the cost of making.
• It is used as a sheathing material directly in contact with the concrete.
• With proper care and treatment of many reuses are possible with plywood.
• IS: 4990–1993 gives specification of the plywood used for the purpose of form lining
and sheathing.
• There are about 20 tests prescribed in IS: 1734–1983 for plywood testing.
• The concrete surface obtained by plywood can virtually eliminate the need of
plastering.
• Plywood can be used in a be er way both in the hot and the cold climatic conditions
as the external heat or cold does not penetrate through plywood unlike metal sheeting.

II) Plywood: Permissible Stresses in Plywood
• Based on the permissible deflection in plywood to be 1:270 (1/270th of the span
between the bearers), the permissible stresses have been worked out in Table
Note- 1. The above loads apply when the concrete is laid on concrete shuttering
plywood as in slabs and beams.
2. The same thickness of concrete in a wall can be held without excess deflection by
thinner boards.
3. According to IS: 4990–1993, the maximum load should be reduced to 75 % if wet
boards are used.

II) Plywood: Commonly Available Sizes of Plywood and Their Properties
Note- Thicknesses of plywood.
1. In India leading plywood manufacturer offers plywood in 4 mm, 6 mm, 9 mm, 12 mm,
16 mm, 19 mm, and 25 mm thicknesses.
2. The 12 mm and 19 mm plywood thicknesses are the most commonly used.
3. A minimum thickness of 12 mm with proper frame work is suggested.
4. The smaller thicknesses of plywood are used for applications such as casting curved
wall, dome etc. where it is desired to bend the plywood.

II) Plywood: Data on the weight in kg/m2 of plywood from a leading Indian
plywood manufacturer is given in Table
The code IS: 4990–1993 also suggests the minimum bending radii for the plywood of
different thicknesses. They are given in Table

II) Plywood: Requirement for Plywood for Formwork Application
As per IS: 4990–1993
1. The moisture absorption should be as low as possible, otherwise the plywood would
absorb the moisture from the concrete.
2. Moisture absorption leads to loss of binding ability of the resins used in the
manufacturing of plywood.
3. The moisture results in peeling off of the veneers of plywood. Hence it is
recommended to use hard wood veneers in conjunction with water insoluble resins.
4. As far as applicable, both the used and unused forms should be stored in shade.
5. The plywood must also be durable under alternate wetting and drying conditions.
6. The surface films in phenolic resin bonded plywood have the following advantages:
i. Makes the plywood invulnerable to moisture.
ii. Protection from deterioration caused by concrete and slurry.
iii. Smooth finish to the concrete.
iv. Large reuses are possible.
v. Very good for fair faced concrete application.

III) Steel: Introduction
• Compared to timber formwork, steel formwork provides smoother concrete surfaces
and is found to be economical if there are a large number of uses.
• It has adequate rigidity and strength.
• These forms can be erected, disassembled, moved and re-erected at a faster pace, if
suitable handling equipments are available.
• It facilitates in maintaining accurate alignment, levels and dimensions.
• The cost advantage would turn into disadvantage if there were less number of reuses
of the steel form.
• These forms offer little or no insulation protection to the concrete placed during cold
weather.

III) Steel: Commonly Used Steel Sections for Formwork and Their Properties
• Steel plates of 3.15 mm and 5.0 mm thicknesses are commonly used for fabricating
the sheathing.
• IS Angle sections 50 x 50 x 6 and 65 x 65 x 6 are commonly used for making steel
panels to be used in the wall, column, and slab and beam applications.
• Channel section ISMC 100 is the common fabrication material for fabricating walers
and soldiers.
• It is very common to use 40 NB circular steel pipes for shoring and scaffolding works.
• Note- Use IS: 2750–1964 for steel tubes of different diameters and their various
properties like cross sectional area (A), moment of inertia (I), section modulus (Z),
and radius of gyration (K) etc.

III) Steel: Permissible Stresses for Steel
Axial stress in compression— The permissible stress in compression can be
obtained from following figs.

III) Steel: Permissible Stresses for Steel
Axial stress in tension— The various stresses on the net cross-sectional area of
tubes shall not exceed the values specified in Table
Note: Some of the relevant IS codes providing specification on steel forms are
IS: 2062–2006, IS: 8500 –1991, IS: 1977–1996, IS: 800–2007, and IS:
1161–1998.

III) Steel: Application of Steel in Formwork
The various applications of steel in formwork and their brief features are given in Table

IV) Aluminium: Introduction and Features of Aluminum Formwork
1. The aluminum forms are similar to steel forms.
2. Because of lower density and being lighter than steel, it is necessary to use larger
sections when forms are made of aluminum as they have lower strength in tension and
compression compared to steel.
3. Very less labor required in erecting, handling, and disassembling. Handling is easy.
4. Pure aluminum is attacked chemically by wet concrete but certain aluminum alloys
are found resistant to wet concrete as well as atmospheric corrosion, and they are used
for making aluminum forms.
5. Lightweight props in aluminum alloy tubes are also used.
6. The aluminum forms are highly favored in countries where mechanical handling
equipment are used on a large scale, labor is in short supply, and the cost of engaging
labor is also prohibitive.
7. Some manufacturers claim upto 2,500 repetitions with aluminum form, provided
reasonable care, cleaning, and maintenance are carried out at reasonable intervals.

V) Plastic Forms:
1. It is a better alternative to wood.
2. Plastic formworks are eco-friendly.
3. It gives better finish, easy transportation, are light in weight, does not rust, easy
cleaning etc.
4. These moulds resist mechanical handling; they do not deform easily and can be
quickly assembled and stripped.
5. The jointing of plastic formworks is also easily achievable.
6. They can be re-used large number of times.
7. It allows greater freedom of design. We can mould even unusual textures and designs.
8. Best known plastic sheathings are those made of PVC, neoprene, and polyester
strengthened with glass fiber.

VI) Other Form Materials:
1. Plaster of Paris Forms- Used for architectural ornamental purposes
2. Hard Boards-Is a board manufactured from wood fiber under the controlled
combination of pressure, heat, and moisture. Tempered hard boards have improved
strength properties, lower rate of water absorption, better abrasion resistance and
therefore, these are used for formwork lining. Being thinner - about 6 mm — they are
flexible, and are used for lining of the curved surfaces.
3. Lost forms- Examples are precast concrete planks, pressed fiber planks, cardboard
tubes, precast reinforced concrete joists and clay filler blocks, ferrocement planks etc.
These forms are left along with the laid in-situ concrete and form an integral part of the
construction.
4. Fiber Forms- These are generally used as lost forms especially for concrete walls,
roofs and slabs. These are left in place on the exposed face of the concrete where it
gives an architectural look and also improves the acoustical and insulating properties.
Fiber boards are made from such substances as glass fibers or wood fibers.

VI) Other Form Materials:
5. Gypsum Boards- Gypsum moulds are generally used to provide artistic design or
ornamental pattern for the exposed concrete face. Gypsum mix is reinforced with
organic fiber or coir or sisal to get structure toughness. Concrete quality may improve
due to the absorption of superfluous water in the concrete. The fragility of gypsum
surfaces impedes its widespread use.
6. Asbestos Tiles- Corrugated asbestos tiles like corrugated sheets have been used as
the re-usable shuttering material where corrugated face is required.
7. Wire Mesh- The wire mesh is a kind of lost form. The wire mesh may act as a form
to take up plaster. Example is ferrocement structure
8. Inflated Membranes- Are used (1) occasionally to form structures of varied shapes
such as spherical, elliptical, cylindrical etc. (2) for providing block-outs or cavities in
areas where access is difficult and the application of other form materials is difficult.
9. Fabric Formwork- Fabric formwork uses a flexible textile membrane in place of the
rigid formwork panels.

Ø Introduction- Impact of Structural Design
on Formwork Cost
“The least amount of permanent materials in the structure will result in the least cost.”
For most economical design, the designer typically analyzes each individual member to
make certain that it is not heavier, wider, or deeper than its load requires. This is done
under the pre-tense that the minimum size and least weight result in the best design.
However, this approach to design neglects the impact of the cost of formwork, the
temporary support structure that must be fabricated and installed to support the
permanent materials. Focusing only on ways to economize on permanent materials, with
little or no consideration of the temporary formwork, can actually increase, rather than
decrease the total cost of a structure.

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