Formworks are used to hold freshly placed concrete at its position until it attains enough strength and with the help of formworks concrete can be easily manipulated and various shapes and forms can be constructed.

There are different materials which may be used to manufacture formworks for example timber, steel, and aluminum in addition to plastic that offer number of advantages compared to other materials for example durability, cost effectiveness, and flexibility.

It is reported that, not only does the construction of formworks takes time but also spent about 20 to 25% of the total cost of the structure. It is one of the major factors that influence the success of construction of any structure in terms of safety, cost, quality and speed.

It is for these advantages and benefits of plastic forms that will be discussed in the following sections.

Fig.1: Plastic Formwork for Concrete Construction

Objectives of Plastic Formworks in Concrete Construction

Form work is not a permanent construction but it requires careful attention to avoid damages of permanent construction. The formwork design and construction are controlled by three principles:

• Safety: It depends on the formwork strength substantially. Life of labors and public might be in danger if the formwork does not possess adequate strength.
• Quality: Concrete shape accuracy and the quality of final finished surface of concrete element.
• Economy: The structural frame, which is the most critical factor during construction, is most crucial cost component.

Advantages of Plastic Formworks in Concrete Construction

Formworks expend large labor force and total cost of the project that is why efforts have made to introducing a formwork system that is cost effective and can be assembled, dismantled and handled easily by labor.

There are several advantages provided by plastic formworks in concrete construction:

• Cost effective
• Labor friendly
• Eco friendly
• Low maintenance
• Versatile

Plastic Formwork System

It is manufactured from specific grade plastic that neither placed materials adhered to it nor chemical reactions occur between poured materials and the plastic form and consequently there will be no any patch on the surface of reinforced concrete element.

Water or freshly placed concrete leakages from different parts of the form is avoided because of the perfect fitness of various parts of the system. Added to that, it is most labor friendly system because not only does it fit and plugged easily but also it is considerably light weight compare with other types of formwork systems.

Nailing and oiling plates are not required prior to concreting because of auto leveling of plugging system that make plates level automatically and consequently it takes about 30% lesser time during installing and dismantling compare with conventional form systems.

Additionally, after its utilization, the plastic forms can be cleaned with water easily and if it breaks due to bad handling, low voltage hot air gun is used to seal it.

Other formwork materials such as wood, steel, aluminum produce various disadvantages which might outweigh their benefits. For example, the application of wood is considerably costly and create substantial environmental impacts due to deforestation.

With regard to aluminum formworks, it can be employed many times but its high cost makes it as not very suitable option to use. In contrary, plastic forms can be recycled and it is environmentally friendly in addition to reduce handling time and great reusability index.

Table-1 provides a comparison between plastic forms and other form types.

Table-1: Comparison Between Plastic, Traditional, And Steel Formworks

Different Types of Commercially Available Plastic Formworks

Siscon Plastic Formwork System

It is one type of commercially available plastic forms which can be used to construction various structural elements such as walls, columns, and slabs. It provides substantial precision.

Plastic Formworks for Walls

It is used for the construction of boundary walls, retaining walls, shear walls, rainwater drains, flood or storm drain walls, irrigation canals, and in concrete banking.

Fig.2: Wall Plastic Formwork Applications at Different Location of the Structure

Fig.3: Plastic Formworks used for Wall Construction

Plastic Formworks for Floors

Fig.4: Plastic Formwork for Concrete Floor Concrete

Fig.5: Plastic Formwork for Slab Construction

Plastic Formworks for Column

Fig.6: Installation of Plastic Formwork for Column

Nova Plastic Formwork System

It is produced from composite plastic material and it is the only plastic form that can be employed in both salt and fresh water without damage. Various application of NOVA plastic forms are shown in Figure-7, Figure-8, and Figure-9.

Fig.7: Application of Nova Plastic Forms

Fig.8: Application of Nova Plastic Form in the Construction of Column

Fig.9: Use of Nova Plastic Form in Construction of Concrete Walls

How to Measure Formworks? 

The formwork is measured in terms of area that is in contact with the concrete surface.

For example, the formwork for concrete footing will be calculated as the surface area of four sides of foundation only. Bottom of the footing is resting on earth, there is no need of any formwork and top of footing is open.

From the above footing plan and elevation, it can be seen that formwork area required will be

2 x (2 + 3) x 0.6 = 6 m²

Similarly, for a reinforced concrete beam, the measurement of formwork will be taken as the combined surface area of two sides and bottom of the beam.

Issues in Formwork Measurements

• Normally, the forms are used more than once in concrete construction. But the payment is calculated based on the total contact area of the formwork with concrete and reuse of the forms is not taken into account. Thus, the price per unit area of formwork can be reduced for reuse of the forms. Aluminium and steel forms are reused for many number of times than wooden forms.
• Complicated shape of concrete makes the formwork installation costlier than the simple formwork installation because of labor cost and inability to reuse these forms.
• A construction plan is required to reuse the forms maximum number of times to make the construction cost effective.

Deduction of formwork area should not be taken for:

• Intersection of beams
• Intersection of beams and columns and walls
• Any openings or cutouts in slabs

Units of Formwork Measurement

Formworks are measured in terms of area. So any unit such as square meter, square foot, square centimetre can be adopted. But generally, square meter and square foot of the contact area with concrete is taken as the unit of measurement.

The dimensions of a formwork should be measure correct to the centimetre or inches whichever the case may be.

Formworks are measured as just contact area, not area of formwork, as shown below:

The measurements of formwork are carried out separately for each type of concrete works such as following:

1. Foundations, footings, bases of columns etc. and for mass concrete and precast shelves.
2. Walls of any thickness including attached pilasters, buttresses, plinth and string courses etc.
3. Suspended floors, roofs, landings, shelves and their supports and balconies.
4. Lintels beams, girders and cantilevers
5. Columns, pillars, posts and struts.
6. Stairs (excluding landings) except Spiral Staircase
7. Spiral staircases (including landings)
8. Arches
9. Domes, vaults, shells roofs, arch ribs and folded plates
10. Chimneys and shafts
11. Well steining
12. Vertical and horizontal fins individually or forming box, louvers and bands
13. Waffle or ribbed slabs
14. Edges of slabs and breaks in floors and walls
15. Cornices and mouldings

Formworks can also be named based on the type of structural member construction such as slab formwork for use in slab, beam formwork, column formwork for use in beams and columns respectively etc.

The construction of formwork takes time and involves expenditure upto 20 to 25% of the cost of the structure or even more. Design of these temporary structures are made to economic expenditure. 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.

A good formwork should satisfy the following requirements:

1. It should be strong enough to withstand all types of dead and live loads.
2. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.
3. The joints in the formwork should be tight against leakage of cement grout.
4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.
5. The material of the formwork should be cheap, easily available and should be suitable for reuse.
6. The formwork should be set accurately to the desired line and levels should have plane surface.
7. It should be as light as possible.
8. The material of the formwork should not warp or get distorted when exposed to the elements.
9. It should rest on firm base.

Economy in Formwork

The following points are to be kept in view to effect economy in the cost of formwork:

1. 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.
2. Design should be perfect to use slender sections only in a most economical way.
3. 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.

Formwork can be made out of timber, plywood, steel, precast concrete or fiberglass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of precast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.

Economy in Formwork

The following points are to be kept in view to effect economy in the cost of formwork:

1. 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.
2. Design should be perfect to use slender sections only in a most economical way.
3. 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.

Formwork can be made out of timber, plywood, steel, precast concrete or fiberglass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of precast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.

Types of Formwork (Shuttering) for Concrete Construction:

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.

Normal sizes of members for timber formwork:

Plywood Formwork

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

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

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. 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.

Steel forms compared with timber formwork:

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

Construction of Concrete formwork:

This normally involves the following operations:

1. Propping and centring
2. Shuttering
3. Provision of camber
4. Cleaning and surface treatment

Order and Method of Removing Formwork:

The sequence of orders and method of removal of formwork are as follows:

1. 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.
2. Shuttering forming soffit of slabs should be removed next.
3. Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.

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 atleast 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.

Table: Period of Removal of Formwork