Formwork Concrete Technology

Formwork

Formwork is used to hold the concrete in any structural element before hard it. The weight of the wet concrete is higher than the dry concrete. Therefore the formwork should be strong enough to carry the load of the wet concrete. So also formwork should be free of voids and concrete or grout should not be going out from the formwork. So also it should be straight enough to maintain the shape of the structural element and can be withstand against the vibration of the equipment used. Therefore the form work is very important in the concreting works.

Definition:

§   As a structure,

§   Temporary which is designed to contain fresh fluid concrete.

§   Form it into the required shape and dimensions.

§   Support it until it cures sufficiently to become self supporting.

The term ‘formwork’ includes the actual material contact with the concrete, known as form face, and all the necessary associated supporting structure.

REQUIREMENTS OF A GOOD FORMWORK SYSTEM

§  How formwork can be erected and de-shuttered fast.

§  How good concrete quality and surface finish can be achieved.

§  What is the optimum stock of formwork required for the size of work force, the specified time schedand flow of materials.

§  What is the overall cost savings that can be achieved using the right type of formwork.

§  How safety can be improved for the site personnel.

To successfully carry out its function, formwork must achieve a balance of following requirements:

·         Containment: formwork must be capable of shaping and supporting the fluid concrete until it cures.

·       Strength: formwork must be capable of safely withstanding without distortion or danger the dead weight of the fluid concrete is placed on it, labour weight, equipment weight and any environmental loadings.

·     Resistance to leakage: all joints in form work must be either close fitting of covered with form tape to make them grout tight. If grout leakage occurs the concrete Will leak at that point. Leakages cause honeycombing of the surface.

·       Accuracy: formwork must be accurately set out so that the resulting concrete product is in a right place and is of correct shape and dimensions.

·       Ease of handling: form panels and units should be designed so that their maximum size does not exceed that which can be easily handled by hand or mechanical means. In addition all formwork must also be designed and constructed to include facilities for adjustments, leveling, easing and striking without damage to the form work or concrete.

·         Finish and reuse potential: the form face material must be selected to be capable of consistently imparting the desired concrete finish (smooth, textured, featured or exposed aggregate etc.) At the same time it should also achieve the required number of reuse.

·         Access for concrete: any formwork arrangement must be provide access for placing of the concrete. The extent of this provision will be dependent on the ease of carrying out the concrete operations.

·      Economy: all the formwork is very expensive. On average about 35% of the total cost of any finished concrete unit or element can be attributed to its formwork; of this just over 40% can be taken for material for formwork and 60% for labour. The formwork designer must therefore not only consider the maximum number of times that any form can be reused, but also produce a design that will minimize the time taken for erection and striking.

No	Description of structural member	Period of time
1	Walls, columns and vertical sides of beams	1 to 2 days
2	Slabs (props left under)	3 days
3	Beam soffits (props left under)	7 days
4	Removal of props to slabs
	(a) For slabs spanning u pto 4.5 m	7 days
	(b) For slabs spanning over 4.5 m	14 days
5	Removal of props to beams and arches
	(a) Spanning up to 6 m	14 days
	(b) spanning over 6 m	21 days
6	Props to cantilever	28 days
7	Beam sides, walls & columns (unloaded)	24 hours
8	Slab (need to provide props)	4 days

Table show : Duration taken for removal of formwork

TYPES OF FORMWORK

There are different types of formwork available for different purposes. Generally, the formworks for vertical concreting are called wall forms and those for horizontal concreting are called slab or floor forms. The various types of formwork available today in the market are discussed in detail.

·         Traditional formwork

·         Climbing formwork

·         Sliding formwork or slip forming

·         Permanent formwork.

·         Special forms

·         Table form

·         Ganged panel form

·         Tunnel form