27-06-2012, 01:02 PM
FORMWORK
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What is Formwork
Formwork is a die or a mould including all supporting structures, used to shape and support the concrete until it attains sufficient strength to carry its own weight. It should be capable of carrying all imposed dead and live loads apart from its own weight.
INTRODUCTION TO FORMWORK
Formwork has been in use since the beginning of concrete construction.
New materials such as steel, plastics and fiberglass are used in formwork.
greater attention is being given to the design, fabrication, erection and dismantling of formwork
DEFENITION:
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 schedule and 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.
In order to successfully carry out its function, formwork must achieve a balance of following requirements:
Containment
Strength
Resistance To Leakage
Accuracy
Ease Of Handling
Finish And Reuse Potential
Access For Concerted
Economy
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.
FORMWORK BASED ON MATERIALS
MATERIALS FOR FORMWORK
Formwork can be made out of a large variety of materials.
The material most commonly being used to date is timber. However, due to the depleting forest reserves and increasing cost of timber the use of alternate materials such as plywood and steel has become prominent.
More recently, materials such as plastics and fiberglass are also being used for pre-fabricating formwork.
The type of material to be used depends on the nature of construction as well as availability and cost of material.
The constraints on the project such as overall cost, time of completion also play a major role in the use of a particular material for formwork.
TIMBER FORMS
Timber is required for practically all jobs of formwork. The timber bring used for formwork must satisfy the following requirements:
I. It should be durable and treatable
II. It should have sufficient strength characteristics
III. It should be light weight and well seasoned without warping,
IV. It should hold nails well.
Advantages of using timber forms:
I. It is economical for small construction jobs
II. It is design flexible and easy to erect
III. It has good thermal insulation which makes it useful to be used in colder
Regions
Iv. It can easily be made into any shape or size
Plywood forms (in combination with timber)
Concrete shuttering plywood is bwp grade plywood, preservative treated and specially suited for use in concrete shuttering and formwork.
The plywood is built up of odd number of layers with grain of adjacent layers perpendicular to each other.
Plywood is used extensively for formwork for concrete, especially for sheathing, decking and form linings.
There are two types of plywood - internal and exterior.
The interior type is bonded with water resistant glue and exterior type is bonded with water proof glue.
Hardboard forms
Hardboard is a board material manufactured of wood fiber, which is then refined or partly refined to form a panel having a density range of approximately 50 to 80 pounds per cubic foot.
Hardboards are standard / non-tempered or tempered.