Formwork

 

Centering, shuttering or formwork is a sort of temporary construction provided for laying cast-in-situ concrete to required shape. A good formwork should satisfy the following requirements.

·      It should be strong enough to withstand all types of dead and live loads such as self-weight, weight of reinforcement, weight of wet concrete, loads due to workmen, construction equipment, other incidental loads and forces caused by placement and consolidation of concrete, imposed upon it during and after casting of concrete.

·      It should be rigidly constructed and efficiently propped and braced so as to retain its shape without undue deflection.

·      The joints in the formwork should be tight against leakage of cement grout.

·      The material of the formwork should be cheap, easily available and should be suitable for reuse several times.

·      The formwork should be as light as possible.

·      The formwork should rest on firm base.

·      The material of formwork should not wrap or get distorted when exposed to sun, rain or water during concreting.

ECONOMY IN FORMWORK

It may be noted that the total cost of concrete construction includes the cost of formwork as well. 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 concrete work. In case of special structures like bridges, all chimneys, dams etc. the cost of formwork may range between 50 to 100% of the cost of concrete work, or even more. Since formwork contributes nothing towards the stability of the finished structure, it cost should be kept barest minimum consistent with safety.

·      The building should be planned in such a way that there are minimum number of variations in the size of rooms, floor area etc. so as to permit re use of the formwork moulds repeatedly.

·      The scheme of the formwork should be efficiently planned and suitably designed to determine the most economical but safe sizes of different components including the supports or props.

·      The formwork should be constructed in such a way that timber is cut to the minimum and it can be struck off with ease and re used with least damage.

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 out of rough timber and is not constructed properly, the resultant concrete surface will be irregular and full of defects.

This to effect economy, the formwork should be made with good quality material and should be constructed properly, besides following the steps referred above.

MATERIALS FOR FORMWORK

Formwork can be made out of timber, plywood, steel, precast concrete or fibre glass, used separately or in combination. The type of material to be used for formwork depends upon the nature of construction as well as the availability and cost of material. In early stages, formwork was made up of timber alone. With the introduction of steel forms, use of timber former is getting reduced specially in case of major construction projects and in situations where large number of re uses of the same forms are possible. However, for small works involving less number of re uses, timber formwork proves economical. Fibre glass is used mainly for making moulds for repetitive castings of pre cast concrete products. Moulds made up of pre cast concrete, fibre glass and aluminium are used in cast in situ construction such as waffle slabs or members involving curved surfaces.

TIMBER FORMWORK

Timber used for the formwork should satisfy the following requirements:

·      It should be well seasoned.

·      It should be light in weight.

·      It should be easily workable with nails without splitting.

·      It should be free from knots.

The sizes of timber sections for different components of form work depend upon the span of the slab or beam, floor to floor height and the centre to centre spacing of the centering supports.

The size of timber planks or joists that can be adopted for different components of formwork for shuttering of 4.5 m span and 3.5 m height.

For normal construction work where repetitive use of shuttering is possible, the quantity of timber shuttering can be worked out on the assumption that one set of shuttering can be used 10 to 12 times.

The timber used for shuttering for exposed concrete work should have smooth and even surface on all faces which are to come in contact with concrete in situations, where concrete surfaces are not exposed, as in case of foundation etc. undressed timber can be used to effect economy. The timber planks for the shuttering should be joined by providing tongued and grooved joints so as to ensure adequate tightness against leakage of cement grout.

PLYWOOD FORMWORK

Use of plywood instead of timber planks is getting popular these days. In this case resin bonded plywood sheets are attached to timber frames to make up panels of required sizes. The panels thus formed can be easily assembled by blotting in the form of shuttering. This type of shuttering ensures quality surface finish and is specially recommended in works where large exposed areas of the concrete are to be constructed such as floor slabs, faces of retaining walls etc.

·      It is possible to have perfectly plain and smooth surface by use of plywood shuttering.

·      By use of large size panels it is possible to effect saving in the labour cost of fixing and dismantling.

·      Number of re uses are more as compared with timber shuttering. For estimation purposes, number of re uses can be assumed as 20 to 25.

STEEL FORMWORK

This consists of panels fabricated out of thin steel plates stiffened along the edges by small steel angles. The panel units can be held together by two or more clamps or bolts provided along each edge and the shuttering can be assembled and kept in alignment by use of horizontal or vertical centering of timber or steel. The panels can be fabricated in large number in any desired modular shape or size. The usual size for wall or slab panel varies from 60 cm x 60 cm x 120 cm.

Steel forms are mostly used in large construction projects or in situations where large number of re uses of the same shuttering are possible. Steel forms are extensively used in repetitive castings of pre cast concrete products. This type of shuttering is considered most suitable for circular or curved shaped structures such as tanks, columns, chimneys etc. and for structures like large sewer, tunnels and retaining walls.

Although steel shuttering costs more initially but in view of its various advantages it may work out to be economical for a medium sized work in the long run.

Advantages of steel formwork over timber forms:

·      Steel forms are stronger, more durable and have longer life s compared with timber forms.

·      They can be put to sufficiently larger number of re uses. For estimation purposes the number of re uses can be assumed to vary from 100 to 120.

·      Steel forms can be installed and dismantled with greater ease and speed which results in saving in labour cost for this item of work.

·      There is no danger of the formwork absorbing water from the concrete and hence the chances of honey combing are minimized.

·      They are not to shrink or distort and hence it is possible to achieve better workmanship and higher degree of accuracy by use of steel forms.

·      The quality of exposed concrete surface obtained by use of steel forms is excellent and it needs no further treatment. On the other hand, construction carried out by use of timber formwork invariably requires plastering to obtain desired finish of the concrete surface. Thus there is saving in the cost of finishing the surface by use of steel forms.

CONSTRUCTION OF FORMWORK

The construction of formwork normally involves the following operations:

1.Propping and Centering:

The props used for centering may be of steel, timber posts or bellies. Pillars made up of brick masonry in mud mortar are also sometimes used as props. In case wooden ballies are used as props, they should rest squarely on wooden sole plates laid either on ground or on brick masonry pillars in mud mortar. The wooden plate should have area at least 0.1 sq.m. and it should be 40 mm in thickness. Double wedges are essentially provided between the sole plates and the timber props with a view to permit accurate adjustment of the shuttering prior to concreting operation and to allow easy removal of shuttering afterwards. In case brick masonry pillars are used at prop, the wooden sole plates are provided at the top of the pillars, and the double wedges are inserted the sole plates and the bottom of shuttering.

2. Shuttering:

The shuttering can be made up of timber planks, or it may be in the form of panel units made either by fixing plywood to timber frames or by welding steel plates to angle framing. In any case, the shuttering should be constructed in such a manner that the joints should be tight against leakage of cement grouts.

Sometimes, the Architect specifies provision of moulding in slab or columns or provision of chamfers at corners of beams and columns. All such mouldings, bevelled edges, chamfers etc., required to be formed in the concrete work should be made in the shuttering itself. Similarly, the opening for fan clamps or other fittings should be provided in the shuttering as per details in the drawing.

3. Provision of camber:

Certain amount of deflection in structure in unavoidable. It is therefore desirable to give an upward camber in the horizontal members of the concrete structure to counteract the effect of deflection. The provision of desired camber should be made in the formwork itself during its erection. Camber for slabs and beams may be adopted @4 mm per metre of span. In case of cantilevers, the camber at free end should be taken @ 1/50^th of the projected length of the cantilever.

4. Surface treatment:

Before laying concrete the formwork should be cleaned of all rubbish particularly the sawdust shavings and chippings etc. All surfaces of timber shuttering that are to come to contact with concrete should be well wetted with water. This is necessary to prevent the chances of dry shuttering timber absorbing water from the concrete which may cause warping, swelling and distortion of timber besides resulting in defect of honeycombing in concrete.

Similarly steel forms that have been exposed to hot weather should be cooled by watering before laying concrete.

In addition, all surfaces of shuttering which are to come in contact with concrete should be given a good coating of raw linseed oil or soft soap solution or any other material so as t prevent the concrete getting struck to the formwork and thus facilitate easy removal. It is, however, important to note that the coating should not come in contact with reinforcement. The selected coating should be impermeable and should neither get flaky on exposure to weather nor stain the surface of concrete.

ORDER AND METHOD OF REMOVING FORMWORK

The formwork should be planned and constructed in such a manner that it is possible to remove the different components in the following order of sequence:

·      Shuttering forming vertical faces of walls, beams and columns sides should be removed first.

·      Shuttering forming soffit to slabs be removed next.

·      Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.

Duration of time up to which the formwork should be kept in place depends upon many factors such as type of cement used, shape and position of the member loads to be carried by the members and the temperature of the air. In general, rapid hardening cement, warm weather and light loading conditions allow early removal or formwork.

Early release of formwork permits its re use in other areas and this results in economy by way of optimum utilisation of material and fast construction. At the same time premature removal of formwork may prove to be dangerous and should not be allowed.

The formwork should, under no circumstances, be allowed to be removed until concrete reaches a strength of at least twice the stress to which the concrete may be subjected to at the removal of formwork.

In normal conditions the formwork may be struck or removed after the expiry of the period indicated against respective member.

All formwork should be eased gradually and carefully in order to prevent the load being suddenly transferred to concrete. The forms can be easily stripped by loosening the double wedges. Sometimes, the swelling of timber may result in shuttering getting jammed at places. In such situation a mall crowbar can be used to strike off the shuttering. In case of cantilever slabs and beams, the centering should be removed only after necessary construction to provide proper counterweight has been carried out and the same has attained sufficient strength.