What is Formwork (Centering & Shuttering) in construction?
Formwork is a process of creating all temporary or permanent forms or moulds required for forming the concrete which is cast-in-situ, together with all temporary construction required for their support.
It is ensured to be strong enough to withstand the dead and live loads and forces caused by ramming and vibrations of concrete and other incidental loads, imposed upon it during and after casting of concrete.
It is made sufficiently rigid by using an adequate number of ties and braces; screw jacks or hardboard wedges where required are provided to make up any settlement in the formwork either before or during the placing of concrete.
Forms are so constructed as to be removable in sections in the desired sequence, without damaging the surface of concrete or disturbing other sections.
Design & Tolerance In Construction
Formwork is designed and constructed to the shapes, lines and dimensions with the tolerances apply to concrete dimensions only, and not to the positioning of vertical steel or dowels. Design is based on structure.
Material for formwork
All propping and centring should be either of steel tubes with extension pieces or built-up sections of rolled steel.
Staging should be as designed with required extension pieces as required to ensure proper slopes, as per design for slabs/beams etc. All the staging to be either to Tubular steel structure with adequate bracings as required or made of built-up structural sections made from rolled structural steel sections.
Shuttering used is ensured to be of sufficient stiffness to avoid excessive deflection and joints are tightly butted to avoid leakage of slurry. Steel shuttering used for concreting is sufficiently stiffened.
The steel shuttering should be stiff, free from bends, dents etc., and properly cleaned to avoid stains, honeycombing, seepage of slurry through joints etc.
Rolled Steel Joist, Mild Steel Channel or any other suitable section of the required size are used as runners.
Beam head is an adopter that fits snugly on the head plates of props to provide wider support under beam bottoms.
Procedure of Formwork
- After completing the column and footing excavation, the size of the pit is checked before placing PCC.
- PCC is done.
- The level of PCC is checked; the centre of the footing is marked in a cement mortar on PCC with the help of the Side rail or Center Line Pillar.
- Footing Form Work is applied with one coat of mould oil and placed in position size and diagonals of the form workbox are checked.
- Check is made for gaps if any between formwork and PCC top level.
- Before concreting, the space inside formwork and PCC is cleaned with a water jet’s help.
- Proper supports to formwork are provided so that its position does not shift during subsequent work.
- Centres of the footing are marked with the help of a nail, on footing box Centre marked on the footing box should match the centre marked on the side rail or centerline pillar.
- Reinforcement of Footing .column and concreting depth of footing are checked before placing concrete.
- Materials should be waterproof.
- There should not be any curvature in the plywood used for formwork.
- Supports to plywood should be sufficient in both vertical and horizontal directions.
- Proper care should be taken before placing column formwork and it is coated with mould oil.
- Dimensions and diagonals of the formwork box of the column are checked.
- MS Clamps / Runners are fixed to prevent bulging of the column during concreting.
- After marking the centerline of the column, the minimum cover and the column reinforcements are checked.
- Bracing supports with adjacent side columns are provided to avoid rotary movement of the column.
- Any gap at the bottom is filled with putty or any other suitable filling material.
- The vertically of the shuttering is checked before and after concreting.
- Centring plates should be of uniform thickness.
- The centring plate should be at a perfect right angle.
- The level of the slab is marked with a dumpy level or tube level.
- Formwork for the slab is done after floor PCC is completed and fully cured.
- Centring plates are coated with mould oil before reinforcement is placed.
- Side edge plank is adjusted along with the beam sides as per the dimensions of each bay & the available size of the plate to avoid voids in between.
- Bracing are provided for each prop to avoid lateral movement.
- AH MS sheets are properly adjusted. It is ensured no portion of the plate enters the beam side width.
- Line, plumb and supports of beam sides are checked.
- Gaps between plates are filled with putty or tape.
- Prop of slab centring sheets is ensured to be vertical at the required spacing.
- The form faces have to be kept at a fixed distance apart and an arrangement of wall ties with spacer tubes or bolts are done.
- The two shutters of the wall are to be kept in place by appropriate ties, braces and studs.
- Surface treatment – Oiling the Surface
- Shuttering gives a much longer service life if the surfaces are coated with suitable mould oil which acts both as a parting agent and also gives surface protection.
- A typical mould oil is heavy mineral oil or purified cylinder oil containing not less than 5% Penta chlorophenol conforming to IS 716 well mixed to a viscosity of 70-80 centipoise.
- After 3-4 uses and also in cases when shuttering has been stored for a long time, it should be recoated with mould oil before the next use.
Inspection of Formwork
The completed formwork is inspected for conformance as per drawing and approved before the reinforcement bars are placed in position and recorded in the pour card. Proper formwork should be ensured for concreting to avoid honeycombing, blowholes, grout loss, stains or discolouration of concrete etc.
Proper and accurate alignment and profile of finished concrete surface are ensured by proper designing and erection of formwork. Shuttering surface before concreting is ensured to be free from any defect/deposits and fully cleaned to give a perfectly straight smooth concrete surface.
Shuttering surface is therefore checked for any damage to its surface and excessive roughness before use. Form Work is erected true to line and the shapes and dimensions required for the work.
It is designed to carry without deformation the full weight of wet concrete and other incidental live loads. It should also withstand the effect of vibration, without deflection, bulging, distortion or loosening of its parts. The site Engineer is responsible for the sufficiency of all formwork, centring and moulds.
All joints are tongued and grooved and sufficiently tight to prevent leakage of cement paste. All formwork is removed without shock or vibration such as would damage the concrete.
Before the soffit forms and props are removed Site Engineer ensures that the concrete has hardened sufficiently and is removed with the approval of the Project Head/Project Engineer.
Concrete surfaces that are to be finished with cement rendering are thoroughly hacked with hand tools immediately after removal of formwork to bring the adequate bond between the concrete and cement rendering.
Removal of Formwork
In normal circumstances and where ordinary Portland cement is used, forms may generally be removed after the expiry of the periods as specified in IS 456:2000 or as per requirements.
- While de- shuttering formwork of columns, it is ensured that the edges of columns are protected.
- While de-shuttering the external sides of the floor beams, it is ensured that nobody is working nearby area, so as to prevent possible accidents. and
- During de- shuttering the internal beam sides, it is ensured that supports of slab and beam bottom are not disturbed.
De-shuttering of slab plates is done after making scaffolding. No plates should be removed from the slab without proper scaffolding, as otherwise plates would be thrown down to ground level. It is ensured all shuttering material like plates, props,
spans, plywood shutters are brought down using rope and pulley so that materials are not damaged.
Types of Formwork
There are 5 types of formwork
- Foundation formwork
- Column formwork
- wall formwork
- Beam formwork and
- Ceiling formwork
Causes of Failure
Formwork is responsible for a large number of construction-related accidents. The fundamental reasons are as follows
- Wet concrete-carrying structure that is unstable, or severe weakness and overstress
- Structure with no diagonal bracing that is subjected to significant lateral loads
- Human error, negligence, and overconfidence are all examples
- Premature form erosion
- Props of insufficient size and spacing
- Due to the heaping of concrete, the load is concentrated.
- Cross bracing and horizontal bracing are insufficient.
- Poor soil SBC causes sinking of supports/props.
- Failure to keep the concrete casting process moving at the right pace and in the right order.
- Placing rolled steel members improperly
- The impact of girder supports being pre-stressed
- Existing structure stabilization
Uses of Formwork
- Provides safety to workers during construction.
- Supports the structure while concreting to the shape required.
- good formwork minimizes the time of construction and provides quality surface finishing.
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