The formwork or shuttering is a temporary construction used as a mould for the structure, in which concrete is placed and in which it hardens.
The cost of formwork may be up to 20 to 25% of the cost of the structure in building work, and even higher in bridges.
To overcome that expenditure, it is necessary to design economical sorts of formwork including mechanizing its construction.
If the concrete has reached a certain required strength, every form is no longer wanted also is removed.
The method of removing the formwork remains commonly called stripping.
When stripping takes spot, the components of formwork are removed including then reused for the forms of a different part of the structure.
Information is classified as wooden, plywood, steel, combined wood steel, reinforced concrete, including plain concrete.
Timber is the most common material used for formwork. The problem with wooden formwork is every possibility of warping, swelling, also shrinkage of the timber.
A steel shuttering is used for important work where everything does mechanized.
Steel formwork has many advantages, such as follows
It can be put to a high number of uses,
It provides ease of stripping,
That ensures an even and smooth concrete surface,
That possesses greater rigidity,
It is not liable to shrinkage or distortion. However, steel formwork is comparatively more costly.
Requirements of Form Work
Good formwork should satisfy the following requirements
The material of some formwork should be cheap also it should be suitable for re-use several times.
This should do practically waterproof so that it does not absorb the water of concrete. Also, its shrinkage and swelling should be minimal.
It should be strong enough to withstand all loads coming on it, such as a dead load of concrete and a live load during its pouring, compaction, and curing.
That should be strong enough so this deflection is minimum.
It should be as to be as light as possible.
Each surface of some formwork should be smooth, also it should afford easy stripping.
Total joints of the formwork should be strong so that lateral deformation below loads is the smallest. Also, these joints should be leakproof.
Loads on Form Work
The formwork has to bear mainly the following loads apart from its weight:
Live load due to labour etc.
The deadweight of wet concrete,
The hydrostatic pressure of the fluid concrete acting against the vertical or inclined faces of form, and
Impact due to pouring concrete.
The temporary live loads of workmen and equipment, including the impact, may be taken equal to 3700 N/m2 for the design of planks and joints in bending and shear.
The hydrostatic pressure due to the fluidity of concrete in the initial stages of pouring depends upon several factors such as the quantity of water in concrete, size of aggregates, rate of pouring and temperature.
Shuttering for Columns is Form Work
Shuttering during a column is presumably the simplest.
It consists of the following main component:
Sheeting all-round the column periphery.
Side yokes and end yokes
Bolt with washer
The side yokes and end yokes consist of two numbers each and are suitably spaced along with the height of the column.
The two-side yokes are comparatively of a heavier section and are connected by two long bolts of 16 mm dia.
4 wedges, one at each corner, are inserted between each bolt also the edge yokes.
Shuttering for Beam and Slab Floor is Form Work
The slab is continuous over some beams.
The shuttering is supported on wooden battens which are laid between the beams, at some suitable spacing.
To reduce deflection, the battens may be propped in the middle of the span through joists.
Single side forms of each beam consist of 3 cm thick shuttering.
The bottom shuttering of the beam form maybe 5 to 7 cm thick.
The effects of the battens remain supported on the ledger which is fixed to specific cleats throughout the length.
Cleats 10 cm x 2 cm to 3 cm are fixed to the side forms at the same spacing as that of battens, so that battens may be fixed to them.
Form Work for Walls
The boarding maybe 4 to 5 cm thick for walls up to 3 to 4 m high.
The boards are fixed to 5 x 10 cm posts, identified while studs are spaced approximately 0.8 apart.
Horizontal walling of size 7.5 cm x 10 cm is fixed to the posts at a suitable interval.
The whole assembly is then strutted as shown, using 7.5 cm 10 cm struts. Some two shutters are kept alone equal over the thickness of the wall.
Each 15 mm plywood is commonly applied instead of boarding. The panels are erected in such a way that the lower panels can be removed when concrete is hard and used higher up the wall.
Framing of area 5 cm x 10 cm is applied to the ply shutter. Some panels are fixed to a central including two end studs. The specific post consists of two pieces of timbers, 5 x 15 cm, blocked.
Formwork for stairs
Deck slabs are carried on cross-joists which are in turn supported on raking ledgers.
The ledger is generally 7.5 cm x 10 cm in size. The cross-joists may be of 5 cm x 10 cm size, suitably spaced.
The riser’s planks are 4 to 5 cm thick and equal to the height of the riser.
These planks are bevelled at the bottom to permit the whole of the tread face to be travelled.
The riser planks are placed only after the reinforcement has been fixed in position.
The outer ends of the risers are carried by a cut string made of a 5 cm thick plank.
The wall ends of the riser planks are carried by 5 cm x 10 cm hangers secured to a 5 cm thick board fixed to or strutted against the wall.
Some treads are left apart to permit concreting including thorough vibration.
A stiffener joist of size 5 cm x 10 cm is placed along the middle of the riser planks. The stiffener is wired to cross-joist through the decking.