In Construction, the formwork has to bear, besides its own weight, the weight of wet concrete, the live load due to labour, and the impact due to pouring concrete and workmen on it The vibration caused due to vibrators used to compact the concrete should also be taken care off Thus, the design of the formwork is an essential part during the construction of the building. For the design of planks and joists in bending & shear, a live load including the impact may be taken as 370kg/m². It is however, usual to work with a small factor of safety in the design of formwork.
The surfaces of formwork should be dressed in such a manner that after deflection due to weight of concrete and reinforcement, the surface remains horizontal, or as desired by the designer. The sheathing with full live load of 370 kg/m² should not deflect more than 0.25 cm and the joists with 200kg/m² of live load should not deflect more than 0.25cm.
In the design of formwork for columns or walls, the hydrostatic pressure of the concrete should be taken into account This pressure depends upon the quantity of water in the concrete, rate of pouring and the temperature.
The hydrostatic pressure of the concrete increases with the following cases:-
- Increase in quantity of water in the mix.
- The smaller size of the aggregate.
- The lower temperature.
- The higher rate of pouring concrete.
If the concrete is poured in layers at an interval such that concrete has time to set, there will be very little chance of bulging.
Aluminium as usual is not a very strong material. So the basic elements of the formwork system are the panel which is a framework of extruded steel sections welded to an steel ribs It consists of high strength special aluminium components. This produces a light weight panel with an excellent stiffness-to-weight ratio, yielding minimal deflections when subjected to the load of weight concrete.
The panels are manufactured in standard sizes with non-standard elements produced to the required size and size to suit the project requirements.