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The formwork sheeting (also called formwork shell)
The formwork sheeting is in direct contact with the concrete and can be considered as mirror image of the concrete components to be produced.
It is the moulding element of the formwork and has the greatest influence on the quality of the concrete surface.
The formwork sheeting has to resist heavy stress. It takes up surface pressure and carries it off to lineary acting supports.
Figure 3 Formwork pressure
distribution
1 formwork sheeting, 2 formwork bearer, 3 main bearer, 4 columns, 5 formwork pressure (surface pressure), 6 carrying off to the columns (linear)
It is typical of the formwork sheeting that, because of the heavy stress and direct contact with the concrete, it is the part of the formwork to wear first.
Sheeting boards and prefabricated wooden panels can be used as sheeting material.
Formwork sheeting of individual boards normally consists of 25 mm thick and approximately 140 mm wide boards. The heartwood side must always lie towards the concrete. Sheeting boards have great advantages because of their easy workability, such as by sawing, planing, boring and nailing.
Disadvantages are:
- the high expenditure of working time required for formwork making,
- the short service life,
- loss by waste wood.
In the event of repeated use of formwork of the same design, such as for foundation strips, sheeting panels can be prefabricated from individual boards using squared timbers as frame.
Figure 4 Sheeting panel for
repeated use
1 sheeting panel frame, 2 holes for steel wedges
The sheeting panels are prefabricated in a workshop. The advantage of sheeting panels is that they considerably reduce the time required for the erection of the formwork on site.
The disadvantages of sheeting panels in terms of service life are similar to those of sheeting boards.
Normally both types of formwork sheeting are used on site in a combined manner.
What are the common types of formwork sheeting?
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The formwork bearers
The formwork bearers directly support formwork sheeting. They carry off the forces through falsework structures to supporting members, the soil and formwork ties. The formwork bearers normally consist of squared timbers. The cross sections of the beams depend on the loads to be resisted. Because of their good workability, wooden beams have a wide field of application. In addition to wooden beams, prefabricated steel parts (steel beams, clamps etc.) are also used. Clamps are mainly used for column formwork.
Figure 5 Steel clamp comer
point for column formwork
1 steel wedge, 2 clamp, 3 holes for steel wedges
The formwork ties
With vertical formwork, the horizontally acting formwork pressure is mostly taken up by tieing the two form-work faces to each other by formwork ties. It is done by slinging a steel wire (3.1 mm to 4.2 mm, annealed) around the formwork bearers, guiding it through boreholes in the formwork sheeting and tightening it by twisting. Immediately near the tie wire a stull (of wood or concrete) is to be mounted to maintain the necessary width of formwork.
When placing the concrete, the stull (if made of wood) is to be remove because the concrete mix assumes the bracing function (pressure) (See Fig. 1 and Fig. 2).
Another way of bracing is screwing by means of steel screws.
The elements of falsework structures
Falsework structures are those parts of the formwork which take up and carry off the load from the formwork sheeting and formwork bearers.
The main elements of such formwork structures are columns and main bearers.
The column takes up the load directly from the formwork bearer and carries it off.
Round timber braces and specially made metal tube structures adjustable in height may be used as columns.
In the case of round timber columns, double wedges are to be placed at the foot to prevent lowering of the formwork when stripping.
Figure 6 Column foot with
wedge support
1 column, 2 wedge support
Other falsework elements are the main bearers which are horizontally arranged. One main bearer takes up the load from several formwork bearers and carries it off to the columns. Main bearers can be made of amply dimensioned squared timbers. But they may as well consist of metal tube structures of lattice type.
Lattice-type metal tube structures are indispensable for big widths where columns cannot be laced at close intervals.
Further falsework elements are bracing members which are required for both vertical and horizontal form-work. They ensure that the admissible unsupported length of columns is kept and secure the formwork against displacement and canting.
Their arrangement and fixing according to the project is decisive for the stability of the formwork.
Bracing members may be:
- boards, squared timbers and round timbers,
- metal tube structures, steel ropes and steel sections.
How is dimensional inaccuracy by compression of the formwork prevented by means of tie wires?
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What is a falsework structure?
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