|Formwork Making - Course: Timberwork techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 14 p.)|
The formwork is the main means of work in the moulding process of the concrete. Basically the process of formwork making has to meet the following requirements:
- The structural component to be produced is to be moulded with the projected dimensions keeping the admissible tolerances.
- The dead loads of the fresh concrete and of the reinforcement as well as the temporary load of persons and working tools must be safely resisted and carried off to the soil or supporting members of the structure.
- The concrete must be protected against too high or too low temperatures as well as vibrations.
- The thin concrete mix must not flow out of the formwork.
- The future concrete component must have a surface finish meeting the required quality after stripping.
- The placement of necessary steel reinforcements must be easily possible during the erection of the formwork.
- Stripping of the concrete components produced must be uncomplicated.
What are the basic principles of formwork making?
Loading of vertical formwork
Vertical formwork is used for strip foundations, concrete walls and columns.
Immediately after placement in the formwork until achievement of its inherent stability, the concrete mix, under the effect of its own load and of compaction by vibration, exerts lateral pressure on the formwork which is called lateral pressure of the concrete mix.
The lateral pressure of the concrete mix depends on the following factors:
- Composition and properties of the concrete mix (density, type of cement, quality of concrete),
- concrete placing technology (concreting speed, compaction, vibration depth, total height of the concrete mix)
- ambient conditions (temperature, air humidity).
Tie wires (tie rods) are used to take up the lateral pressure of the concrete mix. They are to be included in the formwork project.
The maximum lateral load with external vibration occurs at the foot of the formwork and with internal vibration above the foot.
In addition to the lateral pressure of the concrete mix, the concrete mix also produces buoyant forces which may cause lifting of the formwork. This can be the case particularly with foundation formwork. To avoid this, the formwork is to be anchored in the subsoil.
Another way is loading the formwork by means of concrete parts.
Figure 1 Formwork tieing in the subsoil by means of tie wires
1 stulls, 2 sheeting boards, 3 post, 4 upper water, 5 lower waler (additionally supported) 6 tie wire, 7 tieing in the concrete bottom (tie wire), 8 thrust-board, 9 concrete bottom
Figure 2 Formwork tieing by loads on the formwork
1 concrete sheeting, 2 formwork bearer, 3 main bearer, 4 columns, 5 formwork pressure (surface pressure), 6 carrying off to the columns (linear)
What does the terms "lateral pressure of the concrete mix" mean?
Loading of horizontal formwork
Horizontal formwork is used for ceilings and beams. Horizontal formwork is subjects to vertical loads which are to be carried off to solid subsoils through formwork bearers and main bearers as well as columns.
Vertical loads are produced by
- the concrete mix weight in the specified height,
- concrete cones on the concrete pouring spot,
- concrete pouring impact on the formwork,
- persons and working tools,
- dead load of the formwork.
In addition to vertical loads, there are also horizontal loads which are produced by:
- wind effects
- inclined position of columns,
- backing up, etc.
The horizontal forces are taken up by auxiliary structures, such as braces and struts, or rigid connection to existing structural components, such as walls and columns.
Formwork walls and columns are to be backed up from all sides!
Through which components are vertical loads carried off?