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Why is formwork and falsework needed?
What is formwork?
Formwork is used to hold and support floors of a building by containing freshly placed concrete by compacting it until it has gained enough strength to be self-supporting. It is also used to produce a concrete element of the required size, shape and surface texture.
Formwork is made up of shutters consisting of individual pieces of timber, plywood or steel that are joined together and supported. The concept of formwork is similar to that of a jelly mould as the shutters are used as the mould.
Once the concrete has hardened, the formwork is removed and the concrete column duplicates the same shape as the same shutter.
What is falsework?
Falsework is any temporary structure used to support a permanent structure while it is not self-supporting. This may include temporary support structures for formwork. Falsework will need to be strong enough to carry its own weight; properly braced to prevent side sway and buckling; supported on a firm surface and easy and safe to erect and dismantle.
General requirements of formwork
- Forms should be strong enough to withstand the pressure resulting from placement and compaction of the concrete and have enough rigidity to maintain the specified tolerances and the required shapes, finishes, position, levels and dimensions.
- Forms should be set up with the joints tight enough to prevent leakage of the cement paste.
- Form surfaces that are in contact with freshly placed concrete should be treated with a coat of non-staining mineral oil or any other suitable release agent. In the case of timber forms, thorough wetting of the surfaces with water will ensure that the concrete does not stick to the formwork.
- Formwork should be capable of being taken apart and removed from the cast concrete without shock, disturbance or damage to the concrete.
- Exposed concrete surfaces must have the required degree of accuracy after the formwork is removed and any surface defects have been repaired.
- Any rubble and dust that could contaminate the concrete should be removed from the inside of the forms before the concrete is placed.
- All formwork should be reconditioned and cleared before using again.
- The minimum time should elapse before the formwork is removed.
How to ensure accurate and safe formwork
- The formwork must not bend or move under any planned load including the loading of the wet concrete, the actual weight of the wet concrete when it is inside the formwork and other loads such as construction workers and wheelbarrows.
- The formwork should be set out accurately according to the specifications of the engineer who designed it.
- The formwork joints between the faceboards must be properly sealed to prevent unnecessary loss of concrete or unsightly ridges and honeycombing. The joints should be sealed with flexible polyurethane strips or special adhesive tape.
- In order to manage the formwork easily you must be able to set it up, get to, fix and remove it as well.
Typical defects caused by inaccurate formwork
The finished concrete may have an uneven colour when the form faces did not absorb water from the concrete at an even rate. This could be caused by using new form faces together with old ones to make your formwork.
Blowholes could form, which are small holes less than 15mm in diameter caused by air trapped between the formwork and the concrete face. Using a poker vibrator or another suitable type of compaction will help to remove the trapped air. You may also brush or spray release agent onto the inside of the form faces. This is an oil or liquid used in moulding and casting that helps in the separation of a mould from the material being moulded and reduces defects in the moulded surface.