What is Formwork??
- Formwork is construction is the use of support structures and moulds to create structures out of concrete which is poured into the moulds. Formwork can be made using moulds out of steel, wood, aluminium and/or prefabricated forms.
- Formwork is a mould or open box, like container into which fresh concrete is poured and compacted.
- When concrete is fresh and in its liquid state it must be restrained within a mould in order for it to set in its required shape. Formwork is the term used to describe this mould.
- Formwork is an ancillary construction, used as a mould for a structure. Into this mould, fresh concrete is placed only to harden subsequently.
- When the concrete is set, the formwork is removed and a solid mass is produced in the shape of the inner face of the formwork.
- The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable forms are known as panel forms and non-usable are called stationary forms.
- The top of the formwork is normally left open.
Uses of Formwork
- Formwork mold the concrete to the desired shape and size and controls its position and alignment.
- It is used to transfer external temporary loads.
What is Good Formwork??
- There are two factors that decide how ell the concrete will look in the final result, one is how the concrete is placed and compacted, and the other is the quality of the formwork.
- Formwork is made from expensive materials, and requires great skill and experience in its manufactures. Its importance is evident when you consider that the cost of fabrication, erecting and striking that the formwork, often exceeds the cost of the concrete it is designed to shape and support.
- Formwork usually needs to be used many times for it t be cost efficient. This can only be done if it is carefully handled, cleaned and stored, regardless of what material it is made from.
- For anything other than most smallest of concreting jobs e.g. walls over 1m in height, a drawing should be provided, showing what formwork is required. This need not be an intricate design drawing, a simple sketch is usually sufficient.
Requirements of Good Formwork
- Material should be cheap and re usable.
- It should be practically water proof, so that it should not absorb water from concrete.
- The joints in the formwork should be tight against leakage of cement grout.
- Swelling and shrinkage should be minimum.
- Strong enough to with stand all external loads.
- Deflection should be minimum.
- Material of the formwork should not warp or get distorted when exposed to the elements.
- Surface should be smooth, and afford easy striping.
- Light in weight, so that easy to transfer.
- Joints should be stiff, so that lateral deformation and leak is minimum.
Major objectives considered in formwork
- Quality: Forms must be designed and built with sufficient stiffness and accuracy so that the size, shape, position, and finish of the cast concrete are maintained.
- Safety: forms must be built sufficient strength and factor of safety so that they have the capable of all supporting loads.
- Economy: Forms must be built efficiently, minimizing time and cost.
- Most common material used for bracing the member, hence called as the traditional formwork.
- Can easily be cut to size on site. Joist are replaced with engineered wood beams and supports are replaced with metal props. This makes this method more systematic and reusable.
- This is by far the most common material used for the facing panel. It is easily cut to shape on site and it handled an stored carefully, it can be used many times.
- A standard plywood thickness on site is 18mm. this is usually sufficient for most pours.
- However, if the formwork is curved, a thinner plywood is used to facilitate bending.
- Steel forms are stronger, durable and have longer life than timber formwork and their reuses are more in number.
- Steel forms can be installed and dismantled with greater ease and speed.
- The quality of exposed concrete surface by using steel forms is good and such surfaces need no future treatment.
- Often used in pre-fabricated formwork, that is put together on site.
- Aluminium is strong and light, and consequently fewer supports and ties are required.
- The lighter sections will deflect more. But this can be avoided by simply following the manufactures recommendations.
- Glass reinforced plastics and vacuum formed plastics are used when complicated concrete shapes are required.
- Although vacuum formed plastics will always need support, GRP can be fabricated with integral bearers making itself supporting.
Causes of Formwork Failure
- Improper stripping and shore removal.
- Inadequate bracing.
- Unstable soil under mudsills.
- Inadequate control of concrete placement.
- Lack of attention of formwork details.
Advantages of Pre-Fabricated Formwork
- Very little on-site skilled labor needed.
- The ability to reuse forms either as a large section or as individual units.
- All prefabrications systems are designed for light as well as heavy construction.
- Prefabricated forms may be set in any combination, horizontally and vertically, to any wall height.
Safety Precautions in Formwork
- Material used for the construction of formwork must fulfill the specification.
- Formwork is fixed firmly & properly.
- Construction area must be protected to prevent vandalism of formwork.
- Warning sign must be put up at the area where the formwork is fixed to prevent entrance of people that may damage the formwork.
- The formwork must be inspected the concrete is poured.