Formwork is a shape or mould comprising all supporting arrangements, used to outline and backing the concrete until it attains adequate strength to carry its self-weight. It shall accomplished to carrying all compulsory dead load and live load including its self – weight.
1. Requirements of good formwork
- PCC floor : Always the formwork should rest on solid, hard and firm base. E.g. on PCC floor over well compacted soil.
- It should firmly erected, well propped, supported and to retain its structure with proper horizontal and vertical bracers.
- As already told, it should accomplished to carrying all compulsory dead load and live load.
- Water tight : All joints of shuttering work should be water tight against possible leakage of concrete slurry.
- It is always recommended that, the cost of formwork not only to minimum and at the same time to maintain the consistent safety.
- Materials used : The material used for the formwork should be economical, easily available and to produce maximum repetitions after its reuse.
- Design and making of formwork should permit stripping of all components in preferred sequences without damage to the concrete surface.
- The quality and method of formwork is designed in such a way that, it should satisfy the required dimensions, shape, level and finish of the concrete obtained.
- The internal surface of this work, should always maintained with smooth surface by applying branded shuttering oil, so that the stripping of formwork will be easy and also good concrete surface finishing can be expected.
- Shuttering work should be as light as possible and the material used should not warp or get distorted when exposed to the atmospheric elements.
2. Cost of formwork
- The cost is approximately between 30 – 40% of the cost of concrete work in case of regular structural work
- In case of Special infrastructures like Flyovers, underpass, bridges, tall chimneys, dams etc., the cost will go up between 50 – 65 %.
- The building should be scheduled and designed such that there are minimum number of different size rooms, floor area etc. so as to obtain maximum repetitions and reuse of formwork boards.
- Shuttering work should made of cost effective materials, fabrication and labor as far as possible.
- After its usage, immediately shuttering moulds to be carefully released from concrete surfaces and maintenance to be done by cleaning the cement slurry and oiling all surfaces as per specification.
3. Causes of formwork failure :
Formwork failure is happening in the following cases,
- Formwork is not designed for specific purpose, load calculations not dealt accurately.
- Not designed to take horizontal thrust into consideration.
- Adopting improper method of stripping, early stripping and negligence in shore removal. E.g., by removing the free end of cantilever portion first, instead of fixed end of cantilever portion first.
- Inadequate diagonal bracing, and improper packing for vertical props at bottom instead of base plates.
- Due to unable to withstand Vibration forces
- Unstable soil under props and base plates, not maintaining plumb of shoring and giving slant supports.
- Inadequate control of concrete placement over slabs, beams, cantilever (by stacking excess concrete load)
- Lack of attention to formwork details, altering vendor supplied materials.
- Usage of damaged elements likes loosen weld joints for acrospans, broken u clamps, slippery wedges etc.
- Improper vibration and compaction of concrete
4. Safety precaution :
- All Materials used for the erection of shuttering work must satisfy the safety standard and technical specification.
- Always see that, the shuttering work is fixed firmly & properly over a rigid base
- Always note that working area and shuttering work completed area must be protected by means of regular hard barricade,bracing, and additional supports to prevent destruction of shuttering work.
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Building, acrospans, base plates, concrete, concrete slurry, cost of formwork, dead load, Live load, PCC floor, Shuttering work, vertical props, requirements of good formwork, design of formwork, formwork details, causes of formwork failure.