Formwork in construction
Fresh concrete is a freshly mixed material which can be moulded into any desired shape. As it is in the plastic stage it can not even sustain its own load until it becomes hard. Hence to mould the fresh concrete into the desired shape, we need fromwok. Formwork is a provisional structure designed for withstanding the loads coming from freshly poured concrete, live load, its own weight and environmental load. Actually, formwork is a reverse shape of a concrete member which is to be cast. Centering terms are used for circular work such as arch, dome.
Requirements of Formwork
Good formwork in construction should suitably have the following major requirements
1. Easy Removal:
Formwork can be easily removed by the least amount hammering. This can be achieved by applying crude oil or soft soap solution on the inner surface of formwork in construction.
2. Economical Formwork
- It is to be noted that the formwork is not a permanent structure. Hence it does not contribute to the stability of the finished structure. So it is desirable to bring down the cost to a minimum with safety. Reduction in cost can be achieved with the following steps
- Reduction in the number of irregular shapes, Standardizing room dimensions, Use of components parts of commercial sizes. Putting formwork in use again as early as possible, and use of that material which is easily available at low cost and which can be reused for several times
3. Less leakage through joints
4. Rigidity of formwork
so as to retain shape without any appreciable deformation. For visible surfaces in completed work the deflection is limited to L/1000.
5. Smooth surface to the concrete
This can be achieved by applying crude oil or soft soap solution on the inner surface of formwork in construction.
6. Strength of formwork
- Loads on vertical forms are to be assessed from various considerations such as the weight of concrete, the dimension of the section, Reinforcement details, stiffness of forms, Rate of pouring of concrete, vibration and impact of machine delivered concrete, lateral pressure of fresh concrete, unsymmetrical placement of concrete, concentrated load and storage of construction materials etc
- Careful design is required as overestimation of loads result into expensive formwork and underestimation of loads result into failure of formwork
7. Cost of Formwork in construction
- For placing of 1 cum of concrete in a beam of size ( 230 mm X 300 mm) we require 12 sqm (approx) of formwork. The cost of concrete is Rs. 6000 per cum (approx) and the cost of formwork is Rs. 200 per sqm. Hence the cost of formwork varies 40 to 50 percent to the cost of concrete for simple structure. And for special structures like cooling tower, overhead water storage tank, it varies 50 to 60 percent of the cost of concrete.
- Hence it is desirable to reduce the cost of formwork so that we can achieve an overall economy in concreting.
Tolerance in dimension
- Deviation from specified dimension of cross section of column and beams. ( +12 mm to – 6 mm ).
- Deviation from specified dimension of cross section of footing ( + 50 mm to -12 mm )
- Deviation in thickness (± 0.05 times the specified thickness )
- All rubbish, particularly, chippings, shavings, and sawdust shall be removed from the interior of the formwork before placing the concrete.
- The face of formwork in contact with the concrete shall be cleaned and treated with a form release agent.
- The formwork shall be designed and constructed so as to remain sufficiently rigid during placing and compaction of concrete.
- it shall be such as to prevent loss of slurry from the concrete
- Camber shall be provided as per drawing.
Stripping Time of formwork
- Shuttering can be released after the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of formwork.
- when the ambient temperature does not fall below 15 degree C, OPC cement is used and proper curing is done . in such cases stripping time is as below
Types of formworks
- Timber Formwork
- Steel Formwork
- Slip Formwork
- Permanent Formwork
- Special Formwork
- Table Form
- Ganged Panel Form
- Tunnel Form
- Doka Formwork System
We are mainly going to discuss timber and steel formwork in construction
Timber Formwork in construction
- Timber formwork is suitable for small works. It is cheap when compared with steel formwork.
- It should be well seasoned, a timber shall be neither too dry nor too wet. if timber will be too dry, it will absorb water from fresh concrete and if it is too wet, in such scenario timber will shrink and will increase the joint width. the typical moisture content of timber shall be in the range of 15 to 20 percent.
- Nails to be used for joining the formwork should be minimum. Nailing should be done in such a way that nails can be easily removed during the dismantling of formwork.
- The bearing soil should be sound and suitably prepared. The sole plates should fully bear on the ground. without a possible settlement.
- It is a usual practice that timber ballies which support the slab are cut to approximate size and placed vertically and finally adjusted with wedges below to it. It is a point where there might be horizontal displacement due to horizontal forces. as timber ballies are not straight hence they do not transmit the load axially.
Steel formwork in construction
- It is fabricated from Sheets, angle iron and Tee iron
- Such type of formwork is desired in case of reuse of formwork several times. It can be used 10 to 15 times higher than timber formwork.
- As in the case of timber formwork, there are chances of water absorption from fresh concrete and widening of joints but in case of steel formwork there is no such type of issue.
- The initial investment is high, But after its uses, it is cheaper than timber formwork.
- It gives a smooth hardened concrete surface. which does not require further finishing treatment.
- Such formwork can be easily erected or dismantled.
- Please refer IS 14687: 1999 for further details