Formworks are temporary structures (moulds) used at site usually to maintain the shape of the structural members like slab, beam, column and to help placing the concrete in desired form.
Formworks can be made of plywood, steel or timber based on the need and site conditions. It is usually removed after the primary hardening of concrete poured. But the removal of formworks should be done with extreme caution, not to damage the structural members.
Factors Affecting Removal Time of Formwork
The removal time of formworks is affected by various factors like
- Type of concrete used: There are many concrete types available and the based on the materials (e.g. cement, mineral admixtures) used their hardening time can vary, for example the formworks can be removed soon if rapid hardening cement is used in the concrete.
- Quality of materials used: If high quality materials are used especially if cement and mineral admixtures like silica fume are of good quality then the concrete attains early strength quickly, thus formworks can be removed soon.
- The temperature at site: This plays an important role influencing the setting time of concrete and also the strength development. The formwork removal time is more in cold weather and it is less in hot weather concreting.
- Curing: The type of curing and the rate at which it is done influences the development of strength in concrete, for instance if steam curing is used the strength development rate in concrete is rapid thus formwork removal time required will be less.
- Exposure condition: severe exposure conditions can result in slow development of strength in concrete thus delaying the formwork removal time.
In addition to the above factors there is another important criteria which is based on the stresses and load applied on the concrete while removal, thus the formwork removal can be done only after the concrete attains considerable strength approximately twice the load implied during removal.
The type of structural members is also taken into account during the formwork removal. For slabs and beams which carry its own self weight and also the live load in the building, more time is required to remove the formwork and also the formworks should be removed carefully so as to not cause any damage to the member.
Time for Removal of Formwork
In normal circumstances (generally where temperature is above 200C), and where ordinary cement is used, forms may be struck after expiry of following periods depending upon the type of structural member
As already mentioned you can see the formwork removal time for beams and slabs is more comparatively. The above mentioned time varies based on the type of cement used.
|S.No||Structural Member||Time of Removal|
|1||Walls, columns and vertical sides of beam||24-48 hours|
|2||Slabs ( props left under)||3 days|
|3||Beam soffits (props left under)||7 days|
|4||Removal of props to slabs · Span<= 4.5m · Span > 4.5m|
· 7 days
· 14 days
|5||Removal of props to beams and arches · Span<=6m · Span >6m|
· 14 days
· 21 days
how to Properly Remove the Formwork
The formwork removal is one of the most hazardous jobs in the site. If not properly done it not only affects the structure but also can result in high risk for the labours working at site. Thus the following points must be kept in mind during formwork removal
- Proper tests are conducted (mostly NDT) to determine the strength of concrete developed.
- The formworks of vertical members are removed first.
- The props of soffits are not removed until it is ensured to bear its own self weight and external stresses effectively.
- The crowbars are not to be inserted in the sides to remove the formworks as it can damage the edges of concrete.
- Any loose material of concrete found in the edges and corners are to be removed.
- The quality engineers should inspect the site before removal of formworks to ensure suitable conditions.
- To ensure safety of workmen, it is to be inspected weather sufficient bracings and work platforms are provides to remove the formwork at higher elevations.
Thus from the above data we can understand the importance of removal of formwork in time with necessary precautions to be taken care at the site.