Formwork is a temporary arrangement of mould to provide support to the fresh concrete. The basic requirement of formwork is that it should be strong enough to carry a load of concrete, live load and all the other imposed loads apart from its own weight. Once the concrete hardens, the house slab formwork is removed in a particular time frame depending upon the type and size of the component. Removing formwork is called stripping. Design of formwork is given greater attention as it contributes to about 20-25% of the construction cost. It can be reusable or non-reusable. The no. of repetitions of formwork makes it more economical to use. IS code 14687 (1999) can be used to understand more about formwork design.
A house slab formwork includes the following components:
- Sheathing: Sheathing is placed at the bottom of the slab and placed vertically at the sides of the slab of 25-40 mm thick size.
- Joists and ledges: Joists and ledges are placed below the sheathing in horizontal and vertical direction respectively. They are placed to support the sheathing and are of 50 x 70 mm to 150 x 70 mm size.
- Props: Propos of 75-100 mm dia. are placed vertically at regular intervals to support the joists and ledges.
- Base plates: Base plates are placed below the props to keep them in an intact position.
- Bracing: Cross bracings or horizontal bracings are provided to support the props and to keep them in a fixed position.
Slab formwork procedure
The formwork of slab includes 3 steps:
- Assembly: The type of house slab formwork selected should be able to attain the loads imposed on it and the size of forms should match the dimensions of the member.
- Erection: The house slab formwork should be in proper line and level. All the joints should be sealed adequately to prevent any leakage of fresh concrete. Lateral bracing should be provided as per the requirement of assembly. The forms should be cleaned and oiled for easy removal at the time of stripping.
- Removal: Once the concrete hardens, the forms are removed in sequence as mentioned below:
- Slab soffit to be removed after 3 days (props to be refixed immediately after removing formwork)5.
- Props to be removed in 7 days if the slab span is up to 4.5 m and in 14 days if the slab span is more than 7.5 m.
Types of Slab Formwork
- Timber and plywood formwork
- It is made using timber and plywood or particleboard and is light-in-weight.
- It is used in small construction projects and no. of repetitions of forms is less.
- The timber formwork for slab consists of slab sheeting, batten, ledges, head tree, planks, props, wedges and sole plates.
- Beam formwork rests on a head tree and house slab formwork rests on battens supported by props and sole plate at the bottom.
- If the height of the prop used is more than 1 m, cross bracings are provided to support the props.
- Steel Formwork
- Steel formwork is used for larger construction projects and it has a high no. of repetitions compared to timber formwork.
- It consists of panels made of thin steel-plated stiffened by steel angles at the edges.
- Panels are supported by beams at the bottom which are supported by head jack-steel prop assembly at the bottom. Cross bracings are provided as per the requirement.