Short Answer:

Formwork removal time is determined based on the type of RCC element, the strength of concrete gained, curing conditions, and weather. It ensures the concrete has developed enough strength to support itself without depending on the formwork.

The Indian Standard IS 456:2000 gives general guidelines for stripping time, such as 1–2 days for vertical faces like walls, and 7 days or more for slabs and beams. Actual removal is done only after confirming that the concrete has reached safe load-bearing strength through tests or standard practices.

Detailed Explanation:

Formwork removal time determination

Formwork removal, also called stripping, is a critical step in RCC construction. It refers to taking off the temporary molds or supports that hold the fresh concrete until it becomes strong enough to bear loads. Removing formwork too early can lead to collapse, cracks, or deformation. Delaying it too long increases costs and slows down construction. Hence, determining the right time is very important.

The main factor in deciding formwork removal time is concrete strength. The concrete must achieve sufficient early strength so it can support its own weight and any construction loads. This depends on several factors such as temperature, cement type, mix design, size of the structure, and curing practices. Stripping is done in stages—vertical faces first, then slabs, then beams and cantilevers.

How Formwork Removal Time is Determined

1. Based on Indian Standards (IS 456:2000)
   • IS 456 provides minimum time limits for removing formwork under normal conditions (temp >15°C and proper curing):
     • Vertical sides of walls, columns, and beams: 16–24 hours
     • Slabs (props left under): 3 days
     • Beam soffits (props left under): 7 days
     • Removal of props for slabs (spanning up to 4.5m): 7 days
     • Removal of props for beams (spanning up to 6m): 14 days
     • For longer spans, more time is needed.
2. Strength-Based Approach
   • In some cases, stripping time is based on actual compressive strength tests of cubes taken from the same concrete batch.
   • If the concrete reaches 70% of its design strength, formwork may be safely removed.
   • This method is preferred for important or large structures.
3. Weather and Temperature
   • In cold weather, concrete gains strength slowly, so more time is required before removal.
   • In hot weather, curing is quicker, but formwork must be protected from rapid drying.
4. Type of Cement and Mix Design
   • Rapid-hardening cement allows quicker removal.
   • Low water-cement ratio or admixtures can also accelerate strength gain.
5. Structural Member Type and Load Conditions
   • Heavily loaded or longer-span members need more support time.
   • Cantilevers and large beams are stripped later to avoid sagging or cracking.
6. Quality of Formwork and Site Conditions
   • Well-designed formwork ensures uniform support and reduces chances of failure.
   • On uneven or unstable ground, extra care is taken before removal.

Conclusion:

Formwork removal time is carefully decided by considering concrete strength, type of structural member, weather, and code guidelines. Following IS 456:2000 or using strength-based cube testing ensures safety and structural performance. Timely removal helps continue construction efficiently while protecting the structure from early failure.