Stripping Time of Formwork as per IS 456: 2000
The stripping or removal of formwork in concrete construction is a critical step, as it ensures that the concrete has attained sufficient strength to bear its self-weight and any additional loads. The Indian Standard IS 456: 2000 provides clear guidelines on the duration for which formwork should remain in place before being stripped, depending on the structural element and span length.
Clause 11.3.1 of IS 456: 2000 – Formwork Removal Guidelines
The recommended time frame for removing formwork are based on standard curing conditions (use of Ordinary Portland Cement at normal ambient temperatures). Below are the specified stripping times:
1. Vertical Formwork for Columns, Walls, and Beams
Minimum Stripping Time: 16 to 24 hours after casting.
Reason: Vertical members like columns and walls do not bear significant load immediately and can safely support themselves once the concrete sets.
2. Soffit Formwork (Bottom Shuttering) for Slabs
Minimum Stripping Time: 3 days, with props left under the slab for support.
Reason: Slabs bear distributed loads, and early removal of props may lead to deflection or cracking.
3. Soffit Formwork (Bottom Shuttering) for Beams
Minimum Stripping Time: 7 days, with props left under the beam for additional support.
Reason: Beams carry concentrated loads and require longer curing time to gain adequate strength before bearing weight independently.
4. Props Under Slabs
Stripping Time for Props:
For spans up to 4.5 meters: 7 days.
For spans greater than 4.5 meters: 14 days.
Reason: Larger spans exert greater pressure and need longer curing to prevent sagging or structural failure.
5. Props Under Beams and Arches
Stripping Time for Props:
For spans up to 6 meters: 14 days.
For spans greater than 6 meters: 21 days.
Reason: Beams and arches have concentrated loads and require extended time to achieve sufficient compressive strength.
Factors Affecting Stripping Time
The times mentioned above are general recommendations and may vary based on the following factors:
1. Type of Cement:
Cement types other than Ordinary Portland Cement (e.g., PPC or PSC) typically gain strength slower, so longer stripping times may be required.
2. Ambient Temperature:
Concrete sets slower in cold weather, which requires extending the stripping time. In hot weather, early curing may help achieve strength faster.
3. Mix Design and Strength:
High-performance concrete or mixes with accelerators may gain strength faster, potentially reducing stripping times.
4. Structural Span and Loads:
Longer spans and heavier loads demand extended formwork retention to ensure structural integrity.
5. Engineer’s Discretion:
The supervising engineer may modify the stripping schedule based on project-specific requirements.
Best Practices for Formwork Removal
To ensure safe and effective formwork removal, follow these guidelines:
1. Test Concrete Strength:
Ensure that the concrete has reached at least 70% of its design strength before stripping.
Use cube or cylinder tests to verify the compressive strength.
2. Remove Gently:
Avoid sudden shocks, vibrations, or impacts while removing formwork to prevent cracking or surface damage.
3. Check Weather Conditions:
Prolong stripping time during cold weather to compensate for slower strength development.
4. Maintain Curing:
Continue curing after formwork removal to ensure complete hydration and strength development.
Conclusion
The stripping times specified in IS 456: 2000 are crucial for ensuring safety and durability in concrete structures. Adhering to these guidelines minimizes the risk of deflection, cracking, or structural failure. Proper supervision, curing, and testing ensure that the formwork removal process aligns with the project's safety and quality standards.
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