Formwork is one of the most essential temporary structures used in construction. It acts as a mould into which concrete is poured and allowed to harden into the desired shape. The strength, durability, and finish of a concrete structure depend a lot on the quality of formwork and the time at which it is removed. Improper removal can lead to cracks, collapse, or deformation of the structure. Therefore, knowing the correct period of removing formwork and the right techniques is very important for every civil engineer and site supervisor.
What is Formwork?
Formwork is a temporary or sometimes permanent structure used to hold freshly poured concrete until it gains enough strength to support itself. It provides shape, support, and surface finish to concrete elements such as slabs, beams, columns, and walls.
There are various types of formwork depending on the material used and the type of structure:
- Timber Formwork – Traditional and easy to use but not very durable.
- Steel Formwork – Strong, reusable, and provides smooth surfaces.
- Aluminium Formwork – Lightweight, quick to assemble, and ideal for repetitive structures.
- Plastic Formwork – Suitable for small projects and easy to clean.
- Plywood Formwork – Commonly used due to its flexibility and smooth finish.
Importance of Proper Formwork Removal
Removing formwork too early can cause the concrete to crack or collapse because it may not have developed sufficient strength. On the other hand, keeping it for too long may delay the project and increase costs. Therefore, proper timing ensures:
- Safety of the structure.
- Smooth surface finish.
- Cost and time efficiency.
- Prevention of structural damage.
Factors Affecting the Period of Formwork Removal
The period for removing formwork is not fixed for every project. It depends on several factors, such as:
- Type of Cement Used:
Rapid hardening cement gains strength faster than ordinary Portland cement (OPC). - Temperature and Weather Conditions:
In hot weather, concrete sets faster, while in cold weather, it takes longer. - Grade and Mix of Concrete:
Higher grade concrete gains strength quicker. - Type of Structural Element:
Different elements (slab, beam, column) carry different loads, so their formwork removal times vary. - Curing Conditions:
Proper curing helps the concrete achieve strength earlier.
Period of Removing Formwork as per IS 456:2000
The Indian Standard Code IS 456:2000 gives general guidelines for the removal of formwork based on the type of structural member. Below is a summary:
| Type of Formwork | Minimum Period Before Removal |
|---|---|
| Vertical sides of walls, columns, and beams | 16–24 hours |
| Slabs (props left under) | 3 days |
| Beams (props left under) | 7 days |
| Slabs (props removed) | 7 days |
| Beams (props removed) | 14 days |
| Arches (props removed) | 21 days |
These timings assume normal temperatures (15°C to 27°C) and good quality concrete. In colder weather, the time should be increased.
Techniques for Safe Formwork Removal
To ensure safety and avoid damage during removal, proper techniques must be followed:
- Check Concrete Strength:
Before removing the formwork, verify that the concrete has gained the required strength using cube tests or non-destructive tests like rebound hammer. - Remove in the Right Sequence:
Always remove the formwork in the reverse order of erection. For example, start with non-load-bearing parts and end with main supports. - Loosen Gradually:
Do not pull or jerk the formwork. It should be loosened gently to prevent cracking or damage to edges. - Use Proper Tools:
Use wedges and crowbars carefully. Avoid hammering the formwork, as it can damage the concrete surface. - Check for Surface Defects:
Once removed, inspect the surface for honeycombs, cracks, or voids. Immediate repair should be done if needed. - Clean and Store Formwork Properly:
After removal, clean and oil the formwork before storing it for reuse. This extends its life and ensures better finish in future use.
Precautions During Formwork Removal
- Never remove formwork from beams or slabs before they achieve full strength.
- Avoid loading or vibration on freshly cast concrete during removal.
- Ensure proper curing continues even after the formwork is removed.
- Supervision by experienced personnel is mandatory.
Advantages of Proper Formwork Handling
- Ensures structural safety.
- Enhances the surface quality of concrete.
- Reduces wastage and damage.
- Saves cost by reusing materials.
- Improves project efficiency.
Common Mistakes to Avoid
- Removing formwork too early to save time.
- Ignoring weather conditions.
- Using damaged or deformed formwork.
- Not checking the strength of concrete before removal.
Causes of Formwork Failure
Even though formwork is a temporary structure, it must be designed and erected with great care. A failure in formwork can lead to accidents, property loss, and even fatalities. Below are the common causes of formwork failure in construction:
- Improper Design and Planning
- Inadequate design of formwork pressure, spacing of supports, or lack of stability can cause collapse.
- Not following standard design codes and load considerations also leads to failure.
- Poor Quality Materials
- Using damaged or low-quality timber, thin plywood, or rusted steel can reduce the strength of formwork.
- Nails, bolts, and clamps that are weak or corroded can loosen under load.
- Inadequate Bracing and Supports
- Formwork needs proper bracing to resist lateral pressure from fresh concrete.
- Insufficient ties or props may cause bulging or tilting during pouring.
- Improper Alignment and Assembly
- Wrong alignment or incorrect fixing of joints can make the formwork unstable.
- Poor supervision during assembly often leads to uneven surfaces or leakage of concrete.
- Premature Removal of Formwork
- Removing formwork before the concrete achieves the required strength can result in cracking, deflection, or complete collapse.
- Overloading During Construction
- Stacking construction materials or workers standing on unsupported formwork can cause failure due to excess load.
- Vibration or Impact During Concreting
- Excessive vibration during compaction or from nearby equipment can disturb the stability of the formwork.
- Weather Conditions
- Rain, wind, or heat can weaken formwork materials or affect the curing process.
- Wet timber swells and loses shape, while heat can dry and shrink it, affecting its fit.
- Lack of Skilled Supervision
- Most failures occur due to poor monitoring, lack of inspection, or ignoring early warning signs like movement or cracks in formwork.
Relevant IS Codes for Formwork in India
In India, several IS Codes provide guidelines for the design, construction, and removal of formwork. Here are the most relevant ones:
| IS Code | Title / Description |
|---|---|
| IS 456:2000 | Plain and Reinforced Concrete – Code of Practice (Gives details about stripping time and removal of formwork.) |
| IS 14687:2017 | Formwork for Concrete – Guidelines (Covers design, materials, assembly, and safety requirements.) |
| IS 4990:2011 | Plywood for Concrete Shuttering Work – Specification (Specifies quality and thickness of shuttering plywood.) |
| IS 801:1975 | Code of Practice for Use of Cold Formed Light Gauge Steel Structural Members in General Building Construction (For steel formwork and temporary structures.) |
| IS 2750:1964 | Specification for Steel Scaffoldings (Guidelines for scaffolding and support systems used with formwork.) |
| IS 3696 (Part 1 & 2):1987 | Safety Code for Scaffolds and Ladders (Ensures worker safety during erection and dismantling.) |
| IS 14687 Annex C | Recommended stripping times and procedures for formwork removal. |
Tips to Prevent Formwork Failure
- Always use standard materials approved by the engineer.
- Follow IS code provisions strictly.
- Ensure proper supervision during concreting.
- Check formwork stability and tightness before pouring concrete.
- Do not overload or disturb formwork until full strength is achieved.
- Carry out inspection before, during, and after concreting.
FAQs
1. What happens if formwork is removed too early?
If formwork is removed too soon, the concrete may not have enough strength, leading to cracks, deformation, or even collapse.
2. Can formwork be reused?
Yes, steel, aluminium, and plastic formwork can be reused multiple times if properly maintained and cleaned after each use.
3. How is the strength of concrete checked before removing formwork?
It can be checked using compressive strength cube tests or non-destructive tests like rebound hammer and ultrasonic pulse velocity test.
4. Why is curing important after removing formwork?
Curing ensures that the concrete continues to gain strength and prevents surface cracking due to rapid moisture loss.
5. Who decides when to remove the formwork?
The site engineer or project supervisor decides, based on test results, weather conditions, and standard codes.
Conclusion
Formwork plays a vital role in shaping and supporting concrete structures. The correct period of removing formwork is crucial for ensuring safety, strength, and durability. By following the proper removal techniques, adhering to IS code guidelines, and maintaining good curing practices, construction teams can achieve better-quality structures and avoid costly damages. Always remember — patience during formwork removal ensures long-term stability and performance of the structure.