Method and Period of Formwork Removal
In this article, we will discuss the method and period of formwork removal, which are crucial to ensuring the successful completion of concrete construction projects. Proper formwork removal is essential to maintaining the integrity of the structure, achieving the desired finish, and ensuring the safety of workers. Let’s delve into these aspects:
What is Formwork?
Formwork, also known as “shuttering,” is a temporary structure or mold that is used to contain and shape freshly poured concrete until it reaches sufficient strength to support itself and any applied loads. Formwork is a critical component of concrete construction and plays a vital role in ensuring the integrity and quality of the final concrete structure. It provides the necessary support, shape, and surface finish for the concrete during its curing process.
Method and Period of Removing Formwork
The method and period of removing formwork depend on several factors, including the type of concrete being poured, the formwork system used, the ambient conditions, and the desired strength of the concrete. Here’s a general guideline for the method and period of removing formwork:
1. Method of Removing Formwork
The method of removing formwork can vary based on the specific project and formwork system used. Here’s a general sequence:
Vertical Formwork (Walls, Columns, etc.):
Stripping Time: Vertical formwork for walls and columns is often removed once the concrete has gained enough strength to support itself. This is typically within 24 to 48 hours, depending on the concrete mix, ambient conditions, and the specified strength.
Method: Vertical formwork is often removed by using pry bars, hammers, or lifting equipment, as appropriate. Care should be taken to avoid damaging the concrete surface.
Horizontal Formwork (Slabs, Beams, etc.):
Stripping Time: Horizontal formwork for slabs and beams is typically removed after the concrete has reached a specific level of strength to bear its own weight. This can be around 3 to 7 days for normal concrete, but it can vary based on factors such as ambient temperature and concrete mix design.
Method: Horizontal formwork can be removed using mechanical equipment, such as cranes or hoists, to lift and relocate the formwork components. Alternatively, manual methods like pry bars and hammers can be used.
2. Period of Removing Formwork
The period for removing formwork is often defined by certain criteria, such as concrete strength and time, to ensure the structural integrity of the concrete. Here’s a general guideline:
Early Removal: Formwork can be removed earlier if the concrete has reached sufficient strength to support its own weight without deformation or damage. Early removal can speed up the construction process, but caution is necessary to avoid compromising the concrete’s integrity.
Standard Removal: Formwork is typically removed after the concrete has achieved a specified level of compressive strength. This level is often around 70% to 80% of the specified 28-day strength. This usually occurs within the first week to two weeks after pouring, depending on the mix design and ambient conditions.
Delayed Removal: In some cases, formwork may be left in place for a longer period, especially if the concrete is subjected to significant loads or if the structure requires extra curing time. Delayed removal can enhance the concrete’s long-term durability.
Always refer to the project’s specifications, engineering recommendations, and the guidelines provided by the formwork manufacturer when determining the appropriate method and period for removing formwork. Additionally, ensure that proper safety measures are in place for workers involved in the formwork removal process.
Formwork Stripping Time (When Using Ordinary Portland Cement)
|Types of Formwork
|Formwork Removal Time
Wall sides, columns, and beam vertical faces
24 to 48 hours (depending on engineer's decision)
Slab ( props left under)
Beam soffits (props left under)
Removal of Slab Props
Slabs spanning up to 4.5m
Slabs spanning over 4.5m
Removal of Beam and Arches' Props
Span up to 6m
Span over 6m
What Happens If The Formwork Removed Early?
If formwork is removed too early, before the concrete has gained sufficient strength, it can lead to a range of issues that can compromise the structural integrity and appearance of the concrete element. Some of the potential consequences of removing formwork prematurely include:
Premature formwork removal can cause surface defects such as honeycombing, voids, or rough patches on the concrete surface. This happens because the concrete is still relatively soft and vulnerable to damage when the formwork is removed. Properly cured concrete has a smoother and more durable surface.
Concrete gains strength over time through a process called hydration, during which cement particles react with water to form a crystalline structure. If formwork is removed before sufficient strength is attained, the concrete may not be able to bear the intended loads and may develop cracks or even fail.
Early formwork removal can lead to cracking due to the lack of support provided by the formwork. Cracks can develop as a result of both external loads and internal stresses caused by shrinkage and temperature changes. Cracks not only compromise structural integrity but can also allow moisture to penetrate the concrete, potentially causing durability issues.
The weight of the concrete itself, as well as any additional loads or forces, can cause deformation if the concrete is not adequately cured before the formwork is removed. Deformation can lead to uneven surfaces, misalignment, and structural instability.
Concrete undergoes slight dimensional changes as it cures and dries. Premature formwork removal can interfere with these changes and lead to dimensional inaccuracies or mismatches, especially at connection points or joints.
Adequate curing and proper hydration of the concrete are essential for achieving a strong bond between concrete layers. If formwork is removed too early, the bond between layers may be compromised, leading to reduced structural performance.
Prematurely removing formwork can create hazardous conditions for construction workers as well as anyone who might interact with or use the structure. Weak or damaged concrete can pose a safety risk.
Things to Keep In Mind
Removing formwork requires careful planning and execution to ensure the safety of workers and the quality of the concrete structure. Here are some important points to remember when removing formwork: