The process of removing concrete formwork, also known as strike-off or stripping, should only be carried out after the concrete has achieved adequate strength. This strength should be at least twice the stress that the concrete will face when the formwork is removed. Additionally, it is necessary to make sure that the remaining formwork is stable during the formwork removal process.
Concrete Formwork Removal Time
Formwork Removal Time: The rate of hardening of concrete or the concrete strength is dependent on temperature, and this affects the formwork removal time. In winter, the removal of concrete will take longer than it would in the summer.
Special Care for Flexural Members: When dealing with flexural members such as beams and slabs, special attention must be paid to formwork removal. These members are subject to self-load and live load, and if the strength of the concrete is not sufficient, they may begin to deflect.
Estimating Concrete Strength: Before formwork removal, tests on concrete cubes or cylinders should be conducted, and they should be created with the same mix and cured under the same temperature and moisture conditions as the structural members. Only after it has been established that the concrete has gained sufficient strength to withstand the design load should formworks be removed.
Risks of Delay: When delaying the formwork removal, consideration must be given to potential freeze-thaw damage, cracking due to thermal contraction, and collapse of the structure under self-load or design load. If these risks are significant, formwork should be delayed until the structure is properly insulated.
Calculation of Safe Formwork Striking Times:
Calculating Safe Formwork Striking Times: Formwork is an essential part of the construction process, enabling the structure to be built quicker and more efficiently. To ensure the structural integrity of the structure, it is essential to calculate the behaviour of the structure under its self-weight and construction loads. This can be done through the use of calculations or non-destructive tests on the structural members.
The Harrison Formula for Determining Striking Times:
One of the most widely accepted methods for determining the safe striking times of formwork is the formula presented by Harrison (1995). This formula takes into account the characteristic strength of a cube of equal maturity to the structure required at the time of formwork removal. This allows for a more accurate assessment of the structure’s safety when the formwork is removed.
Non-Destructive Tests for Determining Strength: As an alternative to the Harrison formula, non-destructive tests can be conducted on the structural members to determine their strength. This method is particularly useful when the calculations needed for the Harrison formula are not feasible or when a more detailed assessment of the structure’s safety is required. These tests are able to provide an accurate measurement of the concrete’s strength and can be used to determine the safe formwork striking times.
Factors Affecting Concrete Formwork Striking Times
The striking time of concrete formwork is affected by the strength of the structural member. Factors that influence strength development of concrete members include:
Grade of Concrete: Higher grade concrete will allow for faster strength development, resulting in shorter formwork striking times.
Grade of Cement: Using higher grade cement will also result in faster strength development and quicker formwork striking times. Different types of cement can also affect the strength development rate. Rapid hardening cement will have a faster strength gain than Ordinary Portland Cement, whereas low heat cement will take longer to gain sufficient strength.
Temperature: Higher temperatures during the placement of concrete will result in higher strength gain in a shorter period of time. During winter, the strength gain time is prolonged. Higher ambient temperatures also make the concrete gain strength faster. Formwork helps to insulate the concrete from its surroundings, so the longer it remains with the concrete results in less heat of hydration loss and a higher rate of strength gain.
Size of Concrete Member: Larger concrete section members will gain strength in a shorter time than smaller sections.
Accelerated Curing: The application of heat can be used to increase the rate of strength gain with accelerated curing.
Concrete Strength Requirements for Formwork Striking: Generally, the following values of concrete strength are considered for the removal of formwork for various types of concrete structural members.
Table – 1: Strength of concrete vs. Structural Member Type & Span for Formwork Removal
| Concrete Strength | Structural Member Type and Span |
| 2.5 N/mm2 | Lateral parts of the formwork for all structural members can be removed |
| 70% of design strength | Interior parts of formwork of slabs and beams with a span of up to 6m can be removed |
| 85% of design strength | Interior parts of formwork of slabs and beams with a span of more than 6m can be removed |
Table – 2: Formwork Stripping Time (When Ordinary Portland Cement is used):
| Type of Formwork | Formwork Removal Time |
| Sides of Walls, Columns and Vertical faces of beam | 24 hours to 48 hours (as per engineer’s decision) |
| Slabs (props left under) | 3 days |
| Beam soffits (props left under) | 7 days |
| Removal of Props of Slabs: | |
| i) Slabs spanning up to 4.5m | 14 days |
| ii) Slabs spanning over 4.5m | 14 days |
| Removal of props for beams and arches | |
| i) Span up to 6m | 14 days |
| ii) Span over 6m | 21 days |
Formwork Removal Time for Different Types of Cement
Ordinary Portland Cement: The time required to remove formwork when Ordinary Portland Cement is used is shown in Table-2.
Portland Pozzolana Cement: When Portland Pozzolana Cement is used, the stripping time should be 10/7 of the time stated in Table-2.
Low Heat Cement: For Low Heat Cement, the formwork removal time should be 10/7 of the time stated in Table-2.
Rapid Hardening Cement: For Rapid Hardening Cement, the stripping time should be 3/7 of the time stated in Table-2. Vertical sides of slabs, beams and columns should be retained for at least 24 hours.
Concrete Formwork Removal Specification
When removing formwork, the concrete must be strong enough to support the loads placed on it. The strength of the concrete is dependent on the grade of concrete, the type of cement, the water/cement ratio, and the temperature during curing. The engineer must ensure that the concrete has attained sufficient strength before formwork removal is initiated.
Arrangement of Formwork for Easy Removal: Formwork should be arranged in a way that makes it easy to remove without damaging the concrete or formwork panels, so that it can be reused with minimal repair. When separating the formwork, care should be taken not to use crowbars as this may cause damage to the hardened concrete. Wooden wedges should be used instead.
Supervision of Formwork Removal: The formwork removal procedure should be supervised by the engineer to ensure the quality of the hardened concrete in the structural member. Defects such as honeycombing, size and shape defects can influence the strength and stability of the structure, so immediate repair or rejection of members may be necessary.
Sequence and Pattern of Formwork Removal: Beam and joist bottoms should remain in place until the shoring underneath has been completely removed. Joist forms should be designed and removed in a way that the shores can be temporarily removed for the removal of joist forms, and replaced immediately afterwards. The engineer should approve the sequence and pattern of formwork removal.
Reference:
- ACI (1995) In-place methods to estimate concrete strength. ACI 228.1R-95.
- ASTM (1987) Standard practice for estimating concrete strength by the maturity method. ASTM C1074–87
- BS 8110 – code of practice for the structural use of concrete
- IS-456 – Plain and Reinforced Concrete – Code of Practice
Frequently Asked Questions:
When to Remove Formwork?
The removal of concrete formwork, also known as strike-off or stripping of formwork, should not be done until the concrete has achieved adequate strength – at least twice the stress that the concrete may be exposed to when the formwork is removed. It is also important to make sure that the remaining formwork is stable during the formwork removal process.
What are the factors affecting concrete striking time?
The striking time of concrete formwork is heavily dependent on the strength of the structural members. In order to ensure a quick and safe strike, it is important to understand the factors that influence the strength of concrete.
1. Grade of Concrete: The grade of concrete used will impact the time it takes for the concrete to reach its full strength. Depending on the grade of concrete, the time needed for striking the formwork can considerably.
2. Grade of Cement: The grade of cement used plays an important role in the strength development of concrete. The higher the grade of cement, the more quickly the concrete will reach its full strength.
3. Type of Cement: The type of cement used will also affect the time needed to strike the formwork. Different types of cement have different properties that can influence the strength and setting time of concrete.
4. Temperature:The temperature at the time of pouring and curing can also affect the time it takes for the concrete to reach its full strength. Cold temperatures can slow down the curing process, while warmer temperatures can speed it up.
5. Size of the Concrete Member: The size of the concrete member also affects the time needed to strike the formwork. Larger concrete members will require more time to reach their full strength than smaller members.
6. Accelerated Curing: Accelerated curing techniques can be used to reduce the time needed to strike the formwork. These techniques involve using temperature, humidity and other methods to reduce the curing time of concrete.