Concrete formwork removal, also known as strike-off or stripping, is a critical stage in construction. This process must only begin after the concrete has attained sufficient strength, typically at least twice the stress expected to act on the structure upon formwork removal. Ensuring stability and minimizing risks to structural integrity during this stage is paramount. Proper timing and techniques are essential for the success of the construction project.
Factors Influencing Formwork Removal Time
The time required to remove formwork depends on the rate of concrete hardening, which is influenced by environmental and structural factors. Temperature plays a vital role; concrete hardens faster in summer than in winter. Flexural members such as beams and slabs require particular care as they bear self-load and live loads during construction. Improper timing can lead to deflection or structural failure.
Methods for Determining Concrete Strength
The strength of concrete should be assessed before formwork removal. This is typically done by preparing concrete cubes or cylinders from the same mix as the structural members. These samples are cured under similar conditions of temperature and moisture. Non-destructive testing can also be employed to determine in-situ concrete strength. Leaving formwork in place for longer periods is generally beneficial as it aids in curing, enhancing the concrete’s durability.
Precautions During Formwork Removal
During formwork removal, several precautions must be observed:
- Structural Integrity: Ensure the structural element can sustain self-load and design loads without collapse or excessive deflection.
- Avoid Physical Damage: Use appropriate tools, such as wooden wedges, to avoid damaging the hardened concrete during removal.
- Mitigate Risks: Address potential issues such as freeze-thaw damage and cracks caused by thermal contraction by insulating the concrete if formwork is removed prematurely.
Calculating Safe Formwork Removal Times
To determine safe formwork removal times, it is essential to evaluate the structural member’s response to self-weight and construction loads. Where detailed calculations are not feasible, Harrison’s formula (1995) can be used to estimate safe striking times. The formula incorporates the characteristic strength of concrete of similar maturity.
Factors Affecting Striking Times
The following factors influence the appropriate timing for formwork removal:
Concrete Properties:
- Higher concrete grades develop strength more quickly.
- The cement type significantly impacts the rate of strength gain; for instance, rapid hardening cement achieves desired strength faster than ordinary Portland cement.
Environmental Conditions:
- Elevated ambient temperatures accelerate concrete hardening, while lower temperatures slow the process.
- Insulation provided by formwork retains heat from hydration, aiding faster strength gain.
Structural Characteristics:
- Larger sections of concrete generally gain strength more rapidly than smaller sections.
- Accelerated curing techniques can expedite the strength development process.
Guidelines for Formwork Removal
Concrete strength requirements for formwork removal vary depending on the structural member type:
- Lateral Formwork: Can be removed once the concrete reaches a minimum strength of 2.5 N/mm².
- Interior Formwork for Slabs and Beams:
- For spans up to 6m: Remove at 70% of the design strength.
- For spans exceeding 6m: Remove at 85% of the design strength.
Timeframes for Formwork Removal (Using Ordinary Portland Cement):
- Vertical Faces (Walls, Columns, Beams): 24-48 hours.
- Slabs (with props left underneath): 3 days.
- Beam Soffits (with props left underneath): 7 days.
- Props for Slabs and Beams:
- Slabs with spans up to 4.5m: 14 days.
- Beams with spans exceeding 6m: 21 days.
Modifications for Different Cement Types
The formwork removal times outlined above apply to structures using Ordinary Portland Cement. For other types of cement, adjustments are required:
- Portland Pozzolana Cement and Low Heat Cement: Increase the stripping time to ( \frac{10}{7} ) of the OPC time.
- Rapid Hardening Cement: Reduce the stripping time to ( \frac{3}{7} ) of the OPC time, with vertical sides retained for at least 24 hours.
Specifications and Best Practices
When removing formwork for reinforced concrete slabs and other structural elements, follow these guidelines:
- Timing: Do not remove formwork until the concrete has developed sufficient strength to bear all imposed loads. Strength gain depends on factors such as concrete grade, cement type, and curing conditions.
- Damage Prevention: Arrange formwork connections to facilitate safe removal. Avoid using force or crowbars, which can damage hardened concrete.
- Removal Process: Start dismantling from the middle of beams and joists, moving symmetrically toward the supports. Props must remain until all structural loads are adequately supported.
- Engineer Oversight: Obtain approval for the removal sequence and pattern to ensure structural quality and address any casting defects promptly.
Concrete formwork removal is a critical phase in construction that demands careful consideration of timing, strength, and safety. By following proper guidelines, performing strength assessments, and adhering to best practices, construction professionals can ensure the structural integrity and longevity of their projects.