Removal of Concrete Formwork: Know the Correct Time | calculations | specification -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

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May 8, 2021

Removal of Concrete Formwork: Know the Correct Time | calculations | specification

Formwork are temporary models in which concrete is poured and compressed to lay concrete according to the design. The formwork is set up in the correct position to cast the members onto the concrete structure. The formwork is held in place until the concrete hardens and gains strength.


Removing the formwork is only recommended when the concrete has its dead weight and is strengthened without the formwork or any props. Before dealing with the precautionary measures to be taken when removing formwork or props, it is always necessary to know the exact time to remove the formwork. The formwork must be removed after the concrete structural elements supported in the formwork have been removed. Here is a brief description of the exact time and precautions to be taken to remove the formwork.

 

Correct Time of Removal of Formwork

The time from which the concrete is placed in formwork till the time the formwork is removed is called as supporting period. This time period will vary, based on different factors like:

 

TYPE AND GRADE OF CONCRETE

• Type and grade of concrete

• Admixtures used in concrete mix

• Formwork designs

• Temperature

• Type of structural element (Columns / beams / slabs)

• Quality of content

• Curing conditions

The process of removing the formwork occurs after the concrete has turned and gained sufficient strength, which is called formwork striking time. This can cause stress on the structural dimension due to loss of support on or below all sides, which is now the responsibility of a support structure. Properly hardened concrete element can withstand these external stresses when removing formwork. Formwork is left for a long time to aid in the healing process of concrete components.

 

The normal period for which the formwork can be removed in accordance with IS: 456-2000, Section 11 is shown below:

 

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

 

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

Type of Formwork

Formwork Removal Time

 

Structured members are structured based on designed loads. But before a structure is completed and subjected to all the loads considered during structural design, the structural elements will be subjected to its own weight and construction loads during the construction process.

 

Therefore, in order to proceed with construction activities at a rapid rate, it is necessary to calculate the behavior of the structure under self-load and construction load. This can be done and if the configuration member finds it safe, the formwork can be removed.

 

If these calculations are not possible, the following formula can be used to calculate safe formwork strike times:

 

Characterization of the cube of equal maturity to the structure required at the time of removal of the formwork

 

= (Dead load + Construction load) / ( total design load)

= Grade of concrete

This formula was presented by Harrison (1995), which describes in detail the background that determines the time it takes to remove the formwork.

 

Another method of determining the strength of a concrete structure is to perform non-destructive tests on the structural member.

 

Important Note:

 

It should be noted that the time to remove the formwork shown above in Table-2 is only when ordinary Portland cement is used.

Portland Pozzolana cement is used in the normal construction process. Therefore, the time shown in Table-2 should be modified.

 

For cement other than ordinary Portland cement, the time required to remove the formwork is:

 

1.  Portland Pozzolana Cement – Removing time will be 10/7 of the time specified above (Table-2)

2.  Low heat cement - the transfer time is 10/7 of the time mentioned above (Table-2)

3.  Rapid Hardware Cement - 3/7 of the time mentioned above (Table-2) The transfer time is sufficient in all cases except the vertical sides of the slabs, beam and columns, which must be maintained for 24 hours at least an hour.

 

 

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

 

What Happens If The Formwork Removed Early?

(A) Early Removal Of Side Support

if you remove formwork immediately, the columns and beams sides may bulge if the concrete has not hardened.  And sides may go out of plumb or line.

 

(B) Early Removal Of Support

If you do not get enough strength, the system may be distracted or collapse. Please note that if deviations occur at such times it will be permanent and difficult to repair. So be careful.


FORMWORK CHECKLIST



ALSO READ: INSPECTION CHECKLIST FOR COLUMN CONCRETING 


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