How Long Should Concrete Cure Before Removing Forms

How Long Should Concrete Cure Before Removing Forms

How Long Should Concrete Cure Before Removing Forms

What is Formwork?

Formwork is a type of casting mould or box, like container into which fresh concrete is poured and compacted. When the concrete is hardened, the formwork is removed and a solid mass is produced in the shape of the inner face of the formwork.

How Long Should Concrete Cure Before Removing Forms
Removal of Formwork

What is Removal of Formwork:

The removal of formwork also called as strike-off of forms in which concrete is poured. The formwork should be removed only after when concrete component achieves its sufficient strength. The care should be taken during the removal of formwork to ensure the stability of the remaining formwork.

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Factors Affecting Concrete Formwork Striking Times:

The time of removal of formwork depends on how quickly concrete members gains its sufficient strength and it depends on the following factors.

Grade of concrete – As the rich grade of concrete, the rate of development of strength is higher and thus concrete gain the strength in a shorter time.

Grade of cement – a Higher grade of cement offers a quick setting and it gains strength in a shorter time.

Type of Cement – Cement type used in concrete also affects the strength development of concrete. For example, Compare to ordinary Portland cement rapid hardening cement has higher strength gain in shorter. Low heat cement requires more time for setting compare to OPC.

Temperature – High temperature of concrete and surrounding at the time of placing concrete makes it achieve higher strength in shorter times. During winter, the strength gaining process becomes slower.

Size of the concrete member:  Size of the concrete member also matters in strength gaining process. Large-sized concrete section members gain strength in a shorter time than smaller sections.

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How Long Should Concrete Cure Before Removing Forms

 Time of Removing Formwork as per ASTM:

Sr.
No
Type of Cement Time of De-shuttering
1 Type I cement 7 days
2 Type II cement 10 days
3 Type III cement 3 days
4 Type IV or V cement 14 days
5 ASTM  C 1157 C 595, C 845 cements Varies  

Generally, the American Concrete Institute recommends the following:

Sr.
No.
Type of Formwork Minimum Period Before Striking Formwork
1 Walls and columns vertical Supports 24-48 hours
2 Slabs, with their props left under them 3-4 days
3 Soffits, with their props left under them 7 days
4 Props supporting slabs under 15 feet 7 days
5 Props supporting slabs under 15 feet 14 days
6 Props  supporting arches& beams over 20 feet 21 days

How Long Should Concrete Cure Before Removing Forms

Time of Removal of Formwork as per IS 456: 2000

Sr. No. Type of Formwork Minimum Period Before Striking Formwork
1 Vertical formwork to columns, walls, beams 16-24 h
2 Soffit formwork to slabs (Props must be re-fix immediately after removal of formwork) 3 days
3 Soffit formwork to beams (Props must be re-fix immediately after removal of formwork) 7 days
4 Props to slabs: 1) Spanning up to 4.5 m 2) Spanning over 4.5 m   7 days 14days
5 Props to beams and arches: 1) Spanning up to 6 m 2) Spanning over 6 m   14 days 21 days

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How to Properly Remove Formwork:

  • The formwork removal is one of the most hazardous jobs in the site. If not properly done it not only affects the structure but also can result in high risk for the labors working at the site. Therefore following important point should be kept in mind while removing the formwork.
  • Conduct proper test (mostly NDT) on concrete to ensure its strength developed.
  • Removal of formwork should be started from vertical members are removed first.
  • The supports of slab or soffits should not remove until it is ensured to bear its self-weight and external stresses effectively.
  • The crowbars should not fix on the side to remove the formworks as it can damage the edges of concrete.
  •  Loose concrete or any other material found in the edges and corners are to be removed.
  • The safety engineers should inspect the site before removal of formworks to ensure suitable conditions.
  • For workman safety, inspect the formwork position that sufficient bracings and work platforms are provided to remove the formwork at higher elevations.
  • Team removing formwork should know about risk and rule regulation of safety on the construction site.

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