Forms should not be removed until the concrete has hardened enough to be safely subjected to its own weight and any other live loads.
The supporting period (the period between placing the concrete in the forms and removing the forms) varies according to the type of cement used and the design of the formwork. In normal structures bearing construction loads, the side timbers should not be removed within 7 days and the supporting timbers within 28 days after the concrete is laid.
Under no circumstances shall the forms be trapped until the concrete reaches a strength at least twice the stress at which the concrete can be applied when struck.
The referred strength shall be that of concrete with equal proportions, using the same cement and aggregates, and shall cure under conditions of temperature and moisture present at work. Where possible, formwork should be left on longer, as this will aid in curing.
Under normal conditions (usually where the temperature is above 20.)0c), and where ordinary cement is used, the forms may be struck after the expiration of the following periods.
|Form Type||eviction time period|
|Beam walls, columns and vertical sides||24 to 48 hours as may be decided by the Engineer-in-Charge.|
|Slab (bottom left prop)||3 days|
|Beam soffits (props omitted below)||7 Days|
|Removal of props from slab (extends 4.5 m)||7 Days|
|Removal of props from slab (spreading over 4.5 m)||14 days|
|Removal of props for beams and arches (extends 6 m)||14 days|
|Removal of props for beams and arches (spreading over 6 m)||21 days|
For fast hardening cement, 3/7 of the above duration will suffice in all cases except the vertical sides of slabs, beams and columns, which should be kept for 24 hours.
All formwork must be removed without shock or vibration as this will damage the reinforced concrete. Before removing soffits and struts, the concrete surface should be exposed where necessary to ensure that the concrete has hardened sufficiently. Appropriate precautions should be taken to reduce the rate of hardening that occurs with all elements in cold weather.
IS-456 – Plain and Reinforced Concrete – Code of Practice