Forms should not be removed until the concrete has sufficiently hardened in order that it can carry safely its own weight and any other live loads it is subjected to.
The supporting period (period between placing of concrete in forms and removal of forms) differ according to the type of cement used and the design of formwork. In general structures carrying construction loads, side timbers shall not be removed within 7 days and supporting timbers within 28 days of placing of concrete.
In no circumstances shall forms be stuck until the concrete reaches strength of at least twice the stress to which the concrete may be subjected at the time of striking.
The strength referred to shall be that of concrete using the same cement and aggregates, with the same proportions, and cured under conditions of temperature and moisture similar to those existing on the work. Where possible, the formwork should be left longer, as it would assist the curing.
In normal circumstances (generally where temperatures are above 200C), and where ordinary cement is used, forms may be struck after expiry of following periods.
Type of Form
Removal Time Period
Walls, columns and vertical sides of beams
24 to 48 hours as may be decided by the engineer-in-charge.
Slabs (props left under)
Beam soffits (props left under)
Removal of props to slabs (Spanning upto 4.5 m)
Removal of props to slabs (Spanning over 4.5 m)
Removal of props to beams and arches (Spanning upto 6 m)
Removal of props to beams and arches (Spanning over 6 m)
For rapid hardening cement, 3/7 of the above period will be sufficient in all cases except vertical sides of slabs, beams and columns which should be retained for 24 hours.
All formworks should be removed without shock or vibration as this would damage the reinforced concrete. Before the soffit and struts are removed, the concrete surface should be exposed, where necessary in order to ascertain that the concrete has sufficiently hardened. Proper precautions should be taken to allow for the decrease in the rate of hardening that occurs with all elements in the cold weather.
IS-456 – Plain and Reinforced Concrete – Code of Practice