Q: How soon can forms be stripped? We want to re-use our forms the next day if possible.
A: Forms can be stripped when the concrete is strong enough to support its weight and the weight of any loads that will be imposed on it. The required interval will be established by specifications on some jobs.For jobs on which field specifications have not been established for removal of forms and supports, ACI Committee 347 has recommended various minimum time periods. These apply to temperatures above 50 degrees F. When temperatures dip lower the period of low temperature should not be counted as elapsed time. Their recommendations may be summarized as follows:
Pan joist forms up to 30 inches wide
Pan joist forms over 30 inches wide
If design load is greater than dead load
If design load is less than dead load
Soffits of joists, beams and girders
Clear spans less than 10 feet
Clear spans 10 to 20 feet
Clear spans more than 20 feet
Spans over permanent supports less than 10 feet
Spans over permanent supports 10 to 20 feet
Spans over permanent supports more than 20 feet
* But observe requirements for floor slabs or soffits of joists and beams, if the same forms support them.** Half this interval if forms may be removed without disturbing shores.*** 3 days if forms may be removed without disturbing shores.
These periods are all based on the assumption that there will be no live loads, such as construction traffic, applied at this time. Forms for post-tensioned floor systems are allowed to be stripped as soon as full post-tensioning has been applied, but in multistory work enough shoring should be provided for a sufficient number of floors to develop the capacity to support the imposed loads without excessive strain or deflection. Forms for supported slab systems are subject to similar restrictions on minimum time before removal. Where reshores are required they must be placed as soon after stripping as practicable but not later than the end of the working day in which stripping was done. Reshores put in specifically to permit early stripping and not to carry superimposed loads should be specified by the architect-engineer. In general it is good practice in cool weather to leave forms in place as long as possible so that adequate curing is achieved. If it appears necessary to remove the forms earlier in cool weather than the above guidelines of ACI Committee 347 would permit, the concrete should be strong enough to carry both the dead load and live load stresses to be imposed during construction. It should also be firm enough not to be marred on the surface during form removal. The strength should be determined by test and compared to the structural strength requirements that the design engineer has specified must be reached before stripping. A compressive strength of 500 psi should be considered minimum before concrete can be stripped and exposed to freezing temperatures. Strengths should be determined on samples cured under job conditions.