We have built a 9 1/2-inch thick concrete roof spanning 18 feet over a garage, using a 6 1/2-bag mix at 2- to 3-inch slump. The concrete is now 8 days old. How soon would it be safe to strip the forms?
This can be a critical matter that ought to be decided in consultation with the engineer who designed the slab. It is likely, however, that the forms have already been in place long enough for safety, according to ACI 347, "Recommended Practice for Concrete Formwork," Section 18.104.22.168. According to this document, for floor slabs with 10 to 20 feet clear span between supports, and for which the live load is less than the dead load, a recommendation is given for use when field operations are not controlled by specifications: forms and supports should be left in place under ordinary conditions for not less than 7 days. Section 22.214.171.124 also notes that the temperatures should have been above 50 degrees F during this 7-day period. A total of 7 days at 50 degrees F, not necessarily consecutive, meets the requirement.