Q.: The concrete subcontractor on our job is placing grade beams that are 8 inches wide and 40 inches deep. A nonchloride accelerator has been added to the mix, and the subcontractor wants to strip the forms four hours after concrete placement so workers can reset the forms for the next day's pour. Is there any problem with stripping forms this early?
A.: ACI 347R-94, Guide to Formwork for Concrete, suggests that when the engineer/architect doesn't specify a minimum stripping strength, sides of beams can be stripped after 12 hours under ordinary conditions. This time represents the cumulative number of hours, not necessarily consecutive, during which air temperature around the concrete exceeds 50°F. However, the document also says this period can be reduced as approved by the engineer if high-early-strength concrete is used.
Because the concrete subcontractor is using an accelerator, it appears reasonable to strip the forms in four hours if this can be done without damaging the concrete surface. However, you should have the engineer approve this and recommend the proper time to backfill.