Q.: We recently placed concrete walls in metal forms during a period when daily temperatures were around 50° F, but evening temperatures dropped to 25° F. The day after placing concrete, my foreman checked the concrete at the top of the form and found it was still green. How soon can we strip the wall forms?
A.: Guide to Formwork for Concrete (ACI 347R-94) offers some guidance on form stripping time. When air surrounding the concrete is at 50° F for a total of 12 hours (not necessarily consecutive hours), wall forms can usually be stripped. If high-early-strength concrete has been used, this 12-hour period may be reduced as approved by the engineer-architect. However, where wall forms also support formwork for slab or beam soffits, the removal times of that formwork should govern.
Consider several factors in deciding when to strip forms. After stripping, there should be no evidence of wall distortion or damage to the concrete surface caused by pry bars, wood shims, or other stripping tools. You must also protect the wall concrete from damage caused by freezing, both before and after stripping. You can do this by covering the forms with insulating blankets immediately after concrete placement, and covering the walls with such blankets after the forms have been stripped. Also, check specifications to see if additional moisture-retaining measures are required after the forms have been stripped.