Q: How long should the formwork for the arch be left in place before it can be safely removed in typical masonry construction? We would like to reuse the formwork as much as possible, and therefore would like to keep this time to a minimum.
A: The length of time that the arch formwork, called centering, should be left in place will depend on the weather conditions during the time the arch is installed. It will also depend on the type of mortar used to construct the arch. Portland cement, lime mortars installed during warm weather will set up very quickly.
The general rule of thumb offered by the Brick Industry Association in Technical Note 31 is that centering should remain in place for at least seven days after construction of the arch.
In cold weather, or with arches that support concentrated loads or loads from other elements such as floor or roof slabs, the centering may need to remain in place as long as 28 days.