We are currently placing a shotcrete wall in a tunnel. The wall has a minimum thickness of 8.25 in. (210 mm) and is placed against secant piles. Our specs called for a wet cure. To minimize shrinkage cracking, what is the minimum amount of time to allow after shotcrete placement before the wet cure is applied?

There is a difficult balance between wet curing too early or too late. You should not add water too early (before the material sets), as this would increase the water-cement ratio (w/c) of the material on the surface. You also do not want to add water during the finishing process, as this would also work the water into the surface and increase the w/c at the surface. Good practice would be to use an evaporative retardant, which generally also serves as a finishing aid during the finishing process, and then get the wet cure set up as soon as possible.