PoolQ We’re building a pool 25 x 45 ft (7.6 x 14 m) and had a massive cave-in on our deep end. The builder wants to build temporary walls to shoot the shotcrete against, then remove the plywood walls and backfill with gravel. I’m wondering if this will work and if they will be able to remove the plywood without damaging the shotcrete walls. I know with typical forms you would prep the form with oil so the concrete doesn’t stick. Would that be necessary for shotcrete, too? Also, is there a recommended wait time for curing before we backfill?
Shooting shotcrete against a one-sided form (what you called a temporary wall) is a common way to build a shotcrete wall. Once the shotcrete sets and builds strength, the plywood form can easily be stripped off the back of the wall. Form release agents (not oil) can be applied to the plywood to make the stripping easier. Once the forms are removed and the concrete has gained adequate strength, the walls can be backfilled with compacted soil or gravel, depending on the drainage needs.
We recommend 7 days of curing. Continuous water curing is best, but if impractical, applying a curing compound on the exposed surface at twice the manufacturer’s recommended rate for a good seal is acceptable. If they remove the forms before 7 days, they should also water cure or apply curing compound to that newly exposed surface. The shotcrete needs to build up enough strength to resist the external force of the backfill, so check with the pool designer to see what they need for the required strength of the concrete before backfilling. With most good-quality shotcrete materials and placement techniques, you can expect about 4000 psi (28 MPa) compressive strength in 7 days.
