 Setting a fence post, a mailbox, deck footing, or even a small slab is quick and easy with Quickrete Fast Setting Concrete Mix in the red bag. There's no mixing needed when setting posts. Just pour the dry mix into the hole and soak with water. Quickrete Fast Setting Concrete in the red bag sets hard in 20 to 40 minutes. It's that quick and that easy. You'll find Quickrete Fast Setting Concrete Mix at home improvement stores. Look for it in the red bag.