 Setting a fence post, a mailbox, deck footing, or even a small slab is quick and easy with Quickrete's 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's 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's fast-setting concrete mix at home improvement stores. Look for it in the red bag.