As a builder and carpenter for about 45 years I have built a lot of decks. I quit using concrete many years ago for securing deck posts. The posts will shrink and leave a void that fills with water. I have found it is much better using a post hole digger to excavate to a depth below the annual frost depth. I place a half brick in the bottom of the hole that the deck post will sit on. Holding the post level and plumb I then back fill with pea gravel. The gravel is self compacting and will hold the post as tightly as concrete. As the post shrinks the gravel will only continue to hold the post tighter. Water drains thru the gravel and the post does eventually wind up wet on the bottom. Another advantage is you can build on it immediately, not having to wait for concrete to cure.