we just put wooden post into the ground and put cement around it a couple of days ago, and we heard that cement could eat the wood, and we did not put plastic on the end of them.
If you sloped the cement away from pole to let water run off ,your grandchildren will see them out, in other words, a bloody long time. I found that after a week the cement seems to shrink slightly around the top ,you squeeze silicone sealer between cement and timber, cause water will rot the pole before cement, but still a bloody long time.....So sit down with a coldie and enjoy your new fence or whatever without a worry.
cement does not eat wood water sits in the hole you cemented and rots the wood