I just wonder if you cannot fix the poles in the ground without concrete. To me, this material seems rather "definite". Anyway, the pergola looks beautiful! I am thinking about making one for your yard, too.
You could opt to just bury the posts. My thought was that some vines can get rather heavy and I didn't want the arbor to collapse or come apart. I also live where we can get hurricanes each year, so I prefer to have mine securely situated in the ground.
Have fun making yours! Do post photos, as I would love to see how yours turns out!