The long lasting reliability of trapped air is sketchy, but 1 to 2 foot of air trapped in the top of the PVC pipe should keep it upright given that the rest of the structure is made of plastic tubing that has a specific gravity closer to 1 and not trying to drag it all down. PVC is one of the heavier plastics with a SG of 1.4 so it wants to sink. Most other tubing plastic's SG are much closer to 1 which is what water is. These plastics may float, sink slowly, or merely hover depending on what side of a SG of 1 they fall. With all that mumbo-jumbo said...it would not take much tipping of a rather full bucket base of concrete to take it all down with trapped air being its only means of staying upright. I would suggest forgoing the trapped air and putting some scrap pipe/rebar/angle/whatever (approx 4' long each) through the bottom of the bucket to make and "X" before pouring in the concrete. This "X" will give the whole structure some stability and keep it upright without the trapped air.

Fish on!,
Noel