Believe it or not, I saw a tree ring several years ago almost exactly like what you are describing and was so impressed, I asked how they had built it. They had constructed a base about 6" deep around the perimeter by digging out a hole in the ground out to the outer finish diameter , centered the galvanized pipe, then poured quikrete to fill the outer section just below grade. Once that had set (an hour) they simply built up a "rock" around it by hand using a very dry lightweight concrete mix with a lot of fiber added (helped it stay in place easier). You could save a lot of concrete by building up a spray foam core then covering that with your mix as long as you allow for 3-4" of concrete for strength. I also noticed they had formed some lath over the top edge and covered the pipe's upper edge down below the tree's soil level so that it did not show at all once the dirt was in place. The final finish was a mix of dry "shake-on" concrete colors and a little acid color stippled on with a stiff brush. Very well done and virtually identical to real rock (they went for a speckled, reddish granite). As I recall the only negative was that it held a bit too much water because the soil below it was high in clay. The "fix" would have been to simply amend the soil below or dig it out and place rock & gravel for drainage.

Best of luck, I just wish I'd had a camera with me back then...it was really great looking.

Hello Snowdog and welcome to the forum.
Your idea sounds very appealing. I love donuts! :)
Tango has a great idea with building up the center part. Tango, didn't you have a thread telling how to paint tufa to look like granite? I tried to find it but couldn't. It was very informative.
Snowdog, you could form a framework of chicken wire and fill it with styrofoam, packing peanuts, anything cheap...rocks, tin cans, your kid's old toys, stale bread??, what ever you come up with. When you tufa the form, you can do it in stages. It's ok if one stage cures before the next. Just wet the dry parts and brush on an admix and it will be fine. I hope you take pictures for us. We would love to see your progress.

Jo

Ok thanks for the input. Tango88 glad to hear this idea has been tried with good results. I'm mulling that idea of a below ground form. Not sure I completely understand this. After curing was the tree ring then lifted up to sit above ground? Wouldn't that be quite heavy! I love the idea of a rollover top edge to cover the pipe lip. I really wish I could see pics of the proccess. I still have to read up on the coloring steps. So far I've been occupied with the structure and form issues.

Gottatufa when you say working in stages is fine do you mean that in a layering way or more like sections around the pipe can be done at different times? Wouldn't there be craking or pulling apart issues? If it is not a probelm to work in sections I could see making a small form and treating the planter like an Orange and building a wedge or slice at a time. Hmmmm I'll keep mulling this over. My next step is to break up the ground where the planter will sit so that I don't have drainage issues and then get the pipe in position. I don't want to move it at all once I've applied the tufa!
I will start taking pictures once I'm making forward progress and see if I can figure out how to post them here.

The below grade work was only intended to provide a stable base to build the "rock" on top of. My guess is they put a little sand on bottom for drainage under the solid base. Once poured, it stays put and you just build up on top of it.

The drainage issue is a good one that Tango brought up. He is the master. Sand would be excellant for drainage like he says, or even peagravel. You can build in layers at different times. You know how tufa behaves with molds and how we have to use releases? Well think about that when building on layers of tufa. You know it is going to stick!

Jo

Welcome to the mudhole, snowdog1! It's always good to have another Tufahead, hanging around. Here's a link to one of our very favorite Tufaheads, (Marly/Buddyfly) posts from last year. I thought you might find it very interesting.
Make sure you scroll down the page to see more photos.

Here is a link that might be useful: Marly's Well

Wow Puddle of Mud that is a very helpful link indeed. Looks like Marly's well was a great project and it turned out so well indeed (pun intended). I think I can apply a similar technique to my tree planter pipe project!

Thank you.