I've recently taken on the task of building a flue gallery and outer decorative arch from a customer who attempted his own solution which failed. My approach is rather different than building in brick units which have three big drawbacks. Firstly the problem of the alignment of vertical brick joints and secondly the thickness of the brick units contributing to a large thermal mass acting as a heat sink drawing valuable heat from the oven. Lastly it is almost impossible to create a smooth compound curve with brick units and therefore a smooth smoke flow. However casting enables all three of these shortcomings to be avoided as the attached pics show. Also the labour involved is considerably reduced. The first stage was to create a mould plug from 8:1 vermiculite and casting plaster. Then use a suitable form to create the inside diameter for the flue gallery casting.So far only 3 hrs labour.

So today I made the first attempt to repair my flue gallery. This time I purchased a single-wall to double-wall pipe coupler. This coupler seals the interior of the two walls and it has a 3" deep and 8" diameter flang on the base which I felt would make an excellent coupler to the flue gallery.

to start I re-filled my gallery with wet sand. Then I centered this coupler on top and filled it with wet sand. Next I cleaned and dampened the already dried concrete-mix I previously molded in place. Using a concrete bonding adhesive, I made a slury of sand, concrete and adhesive per directions on the bottle and spread that around the top and exterior sides of my existing concrete-mix. Then I mixed up more of the casting home brew and used that to build up the flue gallery around the pipe coupler.

My experience at this point is that the wet concrete wanted to slowly slump. After messing around for a while trying to mitigate this without much success, I finally decided to wrap the damp concrete in plastic cling wrap with the hope that a little pressure from the outside could hold everything in place while it slowly dries. Finally, once everything was satisfactory I pulled the coupler out.

My hope is first that I get a reasonable bond between new and existing concrete. I presume if that works then once the material is set up I'll have to do a little work to get the coupler to fit. If it doesn't work, then I'll have to knock this gallery down and redo it.