We have a concrete ornamental fountain that is in some ways similar to a cistern. It sits above grade. I believe that when it was first built 40 years ago, the basin was poured separately from the sides. I assume the original material was concrete, it’s the light brown you can see in some chipped off areas. It was at one time covered with a black tar like substance that didn’t hold water at all. There was never any way to install a liner. I’ve used a latex waterproofer, and that works quite well if reapplied annually.
There must be movement between the sides and basin, because the cove joint opens up. The pictures show two sections of the cove joint opening, the wider (lighter colored) one is approximately 1 inch high and ¾ inch deep. Throughout the years I’ve tried many repairs, ranging from filling the joint with polymer-additive mortar to various urethane flexible sealants (the one I use says it has 35% expansion/contraction and is listed for underwater use). Where the joint hole is deep I’ve used backing rod. The urethane sealants work, but only for a few years, and then break down. For the last repair I covered the sealant with a waterproof butyl tape, and then painted that with the latex waterproofer. That lasted 3 years.
On a related note, I’ve had problems with water infiltration from the coping stones, even though I’ve reset them twice with special chemical resistant thinset (used in pool copings) and with the urethane flexible sealer as the grout. I think water gets down behind the waterproofing in the vertical sides and that likely contributes to the cove joints failing. I do not know how far below grade the side walls go (if at all) and realize there might be some hydrostatic pressure from there, although there is none that I can see in the basin itself.
Questions:
- Am I expecting too much in terms of how long the urethane flexible sealant should hold up?
- Is there other way I should try to repair the cove joint?
- What should I use to affix and grout the coping stones?
