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Hi, two questions. 1- Do I have to pour the insulating layer while the support layer is still wet, or can I do it in two parts after the support layer has cured?
2- Should I use flashing to isolate the hearth from the stand? I live outside of Boston and we get a lot of temperature fluctuations during the year and I seem to remember seeing this done in the on-line instructions from a few years ago. (I've been dreaming about this for some time)
Answer to the first question is no, you don't have to pour both at the same time...you can pour the hearth, let it dry and then pour the insulating layer. That's how I did it and it seems to work fine. I'm not sure I understand the second question...maybe one of the more experienced guys can chime in on that one...
If I am understanding question #2 correctly, the flashing would act as slip plane, in theory allowing the stand to move independently from the hearth support slab.
I don't think this would be effective, it may help with lateral movement, but does nothing to lessen the effects of frost heave - which is more vertical. The best way to avoid frost heave is a proper foundation - either set a concrete footer below the frost line or, at the very least, set the base slab on a gravel bed over well draining soil. If you have a lot of clay soil and poor drainage, go for the concrete footer below the frost line.
It would be a good idea to check with a local cement/masonry contractor and see how they do it. They will know what is best for the soil/weather conditions in your area.
For what its worth I spent a fair amount of time on some concrete sites and the main take away was keeping the slab damp/wet for up to a week after the pour makes a big difference in the strength (they say 50% stronger) as does letting the slab cure for a month afterwards. My main recommendation is to take your time on this part.
OK, I know about keeping the slab wet. I plan to cover it with plastic to keep the moisture in. But I assumed I could pour the insulating concrete which would keep the first slab sealed and then just keep the second slab wet. Does that make sense? How soon after I pour the insulating slab can I start building the dome? I live in the north east and would like to have it built before Thanksgiving and winter. Am I being too optimistic?
I think you're right on, the insulating layer will keep the concrete layer wet for a good cure. The perlite/vermiculite and portland cement insulating layer will take a good seven days to cure. This is because the perlite/vermiculite mixture absorbs so much water when its being mixed and it takes days to dry out. So I'd say if its nice, warm and dry then things will dry nicely in seven days.
"This is because the perlite/vermiculite mixture absorbs so much water when its being mixed and it takes days to dry out."
Vermiulite concrete and other concretes require much more water that was used in the original mixing to cure. Within 2 days or so, it has used up all the water included in the mixing and has only reached 20-30 % of its potential strength. Concrete should be provided with unlimited access to water when curing for at least 6 days, preferably longer. A practical limit to its strength is reached only after 21 days or so.
Concrete does not "dry" it cures. - if you allowed to become prematurely dry, you will halt the formation of the microscopic crystals that give concrete its strength. The formation of these crystals will not fully restart if you subsequently add moisture to the dried concrete.
If you are using plasitic, make sure to add water under the plasic from time to time.
Some of this might seem a bit pedantic, but I believe we should try to use the correct technical terminology to avoid confusion.