Need help to continue SE MN insanity Tools, Tips and Techniques Discuss Need help to continue SE MN insanity in the Pizza Oven Design and Installation forums; I need some positive reinforcement. I finished ( I hope) my insulating hearth, with is 2 inches of Asian FB ...

[Eric Pfeifer]

I need some positive reinforcement. I finished ( I hope) my insulating hearth, with is 2 inches of Asian FB board set onto an inch of perlcrete. The boards did, as told, absorb water from the perlcrete mixture. They were set in last Thursday...and today (Monday--5 days out) some parts of the boards are a little mushy, and I'm hesitant to start putting hundreds of pounds of bricks on them. Any advice? Wait a little longer to see if they dry out? Replace them?

Thanks in advance,
Eric

[Les]

Eric, I would avoid tossing on the tons of weight until they dry. I did do a test and the board survived - but I would not subject my build to it. Can you lift the boards to give them a bit of air? Where you live is not your friend in respect of humidity (until it is insane cold)...

[Eric Pfeifer]

Thanks Les,

Yes, at 8 pounds a brick I'd be putting close to a ton on there, and at this point I can depress the FB boards 1/4 inch with finger pressure; so, this morning I dragged my basement dehumidifier up to the hearth enclosure and cranked it way down. Hopefully I can pull the water out of those boards by Sat AM, when I hope to set the soldiers. I can't lift them out without really destroying them, I think. They aren't soggy, just damp. We'll see, I may have to go with another plan.

Thanks again,
Eric

[Tscarborough]

Do the calculations as to actual loading and you will be surprised. Assuming a 36" firebrick dome and a 3000 pound load of firebrick, parging, etc, that gives you a floor loading of :

38.25(center line of firebrick for a 36" diameter dome) * 3.1416 = 120.2 (circumference) x 4.5 (width of firebrick) =540.75 Square Inches of bearing surface, more or less.

3000 / 540.75 = 5.54 pounds per square inch compressive weight on your substrate.

Normal concrete has a compressive strength of around 3000 PSI and pink closed cell foam insulation is around 25 PSI so other than shrinkage issues as it dries, no worries.

[Eric Pfeifer]

Many thanks! Your calculation is no doubt closer than my "Fermi" calculation of 10 psi. Your result is even more encouraging. The boards seem to be drying out well, but for a while there they deformed quite a bit under finger pressure. I was worried I had ruined the binder matrix around the fibers, and that the construction would sink. The dry parts of the boards are much, much stiffer. I expect the whole board to shrink a small percentage after some good firings, but this should be constant for the whole structure, and hopefully I wont get any shifts with unwanted stresses.

Again, thanks!