cracks in vermicrete shell

[jaymac2087]

Hello All.
Should I be concerned about cracks in the vermicrete shell?
I have about 3-4" of blanket underneath and about a 1/2" of heatstop under that. The crack is pretty big but seems solid. Will the sbc fill these cracks?
Any thoughts would be great.

[Tscarborough]

Re: cracks in vermicrete shell

I would be more concerned if it DIDN'T crack. No worries!

[jaymac2087]

Re: cracks in vermicrete shell

TSCARBOROUGH,
Thanks for the reply. Should I cover the cracks with heatstop or just cover them with the surface bonding cement? Also how long after I applied the vermicrete to the dome do I have to wait before I can start my curing fires? Then when should I apply the surface bonding cement?
Thanks again.

[Tscarborough]

Re: cracks in vermicrete shell

The SBC will fill them, but if they are bigger than a 1/4" I would mix up more of the insulating mix if you have any left. If not, I would fill them with SBC BEFORE coating the structure, let them dry then coat with SBC, the idea being that you want that to be part of the insulating layer, not the SBC shell to prevent the cracks from mirroring through the shell.

[jaymac2087]

Re: cracks in vermicrete shell

ok, I will fill them with sbc then when dry I will cover. When do you think I can start curing?

[Tscarborough]

Re: cracks in vermicrete shell

I would cure before I did the finish coat so that any cracking would take place before the final coat(s), as well as allow moisture to exit easier.

[ThisOldGarageNJ]

Re: cracks in vermicrete shell

Hey j
I cured before the SBC, I waited about 3 weeks before firing anything, I dont know that waiting that long was necessary, But im sure it didnt hurt.. Some builders cure their dome before the insulation step... All I can add is I still to date dont have any cracks that I can see,, After the week of curing fires, I coated the vermicrete with SBC and it worked out welll with one exception.. I used the liquid colorant, It faded after less than a year... I recently gave my oven a facelift and had extra SBC and recoated the dome... scar has recommended using the "Iron Oxide" I think "is that right scar" Kebwi is now using that in his build and I understand it lasts way beyond the liquid so I will defer to him on that...

Also want to mention, when my dome was only vermicrete, I kept it covered if I thought it was going to rain..
The pic is when the dome was only vermicrete

Cheers
Mark

[Tscarborough]

Re: cracks in vermicrete shell

Yes iron oxide powder. Blues and Greens and very dark reds will still fade eventually, but not like the liquid color. Iron oxides are cheaper too, although you will probably have to buy more than you need.

[jaymac2087]

Re: cracks in vermicrete shell

Thank you guys for the info. I think I will wait two weeks before I start the curing fires. Also thanks for the info on the color additive. Any suggestions for a good curing schedule?

[Tscarborough]

Re: cracks in vermicrete shell

Ya'll have more patience than I do. I lit fires the day the arch was finished, even before there were ends on the oven (It was cold!)

[ThisOldGarageNJ]

Re: cracks in vermicrete shell

Hey J,
You know you can throw almost anything in your oven from a single light bulb to a halogen to generate mild heat to help the oven pre-cure before you get to the fires...Cheers
Mark

[Tscarborough]

Re: cracks in vermicrete shell

You have 2 goals working against themselves. First, cementious mortar needs moisture to cure, second you are trying to remove moisture to cure the oven. Mortar cures on a logarithmic scale, not a linear one. All other things being neutral, the mortar is at better than 50% ultimate strength in 24 hours, and better than 75% in 36. At 28 days it finally reaches 90%+, and in a couple hundred years it may reach ultimate strength, provided there is any moisture available.

Ideally, you would keep the entire oven under water for 7 days, then begin the drying process. In reality, after 48 hours, you can begin drying it out with gusto.

[david s]

Re: cracks in vermicrete shell

The standard is to keep it moist (wrapped in plastic) for one week. When you get to this stage it starts to get exciting and it is tempting to hurry things along. Better to take it slow and safe.

[jaymac2087]

Re: cracks in vermicrete shell

Thank you everyone for the replies. I have been keeping it under tarps at night and during the bad weather. Tomorrow I will put a space heater inside for the next week then this weekend I will bgin my curing fires. That will be two weeks of curing for the vermicrete shell. I have another question, can I use regular mortar to even out the shape of the dome? I have a couple areas that could use some filling. I have a bag of Type S left and was going to use that. Also how long after the curing process do I have to wait before I cover w/ SBC ?
Thanks guys.