Cracking in render

[isolec]

i have been having drying fires in my rendered barrel oven for the last 4 days. Pushing the temperature up a little more each day. As the fire goes out i close the door to let the heat dissapate thro the structure. This morning i slowly took it up to about about 180/200. I notice that i have a couple of cracks in the render. Could they be just expansion cracks or could it have been steam. The reason i ask is if they are from steam, once the oven is fully dry i can grind them out and patch the render. However if they are expansion they are most likely to come back and will continually spoil the waterproof integrity of the render. Any thoughts?

[cannyfradock]

May 10, 2013 12:12:02 GMT isolec said:

i have been having drying fires in my rendered barrel oven for the last 4 days. Pushing the temperature up a little more each day. As the fire goes out i close the door to let the heat dissapate thro the structure. This morning i slowly took it up to about about 180/200. I notice that i have a couple of cracks in the render. Could they be just expansion cracks or could it have been steam. The reason i ask is if they are from steam, once the oven is fully dry i can grind them out and patch the render. However if they are expansion they are most likely to come back and will continually spoil the waterproof integrity of the render. Any thoughts?

isolec

I've just had a look back over your build thread to get a bit more info on your build. You posted many (brilliant) pics of the start of your build but I can't see one of the finished oven.

To give advice on your problem can you let us know....if you have actually used the oven yet......what insulation layers went on your vault, and perhaps a picture of the offending article.

The usual culprit is when the render is against a non insulated chimney...cracks very often start from this area.

Terry

[isolec]

Hi Terry

Yes I'm sorry ive been a bit lapse with the postings and pictures. I will try to catch up with the build story this weekend.

Basically over the storage heater block vault, i put 100mm of vermicrete and then 25-30mm of render. The vermicrete had a couple of weeks of drying with a few small fires before the render was installed. The render was a 6/1/1/ sharp sand,opc,lime done last weekend.

You are correct, my chimney is unisulated and one of the cracks runs from it. The other is a horizontal crack running at the point where the vertical render wall curves to go over the dome.

Should i be worried?

Steve

[isolec]

Hi Terry

Should have said, oven not been above 200/250 yet.

Steve

[cannyfradock]

Steve

I can't give you a straight answer as to why the render cracked in the first place....just a few comments......

(don't know what chimney system you used)..I have used a s/s insulated chimney on 2 occasions. Normally they come with a base which stops the outer casing touching the dome. My 2 were from scrapyards (£7 each...with cowl) so I butchered them to fit. The outer sleeve was touching the dome and consequently the render has cracked where it touched the chimney. If I do this again, I will put a barrier (piece of fire-blanket) between render and chimney and fill the gap with fire-cement.

The horizontal crack?....your side walls will have minimum render in the bed joints....likewise the arches of your vault. The "springpoint" however between the 2 has a wedge shaped mortar joint (unless you cut the bricks)....which needs either refractory mortar (homebrew)...or lime in your mortar mix, and it needs to be fully packed. If you used fire cement this would never dry. (it may have done now)...this may be the problem.

this "springing point" will also be the weakest part of your build so if you haven't yet fully fired your oven, then the winter damp may have caused this area to expand the damp/water/ice? and make the outer render crack. This may have settled now and not cause any more problems?.....it's hard to diagnose.

Normally a domestic build...Pompeii or Vault is fire-brick, thermal blanket, vermecrete and render. You have omitted the thermal blanket, but many people also do this and your vermecrete is 100mm thick so this will work, but it may mean it's still stopping any moisture from evaporating away.....it will eventually dry but it may take more gentle firings....I always say that a new oven won't reach it's maximum performance until the 4th or 5th full firing. Then all the moisture is gone and the oven gets to work it's magic.

Apologies that this long answer to your your question is not a remedy......purely food for thought.

Hope you get some other responses of a remedy.

Terry

[bookemdanno]

I agree with Terry about the chimney and can confirm the cracking there.
I too have a single walled flue section, where a slight crack appeared early on, then got worse still on full firings. I've been tracking the Ebay all winter and finally lucked out on a section of twin wall and a new cowl. These'll be part of my re-fit and repair. All experience!

I wonder if you've suffered from steam pressure and the dreaded Vermicrete. 100mm of the stuff will have used gallons of water i'd say. You may get more exaggeration of this crack too.

Get another 200/250 fire going and listen for hissing, i certainly had this, and weepings of moisture too! I have a completely more relaxed attitude to it all now, than when it happened.
If you get the hiss, then theres more water to get out.

You may have to use the good weather to your advantage, and go for broke. Get a string of decent fires going to drive out the moisture and then repair the damage.
As for the chimney, that one will continue, due to different expansion rates of metal and mortar.

On my Pompeii, I'm thinking that i'll mount the new twin wall i have on a quoit of Stove Rope as a thermal gasket to ensure the outer shell doesn't touch the dome and maybe extend a second course of bricks around the base to avoid the need to meet the dome render up to the flue.
I then hope that i have a smaller sacrifical section of pointing where the bricks meet the flue.

[isolec]

Hi Both

Thanks for the comprehensive answer. I feel a lot more reassured now. As you suggest, i am going to keep pushing on with the drying out and repair damage at the end. I will try and post pictures tomorrow

Steve