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Fairview WFO 05-09-2009 03:38 AM

Crack Control and fiber addmixtures
 
I noticed the Pompeii instructions say you can add a layer of mortar to the outside of the dome to increase the thermal mass, for those so inclined. Has anyone done this with the addition of fiber reinforcement to that mix to aid in crack control? It certainly helps with concrete shinkage cracking and meets the temperature steel reinforcing requirements in those applications.

Scott

MK1 05-09-2009 07:51 PM

Re: Crack Control and fiber addmixtures
 
Stainless steel needles. $

Mark

david s 05-09-2009 09:05 PM

Re: Crack Control and fiber addmixtures
 
Yeah I use a few different fibers. First is the stainless steel needles for high temp application. They are not actually needles but more of a flat bar with ripples for grip and about 2mm wide by 40 mm long, although I guess there are other sizes. They don't present much problem in application. The only drawback is that you will probably have to buy way more than you need and stainless doesn't come cheap.
The second kind of fiber I use is a short and extremely fine polyproelene fiber that melts at 160 C This is no good for high temp stuff if you want it for reinforcing, but if used in high temp application the fibers burn away and leave minute pipes where the water can escape. This was developed as an addition to make concrete buildings fire proof. The third fibre i use is a longer plastic one that is used as a replacement to steel reinforcing in structural concrete. Our local council uses it for all the concrete footpaths and I use it for the outer concete shell. It is far simpler and quicker the chicken mesh over a spherical surface.

david s 05-09-2009 09:10 PM

Re: Crack Control and fiber addmixtures
 
Thx1138 used some sisal rope fibers as reinforcing. That should work but don't know how it went. You could easily chop short lengths with a tomahawk, but make sure ou mix it in really well to distribute the fibers evenly.

wlively 05-10-2009 11:37 AM

Re: Crack Control and fiber addmixtures
 
I bought my stainless needles by the lb, less than $10.

Fairview WFO 05-11-2009 06:43 AM

Re: Crack Control and fiber addmixtures
 
I would think fiberglass fibers would stand the heat? Wade, where did you source the stainless needles by the pound?

Cypress 05-11-2009 07:30 AM

Re: Crack Control and fiber addmixtures
 
Where did you get your needles? Wlively

david s 05-11-2009 01:48 PM

Re: Crack Control and fiber addmixtures
 
Glass has a melting point around 900 C which is way out of our range, but ordinary glass fibers react with the material. You need to get alkaline resistant fiberglass.

Fairview WFO 05-11-2009 03:09 PM

Re: Crack Control and fiber addmixtures
 
Thanks David, I've found a supplier of Alkali resistant fibers and I can get relitively cheep ($32) delivered for 4 - 1# bags (way more than I need). I think this would be the preferable way to go from a cost and workability standpoint.

What I really want to know if this is a good idea, or does it cause more problems?

Ordinarily on any concrete, stucco, or motar work its alway a benifit to add fiber reinforcement. But with this high temp application I wonder if everything will move together and still be contained?

Is the cooefficient of thermal expansion of the glass fibers the same as the firebrick and mortar? Does this really matter?

Just trying to work it arround in my head....cost isn't much and I only want to do this once....not that it hasn't been fun so far. (I'm actually on the last row now and its not fun at this point!)

For those who have done this, did it work out ok?

david s 05-12-2009 01:35 AM

Re: Crack Control and fiber addmixtures
 
I've had trouble with the outside cracking, but since going to fibres, problems gone. There is considerable expansion with the heat and steam pressure build up. A render mixture that has lime in it is apparently more elastic. Try a 4:1:1 sand ,cement, lime mix for the outer shell, only needs to be thin, after all it's only a weather coating.


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