Expansion question... is it a problem?

[Wiley]

Hi all,
I have my wall stacked and thought I was ready to infill the cores. Now I think not. Here's the question: The plans do not have any rebar connecting the walls and the slab supporting the hearth. I expect this is due to an allowance for expansion of the slab as it heats. It seems to me this is not an issue as several people have finished their WFO with stucco and none have reported cracking along this join line.

Has anyone experienced any movement between the walls and the slab?

MY concern is that I live in an earthquake designated area. And here I'm building this WFO on a slab (the total of which will weigh several hundred pounds) and it won't be tied to the support base. I'm expecting some will say this is a non-issue as the oven itself won't be tied to the slab and if it wants to move then it will part company there. This may be true. I have some ideas on addressing that issue but right now I would like to know if anyone has had movement between the slab and the walls. Many thanks in advance.

Oh and yes, I have looked at Alan Scott's plans. He addresses the expansion due to heat issue by supporting the WFO on rebar pegs that stick out from the support slab into the surrounding support walls. He has an air gap and no connection other than gravity. Any sizeable movement during an earthquake and I suspect the oven is out thru the wall (IMHO).

Again many thanks,
Wiley

[Frances]

Re: Expansion question... is it a problem?

Expansion shouldn't be an issue, because you'll have plenty of insulation between the slab and the oven... if the hearth slab starts getting hot enough to expand you'll have got something seriously wrong!

I think Alan Scott's oven has no insulation underneath, which would explain the difference. Doesn't it?

Earthquakes... dunno. ButI thought one of the issues with earthquake proof building that you make it flexible rather than fixed... so it will go with the sway rather than breaking up.

But I haven't had any movement between the slab and the walls.

[asudavew]

Re: Expansion question... is it a problem?

I'm not sure if I follow you.

But, if you are talking about running rebar up through the cells and tying them in with rebar in the hearth slab.. You will find that most builders do exactly that.

Here is a picture of mine, notice the bent rebar exiting from the filled cell.

As for heating the hearth slab... Frances is right.

Also, there is no sense in wasting firewood and time heating the hearth slab up. That's why the insulating layer goes on top.

Also , your oven will weigh a few tons.... not a few hundred pounds. So plan accordingly.

Hope this helps.

Dave

[DrakeRemoray]

Re: Expansion question... is it a problem?

I did not connect my slab to the walls, thinking about the expansion issue...no earthquakes in Denver...at least not frequent ones...looking it up, it appears we are due

BUT I think Alan Scott's book does mention tying the slab into the walls in earthquake prone areas.

I think I would go ahead and put some rebar up into the slab.

Drake

[Wiley]

Re: Expansion question... is it a problem?

Frances and Dave, thanks for responding. I agree and thought I would ask because in the directions the drawings only have the rebar only in the cells and there is no reference to tying the slab to the support walls.

Regarding the expansion: I don't remember the exact post but it was mentioned by someone that their slab only got warm after many hours of firing. That seemed reasonable to me, I am a strong believer in insulation.

Also, Due to my wanting to have my stand in an "H" pattern I have a problem with dry stacking. The problem is that I cannot stack them in any way that does not have three breaks (ends of blocks butting) vertically in some location. With dry stacking any place where two blocks butt is the same as a crack in a wall, to have three in a vertical line (on a four block high wall) is basically a broken wall. I am going to unstack and restack gluing the blocks together with construction adhesive designed for such. I have done this in the past with the stackable garden wall blocks where we wanted the wall to be vertical not stepped back like they are designed. We broke of the lip on the bottom so they would stack vertically and in over five years have had no problems. We even tried to undo a section to make a modification and gave up-- that glue really holds. Odd the things we learn and then forget.

Frances, you are right about wanting limber and flexible construction in earthquake country. Part of my youth was in California near Loma Prieta, where that large quake was back in 1989. My parents still lived there at the time of the quake. At the same time, my wife and I were were just starting construction of our home. After viewing what did and didn't hold up in that quake we redesigned significant portions of our home. That being said, cement block and flexible are (I think) mutually exclusive terms :-) So for this construction I can only make it as strong as I can.

Wiley

[egalecki]

Re: Expansion question... is it a problem?

I have rebar fixing my block walls to the base slab, and I have rebar tying my hearth slab to my walls. Sort of like pegs sticking up out of the block cores and into, but not through, the hearth. We used the hammer drill to make the holes in the base slab for the rebar to fit in before we filled the cores.

Not much in the way of earthquakes here in SW Va, but we didn't want it going anywhere anyway...

[Dino_Pizza]

Re: Expansion question... is it a problem?

Hi Wiley,
I live in Northridge and lived thru a big quake and watched every foot of concrete block wall around my home crumble to the base. Nice way to meet your rear-yard neighbors! But, like the Pompeii plans show, filling every other cell with concrete and rebar does the trick. It's how Los Angeles changed the codes for all concrete block construction and it will hold up very well.
HOWEVER, I'm glad you brought the subject up cuz if you were worried about the stand, I've been sweatin' bullets every time a post says you build the heavy brick oven and it just sits on the insulation board leveled with brick dust and it doesn't move! I do believe them (I've yet to build mine) and they are experts in my mind but when I build mine I'm thinking of:
1. Side support or putting an angle iron against the arch wall and tap-cons into the oven floor (with a piece of insulation) like Ken524 did in "Vent & Chimney" photos about page 8.
2. Spread a 1/2" of high heat mortar over the brick dome. This adds mass (which I don't really want) but I hope it adds some strength too.
3. Do the heavy chicken wire over insulating blankets over the dome and again, attach it to something popping out of the oven floor slab.
Again, having not built one I'm ready to find out how silly I'm being. Besides, an earthquake catastrophe would you give you a change to rebuild it with all those little things you've learned along the way for an even better oven.
Enjoy the process,
Dean

[asudavew]

Re: Expansion question... is it a problem?

2. Spread a 1/2" of high heat mortar over the brick dome. This adds mass (which I don't really want) but I hope it adds some strength too.

Hi Dean,

I've been thinking about a mobile oven, where shaking and bouncing would be a problem.
I would do the same, but why not add a nice fine wire mesh (small hole chicken wire) to the 1/2 inch of mortar. I think that would help to. It would be like rebar in concrete.


just a thought,

dave

[mfiore]

Re: Expansion question... is it a problem?

Also, Due to my wanting to have my stand in an "H" pattern I have a problem with dry stacking. The problem is that I cannot stack them in any way that does not have three breaks (ends of blocks butting) vertically in some location. With dry stacking any place where two blocks butt is the same as a crack in a wall, to have three in a vertical line (on a four block high wall) is basically a broken wall.

Wiley

I am also building an "H" pattern hearth stand. I've encountered the same dilemna, how to tie in the middle bar with only four courses of block. I went as far as using my son's legos (they actually work pretty well, and are much lighter than concrete) to try different approaches. In the end, I'm not sure it matters. There are ovens with support only on the sides, open from both ends in the middle. I think the middle part will be a free standing wall, holding up the concrete slab. Does it need to be tied into the rest?

[christo]

Re: Expansion question... is it a problem?

I have a center wall in my block base and did not tie it in to the rest of the base, per se.... The hearth pour tied into the wall only.

I used fiber reinforced wall bonding cement to coat my entire base - inside and out. I have no cracks on the inner wall and it has been about 9 months since I coated.

I plan to finish stucco a little later this summer.... too much to do at work.

Christo

[Dino_Pizza]

Re: Expansion question... is it a problem?

Hey Dave (asudavew),
Good idea putting the fine wire mesh around the dome for stability for a mobil oven or a mother-nature-earthquake-shake oven. I'll look thru other builds/posts to see if others have done it. If the metal mesh expands differently from the morter, cracking shouldn't be a problem cuz you can't see it anyway.
Thanks, Dino

[CajunKnight]

Re: Expansion question... is it a problem?

Chicken wire mesh and concrete doesn't last. Its good for the initial cure to prevent cracks but will rust away sooner than later. If you are worried about structural integrity then consider fiber reinforcement. It doesnt burn and it doesnt rust. Since you are going to cover this layer anyway it wont matter what it looks like, The fiber reinforced looks furry when finished.

No fiber in the joints just in the shell cover.

[Dino_Pizza]

Re: Expansion question... is it a problem?

Fiber sounds like a winning idea. Do you know if it's loose and mixed in with the high-temp mortar or is it in short width rolls that you lay strips of it on to the shell?
Thanks, Dino

[CajunKnight]

Re: Expansion question... is it a problem?

you can get it premixed or loose. Just make sure its NOT cellulose type. I have heard of people using fiberglass cloth or matting layer in concrete. First layer of mortar then cloth soaked in a slurry mix and applied then more mortar on top. You will have to look and see whats available in your area.

Now keep in mind that if you have a quake strong enough to damage this oven, you probably wont have a house to live in either. As far as making it quake proof I think it would be best to leave it as the design goes. The tougher you make it the harder it will fall. The idea behind loose joints gives flexibility in the structure. Structure able to flex makes it stronger. If the oven falls apart it will give you a reason to build it again. Besides by the time this happens you will have a thousand ideas on what you would have done differently anyway.