New dome build in Perth Australia

[MarkJerling]

Hi John

40mPa concrete is very high strength concrete, probably more than you need. 25mPa to 30mPa will be fine, but of course, nothing wrong with going stronger.
Using bags of pre-mix can be an expensive way to pour a slab, so you may wish to use your own mix, to the correct proportions.

As for photos: There are numerous free photo resizing software programs, some even come for free with the operating system of your computer. Simply re-size your photos before upload.
I find that something like 1400x900 pixels works well and is more than large enough to display well here.

Kind regards,
Mark

[Ricey]

Hi Mark,
Thanks for the feedback. I was watching this guy in Melbourne talking about pouring the slab, it’s very informative
https://youtu.be/BpR1CPuvWfQ
I have a cement mixer so mixing the slab wouldn’t be a challenge, what ratio would you recommend?

[MarkJerling]

1:2:3 (Cement, sand, stone) works fine for foundations. For suspended slabs, I'd use 1:1:2 (1 measure of cement, 1 measure of sand and 2 measures of stone). That should give you a minimum of 25MPA concrete. (Ideally, you should measure by weight but it's generally easier mixing by volume, which gives a reasonably close measure anyway. If you can measure by weight, then do so.) 15-19mm stone works best, but you can go larger. If you go larger, the concrete needs to be vibrated better.

[Pizzarotic]

I have a cement mixer

Forget about bagged concrete, that's about $1,000/m³. Most sand & gravel suppler will sell you 1/2m³ of "Builders mix" which has the sand and blue metal already in correct proportion, just add cement. If your mixer is a 2.5cuf then I add 1/2 bucket of water, 6 large shovels of builders mix, 1/2 bag of cement followed by another 6 shovels of builders mix then water to correct slump.
Gives a good 25mpa mix at a cost of about $160/m³ and don't forget the reo. I use f72 mesh.

[Ricey]

Thanks for this tip. I did end up using the bags as I was happy to pay the $100 for the slab, looking back I probably should have taken your advise on the builders mix but it's done now. I'm having trouble locating Clay for the dome here in Perth and have seen others in Perth post here saying they couldn't get it so will probably have to buy 25kg of refractory mortar for $50, which is also not much more than what I would spend buying lime, sand, cement and Clay with all the driving trying to find clay. I will try and post a picture of where I'm at, thanks again

[Ricey]

[Ricey]

also managed to buy some firebricks off gumtree pretty cheap, I was told to stack them up, giving me a 70mm wall thickness, instead of sideways giving 110mm thickness, wondering what people would recommend? As I see most posts here are on their side?

[david s]

No brick courses are adequate at 70mm. Houses are never built with the bricks on their sides and are not allowed to do so for this reason. Having said this, years ago I built a curved internal wall and laid the bricks on their sides. Because of the curve there was enough structural integrity for it to have sufficient strength. A hemisphere has even more although it’s subject to a lot of thermal expansion and contraction. Brick oven builders have reported failures with walls built with splits on their sides (50mm), so basically you’re in uncharted waters.
Thinner walls results in fewer bricks required and lower thermal mass, which results in faster heat up time, but less heat storage. Most cast ovens which don't rely on multiple mortar courses are around 50mm thick, although the last one I built was 75mm thick. The bottom line is that all ovens crack, they all take time to heat, but all work pretty well.

[Ricey]

Thanks David, appreciate your guidance with this build.
I will lay them the right way

[MarkJerling]

also managed to buy some firebricks off gumtree pretty cheap, I was told to stack them up, giving me a 70mm wall thickness, instead of sideways giving 110mm thickness, wondering what people would recommend? As I see most posts here are on their side?

Those look like they have been deliberately made to suit a dome shape, with a "thinner" and a "thicker" side, which will give a 113mm thickness. I can't see why one would want to stack them at the 70mm thickness.

[Ricey]

That's what I was thinking Mark, they wouldn't stack well straight up.
Do these usually come in half sizes or do people have to cut every single brick in half during the build?

[MarkJerling]

That's what I was thinking Mark, they wouldn't stack well straight up.
Do these usually come in half sizes or do people have to cut every single brick in half during the build?

I've never seen half size ones, and only seldom the "wedged" shaped ones as you have used. (That's not to say they don't exist, but I've never seen half size ones.)
As it is, I would, if it were me, cut them in half and 'wedge them so as to avoid having to use a "wedge" of mortar, horizontally, to form the "semi-circle" of each layer of brick. That would require slightly more bricks, but quite a bit less mortar, in my view.

[david s]

Those are arch bricks and should make the building of the dome easier. Cutting the bricks as in the pic using 3 cuts rather than cutting in half first and then the angles for the sides which is 5 cuts, will save you a lot of work. Either hire a wet brick saw or preferably buy a second hand one. That way you can keep it to cut the bricks at your leisure as building the dome will take you more than a couple of days. When you're done you should be able to on sell it for at least what you paid for it, maybe more.

Using those arch bricks for the floor presents more of a problem because you won't end up with a flat floor unless you use a thick (50mm) leveller of 50/50 dry sand and powdered clay. Even so you'd have to chamfer the ends of some of the bricks. It would be easier to source some 50mm flat firebricks for the floor.

[Gulf]

I believe that the "HA75-F" marking represents the alumina content. If so, they will be very difficult to cut, requiring a very good blade or many less expensive blades on a wet saw. That cost can offset any savings on the price of bricks.

As David suggests, you will want to make as few cuts as possible. However, the 3 cut method wont work with chamfered brick as the narrow edge should always be to the inside face of the dome. So, cutting them in a half first will be the only option imo. From there you can just cut what is needed to adjust for the "upside down V" to fit the inside face and fill the rest of the head joints with mortar.

Also, using double chamfered brick will require that the head of the IT to be tilted upward so that the inside face of the firebrick are perpendicular to the center point of the dome floor. I really can't tell from the pic if they are double or single chamfered?

[david s]

If I've done my maths correctly and taking the thinner edge as 55mm and the thicker one as 75mm, you'll end up with a semicircular arch of 556mm in internal diameter, if the bricks were laid dry with no mortar joints. This should take 16 bricks for a semicircle. Maybe stack them dry to check this. Not sure what size oven you plan to build, but that's a pretty small internal diameter.This means that for a larger oven your mortar joints will need to be thicker on the inside than on the outside, which is opposite to the usual situation where the mortar joints are thicker on the outside. Just trust your IT to tell you how to align the bricks. Yes, you will need to cut the bricks in half first so 5 cuts will be required, sorry, my error, thanks for pointing that out Utah.