36" build in heart of Europe, Czech Republic

[mrotter]

OK guys. Gulf 's photo helped me tremendously. Thank you man.

I have cut all arch bricks and mortared some of them, plan to mortar last three bricks tomorrow, maybe on Thursday. My "fifth" course is now finished, think I managed to avoid infamous droop.

Photos tomorrow or on Thursday.

[mrotter]

Gulf
UtahBeehiver
JRPizza

Guys, how hot can get outside area of dome insulation (I plan 6-7 cm of Insulfrax)? I ask because like someone recommended I want to use some enforcing fibres into concrete which will come onto insulation. I plan to use non-metal plastic fibres which seem to have only advantages - they provide STRONGER reinforcement with LOWER (much lower in my area) price. The only problem is temperature. If the concrete might get semi-hot (?) like 100 deg. C, fibres might fail...

[david s]

No, under normal operation the outside shell will be no more than just cosy warm (around 35C). If the insulation contains moisture it will conduct heat and the outside can get hot to the touch, but never so hot that you can’t hold your hand against it (maybe 50C) Any fibres added to the outer render will not be worried by temperature. I used to use the 48mm long plastic fibres, but now use shorter and thinner 19mm AR fibreglass fibres which work better because you don’t get the odd one sticking out that has to be removed.

[mrotter]

DNo, under normal operation the outside shell will be no more than just cosy warm (around 35C). If the insulation contains moisture it will conduct heat and the outside can get hot to the touch, but never so hot that you can’t hold your hand against it (maybe 50C) Any fibres added to the outer render will not be worried by temperature. I used to use the 48mm long plastic fibres, but now use shorter and thinner 19mm AR fibreglass fibres which work better because you don’t get the odd one sticking out that has to be removed.

Wow thanks.

[mrotter]

DNo, under normal operation the outside shell will be no more than just cosy warm (around 35C). If the insulation contains moisture it will conduct heat and the outside can get hot to the touch, but never so hot that you can’t hold your hand against it (maybe 50C) Any fibres added to the outer render will not be worried by temperature. I used to use the 48mm long plastic fibres, but now use shorter and thinner 19mm AR fibreglass fibres which work better because you don’t get the odd one sticking out that has to be removed.

Yes, I found now both types.

Plastic fibres of various length or AR glass fibres of various lengths. Plastic fibres seem to be much easier to get in my area. So you think for example PP plastic fibres 12 mm long will work just OK and not melt? Specs say max. temp is 160C.

[david s]

There's no way it would get anywhere near 160C, as I said 50C max and only if the insulation is wet. The pp fibres usually come in a pack of 48/19 the longer and thicker fibres impart tensile strength while the shorter and thinner fibres, finer than human hair impart no tensile strength, but do help with reducing slump cracks and and early setting shrinkage cracks. You could add both although the really fine fibres require a lot of mixing to disperse properly. The AR glass fibres (mine are 18mm) and somewhere between the thickness of the thick and fine pp fibres.Being fibreglass they have a melting point of around 900C (which I've tested samples of in my kiln to confirm) They disperse well in the mix, good to handle and don't stick out from the surface like the odd long pp fibre does. The downside is that they are more expensive.

[mrotter]

There's no way it would get anywhere near 160C, as I said 50C max and only if the insulation is wet. The pp fibres usually come in a pack of 48/19 the longer and thicker fibres impart tensile strength while the shorter and thinner fibres, finer than human hair impart no tensile strength, but do help with reducing slump cracks and and early setting shrinkage cracks. You could add both although the really fine fibres require a lot of mixing to disperse properly. The AR glass fibres (mine are 18mm) and somewhere between the thickness of the thick and fine pp fibres.Being fibreglass they have a melting point of around 900C (which I've tested samples of in my kiln to confirm) They disperse well in the mix, good to handle and don't stick out from the surface like the odd long pp fibre does. The downside is that they are more expensive.

Thanks, I bought 30 mm plastic fibres, amount just what I would need to use. We will see how it goes.

Some pics.

[mrotter]

OK, finalized arch - Gulf's photo was of massive help and it allowed me to mark my arch bricks and make all cuts nicely. Photos below. The angle of dome bricks is starting to get bigger and bigger.

I know that masonry-wise the work is not top notch but average, but structurally it all seems to go well.

As I mentioned before, I have calcium-aluminate mortar - now I noticed that if you add rather less water to have more "thick" and more buttery consistency it allows for greater initial "suck-in" bond, so that brick does not slide too much. So for everyone using that mortar, be very careful about water. Also, be very careful about direct sunlight in your mortar bucket. It can ruin whole batch pretty fast.

I will probably make one more course which will for 90% completely cover arch and will be the first completely round dome course, then I will revisit my floor, check if I can replace some bad bricks with some with nicer surface and potentially make clay/sand mix bed which I will ley bricks into to make them perfectly level.

I think I managed to avoid "droop" so far, but noticed some smaller "droops" around the dome, but overally the dome courses are basically level (checked with water levelling tool). My droops are like caused by some bricks (which are re-claimed) which are slightly thinner then other bricks etc.

As for arch. I only have 250 mm angle grinder so my tappering is not of perfect quality but I feel that "lock-in" was achieved and amounts of mortar in arch are definitely smaller than in non-tapered arch.

Also, I backfill all big mortar spaces with firebrick scraps - seems to do the job well.

Also, I fought with bricks stagering in some places, feel free to criticise.

Any opinions welcomed.

JRPizza
david s
UtahBeehiver

[UtahBeehiver]

Be aware that you will start to see what is called "inverted V joints on the inside of the dome. It can be corrected fairly easy with your angle grinder by beveling the inside of the dome bricks wear the two bricks touch. You do not need to bevel the entire length of the brick just the first inch or so. Attached is an example of an inverted v from another build.

[mrotter]

Be aware that you will start to see what is called "inverted V joints on the inside of the dome. It can be corrected fairly easy with your angle grinder by beveling the inside of the dome bricks wear the two bricks touch. You do not need to bevel the entire length of the brick just the first inch or so. Attached is an example of an inverted v from another build.

Yes, I pay attention to the and I already applied minor combination of angle/bevel with my angle grinder to the last course I made.

I will try to try stack some bricks to see if V becomes more visible with next row and if it does, I will mount my diamond blade to my miter saw and will try to get more precise bevel cuts as it seems that those will be more and more important from now on.

Actually, from my experience, I needed to bevel not just first inch of the brick in last course, I needed like 2.5" which seems to be influenced by the fact the ID of my dome is relatively small (35-36").

[JRPizza]

You are making nice progress! With the variable size of your bricks keeping the joints staggered is more difficult - just try not to have too many rows align. That said I have a crack in my oven that ran straight through the center of a few bricks so having staggered joints does not mean an oven won't crack in a straight line. If you do set up your saw with a diamond blade try to make some sort of fixture so you can adjust the angle and make consistent cuts - it will save you much time in the long run as when you dial in the angle you need for a row you can cut all the bricks you need without having to keep setting up and checking. My setup was very basic (no hinges, screws etc) but fairly repeatable.

[mrotter]

You are making nice progress! With the variable size of your bricks keeping the joints staggered is more difficult - just try not to have too many rows align. That said I have a crack in my oven that ran straight through the center of a few bricks so having staggered joints does not mean an oven won't crack in a straight line. If you do set up your saw with a diamond blade try to make some sort of fixture so you can adjust the angle and make consistent cuts - it will save you much time in the long run as when you dial in the angle you need for a row you can cut all the bricks you need without having to keep setting up and checking. My setup was very basic (no hinges, screws etc) but fairly repeatable.

Yes, I pay higher attention to brick staggering now. My saw has some holding screws which can be used to set fixed angles in two planes, but it can cut at most 80 mm of thick material, so I couldn't use it for arch, but will use it for dome from now on.

Btw, may I repeat question for floor leveling mix? Is clay really needed? Here I can only get like 30 kg bag which is excesively big. I will likely only use a few kilograms. On the other hand the fine SI sand is available in many amounts, starting from 1 kg for fine prices.

[david s]

Just sand should do, but I think a 50/50 sand clay mix is less likely to move around under the bricks. Some builders apply it wet which will make it set hard and stop any movement, but that introduces moisture under the floor which has to be removed as well as making brick removal difficult should that be needed down the track.
Maybe not much difference, what do others think?

[david s]

Be very careful to avoid brick dust as it is dangerous to inhale. Really, any cutting should only be done with a wet saw which eliminates airborne dust. Dry cutting with an angle grinder or fitting a diamond blade to a drop saw is dangerous for the operator and your neighbours. It should be kept to a bare minimum and wear a decent respirator, not a dust mask.

[mrotter]

Be very careful to avoid brick dust as it is dangerous to inhale. Really, any cutting should only be done with a wet saw which eliminates airborne dust. Dry cutting with an angle grinder or fitting a diamond blade to a drop saw is dangerous for the operator and your neighbours. It should be kept to a bare minimum and wear a decent respirator, not a dust mask.

Thanks for tips. I will buy the same and will test it.