Newby questions

[nktshane]

Hey guys, been following this forum for some time for tips but I've come across a problem during my build.
I've recently finished my first course and archway, but the mortar is cracking almost immediately, it seems to be ok for the soldier course, but ive taken down the arch as it didnt really dry overnight, was very damp, cracked and offered pretty much no strength.
I followed the forno bravo recipe of 10:6:3:2 and I'm using a solid engineering brick rather than fire bricks. Is there a better combination I could use?
Thanks for any help, it's much appreciated.

[bookemdanno]

To help with the definition of "cracking" and what's happening, can you drop in some photos please?

[nktshane]

Where the mortar meets the brick its cracking and coming away, with the odd horizontal crack across the join, I changed my mic up today, added slightly more cement and less clay, it seems to be a little better, but does this mix always shrink like this?

[jms]

The formula that most people use at FB is 3:1:1:1, Sand:Cement:Hydrated Lime:Fireclay. With enough water to make it like peanut-butter.

Are you soaking your bricks beforehand?

[nktshane]

I haven't soaked the bricks before hand.
I'm using engineering bricks and a builder friend of mine suggested they will take on very little water, creating a film, meaning the mortar wouldn't bond as well.
I tested the theory and it seemed to ring true.
As for the mix, i used a 10:3:3:2 (2 of lime), which is pretty close to the mix you guys have suggested with much better results! Thanks for the help!
On another subject how do i best build a chimney into the front face of the dome (above the arch) , i originally planned to have a longer entrance to make my life a little easier with this part of the build, but space was an issue.

[bookemdanno]

You're right, don't soak Engineering bricks.
I think that the mix you were using was just too rich for the bricks, and it was drawing back from the face of the brick as it dried out. There's a lot of binders there which are all trying to utilise only so much water. Hopefully the bricks were still held in place?
If the 10:3:3:2 is working better, then stick with it. I think the FB 3:1:1:1 is only really suited to firebricks, which are made up of different materials to "normal" bricks. You'd never use a mortar mix that rich for normal brickwork. Well, maybe if you're building a railway bridge!
A brick and a half arch is usually the way to do it, so it leaves a half brick protruding in front of the chimney. Check out a few builds, and you'll soon get it straight in your mind.

C'mon...where's the piccies?

Good luck!

[nktshane]

I'll put some pictures up tonight, been too busy with the build. I only started Tuesday and I've only got the top couple of courses till the dome is finished :/, it's an addictive game this pizza oven building!

[bookemdanno]

It sure is!

[cannyfradock]

Hello Shane.....welcome to the forum.

I also think that your mix is way too rich. I also found from trial and error that the more common formula used by the FB forum of 3:1:1:1 sand, cement, hydrated lime and powdered fire-clay is also too rich. I now use 5 parts (gritty sand) or 6 parts (fine sand) to 1:1:1.

Your class b engineering bricks should be fine and as they are a dense clay shouldn't spall under intense heat....and as danno mentioned ....there's no need to soak them.

The vent for the chimney is best created in your outer arch by replacing the middle full bricks of the arch with....halves or quarter bricks. This is a not very good example but it may give you an idea how we create the hole....



Again.....as mentioned....have a look through other peoples builds for a better idea.

terry

[nktshane]

sorry about the wait guys, anyway, heres a few pics of the build, go easy on me









hopefully ill get big red insulated by this weekend once ive fininshed my curing fires

[cannyfradock]

Shane

Your build is coming along just fine. If you have the time it's always a good idea to start your curing fires before the other insulation layers go over the dome......you can still carry on with the build though, but it may take a few extra curing fires if you build the whole thing first.

No need to apologise about the wait......post as often or as seldom as you want......when we get new posts, they're always welcome.

Terry

[nktshane]

More questions... Sorry,
So I did the insulation and vermiculite later last Saturday (what a pain that was), and was going to render it this weekend. But the wind blew my cover off last night and it's had a good bashing by the elements. What's the best way to proceed? Could I still render it this weekend?
Also, what render mix does everyone favour? I was going to do a traditional 2 skin with a flexible scratch coat and waterproof top coat, but surely waterproof would trap moisture In the oven?
Thanks again.

[bookemdanno]

Hi there!

You're right, the waterproofed layer can trap moisture within the oven.
Best try to rig up a new cover, and as Terry mentioned get some heat into the oven now.
If you trap the water, it gets converted to steam, pressurises and blows out through the render, usually cracking it.
Small kindling fires, gradually building over a few days. Two weeks is probably good, if its had a nights rainfall onto it.
I know you want to finish, but you can use the oven and the heat you put into it for cooking still about half way through these drying and seasoning fires.
The waterproofed layer usually goes on first with rendering, as this helps to stop the top coat from drying out too quickly and cracking, but that's the usual method. Something like a 5:1:1 mix of plastering sand (or 50:50 mix of soft and sharp sand) cement and hydrated lime with waterproofer added as per manufacturers details, which gets keyed or scratched, and then a 6:1:1 mix over that will do well. You can add a dash of plasticiser to this mix, but its not that necessary if you add the lime and mix well.
You've done amazingly well so far, but don't rush it now.

Hope that helps.