small crack - fill or cut out?

[tapir]

Took the tarp of the oven today after completing an arch last week, and found a crack in the fire cement between bricks (pic below). I'm in two minds whether to get the angle grinder out, cut some of the cement and refill. The arch formwork is still in place, btw.
This afternoon I mixed a little water with the fire cement to create a slurry, and filled in as much as I could, but I'm still unsure whether to see how it goes, or to get cutting. I think that the arch will hold up as is, but am concerned what will happen when I fire up.
Any thoughts? this is the final arch, next step is to drop it down to oven entrance, then cover with a couple of inches of concrete and then 2 inches vermicrete.

Cheers all

edit: image here as it wont show in post barrelvaultoven.files.wordpress.com/2012/05/photo.jpg

[cannyfradock]

Tapir

Can't see any pics......please let us know if your having problems with uploading images.

Without finding your build I would guess that if you are using fire-cement to lay the bricks, the outer perp of the brick is greater that 3-5mm. If this is the case it WILL crack and take an eternity to harden. The homebrew mix of 3:1:1:1 sand, cement, lime and powdered fire clay is the best for larger joints. If you haven't got powdered fire-clay, then just for the extreem outer edges you could scratch out the cracks and fill with a mix of 8:1:1 sand cement and lime......not the ideal mix (best with powdered fire-clay) but for larger joints between bricks fire-cement is not man enough for the job.

Terry

[tapir]

Thanks Terry, that all makes sense, as the perps on the joins are around 10mm at their widest point. oh well..

So the plan is:

• Get the angle grinder out, swap the diamond tipped blade for a general purpose blade - I don't want to be damaging bricks if I'm not too accurate
  
• Cut a channel through the middle of each perp leaving a maximum 5mm amount of fire cement
  
• Make up the homebrew mix (I have all the ingredients thankfully)
  
• Fill the channel with a dry-ish homebrew mix, maybe use an orbital sander to provide vibration to allow the homebrew mix to penetrate and settle
  
• Stick a site lite inside the BVO to assist with warming.
  

As there is a structural risk here, I'm going to try it with the top two courses of firebrick that have the least amount of load, and see how things look.

I'll update this post when I have something to report back

[turkey]

I have quite a few cracks in my home brew, I have been making quite large joints tho. Still seems rock solid tho so I have just back filled them.... Time will tell if this gung ho attitude will bite me, your idea sounds much safer in the long run.

[Tom B]

I've filled the cracks in my render coat with a heat proof silicon - Sika Firesil from B&Q. Makes sense to try something heat proof and flexible. Time will tell if its effective or not - we'll see
I'm going to render over again with another Lime and cement render after its cured.

[tapir]

Thanks Terry, Turkey and TomB for your feedback - all very useful.
So an update; I took the angle grinder to the centre perp on my oven - it was overkill really, as the fire cement was very soft and I didn't so much as 'cut' more 'slice'. Once I'd opened up a disk-width channel, I just used a trowel and a screwdriver to loosen the cement.

What I found alarming was just how soft that stuff was. It took about 30 minutes of digging and cleaning with a screwdriver to get as much as I could out - the remaining cement will dry ok, it's around 5mm wide at the widest point. I found that the very first arch I built last year was pretty much rock solid with cement, and as there had been no evidence of cracking, I left it alone, rather than cut it out.

Using the 3:1:1:1 formula above (sand:cement:lime:fireclay) I knocked up a small batch of homebrew. Had to make it wetter than I wanted as it wasn't going to fill the gaps very well. I initially planned to use an orbital sander to vibrate the homebrew into the gaps, but decided that as I had a structural weakness in my arch now (ie no cement) I wouldn't risk it.
Took a long time to get the gaps filled properly (well, I *think* they're filled properly). I might buy a grout gun for the rest of the perps, but I'd rather not spend £15+ on something I wont have a use for later, so I might just use some disposable icing bags instead.

Rain stopped play so I'll report back next weekend.

I was a bit disheartened at this set back; not so much the cost of the wasted fire cement (less than £25 I guess), but the time I'd spent and the time I now have to spend. However, I thought about the original forum, and how one builder, JerryM would experiment and persevere to get what he wanted. I've never built a WFO before, I'm not a builder, so it's all part of the learning.

[tapir]

Another update; sun came out so I managed to clear and refill another course. Tried to use a disposable piping bag to get into the corners, but it split. Ended up using heavy duty rubber gloves, and pushing the homebrew down with my fingers.

Thinking ahead to the remaining courses, I will have a structural issue to contend with; the lower down the arch I go, the heavier the load on the bricks. This means I can't remove an entire course of fire cement, as I'm sure my arch will collapse. I think the solution will be to remove large volumes, but leave supporting sections. Once the homebrew is dry I'll either remove the columns and replace, or possibly just leave them as they are. Will be tricky removing the fire cements from small space, so I might get a drill on the go to make the initial openings, and then get out what I can.