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Old 09-02-2009, 10:28 AM
kebwi's Avatar
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Default Drilling shallow mortar pockets into bricks

See attached image. Is there any utility in doing this? The argument, obviously, is that as a form of "liquid rock", the mortar conforms to the cavity into which it is placed, then solidifies into a "tab" that can no longer slide out of its "slot" (and vs/va that the slot, aka the brick, cannot slide relative to the solidified tab, aka, the mortar).

I realize that 99% of an arch's strength comes from the transfer of force between bricks, but that only works if the bricks are very precisely wedge-shaped in the first place. Might this approach help in situations where bricks are not precisely shaped?...Especially in the top section of the dome where bricks are approaching vertical and the success of the wedge shape and "arch-effect" relies heavily on precise brick shape. Could this help, even a little bit?

Is it a waste of time in that it can't help, but doesn't hurt?

Or is it a fundamentally bad idea in that it is demonstrably worse than the simpler and more conventional approach?

As an addendum, one could imagine drilling small shallow holes (1/4" diameter, 1/2" deep) instead of large pockets and inserting small metal pins such that affected brick to brick faces are joined by a shared pin. It's just another variant on the same idea. If the holes were larger than the pins, it would allow for differences in thermal expansion between the brick and metal (although would mitigate any advantage *except* when one brick started to slide in which case the pin would catch the brick after 1/8" give or take). One major difficulty of the pin idea (which the mortar pocket idea does *not* suffer from) is precise placement of the holes such that bricks line up properly during construction.

I'm just firing ideas off here. Feel free to tell me they're crazy.

Thoughts?
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Drilling shallow mortar pockets into bricks-brick_inset.jpg  
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  #2  
Old 09-02-2009, 10:46 AM
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Default Re: Drilling shallow mortar pockets into bricks

Metal pins will have expansion problems in heat. If they are tight enough to do any good, they could crack the brick. The dimple in the brick is something that brick makers have been doing since the 19th century.

It can't hurt and it might help.
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Old 09-02-2009, 10:46 AM
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Default Re: Drilling shallow mortar pockets into bricks

Sounds reasonable to me. For my arch bricks, I used the angle grinder and ground 3-4 horizontal "grooves" on the faces of the brick that would accept the mortar. They are not seen. My thought was just as yours, the mortar would fill in the groove and "grip" the brick better. Does it make a difference? Who knows.
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Old 09-02-2009, 10:48 AM
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Default Re: Drilling shallow mortar pockets into bricks

Oops, I didn't read the entire post. I would also probably avoid metal pins. The mortar will bind tightly.
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Old 09-02-2009, 10:54 AM
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Default Re: Drilling shallow mortar pockets into bricks

Cool, I'm hearing mild support for the idea. I like the grinder-groove suggestion. I was unsure *how* I would actually create the pockets, i.e., what sort of drill bit would make a large shallow impression. The grinder would be perfect.

Rats, I was kind of hoping to get away without buying a grinder for this project. I already need a table saw and tile saw. Grinders are tons of fun though. I used them all the time when I set routes in a rock-climbing gym. We would use them to "saw" through bolts that locked up and stripped, thus preventing their removal from the wall...which happened a lot (climbing gyms are very dusty, aka, chalky, so the bolts were always locking up on us).

Sigh, the tools alone are going to set me back on this project. Not a problem for folks who already have a decent shop set up.
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Old 09-02-2009, 01:17 PM
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Default Re: Drilling shallow mortar pockets into bricks

Quote:
Originally Posted by kebwi View Post

Rats, I was kind of hoping to get away without buying a grinder for this project. I already need a table saw and tile saw.
naaahhhhh...

Cost was a big issue for me, so I have been following the original plans and the example of people like Lars and simply cutting the bricks with a hammer and chisel. I've only just started, though, so take that for what it's worth...
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Old 09-02-2009, 01:23 PM
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Default Re: Drilling shallow mortar pockets into bricks

If you do what I did, and drag your project out for two years, your expenses are spread out over time (on average)! I've found a way to justify spending way more than I wanted too/should have!
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Old 09-02-2009, 01:33 PM
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Default Re: Drilling shallow mortar pockets into bricks

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Originally Posted by cynon767 View Post
simply cutting the bricks with a hammer and chisel.
Don't forget to factor in the physical therapy bills for your strained wrists...
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Old 09-02-2009, 02:32 PM
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Default Re: Drilling shallow mortar pockets into bricks

You have some novel thoughts, but the idea of all but the shallowest of mortar pockets would be asking for trouble. Intuitively it seems to make sense. But it also has great potential to weaken the mortar joint itself.

Mortar is much weaker than the bricks. Like concrete, most of it's strength is compressive and it will fail under significant tensile loads. Also realize that mortar strength usually decreases at higher temperatures. And aside from any inherent tensile loads in the oven geometry, there will be a tensile element across an arch from the thermal expansion of bricks. Moreover, mortar will expand laterally more than brick under uniaxial loading.

What all of this means is that mortar is subject to a number of stresses and deviating from best practices is asking for mortar failures. Or to put it another way, mortar is only modestly stronger than the normally anticipated loads it is made for. And anything that serves to weaken it is likely to lead to some kind of (hopefully minor) failure. A strong mortar joint is a thin one. Typical masonry building joints shouldn't be more than 10 - 12mm thick. In ovens, refractory mortar usually shouldn't exceed 6mm due to the added thermal stresses. And 3mm would be better. In high temperature kilns mortar joints are sometimes kept very thin by simply dipping the brick in a thin mortar slurry to minimize the joint thickness. It's also common to see mortar failures where thick mortar joints were placed to allow for thermal expansion. It's now common practice go to great lengths to keep thermal expansion loads in compressive stress and build "refractory hinges" with higher strength materials to manage the remaining (predicted) failure points.

A mortar pocket of enough thickness to pin a brick is likely to seriously compromise the strength of the joint. And even if it holds, any bricks that are unequally pinned will be under added stress from thermal expansion. What seems to have more merit (although not enough that I would bother doing it myself) is to grind a series of thin, shallow, parallel grooves in the brick for the mortar to grip to. So that the pinning action is distributed across a number of thin mortar ridges. But again this is a trade off against a somewhat thicker and hence weaker mortar joint. I wouldn't want any such grooves to be more than one or two mm in width or depth.

As to metal pins, the pins will have a different expansion rate than the brick or mortar and are generally a bad idea. Stainless needles i.e. very small pins are sometimes used to reinforce castable refractories, but even these sometimes cause additional problems. The differential expansion is usually minimized by using small needles and typically coating them with wax to form a small cavity around the needle to allow for the greater metal expansion. The purpose of the needles is to distribute peak tensile stress across a greater load bearing section of the castable. There are now some castables & even mortars that use fibers that don't expand as much. I'm not sure if there are any of these textile reinforced mortars that are intended for refractory use though.

In common practice, our ovens are less demanding that something like a huge, ultra high temperature kiln. So even if our mortar joints are on the thick side, the oven usually works fine. But I wouldn't temp fate by designing in thicker joints. If you want to try something along these lines, I'd suggest very shallow grooves or a modest number of shallowly drilled holes.

.
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Old 09-02-2009, 02:35 PM
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Default Re: Drilling shallow mortar pockets into bricks

Quote:
Originally Posted by DrakeRemoray View Post
Don't forget to factor in the physical therapy bills for your strained wrists...
I married the physical therapist to get around that one. But of course that in turn incurred other bills..

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