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Old 11-28-2012, 10:47 AM   #1
cowboy73
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Default Repointing mortar joints concrete block?

The wife seen a mouse in the kitchen last night So after setting traps, I started looking around the outside of the house for possible entry points. I have a few spots in the concrete block foundation where the mortar is gone or loose. The house was built in the early 1920s. Would you guys just use regular mortar mix? I know I should dampen the blocks to keep the mortar from drying out too fast. Would you use a concrete binding agent to improve the mortar sticking? Would hydraulic cement be a better option since it expands slightly? Any advice would be appreciated.

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Old 11-28-2012, 10:49 AM   #2
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Default Re: Repointing mortar joints concrete block?

You want the mortar to be softer than the brick, or it can cause the brick to crack.
So, NO to concrete, and NO to hydraulic cement.
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Old 11-28-2012, 10:53 AM   #3
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Default Re: Repointing mortar joints concrete block?

Thanks. I was wondering about that. Is the premix stuff in a bag okay for the patch job?
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Old 11-28-2012, 11:02 AM   #4
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Default Re: Repointing mortar joints concrete block?

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Originally Posted by cowboy73 View Post
Thanks. I was wondering about that. Is the premix stuff in a bag okay for the patch job?
I've used it without any troubles on my grandmother's house a couple of years ago. Seems to have held up pretty well. Just be careful when you mix it so that you get it wet enough.
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Old 11-28-2012, 11:33 AM   #5
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Default Re: Repointing mortar joints concrete block?

The premix mortar in the bag will have a letter code.
The code designates the strength of the mortar, with N being in the middle of the range, and that is probably what you want (unless you have really old and soft bricks).
Usually N is the regular stuff, so yeah, that's what you probably want.
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Old 11-28-2012, 12:01 PM   #6
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Default Re: Repointing mortar joints concrete block?

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The premix mortar in the bag will have a letter code.
The code designates the strength of the mortar, with N being in the middle of the range, and that is probably what you want (unless you have really old and soft bricks).
Usually N is the regular stuff, so yeah, that's what you probably want.
I think the old timers used more of a lime base for their mortar. I looked at a house from the 1820s that had been partly repointed with modern mortar, and it's basically ruined about 1/3rd of the bricks. If your house was built after 1900, you should be fine.
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Old 11-28-2012, 12:06 PM   #7
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Default Re: Repointing mortar joints concrete block?

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. I looked at a house from the 1820s that had been partly repointed with modern mortar, and it's basically ruined about 1/3rd of the bricks. .
I will be repointing my 1850's house brick foundation in the near future...... what sort of damage did you observe? My bricks are definitely the softer variety.
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Old 11-28-2012, 12:20 PM   #8
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Default Re: Repointing mortar joints concrete block?

The mortar gets it's waterproof qualities, not by it's hardness, but rather, by compression of the mortar surface by striking off the joints with a jointing tool. Start by removing loose mortar by mechanical means including sawing as necessary. Choose a local sand to try to match the existing mortar color and recipe.
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Old 11-28-2012, 01:16 PM   #9
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Default Re: Repointing mortar joints concrete block?

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I will be repointing my 1850's house brick foundation in the near future...... what sort of damage did you observe? My bricks are definitely the softer variety.
Yeah STINEY, I would do some serious research before making a move. Dang 1/3 of the bricks destroyed! That sucks.
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Old 11-28-2012, 01:23 PM   #10
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Default Re: Repointing mortar joints concrete block?

Did you catch the mouse?
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Old 11-28-2012, 01:34 PM   #11
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Default Re: Repointing mortar joints concrete block?

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I will be repointing my 1850's house brick foundation in the near future...... what sort of damage did you observe? My bricks are definitely the softer variety.
Between the end of the Civil War and roughly 1900, brick "technology" advanaced, so that they were relatively hard throughout. Before this time, the face or "outer crust" (if you will) was hard, but the interior wasn't nearly as strong, so if you chipped the face, it would start to degrade the brick.

Save yourself a lot of hassle and heartbreak, and read this:

http://www.nps.gov/hps/tps/briefs/brief02.htm


So, what happens is that water gets into the brick, and the clays expand and crack it worse, and then once you loose that protective outer shell, the brick more or less begins to disintegrate. Using modern mortar can speed this process up, so generally when you're restoring an old brick home, it's best to use a lime mix (still obtainable). Otherwise, you get this:





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Last edited by geologist; 11-28-2012 at 01:36 PM.
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Old 11-28-2012, 02:27 PM   #12
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Default Re: Repointing mortar joints concrete block?

While I agree that that mortar is important on brick, I don't know that it's super important (or as important) on concrete block. I think everyone missed that that was the OP's question. Concrete block vs. brick is a totally different animal.

I just did a basement foundation wall with hydraulic cement and am not concerned at all about it damaging the block.

I'm on our local historic commission & we bring in experts at times to test the mortar to make sure we're using the right mortar for the old bricks in different buildings. A lot of masons will just slap whatever they have in the truck in there, not realizing how much damage they cause.
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Old 11-28-2012, 02:40 PM   #13
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Default Re: Repointing mortar joints concrete block?

Eeww.

Unfortunately, it would appear mine are in the early stages of that state of demise. Not quite that bad, but in the same ballpark.

Thanks for the info!
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