# Bonding agent for repointing old brick building?



## ClarenceBauer (Mar 4, 2005)

I have repointed many Historical buildings and it has never been specified to use a bonding AGENT.
The best approach would be to test the existing mortar for it's make up, any good soil consultant should be able to do this for you. If you skip this step than plan on using a mix that should fall in between a high PSI & a lower PSI mix.
Look at using a 1-2-9 or a 1-1-6 mix if I had to select based on the photos I would use the 1-2-9 mixture. Also rake the existing mortar joints out to no less than 1-1/2 inch depth when repointing do it in two lifts of 3/4 inch each lift making sure that the mortar is packed in the joint NOT just laid in. Also be sure to keep moist so as the bricks don't pull the moisture from the new mortar. If you are pointing a large area use something like burlap draped over the new work area and keep the burlap damp. Use a joint tool that matches the size of the mortar joint. For a very good reference on Repointing Mortar Joints Look at Preservation Brief # 2


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## KnockABlock (Oct 7, 2017)

I found the article you mentioned and it's great. I have a mortar mix which is supposed to be type N (1-1-6 correct?), would that suffice? Could I approximate a 1-2-9 mixture by amending it with some extra lime? 

I have repointed a very small area so far, at a depth of about 3/4" to 1". By 'lifts' do you mean to point in about 3/4" at a time, let it set up, then put in 3/4" more mortar?

The area I did, I tried to keep misted, but it would dry out so fast. I like the idea of covering it, but any cover I'd have to attach to the wall somehow.


Thanks for the advice.

P.S.

If you look at the old mortar in the building, it has small pebbles in it. I'm not planning on replicating that aspect. Have you ever seen anything like that before?


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## ClarenceBauer (Mar 4, 2005)

To adjust the 1-1-6 mix by adding sand I would not advise that be done. Take a look at www.estimators'reference ( Mortar Mixes ) this chart will give you the mix ratios also note the 28 day PSI rating than go to page # 6 of the report and again check the PSI rating. Type N goes to 1500 to 2400 PSI & Type O goes to 750 PSI to 1200 PSI page 6 is why I selected a 1-2-9 mixture. You can purchase a premix pointing mortar depending on your location. Check A. W. Cooks Atlanta, Ga. / U. S. Heritage Group Chicago / Cathedral Stone & Edison, Coatings, Inc.
Yes lifts is repointing in stages of 3/4 inch at a time.
You can fasten the burlap by using roofing nails in the mortar joints.
The aggregate used is most likely local sand that was used with the dark pebbles which are most likely Granite.


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## KnockABlock (Oct 7, 2017)

Those psi values are a little baffling to me. I thought type N was about 750 psi, and type O around 350? Your earlier link -- nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm -- lists these values. These walls are about 14 to 18 feet tall, so I wasn't sure if type O would be strong enough. If it is though, I would gladly use it.

Is there a name for the type of hollow bricks that I have? I realized I didn't mention their size before: they are about 7.5" deep, 12.5" long, and 5" tall.


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## ClarenceBauer (Mar 4, 2005)

The PSI rating is based on the 28 day ASTM testing.
The ASTM PSI is based on the Minimum required compressive strengths. You need to check out the achieved PSI of the mixes.
The brick appear to be Hollow Tile block.


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## stuart44 (Jun 29, 2016)

Possibly terracotta blocks of which there are quite a few different types.
https://www.google.co.uk/search?q=t...i-quHWAhXpK8AKHVkdBMoQsAQIJw&biw=1366&bih=662
The mortar may be a lime mortar with a sharp sand.


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## KnockABlock (Oct 7, 2017)

Thanks for the further info -- I believe what i have are two cell hollow tiles. In looking into hollow tile bricks I found an interesting publication from 1926 where they tested a number of hollow brick designs and mortars to determine their strength -- one of the tested types matched the dimensions and design of my blocks:

nvlpubs.nist.gov/nistpubs/nbstechnologic/nbstechnologicpaperT311.pdf


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## ClarenceBauer (Mar 4, 2005)

KnockABlock said:


> Thanks for the further info -- I believe what i have are two cell hollow tiles. In looking into hollow tile bricks I found an interesting publication from 1926 where they tested a number of hollow brick designs and mortars to determine their strength -- one of the tested types matched the dimensions and design of my blocks:
> 
> nvlpubs.nist.gov/nistpubs/nbstechnologic/nbstechnologicpaperT311.pdf


If that report gave the compression strength rating ( PSI ) your mortar should be no stronger than the lowest test PSI.
During the 1920's and 1930's those type tiles were used as structural walls. The plastering trade named them speed tile reason being is you could lay the tile one day than the next day you could stucco the exterior and plaster the interior. We used to repair schools that had those type tiles for hallway and class rooms.


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