[KnockABlock]

I'm looking for recommendations for a concrete bonding additive that can be used to bond new mortar to old mortar in a tuckpointing project. The use of a bonding agent is recommended here:

heightslibrary.org/materials/hrrc/12-Masonry/TUCKPOINTING.pdf

To quote:

Next, take an old screwdriver or smaller chisel and scrape out the loose mortar. If the remaining mortar still seems to be crumbling, you can stabilize it by applying a solution of one part concrete bonding additive to one part warm water. Brush it on with a “cheapie” paintbrush the night before you start the work.

The building I'm repointing is an old hollow brick building from the early 1900s. I bought some Sika Latex Concrete Bonding Adhesive and Acrylic Fortifier, but I'm not sure if this is the right thing to use.

The mortar I'm repairing is either wearing out, or just wasn't well applied. It's crumbly in places, brittle in others, and in places completely missing.

I've attached pictures of some of the bricks themselves -- I've removed some from a non structural part of the building -- and of an intact area of the old mortar.

 

[KnockABlock]

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?

 

[KnockABlock]

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.

 

[KnockABlock]

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