Replacement mortar cracking on repoint of load bearing masonry

[enggmom]

We are currently repointing the masonry on a 40 yr old building. The mortar that is being removed is very hard (Type M perhaps). We are repointing with Type N and it's cracking. We did a mock up with Type O and it cracked within 20 mins. I suspect the procedure the mason followed for the mock up was not correct. I know for sure he did not wet the joints in advance of placing the mortar. Any ideas on why the mortar is cracking and suggestions on what to use? I am also looking for some reassurance that repointing load bearing masonry with Type O mortar is acceptable.

 

[JAE]

The load bearing question might depend on how deep you are re-pointing and also how thick the overall wall is.

Not pre-wetting the masonry could have a huge effect on the curing and shrinkage of the mortar.

 

[Ron]

Re-pointing older brickwork is not an easy task. Some will tell you that you need to match the ductility of the original mortar...nice thought, but almost impossible to achieve.

If you use a mortar mix that is too strong, the concern is that when it shrinks, it will crack the brick...can happen, but not likely.

I suspect that your masons are using some incorrect techniques.

First, I would suggest that you test a prism from the existing wall, if you can remove one in an inconspicuous location. If not, use a flat jack and test in-place. This will give you an idea of the strength you need in the section. Do your best to match that if you can.

Next, make sure the depth of repointing is sufficient. I usually specify one-third of the depth of the joint.

Next make sure the old mortar as properly removed from the joint, exposing brick on both sides (or top and bottom) of the joint. This is usually best achieved with a dedicated tuck point grinder, not a side grinder with a masonry blade on it.

Next, the mortar constituents must be properly measured and mixed. Make sure the mason uses a container to measure the sand, cement, lime, etc., and DO NOT allow them to simply measure with a shovel. The moisture content of masonry sand changes throughout the day, so if a shovel is used, the mix will typically be over-sanded in the morning (when the moisture content is higher) and under-sanded in the afternoon. This results in inconsistent mix properties.

Also check the masonry sand gradation. In many older buildings, the masonry sand was finer than what you are likely to get when ordered today.

Consider polymer modification for the mortar.

 

[dik]

...and my 2c added to JAE and Ron's...

It's difficult to match/design mortars for older buildings. The age doesn't fall into the realm of historic buildings, where there are often other problems associated with the building envelope and the function and purpose of the mortar. With historic buildings, you often have the budget that is required for this specialised work.

You will not likely get away from minor cracking and the easiest solution is to use a softer mortar... it will minimise cracking of the brick units, which you want to avoid.

Your hard mortar may be Portland Cenent based and not masonry cement. This tends to be harder and often forces cracking through the brick units.

Another item is that when you modify an existing masonry building, including stone, you may have undesireable 'other' effects due to the slight changes in areas of wall stiffness.

Dik

 

[enggmom]

It looks like we're definitely on the right track. All of your responses have given me greater confidence in the path taken to date. I believe the mason is outside of his comfort zone and as such I've asked the contractor to bring in a mason with experience in dealing with these issues. I know this isn't the first time this has ever happened and I beleive its a solvable problem. Thanks to you all for taking the time to respond.