Why did my mortar fail, crumble and de-laminate?


  #1  
Old 05-18-24, 08:53 AM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
Why did my mortar fail, crumble and de-laminate?

I spent a lot of time and effort in repairing and repointing the parapets on an historic building in central Texas. Within a year the mortar had failed as seen in the pictures below. The bricks that made up the building were soft and old, so I used the formula for mortar suggested by the Texas Historical Commission: 3 parts sand, 1 part hydrated lime, 1/16 part Portland cement, and I added a pinch of pigment to match the color of the existing mortar.

I thought that a hard freeze may have caused the mortar to fail before it had time to set up, but it looks like I did most of the work in April and May.

(I hope this isn't a duplicate. I submitted this yesterday, but it's nowhere to be seen.)








The only mortar that is still sound is that which was used years ago using ordinary (hard) concrete.

 

Last edited by Shadeladie; 05-18-24 at 01:04 PM. Reason: Images not showing
  #2  
Old 05-18-24, 09:34 AM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
No images

It looks like my images don't appear. I'll repost if I can figure out what's wrong.
 
  #3  
Old 05-18-24, 11:00 AM
Shadeladie's Avatar
Super Moderator
Join Date: Jan 2005
Location: PA - USA
Posts: 4,932
Received 402 Upvotes on 329 Posts
There's something wrong with images posting. It's not you. Will probably have to wait until the beginning of the week. Sorry.
 
  #4  
Old 05-18-24, 11:12 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,249
Received 1,960 Upvotes on 1,757 Posts
That's pretty funny... you must have misread something because it's certainly not 1/16 cement.

you probably misread a ratio for the three components such as 1:1:6. But 1:1:4 would be stronger.
 
tightcoat voted this post useful.
  #5  
Old 05-18-24, 02:36 PM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
Go here. And read up on repointing old masonry
NPS.GOV. Then look for Preservation Briefs and read Preservation Brief # 3. Repointing Historic Masonry (or something like that). Maybe you did not have the joints clean enough, or maybe you did not have them cleaned deep enough or maybe they were too dry.

I agree with XSleeper that the 1/16th part of cement sounds like it is not enough. Let us know what else you learn.
 
  #6  
Old 05-18-24, 02:40 PM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
My previous post should reference Preservation Brief. #2.
 
  #7  
Old 05-19-24, 06:02 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 28,164
Received 2,270 Upvotes on 2,022 Posts
Can you post the link to the article you want everyone to read?
 
  #8  
Old 05-19-24, 06:38 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,249
Received 1,960 Upvotes on 1,757 Posts
  #9  
Old 05-19-24, 09:20 AM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
Thanks, XSleeper. I could not do it from this computer. Don’t know why,.
 
  #10  
Old 05-30-24, 07:30 PM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
Thanks to everyone for your helpful replies. Sorry it took me so long to get back.

I also thought the 1/16 part Portland might be too little, but I went back and read the architect's specifications, and that's what he said. The bricks are VERY soft.

Nevertheless, even if I used no Portland at all, I wouldn't expect the mortar to fail so completely. After all, before Portland cement was discovered, didn't they just use lime and sand?
 
  #11  
Old 05-30-24, 08:00 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,249
Received 1,960 Upvotes on 1,757 Posts
Architects are often too full of themselves to admit when they are wrong.
 
  #12  
Old 05-31-24, 08:37 AM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
Yep, no argument there. Nevertheless, before Portland cement was discovered, they built houses with only lime and sand. I wouldn't think a pure lime and sand mortar would fail this badly without something else factoring in.
 
  #13  
Old 05-31-24, 10:06 AM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
I have some theories and suggestions.
Yes, lime mortar is a time tested and proven mortar. A couple of things to know are that in the olden days of lime mortar the lime was quicklime that was slaked ahead of time. Now I am unsure it the lime was dry mixed with sand then slaked or if it was slaked and then the lime putty was mixed with sand to make mortar. The ratios must be correct, but there is a lot of room for variance. Soft, weaker mortar for soft, weaker bricks is correct. If you use lime mortar you can use modern hydrated lime but it will work better if you soak the lime for a while. A few days to months is good as long as it does not freeze or dry out. I mix lime for plaster in a bucket then leave a couple of inches of water on top of the lime. I've kept it years. Just put a lid on the bucket so the water does not evaporate. Check once in a while to make sure it is not evaporating. Replenish the water if needed. This mixed lime is known as lime putty. I suppose you could make mortar and treat it the same way, make mortar and soak it.
No mortar will bond to dirty, dusty, moldy, algae covered substrate. This includes the weak, weathered layer of bricks. In other words, you need to get to sound brick material to get a good bond. You have to know where to draw the line but you might need to wire brush some of the outside of the brick to get down to more sound brick, then you have to get it clean from that dust,
It looks from your photos like you made a coping with the same mortar you pointed with. I think that weak mortar might have dried out before it cured. It might have worked better had you covered it and kept it wet for a week or so. But I think your bricks might have had a layer of spalled material that let loose from the rest of the brick. That is a thing about old, soft brick (and some limestone, too.) The outer layer weathers away and mortar does not hold what comes loose under the mortar.
Now I have had some success with similar, but not identical conditions. First, get things clean. Then you need to consolidate the substrate. I have tried two different ways. Both seem to be holding after ten years but I don't know about the long term yet.
One way I did this is to paint the loose, flaking, spalled material with an acrylic binder. I used Acryl 60 by Thoro Products but I don't think it is made any longer. Soak the surface you want to bond to with the acrylic. Do this when it is not too hot so it soaks in before it dries. Acrylic does some remarkable things when used as an admixture with cementitious materials. BUT ACRYLIC IS A BOND BREAKER. So after you consolidate the soft bricks then you need a bonding agent like Weld Crete by Larsens Products. There are other brands that are white when in the jug but dry clear. Weld Crete is blue when dry. For this reason you might want to use a different brand.
Another thing that seems to be working is to paint the soft, spalled material with a slurry of neat Portland and water and place the mortar immediately before the slurry dries out.

Now I am looking at the coping specifically. I think this might be a good place to use Portland cement based material. Do what you have to do to get it clean, consolidated and bonded then spread a coat of the Portland cement material and press it well into the brick. Don't just pile it on but press it in well then add enough more to straighten it and put a slope on it. This can be a slope in only one direction like to the roof side of the parapet so water does not run off it onto the street side of the wall and stain it or you can make it pyramid shaped, that is higher in the center. You want water to run off. And you don't want to dub it off to nothing on the edges. You need some thickness on the edges. It would be a little complicated but if you could put a form around the parapet and make the edges of the coping about an inch thick it would be good. Even better would be for the edges of the coping to project an inch or so beyond the face of the brick with a drip edge on the coping. When your coping is quite set but not hard and dry a slurry of neat Portland and water painted on the surface will give it extra surface hardness and moisture resistance. This above long paragraph applies only to the coping of the parapet not the mortar joints.
I think were I doing this I would be inclined to use Type O masonry cement. Make sure the joints are cut back deeply enough to get a good amount of mortar into the joint. Don't simply spread some on the surface of the existing mortar joints.

This was long and I am sorry. Please let us know what you do and how it works.
 
bnorton voted this post useful.
  #14  
Old 05-31-24, 10:44 AM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
Thank you for taking the time to write out such a thorough answer. Based on your comments, I took another look at the roof and it looks like the mortar used to repoint is holding up pretty well. It was the coping that failed.

I think I will take your advice and use type O masonry cement on the coping. I sure hope this works. I don't want to do it again.

Thanks again for the excellent answer.
 
  #15  
Old 05-31-24, 11:49 AM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
On the coping only I think go with something stronger and harder than Type O. Go with Type N. Maybe even Type S. This is what takes the brunt of the weather and erosion. Use Type O in the mortar joints.

Or you could form a coping block and cast it on the ground then set it with type N. This would let you make it so it is a little larger than the parapet so it extends past the face of the bricks. You could make a steeper pyramid if you chose to do it this way. But how do you get that mass up there?
As a side note: It's my experience that parapets deteriorate more and faster than the walls below the roof line. They are exposed to the elements from three sides, top and outside and roof side while walls below the roof line are only exposed on the outside.
 
  #16  
Old 05-31-24, 11:53 AM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
So Type O for pointing and Type N or S on the coping.
 
  #17  
Old 05-31-24, 03:16 PM
W
Member
Join Date: Jun 2023
Posts: 67
Upvotes: 0
Received 11 Upvotes on 9 Posts
Not sure if it was suggested earlier (long responses and I didn't read through them all) but I would also suggest that you wet the old surfaces before applying new mortar. The old brick is dry and will pull most of the moisture out of the new mortar.
 
  #18  
Old 06-10-24, 12:30 PM
B
Member
Thread Starter
Join Date: Aug 2001
Location: USA
Posts: 66
Received 0 Upvotes on 0 Posts
Type I/II and IL

So,I went off searching for Type O and N cements and I found Type I/II or IL.(See pics.) I guess that's a different type of "type". It looks like you make Type O and N from that. Is that right?



 

Last edited by bnorton; 06-10-24 at 01:26 PM. Reason: Forgot to add pictures.
  #19  
Old 06-10-24, 02:17 PM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,970
Received 53 Upvotes on 45 Posts
Yes, Masonry cement is a blend of Portland cement, lime and sand in different proportions to achieve different strengths and hardness. I think somewhere online there should be tables of different formulae for different types.
You should be able to buy masonry cement. last time I bought some was years ago. Seems like there were bags of typeSs and type N. You might also be able to buy pre-blended mortar to which you need only add water.
As a general rule masonry cement is more workable than mortar made with only Portland cement. It is the lime that makes it more workable but at the expense of reduced strength and hardness.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description: