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Re-inventing the Wheel


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At Eric's request regarding an old post:

Back in the 70's I was working for a rather large masonry contractor - 150+/- masons and laborers. Over the course of three years we worked on numerous schools, prisons and other institutional buildings. These buildings were always solid masonry throughout.

Most of the government work back then was using Type "S" and some Type "M" mortars - both have very high percentages of cement and they were both pretty new on the scene.

Within a couple of years of completion of these schools, they were suffering from severe cracking in the masonry walls. It looked as if they had been through an earthquake!

Unfortunately, the system the architects developed was completely void of any real flexibility. The brick were highly ceramic - so brittle that it was nearly impossible to cut them with a hammer, so most cuts were sent to a table saw. These brick teamed up with Type-S mortar made for a system that was as hard and inflexible as tile.

If you've ever studied a very old masonry building in the city, you may notice that brick coursing may no longer be straight and brick surfaces may no longer be in a flat consistent plane, and yet the mortar seems to be in pretty fair shape - adhering to the brick. This phenomenon exemplifies the opposite extreme and the beauty of the older softer mortars. The lack of Portland cement and the presence of the lime made the softer mortars self-healing - a term known as "Autogeneous healing". As small separations developed in the system through settlement, the lime and calcium in the mortars would slowly re-fill the gap.

I suppose that in the case of the schools, architects thought they were designing a building that would be indestructible. But, the lesson learned was quickly confirmed in the next generation of schools we constructed as evidenced by a retreat to Type "N" mortar.

The main reason for the Type "S" and "M" mortars fall into two categories:

1. PSI Rating - the ability to bear weight

2. The ability to repel water.

The Type "S"and "M' mortars have increasingly high levels of Portland cement, which allows them to bear more weight and, when properly tooled, produce a mortar joint that is very dense and repels water well.

The Type "N" and "O" mortars, have less Portland cement and more lime and other plasticizers, which make them more trowel-able.

One of the main lessons learned was to not make the system any more dense and brittle than was necessary, and equally important, to match the mortar strength to the masonry unit strength.

Matching the materials became evident when masons began to ignorantly re-point old historic buildings with Type N mortar. The mortar was much stronger and denser than the original mortar, which actually tended to wick the moisture out of the brick and promote evaporation. The more cementitious mortar impeded evaporation and kept the brick damp, which in turn caused the bricks to freeze and expand. Whole faces of brick would slowly pop off (spall).

In Summary, the whole architectural experiment with mortar strengths taught the designers an age old lesson - "If it ain't broke, don't fix it."

Even in today's houses we see a great example of the flexibility of brick and type "N" mortar compared to a more brittle cementitious material. That is why we see, almost every day, cracks in the concrete block backing of a foundation wall and no cracks in the outer brick withe. There are six bricks to one block, which means there is a lot more mortar in the outer withe. The smaller units and greater percentage of mortar give the outer brick withe increased flexibility.

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What do you think about Type O when doing a general repointing job on a 3 story common brick warehouse condo conversion?

This is one of those places; several warehouses all joined into a large scale condo project that's had a million screwed up misapplications over decades. We're trying to hit a *middle balance* of compatibilities.

Solid load bearing, triple wythe, commons all the way through to the interior. Some of the interior brick needs replacement also.

We can still get down to original by grinding 3/4". Thoughts on grinding depths?

We're trying to balance a lot of considerations, not the least of which is cost.

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Thanks for the thread!! Do you guys care if I dig up some of the old threads and ask questions?

This is an area were I see lots of failures but know little about.

Funny i've seen castles in Ireland that had no cracks but I find all has gone to hell on a house from the nineties.

Even crazier I have family that has been in this trade for three generations but I fill I still know nothing. Around here I see very old building with quarter or eight inch mortar joints I ask why there not done that way any more. Reply is almost always brick isn't made to the tolerances as they once were.

Again thanks For this Thread.


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I'm afraid you're taxing the limits of my knowledge by moving in the direction of structure, but I'm happy to offer my gut feeling:

The building has been standing for decades with lime mortar, I gather. If there aren't any new increased load bearing requirements, I'd use a blend of "N" tempered with more lime and other plasticizers to equal what is there already. Even if you do that, the presence of Portland cement is going to make it a more durable mortar than the original.

The main concern is to not accidentally initiate or accelerate brick spalling.

Regarding grinding of joints - My father was one of the assistants to the Architect of the Capital (US). He was in charge of the Capital Power Plant, which provided Electricity, heating and cooling to the Capital, The House Office Building, the Botanical Gardens and several other buildings on Capital Hill. One of their buildings had lost some of the mortar joints and the government elected to have the building re-pointed. The contractor came to my father after a week or two into the job begging for more money, because while it was a lime mortar, the blend was so perfect that he was ruining blades about 5 times faster than he expected to. They finally decided to abandon the project realizing it was cost prohibitive.

I would consult with Riverton, now a subsidiary of Martin Marietta out of Front Royal, Virginia. They have a marvelous matching lab and some great chemical engineers. I've had some very insightful conversations with them.

Regarding grinding depth, I had always heard you go for one inch, but I readily admit that's twenty year old information.

I think the most critical single element of re-pointing, because the depth is so slight' is curing the mortar. When I would re-point an old historic building I did the following:

1. I would grind or chisel to a pretty uniform depth of 1"

2. Clean out the joint

3. Thoroughly wet (nearly saturate) the area I would point in the morning.

4. Use a mortar that was as dry as stone mortar (clumpy) because:

A. It was easier to apply without making a nasty mess.

B. Believe it or not, it did not dry as fast.

C. It bonded nicely to the very damp brick

5. I NEVER brought the joint out beyond the worn brick corner because:

A. once beyond that point the joint appears thicker than original

B. as the mortar turns up over that rounded worn brick corner it will dam water into the brick and accelerate brick deterioration.

6. Throughout the day and just before calling it quits, I would wet the wall to slow drying and permit proper curing of the mortar.

I would receive calls from historical societies counties away due to this careful process. The results were beautiful. The work hardly needed cleaning and the look was as close to original as humanly possible.

There's nothing more heart rending than to see a mason re-point an old building by bringing the mortar out to the surface of the brick resulting in a horrifyingly fat mortar joint that nearly dwarfs the brick - tragic...

It's good to hear from you Kurt.

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When I'm asked to contribute to the specs for repointing historically significant buildings, I offer a couple items that I've learned to be important.

Repointing is only necessary for areas that have eroded back more than 3/8".

For joint prep. - remove only to a depth of approximately 2-1/2 times the joint width. There will be NO grinders used, unless absolutely necessary for removing previous inappropriate repairs.

Don't use Type N, O or any other letter type of mortar. It's builders grade, which usually means that it has hydrated lime with a low calcium content and the lime has been absorbing moisture in unconditioned storage for at least several months.

It's best if the original mortar is analyzed to determine the original source of calcium oxide, the proportions of lime to sand, the size and color of the aggregate and what additives, like local clay, may be present. Then, get the needed ingredients from deGruchy's Lime Works, US Heritage, or Virginia Lime Works.

It's not good enough for the repointing to last my lifetime. It needs to perform as well as, and have at least the same longevity as, the original mortar.

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Well there ya go, from a second source - "If it ain't broke, don't fix it." [:-graduat

Yeah, I'm guilty of monkeying with type N and O mortars to move them in the direction of Bill's advice. I have to admit, though, that during my whole career, the one thing I never did was mix mortar completely from scratch - only modify type "O". Although, now that I think about it, most of my mortars i used were custom mixes ala Riverton, so, in truth, I honestly don't know what was in them!? I'd just send a piece of the original mortar along with the sand I planned to use and picked up the bagged mortar a few days later. Maybe I was closer to Bill's stringent standards than I think?...

And, actually never used a grinder - didn't own one. I tried a circulars saw and carborundum blade once for about a minute and realized it was going to make a huge mess - dust everywhere and possibly damage the brick.

I always used a chisel and did the work T & M. If they cared enough they would go with T & M and if they didn't, I was the wrong contractor for the job. [;)]

And, there's no doubt in my mind that Bill will concur that bringing the new mortar out to the face of the brick, if the edges are worn away is anathema...

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