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Heal thyself


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Have you ever wondered why you may not see cracks and voids in an old masonry building's mortar joints even when the masonry has settled? Some assume it is because the old lime mortars are more flexible.

Actually, it is the result of a rather miraculous feature of the old lime mortar called "autogenous healing". Cracks form in the old mortars as easily as in any mortar. The difference is the characteristics of lime. When a crack forms, over time the lime with the assistance of water migrates to and fills the crack much like the formation of stalactites and stalagmites. Autogenous means self-produced or self-generated. The old lime mortar is self-healing.

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Michael,

Please give us some documention for this. I have long been under the impression that autogenuous is a term applied to (new and old) masonry which heals its own minor cracks, but never major ones. Also, lime is not very water soluble, nor is it a coagulant.

It would be a shame if some new guys got the wrong idea from your post.

Thanks,

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Certainly, it was the first search result on the net when I typed "lime mortar autogenous healing"

http://www.slcc.edu/tech/techsp/arch/co ... mortar.htm

http://www.zatang.com/categories/busine ... edlime.htm

http://www.graymont.com/applications_be ... lime.shtml

This information was given to me by an old brick manufacturer in Winchester, VA. He was a pretty interesting man and knew volumes about masonry. And, of course it was my life and passion for about 12 - 15 years depending upon whether you wish to count my estimating and management years.

Coagulant is the word that he used to describe the characteristics of lime but he did qualify it with "it acts more like a coagulant." In fact he also used the word scab as in a human scab. His analogy was indeed stalactite and stalagmite action. Unfortunately, Mr Barr is dead now. I agree that lime is only slightly water soluble. And, maybe Mr. Barr's use of the word coagulant was not the best analogy, but his point and mine is that lime actually will migrate and reset or rebind. If you have a better analogy, Jim, I welcome it.

Of course lime mortar could never fill large cracks. I was referring to slow ongoing settlement. And, the new more cementicious mortars do not enjoy the level of autogenous healing that old lime mortars do. The flip side, lime can and will eventually wash out of a masonry system that is getting too wet or especially has water passing through it due to lack of flashing etc.

I've seen many portions of masonry where the lime was all but gone and the sand could be raked out with your finger.

Along the same lines, when we started using type "S" and "M" mortars in school construction it backfired on the designers. The high levels of cement removed all the flexibility from the system resulting in wicked settlement cracks. Architects quickly retreated to type "N" mortar in most of their applications again.

Thanks for your assistance, Jim.

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"Have you ever wondered why you may not see large cracks and voids in an old masonry building's mortar joints even when the masonry has settled? Some assume it is because the old lime mortars are more flexible".

I would say that could be the case to some degree in certain circumstances. The way I understand it, when structural movement occurs, lime based mortars can better accommodate movement due to its plasticity. Lime mortars also tend to crack in multiple smaller cracks rather than fewer large cracks. These smaller cracks then tend to coagulate congeal self-heal.

I had the pleasure of receiving some education on lime mortar, plaster and render from the Mortar Industry Association in the UK. The U.S. Heritage Group also provides a good program "Understanding Lime Mortar". This presentation will be available at the Traditional Building Conference in Chicago on April 5-8.

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Michael,

My only concern is that people come here to learn, and if a new inspector read your post, they might just walk into tomorrow's inspection thinking old mortar doesn't have big cracks, because it fixes itself. This may not be what you meant, but it is very nearly what you posted.

New inspectors pass what they read here on to their clients. I'd rather they read the truth from us, and not the plaintiff's expert.

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