| Author |
Message |
Roland Noreika, AIA (Unregistered Guest)
Unregistered guest
| | Posted on Friday, August 27, 2004 - 11:09 am: |    |
Looking of Specifications on Masonry Pointing and Mortar content (Type N?)for repointing of a 150 year old stone structure. I have been told that the modern mortar (High in Portland Cement content) cannot be used on a building like this due to the strength of a "Portland" cement joint vs. the adjacent "High Lime" content cement joints. |
Anne Whitacre, CCS CSI
Senior Member
Username: awhitacre
Post Number: 118
Registered: 07-2002
| | Posted on Friday, August 27, 2004 - 12:56 pm: | |
You will need to talk to someone who really specializes in this area. It is true that modern mortars are sometimes stronger than old bricks and will pull them apart when the building starts to move around. They also have a somewhat different appearance than the old high-lime content pointing mortars. Assume you will have several test patches before you get one that looks the same, and will not damage older brick. |
Richard Howard, AIA CSI CCS
Senior Member
Username: rick_howard
Post Number: 30
Registered: 07-2003
| | Posted on Friday, August 27, 2004 - 12:56 pm: | |
I think you may want to consider using Type O cement-lime mortar. The lime content is typically very high for historic mortars of that age, sometimes exceeding the cement content by 2 to 1. Portland cement was not used much before the early part of the 20th century.
Not exceeding the strength of the existing mortar is especially important in doing repointng. The structural mortar is already in the joint, you just want to dress it with a facing that will not shrink or crack. Lime is also somewhat self-healing.
Some of the earliest historic buildings in this country have no cement in the mortar - just lime and sand. While these mortars are relatively low strength, they have lasted hundreds of years without cracking and have done the job required of keeping out the elements.
Read NPS Preservation Brief 2 "Repointing Mortar Joints in Historic Buildings" at this link: http://www2.cr.nps.gov/tps/briefs/brief02.htm
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Dave Metzger
Senior Member
Username: davemetzger
Post Number: 91
Registered: 07-2001
| | Posted on Friday, August 27, 2004 - 01:18 pm: | |
Especially if this is a historically-listed building, you should get the mortar tested to determine its composition, and then use a pointing mortar that is no stronger than the existing mortar, and preferably weaker. Portland cement wasn't much used in the US before about 1870. Generally, mortar for buildings before that date was lime mortar; and from about 1910 on, Portland cement generally was a constituent of mortar. From about 1870 to 1910, it could be either way.
I agree with both Anne and Rick. I wouldn't use a mortar stronger than Type O. And you may want to consider using just a hydraulic lime mortar. Several years ago there was a seminar at the CSI Convention by John Speweik on lime mortars. if you do a Google search on "lime mortar", you will find several sites with information. |
Lynn Javoroski
Senior Member
Username: lynn_javoroski
Post Number: 143
Registered: 07-2002
| | Posted on Friday, August 27, 2004 - 01:24 pm: | |
What I added to our spec was testing for the existing mortar to determine what it is: "Perform a basic analysis of existing mortar in accord with "Masonry: How the Care for Old and Historic Brick and Stone" published by the Preservation Press, 1988 National Trust for Historic Preservation in the United States and available from the International Masonry Institute."
This reference has geat language for determining what to do. I also added this: "Existing Brick: Either Type O mortar or Lime Mortar, depending on the test of the existing mortar." I also sought the advice of IMI and got this information: "Aside from color match considerations the mortar was more than likely no more than a type "N". I have found that some of those old brick were not soft and absorbent (like Charmin) but had a low IRA. I would suggest as low a mortar strength as possible knowing that even a straight lime mortar will provide great weather resistant properties only not as quickly as a mortar with some portland in it." |
Shelby N. Gordonswyth
Senior Member
Username: shelbyng
Post Number: 9
Registered: 08-2004
| | Posted on Friday, August 27, 2004 - 04:05 pm: | |
As a young architect, I noticed the brickwork of the former county jail, built in the mid-to-late 19th century, across the street from my grandparent's home in Bowling Green, Indiana.
It seemed to me to represent "what brick oughta look like." Three things gave it its character: the old terra cotta/salmon-colored brick, its irregular black or charcoal-colored patina (it had apparently never been cleaned), and the mortar color, which I hadn't hadn't paid much attention to before--it was a light tan color, and very sandy. Light rubbing with a finger would turn it to loose fine-grained sand. It was apparently made of lime and sand without portland cement. The sand resembled that used in sand casting.
Ever since, I have realized that portland cement is the culprit that usually keeps brickwork from having the "authentic" (historical) look often desired--it's a dead flat gray mortar that gives masonry a dull and lifeless look. (More recently, too many neo-georgian houses err in two ways, pitting a deep red brick against an off-white, too-bright mortar.)
Salmon brick and light tan/cream-colored mortar make all the difference.
(Is there a shortcut to developing the patina...?) |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 305
Registered: 10-2002
| | Posted on Friday, August 27, 2004 - 04:57 pm: | |
Lab analysis of the mortar and duplication of the original proportions are a must. But equally important is the execution instructions for repointing. How deep do you rake out the existing mortar, do you do the full facade or only specific locations (and who determines those, do you examine them and indicate the area on drawings, or rely on requiring the contractor to do all loose deteriorated areas...the later not advisable) and if you only do small areas, you can have a difference in coloration of new to old that makes it look worse than having done nothing. No mater what, the new will not match the old. I have used 1/2 inch as a good rule of thumb as the raking depth, but you need to see how deep the deterioration goes.
And how do you perform the removal - hopefully hand tools only, no power tools for removal permitted. You need to make sure that the back joint of the raked area is square corners, not ragged or irregular or you get voids in the new mortar. It needs to be cleaned out, brushes, air jets (low pressure - just to remove loose debris from the back of raked areas), and then dampened (but no excess water) during the repointing process.
How thick is the new material being put into the joint, how 'cured' do you let it become before putting in the next layer. 1/4 inch layers, letting each layter come to thumb print hardness before the next layer is placed. Yes, this is more like making pastery than engineering.
You did not mention if you are going to wash the facade, which should be done or any color matching will be to dirty materials. You need to be careful with that, sample panels on the building prior to the construction process that you work out with different specialty people is the best way to go - contractor required to do sample panels on the building to match these. But you can go directly to the creation of contractor sample panels if you want (located in inconspicuous places).
And don't let anyone convince you that any sealers should be used on the completed facade.
William |
D. Marshall Fryer
Senior Member
Username: dmfryer
Post Number: 33
Registered: 09-2003
| | Posted on Monday, August 30, 2004 - 08:59 am: | |
Well covered, William.
May I point out that masonry restoration contractors are notorious for believing (rightfully in some cases) that they know much more about masonry restoration work than does the Architect specifying the work. They will have their own notions of how to do the work, and will most likely ignore any written specifications if given the chance.
A comprehensive pre-construction meeting is a must, mock-ups should be required, and frequent (daily?) construction observation is recommended, by someone who is experienced with restoration work and has a strong spine.
Otherwise, out come the power saws and cheap mortars. |
Helaine K. Robinson CCS
Senior Member
Username: hollyrob
Post Number: 70
Registered: 07-2003
| | Posted on Monday, August 30, 2004 - 11:23 am: | |
There is a CSI Technical Document called TD-2-8 Guide to Preparing Design and Construction Documents for Historic Projects [1996]
by Christine Beall & Deborah Slaton. Go to this link:
http://www.csinet.org/s_csi/doc_bookstore.asp?TRACKID=D4YPDFNS58ZE2YGBPEPS8ELNF73UVDLR&CID=423&DID=6658 |
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