Publisher’s version / Version de l'éditeur:
Columns - The Newsletter of the Alberta, Saskatchewan and Manitoba
Associations of Architects, 6, October 1, pp. 2-3, 1998-10-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Mortar for pointing old masonry buildings
Rousseau, M. Z.; Maurenbrecher, A. H. P.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=f77ebccb-22db-4a13-8af2-60986d81ea0f https://publications-cnrc.canada.ca/fra/voir/objet/?id=f77ebccb-22db-4a13-8af2-60986d81ea0f
Mortar for pointing old masonry buildings
M.Z.Rousseau and Paul Maurenbrecher
Institute for Research in Construction, National Research Council Canada
Originally published in "Columns, The Newsletter of the Alberta, Saskatchewan, and Manitoba Associations of Architect"s, V. 6 No. 1, Octber 1998
Restoration of old buildings has been gaining momentum in Canada in the last twenty years. Repointing mortars for old masonry walls need to be somewhat different than the mortars used for modern masonry. Below is some technical information concerning the desirable properties of repointing mortars, as well as the research work undertaken at IRC to assess their overall performance.
Soft mortars. Mortar mixes used for new buildings are not suitable for old masonry structures, partly because their high Portland cement content gives them fairly high density, compressive strength and hardness, so they cannot act as the sacrificial material of the masonry assembly. Use of a softer mortar, with a low compressive strength, more flexibility and higher porosity, is recommended. Such a mortar can accommodate minor movements in the wall. If larger shifts occur, a soft mortar will take the strain and will “sacrifice” itself to maintain the integrity of the masonry unit. It is simpler and cheaper to repair mortar joints than to replace a spalled brick or stone.
High lime content mortars. Mortars having high lime content (2 to 3 times more lime than cement by volume) have many advantages for pointing old masonry structures. Unlike Portland cement, lime is a binder that helps achieve this desired flexibility in the mortar. Since it is more flexible, a high lime content mortar is less likely to separate from the masonry units. Lime also makes the mortar more malleable, resulting in a larger contact surface between the mortar and the masonry unit when it is placed. These two characteristics reduce rainwater infiltration into the joints, and unlike a dense cement pointing mortar, it will allow any moisture that gets into the wall to dry out much faster. However the frost resistance of high lime content mortars may not be sufficient for long-term durability in our cold climate. In Canada, the preliminary results of research work now in progress indicate that air-entraining agents similar to those used for concrete increase the frost resistance of this type of mortar.
Durability. In the last few years IRC, Public Works and Government Services Canada’s Heritage Conservation Program and lime manufacturers have joined forces to study the durability of high lime content mortars for pointing masonry structures in old and historic buildings. This research indicates that the mortar mix 1-2-8 (white Portland cement-lime-sand by volume) shows promising characteristics with respect to freeze-thaw resistance and other physical and mechanical properties. Resistance to salts (such as de-icing salts & sulphates) needs further assessment as well as bond to masonry units (some stone units had good bond while others almost none). Debonding at the mortar masonry unit interface allows greatly increased water ingress. Lime leaching from within the mortar out onto the surface of the masonry is another factor being investigated because it can affect the appearance of the masonry work. When lime leaches onto the surface, it can leave behind a white deposit (it reacts with carbon dioxide to form calcium carbonate, almost non-soluble in pure water although with time will gradually dissolve in acidic rain water). Water seeping out from within the masonry is the main culprit. Thus masonry should not be excessively wetted before repointing and extra care must be taken in exposed situations such as chimneys and free standing walls. Future stages in this project include studying other mortar mixes in the laboratory, tests on outdoor exposure sites and monitoring the performance of mortar in restored masonry.
Additional sources of information: The most recent issue of the magazine of the Association for Preservation Technology discusses past work on repointing mortars (APT Bulletin Vol 29, No 2, 1998). The Heritage Preservation Service for Historic Buildings, US National Park Service offers its 41
Preservation Briefs on the internet (http://www2.cr.nps.gov/tps/briefs/presbhom.htm); Brief No 2 is about repointing mortars. Last year IRC set up a Working Group on Repointing Mortars for Masonry Buildings bringing interested manufacturers, designers, scientists and masons to one table. It provides a platform for informal exchange of experience and for discussion of new findings regarding the performance of
2
Page 2of 2
Information: Paul Maurenbrecher and Madeleine Rousseau, IRC/NRC
For further information or to join the Working Group on Repointing Mortars for Masonry Buildings, contact Paul Maurenbrecher, researcher at IRC/NRC at (613) 993-0073; fax (613) 952-8102; e-mail paul.maurenbrecher@nrc.ca
Stucco and detergent don’t mix
It appears that adding detergent to stucco mixes to improve workability has become increasingly common. This practice is unacceptable. Detergents are unsuitable for air-entraining purposes and can compromise the ability of stucco-clad walls to control rain penetration. Detergents increase the wetting properties of the stucco and facilitate the absorption and transfer of water through it. When rain reaches the stucco, the detergent migrates inward and has similar effect on the materials of the inner wall; that is, they can get wet and absorb water more easily. This may compromise the water-resistance properties of polymer-based sheathing membranes. In other words, detergents in stucco can reduce the ability of the stucco and the sheathing membrane to control rain penetration. For further information on the codes and evaluation aspects of stucco, contact Mr. Fadi Nabhan at the Canadian Construction Material Centre of NRC, Tel (613) 993-7702 , fax (613) 952-0268 e-mail fadi.nabhan@nrc.ca.
This article is an extract from IRC newsletter Construction Innovation, Spring 1998. The entire article is available on the World Wide Web at http://www.nrc.ca/irc/newsletter/toc.html
