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Permeability in lime plaster in relation to durability of covered materials
Chalmers. (Kulturvård)ORCID iD: 0000-0002-2994-1766
Gotland University, School of Culture, Energy and Environment. (Kulturvård)
2004 (English)Conference paper, Published paper (Refereed)
Abstract [en]

In restoration of old buildings, lime plasters are used for reasons of authenticity and for technicalreasons as well. In many traditional constructions, there is wood behind the plasters or there is old limemortar in the masonry. For the durability of both these underlying materials, it is important that therelative humidity, RH is not too high under a long time of exposure. This can lead to an accelerateddegradation process of wood and leaching of lime in mortars. To prevent this, the plaster must have aproper permeability.The permeability of a material determines its ability to transport gas and moisture. In lime plasters,permeability affects hardening, carbonation as well as durability of the underlying materials. In thepresent paper, eleven mixtures of lime mortar were studied. The binders used were lime slaked in fourdifferent ways, hydraulic lime and cement.The following measurements were made to analyse the permeability and the pore structure of thesamples. Gas permeability and draining curves give an indication of rate of carbonation. Vapourpermeability shows the ability of plaster to transmit moisture at different RH. Thin sections were madefor microscopically investigations of the samples.The investigations show that both the vapour permeability and gas permeability decreases with anincreased amount of hydraulic binder in the lime plasters. Even small amounts of hydraulic binderhave a significant effect.By connecting these results with examples from reality, the picture becomes clearer. In the medievaltown Visby in Sweden, there are several examples of both rotten wood and disintegrated lime mortars.It is constructions from 13th to 19th centuries and low permeable cementitious plasters have coveredthem all, during the middle of the 20th century.

Place, publisher, year, edition, pages
2004.
Keyword [en]
lime, cement, plaster, pore structure, permeability
National Category
Other Natural Sciences
Research subject
Conservation (HGO)
Identifiers
URN: urn:nbn:se:uu:diva-220075OAI: oai:DiVA.org:uu-220075DiVA: diva2:704001
Conference
Proceedings of the 32nd IAHS World Congress on Housing – Sustainability of the Housing Projects. Trento, Italy 2004.
Funder
Knowledge Foundation
Available from: 2014-03-10 Created: 2014-03-10 Last updated: 2016-01-28
In thesis
1. Kalkputs: porstrukturens betydelse för beständighet
Open this publication in new window or tab >>Kalkputs: porstrukturens betydelse för beständighet
2005 (Swedish)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

When restoring historic lime plaster it can be difficult to reach the required durability. Today newly made lime plaster can fall off due to frost damages after only a couple of winters. To understand and solve these problems, the subject has been studied from different perspectives. Several factors in the production step are making an influence on the durability of lime plaster: 1. A binder, suitable for the building, must be chosen along with sandthat gives good material properties in both fresh mortar and in plasters. 2. The lime/sand ratio and the mixing technique should be chosen from the properties in the lime and the sand, in order to get a mortar with good workability. 3. The craftsmanship should be adjusted to the mortar, the weather and the underlying materials. A plaster with good frost resistance has a certain pore structure. The plaster should have some round air pores that contain air even when the material is filled capillary with water. Such air pores give the water a free space to expand when it is freezing. If the air pores are missing, the ice crystals may damage the plaster when expanding. To increase the chance of making a frost resistant lime plaster, the lime/sand ration must be adjusted so the lime can fill up well in the sand. Otherwise a collapsed pore system can easily occur, which means an open pore system without distinct air pores. A collapsed pore system contains many pores well connected with each other. Such a system is easily damaged by frost. To increase the chance of success, it is of great importance to work the surface of the lime plaster at the right time and in the correct way. Before working on a surface the mortar must be aloud to stiffen. Only then a homogeneous material can be created; cracks due to shrinkage can be pressed together and the result is a more compact material with an open surface. The time necessary for mortars to stiffen is related to the water content of the mortar, the suction of the underlying surface and the weather conditions. If the surface is worked on while the mortar is still fresh, the binder can form a hard lime shell on the surface. Inside thematerial a lack of binder can appear. Such plasters have a very low frost resistance. Other durability problems related to plaster are damages in the underlying materials, i.e. rotten wood in covered constructions or leached lime in old joints. Such damages can occur if the covering mortar is made of strong hydraulic lime or cement, since they form plasters with a low permeability in comparison to lime. Due to mentioned findings, it is important to study how a pore structureis influenced by materials, mixture and craftsmanship. Only then it is possible to understand how damages can be decreased.

Place, publisher, year, edition, pages
Göteborg: Chalmers Tekniska Högskola, 2005
Keyword
kalkputs, kalkbruk, murning, putsning, hantverk, frostresistens, porstruktur
National Category
Other Natural Sciences
Research subject
Conservation (HGO)
Identifiers
urn:nbn:se:uu:diva-220085 (URN)
Presentation
2005-04-01, Chalmers, Göteborg, 10:00 (Swedish)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2014-03-11 Created: 2014-03-10 Last updated: 2014-03-11Bibliographically approved
2. Traditional lime mortar and plaster : Reconstruction with emphasis on durability
Open this publication in new window or tab >>Traditional lime mortar and plaster : Reconstruction with emphasis on durability
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lime mortar and plaster have been investigated with the aim to improve the knowledge on how to make them as durable as before the cement technology was developed. The background was the durability problems experienced for newly produced lime plaster on the medieval churches on the island of Gotland, Sweden. In some cases the new lime plaster façades showed severe frost damages after only one winter. Although the lime was burnt and produced according to old local traditions, the lime mortar was still mixed and worked onaccording to methods developed for lime-cement mortar. This often led to a very porous lime plaster with a lime shell in the surface and such a plaster has been shown to be sensitive to frost expansion. Field studies were combined with laboratory studies of thin section specimens. Optical microscopy and scanning electron microscopy have been important analytical methods showing the porosity and the structure of the binder and aggregate materials. The investigations have been carried out on both historic and on newly made reference mortar and plaster. The field studies were carried out mainly on Gotland, using local materials. The influence of the raw materials, i.e. lime, aggregate and blending ratio was investigated. The focus has been on the workability of the fresh mortars as well as the pore structure of the carbonated plaster. The craftsmanship, meaning mixing and application of mortar and working the plaster surface, was studied in order to clarify its final pore structure. The pore structure in a material determines many of its technical properties, such as moisture transportation, compressive strength, permeability and frost resistance. All these properties are closely connected to the durability of the mortar and plaster. The permeability of the plaster has an impact also on the durability of the covered construction materials. Behind low-permeable plasters made with hydraulic binder, examples of extensive damages of rotten wood and leached lime have been shown. The investigations have shown the importance of choosing a mortar adjusted to the building construction. They also showed the importance of choosing a blending ratio adjusted to the specific binder and sand used in order to get a mortar with a suitable pore structure and good durability. It has also shown the importance of knowing when and how to work on the plaster surface in order to obtain a homogenous material that is well receptive for lime wash and has a good frost resistance. The combination of all the investigations has led to a method for reconstructing historic mortar and plaster with good durability.

Place, publisher, year, edition, pages
Göteborg: Chalmers Tekniska Högskola, 2007. 94 p.
Keyword
lime plaster, lime mortar, masonry, craftsmanship, pore structure, frost resistance, historic mortar
National Category
Natural Sciences
Research subject
Conservation (HGO)
Identifiers
urn:nbn:se:uu:diva-220119 (URN)9789172919907 (ISBN)
Public defence
2007-09-27, Wisby Strand, Visby, 09:00 (Swedish)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2014-04-16 Created: 2014-03-11 Last updated: 2014-04-16Bibliographically approved

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