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The influence of craftsmanship on the inner structures of lime plasters
Chalmers. (Kulturvård)ORCID iD: 0000-0002-2994-1766
2005 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The use of traditional lime plaster in restoration of old buildings is motivated by bothtechnical and historic reasons. The plasterer deals daily with a multivariable problem wherethe variables are related in such a complex way that we are not even close to findingconverging solutions by traditional scientific methods. The decisions and actions of thecraftsman will have an influence on both the surface structure and the inner structure of thelime plaster. The present paper will focus on the time between application of mortar andworking on the surface to make a floated finish. In total, 32 lime plaster surfaces have beenanalysed. Half of them have been worked on after they have stiffened up and half of themhave been worked on while they are still fresh. The results clearly show that the time betweenapplication of mortar and working the surface is a critical parameter. If you wait until theright time, you receive a more homogeneous and compact material with an open surface anda higher frost resistance.

Place, publisher, year, edition, pages
Rilem publications, 2005.
Keyword [en]
lime, plaster, pore structure, frost damages, craftsman
National Category
Other Natural Sciences
Research subject
Conservation (HGO)
Identifiers
URN: urn:nbn:se:uu:diva-220077OAI: oai:DiVA.org:uu-220077DiVA: diva2:704005
Conference
Proceedings of the International RILEM Workshop Repair Mortars for Historic Masonry. Delft, Holland 2005
Funder
Knowledge Foundation
Available from: 2014-03-10 Created: 2014-03-10 Last updated: 2014-04-16
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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