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The Smeaton Project - mortars for historic buildings - research




Properties of

Factors Affecting Lime-Based Mortars




The broad objective of the Smeaton Projectis to contributeto the understandingof the characteristics and behavior of lime-based mortarsfor the repairand conservation of historic buildings. This article presents the first phase results of a joint research program of ICCROM, EnglishHeritage,and BournemouthUniversity.

PrefaceThe first phase of the Smeaton Project was intended to be a rangefinding exercise, which would 1) assist the project team in refining the methods used for preparation, cure, and testing of samples and 2) establish some trends so as to better define and limit future phases of the research. In general, these objectives were achieved. However, the authors wish to emphasize the very preliminary nature of the findings reported in this paper. They are published primarily to promote dialogue and, perhaps, more widespread collaboration in the future. In no way should the results be considered definitive or conclusive. The paper should, thus, be read as an exploratory exercise that will be used to focus and refine future work, rather than as a conclusive scientific study. In the first phase, insufficient samples were produced to statistically validate trends. The consistency of results was also affected by both the method of preparation and cure of the samples. Problems with demoulding and cutting of the large samples used in Phase I, for example, meant that certain samples were not available for all test procedures. Based on this experience, both preparation and cure methods have been altered in Phase II, which is now underway. Similarly, there is controversy about some of the test procedures utilized. In particular, the salt crystallization test that is still widely used to assess durability in the U.K. is no longer recognized by the Canadian Standards Association and is currently under review by ASTM.

There is much discussion regarding this test and freeze-thaw durability tests in general on a European and international level. Again, it was decided to utilize the salt crystallization test largely to assess its suitability and to establish some basis for comparison of samples. Modifications have been made in the experimental design of Phases II and III based on this experience. Finally, it was impossible, in this very preliminary stage of research, to consider all variables that might have affected the behavior of the mortars tested or to explain all mechanisms observed. This is the nature of scientific enquiry. It is hoped that some questions may be answered through more detailed laboratory studies later in the project as well as through comparison with long-term results observed on the exposure site and in field testing.

IntroductionInterest (or perhaps renewed interest) in the use of lime based materials mortars, grouts, plasters, renders and paints - for use in the repair and maintenance of historic buildings and monuments has been growing steadily in the international conservation community for the past 15 to 20 years. To a large extent, current practices have evolved through trial and error informed by only limited scientific and academic research. Although there is a significant body of experience in the use of lime-based materials in certain parts of the world, it is apparent that practice is not always matching theory and that there are still many partially unsolved problems.




of The characteristics mortarsmay definedin severalways. In practibe cal terms,the ones that concernus most are: 1. Of the freshmortar * workability * moisturecontent * rate of hardening shrinkage ? 2. Of the hardenedmortar * appearance * moistureand air content * permeability * mechanical properties including * adhesion * abilityto tolerate movement strength ? * durability(resistance damto age by frost and salts) These,togetherwith the chemical propertiesof the mortar,must clearly be compatiblewith existinghistoric materialsand appropriate the conto text in which they are to be used. The factorsaffectingthe characteristicsand behaviorof lime mortars are derivednot solely from the mortar constituentsbut also from the techniquesused in processingthe and and ingredients in preparing the mortars. The properties placing of materialsin contactwith the mortar and ambientenvironmental conditions at the time of placingand hardeningof the mortarare also influential. In summary, thereare probably threecategoriesof problemsthat need to be addressed. These are :1. Mortar analysis and the components of historic mortars 2. Performance criteria and components of specification mortars 3. Mortar preparation and utilization

The SmeatonProject- a joint researchprogramof ICCROM(the International Centrefor the Study of the Preservation the and Restorationof CulturalProperty), EnglishHeritage(the Historic Buildingsand Monuments Commissionfor England),and Bournemouth University was set up in responseto the need to address such issuesin a systematicway throughboth laboratoryand field research. The broadobjectiveof the SmeatonProjectis to contributeto the understanding the characterisof tics and behaviorof lime-basedmortars by attempting identify- and to where possiblequantify- the material and practiceparameters that affecttheirproperties. Specificemphasishas beenplaced in the first stage of the research on materialand practiceparameters that will improvethe durability and of frost-resistance lime-based mortars in harshclimates. It is hoped that the research will ultimatelylead to the productionof practicalguidelines for specifying,preparing, and mortarsin a utilizingconservation wide varietyof regionalconditions. The name of the projectrefersto John Smeaton,a mathematical makerand engineer, instrument who in 1756, afterexperimenting with limes and additivesfor "waterbuilding" (mortarcapableof hardening underwater),decidedto lay the coursesof stone for the Eddystone lighthouse(20 miles off the coast of Devon, England)in a mixtureof pozzolanafrom Italyand Liaslimefrom the southwest of England. His report on this work1 was the first of many studies preceding the current project that, in a sense, brought together England and Italy in seeking to identify and quantify pozzolanic additives for optimum performance.

TO BACKGROUND CURRENT PROJECTThe Smeaton Project grew out of experimental work begun by English Heritage in 1986 to identify suitable mortars for use in the conservation of Hadrian's Wall (Fig. 1) in the north of England. Sections of this Roman wall are on high ground and exposed to severe weather conditions. In the recent past, relatively strong mortars gauged with portland cement had been used for pointing and repair. This mortar was itself sufficiently durable to withstand the extreme exposure to which the Roman wall is subjected but contributed to the deterioration of the wall in a number of ways that are typical where cement is used in contact with weaker and more porous materials. These deleterious effects include: 1. Concentrating cycles of wetting and drying through the stone faces because water cannot escape through the relatively impervious pointing mortar 2. Trapping water, thereby causing leaching of original core mortar and increasing the risk of frost damage 3. Increasing the likelihood of mechanical damage to stone on removal of high-strength mortar in the course of maintenance works Recognition of these problems led to an attempt to exclude all cement in mortars used on the Roman wall. However, early trials with lime mortars were not completely successful because the mortars had inadequate frost resistance. While their behavior as a sacrificial material was technically satisfactory, such mortars had to be replaced at a frequency that was not economically sustainable for English Heritage.




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