Review of Factors Affecting the Durability of Repointing Mortars For
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REVIEW OF FACTORS AFFECTING THE DURABILITY OF REPOINTING MORTARS FOR OLDER MASONRY MADELEINE ROUSSEAU INSTITUTE FOR RESEARCH IN CONSTRUCTION, NATIONAL RESEARCH COUNCIL CANADA, OTTAWA, ON B UILDING E NVELOPE T ECHNOLOGY S YMPOSIUM • O CTOBER 2 0 0 6 R OUSSEAU • 7 7 ABSTRACT In older masonry structures, the mortar properties need to be compatible with the exist ing mortars and be “sacrificial” to the masonry. Inappropriate repointing materials and exe cution practices can result in permanent damage to older masonry assemblies, affecting their aesthetics qualities, historical value, performance, and service life. Public Works and Government Services Canada (PWGSC), which is faced with the maintenance and refur bishing of the Canadian government’s older building stock, including the Parliament build ings, has been grappling with this problem for years. PWGSC and the National Research Council of Canada’s Institute for Research in Construction (NRCIRC) have carried out con siderable research in the last two decades to evaluate mortar mixes properties for applica tion in cold climates susceptible to freezethaw action and salt damage. This paper will describe critical elements of mix selection, onsite execution, and curing to obtain a durable mortar that has no detrimental effects on the masonry. SPEAKER MADELEINE ROUSSEAU – INSTITUTE FOR RESEARCH IN CONSTRUCTION, NATIONAL RESEARCH COUNCIL CANADA, – OTTAWA, ON Madeleine Rousseau is an architect with 25 years experience in the area of moisture migration in exterior walls at the National Research Council Canada (NRC). For the last 10 years, Madeleine has participated in NRC research for durable repointing mortars that do not have a detrimental effect on older masonry, in the context of cold climates experiencing freezethaw action. In 1998, Madeleine also initiated a multidisciplinary forum of masonry restoration specialists aimed at enhancing exchange of technical information on repointing mortar technologies. The working group, made up of trades, design professionals, building owners and managers, and manufacturers involved in masonry repointing work, has met twice a year since 1998. 7 8 • R OUSSEAU B UILDING E NVELOPE T ECHNOLOGY S YMPOSIUM • O CTOBER 2 0 0 6 REVIEW OF FACTORS AFFECTING THE DURABILITY OF REPOINTING MORTARS FOR OLDER MASONRY INTRODUCTION WHAT IS REPOINTING MORTAR? strength mortars are often not appropriate The selection of repointing mortars for Repointing mortars replace the outer and may cause damage. older masonry in cold climates is a subject deteriorated or damaged mortar in masonry Challenges to mortar selection include: of debate. Standards for modern masonry joints. The deteriorated mortar needs to be • Original mortar materials are no construction are not necessarily appropri carefully removed without damaging the longer available. ate for the needs of older masonry struc masonry units. The resulting gap is then • Difficulty in determining the exact tures, which can have unique performance cleaned and filled (repointed) with a com composition and properties of the requirements. As such, determining an patible repair mortar to stop further deteri existing mortar. appropriate mortar mix to use in the repair oration and water penetration (Figure 2). • Issues of historic authenticity. of masonry buildings can be a challenge. Repointing mortar mixes are drier and • Lack of trades specializing in old Repointing mortars should be durable, stiffer than bedding mortars to reduce materials and techniques. Many of practical in application (e.g., workmanship shrinkage and avoid staining of the mason the materials and techniques for and quality control), and not have a nega ry. For the repair of older brick and stone producing and applying traditional tive effect on the durability of the existing masonry walls, there is an increasing use of mortars were lost and substitutes masonry. Durability is not only dependent traditional mortar mixes, largely driven by may have to be found and tech on the mortar mix used, but also on how it the wish to have compatibility with the orig niques learned again. is installed and cured (workmanship) and inal mortar used and the existing bedding • Little published information on the on the severity of the environmental expo mortar (from historic, aesthetic, and mater performance of traditional mortars sure, which in turn depends on weather, ial property perspectives). Modern, higher in the Canadian climate. design, construction, operation, and main tenance. Public Works and Government Services Canada Heritage Conservation Directorate (PWGSC HCD) is responsible for the main tenance and repair of a considerable num ber of older masonry buildings, including the Parliamentary Precinct in Ottawa (Figure 1). For the past two decades, PWGSCHCD has teamed up with NRCIRC to carry out research projects which will help to define performance objectives of repointing mortars and assess a variety of mortar mixes in order to determine appro priate mixes for use in the repair of older masonry buildings. This ongoing work has been carried out to help fill a need for clear performance objectives for repointing mor tars so that an appropriate repointing mor tar can be chosen for a particular building. Performance objectives then need to be translated into set criteria that in many cases are still a subject of debate and research. This paper reviews literature dealing with the selection and performance of mor tars used in the repair of older masonry. Particular emphasis is given to factors af fecting the resistance to freezethaw dam age, a major consideration in a cold climate. Examples from Parliament Hill in Ottawa will be given as illustration of the text. (See Figure 1.) Figure 1 – The Center Block of Parliament Hill in Ottawa, Canada. B UILDING E NVELOPE T ECHNOLOGY S YMPOSIUM • O CTOBER 2 0 0 6 R OUSSEAU • 7 9 and at the mortar/unit interface). These, in turn, are caused by frost action, salt crys tallization (sulphates and chlorides – Figure 3), movement (settlement, differential, ther mal, and moisture), dissolution, environ mental pollution (acid rain), water migra tion (Figure 4), and biological attack in wet areas of the building. Moisture is the environmental factor commonly associated with most of these problems, although temperature also has a significant influence on the rate and extent of damage caused by the moisture. Temperature and moisture also directly affect expansion and contraction move ments in the masonry. Mortar can also cause problems for the masonry units such Figure 2 – Example of a mortar joint with repointing mortar. as lime leaching out of the mortar causing staining (Figure 5) or spalling with too • Traditional mortars, usually weaker POSSIBLE FORMS OF FAILURE strong a pointing mortar. than modern mortars, are less for Potential problems with mortar include Traditional mortars with high lime con giving of poor construction prac spalling, crumbling, efflorescence, biologi tent have more initial flexibility and higher tices; good quality control and site cal growth, and cracking (within the mortar porosity than modern mortars, and hence, supervision are needed they can better accom to ensure success. modate minor move • Difficulty in correlating ments in the wall with laboratory tests to actual out cracking. Cracks field exposure condi lead to increased in tions. gress of water into the • Few standards on use and testing of traditional mortar mixes. Modern Below: Figure 4 – standards have often Watershedding deleted reference to details deflect water older mortars but this off the upper surface trend is beginning to of the masonry. reverse. Unfortunately, the water is deflected Mortars are basically com onto lower surfaces. posed of a binder, aggregate (sand), water, and additives. Traditional mortars usually have lime as the major compo nent of the binder. Today, many restoration mortars have a cement/lime binder – usually with more lime than cement. There is also increasing interest in the use of pure lime mortars, hydraulic lime mortars, and proprietary premix mortars to which only water needs to be added. In contrast, binders for modern mortars have cement as the major component with either lime added in equal or less proportion, or with propri etary additives (e.g., masonry cement). Figure 3 – Salt damage to stone and mortar is apparent beside an entryway where salt is used to remove ice in the winter. 8 0 • R OUSSEAU B UILDING E NVELOPE T ECHNOLOGY S YMPOSIUM • O CTOBER 2 0 0 6 masonry that in turn hang has less risk of dam affects durability. If age than mortar in a cracks do occur, they are chimney). more likely to occur along North America has a the weaker mortar joints large climatic diversity. and not through the ma Climates vary from hot sonry units (this is prefer summers to cold winters. able, simply because it is In winter, some areas can easier and cheaper to be cold and relatively dry, repoint mortar joints than while others have large to repair or replace dam amounts of precipitation. aged brick or stone). Fine Regions with more precip cracks may reseal due to itation (snow and rain) redeposition of lime with and many freezethaw cy in the crack. cles are more severe for the masonry than regions DURABLE that tend to stay cold or REPOINTING mild over the entire win Repointing mortars ter. Building orientation should be as durable as affects the local climate possible without causing (e.g., prevailing driving damage to the existing rain and solar radiation). masonry (a dense mortar Not much can be done could retard the drying of about the weather, but the masonry assembly the design of the building, and cause frost or salt especially building de crystallization damage in tails, can have a large the bedding mortar or influence on the local masonry units). It is environment experienced preferable to repoint mor by the masonry. tar joints at more frequent Attention to water intervals than to have to shedding architectural Figure 5 – Lime leached out of the mortar joints onto the masonry repair damaged masonry features, including their surface leaves an unsightly stain.