Repointing mortars for older masonry

Publisher’s version / Version de l'éditeur:

Canadian Architect, 45, Sept 9, p. 44, 2000-09-01

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Repointing mortars for older masonry

Maurenbrecher, A. H. P.; Rousseau, M. Z.

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Mortars for repointing older masonry buildings

by Paul Maurenbrecher and Madeleine Rousseau

Institute for Research in Construction, National Research Council Canada

A version of this paper was published in the Canadian Architect, 45, (9), Sept pp. 44, 2000

Résumé

Mortars are basically composed of binders such as lime and Portland cement combined with sand and water together with any additives. For the restoration of older masonry there is an increasing use of pointing mortar mixes with lime as the major binder, largely driven by the wish to have compatibility with the original mortar (from a historic, aesthetic or material property perspective). Mortars having lime as the major component of the binder have more flexibility and higher porosity than those using Portland cement as the major component (common in modern masonry construction) and hence they can better accommodate minor movements in the wall without cracking. With larger movements, cracks are more likely to occur within the mortar joints and not through the masonry units; this is preferable, simply because it is easier and cheaper to repoint mortar joints than to repair or replace damaged bricks or stones. Today many restoration mortars are based on a mixture of lime and Portland cement (for example: 1 part Portland cement, 2 parts lime and 9 parts sand by volume). There is also increasing interest in the use of pure lime and hydraulic lime mortars (see boxed text).

Hydraulicity of mortars

Lime and Portland cement are both derived from limestone. However, their hydraulic properties are different. Lime (non-hydraulic), obtained by firing pure limestone at 900°C, is available as lime putty or dry hydrated powder. Lime in mortar hardens by reacting with carbon dioxide in the air (carbonation). This is a slow process and allowance needs to be made for it. Hydraulic lime is obtained by firing limestone containing clay ingredients at temperatures up to 1250°C. It hardens by reacting both with carbon dioxide in the air and with water in the mortar mix (hydraulic property). By increasing the clay content and the firing temperature to 1450°C, Portland cement is produced. It gains nearly all its strength by reacting directly with water (and gains it more rapidly). But increasing hydraulicity of the binder decreases the porosity and water and vapour transmission rate of the mortar, which reduces the drying rate should the wall get wet.

Mortars with high lime content present definite advantages for the restoration of older masonry buildings; however, the designer must be aware of current challenges to their use. Many of the techniques for producing and applying traditional lime mortars were lost and have to be learned again. These mortars are also less forgiving of poor construction practices; good quality control and site supervision (material proportioning, mixing procedure, joint preparation, pointing procedure and curing conditions) is needed to ensure success. The use of pre-mixed mortars to which only water needs to be added simplifies on-site preparation.

Performance requirements

Repointing mortars should be as durable as possible, without causing damage to the existing masonry (a dense mortar, for example, could retard drying of the wall and cause frost or salt crystallisation damage in the bedding mortar or masonry units). The mortar should be adapted to the particular masonry assembly under consideration (there is no miracle mortar mix suitable for all masonry). Desired properties of mortars for repointing older masonry include:

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existing bedding mortar (if the pointing mortar is too strong, stress concentrations could cause spalling of the masonry units).

• Water absorption and vapour transmission rates similar to or greater than those of the bedding mortar and masonry units. The pointing mortar should facilitate the drying of the masonry assembly through the mortar joints. Any salts in the masonry will then also migrate out through the mortar (salts can lead to damage or efflorescence).

• Little if any shrinkage after placement. Well graded and washed sand, with no clay fines, will reduce shrinkage, as will low water-to-binder ratios and adequate curing.

• Good (not necessarily strong) bond between mortar and masonry units. The mortar should be packed well into the joint. Good bond and low shrinkage reduce the risk of fine cracks forming at the interface between the masonry units and mortar. Most water infiltration through a masonry assembly occurs at this interface and at poorly filled joints.

• Resistance to frost action and salts (e.g., sulphates and de-icing salts) where needed. High lime content mortars have less resistance to freeze-thaw action when they become saturated (they are most vulnerable early in their life because they take a longer time to harden). On the other hand, the more porous mortars tend to dry faster, thus reducing the risk of damage. An air-entraining agent added to the mortar will improve frost resistance (these agents do not work well with very dry pointing mortar mixes). Great care must be taken in selecting mortars for areas of severe exposure such as chimneys, parapets, free-standing walls, exterior steps, and masonry below or at ground level (base selection on experience and/or testing). Exposure to de-icing salts will further reduce freeze-thaw resistance. Water management, a key to durable masonry

Mortar selection must take into account the severity of the environment in which it will be used. For example, in regions with freezing temperatures, a chimney will require a mortar exhibiting higher freeze-thaw durability than a wall sheltered by a roof overhang. Water shedding details -- roof overhangs, gutters, flashings, sills with proper drip edges-- reduce the accumulation of water on the façade. In colder climates, designers also need to control air exfiltration from inside the building to avoid condensation-related wetting within the masonry (it affects the service life of its constituents in many ways).

The selection of repointing mortars for older masonry buildings subjected to freeze-thaw action is still a subject of debate. New products are adding to this debate. In light of this, the authors brought together an informal working group on repointing mortars to share knowledge and experience. The group, made up of designers, restoration specialists, manufacturers, masons and researchers, meets twice a year. New

members are welcome.

For further information, visit the NRC/IRC masonry web site at http://www.nrc.ca/irc/bes/masonry.

Paul Maurenbrecher (paul.maurenbrecher@nrc.ca) and Madeleine Rousseau

(madeleine.rousseau@nrc.ca) are researchers in the Building Envelope and Structure Program at the Institute for Research in Construction (IRC) at NRC in Ottawa.