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Repointing Historic Masonry Structures

by Edward A. Gerns and Thomas R. Wegener

ASTM Subcommittee E06.24 on Building Preservation and Rehabilitation Technology, part of Committee E06 on Performance of Buildings, has recently completed E 2260, Guide for Repointing (Tuckpointing) Historic Masonry. The guide was prepared with input from ASTM Committees C15 on Manufactured Masonry Units and C12 on Mortars and Grouts for Unit Masonry. Although many portions of the guide are relevant to repointing newer masonry structures, the guide is intended for historic masonry and does not necessarily address all of the concerns regarding newer masonry structures.

Repointing is the process of removing deteriorated or distressed mortar from joints between masonry units and installing new mortar into the joints. Repointing is likely to be necessary for most exterior masonry work at some point in its life, and is one of the basic processes in the refurbishment of, and reducing weathering effects on, masonry. Repointing may improve the weather resistance by reducing the amount of water that penetrates the masonry, thus increasing its longevity as well as other components of the structure, which can deteriorate due to water infiltration through the masonry.

Proper repointing can last 25 to 30 years, however, improper repointing will do little to extend the life of the masonry and may lead to irreversible damage to masonry units. Thus, proper repointing procedures are important to protect historic masonry structures and enable current and future generations to appreciate them.

The new standard on repointing provides guidance on determining which joints to repoint, evaluating in-situ masonry, evaluating techniques for mortar removal, determining appropriate repointing mortar, and executing and inspecting repointing work.

Historical Background

Mortar has been used to hold together various masonry materials from fieldstones to man-made brick for more than 4,000 years. The earliest mortars were typically combinations of various common local materials such as clay and mud. The clay mortars tended to be very soft and somewhat soluble and therefore were not very durable except in hot, arid climates or in protected environments.(1) Later, mortars that were composed of water, lime and sand were developed, and were known as sand-lime mortars.

Until the 1900s, almost all load-bearing masonry construction used sand-lime mortar. These mortars develop their strength through a chemical reaction called carbonation between the lime and carbon dioxide in the air. The carbonation of the lime is a very slow process, therefore the hardening process is very slow. In the construction of older structures, where schedule was not significant, a slow hardening process tended not to be a significant issue.

Within the past 150 years, the construction of larger civil structures, such as bridges and dams, led to a need for quicker setting mortar with greater compressive strength. Sand-lime mortars continued to be used, but natural cements were sometimes added to increase compressive strength and reduce curing times.

Portland cement was first developed in England in the 1820s and in the United States in the 1870s.(2) Cements are produced using limestone, clays and shales that are ground to a powder. The ground rock is then burned in a cement kiln to produce a clinker. The clinker is then ground into a fine powder and gypsum is added.

Mortars made with portland cement were made in a way similar to sand-lime mortars. The materials were brought to the project site in bulk and mixed together in the specified proportions on site.

As the need for increased production, cost saving and product consistency became significant issues in the construction industry, the development of premixed materials was inevitable. The first of these types are masonry cements that were developed in the 1920s; they consist of premixed and bagged cementitious ingredients and other additives and fillers of various proprietary compositions.(3) The bagged material is brought to the site and then mixed with sand and water.

In the 1970s, ready-mix mortars were perfected following the development of set retarders which extended the board life of mortar to as much as three days. The mortar could now be mixed in much the same way as concrete at central batching plants and then distributed in bags or barrels to the construction site.

Today’s Mortar

The majority of mortars used today are sand-lime-cement mortars and masonry cement. Sand-lime mortars are still used in some restoration projects when a very soft mortar is necessary for use between very soft stones or bricks. Sand-lime mortars are much more commonly used in Europe for both new and restoration projects.

Cement, lime, and sand are used in the creation of a mortar that complies with ASTM C 270, Specification for Mortar for Unit Masonry. This specification defines mortar either by composition or physical properties. Composition is defined by volume as the ratio of portland cement or blended cement lime, lime, and aggregate to create cement-lime mortars; and ratios of masonry cement and aggregate to create masonry cement mortars. The difference in mortar types is achieved by varying the relative proportions of the sand, lime and cement. Physical properties include 28-day compressive strength, water retention and air content. Properties of four types of mortar are defined by the specification: S, M, N, and O, with Type S being the hardest with an average compressive strength of 2500 psi (1.7 MPa) and Type O being the weakest at 350 psi (2.4 MPa). Type K mortar is another mortar that since 1982 is no longer included in ASTM C 270, but which is still used for some repointing projects. Type K mortar has the lowest compressive strength, and some historic masonry structures require the softness of a Type K mortar.

Pigments are often added to repointing mortar to change its color to match the existing mortar. Pigments must conform to ASTM C 979, Specification for Pigments for Integrally Colored Concrete.

Repointing

The most common misconception regarding mortar and repointing is that stronger mortar is better mortar. In fact, not only is this not true, but when the mortar is stronger than the adjacent masonry materials, it can actually induce damage to the masonry units and reduce the wall’s long-term durability. If the repointing mortar is too strong, stresses induced into the walls from applied structural loads to thermally induced stress are concentrated along the edges of the stone or brick in contact with repointing mortar. This concentration of stress may cause fracturing of the face of the masonry units. Mortar must have some flexibility or softness to accommodate movement while still maintaining a bond with the adjacent masonry materials. Mortar used for building a wall and for repointing should always be softer and more permeable than the adjacent masonry materials. The strength of the masonry units and of the existing mortar should be taken into consideration when specifying the strength of the repointing mortar.

Repointing of masonry is typically necessary to replace deteriorated mortar in joints of the masonry. Further information for determining what is considered a deteriorated joint is provided in the guide: “When considering repointing, especially on masonry of artistic, architectural, cultural, or historical significance, guidance from a specialist experienced in historic masonry and repointing should be sought.”

In every repointing project, proper joint preparation is critical to the long-term durability and performance of the repointing work. Frequently, improper joint preparation only consists of hand-scraping loose mortar from joints and then applying a new, very thin layer over the existing mortar. Mortar applied with this technique typically falls out of the joints within five years. Mortar installed into a properly prepared joint can last between 25 and 30 years.

Proper preparation of mortar joints should include the removal of old mortar to a minimum depth of two to three times the width of the joint to ensure an adequate bond and performance of the mortar. With joints typically being 3/8 in. (10 mm) wide in most brick masonry, this requires removing existing mortar to a depth of at least 3/4 in. (20 mm) from the face of the masonry. Additionally, deteriorated mortar encountered behind the minimum depth should also be removed. The mortar should be removed to a uniform depth from all joints in the area to be repointed, although this can sometimes be very difficult particularly at the intersection of vertical (head) and horizontal (bed) joints.

Mortar can be removed using hand tools such as hammers and chisels, but typically, electric grinders are used. Extreme care should be taken when using power tools to prevent damage to the adjacent masonry material. The thickness of the blades used on electric grinders should never be more than half the thickness of the joint being cleaned. The grinder should be used to remove the mortar in the center of the joint; the mortar adjacent to the masonry, on either side of the cut, should be removed using hand tools. Methods for the evaluation of mortar removal techniques are included in the guide.

Once the mortar has been properly removed and mortar dust and debris have been properly cleaned out of the joints, an appropriate mortar should be selected. For most walls, a Type N mortar provides good durability as well as enough flexibility to not damage the masonry. If the wall is constructed of sandstone, brownstone or other soft stones, or soft masonry units, a softer mortar may be necessary.

In some instances, the properties of a sand-lime mortar may be preferable because of the characteristics of the masonry materials. Sand-lime mortars require special procedures for preparation, installation and curing, which are beyond the scope of this article and ASTM E 2260. Representative references for sand-lime mortar are included as references in the guide.

Controlling mortar shrinkage is one issue that requires greater attention in repointing masonry vs. new construction of masonry. In repointing projects, mortar shrinkage can lead to cracking of the mortar and thus allow greater amount of water into the masonry. To minimize shrinkage cracking of the mortar once it is in place in the joints, the mortar should be prehydrated. Prehydration consists of mixing dry ingredients together with only so much water that the mortar can be formed into a ball by hand. The mortar should then be allowed to sit undisturbed for 1/2 hours (hydration of the cementitious material), then water is added until the mortar reaches a workable consistency.

Mortar should be pressed into the joints in three successive layers, installing each new layer once the previous has set so that it doesn’t deform under thumb pressure.

Repointing should be performed when ambient air temperatures are between 40° and 90° F (6° to 30° C). To prevent the mortar from freezing, repointing should not be performed when the outside temperatures are expected to be below 40° F (6° C) for at least 48 hours during the curing process after the mortar has been installed. Mortar that freezes during the curing process is less durable and less weather resistant. In very hot weather, it is important that enough water is maintained in the mortar to ensure proper curing and minimize shrinkage cracking. This may require periodically wetting the wall during the curing process.

An effective repointing project should include an evaluation of the existing materials, determination of the appropriate mortar to be used, determination of the extent of repointing that is necessary and installation of a mock-up to evaluate the repointing process as well as the aesthetics of the final product. If proper evaluation and techniques are used to repoint masonry walls, aesthetically pleasing and long lasting results can be achieved. //

References

(1) Beall, C., “Masonry Design and Detailing for Architects, Engineers, and Contractors, Third Edition,” McGraw-Hill, Inc. New York, pp. 113-122. 1993.
(2) Speweik, J. P., “The History of Masonry Mortar America 1720-1995,” National Lime Association. 1995.
(3) Isberner, A. W. “Properties of Masonry Cement Mortars” Portland Cement Association Research and Development Bulletin RD019.01M. 1974.

Copyright 2003, ASTM

Edward A. Gerns and Thomas R. Wegener, co-chairs of ASTM Subcommittee E06.24 on Building Preservation and Rehabilitation Technology, are masonry specialists with Wiss, Janney, Elstner Associates, Inc. in Chicago, Ill.