Type N mortar mix has a medium compressive strength and it is composed of 1 part Portland cement, 1 part lime, and 6 parts sand. Five mortar types, each with a corresponding recommended mix, have been established by ASTM to distinguish high strength mortar from soft flexible mortars. Analysis requires interpretation, and there are important factors which affect the condition and performance of the mortar that cannot be established through laboratory analysis. The exact physical and chemical properties of the historic mortar are not of major significance as long as the new mortar conforms to the following criteria: This mortar is the proper consistency for repointing historic brick. It is described more fully in "The Russack System for Brick & Mortar Description" referenced in Reading List at the end of this Brief. Some late-19th and early-20th century buildings, for example, have horizontal joints that were raked back while the vertical joints were finished flush and stained to match the bricks, thus creating the illusion of horizontal bands.

There may be several colors of mortar dating from different construction periods or sand used from different sources during the initial construction. But it was not in common use throughout the country until the early 20th century. on location for its use. 29 Slate Roofs the proportions of mix are identified and the ratio of mix to sand 38 Removing Graffiti The strength rises to 15 MPa at 3 days, 23 MPa at 1 week, 35 MPa at 4 weeks, and 41 MPa at 3 months. It is made from Portland cement, lime, sand, and water in varying ratios. Thus, a Type K mix, for example, would be referred to as 1-3-10, or 1 part cement to 3 parts lime to 10 parts sand. Here, a hammer and chisel are being correctly used to prepare a joint for repointing. The scope of the earlier Brief has also been expanded to acknowledge that the many buildings constructed in the first half of the 20th century are now historic and eligible for listing in the National Register of Historic Places, and that they may have been originally constructed with portland cement mortar. Technology and Conservation. After the initial setting, immersion in warm water will speed up setting. mortar that is "type N" based on composition, but have a much lower

In addition, a bonding agent is difficult to remove if smeared on a masonry surface. and type M mortar. Acceptable guidelines on particle size distribution may be found in ASTM C 144 (American Society for Testing and Materials).
Sand must be added in a damp, loose condition to avoid over sanding. Examine the powdered portion—the lime and/or cement matrix of the mortar.

After tooling, excess mortar can be removed from the edge of the joint by brushing with a natural bristle or nylon brush.

7F. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. The mortar may also have been tinted to create a colored mortar, and this color should be identified at this point. analysis, mortar composition testing, and custom mortar color matching. Mortar Matching | It may also be used in emergency construction and repairs, and construction of machine bases and gate installations. Photo: NPS files. Dry ingredients are measured by volume and thoroughly mixed before the addition of any water. crushing the sample and analyzing the chemical reaction with acid,

This type of cement costs about the same as type I. First, the mortar should be fully hardened before cleaning. Because some of the kinds of lime, as well as other components of mortar, that were used historically are no longer readily available, even when a conscious effort is made to replicate a "historic" mix, this may not be achievable due to the differences between modern and historic materials. 16 Substitute Materials Surface grouting is sometimes suggested as an alternative to repointing brick buildings, in particular.

447-452. AMR Labs

Masonry deterioration caused by salt deposition results when the mortar is less permeable than the masonry unit. High sulphate resistant; generally develops strength less rapidly than the other types. Historic mortars were not prepared to narrowly defined specifications from materials of uniform quality; they contain a wide array of locally derived materials combined at the discretion of the mason. 8A Revised. Most mortars dating from the mid-19th century on—with some exceptions—have a fairly homogeneous texture and color. 11490 Commerce Park Drive Methods for analyzing mortars can be divided into two broad categories: wet chemical and instrumental.

The owner or administrator of a historic building should remember that repointing is likely to be a lengthy and expensive process.

9 (September 1994), pp. Aberdeen's Magazine of Masonry Construction. Instrumental analysis methods that have been used to evaluate mortars include polarized light or thin-section microscopy, scanning electron microscopy, atomic absorption spectroscopy, X-ray diffraction, and differential thermal analysis. The consultant must also be prepared to spend more time in project inspections than is customary in modern construction. (Plastic may be used, but it should be tented out and not placed directly against the wall.) Examination and analysis of the masonry units—brick, stone or terra cotta—and the techniques used in the original construction will assist in maintaining the building's historic appearance. 35 Architectural Investigation

This relative analysis which is derived from the 10-point hardness scale used to describe minerals, provides a good starting point for selection of an appropriate mortar. Photo: NPS files. Qualified contractors then can provide lists of other repointing projects for inspection. If the joint is properly prepared, there will be a good bond between the new mortar and the adjacent surfaces. composition and ratio of mix.

sand used. Many illustrations are new and in color; Captions are simplified and some complex charts are omitted. More recent, 20th-century mortar has used portland cement as a primary binding material. AMR Sickels-Taves, Lauren B. Several precautions should be taken if a freshly repointed masonry wall is to be cleaned. Several layers will be needed to fill the joint flush with the outer surface of the masonry. Vol.

Thus, the mortar found in masonry structures built between 1871 and 1930 can range from pure lime and sand mixes to a wide variety of lime, portland cement, and sand combinations. Penciling is a purely decorative, painted surface treatment over a mortar joint, often in a contrasting color. The Ancient Egyptians were the first to use lime mortars. The sand analysis provides detailed information about the original The drawback of most mortar analysis methods is that mortar samples of known composition have not been analyzed in order to evaluate the method. It is usually similar to the sand in the historic mortar and provides a better visual match. In this situation it is important to ensure that the specifications stipulate that masons must have a minimum of five years' experience with repointing historic masonry buildings to be eligible to bid on the project. Davidson, J.I. It may liberate additional salts, which, in turn, can lead to more efflorescence. Robert G. Blezard, "The History of Calcareous Cements" in Hewlett, Peter C., ed.. Saikia, Mimi Das. 1.25 mix) with more sand or fine rounded sand particles will make a But mixing, in the familiar sense of turning over with a hoe, sometimes may not be sufficient if the best possible performance is to be obtained from a lime putty mortar. Although spot repointing is generally preferable and some color difference should be acceptable, if the difference between old and new mortar is too extreme, it may be advisable in some instances to repoint an entire area of a wall, or an entire feature such as a bay, to minimize the difference between the old and the new mortar. Mortar pan mills which have a long tradition in Europe produce a superior lime putty mortar not attainable with today's modern paddle and drum type mixers. Total repointing may also be more sensible when access is difficult, requiring the erection of expensive scaffolding (unless the majority of the mortar is sound and unlikely to require replacement in the foreseeable future). 4, No. A mortar composition test is the only way to identify the 8 Revised II. It is generally assumed unless another type is specified. 364-368.
47 Maintaining the Exterior Mortar should be used within approximately 30 minutes of final mixing, and "retempering," or adding more water, should not be permitted. The mortar joint in a historic masonry building has often been called a wall's "first line of defense." Because masonry cements are not required to contain hydrated lime, and generally do not contain lime, they produce high strength mortars that can damage historic masonry. mortar testing, sand Usually a 3 foot by 3 foot area is sufficient for brickwork, while a somewhat larger area may be required for stonework. The three key characteristics of sand are: particle shape, gradation and void ratios. Preliminary research is necessary to ensure that the proposed repointing work is both physically and visually appropriate to the building.

Bulletin of the Association for Preservation Technology. Therefore, when specifying sand for repointing mortar, it may be necessary to obtain sand from several sources and to combine or screen them in order to approximate the range of sand colors and grain sizes in the historic mortar sample. CBD 163.

43 Historic Structure Reports


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