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Brief History
Traditionally, the first mortar was made with lime and sand, which would make lime putty. Starting in the early 20th century, however, masons started using Portland Cement, a strong, fast drying cement, not purely on its own though. Masonry cement made its appearance in the 1930s, which is a combination of Portland cement and ground limestone.

Getting Started
Before starting any actual work, examine the masonry units, mortar, and the techniques used in the original construction. This will help identify the true problem. Find out what were the causes of the deterioration or cracks. If there are cracks or problems in the actual bricks or stone masonry you could have a larger problem that also needs to be addressed. Repointing may cause further damage. If working with a historic structure, hire an architectural historian or conservator to help pinpoint the problems. If the crack is smaller than 2mm and not causing any major defects, than it is better to leave it and not repoint. (Holland) It is common to see cracking along old repairs because of the shrinkage of hard mortar and the seasonal flexing along the joint. (Holland)

Establishing Materials
Examining the structure before working will also help you establish the strength and permeability of the new mortar similar to or better than the original. It helps to establish what the original components of the old mortar are so you can match it. "It is essential that the mortar used for repointing have similar performance characteristics to the original mortar used in a building. Such performance characteristics include permeability, compressive strength, and coefficient of thermal expansion. The mortar must have greater vapor permeability and softer compressive strength than the original mortar. The mortar should also not be stronger (in compressive strength) than the masonry units because it will not have give. Rather than the mortar relieving the stress, the masonry units will, which will cause further damage to the masonry unit, such as cracking or spalling. This is when the face or outer section of a masonry unit breaks away from the rest of the unit. This will be more expensive and strenuous to fix. So for example, if a soft lime-based mortar was originally used, the most appropriate repointing mortar is likely to also contain a large amount of lime.

An architectural conservator can perform a mortar analysis in order to make recommendations for replacement mortar that is both physically and aesthetically compatible with the building. There are two common methods of analyzing mortar. The first is called "wet chemical." This is when a sample of the mortar is crushed and mixed with a dilute acid. The mortar will be broken down, and the type of mortar and sand used will be determined by the color and the texture. Another form of "wet chemical" analysis is the same process by the carbon dioxide gas that is given off by the digestion will be collected and the type of mortar will be determined by its volume. The amounts of each component will also be determined. The second method to analyzing mortar is "instrumental." There are several different forms of "instrumental" analysis; however, the most commonly used is thin-section microscopy. This is when thin slices of mortar are examined by a transmitted light microscope. This process can provide more information than "wet-chemical" examination. Other examples of "instrumental analysis are scanning electron microscopy, X-ray diffraction, and atomic absorption spectroscopy. Your analysis should not be solely based on lab work, however. There are important performances of mortar that can not be determined in a lab: original water content, rate of curing, weather conditions during original construction, method of mixing and placing the mortar, and cleanliness of sand.

It is important to also match the color of the mortar. However, "in the past lime mortar tended to be mixed on site with whatever sand was locally available. Since the sand influences the color of the lime mortar, colors of pointing mortar can vary dramatically from district to district." You can allow weathering to match the new mortar to the old mortar. (Holland) The tooling should also match the tooling of the historic mortar.

Test Panels
Again, before starting any work, start by testing your methods and materials in a test panel. A test panel is an area on the structure out of plain site where you can test the repointing methods you will use, the color of the mortar, and the skills of your mason. This are will also be good to determine the types of tools you will need, which will be discussed in the next paragraph. For a brick structure, the panel should not be any bigger than 3' x 3'. For other masonry units, the test panel can be a little larger. It is also important to pick the right season to do the repointing. Extremely high or low temperatures can cause rapid drying which can have negative effects on the mortar, masonry units, and the structure itself.

Removal of Old Mortar
After examination, the next step is cutting away the old mortar. '''"The old mortar is removed to a depth equal to or slightly more than the width of the joint, or to the point where sound mortar is reached." "Removal of old mortar between joints is done carefully to avoid damage to masonry units. On buildings with soft materials, such as under-fired brick, lime mortar, or terra cotta, removal by hand is often the most effective to avoid damage. Hard Portland cement mortar is usually removed with a grinder or power circular masonry blade, taking care not to damage the masonry units. Vertical joints or thin joints are done by hand or with small power chisels."''' If a power saw needs to be used, only use it on wide and uniform horizontal joins. (Maurenbrecher) Only use the power saw in the middle of the joint. Use a hammer and chisel for the rest closer to the masonry units. (Maurenbrecher). Caulking cutters with diamond blades are an acceptable power tool to remove mortar in wide horizontal joints. They are slow, have no rotation, and vibrate so they are likely to cause less damage to the masonry units.

After the mortar has been cut out, the joints should then be vacuumed or brushed clean. After vacuuming, the remaining debris can be removed with a water jet from the top of the masonry area down. (Maurenbrecher)

Filling in New Mortar
The areas with the deepest recession of mortar should be filled first. The mortar should be out in in several layers, starting with the first layer of 1/4 inch. Let it harden to a desired strength then add another 1/4 inch layer, and continuing this process until flush with the masonry. High-lime mortar dries rapidly, so after to reaches a particular strength it needs to be sprayed with water to avoid chalking, poor adhesion, or poor durability. Mist the structure every two hours for tow days, then every four hours for two days. It is important that the mortar stay wet for at least a couple of days while it is setting. The local conditions will control how long you should keep the structure wet, however. Burlap or plastic should be placed over the repointed wall or structure. This keep the building damp and eliminates direct sunlight. The burlap and the plastic should not be resting against the structure, however. The burlap or plastic can be anchored to the ground to avoid this.

If the color of the new mortar still is not similar to the old mortar after repointing and setting, you can clean the non-repointed areas of the structure. This may bring the colors closer together. To do this, use a low pressure water jet to clean the old pointing. Do not use a high powered water jet because this could damage the masonry and mortar. Do not use chemicals with the water, but if they are needed rinse the structure afterwards with fresh water and natural or nylon brushes. Another method to match the colors is to stain the new mortar. This is not recommended, though, because this can be harmful. If you do decide to stain, test the application in the test panel.

Old article

Construction Details

Bricks, stone blocks, and other masonry units are typically laid in a bed of mortar. The process of placing a masonry unit in a mortar bed causes the mortar to be extruded between units. This excess mortar is cleared and shaped before the mortar hardens. In some cases, the bedding mortar is meant to be seen and the mortar joint is immediately given a finish profile. In other cases, especially where the mortar is pigmented, the bedding mortar is removed from the joint before it hardens (usually a depth that averages about one inch), and then pointing mortar is applied in the void and shaped with a finish profile.

Process

Sound mortar normally does not need to be removed from a building during the repointing process, although such a practice is common. The new mortar should be designed to match the mortar type, color and texture of the existing mortar to avoid incompatible materials and visual aberrations. The professional repointing process begins with a mortar analysis where a sample is taken and examined to identify the components, ratio, and aggregate. The old mortar is removed to a depth equal to or slightly more than the width of the joint, or to the point where sound mortar is reached. Depths greater than 2-1/2 inches or 4cm should be filled in several passes, allowing the mortar to cure for at least 24 hours depending on the mortar type. The joint profile would be as rectangular as possible as the new pointing mortar should be allowed ample opportunity to bond with the masonry unit.

Removal of old mortar between joints is done carefully to avoid damage to masonry units. On buildings with soft materials, such as under-fired brick and lime mortar, removal by hand is often the most effective to avoid damage. Hard Portland cement mortar is usually removed with a grinder or power circular masonry blade, taking care not to damage the masonry units. Vertical joints are done by hand or with small power chisels.

Poor repointing work often raises the level of the mortar joint above the face of the masonry unit, which causes the mortar edge to feather. Such a process is aesthetically undesirable and can cause performance problems as a thin layer of mortar will quickly erode. In addition, depending on the nature of the mortar, mortar that rises above the level of the face of the masonry unit can participate in damaging the arris or corner of the masonry unit. In these cases, deteriorated mortar is often not removed to a sufficient depth.

Materials

It is essential that the mortar used for repointing have similar performance characteristics to the original mortar used in a building. Such performance characteristics include permeability, compressive strength, and coefficient of thermal expansion. For instance, if a soft lime-based mortar was originally used, the most appropriate repointing mortar is likely to also contain a large amount of lime.

An architectural conservator can perform a mortar analysis in order to make recommendations for replacement mortar that is both physically and aesthetically compatible with the building. In the past, lime mortar tended to be mixed on site with whatever sand was locally available. Since the sand influences the colour of the lime mortar, colours of pointing mortar can vary dramatically from district to district.

Sources:

Malcolm Holland, Practical Guide to Diagnosing Structural Movement in Buildings, John Wiley and Sons, Incorporated, 2012. West Sussex UK, pages 189-190.
 * common to seeing cracking along the repair because of the shrinkage of hard mortar and the seasonal flexing along the joint
 * allow weathering to match the new mortar to the old mortar
 * if the crack is smaller than 2mm and not causing any major defects, it is better to leave it alone

Robert C. Mack FAIA and John P. Speweik, Preservation Briefs 2, “Repointing Mortar Joints in Historic Masonry Buildings”, Washington, DC: National Park Service, 1998.
 * don't get confused with tuck pointing: decorative application of mortar on top of existing flush mortar
 * traditionally done: mortar made with lime and sand, lime putty; starting in the early 20th century masons started using Portland Cement, a strong, fast drying cement, not purely on its own though; masonry cement made its appearance in the 1930s, combination of Portland cement and ground limestone
 * identify the problem before starting the work: find out the causes of the deterioration or cracks; there could be a larger problem that also needs to be addressed; use an architectural historian or conservator to help you pinpoint the problems
 * use unweathered mortar to model the new mortar off of
 * the repointing process begins with simple examination of the masonry units and mortar and the techniques used in the original construction, which will establish strength and permeability; also simple visual analysis to develop new mortar mix: the different components
 * sometimes can use lab analysis to obtain original ingredients: sand gradation and color; shouldn't be solely based on lab work because important performance of mortar that can not be established in lab: original water content, rate of curing, weather conditions during original construction, method of mixing and placing the mortar, and cleanliness of sand
 * new mortar must match color, texture, and tooling of historic mortar; the sand must match the sand; it must have greater vapor permeability and softer (compressive strength)
 * the mortar shouldn't be stronger (compressive strength) than masonry units because it won't have give and rather than the mortar relieving the stress the masonry units will, which will cause further damage to the masonry unit which is harder and more expensive to fix, such as cracking and spalling (when the face or outer section of a masonry unit breaks away from the rest of the unit)
 * Two methods of analyzing mortar:
 * 1) wet chemical: acid digestion, a sample of the mortar is crushed and mixed with dilute acid, the mortar is broken down and the type of mortar and sand is determined by the color and texture; another way is to collect the carbon dioxide gas given off by the digestion and based on the volume, the mortar can be determined and the amounts of each component
 * 2) Instrumental:  most commonly used is thin-section microscopy- examining thin slices of mortar in transmitted light microscope; this method can provide more information than wet-chemical examination; other examples: scanning electron microscopy, X-ray diffraction, and atomic absorption spectroscopy
 * Test panels before starting work (as stated below), 3'x3' for brick and larger for other masonry; good time to determine if power tools can be used
 * caulking cutters with diamond blades are acceptable power tool to remove mortar in wide, horizontal joints: slow, no rotation, vibrate, less likely to damage the masonry units
 * the areas with the deepest recession of mortar should be filled first: the mortar will be put in in several layers, starting with the first layer of 1/4 inch, letting it harden to a desired strength then adding another 1/4 inch layer, and continuing this process until flush with the masonry
 * high-lime mortar dries rapidly, so after it reaches a particular strength it needs to be sprayed with water to avoid chalking, poor adhesion, or poor durability
 * Local conditions will control how long you should keep the structure wet, but mist every two hours for two days, then every four hours
 * burlap or plastic (not resting against the structure) should be places over the repointed wall or structure: keeps building damp and eliminates direct sunlight
 * if color of mortar is not similar to old mortar, spot pointing is not advisable because the difference between the new and the old will be to obvious; another way to fix this issue is to clean the non-repointed mortar (use low pressure, no chemicals, if needed rinse afterwards with fresh water, natural or nylon brushes) or stain the new mortar (not recommended because can be harmful, but test this before application)

A.H. P Maurenbrecher, K. Trischuk, M.Z. Rousseau and M.I. Subercaseaux, National Research Council Canada, "Repointing Mortars for Older Masonry Buildings- Site Considerations", March 2008, nrc-cnrc.gc.ca/ctu-sc/ctu_sc_n68
 * Before work it is important to select a trained contractor. The contractor should have at least 5 years experience; assess their previous jobs to assess their qualifications.
 * pick the right season to do the work; extremely high or low temperatures can cause rapid drying which can have negative effects
 * start with a test area not in a high traffic area or dominant view to test for the color and the mason's skills
 * do not use power saws or grinders on thin joints (or softer masonry like brick or terra cotta Preservation Brief); only on wide and uniform horizontal joints: use in middle of joint then use hammer and chisel for the rest
 * joints should then be vacuumed or brushed clean after the old mortar is removed. remaining debris should be removed with a water jet from the top of the masonry area down.