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Repointing is the process of renewing the pointing, which is the external part of
mortar joint In masonry, mortar joints are the spaces between bricks, concrete blocks, or glass blocks, that are filled with mortar or grout. If the surface of the masonry remains unplastered, the joints contribute significantly to the appearance of the mas ...
s, in
masonry Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...
construction. Over time, weathering and decay cause voids in the joints between masonry units, usually in
brick A brick is a type of block used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a block composed of dried clay, but is now also used informally to denote other chemically cured cons ...
s, allowing the undesirable entrance of water. Water entering through these voids can cause significant damage through
frost weathering Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...
and from salt dissolution and deposition. Repointing is also called pointing,"pointing, vbl. n 1. " def. 5 ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) © Oxford University Press 2009 or pointing up, although these terms more properly refer to the finishing step in new construction.


History

Traditionally, mortar was made with lime and sand, producing lime putty. In the early 20th century, masons began using
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
, a strong, fast drying cement. Masonry cement made its appearance in the 1930s, which is a combination of Portland cement and ground
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
.


Repointing process


Examining the structure

Before starting any actual work, building owners examine the masonry units, mortar, and the techniques used in the original construction. They try to identify the true problem they are facing and find out what were the causes of the deterioration or cracks. If there are cracks or problems in the actual bricks or stone masonry there could be a larger problem that also needs to be addressed. If there is a larger issue, repointing may cause further damage. If a historic structure needs repointing, building owners usually hire an architectural historian or conservator to help pinpoint the issues. If the crack is smaller than 2mm and not causing any major defects, then it is better to leave it and not repoint. It is common to see cracking along old repairs because of the shrinkage of hard mortar and the seasonal flexing along the joint.


Establishing materials

Examining the structure before working will also help establish the strength and permeability of the original mortar in order to match the new. It helps to establish what the original components of the old mortar are in order to find the best match. 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 In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...
, and
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...
. 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 Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...
-based mortar was originally used, the most appropriate repointing mortar is likely to also contain a large amount of lime. An
architectural conservator Conservation and restoration of immovable cultural property describes the process through which the material, historical, and design integrity of any immovable cultural property are prolonged through carefully planned interventions. The indivi ...
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 but 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. Analysis is not 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. Weathering of the new mortar will also match it to the old mortar. The tooling should also match the tooling of the historic mortar.


Test panels

Again, before starting any work, the methods and materials for the repointing will be applied in a test panel. A test panel is an area on the structure out of plain sight where the owner or conservator can test the repointing methods they will use, the color of the mortar, and the skills of the mason. This will also be good to determine the types of tools that should be used, 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.


Construction process


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 Lime mortar or torching is composed of lime and an aggregate such as sand, mixed with water. The ancient Egyptians were the first to use lime mortars, which they used to plaster their temples. In addition, the Egyptians also incorporated various ...
, or terra cotta, removal by hand is often the most effective to avoid damage. Hard
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
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, it's only used on wide and uniform horizontal joints and in the middle of the joint. A hammer and chisel are then used for the rest of the mortar, closer to the masonry units. 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 are vacuumed or brushed clean. After vacuuming, the remaining debris is removed with a water jet from the top of the masonry area down.


Filling in new mortar

The areas with the deepest recession of mortar are filled first. The mortar is placed in several layers, starting with the first layer of 1/4 inch. It is hardened to a desired strength then another 1/4 inch layer is added. The process is continued until the mortar is 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. The structure is misted every two hours for two days, then every four hours for two days. It is important that the mortar stays wet for at least a couple of days while it is setting. The local conditions will control how long the structure should be kept wet, however. Burlap or plastic is placed over the repointed wall or structure. This keeps the building damp and eliminates direct sunlight. The burlap and the plastic are not resting against the structure, however. The burlap or plastic are anchored to the ground to avoid this. There are different ways to lay the
mortar joint In masonry, mortar joints are the spaces between bricks, concrete blocks, or glass blocks, that are filled with mortar or grout. If the surface of the masonry remains unplastered, the joints contribute significantly to the appearance of the mas ...
close to the edge of the brick. Some contractors bring the mortar to the very edge, flush with the face of the brick. Others leave it recessed back to allow water to shed off the building. There are three types of recessed mortar: bucket handle, weatherstruck, and simply recessed. The bucket handle is when the mortar is in a curve shape away from the face of the brick. Weatherstruck is when the mortar starts close to the bottom brick and recesses back as it goes up towards the upper brick. The third, recessed, is when the mortar sits back from the face of the brick. There is also
tuckpointing Tuckpointing (also called tuck pointing or tuck-pointing) is a way of using two contrasting colours of mortar in the mortar joints of brickwork, with one colour matching the bricks themselves to give an artificial impression that very fine joints ...
, where a mortar of a contrasting colour is 'tucked' into the masonry joint. If the color of the new mortar is still not similar to the old mortar after repointing and setting, the non-repointed areas of the structure can be cleaned. This may bring the colors closer together. To do this, a low pressure water jet is used to clean the old pointing. A high powered water jet is not to be used because this could damage the masonry and mortar. Chemicals are also not used with the water because this could be harmful to the masonry or mortar. However, if chemicals are used, the structure is rinsed 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 by professionals though, because it can be harmful. If stain is used, the application is tested in the test panel.


See also

*
Stonemasonry Stonemasonry or stonecraft is the creation of buildings, structures, and sculpture using stone as the primary material. It is one of the oldest activities and professions in human history. Many of the long-lasting, ancient shelters, temples, mo ...
*
Tuckpointing Tuckpointing (also called tuck pointing or tuck-pointing) is a way of using two contrasting colours of mortar in the mortar joints of brickwork, with one colour matching the bricks themselves to give an artificial impression that very fine joints ...


References


External links

* * Article on lime pointing repairs to historic masonry and brickwork. {{Stonemasonry Stonemasonry Masonry