Repointing is the process of renewing the pointing, which is the external part of mortar joints, in masonry construction. Over time, weathering and decay cause voids in the joints between masonry units, usually in bricks, allowing the undesirable entrance of water. Water entering through these voids can cause significant damage through frost weathering and from salt dissolution and deposition. Repointing is also called pointing, or pointing up, although these terms more properly refer to the finishing step in new construction.
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, than 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.
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 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.