Use this CSI 3 part SimpleSpecs™ master specification to specify restoration mortar (replacement of mortar) for historic masonry. Mortar is specified by attribute rather than by compressive strength as new mortar is typically specified.
There are several methods available to assist in determining mortar’s attributes. The most common include a simple visual examination, which can often provide an acceptable match to existing mortar. The petrographic analysis involves laboratory testing of samples under controlled conditions. This method may be expensive, may not be available in all locations, and may produce questionable results. Many preservationists question the applicability of petrographic testing to historic restoration work.
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, 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.
To learn more about restoration mortar, visit Wikipedia.
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Related SimpleSpecs™ master specifications: