Shrinkage in lime stucco

One of the most common problems with lime stucco, used in building conservation, is shrinkage during the early stages of curing. In most situations, shrinkage is seen as cracking and/or separation from the underlying masonry wall. According to much of the published literature, shrinkage in lime-based materials is often associated with the amounts of both water and aggregate that are used in the mix. Early manuals of practice and historical builders’ notes suggest that lime mortars (and therefore stuccos) should not be used right after they are made, and that instead they should be kept for a period of time prior to application. In traditional construction, numerous practices were developed, sometimes based on the specific needs of a given site. Some of these practices were passed down without a complete understanding of the underlying principles. It has been said that the “banking” of lime mortar will help mitigate shrinkage in stucco. This was based on the belief that short-term storage of the components (lime putty and sand) would somehow cause them to be in a more stable state to be used than as freshly mixed materials. Interviews with practicing masons as well as a literature review show that although “banking” has been in practice for centuries, there is a lack of research of the actual mechanism and its specific effect in mitigating shrinkage problems. This thesis has explored the practice of “banking” and the role it plays in mitigating shrinkage. A series of laboratory experiments was developed to study selected parameters: type of lime, amount of sand and amount of water. With “banking” as central procedure in the research, variations in these parameters were tested to better understand if the practice will have effect in one mix and not another, or if the practice will affect all mixes. Although the mechanism of “banking” mortar for a relatively short period of time is not well understood, observations derived from this research support the masons’ belief that “banking” helps mitigate shrinkage.