HPC of South Bend and St. Joseph County - Standards Update Draft

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City of South Bend and St. Joseph County Historic Preservation Guidelines 103 For More Information: Masonry Mortar Historically, mortar was composed of sand, lime and water, and possibly additives such as animal hair or oyster shells to contribute to the strength and/or color. Starting in the mid-nineteenth century, a small amount of Portland cement was added into the mix to improve the workability and hasten the setting time . In the early twentieth century, the amount of Portland cement in mortar was increased, resulting in harder mortar corresponding with the manufacturing of harder brick . Sand is by far the largest component of mortar and defines its color, character, and texture. Since masons would use products that were readily available, sand from historic mortars tended to have weathered, rounded edges and was available in a great variety of grain sizes and shades of white, grey, yellow, and brown . Most sand available today is sieved into standard sizes . As a result, mixing sand colors and sizes might be needed to match historic mortar . Lime and Portland Cement act as binders for the mortar . Temperature changes cause masonry units to expand when heated and contract when cold . When masonry units expand in warm temperatures, they press against the harder cement mortar and tend to spall at the edges . During colder temperatures, masonry units tend to pull away from mortar, resulting in open cracks that can allow moisture penetration . Lime-based mortar is pliable and is more likely to compress and flex through temperature cycles. If properly installed, it should also be softer than the adjacent masonry. Because lime is slightly water-soluble, high lime mortars can be self-healing and reseal hairline cracks. By contrast, Portland cement can be extremely hard, resistant to water movement, shrinks significantly upon setting and undergoes relatively large thermal movements . In general, high lime mortars are recommended for nearly all repointing projects at nineteenth and early twentieth century construction to ensure a good bond with original mortar and masonry . It is possible to add a small percentage of Portland cement to a high lime mixture to improve workability and plasticity . Portland cement can generally be increased when repointing masonry of the post-World War I period and later. Portland cement is available in white or grey, and the two colors can be mixed to achieve a desired color . All masonry repointing projects should test the existing mortar for composition before beginning work . Water needs to be clean and free of salts, harmful minerals, and acid . If not, it can break down the mortar and adjacent masonry and discolor finished surfaces. Historic additives included oyster shells, animal hair, clay particles, etc . To duplicate the character of historic mortar, it might be necessary to include additives to match the original . It should be noted that there are several types of chemical additives available today including those that increase or reduce the setting time or expand the recommended temperature installation ranges . The use of newer chemical additives is strongly discouraged unless they have been specifically tested over an extended period with similar historic materials as the proposed installation conditions . Masonry Problems Two Most Common Masonry Problems Defined Efflorescence occurs when water-soluble salts leach out of masonry or concrete by capillary action and deposit on a surface by evaporation. Efflorescence shows up on brick as a white, powdery surface. Its existence indicates that water is coming through the building—usually from a roof or flashing leak. Efflorescence can be cleaned off brick once the water source is discovered and repaired. Spalling is the chipping of masonry, usually referring to the whole or partial face popping off. Spalling is a permanent damage to the masonry material. It can be a sign of improper mortar, water infiltration or previous cleaning that damaged the masonry unit. The damaged unit must be replaced . Typical Masonry Problems It is important to identify masonry problems early to minimize damage . This is particularly true of masonry that is exposed to moisture . Once water is permitted to penetrate a masonry wall, the rate of deterioration accelerates very quickly, becoming more severe and costly. The following examples include some typical masonry problems and their possible repairs. Specific conditions might require professional evaluation by an architect or engineer familiar with historic masonry, particularly to assess settlement issues .DRAFT