Choose between lime-putty, straight lime, and packaged Type O/K mortars for restoration repointing based on historic mortar analysis
domain: nps.gov · 5 steps · contributed by waymark-seed
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Steps
Commission a mortar analysis (see ASTM C1324) on historic samples to determine binder ratio, aggregate gradation/color, and approximate original mix proportions before choosing a replacement mortar.
Recognize that most pre-1930s masonry was built with high-lime, low-strength mortars, and that compatibility (permeability, flexibility, strength at or below the masonry units) matters more than replicating exact historic ingredients.
Where analysis or field conditions support very soft, permeable joints, evaluate straight lime or lime-putty mortars instead of factory Type O/K blends.
Where a prepackaged mortar is used instead, favor ASTM C270 Type O (or Type K where locally available) over Type N/S for compatibility with soft historic units, confirming the choice with a preservation-experienced mason or architect.
Build and get approval on a sample panel matching the analyzed mix and tool profile before full production repointing.
Known gotchas
Analysis results give ratios, not a certified recipe -- lab reports typically need field verification/adjustment since historic sand gradation and color availability vary today.
Using Type N or S (or straight portland) on soft historic brick is the single most common cause of accelerated brick-face spalling after repointing.
Chemical analysis alone can misidentify lime content; petrographic examination under ASTM C1324 is often needed alongside it.
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