November 6, 2020
Deborah Slaton and David S. Patterson, AIA
As noted in previous columns, successful long-term performance for repointing mortar joints in an existing masonry structure requires the development of an appropriate mortar mix, proper installation, and correct designation of the type and extent of work required. Work on older and historic structures often involves developing a repointing mix that matches the original mortar, as non-matching mortar can be visually intrusive as well as potentially problematic in terms of performance.
To provide information for development of a repointing mix, testing is often specified to evaluate the composition of the original mortar in accordance with ASTM C1324, Standard Test Method for Examination and Analysis of Hardened Masonry Mortar. This standard includes sampling, petrographic examination, chemical analysis, and the calculations of proportions of the components of mortar—cement, lime, and sand—as well as evaluation of other mortar characteristics. ASTM C1324 cites several other ASTM standards, such as ASTM C270, Specification for Mortar for Unit Masonry; ASTM C144, Specification for Aggregate for Masonry Mortar; ASTM C295, Guide for Petrographic Examination of Aggregates for Concrete; and ASTM C856, Practice for Petrographic Examination of Hardened Concrete.
ASTM C270 distinguishes mortar types (M, S, N, and O) and their proportions and characteristics, and guides standard mix designs.
Sometimes the full range of ASTM C1324 testing is needed to understand the composition of an existing mortar; in other cases, selective information can be obtained in order to address particular challenges in matching the historic mortar. For example, the type, color, size, and the gradation of sand aggregate contribute significantly to the appearance and texture of the repointing mortar. Petrographic studies assist in determining these factors in the mortar aggregate, and can help identify specific sources of sand if it is needed. ASTM C144 provides guidance for fine aggregate in mortar (where larger sand aggregate is present in the original mortar, ASTM C33, Standard Specification for Concrete Aggregates, provides guidance.)
A skilled petrographer, using techniques outlined in ASTM C856 and ASTM C295, can evaluate the sand present in mortar samples through microscopic examination of thin sections. (Per ASTM C1324, acid-insoluble aggregate components can also be assessed by dissolving the paste matrix in hydrochloric acid and washing and filtering the residue, which can then be sorted by size using sieving methods per ASTM C144, ASTM C33, or a combination thereof.) Even if the surface of the joints is weathered (or in some cases has been previously repointed with a mortar inconsistent with the original), petrographic studies can help to determine the character of the sand used in the original mix so that the new repointing mortar matches the original rather than any previous non-matching repointing campaign. In the examples shown, petrographic studies provided information to adjust the sand included in a Type N mortar mix to closely match the variety of aggregate sizes and color present in the original mortar as illustrated.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at firstname.lastname@example.org.
David S. Patterson, AIA, is an architect and senior principal with WJE’s office in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at email@example.com.
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