Terra cotta trial repairs

by Erik Missio | November 2, 2015 10:18 am

FAILURES
Deborah Slaton and David S. Patterson, AIA
With any façade repair, trials are key to refining approaches and protocols, and to assessing the effectiveness and appearance of the work to be done. Trial repairs can provide an important opportunity to evaluate the repair process, and conduct further investigation of concealed conditions—including those that direct refinement of specific work to be performed at certain locations.

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In the example shown here, cracking had occurred at the terra cotta window head-jamb interface at several locations. In some cases, replacement or patching of the terra cotta unit was required. Preparation for patch installation included removal of the terra cotta at the spall location to provide a clean, rectangular repair location, and installation of stainless steel pins to anchor the patch. The process of preparation of the substrate, including localized removals, and installation of patches as part of trial repairs revealed two issues that resulted in refinement of repair approaches.

First, although samples of the cementitious patch material provided by the manufacturer were a good color match to the existing terra cotta, the material as installed at trial repairs was distinctly darker than the offsite samples. Although the contractor carefully followed the manufacturer’s recommendations for mixing and installation, repeated onsite trials continued to result in poor color matches. Not until the contractor used bottled water in mixing the patch material—avoiding the minerals present in local potable water—did the installed patches more closely match the samples provided by the manufacturer and thus the existing terra cotta. (This led to the use of bottled water in the patch-mixing protocol.)

Second, at certain locations, cracked units had been identified for patch repair, although depending on the extent of cracking revealed during preparation for the patch, the documents note replacement might be necessary. Repair to units exhibiting minor cracking, including those located directly above units designated for patching, was not indicated.

However, when material was removed from selected units for patch preparation, at numerous locations, the existing minor crack in the unit above was found to continue not only perpendicular to the face of the unit, but also parallel, extending upward through its body.

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This finding led to the decision to further assess similar cracked units to determine whether cracking parallel to the face had also occurred—the condition was found at several locations. Where repairs to the cracked units were unfeasible due to lack of sound backup material or other conditions, those units were replaced rather than repaired.

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 via e-mail at dslaton@wje.com[3].

David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be reached at dpatterson@wje.com[4].

Lawson Newman of WFT Architects in Jackson, Mississippi, contributed to this article.

Source URL: https://www.constructionspecifier.com/terra-cotta-trial-repairs/

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