AIC's 52nd Annual Meeting

The building in our study is the focal point of a prominent site and was constructed over a period of forty years. Dedication of the site occurred on February 14, 1853, and completion and dedication of the interior of the building occurred on April 6, 1893. The building was designated a National Historic Landmark in 1964 and is a contributing building in a historic district that was listed in the National Register of Historic Places in 1983. The above-grade walls of the building are constructed of quartz monzonite quarried from a nearby canyon. Quartz monzonite is an igneous rock that is similar in composition and texture to granite but contains lesser volumetric proportions of the common granite-forming mineral quartz.

Unexpected soiling deposition at the building’s exterior masonry facades was evaluated as part of a seismic retrofit and stone masonry repair program. Onsite investigative and laboratory work was performed to identify the likely source of the soiling deposits that were created through the ongoing retrofit and repair program. After onsite assessments and sampling began, additional field and laboratory studies were conducted that resulted in an extensive evaluation to characterize the soiling and examine several potential cleaning approaches.

Over the course of the retrofit and repair program, multiple conditions of soiling were identified at the quartz monzonite stone masonry. Soiling patterns were determined to be based on a range of exposures and correlated to varying moisture conditions. The soiling patterns were consistent with locations where water was likely introduced into the wall system through ongoing seismic retrofit operations. Subsequent field and laboratory studies identified that soiling contained oxidized forms of iron and/or iron compounds bound in common masonry-derived salts. Water introduced into the wall system from retrofit activities most likely resulted in visible migration of soiling components to the exterior surface of the stone masonry. A program of cleaning studies was developed with an aim to isolate and target soiled portions of the stone masonry. Cleaning studies included multiple chemical cleaning systems, such as poultices, proprietary cleaners, and micro-abrasive cleaning systems, to evaluate their effectiveness prior to conducting large-scale cleaning mockups with the project masons.

This presentation considers the procedures of the assessment and the results and findings of the field and laboratory studies used to identify and determine the cause of the soiling. These studies were essential in developing customized cleaning processes to mitigate the targeted soiling and its potential for recurrence.