Helena M. Currie, P.E.1 and Matthew B. Bronski, P.E.2
1)  Senior Staff I – Simpson Gumpertz & Heger, Inc.
41 Seyon Street, Waltham, MA 02453(USA)
e-mail: hmcurrie@sgh.com
2)  Principal – Simpson Gumpertz & Heger, Inc.
41 Seyon Street, Waltham, MA 02453(USA)
e-mail: mbbronski@sgh.com

Keywords: Concrete, Facade, Color-Matching, Weathering, Mid-Century Modernism, Restoration.

Abstract. Boston (USA) has a rich heritage of architecturally significant mid-century modernist  concrete buildings. Built primarily in the 1960s, with exposed, unpainted concrete facades, these “brutalist” buildings are now at the age when we have begun to regard them as historic, and thus deserving of careful restoration. However, they are also now at an age when they suffer from deterioration from a half century of exposure to weather, and where spalling can pose falling hazards to the public. Drawing from their extensive experience leading concrete facade investigation and restoration efforts on buildings designed by Jesep Lluis Sert, Paul Rudolph, IM Pei, Eero Saarinen, the firm of Skidmore Owings & Merrill (SOM), and others, the authors briefly summarize and categorize common deterioration mechanisms that can befall exposed concrete, as well as different treatment and repair options, while considering how each may impact the aesthetics of the exposed concrete facade.

The authors discuss the long-term problems with methods that attempt to color-match concrete repairs to existing concrete facades by focusing primarily on the cement paste color, and/or utilizing pigments to match unpigmented concrete. The authors present their new research, conducted in-situ and in their petrographic laboratory, which quantifies the aesthetic significance of the constituent sand and coarse aggregate in concrete and their contribution to the variation of the perceived surface color of concrete over time from weathering. Having applied this new research to the challenging practical problem of matching the color of concrete repairs to aged, weathered historic concrete, the authors present their recommended procedures for designing constituent-based site-mixed structural concrete repair material to custom-match existing weathered concrete facades, while accounting for the effects of weathering on color and texture, and thereby describe how to provide the best long-term match as both the original concrete and the repair material continue to weather.