H.M. CURRIE(1) and M.B. BRONSKI(2)

(1) Senior Staff II – Building Technology, Simpson Gumpertz & Heger Inc., Waltham, Massachusetts, 02453, United States, hmcurrie@sgh.com (2) Principal, Simpson Gumpertz & Heger Inc., Waltham, Massachusetts, 02453, United States, mbbronski@sgh.com

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 Josep Lluis Sert, Paul Rudolph, I.M. Pei, Eero Saarinen, the firm of Skidmore Owings & Merrill (SOM), and others, 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 repair material and original concrete continue to weather. The fundamental processes of weathering and color change over time, and the principles for color matching described herein also apply directly to matching mortar, stucco, and exterior renders in historic masonry structures.

KEYWORDS: concrete, façade, mortar, color-matching, weathering, mid-century modernism, restoration, stucco, render.