Deborah Slaton and David S. Patterson, AIA
Repairing and restoring historic concrete often involves dealing with concealed conditions contributing to distress. This 1920s Pacific Northwest building features cast-in-place concrete façades finished with facing mortar. The original steel-framed windows were previously replaced with aluminum retrofit units; the new windows were smaller in height than the original openings, with the resultant gap at the head infilled with wood framing and plywood.
An investigation conducted to support façade restoration revealed the cracked and spalled concrete was associated with corrosion of embedded metal, as well as cyclic freezing and thawing. The steel corrosion was determined to be primarily related to carbonation—the reduction of the alkalinity (pH) of the concrete protecting the embedded ferrous metal.
Concrete may become carbonated over time from exposure to acidic components of the atmosphere (particularly carbon dioxide), resulting in a decrease in alkalinity. Corrosion of embedded reinforcing steel occurs due to carbonation in the presence of moisture, especially in areas of minimal concrete cover. (Chlorides from the marine environment, or possibly present in the original mix, may also contribute.)
When embedded ferrous metal corrodes, it expands up to approximately 10 times its original volume. Stresses exerted on the surrounding concrete then cause it to crack. Spalling may expose the corroded steel, allowing further moisture to enter the system. Corrosion would be expected to continue, and accelerate, without repairs.
Lab studies that were conducted on several cores removed from the building found that while compressive strength and the general condition of the concrete were good, the material was not air-entrained, as is common with 1920s construction. Therefore, it was more susceptible to damage from cyclic freezing and thawing in wet conditions.
Cracking and spalling of the concrete adjacent to the windows were observed to be coincidental with the locations of embedded concrete reinforcement; however, removal of the plywood infill at the window head and an inspection opening during the investigation revealed another significant contributor to the concrete deterioration. A continuous L-shaped ferrous metal angle was found embedded in mortar at the head and jambs of the window opening. The angle, part of the original window frame, had been abandoned in the wall when the original windows were replaced.
Following installation of the replacement windows, water entering the wall assembly through cracks in the concrete and open joints at the window perimeter caused severe corrosion of the remnant metal angle. Cracking of the concrete associated with the remnant window frame necessitated an expanded repair program to address concrete deterioration.
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 the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be e-mailed at email@example.com.
Rocco Romero, AIA, and Paul Gaudette of WJE contributed to this article.