Deborah Slaton and David S. Patterson, AIA
The effects of corrosion on metal building components range from aesthetically undesirable appearances to hazardous structural conditions. This is particularly the case for masonry buildings constructed in the early 1900s, where unprotected structural steel is often in contact with exterior wall construction. Thus, the masonry as well as the steel can be affected, and ongoing corrosion of steel may be concealed within the wall.
In masonry buildings, corrosion of steel is typically associated with water that migrates through the exterior cladding and comes in contact with the embedded steel, potentially affecting steel beams, columns, lintels, and anchorages when part of the exterior wall assembly. Unprotected mild steel oxidizes rapidly when exposed to moisture. This oxidation produces rust scale, an iron oxide. Rust scale can occupy up to 10 times the volume of the original metal. The increased porosity of the corroded surface can trap or hold moisture, leading to further corrosion. Forces exerted by corrosion can cause cracking and spalling of adjacent masonry (commonly referred to as rust jacking). The resulting cracks and gaps in the masonry can allow more water to enter the wall assembly, thereby increasing the potential for further corrosion of the metal.
Significant accumulation of rust scale, while visually notable and potentially causing damage to masonry in contact with the corroding steel, does not necessarily result in loss of sufficient original material to render the member, or element, structurally inadequate. Section loss of 10 to 20 percent is often cited as the threshold at which further assessment of the element should be conducted. Where the accumulation of rust scale is extensive, removal of corrosion product is required to determine the section loss of the steel. The location as well as the extent of corrosion determines the need for repair or replacement—for example, whether the corrosion and subsequent section loss have occurred at the web or flanges of a beam, mid-span, or at end conditions. Determining the extent of corrosion and section loss, and implementing repairs, can be costly as it often requires removal of cladding materials in contact with the steel member (or anchorage) in question.
While preventing corrosion-related damage to steel members engaged within a masonry exterior wall can be challenging, the rate (and extent) of corrosion can be limited by maintaining the exterior wall in serviceable conditions to minimize the amount of water infiltrating the wall. Such maintenance typically includes repointing masonry joints, installation of sealant repairs as appropriate, and repairing cracks in the masonry regularly.
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 WJE’s office
in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at email@example.com.