PAINTSQUARE BLOG

Sealants used as weather seals on the exterior envelopes of buildings have been formulated to achieve tenacious bond to many building cladding materials that commonly include aluminum, steel, concrete, masonry, glass, plaster and EIFS.

Occasionally, less common cladding materials are encountered that require special attention in order to achieve good adhesion of the sealant to the substrate. One such material is weathering steel, often referred to by its U.S. Steel trademarked name, Cor-Ten steel.

Photos: WJE

Silicone sealant installed at the perimeter of the glazing, and between the window frames and the spandrel panels is performing well after 16 years on the Daley Plaza in Chicago.

One of the most well-known weathering steel clad buildings is Daley Plaza in Chicago, not only because the building is clad in weathering steel, but the Picasso sculpture that resides in the building’s plaza is also constructed of the material.

Rust-Like Appearance

One’s initial reaction when viewing a weathering steel building is that the building is corroding, which is partially correct. Weathering steel alloys were developed to form a stable patina, or rust-like surface appearance, when exposed to moisture and oxygen, and do not require painting or other coating for protection of the steel.

In most climates, the thin layer of rust that forms provides protection against further corrosion of the underlying steel. In humid climates, or where the steel may be exposed to elevated levels of salt, such as near an ocean, the patina may be insufficient to prevent further corrosion of the steel.

Like other building cladding materials, weathering steel clad buildings will require the use of sealants to prevent air and water infiltration, and air exfiltration. Often, during construction, the sealant is installed prior to the development of the patina, or, the surface of the steel is cleaned of the rust layer prior to installing the sealant, so that the sealant is adhering to the un-corroded surface.

Sealant materials, however, have a finite service life, and will require replacement periodically. With most, more common, cladding materials, the existing sealant is removed down to the substrate surface, the surface is cleaned and possibly primed, and the new sealant is installed; however, the question arises relative to proper surface preparation of the weathering steel when replacing the sealant.

Surface Preparation Approaches

There seems to be a difference of opinion among sealant manufacturers regarding the proper substrate surface preparation relative to sealant replacement on weathering steel. One opinion is that after the existing sealant is removed, a protective coating be applied to the steel prior to installing the new sealant.

Some sealant manufacturers will not warrant the use of their sealant materials adhered to weathering steel for weather seals. One manufacturer’s current installation manual states, “…weather sealants have been known to have limited weather sealing [service] life when applied to Corten [sic] steel panels, due to continued corrosion of the steel at the bond line. This disintegration of the underlying substrate is excluded by the limited weather seal warranty . . . High-performance coatings that act as corrosion inhibitors and primers may be used in these applications.”

The other opinion relative to replacing sealant on weathering steel is to remove the existing sealant and then clean the loose patina material from the steel surface using a wire brush or light media blasting with sand or walnut shells, followed by a solvent wipe using the two-rag method and isopropyl alcohol to remove loose material remaining on the surface. It should be noted that ASTM C1193 Standard Guide for Use of Joint Sealants provides detailed information relative to preparing substrates for the application of sealant.

Adhesive failure of the previous sealant was resulting in problems with the insulating glass units. The current sealant, installed in 2001, is performing well.

Another sealant manufacturer recommends that its sealant materials may be satisfactorily adhered directly to weathering steel if the loose oxidation film is removed at the sealant bond line. The company recommends against removing the oxidation down to bright metal because generally, the oxidation is stable. Removing the oxidation and exposing the bright metal will create a condition where the newly cleaned steel surface will oxidize again when exposed to environmental moisture. That oxidation will occur at the sealant bond line, thereby adversely affecting the adhesion of the sealant to the substrate.

In support of this installation approach, a document from 1975 cites that specimens of weathering steel to which silicone sealant had been applied, after wiping the surface with Xylene, and, with and without primer, were placed in a weatherometer to evaluate adhesion after exposure. Good adhesion was reported for specimens applied with and without primer, after 700 hours in the weatherometer, and before and after water immersion. The manufacturer goes on to recommend sealant adhesion evaluation, which is a wise recommendation for all sealant applications on all substrates. ASTM C794 Standard Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants can be used in laboratory situations, and ASTM C1521 Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints can be used in the field for the evaluation of sealant adhesion.

Sealant Repair Project

In 2001, WJE was involved with the sealant repairs at the Daley Plaza building.

Working with representatives of the sealant manufacturer, the firm documented 27 laboratory adhesion mockups and four field adhesion evaluations during the investigative stage of this project in an effort to develop the best surface preparation method, and to confirm that the direct adhesion of the sealant to the weathering steel was satisfactory.

Select portions of the sealant were evaluated in 2015 with no observed adhesion failures, and as of 2017, the sealant is performing as expected and has no reported adhesion failure.

In Conclusion

Considering the two different approaches to replacing sealant on weathering steel cladding, the fact that not all weathering steel is exactly the same, and that the environments in which the weathering steel clad buildings reside differ, it would be prudent to first pursue the less onerous method, including performing ample adhesion evaluations with and without the use of primer. If, by following this approach, satisfactory adhesion cannot be achieved without removing the weathering steel patina, additional investigation should be performed that includes evaluating the need to remove the patina and applying a high-performance coating to which the sealant will adhere.

About the Author

Christopher Sass, an associate principal-level architect with Wiss, Janney, Elstner Associates Inc., has worked on a variety of projects related to the investigation and repair design of distressed conditions in buildings. He has directed and participated in numerous investigations and design of repairs involving plazas, roofing, masonry walls, EIFS cladding, curtain walls, windows, sealants and water infiltration. His work has been focused on water infiltration, masonry, curtain walls and roofing. Sass is the Chairman of ASTM C24 Building Seals and Sealants, and the president of the board of the Masonry Institute of Michigan.

You can reach him at csass@wje.com.

ABOUT THE THE BLOGGER Wiss, Janney, Elstner Associates Inc. “Solving for Why” is written by professionals at Wiss, Janney, Elstner Associates Inc. Since 1956, WJE’s primary goal has been to provide the best solutions for its clients’ new and existing construction-related problems. The firm’s highly qualified engineers, architects, and materials scientists possess a collective knowledge gained from solving, as well as helping clients avoid, thousands of problems. Author information is available at the bottom of each blog entry. SEE ALL CONTENT FROM THIS CONTRIBUTOR