For old buildings, deterioration is part of the building's life cycle. No matter how well a building is constructed, the combined forces of time and weather will inevitably cause wear and tear. When an old building carries a particular historic or cultural significance, restoring it requires solutions that will not only extend the life of the structure, but also preserve its historic integrity.
Such was the case with a 1904 brick building in Washington, D.C. Now a National Historic Landmark, the building was originally constructed as a printing facility. As such, the building featured impressive ceiling heights, expansive windows, and an ornate terra cotta cornice supported by an embedded steel armature.
In 2005, a condition report revealed that the century-old structure was in need of serious repairs. The scope of work recommended by the report included an entire exterior envelope repair, re-pointing of the brick, patching and repair of the terra cotta elements, cleaning of the exterior brick façade, removal of old scaffolding pins, and cleaning of the decorative metal elements on the windows. Because of the building's National Historic Landmark designation, all restoration work had to adhere to the Secretary of the Interior's Standards for Rehabilitation and Guidelines for Rehabilitating Historic Buildings.
While many of the repairs identified were typical of a thorough historic preservation project, a distressing discovery was made when inspecting the building's terra cotta cornice with its projecting modillions. The team discovered that the main mortar bed joint was cracked and open and previous repairs comprised a hard Portland cement-rich mortar. The improper use of a rigid cement mortar had caused it to adhere to the masonry instead of becoming a sacrificial building material allowing differential movement and the protection against freeze-thaw action. Supporting steel elements located within the cornice were corroding and the open mortar joint had allowed exposure of the steel to moisture and oxygen, creating an ideal environment for corrosion. As a result, expansive corrosion of the steel supports was causing the formation of large downward stresses, leading to cracking of the modillions and presenting a future risk of masonry falling from the historic building.
Seeking a lasting solution for the terra cotta cornice that would both protect the historic integrity of the building and prevent further damage, the museum and the repair team worked to find a solution that wouldn't require the expensive and historically detrimental removal and replacement of the terra cotta modillions.
A non-destructive solution
After assessing invasive and non-invasive treatments, the project team contacted Electro Tech CP to assess the damage and to verify that an impressed current cathodic protection (ICCP) System was a viable solution. Electro Tech CP conducted a week-long investigation and trial on the cornice, ultimately finding that an ICCP system would be the best method for preserving this landmark building's cornice.
ICCP systems consist of installing discrete titanium anodes into the masonry and making separate connections to the corroding steelwork. The anodes and steel are then connected to a sophisticated power supply system with anodes and steel connected to the positive and negative terminal respectively. The charges are precisely controlled and are programmed to deliver an optimal protective current to the steel within the structure. As a result of the current flow, the steel polarity is shifted in a negative direction to remove the harmful electrochemical reactions that result in rusting of the steel. Although ICCP has been used successfully on many structures such as bridges, jetties, and pipelines worldwide, and more than 100 steel-framed buildings in the United Kingdom, it had not been widely used in the United States (though ICCP was recently used at the flagship Marshall Field's store in Chicago) for masonry buildings. However, in this case, the repair team recognized that the ICCP system would not only minimize invasion of the historic structure, but would also offer cost-savings over replacement and provide a long-term solution to corrosion-related damage.
To properly design the ICCP System, metal detection was performed to show where metal was located in the structure, while continuity tests ensured that the steel armature was electrically continuous. The investigation also determined areas that would need to be electrically bonded. A real-life mock-up was created to test the performance of different anode types and methods of installation.
Permanent protection
Once some of the mortar above the modillions had been removed, it became apparent that a few of the outriggers were in far worse condition than previously anticipated. This required additional supports to be added to the cornice, which required careful planning on the part of the entire project team. All supports had to be carefully bonded to the ICCP system so that the new steel was electrically continuous with the old steel.
Based on the system design, anodes were inserted to deliver corrosion protection to the steel outriggers. The mortar joints were re-pointed simultaneously, masking all signs of the system and eliminating prolonged exposure to oxygen and moisture. The installation required only one major joint to be chased out, just above the modillions on each elevation. The anodes and reference electrodes were installed within this joint, and external wiring was placed within the brickwork just below the terra cotta cornice. The anodes were connected to a DC power source, with the potential changes measured by strategically placed reference electrodes. Internal components-including the power supply and main control unit, which can be accessed via the Internet for remote monitoring-were installed in the mechanical space within the attic, with all wiring and internal connections terminating at distributed power supplies.
Although the design and installation of the ICCP system added more work to the overall facade restoration and therefore was subject to an aggressive completion schedule, the project was an overwhelming success, thanks in part to the open communication and teamwork between the conservators, engineers, and the repair contractor. The system has been running successfully since January 2007, proving that ICCP technology is a state-of-the-art solution for creating cost-effective, lasting repairs to historical structures while still preserving their integrity.
Paul Noyce is chief engineer and Gina Crevello is an architectural conservator for Electro Tech CP in Tequesta, Fla. They can be reached at 561-744-2258.