Special points of interest:
- One of the world's first department stores
- Steel-frame corrosion
- Terracotta conservation
- First ICCP system in an American steel-framed building
- Discrete anodes
- 24 monitoring points
- Four anode zones
- Remote monitored
Marshall Field's flagship store, located on South State Street, is probably the most important store in downtown Chicago. A cornerstone of the city, Marshall Field's is one of the world's first department stores and it has an international reputation for quality. The building was designed by Daniel Burnham, a renowned Chicago architect and city planner responsible for the plan of the 1893 World's Columbian Exposition. This neo-classical building is a tribute to the prominence and wealth of Chicago at the end of the 19th Century. Marshall Field spared no expense during the construction of his flagship store. It was the first building ever to have a Tiffany favrile glass mosaic ceiling. The first building was erected in 1893, with further additions made in 1907. As the company expanded, the structure grew through the 1920s to its current configuration.
The Marshall Field's Department Store is comprised of a semi-rigid structural steel skeleton frame encased in granite up to the third floor and white glazed terracotta to the remaining ten stories. The form of construction was typical of the period with the terracotta bonded to the steel frame and a mortar or brick in-fill. The white terracotta blocks are individually tied back to the infill material using ¼-inch square section anchor bars approximately 6-inches in length. The anchors are located in pre-formed holes in the terracotta blocks at the corner of each block.
The Challenge
As commonly found in this type of construction, the steel frame was corroding. The expansive forces generated by the corrosion process were causing deterioration of the terracotta units in the form of cracking and spalling, resulting in ongoing maintenance and safety issues for the building owner.
In 2002, Electro Tech CP was commissioned by Wiss Janey Elstner Associates, Inc., on behalf of the Target Corporation, to conduct a Cathodic Protection (CP) Feasibility trial of the fluted terracotta columns between the 11th and 13th floors of the North Wabash building, constructed circa 1914. The goal of the trial was to examine the possibility of installing a CP system that would prevent continued corrosion of embedded structural steel stanchions and associated steel/iron masonry anchors. In the trials, the following important issues were addressed:
- Method of construction and presence of voidage in the in-fill masonry
- Corrosion rates for the steelwork
- pH and chloride levels of the infill mortar in contact with the steel frame
- Conductivity of the masonry
- Level of continuity in the steel frame and associated masonry anchors
- Level of difficulty in establishing electrical continuity with electrically discontinuous metal components
- Ease of polarizing the steel with a Cathodic current indicating the suitability of CP techniques
- Ability to train local masonry workforce in the application of CP
As a result of the trial, it was concluded that a CP system could be developed as a terracotta conservation technique. The application of CP would control corrosion and allow in-situ stabilization of deteriorated terracotta. In addition to the conservation benefits of this approach, the owner would save approximately 50 percent as compared to repairs involving widespread terracotta replacement. Continuing in its line of distinction, Marshall Field's was selected to be the first American building to have an Impressed Current Cathodic Protection system installed.
The Solution
Due to the success of the feasibility investigations, commercial viability and conservation benefits, Electro Tech CP was commissioned to design a full-scale CP system to protect the fluted columns between the 12th and 14th floors. In addition, supervision of both local masonry and electrical contractors was to be provided. This was to ensure all design requirements were fulfilled by the contractors who were unfamiliar with the technology.
The scope consisted of drawings, specifications and material schedules, detailing the safe extra-low voltage electrical circuits, 110VAC electrical circuits and masonry installation works. The final design consisted of small ½-inch diameter by 4-inch long inert anode electrodes installed along the 3/8-inch mortar joints. These were interconnected by a small 16AWG titanium wire buried in the mortar joining and a separate wired connection to the steel frame. A monitoring sensor was installed in each column and discrete connections to the terracotta anchors were made to ensure their protection by the CP system.
The anode electrodes were wired back to the positive terminals of an intelligent power supply and monitoring system. The steel was wired separately to the negative terminals. Protective currents were then applied to the steel via the terracotta masonry using a computer system to control the intelligent power supplies.
A particular feature of the installed CP system was its presentation of the data in real-time CAD layouts on the embedded computer screen in the main control unit. The system utilized touch-screen computer technology, allowing the user to access and understand the CP system layout simply by touching menus and drawings/images on the PC screen.
The CP system installation was completed ahead of schedule and under budget in August 2004. In August 2005, a full survey and inspection of the building was performed. The visual inspection of the terracotta and data collected by the CP system confirmed successful control of the underlying corrosion problems. The Marshall Field's project has demonstrated that Electro Tech CP is able to successfully provide Cathodic Protection technology to high-rise buildings in the U.S.