Track 4: This New World: Preservation technology and emerging issues within our historic buildings and built landscapes
3 - Lessons in Cathodic Protection Technology from Soldier Field and the Franklin Avenue Bridge
Tuesday, September 25
10:30 AM - 12:00 PM
Cathodic protection is one of several technologies used to prolong the life of historic concrete structures that are prone to damage from corrosion of embedded reinforcing steel. The author (Johnson) previously published a comprehensive study of corrosion mitigation systems for the historic concrete of Soldier Field, constructed 1922–1926 in Chicago, Illinois (“Soldier Field Stadium: Corrosion Mitigation for Historic Concrete,” APT Bulletin: Journal of Preservation Technology 35: 2–3, 2004). Field trials and testing of the following systems were included in that study: corrosion inhibitors (spray applied and admixed in patches); realkalization (a treatment to mitigate carbonation); and passive cathodic protection using embedded zinc anodes, arc-sprayed zinc and adhered zinc sheets. Although cathodic protection was not implemented across the structure due to the owner’s budgetary limitations, the trial installations were left in place. The studies at Soldier Field were widely recognized, including a reference in National Park Service Preservation Brief 15: Preservation of Historic Concrete (Gaudette and Slaton, 2007).
Annually since that time, the historic concrete surfaces at Soldier Field have been inspected close-up and documented for distress. In 2016, the authors performed follow-up testing to examine how the corrosion mitigation trials were performing 15 years after their installation. Testing revealed that the discrete zinc anodes embedded in the colonnade concrete had afforded little if any long-term protection to the concrete, even though testing in 2001 showed good initial performance. Based on analysis of material samples and consultation with the system manufacturer (Vector Corrosion Technologies), the poor long-term performance was attributed to the following factors that were not known or fully understood at the time: resistivity of the anode embedding mortar; resistivity, density and permeability of the native concrete; and proximity of the anodes to the long-term moisture source, chloride-contaminated concrete, and reinforcing steel.
Recently, the authors designed a cathodic protection system as part of a major rehabilitation of the historic Franklin Avenue Bridge, constructed 1919–1923 over the Mississippi River in Minneapolis, Minnesota. The cathodic protection design, which involves continuous zinc anodes placed along the arch rib corners, illustrates proper consideration of the design factors that hindered the long-term performance of the galvanic anodes at Soldier Field. Rehabilitation of the Franklin Avenue Bridge was recently completed, and ongoing continuous monitoring at permanently embedded test stations has shown excellent performance of the cathodic protection system since it was installed three years ago.
This presentation will summarize the details and performance of these two examples of cathodic protection technology used at historic concrete structures, and then use these examples to explain all of the design factors that must be considered in order to achieve long-term protection of concrete. These factors are particularly relevant to the successful use of this emerging technology at historic structures.
- Upon completion, participant will be able to list several different types of corrosion mitigation technologies used to protect historic concrete.
- Upon completion, participant will be able to describe, in general, the cathodic protection trial systems installed at Soldier Field in 2001 and their performance as measured in 2001 and 2016.
- Upon completion, participant will be able to describe, in general, the cathodic protection system installed at the Franklin Avenue Bridge in 2015 and its performance as measured continually since then.
- Upon completion, participant will be able to list the primary factors that must be considered in the successful design of a sacrificial anode cathodic protection system for a historic concrete structure.