By: David Whitmore and Rachel Stiffler
Philadelphia’s Vine Street Expressway, part of I-676, is a 1.75-mile limited-access highway traveling east-west across the northern edge of the city’s central business district.
The expressway connects I-95 and I-76, and was designed to easily move traffic between the surrounding counties in Pennsylvania and New Jersey. The seven bridges spanning the western end of I-676 between 18th and 22nd streets were built in 1959, reconstructed in 1989, and more recently deemed to be structurally deficient and reaching the end of their service lives. Previous inspections of the bridges documented sufficient deterioration to indicate that replacement was the best course of action.
Ideally, the concrete bridge abutments could be repaired and the useful service extended to more closely match the newly constructed superstructure.
Preservation of the concrete abutments could lead to substantial savings in the project budget and schedule. In order to make this determination, the Pennsylvania Department of Transportation’s (PennDOT) consulting engineering team performed a concrete and corrosion survey of the abutments. The conventionally reinforced concrete abutments were exposed to corrosive deicing salts, and the objective of the survey was to determine the magnitude and extent of corrosion. This condition survey also provided an overview of the condition of the concrete as well as information to determine the suitability of various repair options to protect and extend the service life of the concrete abutments.
Looking at the potential
The abutments were visually inspected for cracks and concrete spalling, and sounded to detect delaminations to locate existing corrosion-related concrete damage. In total there was approximately 1,200 sq ft of delaminated and spalled concrete, and 830 lineal ft of cracking in the concrete abutments. This amounted to approximately 4% of the total surface area of the abutments.
Concrete samples were taken to at least the depth of the reinforcing steel and tested for acid soluble chloride ion concentration. To analyze the results, a chloride threshold of 0.035% chloride ion by weight of concrete was used. Chloride ion concentrations below 0.035% by weight of concrete at the level of the rebar were deemed to be below the corrosion initiation threshold and thus had a low corrosion risk. Locations with samples above the threshold were deemed to be a high risk of corrosion. The test results indicated that 76.5% of the samples were found to be above the corrosion threshold.
Corrosion potential measurements were taken using a copper-copper sulfate reference cell placed on a grid pattern across the full surface area of each abutment. Corrosion potential readings greater than -200 mV are indicative of a low probability of active corrosion; readings less than -350 mV indicate a high probability (>90%) of active corrosion. On average, the abutments had 10% of the surface area with a high probability of active corrosion and only 18% of the surface area with a low probability of active corrosion.
One core was extracted from each of the 15 abutments for carbonation. Carbonation of concrete occurs when its pH (normally pH 11 or 13) drops below pH 10. If the reinforcement is in carbonated concrete, corrosion can initiate because insufficient alkalinity exists to support the naturally occurring passive oxide film. The test results indicated that the maximum depth of carbonation was 0.75 in. and that carbonation was not causing corrosion.
One core was extracted from each abutment and tested for concrete resistivity. The lower the resistivity, the higher the potential for corrosion, as it is easier to transfer ions through concrete. Resistivity testing also aids in the selection and design of various types of galvanic and impressed current cathodic protection. The average saturated resistivity for all samples tested was 8,386 Ω*cm and the average dry resistivity was 25,801 Ω*cm.
In summary, the condition investigation indicated that the abutments were sufficiently chloride contaminated to cause corrosion of the reinforcing steel. The structures are in the early stages of deterioration with 10% of the area with a high probability of active corrosion but only 4% of the surface area currently experiencing concrete damage. Over time, the structures would continue to corrode, and the deterioration was expected to increase exponentially. Given their current state, the abutments could be preserved and the service life extended with an appropriate intervention.
From May 2016 to September 2017, Vector Construction performed electrochemical chloride extraction on 11 of the Vine Street Expressway abutments.
Trying to manage
After the initial testing phase was completed, it was clear that localized concrete repairs were needed, and that removal and replacement was not necessary. The question that remained was how to manage the corrosion to preserve the entire abutment and meet the service life needs. The collected survey information was used to analyze several different options to rehabilitate the reinforced concrete abutments including embedded galvanic anodes, surface applied galvanic anodes, impressed current cathodic protection and electrochemical treatments such as electrochemical chloride extraction (ECE).
After considering all factors, ECE in combination with localized concrete repair was chosen as the most appropriate corrosion protection technology for this structure. ECE is an electrochemical process used to mitigate reinforcement corrosion in deicing salt-contaminated concrete structures and was deemed to be one of the most promising technologies coming out of the 1987 FHWA Strategic Highway Research Program.
Unlike a cathodic protection system, which is permanently installed onto the structure, the ECE process is a short-duration treatment that provides long-term corrosion mitigation by directly addressing the cause of corrosion. The ECE treatment simultaneously reduces chloride levels within the structure and generates higher alkalinity around the reinforcing steel to mitigate active corrosion and reinstate the passivity of steel reinforcement. No permanent system is left in place on the structure including equipment or wiring that can be vandalized or otherwise damaged. ECE maintains the original appearance of the structure and eliminates the need to operate, monitor, and maintain a cathodic protection system over the life of the structure.
In April 2015 PennDOT launched the major bridge rehabilitation project to ensure the continuing safety and stability of the expressway. This program included the replacement of seven deteriorating bridges in Center City. The existing bridges (two-span non-composite, prestressed adjacent concrete box beam superstructures with a center pier) were to be replaced with single span structures. The abutments were to be repaired and preserved to extend their life using the ECE process. Construction began in the spring of 2015, and an accelerated schedule was implemented to allow the overall project to finish by the fall 2018, more than a year earlier than originally planned.
Putting a charge in it
From May 2016 through September 2017, Vector Construction Inc. performed ECE on 11 of the Vine Street Expressway abutments. The ECE system was installed as follows:
- All delaminated or loose areas of concrete were located and repaired prior to the installation of the system;
- All cracks in the concrete were repaired or sealed prior to installation of the system. This was necessary to avoid electrical shorts and ensure a uniform treatment to the entire concrete surface area;
- The existing paint coating was removed prior to the treatment. Heavy sandblasting was used to remove the coating. No additional surface preparation was required;
- Connections to the reinforcing steel were made by locating the reinforcing steel and drilling to make a mechanical/electrical connection. The connections were then sealed with a two-part epoxy to protect the connections from corrosion. The rebar connection wires were routed to a junction box which was then connected to the rectifier;
- Wooden strips were attached to the surface of the concrete using non-conductive anchors to avoid electrical shorts with the rebar. The strips act as a non-conductive spacer for the anode;
- A medium-gauge welded wire steel mesh was attached to the wooden strips and secured using non-conductive straps. The welded wire mesh acts as the anode when the ECE system operates;
- An electrolytically conductive media was sprayed over and through the steel mesh to the surface of the concrete, and brought to a level of approximately 1 in. beyond the surface of the wire mesh. The conductive media acts as an electrolyte that distributes the current uniformly over the entire surface of the concrete;
- To keep the conductive media wet throughout the treatment, a drip hose was installed along the top of the structure with intermediate lines installed incrementally along the height of the abutment;
- The steel mesh was divided into sub-zones of approximately 100 sq ft each. The sub-zones were installed such that they were electrically isolated from each other. This was necessary for monitoring purposes to be able to locate electrical shorts or any problems with electrical connections;
- The structure was then covered and wrapped with 6-mil plastic sheeting to retain moisture and prevent the system from drying out; and
- The sub-zones were connected to the rectifier. All wires were tested for continuity to ensure all connections were good.
Before the rectifier was turned on, the conductive media was soaked with potable water by turning on the watering system for approximately 24 hours. The flow of water through the cellulose fiber provided an electrically conductive electrolyte while simultaneously washing away any chloride ions brought to the surface. The rectifier was switched on and operated in the constant voltage mode. The DC output was operated and controlled according to the specification and industry standards.
During the treatment time, electrical measurements and system inspections were necessary. Current and voltage output measurements at the rectifier and current measurements for each sub-zone were taken daily to document progress of the treatment. A visual inspection also was carried out daily to ensure that there were no visible problems.
The ECE treatment passed a total charge of over 58.0 A-h/sq ft of concrete surface area. The process reduced the chloride ion concentration at the level of the reinforcing steel to below the specified value at test locations. The 60-day treatment period specified in the construction documents also was exceeded. Based on the pre- and post-ECE data collected, all abutments were successfully treated and the project specification requirements were met.
Current and voltage output measurements at the rectifier and current measurements for each sub-zone were taken daily to document progress of the treatment.
Adding years
The Vine Street Expressway bridge replacement and abutment rehabilitation project was completed on seven bridges which span across the western end of the Vine Street Expressway from 22nd to 18th streets. The bridgework for this project included the replacement of the existing two-span box beam superstructures with single-span superstructures while maintaining and retrofitting the existing reinforced concrete abutments. The bridge superstructures were replaced, and the highly chloride-contaminated reinforced concrete abutments, which had concrete damage and active reinforcing steel corrosion, were preserved.
Field corrosion evaluation testing of the substructures was done to ensure that the service life of the substructures could be increased to match the desired service life of the new superstructures. ECE was selected as the most appropriate long-term repair solution as it met the desired project schedule and allowed the existing abutments to be rehabilitated and reused for the new superstructures.
The new 18th Street Bridge over I-676 was opened to traffic on May 4, 2018, marking the substantial completion of work to replace seven bridges over I-676 in Center City. The Vine Street Expressway continues to be a crucial and integral link in the highway system serving Philadelphia and the region.
About The Author: Whitmore and Stiffler are with Vector Corrosion Technologies.
