Well-documented efforts undertaken two decades ago to mitigate corrosion of a Highway 394 reinforced concrete bridge have given researchers the perfect scenario for evaluating the treatments’ long-term effectiveness. The test results are mixed: State-of-the-art methods for electrochemical chloride extraction and fiber-reinforced polymer wrapping of bridge elements performed well in combination, but poorly in isolation.
In climates like Minnesota’s, corrosion from exposure to water and deicing salts containing chlorides can shorten the service life of reinforced concrete bridges. Corrosion begins once chlorides become concentrated at the depth of reinforcing steel.
While corrosion primarily affects bridge decks, exposure to plowing spray as well as leaking expansion joints and drainage systems can lead to damage in reinforced substructures. Left unchecked, corrosion can significantly reduce load capacity of bridges because of a decrease in the volume of steel or by rust expansion that causes concrete to crack or spall. Repair typically entails removing damaged concrete, cleaning exposed reinforcement and placing repair material.
These repairs may be short-lived if nearby concrete is also chloride-contaminated or corrosion activity is latent. To enhance repair durability or limit corrosion recurrence, crews may seal or cover concrete to prevent water ingress, or may remove chloride ions from otherwise sound concrete. For many strategies, however, long-term effectiveness remains unclear.
“These were new technologies at the time. This was a unique opportunity to evaluate actual performance after 20 years of service in our environment. Results were mixed, but not surprising,” said Mark Chauvin, associate principal, Wiss, Janney, Elstner Associates Inc.
In 1998, researchers tested several corrosion mitigation strategies on support piers of a Highway 394 bridge built in 1970 over Dunwoody Boulevard west of downtown Minneapolis. Researchers evaluated five piers—each with three columns and a pier cap—treated with electrochemical chloride extraction (ECE), a fairly new option in the late 1990s; various coatings and conventional repairs; and fiber-reinforced polymer (FRP) wraps.
What Was Our Goal?
The MnDOT Bridge Office needed to evaluate the performance of these treatments after 20 years. Researchers aimed to determine the effectiveness of ECE and FRP wraps in terms of costs and benefits.
What Did We Do?
Investigators, led by a researcher involved in the 1998 study, generally repeated the inspection and testing strategy employed 20 years earlier. Researchers visually inspected and conducted extensive hammer sounding of pier caps, and mapped 1998 and current deterioration assessments on the concrete. They also performed corrosion potential testing and extracted 46 concrete cores near areas of 1998 concrete samples for laboratory analysis of chloride content. Using data from both 1998 and 2018, researchers compared mapped deterioration, embedded chloride levels and other corrosion indicators.
What Did We Learn?
Together, ECE and FRP wrap worked very well in arresting 1998 chloride-related damage and resisting new corrosion-related deterioration. None of the other treatment combinations or individual applications of ECE or FRP wrap performed as well.
The greatest recurrence of distress occurred in sections that received ECE followed by penetrating sealers, which performed comparably to control areas that received no ECE, FRP or surface-protecting treatment. Exposure to new moisture appeared to influence repair longevity more than any specific treatment approach.
Chloride reductions were not apparently sustained where chloride exposure persisted. All piers showed significant increases in chloride contamination over 20 years, indicating that wraps and sealers alone did not prevent chloride infiltration for extended periods. Although ECE achieved significant and immediate chloride reductions in 1998, chloride levels exceeded pretreatment levels almost uniformly by 2018, regardless of wraps and sealants used.
FRP wraps did not effectively waterproof treated elements, and they inhibited visual inspection. Where FRP was used over repairs or chloride-contaminated concrete, new distress behind wrap exposed to significant moisture was identified by hammer sounding, a technique not routinely performed during inspections.
“The chloride treatment done in 1998 was successful at first, but now the chloride levels are high again. The biggest lesson was to try to keep the leak sources, the joints, in good condition,” said Paul Pilarski, bridge construction and scoping engineer, MnDOT Bridge Office.
Conventional concrete repair treatments showed a high distress recurrence rate, but appeared to be the most cost-effective approaches for 1998. Though combining ECE and FRP was the most successful approach, it was also the most expensive. ECE treatment also requires long-term access that is impractical for areas where traffic cannot be diverted for several weeks. The high expense, construction time and access requirements limit viable locations as candidates for ECE application.
This study confirmed that the best approach to corrosion mitigation is to minimize water and chloride exposure through maintenance and routine repair of deck joints and drainage systems. Chloride contamination levels found at this site exceeded thresholds at which corrosion is traditionally seen. Further research may help improve guidance on chloride tolerance of steel in concrete in northern climates.
Distress areas found in 2018 will be repaired in spring 2020. FRP wrap will be removed and concrete distress and steel corrosion will be identified, allowing evaluation of the impact of FRP wrap and corrosion expectations predicted by current chloride guidance.
This post pertains to Report 2019-45, Evaluation of Electrochemical Chloride Extraction, Fiber Reinforced Polymer Wraps and Concrete Sealers for Corrosion Mitigation in Reinforced Concrete Bridge Structures, published December 2019.