October 25, 2007
Piers in distress? Throw them a Lifejacket
By DOUGLAS L. LENG
Corrosion Restoration Technologies
Corrosion of reinforcing steel in concrete is an insidious problem. It significantly affects the functionality of concrete by inducing cracks and spalls, which compromise structural integrity of the concrete member. Engineers looking for ways to solve the corrosion problem are focusing on conserving the structures to avoid the increasingly high costs of new construction.
To deal with this challenge, a system for the repair of concrete and corrosion control, called Lifejacket, is a cost-effective, minimally invasive solution.
Sacrificial anodes are the key to the system. A specially designed anode is encapsulated within an annular cavity created between the existing structure and a two-piece fiberglass form. The approach is particularly advantageous for deteriorating components requiring concrete restoration since it restores the concrete and provides corrosion protection in a single operation, reducing the time necessary to execute repairs. The system is appealing to owners because contractors can install it while the structure remains in use, in most cases.
The corrosion problem
Steel reinforcement in concrete is generally protected from corrosion by the development of a stable oxide film on its surface. This film is formed by the chemical reaction between the highly alkaline concrete, the pore water and the surface of the steel. Corrosion is negligible until the protective layer becomes saturated with chloride ions or by carbonation, which lowers the pH of the concrete. Once this occurs, the steel will begin to corrode rapidly.
For this reason, coastal structures tend to suffer more quickly due to their physical location. In marine environments, chloride ions from seawater migrate into the porous concrete by diffusion and eventually reach the steel reinforcement.
In regions of low resistance, chlorides attack the oxide film and develop anodic and cathodic sites on the steel. The most oxygen-rich regions become the cathode and will fuel the corrosion reaction.
In 1982, the Federal Highway Administration described cathodic protection as the only rehabilitation technique proven to stop corrosion in salt-contaminated structures, regardless of the chloride content in the concrete.
There are two forms of cathodic protection: impressed current and sacrificial (galvanic). Although both systems control corrosion by providing an electrical current to an affected region, there are significant differences in how they are maintained over the life of the system.
A galvanic system, such as the Lifejacket, provides its own power and regulates its current output according to the changing environmental conditions. There is little need for post-installation maintenance and monitoring.
Impressed current systems require maintaining rectifier currents and adjustments to changing conditions. The burden of this ongoing maintenance has serious implications. According to a recent survey, although 80 percent of the impressed current systems are still functional, several problems have occurred due to their complexity and lack of proper service and monitoring.
Sacrificial cathodic protection offers a noticeable advantage on structures where the owners do not have cathodic protection experts on staff.
Simple installation, economical
One of the biggest advantages in using the Lifejacket system is that it restores concrete section loss and provides structural strengthening. Also, there is no need for wiring and complex conduit systems. The system operates maintenance-free over its design life with no additional utility bills, consultant fees or reapplication costs.
The system can be installed using simple construction procedures with qualified contractors. The deteriorated concrete is removed with chipping hammers and pneumatic demolition equipment. Once the corroded steel is exposed and all loose concrete has been removed, the piling is sand- or hydro-blasted to produce a good conductive interface for making the system active once the filler material is added.
In order for the system to work properly, the steel reinforcement must be continuous. If not, continuity must be established throughout the structure prior to making the connections. The connection can be made through a single excavation to a sound reinforcement bar or strand that is in the region requiring protection. This will become the anode-cathode connection and must be done in compliance with design specifications. A standard junction box is used to house all the functional wired connections and may serve as a site for shunting the circuit when current and voltage measurements are required.
In a truly galvanic system, the current output will self-adjust depending on environmental conditions and will achieve electrical equilibrium with the embedded steel. Once this equilibrium has been met, the system will continue to regulate its current output and will not over-protect the steel. In certain cases, impressed current systems that are not properly maintained have produced excessive currents that led to hydrogen over-voltage and embrittlement of the steel. This is a significant benefit of galvanic technology and should be considered when designing a repair strategy to protect prestressed or post-tensioned structures.
The Florida Department of Transportation played a key role in the development and testing of the Lifejacket system partly because the state has a significant stake in the battle against corrosion. With its many bridges in saltwater environments, Florida constantly must repair and protect pilings. In the mid-1990s, Florida began using the Lifejacket system.
“In 1994, the first Lifejackets were installed at the Broward River Bridge on an experimental basis,” says Ivan Lasa, FDOT corrosion mitigation and rehabilitation technologist. Following the successful implementation at Broward River, FDOT installed additional Lifejacket systems in 1995 at Orange Avenue and Main Street Bridges in Volusia County.
“Ten years later, these bridges are still functioning in a satisfactory manner with no additional corrosion damage observed,” says Lasa.
Since the first Lifejacket was installed on the Broward River Bridge, FDOT has specified approximately 3,000 Lifejackets for 70 bridges throughout Florida.
Galvanic cathodic protection has proven itself as a technology for repair and protection against corrosion in marine environments. Based on the findings in Florida, the technology is well suited for similar marine applications.
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