Corrosion of reinforcing steel in concrete is a dangerous problem—and all too common for the many bridges in Florida's salt-water environments. It induces cracks and spalls, which compromises structural integrity and public safety. Florida's warm climate and abundance of chlorides make the bridges highly susceptible to corrosion.
Since construction costs are on the rise, the ideal solution should solve the corrosion problem without having to replace structures. In the mid-1990s Florida DOT (FDOT) turned to a minimally invasive concrete repair and corrosion control system for steel-reinforced concrete on bridges and other saltwater structures, called the Lifejacket system.
“Ten years later, we see no additional corrosion damage,” says Ivan Lasa, FDOT corrosion mitigation and rehabilitation technologist. This is a far cry from previous patch repair methods that only restored absent concrete and did very little to address corrosion or maintain structures.
The system is now one of FDOT's standard methods for repairing pilings in marine environments.
Battle Of The Currents
FDOT must constantly repair and protect pilings in corrosion-damaged bridges. Since corrosion is caused by an electrochemical reaction between the metal and its environment (the metal loses electrons), one way to combat it is with cathodic protection, where corrosion is controlled by sending an electrical current to an affected region.
There are two forms of cathodic protection: impressed current and sacrificial (galvanic). Impressed current systems use a rectifier or permanent external power source. Sacrificial cathodic protection uses the principal of galvanic cells (dissimilar metals) as the energy source. Both systems establish a current flow from an externally placed anode that negatively polarizes the corroding steel, creating electrical equilibrium with the embedded steel.
The difference between these systems lies in how they are maintained. With impressed current systems, making adjustments to changing environmental conditions, such as temperature and humidity, becomes an ongoing burden. But a galvanic system provides its own power and regulates its current output according to conditions, so it requires minimal maintenance and monitoring.
FDOT uses both systems, but decided on the low-maintenance, galvanic Lifejacket system for bridges that need a limited number of piles repaired.
“In 1994, the first Lifejackets were installed at the Broward River Bridge on an experimental basis,” says Lasa. Since then, FDOT has specified approximately 3000 Lifejackets for 70 bridges throughout Florida.
Nuts And Bolts
The Lifejacket system restores concrete section loss, offers structural strengthening, and provides corrosion protection in a single operation. In most cases, qualified contractors can install the system while the structure remains in use.
A durable, stay-in-place fiberglass form positions the anode relative to the embedded steel and creates the essential annular space for filling with an approved material. Plus, a supplemental bulk anode protects the submerged portion of the structure against corrosion and minimizes the current demand on the lower portion of the anode mesh, which is subjected to more frequent wetting by tidal action.
During installation, deteriorated concrete is removed to expose the corroded steel. Next, the piling is sand- or hydro-blasted to produce a clean, conductive interface. A wooden bottom form retains the grout during pouring and curing, and is later removed. The final wire connection is made, and the system becomes operational.
For Lifejacket to work properly, continuity must be established 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 becomes the anode-cathode connection and must be done in compliance with design specifications.
Once installed, it operates maintenance-free over its design life with no additional utility bills, consultant fees, or reapplication costs.
In certain situations, the Lifejacket is not the best solution.
“Because of the limited voltage output of the zinc mesh anode, it is not the best approach for high electrical-resistance concretes,” says Lasa. “It is also not recommended for piles other than those in marine environments in direct contact with water. Under dry conditions, the zinc mesh tends to passivate. The fiberglass stay-in-place forms retain moisture to keep the anode active.”
In particular, a galvanic system needs the saltwater (electrolyte) to complete the circuit (battery cell). This is what makes the Lifejacket well suited for the splash area of marine structures, says Lasa. “It combines concrete restoration and corrosion control, which reduces the time necessary to execute repairs.”
— Douglas Leng heads business development with Tequesta, Fla.-based Corrosion Restoration Technologies, and is a co-inventor of the Lifejacket system.